CHAPTER 3. SYNTHESIS AND CHARACTERIZATION OF MgB 2, AlB 2 AND NbB 2

Transcription

1 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION OF MgB 2, AlB 2 AND NbB 2

2 3.1 INTRODUCTION Discovery of superconductivity in MgB 2 with the critical temperature of 39 K [1] had been of prime importance for the scientific community in superconductivity regime. The relatively higher T c [1], simpler structure with lower anisotropy [2] and nearly transparent grain boundaries [3] attracted a huge interest in theoretical and experimental research [4-6] in this compound. Various properties like thermal and electrical conduction [7-9], specific heat [10-11], isotope effect [12-13] and doping effects [14-16] were studied. The reason behind the superconductivity was the strong electron phonon interactions in this compound because of lighter elements like Boron. In fact, evidence of isotope effect yielded a clear indication that phonons play an important role in pairing mechanism of this compound [12-13]. This idea leads us to think about the existence of superconductivity in other Boron related compounds i.e other diborides. AlB 2 and NbB 2 are the diborides whose crystal structure is exactly same as that of MgB 2. These three crystallize in the same space group P6/mmm. But in comparison of MgB 2, very few reports exist on other diborides; even the existence of superconductivity is suspected in some of the diborides. For example a very few reports exist on AlB 2 [17-19]. Similarly, Gasprov et al. and others [20, 21-23] have reported no observation of superconductivity in TaB 2, while Kackzorowski et al. [24] report a transition temperature of 9.5 K. The results for NbB 2 are even more diverse. Gasprov et al. [20] and Kackzorowski et al. and others [24, 25] report no superconductivity, while many others [21, 26, 27] report different values of transition temperature in the range of K. In order to search for superconductivity in other diborides, we first of all synthesized these three in their phase pure form giving various heating schedule followed by characterization on the best-optimized sample. In particular, the focus is given on synthesis, structure/microstructure, resistivity, susceptibility, Raman spectroscopy, thermoelectric power S (T), and magnetization properties. 3.2 SYNTHESIS AND PHYSICAL PROPERTY CHARACTERIZATION OF MgB 2 47

3 3.2.1 EXPERIMENTAL Synthesis of samples The details of sample synthesis are mentioned in section 2.2 under Chapter 2. Briefly, polycrystalline MgB 2 samples were synthesized by solid-state reaction route using ingredients of Mg and B at different set of temperature and holding time periods. For individual sample, the constituent powders of Mg and B were taken in stoichiometric ratios according to the formula unit MgB 2 and were mixed thoroughly by continuous grinding for 1-2 hours. The mixed homogenous powders were pelletized in rectangular pellets and were enclosed in soft Fe-tube subsequently placed in Alumina boat. The encapsulated system was placed in a tubular furnace equipped with an arrangement of continuously flowing Argon. The samples were heated at different temperatures in the range from C to C and held at that temperature for different time periods varying in range from 1 to 3.5 hours in flow of Argon gas at ambient pressure and subsequently allowed to cool up to room temperature in same atmosphere. Both ends of Fe-tube were open for the continuous passage of Argon gas. The resultant samples were bulk polycrystalline black compound Characterization of the samples The characterizations of the samples were carried out by the following techniques: X-ray diffraction Studies Details of X-ray diffraction technique are described in Chapter 2 under Section Briefly, the X-ray diffraction patterns of all the synthesized samples were recorded using CuK radiation Scanning Electron Microscope (SEM) Studies The micro structural investigation of the MgB 2 samples was carried out using scanning electon microscopy, details of which are described in Chapter 2 under Section The Scanning Electron Microscopy (SEM) studies were carried out on these samples using a Leo S-440 (Oxford Microscopy: UK) instrument. 48

4 Raman Spectroscopy Measurements Details of Raman Spectroscopy are given in Chapter 2 under Section Raman measurements on MgB 2 sample were performed on a dispersive single Horiba Jobin Yvon Hr-800 mono-chromator coupled to a charge couple device. The 488 line of an argon ion laser was used as a probe beam that is focused on to a ~2 μm spot. The power was kept to a minimum of ~2 mw at the sample, and all the measurements were carried out in a back scattering geometry with detection in the un-polarized mode Thermoelectric power measurements Thermoelectric Power (TEP) measurements on MgB 2 sample were carried out by dc differential technique over a temperature range of 5 300K, using a homemade set-up. Temperature gradient of ~1K is maintained throughout the TEP measurements. Details of this technique are given under Section in Chapter Resistivity measurements Temperature dependent resistivity measurements on MgB 2 sample were carried out using four-probe technique on a closed cycle refrigerator in the temperature range from K. Details of temperature dependent resistivity measurements are given in Chapter 2 under section Resistivity measurements on MgB 2 sample were also done under applied magnetic fields of up to 14 Tesla with field applied perpendicular to current direction, using four probe technique on Quantum Design Physical Property Measurement System (PPMS)( Chapter 2, Section ) Magnetization Studies Details of magnetic characterization of samples are given in Chapter 2 in section and section Briefly, Magnetization measurements on MgB 2 sample were carried out with a Quantum-Design Physical Property Measurement System (PPMS) having Vibrating Sample Magnetometer (VSM) attachment RESULTS AND DISCUSSION Structure and Microstructure 49

5 I (arb. units) I (arb. couns) 750 o C 3.5h 750 o C 2.5h 750 o C 1h 650 o C 3.5h 650 o C 1h (deg.) MgB 2 Ar annealed MgB 2 Mg MgO Fig. 3.1 X-ray diffraction patterns of MgB 2 samples sintered at different temperatures in the range o C for different time spans. 950 o C 3.5h 950 o C 1h 850 o C 3.5h 850 o C 2.5h (100) (001) 850 o C 1h (101) MgB 2 Ar annealed (002) ( deg.) Fig. 3.2 X-ray diffraction patterns of MgB 2 samples sintered at different temperatures in the range O C for different time spans. Fig. 3.1 depicts the room temperature X-ray diffraction (XRD) pattern of variously synthesized MgB 2 samples. Fig. 3.1 shows the samples synthesized at 650 o C and 750 o C for different time periods of 1h, 2.5h and 3.5h. Various Mg peaks are observed along with the MgB 2 phase peaks in all these XRD patterns. It means that Mg reacts with Boron partially to form MgB 2 phase so that unreacted Mg remains as it is in the sample. Moreover, it is observed that samples synthesized at 750 o C have more prominent MgB 2 peaks as compared to the samples synthesized at 650 o C. It prompts us to synthesize samples at further high temperature greater than 750 o C. Fig. 3.2 shows the X- ray diffraction patterns of MgB 2 samples synthesized at 850 o C and 950 o C for time periods of 1.5h, 2.5h and 3.5h. Although MgB 2 phase is formed completely and all characteristic bragg peaks are observed in the sample synthesized at C with holding (110) (102) (200) (111) (201) Mg MgO 50

6 I (arb. units) time of 1h but it is accompanied with the unwanted phase of Mg. The problem sorts out on increasing the time span up to 2.5 h at the same temperature. It is found that the sample synthesized at 850 o C with holding time of 2.5 h possesses all peaks of MgB 2 phase in appropriate relative intensity and does not have any Mg peak. Only a minor peak of MgO is obtained at 63 o. Increasing the temperature and time beyond this (850 o C, 2.5h), adversely affects the phase purity of the samples. The other unwanted phase like MgO starts appearing and a multi phase compound is obtained at further high temperature. Thus, it is observed that the sample synthesized at a temperature of 850 o C MgB o C Ar annealed MgO Atomic positions: Mg(0,0,0) B (1/3,2/3,1/2) P6/mmm a = (23)A o c = (30) A o (deg.) Fig. 3.3 Rietveld analysis of Ar annealed MgB 2 sample sintered at 850 O C and 2.5 h is the phase purest among all synthesized samples. Except for a weak reflection at 2θ = 63 0, corresponding to pure MgO [6, 14, 26-29], the rest of the Bragg reflections are characteristic of the hexagonal MgB 2 structure. MgO peak is marked in the pattern in Fig The indexing of respective XRD peaks is also done which corresponds to known hexagonal Bravis lattice. The structure of MgB 2 belongs to space group P6/mmm. The asymmetric unit of the structure consists of Mg at (0, 0, 0) and B at (1/3, 2/3, 1/2). Preliminary Rietveld refinement is carried out using the program Fullprof for the bestsynthesized sample and is shown in Figure 3.3. The occupancy parameters of the various atoms and their positions are fixed at their nominal values. The points correspond to the experimentally observed data while the line corresponds to the theoretically fitted curves obtained from Rietveld refinement. The differences between the experimental and 51

7 calculated XRD patterns are very small and are shown by a line curve at the bottom of the graph. Bragg peaks obtained from Rietveld refinements are shown by bars below the experimental and theoretical curves. It can be clearly seen that all the bragg peaks are obtained in the experimental data thus confirming the pure phase formation of MgB 2. The lattice parameters calculated from rietveld refinement are a = (23) Å, and c = Fig. 3.4(a) Fig. 3.4(b) Fig. 3.4 Scanning electron microscope (SEM) pictures of C Argon annealed MgB 2 compound at (a) 2 K X and (b) 10 K X (30) Å, with c/a ~ The lattice parameters are though close to the vacuum annealed MgB 2 [26], the presence of MgO is comparatively less in Argon annealed sample, as seen by XRD pattern (Fig. 3.3). As far as the presence of minute amount of MgO is concerned, the same is seen in earlier reports as well [6, 26-35], but is not always marked on the respective XRD pattern. Scanning Microscope Electron (SEM) images of present sample are shown in Fig. 3.4(a) and (b) at different magnifications of 2KX and 10KX respectively. Nearly homogenous distribution of crystallites can be seen in SEM picture in Fig. 3.4(a). The average grain shape is like platelets with size 2-6 m. The enlarged view can be seen

8 Raman Intensity in Fig. 3.4(b) where some porous regions can be seen. The shape and size of observed grains for present MgB 2 are in general agreement with the reported literature on this compound [30-32] Raman studies Room temperature Raman spectrum of the optimized (850 0 C 2.5 hours) pristine MgB 2 7x10 3 6x10 3 5x10 3 4x10 3 3x sample is depicted in Fig The phonon peak occurs at 600 cm -1, this is in confirmation with various earlier reports on this compound [33-35]. The 600 cm -1 is the characteristic e-ph (electron-phonon) coupling peak for superconducting MgB 2. This arises from E 2g phonon mode, being ascribed to the in-plane B bond stretching. Any disorder in terms of onsite substitution in MgB 2 results in weakening of e-ph coupling and a shift in peak position to higher energies. The 600 cm -1 e-ph peak in MgB 2 is unusual for AlB 2 class (hexagonal P6/mmm) materials. Strong electron-phonon coupling due to softening of e-ph modes is one of the reasons behind 39K superconductivity of MgB 2. As mentioned in introduction the phonons play an important role in pairing mechanism in this compound [12, 13], and the 39 K superconductivity could be just at the BCS strong coupling limit. An early indication of the strong phonon contribution in MgB 2 can be presumed on the basis of its stretched c-lattice parameter. The phonon peak at 600 cm -1 is seen only in superconducting MgB 2, which moves towards higher wave number side accompanied by suppression in the superconductivity with any substitution either at Mg or at B site. MgB Raman Shift (cm -1 ) Fig. 3.5 Raman spectra for C, 2.5 h Argon annealed MgB Thermoelectric power Analysis

9 S ( V/K) S = AT+BT 3 Chi 2 = A = ± V/K 2 B = 9.472E-7 ± E-8 V/K 4 MgB T (K) 54 The S (T) plot of present MgB 2 is shown in Fig The absolute value of S is positive, which indicates towards the hole type conductivity in this system. The binding energy of Mg in MgB 2 is reported less than as expected for Mg +2 [36]. Lowering of Mg charge results in to electron transfer in Boron (B) giving rise to holes in the band [36]. It is the to electron transfer which gives rise to hole superconductivity in MgB 2. Superconducting transition (T c ) is seen as S=0 at 38K. The room temperature thermoelectric power S 300K is around 7 V/K, which is comparable to that as reported in refs. [31, 37-39]. However the shape of S(T) plots is slightly different in all the references, in particular the change in slope of S(T) before onset of T c. Shape of our S(T) plot is more close to that as in ref. [31, 39 & 40] but slightly different from that as in refs. [37 & 38]. S (T) has mainly the contributions from electrons and phonons. The normal state thermopower in MgB 2 in the range of ~40 to ~100K may be explained by S 3 Sd Sg AT. B. T (1) Where S d is the electronic diffusion term and S g is the phonon drag term. 2 K B coefficients A and B are given by γ/ne and 3 e na 5 D The respectively where n a is the number of free electrons per atom and D is the Debye temperature. Generally, the phonon relaxation time for interaction with other phonons and impurities is much larger than the relaxation time for the phonon-electron interaction. This is quite important below the Debye temperature ( D ). Theoritical Curve Experimental curve Fig. 3.6 Experimental S(T) plot of C Argon annealed MgB 2 fitted with single band model The theoretical curve of S(T) is drawn using the equation S = AT + BT 3. By fitting the

10 experimental curve with theoretical equation, we found the fitting parameters A = V/K 2, B = V/K 4. The estimated Debye temperature ( D ) is around 1407 K. Besides the D, the carrier density (n) is also estimated from the S(T) equations and A/B constants. The estimated Debye temperature ( D ) and carrier density (n) are found to be comparable ( D = 1407K and n = /m 3 ). The equation (1) for thermoelectric power corresponds to a single band system while MgB 2 has two bands and. So, we can modify equation (1) so that it is applicable for two band system and we obtain the fitting of thermoelectric power data in the whole temperature range from T c to room temperature. As discussed earlier, thermoelectric power has two terms; diffusion term and the phonon drag term. The diffusion term here is also same i.e. S d = AT (2) but the phonon drag term is subdivided into two; phonon drag term due to pi band, S pd, and phonon drag term due to sigma band, S pd,. For the 3D -band the phonon drag contribution will vary like T 3 for low T [37], and like T -1 for high T [41]. However, for the intermediate temperature values, the behavior of the phonon drag TEP is given by a very complicated expression [41-42], but still the same varies smoothly from low T values to the high T values. We thus hope that a simple interpolation will provide a reasonable phenomenology of the variation of the phonon drag contribution to TEP from T 0 to T. Since TEP varies as T 3 for T 0 and as T -1 for T, we consider the interpolation S pd, = T 3 /(B+CT 4 ) (3) for the 3D - band. Here the suffix pd on S implies phonon-drag and the suffix implies the 3D - band. From Eq. (3), we see that, S pd, T 3 /B for T 0. Here B should vary like D 3 [43]. Here 55

11 S ( V/K) D is the Debye temperature. C is also a constant, but not dependent on D. In order to work out an expression for the phonon drag contribution of the 2D -band, we notice that the 2D character of the -band will allow the phonons to drag only in a 2D plane. Because of this, the reason that led to a T 3 variation of the phonon drag contribution from a 3D band, will lead to a T 2 variation for a 2D band. Thus using the interpolation similar to that of Eq. (3), we may express the phonon drag TEP due to the -band by S pd, = T 2 /(D+ET 3 ) (4) where D is expected to vary like 2 D. Combining all the contributions from Equations (2) - (4), we obtain the following expression for the TEP of MgB 2 samples. S = AT +T 3 /(B+CT 4 ) + T 2 /(D+ET 3 ) (5) 8 6 S=AT+T 3 /(B+CT 4 )+T 2 /(D+ET 3 ) MgB 2 Using Eq. 5, the experimental data is fitted and shown in Fig The Chi 2 = A = ± V/K 2 B = ± K 4 / v C = ± ( V) -1 D = ± K 3 / V E = -9.7E-42 ± -- ( V) T (K) Fig. 3.7 Experimental S(T) plot of C Argon annealed MgB 2 fitted with two band model experimental data is shown by open circles while the fitted curve is shown by the solid line. From Fig. 3.7 we see that Eq. (5) provides good agreement of the theoretical data with the observed data in the whole temperature range. The fitting parameters are given in the inset of Fig The parameter E acquires practically zero value. Now, we can also know about the relative contributions of the diffusion process and phonon drag process. For this purpose, 56

12 we take a specific temperature T = 100 K, and consider the values of S for MgB 2 sample. From the calculated values, it turns out that the pure MgB 2 sample corresponds to S d = 4.0 V/K, S pd, = V/K and S pd, = V/K. We see that the phonon drag term also corresponds to significant contribution Resistivity Analysis The resistivity (T) plot of polycrystalline MgB 2 is shown in the Fig The critical ( -cm) MgB T (K) temperature T c and room temperature resistivity ( 300K ) are found to be 37.8 K and cm respectively. The superconducting transition temperature (T c ) is defined as temperature where 0. For phase pure polycrystalline MgB 2, the 300K value in literature is Fig. 3.8 (T) plot for C Argon annealed MgB 2 reported around cm [12, 31, 44]. In polycrystalline samples, the connectivity of grains affects the conduction in a major way and comparatively higher value of room temperature resistivity indicates towards lesser grain connectivity in MgB 2 sample. The porous nature of MgB 2 makes this problem more serious. Though the Arannealed MgB 2 samples are comparatively less porous than the vacuum annealed ones [29], but the porous regions are still seen clearly in SEM pictures, see Fig The presence of insulating MgO (Fig. 3.3) may also be a factor behind less connectivity of grains in polycrystalline MgB 2 but the main cause is its porous nature. In normal state i.e., above T c onset, the compound is metallic with residual resistivity ratio ( 300K / onset ) of 3.6, which is generally defined by RRR (residual resistivity ratio). For disordered samples, the RRR comes down rapidly, for example, RRR is only 1.5 for 57

13 MgB 2-x C x samples [44]. The RRR of polycrystalline bulk MgB 2 in ref. 31 is 3.0, which is close to the present value. The comparatively higher 300K and low RRR values demonstrate that present MgB 2 sample is disordered. The presence of small amount of MgO (Fig. 3.3) and the various types of defect features (Fig. 3.4) in present sample are responsible for the above observation Magneto transport Resistivity versus temperature under magnetic field, (T)H plots of present MgB 2 sample, are shown in Fig The superconducting transition temperatures (T c ) are defined by that temperatures at which 0. The (T)H measurements show T c ( 0) is 37.8 and 5.8 K under zero and 14T(Tesla) applied fields respectively. The transitions are single step in both the absence and presence of field. The transition is quite sharp with a transition width of 0.9 K at zero field, but with the application of field, transition broadens and ( -Cm) MgB transition width increases up to 10 K for 14 T field. Fig represents the Upper critical field, H c2 versus T plot in the temperature range from K, where upper critical field (H c2 ) is defined from the 90% of the resistive transitions [45] i.e. H c2 = H at which =90% 40. Using this criterion, H c2 is determined from (T)H data. Values of T c ( 0) and T c ( =90% 40 ) corresponding to applied magnetic fields are tabulated in table 3.1. Values of Column 1 correspond to upper critical field (H c2 ) values at corresponding temperatures shown in Column 3 as per the criterion of 90% resistive transition. 58 T (K) 0 T 1 T 3 T 5 T 7 T 9 T 11 T 13 T 14 T Fig. 3.9 (T) plots for C Argon annealed MgB 2 with H = 0 to 14 T

14 H c2 (Tesla) Column 1 Column 2 Column 3 H(Tesla) T c ( 0) (K) T c ( =90% 40 )(K) Table 3.1 Values of T c ( 0) and T c ( =90% 40 ) corresponding to applied magnetic fields. Values of Column 1 also correspond to upper critical field (H c2 ) values at corresponding temperatures shown in Column 3 as per the criterion of 90% resistive transition. The same is plotted in Fig As temperature decreases, value of upper critical field increases which is obvious. H c2 vs T curve directly determines the extent of maximum field applied up to which the sample remains superconducting at a particular temperature. Though the present sample is neither doped with nano-particles nor synthesized by special techniques still its, H c2 (T) values are significantly high and are comparable with other reports [14, 28-30,46]. The H c2 (T) value for this sample is 14 T at 12.5 K. In addition, in doped and neutron irradiated [47-48] MgB 2 bulk samples the reported H c2 values are comparable to the present sample e.g. H c2 is around 10 Tesla at 21.5K [47]. In our case (pure MgB 2 ) and in ref. 47, the MgB 2 H c2 =H at ( = ) same criterion of determination of H c2 (90% of the resistive transitions) is used T (K) Fig Upper critical field vs temp. plot Magnetization studies Figure 3.11 depicts the dc susceptibility versus temperature (T) plots in an applied field of 10 Oe, in both zerofield-cooled (ZFC) and field-cooled (FC) situations. It is evident from 59

15 M(emu/g) M (emu /g) MgB FC ZFC H=10 Oe T (K) Fig (T) plot of C Argon annealed MgB MgB2 this figure that MgB 2 undergoes a sharp superconducting transition (diamagnetic, T dia c ) at 37.2 K within less than 1 K temperature interval, without any unusual rounding occurring down to 5K. In fact, the diamagnetic signal remains more or less constant below 36K down to 5 K. It is found that the superconducting critical temperature (T dia c ) being seen from ZFC (T) measurements is in agreement with the T c as seen from (T)-H plots K 20K Fig. 3.12(a) M(H) plots for C Argon annealed MgB 2 sample at different temperatures. 60 The FC (T) plot is rather merged with the zero base line. In fact the FC (T) is very small and close to the instrument detection limit of 10-6 emu. We found that this has happened in most of pinned MgB 2 compounds with the FC signal being very small in comparison to the ZFC. Seemingly the present MgB 2 itself is in the pinned state. Very low value of FC observed in comparison to ZFC signal (Fig. 3.11) in superconducting state i.e., below say 38 K is an indication in this direction. 5K H(Tesla) 20 K 10 K 5 K As far as superconducting volume fraction is concerned, in the case of strong pinning, the

16 J c (A / cm 2 ) FC is too small and hence its estimation is near impossible. One may, however, estimate the volume fraction from ZFC, but that would clearly be incorrect, as this would include a large contribution arising from shielding currents. Figure 3.12(a) shows the 10 5 MgB 2 J c 5K J c 10 K J c 20 K K magnetization (M-H) plots for present MgB 2 sample at 5, 10 and 20 K with applied fields (H) of up to 8 T. The magnetization K 10 K M(H) grows (as usual) slowly with H and falls sharply to near zero H (Tesla) Fig. 3.12(b) Critical current density as a function of the applied magnetic field for MgB2 sample at 5, 10 & 20 K. moment value, and further grows again in a common way. M-H loop for pristine MgB 2 sample closes at 5, 7.5 and 8 T at 20, 10 and 5K respectively. The flux avalanches were seen quite symmetric in both increasing/decreasing fields in all the four quadrants of the M(H) loops which can be seen prominently in Fig for 5K curve. Fundamentally, very low heat capacity and relatively high J c values in MgB 2 are seemingly the cause for the observed complex vortex dynamics resulting in flux avalanches [49-51]. The critical current density, J c is calculated from M-H data using Bean s critical state model. The J c (H) curves for MgB 2 sample at 5, 10 and 20 K are shown in Fig. 3.12(b). It can be seen from the curves that J c is although high of the order of 10 5 A/cm 2 at low field of less than 1 Tesla but it decreases sharply with the application of field. More specifically, the estimated value of J c is around 1.03 x 10 5 A/cm 2 at 10 K in the field of 0.6 Tesla and then it decreases sharply and remains of the order of 10 2 A/cm 2 at 7.5 Tesla. Similarly, J c falls to 10 2 A/cm 2 at only 4.5 T at 20 K. It means that J c falls very sharply with application of field at all temperatures and hence high field performance of MgB 2 61

17 does not seem to be very good and needs to be improved. As per the Literature Review under section 1.5 in Chapter 1, the J c and other critical parameters of MgB 2 can be enhanced either by specific additives or by substituents. This will be discussed in Chapter 6 where significant improvement in critical properties of MgB 2 is observed with nano Carbon substitution at B site. 3.3 SYNTHESIS AND PHYSICAL PROPERTY CHARACTERIZATION OF AlB EXPERIMENTAL Synthesis of samples The details of sample synthesis are mentioned in section 2.2 under Chapter 2. Briefly, polycrystalline AlB 2 samples were synthesized by solid-state reaction route using ingredients of Al and B powders as the precursor materials. The ingredients were taken in stoichiometric ratio according to the formula unit AlB 2 and were weighed precisely using chemical balance of least count g. The powders were ground properly and continuously for 2-3 hours so as to obtain a homogenous mixture. The mixture so obtained was pelletized using a hydraulic press. The pellets were encapsulated in iron tubes so as to inhibit the Al diffusion. The encapsulated system was placed in a tubular furnace equipped with an arrangement of continuously flowing Argon. The samples were heated at different temperatures in the range from C to C and holded at that temperature for different time periods varying in range from 1 to 3.5 hours in flow of Argon gas at ambient pressure and subsequently allowed to cool up to room temperature in same atmosphere. Both ends of Fe-tube were open for the continuous passage of Argon gas. The resultant samples were bulk polycrystalline grey compound Characterization of the samples The characterizations of the samples were carried out by the following techniques: 62

18 X-ray diffraction Studies Details of X-ray diffraction technique are described in Chapter 2 under Section Briefly, the X-ray diffraction patterns of all the synthesized samples were recorded on a diffractometer using CuK radiation Scanning Electron Microscope (SEM) Studies The micro structural investigation of the AlB 2 samples was carried out using scanning electon microscopy, details of which are given in Chapter 2 under Section The Scanning Electron Microscopy (SEM) studies were carried out on these samples using a Leo 440 (Oxford Microscopy: UK) instrument Thermoelectric power measurements Thermoelectric Power (TEP) measurements on AlB 2 sample were carried out by dc differential technique over a temperature range of 5 300K, using a homemade set-up. Temperature gradient of ~1K is maintained throughout the TEP measurements. Details of this technique are given under Section in Chapter Resistivity measurements Temperature dependent resistivity measurements on AlB 2 sample were carried out using four-probe technique on a closed cycle refrigerator up to the temperature as low as 20 K. Details of temperature dependent resistivity measurements are given in Chapter 2 under section Magnetization Studies Details of magnetic characterization of samples are given in Chapter 2 in section and section Briefly, Magnetization measurements on AlB 2 sample were carried out using Quantum-Design 14 Tesla Physical Property Measurement System (PPMS) having Vibrating Sample Magnetometer (VSM) attachment RESULTS AND DISCUSSION 63

19 I (arb. units) I (arb.units) Structure and Microstructure 750 o C 3.5h 750 o C 2.5h 750 o C 1h 650 o C 3.5h 650 o C 1h (deg.) Al AlB 2 AlB 12 Figure 3.13 X-ray diffraction patterns of AlB 2 samples sintered at different temperatures in the range O C for different time spans. 950 o C 3.5h 950 o C 1h 900 o C 2.5h (100) (001) 900 o C 1h 850 o C 2.5h 850 o C 1h (101) (002) (deg.) AlB 2 Figure 3.13 depicts the room temperature X-ray diffraction (XRD) pattern of AlB 2 samples synthesized at 650 o C and 750 o C for different time periods of 1h, 2.5h and 3.5h. In the case of the sample synthesized at 650 o C for 1h time, the sample is completely multi phase and comprises of three main phases identified as Al, AlB 2 and AlB 12. The phases of AlB 2, Al and AlB 12 are respectively marked by, and in Fig AlB 2 peaks are marked only in the pattern at the bottom i.e. for the sample synthesized at 650 o C for 1h. In rest of the samples only impurity peaks are marked. The main peak of maximum intensity in the X-ray diffraction pattern corresponds to the Al phase in the sample synthesized at 650 o C for 1h. Thus, we can say that although AlB 2 peaks are there but it exists as a minor phase in the sample. As the temperature and time are increased further, Al phase suppresses and growth of AlB 2 phase increases which is Al AlB 12 (110) (111) (201) (102) (200) Figure 3.14 X-ray diffraction patterns of AlB 2 samples sintered at different temperatures in the range O C for different time spans. 64

20 I (arb. units) indicated by the intensity of peaks. Now AlB 2 exists as a major phase in samples synthesized at 750 o C but still it is accompanied by the minor phases of Al and AlB 12. This improvement in growth of AlB 2 phase prompts us to synthesize samples at further high temperature greater than 750 o C. Figure 3.14 shows the X-ray diffraction patterns of AlB 2 samples synthesized at 850, 900 and 950 o C for times of 1.5, 2.5 and 3.5h. Although AlB 2 phase is formed completely in the sample synthesized at C, but it is accompanied with the minor peaks of Al and AlB 12. Increasing the temperature up to 900 o C and time up to 2.5h sorts out the problem, i.e. the sample synthesized at 900 o C with holding time of 2.5 h possesses all peaks of AlB 2 phase in appropriate relative intensity and does not have any Al and AlB 12 peak of considerable intensity. A very small AlB 12 peak of about negligible intensity can be seen at an angle of 39.4 o. Increasing the temperature and time beyond this limit adversely affects the phase purity of the samples. The other unwanted phases start appearing and a multi phase compound is obtained at further high temperature of 950 o C. Thus, it is observed that the sample synthesized at a temperature of 900 o C and 2.5 h is the phase purest among all synthesized samples. The indexing of peaks is done in Fig for the Atomic positions: Al (0,0,0) Mg (0,0,0.5) B (1/3,2/3,1/2) AlB 2, P6/mmm 900 O C, Ar annealed a = (17) A o c = (23) A o optimized sample synthesized at 900 o C for time of 2.5 h. All the peaks are characteristic of the known hexagonal Bravais lattice of AlB 2 structure except minor peaks of AlB 12, which are only visible in enlarged pattern in Fig at about 39.4 o (deg.) Fig Rietveld analysis of Ar annealed AlB 2 sample sintered at 900 O C The structure of AlB 2 belongs to space group P6/mmm. The asymmetric unit of the structure consists of Al at (0, 65

21 0, 0) and B at (1/3, 2/3, 1/2). Preliminary Rietveld refinement is carried out using the program Fullprof [52] for the phase pure sample and is shown in Fig The occupancy parameters of the various atoms and their positions are fixed at their nominal values. The points correspond to the experimentally observed data while the line corresponds to the theoretically fitted curves obtained from Rietveld refinement. The differences between the experimental and calculated XRD patterns are very small and are shown by a line curve at the bottom of the graph. Bragg peaks obtained from Rietveld refinements are shown by bars below the experimental and theoretical curves. It can be clearly seen that all the bragg peaks are obtained in the experimental data thus confirming the pure phase formation of AlB 2. All the Bragg peaks are found with appropriate intensity with a minor peaks of AlB 12. The lattice parameters calculated from reitveld refinement are a = (17) Å, and c = (23) Å, with c/a ~ 1.08, which are in agreement with the reported literature [43, 53-55]. As compared with MgB 2, c- parameter is lesser and consequently c/a parameter decreases. a parameter also decreases slightly in comparision to MgB 2 and produces a slight increment in c/a parameter but even then c/a is quite less in comparison to MgB 2 thus confirming that MgB 2 lattice is stretched in c direction as compared to AlB 2. Fig Scanning electron microscope (SEM) pictures of C Argon annealed AlB 2 compound. 66 Scanning electron microscope (SEM) picture of present sample is shown in Fig Nearly homogenous distribution of crystallites can be seen in SEM pictures. The average grain size is about 1-3 m. The overall sample seems to be porous and the presence of secondary phase is also visible. So, at 900 o C, a melt of AlB 12 phase forms and appears in the SEM image. But we have seen that only one peak of very minor intensity appears in XRD pattern of sample, which confirms that AlB 12 phase is present as a

22 S ( V/K) minority and does not have any impact on the physical properties of the main phase AlB 2. The formation of AlB 2 is generally accompanied by the formation of AlB 12 and it is also discussed in Ref. [56] Thermoelectric power analysis The S(T) plot of phase pure sample of AlB 2 is shown in Fig The absolute value of S T (K) AlB 2 Fig Thermoelectric power vs temperature curve for Argon annealed AlB 2 compound. is negative, which indicates towards the electron type conductivity in this system. No superconducting transition (T c ) is seen as S=0. S(T) has mainly the contributions from electrons and phonons. The room temperature thermoelectric power S 300K is around -0.5 V/K. As the temperature decreases from room temperature, the absolute value of thermoelectric power, S increases up to 1.2 V/K at 130 K. Then it remains almost constant up to a temperature of 85 K and decreases further with the decrease in temperature. The sign of TEP is negative throughout the whole temperature range. It indicates that electrons exist as the majority carriers in AlB 2. As discussed under section , majority carriers observed in MgB 2 are holes. This changeover of majority carriers occurs due to the extra electron provided to the boron layer by Al 3+ ion in comparison to Mg 2+ in MgB 2. Thus, the holes of sigma band are completely filled resulting in electron type conductivity Resistivity Analysis Fig shows the temperature variation of resistivity for AlB 2, which was synthesized at 900 o C. As temperature decreases, resistivity of the sample decreases showing metallic behavior. The absolute value of room temperature resistivity of pure AlB 2 is about 48-67

23 ( -cm) M (emu/g) cm. It decreases to 35 -cm up to a temperature of 30 K. Thus, we can say that the 48 AlB o C, Ar annealed x x x x x10-5 T (K) Fig Variation of resistivity with temperature for C Argon annealed AlB 2 compound. AlB 2 H =10 Oe 1.4x x T (K) Fig M-T plot for optimized AlB 2 sample relative decrease in resistivity is very less as compared to that of MgB 2. It does not show any superconducting transition. Hence, AlB 2 is a conductor but does not possess any superconductivity Magnetizaton Studies Fig shows the magnetization vs temperature curve for AlB 2 in zero field cooled situation in the temperature range from 5-200K at H=10 Oe. The sample shows a small positive magnetic moment, which increases slightly with the increasing temperature. No diamagnetic transition is shown by the sample. Thus, it corroborates with the resistivity and thermoelectric power measurements and confirms that pure AlB 2 does not possess any superconductivity. Fig depicts the magnetic behavior with the increasing and decreasing directions of field. As expected, AlB 2 does not show any diamagnetic signal rather shows a weak paramagnetic signal, which might be due to some iron impurity in Boron powder. Thus, 68

24 M (emu) 6.0x x x10-5 AlB 2 5 K Fig Magnetization curve M(H) with continuously varying field at a fixed temperature of 5 K for C Argon annealed AlB 2 sample x x x H (Oe) M-H behavior also corroborates with resistivity and M-T measurements and confirms that AlB 2 is non-superconductor. 3.4 SYNTHESIS AND PHYSICAL PROPERTY CHARACTERIZATION OF NbB EXPERIMENTAL Synthesis of samples The details of sample synthesis are mentioned in section 2.2 under Chapter 2. Briefly, the polycrystalline bulk samples of Niobium boride were first prepared by In situ method following the same solid-state reaction route as in case of MgB 2 and AlB 2. The Niobium and Boron powders were used as a precursor material. The samples were prepared in a temperature range from o C and sintering time was taken as 24h. Although, the phase purity is obtained at a temperature of 1100 o C and sintering time of 24h but all the samples obtained from in-situ method were completely in powder form and were not suitable for various physical property characterization techniques like resistivity measurements, thermoelectric power etc. Literature survey also confirms that in situ method requires critical conditions of very high temperature and high-pressure applications. Due to unavailability of these critical conditions, it was decided to synthesize the sample by Ex-situ method. So, the commercially purchased powder of 69

25 NbB 2 was taken as a precursor material. The powder was pelletized by applying a pressure of 10 tons/cm 2 using hydraulic press. The pellets were then put in a tubular furnace under Ar flow. The temperature of sintering was taken as o C and holding time was taken as 20 hrs since higher sintering temperature leads to the emergence of extra phases as observed in case of insitu method. The phase purity was 100% for all the samples, but the samples synthesized at temperature lesser than 1100 o C were very brittle. The sample synthesized at 1100 o C was comparatively harder and was fit for doing resistivity, magnetization and thermoelectric analysis. So, the NbB 2 sample synthesized at 1100 o C with holding time of 20 h using ex-situ technique was chosen as the optimized sample Characterization of the samples The characterizations of the samples were carried out by the following techniques: X-ray diffraction Studies Details of X-ray diffraction technique are described in Chapter 2 under Section Briefly, the X-ray diffraction patterns of all the synthesized samples were recorded on a Diffractometer using CuK radiation Scanning Electron Microscope (SEM) Studies The micro structural investigation of the NbB 2 sample was carried out using scanning electon microscopy, details of which are described in Chapter 2 under Section The Scanning Electron Microscopy (SEM) studies were carried out on these samples using a Leo 440 (Oxford Microscopy: UK) instrument Raman Spectroscopy measurements Details of Raman Spectroscopy measurements are described in Chapter 2 under Section Raman measurements on NbB 2 sample were performed on a dispersive single Horiba Jobin Yvon Hr-800 mono-chromator coupled to a charge couple device. The 488 line of an argon ion laser was used as a probe beam that is focused on to a ~2 μm spot. The power was kept to a minimum of ~2 mw at the sample, and all the measurements 70

26 were carried out in a back scattering geometry with detection in the un-polarized mode Thermoelectric power measurements Thermoelectric Power (TEP) measurements on NbB 2 sample were carried out by dc differential technique over a temperature range of 5 300K, using a homemade set-up. Temperature gradient of ~1K is maintained throughout the TEP measurements. Details of this technique are given under Section in Chapter Resistivity measurements Temperature dependent resistivity measurements on NbB 2 sample were carried out using four-probe technique and the probe was inserted in a container having Helium liquid so as to achieve temperature as low as 4.2 K. Resistivity measurements are carried out in the temperature range K. Details of this technique are given under Section in Chapter Magnetization Studies Details of magnetic characterization of samples are given in Chapter 2 in section and section Briefly, Magnetization measurements on NbB 2 sample were carried out using a Quantum-Design 14 Tesla Physical Property Measurement System (PPMS) having Vibrating Sample Magnetometer (VSM) attachment RESULTS AND DISCUSSION Structure and Microstructure X- ray diffraction patterns for variously synthesized NbB 2 samples in the temperature range o C are shown in Fig All these samples are synthesized by In situ method i.e. the precursor materials taken were Niobium and Boron powder. The samples synthesized at 1000 and 1100 o C seems to be good enough as far as phase purity is concerned. With further increase in temperature, some impurity peaks are obtained. 71

27 I (arb. Units) I (arb. units) Sample synthesized at 1100 o C is the phase purest among all these. But as we discussed earlier, all these samples are obtained in powder form. Thus, the transport measurements were not possible. So, it was NbB o C 24h 1300 o C 24h decided to synthesize the sample by Ex-situ method (deg.) 1200 o C 24h 1100 o C 24h 1000 o C 24h Figure 3.21 X-ray diffraction patterns of NbB 2 samples sintered at different temperatures in the range O C following in-situ method o C 950 o C 800 o C Non-sintered NbB (deg.) Fig X-ray diffraction patterns of NbB 2 samples sintered up to 1100 o C following Ex-situ method Fig shows the X-ray diffraction pattern of NbB 2 sample synthesized by Ex-situ technique. Here the commercially purchased powder of NbB 2 is used. The pellets were sintered in a temperature range from o C. As far as phase purity is concerned, all the samples are phase pure and no impurity is formed during sintering. The samples sintered at temperature lower than 1000 o C were soft while the sample sintered at 1100 o C was hard enough to carry out all characterization measurements. Thus, the NbB 2 sample synthesized by Ex-situ method and sintered at a temperature of 1100 o C and holding time of 20 h is hence found as a optimized sample. The further characterization 72

28 I (arb. Units) measurements are carried out on this sample. The structure of NbB 2 belongs to space group P6/mmm. The asymmetric unit of the structure consists of Nb at (0, 0, 0) and B at (1/3, 2/3, 1/2). Preliminary Rietveld refinement is carried out using the program Fullprof [52] for the NbB 2 P6/mmm Nb (0,0,0) B (1/3,2/3,1/2) a = (13) A o c = (17) A o best-synthesized sample and is shown in Fig The occupancy parameters of the various atoms and their positions are fixed at their nominal values. In Fig. 3.23, the points correspond to the experimentally observed data while the line (deg.) Fig Rietveld analysis of NbB 2 sample sintered at 1100 O C by Ex-situ method corresponds to the theoretically fitted curves obtained from Rietveld refinement. The differences between the experimental and calculated XRD patterns are very small and are shown by a line curve at the bottom of the graph. Bragg peaks obtained from Rietveld refinements are shown by bars below the experimental and theoretical curves. It can be clearly seen that all the bragg peaks are Fig (a) Scanning electron microscope (SEM) pictures of C synthesized NbB 2 compound at 5KX magnification obtained in the experimental data thus confirming the 73

29 Intensity (arb. units) pure phase formation of NbB 2. All the Bragg peaks are found with appropriate intensity with no impurity peak. The lattice parameters obtained for NbB 2 are a = Å, and c = Å. The Scanning electron microscope images of NbB 2 sample sintered at 1100 o C Fig (b) Scanning electron microscope (SEM) pictures of C synthesized NbB 2 compound at 10 KX magnification NbB 2 by Ex-situ method are shown in Fig. 3.24(a) and 3.24(b) at 5KX and 10KX magnifications. The sample seems to be dense enough. The grains are of variable shape and grain size is not uniform. It varies from 4 m to 20 m. Grains of small sizes are distributed among large sized grains so as to decrease the porosity of the sample unlike MgB 2. No impurity phase can be seen from SEM images. It is in confirmation with the XRD pattern Raman studies Room temperature Raman spectrum of the optimized Raman shift (cm -1 ) Fig Raman spectra for C synthesized NbB 2. ( C 20 hours) NbB 2 sample is depicted in Fig The electron-phonon peak is obtained at 980 cm -1 74

30 S ( V/K) while the same was observed at 680cm -1 in case of MgB 2. It shows that electron phonon modes are weak in NbB 2 as compared to MgB 2. An early indication of the strong phonon contribution in MgB 2 can be presumed on the basis of its stretched c-parameter Thermoelectric power analysis NbB T (K) Fig.3.26 Thermoelectric power vs temperature curve for NbB 2 compound NbB 2 The S(T) plot of phase pure sample of NbB 2 is shown in Fig The absolute value of S is negative, which indicates towards the electron type conductivity in this system. No superconducting transition (T c ) is seen as S=0. S(T) has mainly the contributions from electrons and phonons. The room temperature thermoelectric power S 300K is around -6.2 V/K. As the temperature decreases from room temperature, the absolute value of thermoelectric power, S decreases. The sign of TEP is negative throughout the whole temperature range. ( -cm) Non- Superconducting T (K) Fig Variation of resistivity with temperature for C synthesized NbB 2 sample Resistivity measurements Fig shows the temperature variation of resistivity in the temperature range of K for the phase pure sample of NbB 2, which was synthesized at 1100 o C. As temperature decreases, 75

31 M (emu/g) M (emu/g) resistivity of the sample 5.8x10-5 NbB 2 decreases showing metallic 5.6x10-5 behavior. The absolute value of room temperature resistivity of 5.4x10-5 pure NbB 2 is about 125 -cm. It 5.2x10-5 decreases linearly from 300K to 80K and achieves a value of 5.0x T (K) about 92 -cm. Below 80 K, Fig M-T plot of C NbB resistivity decreases very slowly 2 and achieves almost saturation but no superconducting transition is observed down to liquid Helium temperature. The relative decrease in resistivity is very less as compared to that of MgB 2. Residual Resitivity ratio, RRR is defined as 300 / 4.5, which is found to be 1.37 for 1100 o C synthesized optimized sample of NbB Magnetization measurements 1.5x x x x x x10-2 NbB H (koe) 76 5 K Fig M(H) plot for C NbB 2 sample at 5 K. Fig shows the Magnetization vs temperature variation for optimized sample (1100 o C, 20h) of NbB 2 in the range of 5-14K. Sample shows a very low positive signal with an applied field of 10 Oe. The magnetic moment decreases with decrease in temperature but does not show any diamagnetic behavior rather NbB 2 sample indicates a weak paramagnetic character. So, magnetization measurements clearly confirm that the sample is not diamagnetic and hence does not possess any bulk superconductivity down to 5 K. Thus, magnetization measurements corroborate with the resistivity measurements.