3. STUDIES ON POTASSIUM LEAD BROMIDE SINGLE CRYSTALS

Transcription

1 3. STUDIES ON POTASSIUM LEAD BROMIDE SINGLE CRYSTALS 3.1 INTRODUCTION Ternary alkali lead halide single crystals have become important because of their potential applications in acousto-optic and opto-electronic devices.lead bromide crystals hold much promise in applications for acouto-optic devices in signal processing and optical spectrum analyzing systems. Single crystals of this material have favourable acousto-optical properties, the most significant of which are its a) spectral transmission range, (b) photo-elastic co-efficient, (c) acousto-optic figure of merit, (d)acoustic velocity and (e) acoustic attenuation, although its use has been hampered by difficulties in growing crystals of high optical quality. Recently, it has been found that ternary alkali halide single crystals can be grown by the melt method and they become important due to their potential applications. Monoclinic KPb 2 Br 5 (KPB) is among the most promising bromide host materials because this material possesses an incorporation of Nd 3+, Tb 3+, Dy 3+ and Er 3+ doping ions and provides better homogeneity and quality of doped single crystals [57]. The crystal structure of KPB, (having spacegroup P2 1 /c, lattice parameters a=8.854(2) Å, b=7.927(2) Å, c=12.485(3) Å, β=90.05(3)å and Z=4), is shown in Figure 3.1 [124]. Complex polyhedral coordination by bromine atoms was found for both potassium and lead atoms. An important step towards practicality was made when the rare-earth-doped alkali-lead halide crystals MPb 2 Hal 5 (M = Rb,K and Hal = Cl, Br) were identified as promising new low-phonon-energy host materials for mid-ir applications. 44

2 The present investigation deals with the growth of lead bromide and potassium bromide mixed crystals by slow evaporation technique. The grown crystals (expected to be KPb 2 Br 5, KPbBr 3, K 2 PbBr 4 and K 3 PbBr 5 ) were subjected to powder X-ray diffraction (PXRD), single crystal XRD, AAS, EDAS, SEM, TGA/DTA, UV-Vis-NIR spectral and electrical (both AC and DC) measurements. The results of these experiments are reported and discussed in this chapter. Figure 3.1: The crystal structure of KPb 2 Br 5 single crystal 45

3 3.2 GROWTH OF SINGLE CRYSTALS Analytical reagent (AR) grade samples of Lead Bromide (PbBr 2 ), and Potassium Bromide (KBr) along with double distilled water were used for the growth of Potassium Lead Bromide single crystals. Lead Bromide and Potassium Bromide were taken in the ratios 1: 0.5, 1:1, 1:2 and 1:3 dissolved in double distilled water and maintained at 80 o C for about 60 minutes with continous stirring to ensure homogenous temperature and concentration over the entire volume of the solution. Temperature as low as 80 o C was maintained in order to avoid decomposition of the salt. The supersaturated solutions were filtered using 4 micro watman filter paper. Then the filtered solutions were kept for free evaporation. Clear tiny needle like crystals were obtained in about 20 days. A photograph of the grown crystals is shown in Figure CHARACTERIZATION The powder X- ray diffraction (PXRD) analysis was carried out using an X- ray powder diffractometer (PANalytical) with scintillation counter and monochromated CuK α (λ = Å) radiation. The samples were scanned over the 2θ range at a rate of one degree/minute. The single crystal XRD data were collected using an automated 4-circle diffractometer (Enraf Nonius CAD4). Atomic absorption spectra were recorded using Perkin Elmer spectrophotometer. The UV-Vis- NIR spectrum was recorded in the range of nm using a Shimadzu UV-2400 PC spectrometer. SEM and EDAS analysis were carried out to study the morphology and elemental compositions.the thermo gravimetric analysis (TG) of the crystal was carried out using an Universal V4.1 DTA Instruments, in the temperature range from 50 to 700 o C in nitrogen atmosphere at a scanning rate of 10 K/min. 46

4 The AC conductivity, dielectric constant and dielectric loss of the samples were determined to an accuracy of ± 2% using an LCR meter (Agilent 4284A) with five different frequencies (100 Hz, 1 khz, 10 khz, 100 khz and 1 MHz) at various temperatures ranging from C. The measurement of DC electrical conductivity was done using the conventional two-probe technique using a million megohm meter for temperatures ranging from C. The crystals grown are needle shaped ones with small thickness. So, crystal portion with sufficient size cannot be out and polished for the use of electrical measurements. Hence, in order to make the electrical measurements, we have made pellets of the grown crystals and used as the sample for the AC and DC electrical measurements. The flat surfaces of the pellet were coated with graphite to have a good conductive surface layer. 47

5 Figure 3.2: Photograph of the sample crystals grown [From left are: KPb 2 Br 5 K PbBr 3, K 2 PbBr 4 and K 3 PbBr 5 ] 48

6 3.4 RESULTS OBTAINED Single Crystal XRD Analysis It is observed from the single crystal XRD data that all the crystals crystallize in the orthorhombic system except KPbBr 3. The KPbBr 3 crystal belongs to the monoclinic system.the single crystal XRD data for the samples prepared are presented in Table Powder X-ray Diffraction Analysis X-ray diffraction data were collected from powder samples using an automated X-ray powder diffractometer. The reflections were indexed using a homely designed two theta software [125,126]. Figures show the indexed XRD patterns. 49

7 Table 3.1: Single crystal XRD data for potassium lead bromide crystals grown in the present study Crystallographic data KPb 2 Br 5 KPbBr 3 K 2 PbBr 4 K 3 PbBr 5 a (Å) b (Å) c (Å) α(º) β(º) γ(º) Volume (Å 3 ) Crystal system orthorhombic monoclinic orthorhombic orthorhombic 50

8 51

9 52

10 53

11 54

12 3.4.3 Atomic Absorption Spectra The AAS measurements were carried out using a Perkin Elmer spectrophotometer to determine the K and Pb atom contents in the grown crystals. The AAS results are given in Table 3.2, which reveal the presence of K + and Pb 2+ ions in the crystals Energy Dispersive X-ray Absorption Spectra The EDAS spectra observed are shown in Figures ( ). Results are summarized in Table 3.3. The dominant peaks correspond quite well to the energies of lead and bromine while a small hemp at 3.2 kev corresponds to K line of potassium (reported in the EDAS international chart), giving a clue that lead is dominant over potassium in the crystals grown. 55

13 Table 3.2: Atomic absorption spectral data Atomic content (ppm) Sample Pb K KPb 2 Br KPbBr K 2 PbBr K 3 PbBr Table 3.3: Energy dispersive X-ray absorption spectral data for potassium lead bromide crystals Atomic % of Sample Pb K Br KPb 2 Br KPbBr K 2 PbBr K 3 PbBr

14 57 Figure 3.7: EDAS spectrum for K Pb2Br5

15 58 Figure 3.8: EDAS spectrum for KPbBr3

16 59 Figure 3.9: EDAS spectrum for K2PbBr4

17 60 Figure 3.10: EDAS spectrum for K3PbBr5

18 3.4.5 Scanning electron microscopic pictures The quality of the grown crystals can be inferred to some extent by observing the surface morphology of the cut and polished crystals. The SEM image of all the 4 crystal samples observed are shown in Figures It is observed from SEM photographs that all the crystals are free from cracks and significant visible inclusions. They have rod like morphology UV- Visible Absorption Spectra The observed UV- Visible spectra for the four grown potassium lead bromide crystals are shown in Figure All the four crystals exhibit absorption edges at nearly 370 nm and good transmittance in the visible region. The transmittance (T) in the order of T for KPb 2 Br 5 > T for K 2 PbBr 4 >T for K 3 PbBr 5 >T for KPbBr 3. 61

19 Figure 3.11: SEM photograph of KPb 2 Br 5 crystals Figure 3.12: SEM photograph of KPbBr 3 crystals 62

20 Figure 3.13: SEM photograph of K 2 PbBr 4 crystals Figure 3.14: SEM photograph of K 3 PbBr 5 crystals 63

21 5 KPb 2 Br 5 KPbBr 3 K 2 PbBr 4 K 3 PbBr 5 absorption(arb.unit) Wavelength(nm) Figure 3.15: UV-Vis spectra observed for the grown crystals 64

22 3.4.7 Thermal Studies The thermo gravimetric and differential thermal analysis [ ] were carried out for all the four crystals and the patterns observed are presented in Figures 3.16 to The plots are marked with temperature against weight loss percentage. The TGA patterns show that all the grown crystals were thermally stable up to 500 o C. The exothermic peak at 373 o C for KPb 2 Br 5 single crystal corresponds to the phase transition [130]. For the remaining crystals the phase transitions occur at o C, 373 o C and 368 o C respectively. 65

23 66 Figure 3.16: TG / DTA pattern of KPb2Br5 single crystal

24 67 Figure 3.17: TG / DTA pattern of KPbBr3 single crystal

25 68 Figure 3.18 : TG / DTA pattern of K2PbBr4 single crystal

26 69 Figure 3.19: TG / DTA pattern of K3PbBr5 single crystal

27 3.4.8 Dielectric Parameters The dielectric parameters, viz. the ε r, tanδ and σ ac values obtained in the present study for the pelletised samples are provided in Tables and also shown in Figures 3.20 to They are found to increase with increasing temperature for all the four crystals considered in the present study. The ε r and tanδ values decrease while σ ac value increase with the increase in frequency of the applied field. This shows that all the four crystals grown exhibit the normal dielectric behavior The DC conductivities Table 3.16 provides the σ dc values obtained in the present study for the pelletized samples. Also σ dc values are shown in Figure The DC electrical conductivity (σ dc ) increases, in all the four crystals studied, smoothly with the temperature increase through the temperature range considered in the present study. It should be noted that the σ dc values are more than the σ ac values at all temperatures for all the four potassium lead bromide crystals studied in the present investigation. 70

28 Table 3.4: The dielectric constants for KPb 2 Br 5 crystal Temp ε r with frequency ( C) 100 Hz 1 khz 10 khz 100 khz 1 MHz Table 3.5: The dielectric constants for KPbBr 3 single crystal Temp ε r with frequency ( C) 100 Hz 1 khz 10 khz 100 khz 1 MHz

29 Table 3.6: The dielectric constants for K 2 PbBr 4 crystal ε r with frequency Temp ( C) 100 Hz 1 khz 10 khz 100 khz 1 MHz Table 3.7: The dielectric constants for K 3 PbBr 5 crystal Temp ε r with frequency ( C) 100 Hz 1 khz 10 khz 100 khz 1 MHz

30 Table 3.8: The dielectric loss factors for K Pb 2 Br 5 crystal Temp tanδ with frequency ( C) 100 Hz 1 khz 10 khz 100 khz 1 MHz Table 3.9: The dielectric loss factors for KPbBr 3 crystal Temp tanδ with frequency ( C) 100 Hz 1 khz 10 khz 100 khz 1 MHz

31 Table 3.10: The dielectric loss factors for K 2 PbBr 4 crystal Temp ( C) tanδ with frequency 100 Hz 1 khz 10 khz 100 khz 1 MHz Table 3.11: The dielectric loss factors for K 3 PbBr 5 crystal Temp tanδ with frequency ( C) 100 Hz 1 khz 10 khz 100 khz 1 MHz

32 Table 3.12: The AC electrical conductivities for K Pb 2 Br 5 crystal Temp σ ac (x 10-7 mho/m ) with frequency ( C) 100 Hz 1 khz 10 khz 100 khz 1 MHz Temp ( C) Table 3.13: The AC electrical conductivities for KPbBr 3 crystal σ ac (x 10-7 mho/m ) with frequency 100 Hz 1 khz 10 khz 100 khz 1 MHz

33 Table 3.14: The AC electrical conductivities for K 2 PbBr 4 crystal Temp ( C) σ ac (x 10-7 mho/m ) with frequency 100 Hz 1 khz 10 khz 100 khz 1 MHz Table 3.15: The AC electrical conductivities for K 3 PbBr 5 crystal Temp ( C) σ ac (x 10-7 mho/m ) with frequency 100 Hz 1 khz 10 khz 100 khz 1 MHz

34 Hz 1kHz 10kHz 100kHz 1MHz ε r Temperature( o C) Figure 3.20: Temperature dependence of dielectric constant for KPb 2 Br 5 crystal for various frequencies Hz 1kHz 10kHz 100kHz 1MHz ε r Temperature( o C) Figure 3.21: Temperature dependence of dielectric constant for KPbBr 3 crystal for various frequencies 77

35 Hz 1kHz 10kHz 100kHz 1MHz 50 ε r Temperature( o C) Figure 3.22: Temperature dependence of dielectric constant for K 2 PbBr 4 crystal for various frequencies Hz 1kHz 10kHz 100kHz 1MHz 60 ε r Temperature ( o C) Figure 3.23: Temperature dependence of dielectric constant for K 3 PbBr 5 crystal for various frequencies 78

36 Hz 1kHz 10kHz 100kHz 1MHz tanδ Temperature( o C) Figure 3.24: Temperature dependence of dielectric loss factor for KPb 2 Br 5 crystal for various frequencies B B B B B tanδ Temperature( Temperature( o C) o C) Figure 3.25: Temperature dependence of dielectric loss factor for KPbBr 3 crystal for various frequencies 79

37 Hz 1kHz 10kHz 100kHz 1.2 1MHz tan δ Temperature( o C) Figure 3.26: Temperature dependence of dielectric loss factor for K 2 PbBr 4 crystal for various frequencies Hz 1kHz 10kHz 100kHz 1MHz 0.9 tan δ Temperature( o C) Fig 3.27: Temperature dependence of dielectric loss factor for K 3 PbBr 5 crystal for various frequencies 80

38 Hz 1kHz 10kHz 100kHz 1MHz 60 σ ac Temperature( o C) Figure 3.28: The AC electrical conductivities (x10-7 mho/m) for K Pb 2 Br 5 crystal for various frequencies Hz 1kHz 10kHz kHz 1MHz σ ac Temperature( o C) Fig 3.29: The AC electrical conductivities (x10-7 mho/m) for KPbBr 3 crystal for various frequencies 81

39 Hz 1kHz 10kHz 100kHz 1MHz 120 σ ac Temperature( o C) Fig 3.30: The AC electrical conductivities (x10-7 mho/m) for K 2 PbBr 4 crystal for various frequencies σ ac Temperature( o C) Fig 3.31:The AC electrical conductivities (x10-7 mho/m) for K 3 PbBr 5 crystal for various frequencies 82

40 Table 3.16: The DC electrical conductivities for potassium lead bromide crystals Temperature ( o C) σ dc ( x 10-5 mho / m ) for K Pb 2 Br 5 KPbBr 3 K 2 PbBr 4 K 3 PbBr KPb 2 Br 5 KPbBr 3 K 2 PbBr 4 K 3 PbBr σ dc Temperature( o C) Figure 3.32: The DC electrical conductivities (x10-5 mho/m) for potassium lead bromide crystals 83

41 3.5 DISCUSSION All the four single crystals (KPb 2 Br 5, KPbBr 3, K 2 PbBr 4 and K 3 PbBr 5 as per the initial composition considered for crystallization) grown are of needle shape. The grown crystals show considerable transparency and mechanical and thermal stabilities. Growth of high quality crystals with uniform composition is of great importance for high performance devices manufacturing. Among the requirements to crystal properties, well-defined composition, macro- and micro- uniformity should be mentioned in the first instance. For example, in electronic and optoelectronic applications the quality of the active epilayers often depends directly on the chemical homogeneity of the substrate. In case of quasibinary solid solutions (A 1-x B x ) 1-s X 1+s, the composition is characterized by the mole fraction x (which defines the energy band gap) and the deviation from stoichiometry δ (which influences the carrier concentration) [133]. It should be noted that in the case of lead chalcogenides, the deviation from stoichiometry can be effectively controlled by a post -growth annealing under Pb or chalcogen vapour, whereas the x value should be fixed during the growth process. Axial or radial segregation, both at the macroscopic and the microscopic scale, is one of the major factors limiting the yield of bulk crystals grown from the melt or from the vapour. Besides, it should be mentioned that essential axial and radial segregation causes noticeable increase of the dislocation density in the grown crystals. The crystals of alloys are frequently subjected to serious distillation-like (i.e., thermodynamically imposed) segregation [134] leading to essential variation in composition between the initially and finally grown fragments of the crystals, which restricts the applicability of the obtained materials for the device manufacturing. 84

42 Shtanov and Yashine [133] have illustrated using (Pb 1-x Sn x ) 1-δ Se 1+δ solid solutions as an example the application of T-x-y phase diagram for the control of the crystal composition of alloy crystals during Bridgman growth. The alloying of two or more metals has always been systematically used in order to modify and improve the properties of the metallurgical materials. The mixing of ionic solids has been equally investigated in the purpose of obtaining new materials with specific properties. A very important situation that is special to ionic crystals arises when these crystals are doped (or added) with impurities. The behavior depends on the valence state of impurity ions. When an ion like Ca 2+ replaces a Na + ion in NaCl crystal it results in the creation of a positive ion vacancy or a negative ion interstitial. Anion impurities also produce corresponding charge compensating point defects. Whether an impurity ion goes to substitutional position or interstitial position, is determined by the ionic radius of the doped (or added) ion and also on the electronic configuration of the ion. If the impurity ion behaves in the same way as the lattice ion, a wide range of solubility may be possible. To describe this, the term mixed crystal is used. It should be realized, however, that the impurity ions are all distributed at random throughout the lattice so that the term solid solution is more appropriate. Two compounds or elements are said to form a continuous solid solution if a single lattice parameter as measured by X-ray powder diffraction patterns, can be assigned to the solid solution at all compositions. In the continuous solid solutions of alkali halides, Retger s law (additivity of molar volumes) [135] and Vegard s law (linear variation of lattice parameter with composition) [136] are closely followed as indicated by X-ray diffraction studies. 85

43 Potassium and lead halides are soluble in water. It is possible to grow, in certain cases, mixed crystals by evaporation of aqueous solution. However, the melt technique is the commonly employed technique to grow mixed crystals. Tobolsky [137] showed that for ionic crystals like alkali halides, complete miscibility is possible only above a particular temperature given by T=4.5δ 2, where δ being the percentage deviation in the lattice parameter. As per this, alkali halide solutions have got only limited miscibility at room temperature. Vertical Bridgman technique (melt technique) is mostly used for growing single crystals of alkali lead halides and alkali halides. At temperatures nearer to the freezing point, the crystals are observed to be fairly transparent. When the crystals are cooled from high temperature to the room temperature in a relatively short time the transparency of the crystals is found to be reduced and becoming white. This is partly due to the introduction of thermal defects since the rate of cooling is high. Transparency can be improved by reducing the rate of cooling and consequently reducing the introduction of thermal defects. In this situation, growth of crystals by the solution methods at near ambient temperatures can be considered to be useful. A 3 MX 5.2H 2 O (where A is a univalent cation, M is a divalent metal and X is a halogen) crystals exhibit unusual physical properties. They have attracted a great deal of attention owing to the occurrence of varying stoichiometries in these compounds [138]. A 3 MX 5.2H 2 O crystals are closely related to A 2 MX 4 and both represent the largest known group of insulating crystals with structurally incommensurate phases [139]. Byrappa et al [140] have mentioned that no detailed X-ray crystal structure (refinement) is available for A 3 MX 5.2H 2 O type crystals. However, Krishna kumar et al 86

44 [141], without giving any experimental details, have described in brief the crystal structure of Na 3 BaCl 5. 2H 2 O crystals. The structure described by them is as shown in Figure The Na 3 BaCl 5. 2H 2 O crystals consist of metal ions such as Na and Ba, Cl - ions and two H 2 O molecules. The chlorine atoms lie at the vertices of trigonal bipyramidal geometery. Three Cl - ions form electrovalent bonds between the adjacent Na + and central Ba 2+ ions. This bond is naturally the attractive electrostatic force existing between positive and negative ions when they are brought into a closer distance. The two H 2 O molecules are stacked diagonally up and down, which may have a linkage with the adjacent Na + ions. Figure 3.33: Crystal structure of Na 3 BaCl 5.2H 2 O Manonmani et al [142,143,113] have attempted to grow from aqueons solutions by the slow (free) evaporation of solution method single crystals of (composition considered in the solution) K 3 BaCl 5.2H 2 O, K 3 CaCl 5.2H 2 O, and Na 3 CaCl 5.2H 2 O and characterize them. They have confirmed by experimental means (XRD, TGA, AAS and FTIR and Raman spectroscopic measurements) that non 87

45 stoichiometry is present in all these crystals grown. These compositions were estimated as K Ba Cl H 2 O for K 3 BaCl 5.2H 2 O, K Ca Cl H 2 O for K 3 CaCl 5.2H 2 O and Na Ca Cl H 2 O for Na 3 CaCl 5.2H 2 O. The variation of DC electrical conductivity with temperature observed by them indicates that KCl-BaCl 2 is a dielectric material while the others (KCl-CaCl 2 and NaCl-CaCl 2 ) are ionic conductors. Less non stoichiometry retains the dielectric nature (usual for ionic substances) and higher non stoichiometry leads to ionic conductors. Keller [144] has reported that orthorhombic symmetry is shown by single crystals of K 2 PbBr 4.H 2 O: a=8.537 Å, b=13.083å,c=4.594å. Z=2, space group P He has demonstrated the analogy between the crystal structure of K 2 PbBr 4.H 2 O and KPb 2 Br 5 by group subgroup relations of space groups. Iwadate et al [145] investigated the complex formation and ionic aggregation in PbBr 2 -NaBr and PbBr 2 -KBr melts by Raman spectroscopy with supplementary use of 2- molecular orbital calculations (MO). Their results suggest that there existed PbBr 4 complex ions in the mixture melts, which might not form further clustering or network. Kusumoto et al [146] have mentioned that as PbBr 2 hardly dissolves in water (0.97g/100g water), it is not suitable for aqueous solution growth. So, they have grown PbBr 2 single crystals in silica gel and obtained the following results: i) Transparent PbBr 2 single crystals were obtained in a high-acidic gel, ii) sizable single crystals of PbBr 2 were also grown in the liquid placed over a gel because the gel barrier had the task of slowing down the diffusion rate of reacting ions. Also, they have mentioned 88

46 that it was difficult for them to grow a PbBr 2 crystal of optical high quality from the melt even though they used a % purity material. Rademaker et al [72] observed that the KPb 2 Br 5 (KPB) crystal grown by the Bridgman (melt) method is biaxial and has a monoclinic crystal structure with a space group symmetry P 2 1 / c. From an X-ray single crystal diffraction study of KPB, they determined the lattice parameters to be a=9.256 (2) Å, b=8.365 (2) Å, c= (3) Å and β=90.00 (3), Z=4. These values were obtained for crystals evidencing substantial micro twinning. For crystals with no twinning structures, the given lattice parameters will change, but further research is needed to clarify this situation. Determined from lattice constants, the density was found to be 5.62g/cm 3 which matched with that available in other literature, 5.60g/cm 3 [91]. Rademaker et al [72] also have observed a phase transition in KPB at a temperature of 249 C which matched with that of 242 C reported in other literature [89,91]. Hommerich et al [147] have investigated KPb 2 Br 5 (KPB) as a potential new solid state laser host material. The fundamental absorption edge of KPB is located at ~400nm. At longer wavelength the transmission ranged between ~75-77% without any significant absorption features. According to Beck et al [90] KPb 2 Br 5 (KPB) is monoclinic (space group P 2 1 / c ) with an angle β very close to 90. The unit cell parameters are a=9.264, b=8.380, c= Å and β=90.06 ; Z=4. Pb 2+ ions occupy two non-equivalent lattice sites of low symmetry, one site is a distorted octahedron and the second site is a distorted trigonal prism. 89

47 Lead bromide belongs to the orthorhombic symmetry class D 2h and mmm space group [148]. The lattice parameters are: a=8.0620(1)å, b= (13) Å and c= (6)å. V= Å 3, Z=4, ρ=6.695gcm -1. PbBr 2 exhibits extraordinary properties, including a very large optical transparency range, an anomalously slow longitudinal wave velocity in the [010] direction, a large birefringence and a high figure of merit (M2-550, about twelve times higher than that of PbMoO 4 ). Therefore this material has good application potential, especially for infrared devices where large diffraction efficiencies are needed. Crystals were grown by the vertical Bridgman method. Singh et al [49] observed that lead bromide crystals severely cracked during the cool down period after the growth, due to destructive phase transformation. The energy of phase transformation was suppressed by silver doping and large crystals were grown from the melt. The acoustic attenuation constant, an important parameter for the devices, was almost identical for doped (below 3000 ppm) and undoped crystals. In the present study, the results obtained through X-ray diffraction, AAS and EDAS measurements indicate the absence of proper mixing of KBr and PbBr 2 in all the four potassium lead bromide crystals grown. The grown crystals may be considered as K + doped PbBr 2 single crystals. However, the thermal stability and the temperature at which the phase transition occurs in all the four crystals studied are similar. The phase transition occurs at ~370 C (see section 3.4.7) which is largely deviated from that observed for KPb 2 Br 5 crystals grown by the melt method (~245 C) [16-18]. Singh et al [49] have presented a solid/solid phase transformation observed by DTA in PbBr 2 at 365 C. So, the results obtained in the present study through 90

48 thermal analysis also evidence the formation of KBr added PbBr 2 crystals and not the proposed mixed crystals. So, the chemical formulae used to represent the grown potassium lead bromide crystals are not correct. However, we use here as the sample representation. Since the initial composition used for the growth of crystal is the same. The lattice parameters obtained in the present study for KPb 2 Br 5, K 2 PbBr 4 and K 3 PbBr 5 are nearly same with the orthorhombic crystal system. However, the lattice parameters obtained for KPbBr 3 are highly deviated and also with a different crystal system (monoclinic). This may be due to lattice distortion which is evident from the considerably lower Br - and higher Pb 2+ contents when compared to the other three crystals considered (see table 3.3). The optical absorption edges observed for all the four potassium lead bromide crystals grown in the present study are nearly 370 nm which is significantly less than that observed for the melt grown KPb 2 Br 5 (~400 nm) [147]. Like PbBr 2 crystal, the four crystals considered in the present study exhibit a large optical transparency. Moreover, the transmittance observed is significantly more than that observed for PbBr 2 [148]. Even though they are not properly mixed potassium lead bromide crystals, all the four single crystals grown in the present study exhibit superior optical characteristics required for acousto-optical (AO) devices. The large optical transparency range of these crystals is very useful for wide band or multiple band AO tuneable filters (AOTF) applications. The intrinsic point defects in lead bromide are supposed to be either of the Schottky or of the Frenkel type. Tubandt et al [149] concluded from transport measurements that the electric current in lead bromide is carried exclusively by the 91

49 bromine ions. Therefore it is not necessary to consider the lattice defects in the lead ion sub-lattice as charge carriers. The crystal structure of lead bromide was determined by Brackken and Harang [150] and by Nieuwenkamp [151] and shown a coordination structure formed by a disturbed hexagonal packing of bromine ions between which the lead ions are placed. These lead ions are surrounded by 9 bromine ions at different distances (3.0 to 4.1 Å). In lead bromide the ions at interstitial sites might occur only in the mirror planes (100) 0 and * ( 100), while in the neighbourhood 1 2 of the gliding mirror planes at (001) 1/4 and * ( 001) bromine ions at 4.1 Å have left 3 4 enough space for ions with a radius of at most 0.94 Å. The Pauling radii of bromine and lead ions are 1.95 and 1.21Å, respectively, so we may disregard the occurrence of interstitial bromine and lead ions and so we consider anion and cation vacancies to be the only intrinsic point defects in lead bromide. According to a Schottky mechanism their thermal generation is given by O V + + V Pb 2 2 Br, where V Pb 2+, V Br - denote a missing lead ion at a lead ion site and a missing bromine ion at bromine ion site, respectively, and O denotes the perfect lattice. We assume that the foreign ions keep their normal valency states. The electroneutrality condition upon doping with monovalent cations Me +, divalent ions A 2-, or trivalent cations Me 3+, according to the Koch and Wagner system is then given by [ V ] + [ Me ] = 2[ V ] + [ Me ] + [ A ], Br Pb 2+ 92

50 where square brackets denote concentrations. Upon doping with monovalent cations in concentrations well above those of the intrinsic lattice defects this relation becomes [ V + ] [ ] Br = Me All foreign ions have radii greater than 0.94Å, so in all cases the bromine ion vacancies are to be considered to carry the electrical current in lead bromide [152]. In the case of potassium doped PbBr 2 crystals the K + ions may not occur at interstitial sites since the Pauling radius of the monovalent potassium ion is 1.51Å. 93