CHAPTER 3 GROWTH AND PROPERTIES OF RARE EARTH METALS DOPED GLYCINE PHOSPHITE SINGLE CRYSTALS

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1 72 CHAPTER 3 GROWTH AND PROPERTIES OF RARE EARTH METALS DOPED GLYCINE PHOSPHITE SINGLE CRYSTALS 3.1 INTRODUCTION Ferroelectricity in glycine phosphite single crystals (hydrogenbonded compounds) was first observed in the year 1996 (Dacko et al 1996). GPI and deuterated GPI undergo a continuous ferroelectric phase transition at 224 K and 324 K respectively. Both these crystals are the order-disorder nature of the ferroelectric phase transition of the second order type, belong to the monoclinic system and the space group P2 1 /a in paraelectric phase and P2 1 in ferroelectric phase (Baran et al 2002, Lapsa et al 2000) characterized by the linking of hydrogen bonds of the inorganic HPO - 3 tetrahedra to zig-zag chains, each glycine molecule being attached via OH---O bonds to these inorganic units. Cell parameters of GPI are a = (3), b = 8.465(3) and c = (3) Å, = = 90 o, = o and V = 599.4(4) Å 3 (Lapshin et al 2005). GPI was synthesised from Glycine and H 3 PO 3 and growth of large size bulk crystals of pure GPI has been reported recently. Effect of rare earth metals plays a vital role in improving the physical properties of organic ferroelectric materials such as TGS, TGSe and BPI (Muralidharan et al 2003, Batra et al 2003). Many efforts have been made in the past to understand the growth mechanism and improve its piezoelectric, pyroelectric, mechanical, optical, and ferroelectric properties of ferroelectric materials and to prevent depolarization (Lal and Batra 1993, Gaffar et al 1989, Mihaylova and Byrne 2000). Doping with heavy rare earth (Ho, Tm and Yb) ions create structural and chemical defects in the crystals, where as in the case of light rare earth

2 73 ion doping, the crystal structure remains unaltered however, growth aspects, morphology, domain structure and hysteresis behavior altered significantly. Ferrolelectric behaviour of rare earth doped TGS (Mihaylova and Stoyanov 1996), indicate well saturated hysteresis loop and enhanced ferroelectric properties were observed. Investigation on the doping effect of La 3+, Ce 3+, and Nd 3+ ions on the electrical and other physical properties of TGS crystals were reported (Muralidaran et al 2002, 2003). Studies on rare earth dopants of Sm, Yb and Tb were also reported to understand the electrical, piezoelectric and pyroelectric properties of TGS crystals (Batra et al 2006). Efforts were made in the present investigation to understand the growth aspects of rare earth metal ions (La 3+, Ce 3+, and Nd 3+ ) doped GPI and to realize the physical properties such as structural, piezoelectric, ferroelectric and mechanical properties. 3.2 SINGLE CRYSTAL GROWTH Synthesized GPI was obtained from equimolar ratio of amino acid (Glycine) and orthophosphorous acid using triple distilled water as solvent. Synthesised pure GPI material was subjected to multiple recrystallization for purification. The trivalent rare earth metal ions of 0.2 mol % of Ce and Nd and 1 mol % of La in the form of nitrates were doped with pure GPI material. Solutions were poured into petri dishes and kept without disturbances. Good quality seed crystals were harvested by means of slow evaporation methods. Carefully selected and b-axis oriented seed crystals were used for bulk crystal growth. According to the growth kinetics of GPI, wider metastable zone width was observed between 40 o C to 45 o C. Hence for temperature lowering methods, the supersaturated solution was prepared at 45 o C and placed in a constant temperature bath with an accuracy of 0.01 o C and the solution was maintained for one day for homogeneous dissolvation. Test seed was inserted and it allowed for a day, after attaining exact supersaturation point, ferroelectric b-axis oriented seed crystals were inserted in the solution and

3 74 growth begins at 45 o C. The rate of cooling was maintained in the present investigations as 0.1 o C / day for about a week afterwards it is increased to 0.2 o C / day till the end of the growth period. Thus the bulk crystals of rare earth metal ions doped GPI crystals were grown from solvent evaporation as well as temperature lowering methods. The grown crystals were found to be non-hygroscopic. The crystal dimensions of Ce-GPI, Nd-GPI and La-GPI are 19 mm 17 mm 13 mm, 15 mm 14 mm 11 mm and 22 mm 14 mm 12 mm respectively and are shown in Figure 3.1. (a) (b) (c) Figure 3.1 As grown crystals of (a) Ce, (b) Nd and (c) La doped GPI grown by temperature lowering method

4 RESULTS AND DISCUSSION Single Crystal X-ray Diffraction Analysis Single crystal X-ray diffraction studies of the rare earth doped GPI crystals were carried out using ENRAF NONIUS CAD 4 single crystal X-ray diffractometer with MoK ( =0.717 ) radiation at room temperature. The crystal specimens of dimension mm 3 were used for the analysis. Least-square refinement of 138, 156 and 164 reflections were made for Ce, Nd and La doped GPI crystals respectively and the structure was solved by direct method and refined by the full matrix least-square technique using the SHELXL program. Rare earth metals doped GPI belong to monoclinic structure with the space group P2 1 /a. The lattice parameters of pure and rare earth metals doped GPI crystals were calculated and presented in Table 3.1. It was observed that volume of the doped crystals are reduced since the rare earth metal ions occupies the void spaces of pure GPI crystalline matrix and there will be a local compressive strain in the lattice. Table 3.1 Lattice parameters of pure and rare earth metals doped GPI crystals Cell Parameters / Crystal Name a (Å) b (Å) c (Å) V (Å 3 ) Pure GPI 7.391(2) 8.477(3) 9.774(4) 90 o o 90 o 616 Ce-GPI 7.408(3) 8.472(6) 9.778(4) 90 o o 90 o Nd-GPI 7.367(6) 8.444(6) 9.732(7) 90 o o 90 o La-GPI 7.380(6) 8.456(7) 9.752(11) 90 o o 90 o 598.6

5 Powder X-ray Diffraction Analysis The effect of rare earth metal ions on the structure of pure GPI crystals was analysed by recording the powder X-ray diffraction pattern of doped crystals using SEIFERT JSO DEBYE FLEX 2002 diffractometer. The grown crystals of doped GPI were crushed and fine powders were subjected to intense X-rays of wavelength Å (Cu K ) at a scan speed of 1 0 /min with a maximum scan range of The powder diffraction pattern of pure, Ce, La and Nd doped GPI crystals are shown in Figure 3.2. The presence of rare earth dopants within GPI crystal lattice indicates the changes in a relative intensity of the peak and their positions. Dopants diffuse into voids in parent crystal matrix and influences the lattice distortion which reveals that the planes such as (11-2), (120), (112) and (013) are broadened compared to pure GPI. The planes such as (103) and (023) were diminished in the case of dopants, however new peaks such as (125) and (1-51) arises in Nd doped GPI crystal.

6 77 Figure 3.2 Powder XRD pattern of Pure, Ce, Nd and La doped GPI crystals X-ray Rocking Curve Analysis To realize the full efficiency of the device properties, the crystals should be free from defects (Bhagavannarayana et al 2005a). High-resolution X-ray diffraction is one of the most widely used techniques for determining the crystalline perfection or the defect studies in single crystals. Evaluation of crystalline perfection is very important, particularly when they are doped, as

7 78 these dopants influence the crystalline perfection particularly at higher concentrations and at their larger size (Bhagavannarayana et al 2008). Figure 3.3 shows the Rocking curves recorded for 0.2 mol% of Ce and Nd doped crystal specimens. The RCs were quite sharp without any satellite peaks which may otherwise be observed either due to internal structural grain boundaries (Bhagavannarayana et al 2005). FWHM of the RCs were 9 and 8 arc sec for Ce and Nd doped GPI crystals which were very close to that expected from the plane wave theory of dynamical X-ray diffraction (Batterman and Cole 1964). The single sharp diffraction curve with very low FWHM of doped GPI crystals indicates that the crystalline perfection was quite good. The crystal specimens are nearly perfect single crystals without having any internal structural grain boundaries. Diffracted X-ray intensity [c/s] Ce-GPI (120) Plane MoK 9" Glancing angle [arc sec] (a) Figure 3.3 (Continued)

8 79 Diffracted X-ray intensity [c/s] Nd-GPI (120) Plane MoK 8" Glancing angle [arc sec] (b) Figure 3.3 X-Ray Rocking curve of (a) Ce and (b) Nd doped GPI crystals Figure 3.4 shows the Rocking Curve recorded for typical solution grown 1 mol% of La doped GPI crystal. The curve contains a single peak and indicates that the specimen is free from structural grain boundaries. FWHM of the curve was 18 arc sec. This value of FWHM is somewhat more than that expected from the plane wave theory of dynamical X-ray diffraction (Batterman and Cole 1964) for an ideally perfect crystal but close to that expected for nearly perfect real life crystals. This broadness with good scattered intensity along the wings of the diffraction curve on both sides of the peak indicates that the crystal contains both vacancy and interstitial type of defects. Such defects are very common to observe in almost all real crystals including nature gifted crystals and are many times unavoidable due to thermodynamical conditions. It is worth to mention here that the observed scattering due to point defects is of short range order as the strain due to such minute defects is limited to the very defect core and the long range order

9 80 could not be expected and hence one cannot observe any change in the lattice parameter. Diffracted X-ray intensity [c/s] La-GPI (120) Plane MoK 18" Glancing angle [arc sec] Figure 3.4 X-Ray Rocking curve of La doped GPI crystal X-ray Fluorescence Spectroscopic Analysis X-ray fluorescence spectra are used in the chemical analysis of both bulk samples and submicroscopic particles. Finely crushed and powdered samples were used and pellatized for the measurement. These pellatized samples were bombarded with high energy X-rays and the resulting emission spectrum recorded. From the spectral peak positions the elements present in the crystals were identified and from their intensities a quantitative analysis was made. Concentrations of metal ions in the pure GPI crystalline material were quantitatively determined by Bruker S4-Pioneer X-ray Fluorescence spectrometer and were found to be 118 ppm of Ce, 0.23 weight % of Nd, and 0.87 weight % of La in their respective doped GPI crystals.

10 Optical Spectral Analysis A SHIMADZU UV-Spectrometer 1601 was used for recording the transmission spectra of rare earth metals doped GPI crystals. The transmission spectra of Ce, Nd and La doped GPI crystals are shown in Figure Transmittance (%) Wavelength (nm) Pure GPI Ce-GPI Nd-GPI La-GPI Figure 3.5 UV-Visible Spectra of Pure, Ce, Nd and La doped GPI crystals Optical spectra indicate that the lower cut-off wavelengths for all the crystals are at 235 nm. The maximum transmittance of pure GPI was 75%, Ce and Nd doped GPI crystals it was around 85%. For La doped GPI the maximum transmittance reduced to 50% Differential Scanning Calorimetric (DSC) Analysis Differential scanning calorimetry is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. Both the sample and reference are maintained at nearly the same temperature throughout the experiment. Generally, the temperature program

11 82 for a DSC analysis is designed such that the sample holder temperature increases linearly as a function of time. The reference sample should have a well-defined heat capacity over the range of temperatures to be scanned (Wunderlich 1990). The basic principle underlying this technique is that, when the sample undergoes a physical transformation such as phase transitions, more or less heat will need to flow to it than the reference to maintain both at the same temperature. The nature of heat flow to the sample depends on whether the process is exothermic or endothermic. For example, as a solid sample melts to a liquid it will require more heat flowing to the sample to increase its temperature at the same rate as the reference. This is due to the absorption of heat by the sample as it undergoes the endothermic phase transition from solid to liquid. Likewise, as the sample undergoes exothermic processes (such as crystallization) less heat is required to raise the sample temperature. By observing the difference in heat flow between the sample and reference, differential scanning calorimeters are able to measure the amount of heat absorbed or released during such transitions. DSC may also be used to observe more subtle phase changes, such as glass transitions, ferromagnetic and ferroelectric transitions (Dean 1995, Pungor 1995, Douglas et al 1998). Paraelectric to ferroelectric transition temperature (T c ) of rare earth doped GPI crystals were identified by DSC measurements using NETZSCH DSC 204 differential scanning calorimeter with a cooling/heating rate of 10 K/min. In this study, crystalline powder of 2 mg was taken and experiment was performed in nitrogen atmosphere and the resultant DSC spectrum of the rare earth doped GPI is shown in Figure 3.6. A sharp endothermic peak at 230 K was observed Ce and Nd doped GPI crystals and 226 K for La doped GPI crystal, which implies the second-order ferroelectric phase transition of the material.

12 La-GPI K Heat flow (mw / mg) K Nd-GPI Ce-GPI K Temperature (K) Figure 3.6 DSC curve of Ce, Nd and La doped GPI crystals Piezoelectric Properties Piezoelectric charge coefficient was measured for rare earth metals doped GPI crystals in paraelectric phase i.e at room temperature. For the d 33 piezoelectric measurement using a Precision Piezo Meter System PM 300, rare earth doped GPI crystals were selected with smooth surface and (100) plane oriented crystals were used for the measurement. Crystal plates were coated with dry silver paste for the electrical contacts. The measurements were performed with the dynamic force of 0.25 N and the frequency of 110 Hz. Piezoelectric d 33 coefficients were determined using the relation, d 33 = [ (Q/A)(F/A) ] = Q / F = CV/F (C/N) (3.1) where A is the area stressed by a force F, C is the capacitance in the circuit and V is the voltage generated. Poling was made on the crystal samples with

13 84 the electric field of 10 kv / mm for 10 minutes duration on the same direction in which d 33 coefficients were measured. Piezoelectric charge coefficients of rare earth doped GPI (unpoled and poled) crystals were presented in Table 3.2. Table 3.2 Piezoelectric charge coefficients (d 33 ) of pure and rare earth metals doped GPI crystals Crystal Name Before Poling d 33 (pc/n) After DC Poling Pure GPI Ce-GPI Nd-GPI La-GPI Ferroelectric Hysteresis Loop Measurements P E Hysteresis loop (Figure 3.7) for pure and rare earth metals doped GPI crystals was traced by automatic P E loop tracer which includes modified Sawyer Tower circuit. Hysteresis loop was traced at 193 K and at the frequency of 50 Hz with the biasing field of 4 kv/cm, 6 kv/cm and 8 kv/cm. Ferroelectric b axis and (100) oriented crystal plates were selected for the measurements. Electrodes were made by painting dry silver paste on the polished thin rectangular samples (12 mm 10 mm 3 mm).

14 85 10 Polarization (µc/cm 2 ) Pure GPI Ce-GPI Nd-GPI La-GPI Electric Field (kv/cm) Figure 3.7 P E Hysteresis loop of pure and rare earth metals doped GPI crystals In the present investigation elliptical shaped loops were observed which indicates that the rare earth doped GPI crystals have good ferroelectric nature. Retentivity, coercivity and area of the hysteresis curve increases with the applied electric field. The value of squareness of hysteresis loop evaluates the hysteresis curve of a ferroelectric materials and is given by the relation, R sq P P r 1.1E c (3.2) s P P r where, P r, R sq, P s and P 1.1Ec are the remanent polarization, squareness of hysteresis loop, saturation polarization and polarization at an electric field equal to 1.1 times the coercive field, respectively. For an ideal hysteresis loop the squareness parameter is 2. The squareness parameters for rare earth metals doped crystals were found to be nearer to 2. The ferroelectric parameters were calculated and presented in Table 3.3.

15 86 Table 3.3 Ferroelectric parameters of pure and rare earth metals doped GPI crystals Crystal Coercive Field E c (kv/cm) Remanent Polarization P r µc/cm 2 ) Saturation Polarization P s (µc/cm 2 ) Squareness of Polarization R sq Pure GPI Ce-GPI Nd-GPI La-GPI Indentation Studies In order to study the mechanical properties of the rare earth doped GPI crystals indentation studies were carried out using MVH-1 (METATECH) Vicker s Microhardness tester. The indentation was made at different loads from 5, 10, 20, 30, 40 and 50 g. The duration of indentation was kept constant (10 s) for all indentations. The maximum value of hardness H v was calculated from the relation, H v = P/d 2 kg/mm 2 (where P is the applied load in kilograms, d is the diagonal length of the indentation impression in micrometers, and is a constant of a geometrical factor for the diamond pyramid). The plot of hardness H v versus load P for pure and rare earth doped GPI crystals is shown in Figure 3.8. The value of hardness increases with the applied load up to 20 g for pure and rare earth doped GPI crystals, which corresponds to work hardening of the material (Dhanaraj et al 1994). The plots of H v vs load also satisfy the indentation size effect (ISE), i.e. decrease in H v with increase in load in higher load region (above 20 g of load).

16 87 Microhardness (H v ) Load (g) GPI Ce-GPI Nd-GPI La-GPI Figure 3.8 Load vs Hardness plot of pure, Ce, Nd and La doped GPI crystals The maximum values of H v for pure and doped GPI crystals are presented in Table 3.4. The value of H V increases on doping because dopant ions enter into the lattice and hinder the formation of dislocation. Above 20 g load, plastic flow causes crack formation and hence reduces the hardness (Subhadra et al 2000). Elastic stiffness constant (C 11 ) was calculated from C 11 = H 7/4 for pure and doped GPI crystals using Wooster s empirical relation (Wooster et al 1953) and tabulated in Table 3.4. The relation between applied load and size of indentation was given by Meyer s law P = k 1 d n, where k 1 is the material constant and n is the Meyer's index. The plot of log P versus log d for all crystals gives a straight line which is in good agreement with Meyer's law. The slope of the graph yields the value of n. This was determined for pure and doped GPI crystals and presented in Table 3.4. Meyer's index n is a measure of ISE, if the hardness is independent of load, then, n=2. If the hardness increases when the applied load decreases, n < 2. If the hardness decreases when the applied load

17 88 decreases, n > 2. Deviations from n = 2 are the measure of ISE and in a specific case will be a measure of microstructural variations, surface layers, mechano-chemical effects etc (Yurkov et al., 1994) for certain materials. According to Onitsch and Hanneman, n should lie between 1 and 1.6 for hard materials and above 1.6 for softer ones. Meyer's index n was calculated for pure and doped GPI crystals and is presented in Table 3.4. The n value observed in the present study was more than 1.6 for pure and doped GPI crystals suggesting that rare earth doped GPI crystals are relatively soft materials. Yield strength can be calculated from hardness values for Meyer s index using the equation y = H v / 3 (for n < 2) (Banwari Lal et al 2002). Since n, being less than 2 for pure and rare earth doped GPI crystals. The mechanical parameters are presented in the Table 3.4. Table 3.4 Mechanical properties of pure and rare earth metals doped GPI crystals Crystal Maximum value of H v (kg/mm 2 ) Meyer s index n Stiffness constant C 11 (MPa) Yield strength y (MPa) GPI Ce-GPI Nd-GPI La-GPI CONCLUSION Good quality and bulk crystals of rare earth metals doped GPI crystals were grown by slow evaporation and slow cooling methods. Various characterization were carried out on doped GPI to realize the physical

18 89 properties of doped GPI materials. Single crystal XRD results reveal the variation of cell parameters and structural morphologies of the doped crystals with pure GPI crystal. X-ray powder diffraction method was used to identify the crystalline phases of pure and rare earth metals doped GPI crystals. X-ray rocking curves were plotted for pure and doped crystals using HRXRD analysis, which reveals the crystalline perfection of crystals. FWHM of Ce, Nd and La doped GPI crystals were 9, 8 and 18 arc sec respectively. Ce and Nd doped GPI crystals are nearly perfect single crystals without having any internal structural grain boundaries. However, grain boundaries, low angle tilt boundaries and interstitial defects were observed for La doped GPI crystal specimen. The incorporation of rare earth metal dopants in the pure crystalline matrix was quantitatively identified by XRF spectral analysis. Influence of dopants on optical properties of crystals was determined using UV-Visible spectral analysis. Paraelectric to ferroelectric transition temperature of pure and doped GPI crystals was determined by DSC. Transition temperature (T c ) was improved significantly for Ce doped GPI while it is slightly improved for Nd and La doped GPI crystals. Piezoelectric charge coefficient (d 33 ) was measured for pure and doped GPI (unpoled and poled) crystals. After poling there will be slight increment in d 33 values for doped GPI. P E hysteresis loop of pure and doped GPI crystals were measured and the ferroelectric parameters were calculated. Mechanical stabilities of pure and doped GPI crystals were analyzed using Vicker s microhardness analysis and mechanical properties of crystals were calculated.