CHAPTER 4 GROWTH AND CHARACTERISATION OF METAL THIOUREA COMPLEX DOPED KDP CRYSTALS

Transcription

1 77 CHAPTER 4 GROWTH AND CHARACTERISATION OF METAL THIOUREA COMPLEX DOPED KDP CRYSTALS 4.1 INTRODUCTION Single crystal growth has prominent role to play in the present era of rapid technical and scientific advancement, where the application of crystals has unbounded limits (Elwell and Scheel 1975, Robey et al 2000). Frequency conversion is an important and popular phenomenon for extending the useful wavelength range of lasers. This has led to the production of various devices such as harmonic generators, optical parametric oscillators, electro-optic modulators and amplifiers for high power lasers (Xin-Guang Xu et al 2001). Hence efforts were focused at the development of new and efficient frequency conversion materials (Liu et al 2006). The search for new materials was primarily focused on increasing the nonlinearity (Schmidt et al 2000). With progress in crystal growth technology, materials having attractive nonlinear properties are being discovered at rapid pace (Chemla et al 1987). KDP crystal offers high transmission throughout the visible spectrum and meets the requirement for optical birefringence, large enough to bracket its refractive index for even extreme wavelength over which it is transparent (Podder 2002). Among the non-linear optical phenomena, frequency mixing and electro-optic are important in the field of optical communication (Miroslow Rak et al 2005). Improvement in the quality of the KDP crystals and the performance of the KDP based devices can be realized with suitable dopants (Newman et al 1990). In this connection, efforts were made to dope copper- thiourea complex in KDP single crystals.

2 78 The optical quality copper-thiourea complex doped KDP crystals were grown by the slow evaporation method. The grown crystals were characterized by single crystal X-ray diffraction, FT-IR, TG, DTA and UV VIS spectroscopy. SHG efficiency of the grown crystals was measured by powder Kurtz method using Nd:YAG laser. Influence of copper-thiourea complex doped in KDP, non-linear performance of KDP was analysed. 4.2 CRYSTAL GROWTH Copper-thiourea complex doped KDP crystals were grown from aqueous solution by slow evaporation method. The solubility of copperthiourea complex doped KDP in water was measured. It was found to be 31.5 g/100 ml at 40 o C for KDP and g/100 ml at 40 o C for copperthiourea complex. The amount of KDP salt to be dissolved was determined from its solubility curve at an average temperature of 38 o C. The solution was stirred long enough to ensure complete dissolution of the solute, and filtered. Subsequently the solution was cooled at a rate of 0.1 o C/day. The seed crystals were prepared at low temperature by spontaneous nucleation (Figure 4.1(a)). (a) Figure 4.1(a) Seed crystals Cu-thiourea doped KDP

3 79 The seed crystals with perfect shape and free from macro defects were used for growth experiments. Seed crystals of pure KDP and doped KDP (copper- thiourea complex) were grown using constant temperature bath controlled with an accuracy of ± 0.01 o C. A supersaturated solution of Cu-thiourea complex doped KDP powder was prepared in distilled, deionized water. Seed crystals were introduced into the solution using thin nylon thread at the appropriate supersaturation condition. Experiments were allowed to run for considerably longer duration of the time (20 days) would grow large crystals (Figure 4.1 (b)). (b) Figure 4.1(b) Bulk crystal of Cu-thiourea doped KDP 4.3 POWDER X-RAY DIFFRACTION ANALYSIS Powder X-ray diffraction studies were carried out for the as grown crystals using a Rich Seifert X-ray diffractometer with CuK α (λ = Å) radiation (Kurtz 1968). Powder X-ray diffraction spectra of the grown crystals from pure and doped (Cu-thiourea complex) KDP are shown in Figure 4.2.

4 80 Figure 4.2 XRD Spectra of (a) pure and (b) Cu-thiourea complex doped KDP crystal Powder XRD spectra for the pure and Cu-thiourea complex doped KDP revealed that the structures of the doped crystals were slightly distorted compared to the pure KDP crystal. This may be attributed to strain on the lattice by the absorption or substitution of Cu-thiourea complex. It is observed that the reflection lines of the doped KDP crystal correlate well with those observed in the individual parent compound with a slight shift in the Bragg angle.

5 OPTICAL STUDIES FT-IR Studies The FT-IR spectra of pure and Cu-thiourea complex doped KDP crystals on a Bruker IFS 66V model spectrophotometer using 1064 nm output of a cw diode pumped Nd: YAG laser as a source of excitation in the region cm 1 operating at 200 mw power at the samples with a spectral resolution of 2 cm 1. The observed FT-IR spectra of pure and doped KDP are shown in Figure Figure 4.3 FT-IR spectra of (a) pure and (b) Cu-thiourea complex doped KDP crystal

6 82 The frequencies with their relative intensities obtained in FT-IR of pure and doped KDP and their most probable assignments are presented in Table 4.1 Table 4.1 FT-IR Assignments of pure and Cu-thiourea complex doped KDP single crystals Calculated frequency (cm -1 ) Pure KDP KDP + Cu- thiourea complex Assignments (w) 3650(w) Free O-H stretching (vw) N-H asymmetric stretching (br) (br) (br) - Intramolecular H-bonded O-H stretching N-H symmetric stretching in NH 2 group O = P OH asymmetric stretching of KDP (w) 2250(br) P-O-H asymmetric stretching (br) 1650(vs) (sh) 1350(vs) O = P OH symmetric stretching of KDP P=O symmetric stretching (aliphatic) (sh) 1150(vs) P-O-H symmetric stretching (s) - O = P OH bending (sh) 600(sh) HO P OH bending w-weak vw-very weak s-strong vs-very strong sh-sharp br-broad In doped KDP spectra, broad peak around 3650 cm -1 was due to free OH stretching vibration. It reveals that at least one of the OH group of KDP was remain unaltered after that doped with Cu-thiourea complex. Due to the greater mass of sulphur in copper-thiourea complex the C=S stretching vibration was expected to occur at 625 cm -1, it was considerably lower frequency than the usual C=O stretching vibration at 1710 cm -1 because of the

7 83 C=S group is less polar than the C=O group and has a considerably weaker band. The strong vibrational coupling was operative in the case of nitrogen containing thiocarbonyl group of thiourea and that the C=S vibration was not located in the spectra UV-Visible studies The transmission spectra were taken at room temperature using Varian Cary 2300 Spectrometer (UV-VIS-NIR). The transmission spectra were recorded in the range nm for 1 mm thick c-cut plates of pure and doped KDP was shown in Figure 4.4. Figure 4.4 UV-Visible spectra of (a) Cu-thiourea complex doped KDP and (b) pure KDP crystal

8 84 All the crystals irrespective of the dopants are transparent in the entire visible region. The UV-Visible spectrum of Cu-thiourea complex doped KDP crystal, maximum wavelength of absorption (λ max ) appeared at 780 cm -1. Therefore, it reveals that after incorporation of Cu-thiourea complex, the UV absorption was shifted to Red region. The Bathochromic shift (Red shift) increases in accordance of mole fractions of dopants. The pure KDP crystal has about 70% of transmission. The Cu-thiourea complex doped KDP crystal is invariably has higher transmission percentage compared to pure KDP crystal. From the UV-Visible spectrum, transmission percentage increases due to addition of Cu-thiourea complex in KDP crystal, that would enhances the optical property of KDP crystal. 4.5 MICROHARDNESS STUDIES Microhardness is one of the important mechanical properties of the KDP crystals. It can be suitably used to measure the plastic properties and strength of a material. Microhardness measurements were carried out using Leitz Weitzler hardness tester fitted with a diamond indenter. The wellpolished doped KDP crystal was placed on the platform of the Vickers micro hardness tester and the loads of different magnitudes were applied over a tester at a fixed interval of time. The indentation time was kept as 8 sec for all the loads. The microhardness value was calculated using the relation H v = X P/d 2 kg/mm 2, where P is the applied load in kg and d is the diagonal length of the indentation impression in mm. The applied load was 5,10 and 25 g. Cu-thiourea complex doping in KDP improves the mechanical strength of KDP. The microhardness value of pure and Cu-thiourea complex doped KDP are tabulated in Table 4.2.

9 85 Table 4.2 Microhardness values of doped KDP S.No Crystal Microhardness (kg/mm 2 ) 1. KDP Cu-thiourea doped KDP THERMAL STUDIES Figure 4.5 illustrates the Differential Thermal Analysis (DTA) and Thermo-Gravimetric Analysis (TGA) curves for the grown copper-thiourea complex doped KDP crystal. The DTA curve implies that the material undergoes an irreversible endothermic transition at 200 C where the melting begins. This peak was endothermic peak, represents the temperature at which the melting terminates which corresponds to its melting point at 210 C Figure 4.5 TGA-DTA curves of copper-thiourea complex doped KDP Crystal

10 86 Ideally, the melting point of the trace corresponds to a vertical line. The sharpness of the endothermic peak shows good degree of crystallinity of the grown ingot. The exothermic peak at 290 C indicates a phase change from liquid to vapour state as evident from the loss of weight of about 87% in TG curve. 4.7 NLO STUDIES The Cu-thiourea complex doped KDP crystals are used for the generation of second harmonics of Nd-based near-infrared solid-state lasers. The fundamental of an Nd:YAG laser (1064 nm) can be converted to 532 nm of second harmonic or its 355 nm of third harmonic or its 266 nm of fourth harmonic by using KDP crystals. The performance of these frequency conversion devices can be seriously degraded if there are defect-associated absorption bands in the crystal which overlap the fundamental pump wavelength or one of the output wave lengths. Thus, it is important to identify and characterize all potentially harmful absorption bands in non-linear optical crystals (Munn and Ironside 1994). In order to confirm the suitability of the doped KDP crystal, the non-linear applications, harmonic generation was tested using the Nd-YAG laser. A small crystal was placed on the sample holder and the YAG laser beam was made to pass through the crystal and the output conversion of input as green light SHG was analyzed. The efficiency of doped KDP crystals were compared with pure KDP and also show that Cu-thiourea complex doped KDP crystal has higher efficiency. A sample of potassium dihydrogen phosphate (KDP), also powdered was used for the same experiment as a reference material in the SHG measurement. It was found that the frequency doubling efficiency of the doped KDP is better than KDP. A comparison of SHG property of KDP crystal with Cu-thiourea complex doped KDP crystal is presented in Table 4.3.

11 87 Table 4.3 Comparison of SHG of pure and Cu-thiourea complex doped KDP Crystals S.No. Compound SHG efficiency 1. KDP Cu-thiourea complex doped KDP FERROELECTRICS The poling apparatus consists of a constant temperature silicone oil bath and a high voltage power supply. The sample holder to be used in the silicon oil bath is designed and fabricated using Teflon and suitable electrodes. It is immersed in the silicone oil bath and leads are introduced to the sample of high voltage power supply. The temperature of the bath is controlled by a temperature controller. The temperature of the bath could be set at temperature in the range 30 ºC- 400 ºC to an accuracy of ±1 ºC. Heat sink copper tube was incorporated in the silicone oil bath for quick cooling to reduce the cooling time between two successive poling processes. The set up of the poling apparatus is shown in Figure 4.6. Figure 4.6 Poling apparatus with sample holder

12 88 The power supply was capable of delivering varying voltages from 2 to 50 kv insteps of 0.1 kv. A Teflon disc of a few millimeters thickness (depending on the thickness of the sample) along with the silicone sealant was used in the poling cell (sample holder) in order to increase the arcing length and thus avoid arcing. Cu-thiourea complex doped KDP possess a high dielectric constant along the polar axis, which was a function of temperature and reaches to a peak value (theoretically infinite) at the critical temperature (123 K). The ferroelectric transition at the Curie temperature was associated with either the latent heat or anomalous behavior of specific heat and at critical temperature (T c ) the surface charges appear abruptly (Laulicht 1990). It possess pseudo symmetric structure of polar symmetry and have polar structure in ferroelectric state while non polar structure in para electric state and the ferroelectric crystal possess domain structure which is visible in polarized light (Schmidt et al 2000). The domain structures in KDP crystal due to the presence of Cu-thiourea complex were analyzed. It was observed that the dopant improves the ferroelectric property in KDP crystal. 4.9 RESULTS AND DISCUSSION X-ray diffraction studies have been carried out to confirm the crystallinity and to calculate the lattice parameters of the grown samples. Using the monoclinic crystallographic equation, the lattice parameter values of doped KDP crystals were calculated and compared with the literature values. This confirms that the doped KDP single crystal retain its own crystal system. The TG curve of this sample indicates that the sample is stable up to 210 C and above this temperature the weight loss occurred due to selfdegradation of doped KDP but merely to its evaporation after its melting (Bohenck et al 1997). Three prominent and interesting features of ferroelectrics of doped KDP crystals are their reversible polarization, anomalous behavior (optical,

13 89 thermal, elastic etc.,) and their non-linearities. The ferroelectric materials cease to be ferroelectric above a certain temperature, known as transition temperature and show normal dielectric behavior (Zheshuai Lin et al 2003). The anomalous behavior near the transition temperature is probably as significant as the reversible polarization, but it is not definitive of ferroelectric. At the transition temperature the Cu-thiourea complex doped KDP crystal undergoes a transition from ferroelectric phase to a high symmetry phase and permittivity increases sharply to a very high value which was referred to be anomalous in the neighborhood. The highest symmetry phase compatible with the ferroelectric structure is termed as the prototype phase also called para electric phase (Zaitseva et al 1998). Although it is not necessary of non-polar character, it was proves to be the greatest majority of known ferroelectrics so far. Most of the phases actually exist as the highest temperature phase of the crystal, although in some instances the structure may melt before the prototype phase would otherwise become stable (Smolsky et.al 1998). As a result of its small structural displacement from the prototype, a typical ferroelectric material possesses a spontaneous polarization P s which decreases with the increase of temperature T c CONCLUSION Good optical quality single crystals of semi-organic (Cu-thiourea complex) doped KDP have been grown from solution by slow solvent evaporation technique for the first time. Experiments were allowed to run for considerably larger duration of the time (20 days) can grow large crystals. The functional groups present in the grown crystals have been confirmed by FT-IR spectral analysis. The observed frequencies were assigned on the basis of symmetry operation on the molecule and

14 90 normal coordinate analysis. The crystallinity of the grown sample was confirmed by single crystal X-ray diffraction analysis. Thermal stability of the grown sample was studied by TG and DTA analysis. The results from TG and DTA studies, Cu-thiourea complex doped KDP crystals were stable upto 210 C. The exothermic peak at 290 C indicates a phase change from liquid to vapour state as evident from the loss of weight of about 87% in TG curve. Optical transmission UV-Visible spectral) range of Cu-thiourea complex doped KDP was measured and the doped KDP crystal has a good optical transmission in the entire visible region. The powder SHG measurement shows that the grown doped KDP crystal has 1.89 times higher SHG efficiency than KDP. Vickers micro hardness was calculated in order to understand the mechanical stability of the grown crystals. The non-linear response of the physical properties with respect to the electric field and temperature is a unique characteristic of ferroelectric materials. This makes them extremely attractive materials for a variety of applications, particularly based on their anomalous electrical, optical and piezoelectric properties.