Electrical conductivity and dielectric measurements in Au + doped/undoped KDP crystals with KCl and NaCl as additives

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1 Indian Journal of Pure & Applied Physics Vol. 43, February 2005, pp Electrical conductivity and dielectric measurements in Au + doped/undoped KDP crystals with KCl and NaCl as additives R Ananda Kumari 1 & R Chandramani 2 1 Department of Physics, Sree Siddaganga College for Women, Tumkur Department of Physics, Bangalore University, Bangalore Received 6 May 2004; revised 13 August 2004; accepted 3 January 2005 Pure potassium dihydrogen orthophosphate (KDP) containing KCl or NaCl with and without gold doping were grown by slow evaporation technique. All the grown crystals were γ-irradiated using 60 Co source. Electrical conductivity measurements were carried out perpendicular to the unique direction before and after γ-irradiation. The conductivity of KDP crystals increases with the addition of KCl and NaCl and with gold doping as well as temperature. Computed values of activation energies from the conductivity measurements are given. The dielectric constant and dielectric loss have been measured as a function of frequency. The study confirms the contribution of space charge polarization. [Keywords: KDP crystals, Doping, Dielectric constant, Electrical conductivity, Activation energy] IPC Code: H01 L 41/00 1 Introduction Electrical conductivity of ionic crystals yields useful information regarding the mobility and production of lattice defects in these materials. In potassium dihydrogen orthophosphate (KDP) type of crystals, the possible type of point defects which help the electrical conduction process are the ionization defects, viz. (HPO 4 ) -2 and H 3 PO 4 produced as a result of proton jump from one phosphate group to another along the same bond 1. The electrical conduction in KDP crystals has been established to be protonic 2,3. Earlier reports on KDP crystals doped with oxalate and chloride impurities have shown increase in conductivity, which has been explained as due to the replacement of (H 2 PO 4 ) -1 ions by (C 2 O 4 ) -2 and Cl - 1 ions 4,5. It was also found that the activation energy for KDP crystals does not vary much on adding oxalate impurity of various concentrations 6. It has been observed that the KDP system has become complex after ion irradiation and it shows irregular behaviour with regard to conductivity property 7. At any temperature, the Gibb s free energy of a crystal is minimum when a certain fraction of ions leaves the normal lattice. As the temperature increases, more and more defects are produced which in turn increase the conductivity 8. smshashidhara@yahoo.com; Fax: No The KDP crystals have been modified by adding KCl and NaCl in definite ratios and doping with gold with the aim of discovering new useful materials for academic and industrial use. The present paper reports the studies of electrical conductivity and dielectric constant of solution grown KDP crystals containing KCl and NaCl with/without gold doping. 2 Experimental Details Single crystals were grown from aqueous solution by slow evaporation technique at room temperature using AR grade samples of KDP, KCl and NaCl. KDP was added with KCl and NaCl separately, each in three different molecular ratios, viz. KDP:X (X represents the KCl or NaCl) 10:0.0 (pure KDP), 9:1 and 8:2. Au + doped crystals were grown by adding a definite volume of Tetra-Auro chloric acid solution to the mixture of KDP containing KCl or NaCl. Grown crystals were found to be transparent and exhibited scalenohedral (twelve sided polyhedron) morphology. The crystals with high transparency were used for electrical conductivity and dielectric measurements. The KDP crystals modified with KCl and NaCl were analysed by XRD and energy dispersive X-ray spectroscopy (EDAX) to check the presence of doped elements. Figures 1 and 2 indicate that the impurities have entered into the lattice of the KDP crystals.

2 124 INDIAN J PURE & APPL PHYS, VOL 43, FEBRUARY 2005 Fig. 1 EDAX data of (KDP) 0.8 (KCl) 0.2 crystals Fig. 2 EDAX data of (KDP) 0.8 (NaCl) 0.2 crystals

3 ANANDA KUMARI & CHANDRAMANI: ELECTRICAL CONDUCTIVITY OF KDP Dielectric measurements The extended portions of the crystals were removed completely and the samples were ground to proper thickness and polished. The final dimensions of the samples were about 1.5 to 2 sqcm and 0.1to 0.3 cm in thickness. Each sample was electroded on either side with air-drying silver paste so that it behaved like a parallel plate capacitor. A 4275A, Multi frequency LCR meter (Hewlett-Packard) was used to measure capacitance (C) and dissipation factor (D) of the sample as a function of frequency. The dielectric constant (ε) and dielectric loss (tanδ) were calculated from C and D using the relations, Cd ε = and tanδ = Dε, where C is the capacitance Aε 0 of the sample, d the thickness of the sample, A the area of the face in contact with the electrode and ε o is the permittivity of free space. 2.2 Conductivity measurement Both undoped and Au + doped crystals were subjected to γ-irradiation using 60 Co source to a dosage 5 Mrads. The conductivity measurements were carried out for both irradiated and non-irradiated crystals along a-direction (perpendicular to the unique axis) using the conventional two-probe technique at various temperatures in the range from room temperature to 150 o C. The conductivity values were fitted into the equation σ (T) = σ o exp (-E/kT), where k is the Boltzmann constant, T the absolute temperature, σ o a constant and E is the activation energy. Fig. 3 Variation of Dielectric Constant with Frequency in KDP crystals containing KCl with and without Au + doping. Fig. 5 Variation of Dielectric Loss with log (Frequency) in KDP crystals containing NaCl with and without Au + doping. Fig. 4 Variation of Dielectric Constant with Frequency in KDP crystals containing NaCl with and without Au + doping. Fig. 6 Variation of Dielectric Loss with log (Frequency) in KDP crystals containing KCl with and without Au + doping.

4 126 INDIAN J PURE & APPL PHYS, VOL 43, FEBRUARY 2005 Table 1 Values of the dielectric constant of pure KDP, Au + doped KDP, KDP containing KCl and NaCl with and without Au + doping Sample KDP containing 10% KCl or NaCl KDP containing 20% KCl or NaCl At 100 khz At 1 MHz At 100 khz At 1 MHz Pure KDP Au + doped KDP KDP+KCl Au + doped KDP+KCl KDP+NaCl Au + doped KDP+NaCl Results and Discussion (i) The variation of ε and tan δ at room temperature for KDP, Au + doped KDP and KDP containing KCl and NaCl with and without gold doping are shown in Figs 3-6. It is observed that the value of ε decreases with the increase in the frequency. Values of ε and tan δ at 100 khz and 1 MHz frequency for pure KDP, Au + doped KDP and KDP crystals containing KCl and NaCl with and without gold doping are given in Table 1. The dielectric constant of a material is generally composed of four types of contributions, viz ionic, electronic, orientational and space charge polarizations. At low frequencies, all polarizabilities are operative hence ε is high. As frequency increases one polarization mechanism after another is frozen out. The first to stop contribution to ε is orientational component, then the ionic and lastly the electronic 9,10. The dielectric loss (tan δ) is due to the resistive component that makes them lossy, so that they dissipate some of the applied ac energy. The value of tan δ was found to initially decrease with frequency and later almost a constant over a range of frequency. Fig. 8 Variation of log (σ T ) with 1000/T for KDP crystals containing KCl with and without Au + doping. Fig. 7 Variation of log (σ T ) with 1000/T in KDP and Au + doped crystals before and after irradiation Fig. 9 Variation of log (σ T ) with 1000/T for KDP crystals containing NaCl with and without Au + doping

5 ANANDA KUMARI & CHANDRAMANI: ELECTRICAL CONDUCTIVITY OF KDP 127 Fig. 10 Variation of log (σ T ) with 1000/T in KDP crystals containing KCl with and without Au + doping before and after irradiation. Fig. 11 Variation of log (σ T ) with 1000/T in KDP crystals containing NaCl with and without Au + doping before and after irradiation. Sample (ii) The σ values obtained along the a-direction are within the experimental error. Figures 7-11 show the plots of logσ (T) versus 1000/T for the pure and KCl and NaCl added KDP crystals with and without gold doping before and after γ-irradiation when the field is along the a-direction. The computed values of the activation energy E for pure and impurity added crystals are given in Table 2. It can be seen from Figs 7-11, that the electrical conductivity increase with addition of KCl and NaCl and with irradiation. The conductivity graphs show the usual intrinsic and extrinsic regions. The conduction at high temperature is the intrinsic conduction region, which is due to the thermally created vacancies as well as the variation of their mobility with temperature. The low activation energy suggests that oxygen vacancies are responsible for conduction in this region. Extrinsic region i.e. the low temperature region is structure sensitive region. Hence, the addition of impurities further increases the electrical conductivity in the extrinsic region. Conductivities obtained in the present study are in good agreement with those obtained by previous researchers for the KDP crystals The electrical conduction in dielectrics is mainly a defect-controlled process in the low temperature region (structure sensitive region). The defect concentration will increase exponentially with temperature and consequently the electrical conduction also increases. The addition of impurity further increases the electrical conduction in the temperature region considered. For the concentrations of the impurities studied, the electrical conductivity increases with increase in the impurity content of the crystal. In the case of KDP, the most important contribution to an understanding of the structure is given by West 7. The atomic arrangements are such Table 2 Activation energies of pure and impurity added KDP crystals Before irradiation KDP containing 10mole% of alkali halides Before irradiation KDP containg 20mole% of alkali halides After irradiation KDP containing 10 mole % of alkali halides Pure KDP Au + doped KDP KDP + NaCl Au + doped KDP + NaCl KDP +KCl Au + doped KDP + KCl

6 128 INDIAN J PURE & APPL PHYS, VOL 43, FEBRUARY 2005 that each P atom is surrounded by four O atoms at the corners of a regular tetrahedron. Each PO 4 group is linked with four other PO 4 groups by H bonds. The upper oxygen of one PO 4 group is linked with the lower oxygen of the neighbouring PO 4 group by an H bond. Neutron diffraction studies revealed that each PO 4 group has two nearest Hs, and as a group they form (H 2 PO 4 ) ions. The PO 4 group and the K ions are built up in such a way that K and P atoms alternate each other at a distance of c/2 along the C-axis. Each K is surrounded by eight Os and four of them nearer than the other four. For electrical conduction it is proved 4,6 that K ion does not contribute much. The conduction is mainly due to the anions namely (H 2 PO 4 ) ions. The H bond with the PO 4 group is in such a way that one H bond is associated with the upper O and the other H bonds with lower O. This arrangement gives a favourable picture of conduction mechanism. As the temperature increases, the spontaneous polarization decreases and becomes zero at and above the Curie temperature (123K). This change is due to the change in the configuration of the PO 4 group. The H bonds, which are associated with either the upper two Os or lower two Os, before the transition temperature, change their configuration in such a way that one H bond is associated with one of the upper Os and the other with one of the lower Os. This type of transformation of H bonds is called M defect. In a real crystal, the presence of M defect creates an H bond with two hydrogen at one end and an H bond with no hydrogen at the other end. These defects are known as D and L defects, respectively. O Keeffe and Perrino 2 introduced another type of defect (HPO 4 ) - and (H 3 PO 4 ) + formed by self-ionization of H 2 PO 4. These charges combine with the H 2 PO 4, giving rise to an M defect. This M defect, in turn by intra bond jump annihilates itself and yields L and D pairs. These defects are responsible for the electrical conduction in KDP crystals. The increase in conductivity of KDP crystals containing KCl and NaCl impurities can be explained by considering the replacement of (H 2 PO 4 ) -1 ions by Cl - ions. This is in agreement with the results obtained by the previous workers in the case of doped KDP crystals 4, Computed values of activation energies from the plots of σ (T) versus 1000/T confirm higher values of activation energy in case of KDP crystals containing NaCl and KCl and also with Au + doping. Irradiated crystals have answered for still higher activation energies. This increase in activation energy gives a clue regarding the diffusion of impurities to the regular position, hence the increase in E. 4 Conclusions Pure and NaCl and KCl added KDP single crystals with and without gold doping were grown by slow evaporation technique. EDAX data confirm the impurities have gone into the lattice of the crystal. Electrical conductivities were measured along the a- direction at various temperatures ranging from room temperature to 150 o C. The present study gives further evidence that the conduction in KDP is mainly due to the anions and not the cations. The present study indicates that the conductivity increases with the addition of KCl and NaCl and also with gold doping. Also confirms the further increase in conductivity as due to the effect of radiation-induced defects. Higher value of activation energy in case of KDP crystals containing NaCl and KCl, and also with Au + doping gives a clue regarding the diffusion of defects either to the regular position or to the interstice. References 1 Jain S C & Dahake S L, Indian J Pure & Appl Phys, 2 (1964) Shanmugham M, Gnanam F D & Ramasamy P, Indian J Pure & Appl Phys, 23 (1985) Udupa K S, Mohan Rao P, Sriramana Aithal, Bhat A P & Avasthi D K, Bull Mater Sci, 20 (1997) Shanmugham M, Gnanam F D & Ramasamy P, Indian J Pure & Appl Phys, 23 (1985) Ramasubramanian P S & Mahadevan C, Indian J Pure & Appl Phys, 29 (1991) Harris L B & Vella G J, J Appl Chem, 17 (1966) Udupa K S, MohanRao P, Sriramana Aithal, Bhat A P & Avasthi D K, Bull Mater Sci, 20 (1997) Keeffe M O & Perrino C T, J Phys & Chem Solids, 28 (1967) Shanmugham M, Gnanam F D & Ramasamy P, Indian J Pure & Appl Phys, 20 (1982) George A W, Nucl Instrum Meth Phys Res, B 39 (1989) J Albin Sancta, A Gnana Sutha, T H Freeda C Mahadevan & C Balasingh, Indian J Phys, 75A(3) (2001) R Ananda Kumari & R Chandramani, Bull Mater Sci, 26(2) (2003) R Ananda Kumari & R Chandramani, Indian J Phys, 77A(4) (2003) 397.