Dielectric behavior of mixed cadmium magnesium hydrogen phosphate crystal

Magnesium hydrogen phosphate (MHP) and transition metal doped cadmium magnesium hydrogen phosphate (CdMHP) was synthesized in the form of crystalline material by room temperature solution technique known as gel encapsulation technique. The synthesized crystals were then characterized for their structural, mechanical and electrical investigations using various chemical and physical methods. X - ray diffraction analysis (XRD) establishes magnesium hydrogen phosphate and cadmium magnesium hydrogen phosphate belonging to orthorhombic crystal system with space group Pbca. The mechanical behaviour of these crystals was studied by calculating Vicker’s hardness number. The behaviour of microhardness with applied load was observed to be complex. The electrical behaviour was carried out by calculating dielectric constant at different temperatures and for different frequencies. The dielectric constant (ε / ) was found to be strongly dependent on temperature and frequency. The transition metal doping of cadmium in magnesium hydrogen phosphate remarkably decrease the value of dielectric constant from 68 to 23. The transition temperature also decreases from 330˚C in case of magnesium hydrogen phosphate to 310˚C in case of cadmium magnesium hydrogen phosphate. Copyright © 2014 VBRI press.


Introduction
Phosphates are considered as the most interesting class of materials which have attracted more and more interest on synthesis, crystallography, electro-optical properties and so on [1,2]. Over the past few decades, the technology and science of materials has advanced rapidly for the synthesis and characterization of new non-linear optical materials. Potassium dihydrogen phosphates (KDP), an important class of material belonging to phosphate group have gained special interest due to their vide applications as ferroelectric, piezoelectric, pyroelectric as well as laser modulator [3,4]. Studies on the preparation and characterization of calcium phosphate have been widely investigated [5][6][7][8]. Synthesis of some transition metal phosphates by different synthesis conditions and medium agents have been reported [9,10]. The precipitation of calcium phosphates in the presence of increasing cadmium amount was studied at 25 0 C in dilute ammonia solutions [11]. An amorphous precipitate, an apatitecalcium phosphate and the compound Cd 5 H 2 (PO 4 ) 4 .4H 2 O are formed according to pH and cadmium concentration. The effect of cadmium substitution on various properties of pure calcium hydrogen phosphate has been studied by Bamzai et al [12]. Recently, effect of rare earth on thermo luminescence properties of CaSO 4

phosphors have been investigated by Wani et al [13]
The alkaline earth phosphates have received enormous importance with respect to their use as phosphor matrices [14]. The effect of doping on gel grown crystals has been studied by many investigators [15][16][17]. Inorganic phosphates encompass a large class of diverse materials with a vide range of applications which includes catalysts, linear and nonlinear optical components, solid electrolytes for batteries, in synthetic replacement for bone and teeth and many more [18,19] In the present investigations, the main objective of the work is to report growth, characterization, mechanical and dielectric behavior of pure and cadmium (Cd) mixed magnesium hydrogen phosphate crystals. Growth of mixed crystals is interesting because their characteristics change from those of the crystals of single components grown separately, e.g., mixed crystals are found to be harder than pure ones [23,24]. Cadmium (Cd 2+ ) ion which is transition metal has been selected for the modified magnesium hydrogen phosphate because its ionic radius (0.97Ǻ) is very close to that of magnesium (0.65Ǻ) ion. According to Goldsmith rule, the dopant cation enters into the sites if the radius of the substituted ion and the replaced ion does not differ by 15 -20%. Also, literature survey unfolds that no work has so far been reported on growth of mixed crystal of Cd modified magnesium hydrogen phosphate in gels at room temperature. The effect of substitution on various characteristics of the parent material has been investigated by carrying out systematic study on the crystal of the modified composition.

Materials
To carry out the growth of magnesium hydrogen phosphate and cadmium magnesium hydrogen phosphate, the starting materials were the following: orthophosphoric acid (H 3

Method
The growth of magnesium hydrogen phosphate (hereafter referred as MHP) and cadmium mixed magnesium hydrogen phosphate (referred as CdMHP) has been carried out by using room temperature solution technique i.e., gel encapsulation technique. The method involves incorporating one reactant called the lower reactant into the gelling mixture and later diffusing another reactant called the upper reactant into the gel medium. This leads to a very high supersaturation condition initiating the nucleation necessary for the growth of crystals.

Preparation of hydrosilica gel
In order to grow crystals from silica gel, first of all silica gel solution of desired molarity was prepared by dissolving sodium metasilicate (SMS) powder in distilled water. The pH of the SMS solution must be lowered in order to have proper gelation. This can be achieved by mixing an acid with the solution. Since in the present case phosphate crystals are to be grown, therefore, orthophosphoric acid of desired molarity was added gently to the prepared SMS solution with constant stirring to adjust the pH of the solution and also as the source of anions for crystallization of the compound. The dissociation of orthophosphoric acid system can be represented by three dissociation equilibrium and the presence of various ions at various pH values [25]. Based on these results, the gel pH in the range 5-7 has been used in which the HPO 4 2ions dominate or exist alone. The acidified gel was then poured into the test tube. This gel was allowed to set in the crystallizer and then aged for desired time. In our experiments, we used gels of ages 72, 96, 120, 144 and 198 h.

Synthesis of pure MHP
After the gel was set and aged, the supernatant (magnesium chloride) solution of desired molarity was poured along the sides of the tube, ensuring that this process does not break the gel. As soon as upper reactant comes in contact with the lower reactant, precipitation starts immediately suggesting spontaneous nucleation. All the experiments were conducted in the room temperature range 30-40C. The expected chemical reaction in case of magnesium hydrogen phosphate (where lower reactant is orthophosphoric acid and upper reactant is magnesium chloride) is:

Synthesis of CdMHP
For the growth of 10% cadmium mixed magnesium hydrogen phosphate crystals, lower reactant remains the same. However, for upper reactant a mixture of CdCl 2 and MgCl 2 in proper ratio was used. The diffusion of Cd 2+ and Mg 2+ ions through narrow pores of silica gel leads to the reaction between these ions and HPO 4 2ions of lower reactant already present in the gel suggesting spontaneous nucleation and the expected chemical reaction is:

CdMgHPO 4 + by-products
In order to obtain the optimum conditions conducive for the growth of MHP and CdMHP, number of experiments were carried out by varying the various growth parameters including upper reactant concentration, lower reactant concentration, gel concentration, gel pH and gel ageing. The optimum condition for the growth of good quality single crystal is given in Table 1.

Characterization
The incident light microscope "EPIGNOST" was used for the rapid examination of the grown crystals. Single crystal X'Calibur Oxford X-ray diffractometer was used to find out the crystal system, whereas the powder pattern was also obtained using powder X-ray diffractometer (Rigaku Co. Ltd. Japan) with Cu Kα radiation ( = 1.5406Å) with a scanning rate of 2 ο /min. Hardness studies have been carried out by using auto detection microhardness analyzer (HMV-2 of Shimadzu, Japan). The dielectric measurements were recorded with the help of automated impedance analyzer (4192A LF model) interfaced with USB -GPIB converter 82357 (Agilent) and further automated by using a computer for data recording, storage and analysis.

Structural characterization
Some of the good quality single crystals of MHP and CdMHP placed on a microslide is shown in Fig. 1 (a & b) respectively. For the micromorphological studies, the crystals were placed under the incident light microscope "EPIGNOST". The optical micrographs of MHP and CdMHP is shown in Fig. 2 (a & b) respectively. Fig. 2 (a) shows the formation of a given plane on the surface of pure MHP crystal, whereas Fig. 2 (b) represents the optical micrograph of CdMHP crystal showing clearly the hexagonal morphology along with a clear plane. The powder Xray diffraction pattern for MHP and CdMHP is shown in Fig. 3 (a & b) respectively. The occurrence of highly resolved intense peaks at specific 2θ Bragg angles indicates the crystallinity of the grown material.  The details of the Xray diffraction plot depicting dspacing and corresponding [hkl] planes for MHP and CdMHP crystal is given in Table 2. From the diffraction pattern, it can be clearly seen that the substitution of Cd in place of Mg ions in pure MHP crystals lead to shift in the positions of peaks indicating a change in the internal structure of crystals due to change in bond lengths as a result of doping. Thus, it has been observed that doping shifts peak value towards higher angle, indicating an increase in the value of lattice constants. Single crystal X -ray analysis establishes that MHP and CdMHP belong to orthorhombic crystal system. The lattice parameters obtained for both the systems is given in Table 3. From the powder as well as single crystal Xray diffraction results, it can be concluded that MHP and CdMHP have the orthorhombic crystal system with space group Pbca. Thus, it is clear that doping of about 10% Cd into MHP does not change the crystal system. Table 3. Lattice parameters for magnesium hydrogen phosphate (MHP) and cadmium magnesium hydrogen phosphate (CdMHP) crystals.

Mechanical characterization
The mechanical strength of the grown crystals was done by microhardness measurement technique. For carrying out the hardness studies on MHP and CdMHP, smooth and clean face of the crystal was selected and subjected to indentation test at room temperature. Loads ranging from 0.2452 N to 1.961 N were used for indentation keeping the indenter at right angles to the surface of the crystal for 10 sec in all cases. The distance between any two consecutive indentations was suitably adjusted in order to ensure that the effects were independent of each other. The Vicker's microhardness value (H v ) was calculated using the formula: where 'P' is the applied load in Newton and 'd' is the diagonal length of the indentation mark in micrometer. The error in H v was estimated through the relation: where Y = d 2 , Y = 2dd, P, Y and d being error in P, Y, and d respectively. The study on variation of microhardness with applied load for MHP crystals has already been carried out and reported [26]. It was found that microhardness value first increases and then becomes independent of load. However, there is no report regarding the microhardness behaviour with applied load for CdMHP crystals. A survey of literature suggests that different materials behave differently so far their dependence of microhardness on applied load is concerned. It is reported that microhardness is: i) independent of load Therefore, it is interesting to see how Cd doping in MHP system responds to the indentation. Fig. 4 shows the indentation impression at a load of 0.49 N. Table 4 gives the compiled data on the microhardness studies of CdMHP crystal. The behaviour of Vicker's hardness number with applied load is somewhat complex.  It shows increasing trend with increase in applied load, where at first, the increase for smaller loads is less as compared to the higher load. The value of Vicker's hardness ranges from 4319 -52680 MN/m 2 in the load ranging from 0.245 -9.807 N, respectively. The explanation for the variation of H v with load in complex fashion can be given on the basis of role played by the surface layer in deciding its hardness property. The diamond pyramid indenter used in the present investigation for calculating microhardness values penetrate considerably deeper than the thickness of the surface layer. However, as the depth of penetration increases with increasing load, the effect of the surface layer of the crystal becomes less marked, thereby, indicating the increase in the hardness value. For much larger loads, when the indenter reaches a depth at which undistorted layers of the material exists and the elastic properties of the inner material predominate over the surface properties, the microhardness is expected to cease its dependence on load which in the present case, for CdMHP has not been obtained. Berzina et al [31] reported for various alkali halide crystals, the anomalous behaviour of microhardness at low loads is greatest in crystals with large surface energy and thus related the microhardness particularly, in low load region on the surface energy of the solids. Thus, we can say that in the present case also, the surface energy plays an important role for the increase in the values of microhardness.

Electrical characterization
The electric characterization includes the response of dielectric constant to an applied electric field. The variation of dielectric constant (ε / ) can be attributed to different types of polarization which comes into play at different stages of its response to varying temperature and frequency of the applied alternating field. A study of variation of dielectric constant with temperature and frequency of the applied electric field is very useful in the study of phase transitions taking place in materials. For MHP and CdMHP, the dependence of dielectric constant on temperature was studied in the temperature range of 30 -400˚C and in the frequency range of 1 -1000 kHz of the applied a.c field.

Variation of dielectric constant with frequency at different temperature
The dependence of dielectric constant on frequency of the applied a.c field in case of MHP and CdMHP is shown in Fig. 5 (a & b) respectively. It can be seen from the figure that the dielectric constant decreases continuously with increasing frequency and attains almost saturation at higher frequencies. The decrease in the value of dielectric constant with increasing frequency is a normal dielectric behaviour and has been reported earlier [32][33][34][35]. At each particular temperature, the dielectric constant has a maximum value at lower frequency. This type of behaviour indicates higher space charge polarizibility in the low frequency region. As the frequency increases, the dipole do not comply with the varying external field resulting in the decrease in polarization which, in turn results in the decrease in the value of dielectric constant with increasing frequency [36].   Variation of dielectric constant with temperature at different frequencies Fig. 6 (a & b) shows the behaviour of dielectric constant (ε / ) with temperature in case of MHP and CdMHP respectively. From the graph, it is clear that MHP has a high dielectric constant as compared to CdMHP. In case of MHP, the dielectric constant (ε / ) under the application of frequency varies little upto the temperature of 200˚C. As the temperature increases, the value of dielectric constant increases and reaches a maximum value of 68 for a frequency of 10 kHz at a temperature of 330˚C. The value of ε / decreases after 330˚C and reaches upto a value of 52 at 390˚C. For other frequencies i.e., from 20 kHz -1 MHz, the peak value of dielectric constant is also observed at a temperature of 330˚C. Thus, the peak value of 330˚C is observed for all the frequencies considered, thereby, suggesting that the transition temperature in case of MHP is 330˚C. In case of CdMHP, the variation of dielectric constant with frequency of applied ac field is very little upto the temperature of 190˚C. As the temperature increases, the value of dielectric constant increases and reaches a maximum value of 23 for a frequency of 10 kHz at a temperature of 310˚C. Beyond 310˚C, the value of ε / decreases and reaches upto a value of 13 at 390˚C. For other frequencies i.e., from 20 kHz -1 MHz, the peak value of dielectric constant is also observed at a temperature of 310˚C. Thus, the peak value of 310˚C is observed for all the frequencies considered, thereby suggesting that the transition temperature in case of CdMHP is 310˚C.