Solid state synthesis and structural refinement of polycrystalline phases: Ca_1−2xZr₄M_2xP_6−2xO₂₄ (M⚌Mo, x = 0.1 and 0.3)

2.1 Ceramic route synthesis of Ca_1−2xZr₄M_2xP_6−2xO₂₄ phases

In the present work the solid solution of Ca_1−2xZr₄M_2xP_6−2xO₂₄ with x = 0.1, 0.2, 0.3, 0.4 and 0.5 compositions was synthesized by the conventional solid state reaction method. The stoichiometric quantity of fine dry powders of precursor nitrates/carbonates and ammonium dihydrogen phosphate was mixed in a mortar-pestle in glycerol medium. The selected chemical compounds were AR grade CaCO₃, (NH₄)₆Mo₇O₂₄·4H₂O, ZrO(NO₃)₂ and (NH₄)H₂PO₄. The glycerol paste was gradually heated initially at 600 °C for 8 h in a platinum crucible. The initial heating is done to decompose CaCO₃, (NH₄)₆Mo₇O₂₄ and (NH₄)H₂PO₄ with the emission of carbon dioxide gases, ammonia and water vapors.

The powders were then compacted into small disks of 12.5 mm diameter and 2–3 mm of thickness under a load of four tons. Then pellets were sintered in a platinum crucible at 1250 °C for 72 h. The process was repeated to get a dense, polycrystalline material.

2.2 Characterization

The phase purity of the synthesized samples was checked by X-ray diffraction on a Pan Analytical diffractometer (XPERT-PRO) using Cu Kα radiation (λ = 1.54 Ǻ) at a step size of 2θ = 0.02° and a fixed counting time of 5 s/step and was refined by the Rietveld method using the General Structure Analysis System (GSAS) program Agrawal and Stubican, 1985. The GSAS program with the EXPGUI (Larson and Von Dreele, 2000) graphical user interface was used to carry out crystal structure determination which allows refinement of atomic coordinates, site occupancies and atomic displacement parameters as well as profile parameters (lattice constants, peak shape, peak height, instrument parameters and background).

The homogeneity and chemical compositions of the samples were checked by Scanning electron microscopy (SEM). Scanning electron microscopy (SEM) has been carried out on a HITACHI S-3400n electron microscope system attached with Thermonoran ultra dry detector facility for energy dispersive X-ray (EDAX) analysis. The images presented are recorded in backscattered electron (BSE) mode and electrons of energy 20 keV were used in all experiments. To confirm functional compositions of the phosphates, their IR spectra were recorded using a SHIMADZU FTIR-8400S instrument. Samples were prepared by finely dispersing powder material on a KBr carrier.

3.1 Rietveld refinement and crystallographic model of the phases

The powder XRD data showed that monophases of composition Ca_1−2xZr₄M_2xP_6−2xO₂₄ (M⚌Mo and x = 0.1 and 0.3) are isostructural to CaZr₄(PO₄)₆ Toby, 2001 but beyond x = 0.3, a secondary phase of molybdenum zirconium phosphate (marked asterisk) (International Tables for X-ray crystallography, 1984; International tables for X-ray crystallography, 1983) is evident along with CZP (Fig. 1). Mo modified CZP crystallizes in the rhombohedral system (space group R-3). The conditions for the rhombohedral lattice: (i)−h + k + l = 3n (ii) when h = 0, l = 2n and (iii) when k = 0, l = 2n have been verified for all reflections of 2θ = 10–90°. The intensity and positions of the diffraction pattern match with the characteristic pattern of parent compound calcium zirconium phosphate, which gives several prominent reflections of 2θ = 12–61° (Pet’kov et al., 2003). Ca atoms were assumed to occupy the M1 (6b) site. The occupancy of Ca(1) was allowed to vary but the total P and Mo contents were constrained and refined according to their theoretical molar ratios. The structure refinement leads to rather good agreement between the experimental and calculated XRD pattern (Fig. 2) and yields acceptable reliability factors: RF², Rp and Rwp (Kojitani et al., 2005; Chourasia and Shrivastava, 2011). The normal probably plot for the histogram gives nearly a linear relationship indicating that the I_o and I_c values for the most part are normally distributed (Fig. 3). The cell parameters of the specimens register a slight increase in the c direction. Simultaneously, the structure shows a slight contraction along a direction (Table 1). This is due to angular distortions as a result of the coupled rotation of ZrO₆ and PO₄ polyhedrons (Lenain et al., 1987). Alteration in lattice parameters shows that the network slightly modifies its dimensions to accommodate the cations occupying M1 and T sites without breaking the bonds. The basic framework of CZP accepts cations of different sizes and oxidation states to form solid solutions but at the same time retaining the overall geometry unchanged. The final atomic coordinates and isotropic thermal parameters (Table 2), inter-atomic distances (Table 3) and bond angle (Table 4) are extracted from the crystal information file prepared after the final cycle of the refinement. Selected h, k, l values, d-spacing, and intensity data along with observed and calculated structure factors have been listed in Table 7.The refinement leads to acceptable Zr–O, P–O bond distances. Zr atoms are displaced from the center of the octahedron due to the Ca²⁺–Zr⁴⁺ repulsions. Consequently the Zr–O(2) distance, neighboring the calcium Ca(1), is slightly greater than the Zr–O(1) distance, however, average Zr–O distances are smaller than the values calculated from the ionic radii data (2.12 Å) Shannon, 1976. The O–Zr–O angles vary between 79.334° and 178.120°. The angles implying the shortest bonds are superior to those involving the longest ones due to O–O repulsions which are stronger for O(1)–O(1) than for O(1)–O(2).

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Figure 1

Powder XRD pattern of Ca_1−2xZr₄M_2xP_6−2xO₂₄ (M⚌Mo and x = 0.1–0.5) ceramic samples. ^∗Marked peaks are due to MoZr₄P₅O₂₄.

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Figure 2

(a and b) Rietveld refinement plot for Ca_0.4Zr₄Mo_0.6P_5.4O₂₄ ceramic sample showing observed (+), calculated (continuous line) and difference (lower) curves. The vertical bars denote Bragg reflections of the crystalline phases.

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Figure 3

(a and b) Probability plot between observed intensity (I_o) and calculated intensity (I_c) for Ca_0.4Zr₄Mo_0.6P_5.4O₂₄ ceramic sample.

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The P–O distances are close to those found in Nascicon type phosphates (Anantharamulu et al., 2011; Navulla, 2010). The O–P–O angles vary between 107.522° and 119.055°. Fig. 4 illustrates the Diamond view showing the ZrO₆ inter ribbon distance in the structure of the title phase which is a function of amount and size of alkali cation in the M site of the 3D framework, built from ZrO₆ octahedrons and corner sharing PO₄ tetrahedrons.

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Figure 4

DIAMOND view of crystal structure of Ca_0.8Zr₄Mo_0.2P_5.8O₂₄ ceramic phase.

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3.2 SEM and EDAX analysis

The microstructure of the Ca_1−2xZr₄M_2xP_6−2xO₂₄ (x = 0.1 and 0.3) ceramic phases has been examined by SEM and EDAX analysis of the specimen. The morphology of MoCZP phases can be seen clearly in the electron micrographs of the ceramic sample (Fig. 5). Within the limits of experimental error, the EDAX analytical data on atomic and wt.% of Ca, Zr, P and Mo are found agreeable with their corresponding expected molar ratios. Simultaneously, the particle size was also determined using Scherrer’s equation where broadening of peak is expressed as full width at half maxima in the recorded XRD pattern. The particle size varies between 35.14 and 152.28 nm (Table 5) Chourasia et al., 2010.

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Figure 5

(a and b) Scanning electron micrographs and EDAX spectrum of Ca_0.8Zr₄Mo_0.2P_5.8O₂₄ ceramic phases. (c and d) Scanning electron micrographs and EDAX spectrum of Ca_0.4Zr₄Mo_0.6P_5.4O₂₄ ceramic phases.

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3.3 IR analysis

The presence of orthophosphate anions in the crystal structure was confirmed with the IR spectroscopy. Table 6 lists the IR assignments for Ca_1−2xZr₄M_2xP_6−2xO₂₄ (x = 0.1 and 0.3) ceramic phases. The IR spectra of CZP compounds of the formula Ca_1−2xZr₄M_2xP_6−2xO₂₄ (x = 0.1 and 0.3) are very similar and shown in Fig. 6. In both IR spectra, the absorption bands in the range between 12501022 cm⁻¹ and 650–507 cm⁻¹ are assigned to stretching and bending vibrations of P–O bonds of the PO₄ tetrahedron, respectively. The stretching vibrations occur between 1270 and 1020 cm⁻¹ as υ₃ band, the symmetric stretching υ₁ and anti symmetric bending υ₄ vibrations are observed in the regions 990–900 cm⁻¹ and 640–505 cm⁻¹, respectively (Barj et al., 1983; Mbandza et al., 1985; Thomas and Andrews, 1974; Buvaneswari and Varadaraju, 1999).

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Figure 6

Infrared spectra of Ca_1−2xZr₄Mo_2xP_6−2xO₂₄ (a) x = 0.1 and (b) x = 0.3.

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