Synthesis and Investigation of Na_{1 – x}R_0.33xTiO₂(PO₄)₃ (R = Y or La) Phosphates

Abstract

Phosphates Na_{1 – x}R_0.33xTi₂(PO₄)₃ (R = Y or La; 0 ≤ х ≤ 1) were synthesized by the Pechini process and characterized by X-ray diffraction, electron microscopy with microprobe analysis, and IR spectroscopy. The systems exhibit isodimorphism to form a series of solid solutions belonging to the NaZr₂(PO₄)₃ (NZP) structural type and crystallizing in space group R$\bar {3}$c or R$\bar {3}.$ The Rietveld structural studies confirmed the isomorphic miscibility of sodium and the rare-earth element in the interstices of the NZP structure. The unit cell parameter с in the phosphates studied tends to increase, and the parameter а tends to decrease, in response to rising temperature, which trends are typical of NZP phosphates.

INTRODUCTION

Interest in phosphates of the NaZr₂(PO₄)₃ (NZP/NASICON) structural type is due to their potential for use as high-tech ceramic materials with tunable thermal expansion, including low values; they can be used in items requiring high thermal shock resistance [1–4] and in solid materials with high ionic conductivity [5], which are in demand for the design of rechargeable batteries [6]. In addition, NZP phosphates containing rare-earth elements (REE) are used as converters of UV rays, X-rays, and gamma-rays to the visible light [7–9]; as biologically compatible additives for the detection and imaging of tumors [10]; and as luminescent materials [11–16] resistant to damaging environmental factors.

The crystal structures of NaZr₂(PO₄)₃ [17] and NaTi₂(PO₄)₃ [18] are three-dimensional frameworks built of ZrO₆ (TiO₆) octahedra and РО₄ tetrahedra, which form columns lying parallel to one another. The cavities inside the columns are populated by Na⁺ ions. The incorporation of cations(3+) into interstices of the NZP structure distorts the unit cell symmetry compared to the parent structure [19–23]. In R_0.33Zr₂(PO₄)₃ (R = Y, La, … Lu) samples, for examples, cationic positions located in the cavities inside the columns of polyhedra are split into three types, and R³⁺ cations orderly populate one of them. The cell symmetry in REE-containing phosphates lowers from space group R$\bar {3}$c of the parent structure to P$\bar {3}$ (for La [20]) or $P\bar {3}$c (for Ce–Lu and Y [19]). Limited solid solutions A_{1 – x}Eu_0.33xZr₂(PO₄)₃ (A = Na, K, or Rb), which have different unit cell symmetries, were observed for both end-members [24].

The studies of R_0.33Ti₂(PO₄)₃ (R = La, Pr, or Gd) phosphates [25, 26] showed that the interstitial positions inside polyhedral columns are split into two types, where one type is 2/3 populated by REE ions and the other remain vacant. The alternation of these two types of interstices is implemented within the framework of the NZP structure with space group R$\bar {3}.$

Obviously, the reason for the difference between the structures of zirconium and titanium phosphates containing rare-earth elements lies in the sizes of the framework-forming ZrO₆ or TiO₆ octahedra (the ionic radius is 0.72 Å for Zr⁴⁺ and 0.61 Å for Ti⁴⁺ [27]). In this regard, it was of interest to study the effect of the framework-forming cations on phase formation and the properties of triple phosphates of alkali and rare-earth elements. The purpose of this work was to study the phase formation, structure, and thermal expansion of Na_{1 – x}R_0.33xTi₂(PO₄)₃ phosphates. Since the REE ionic radii in octahedrally coordinated interstices vary in the range of 0.87–1.03 Å, yttrium (0.90 Å) and lanthanum (1.03 Å) were chosen as the R elements substituting for sodium.

EXPERIMENTAL

The Na_{1 – x}R_0.33xTi₂(PO₄)₃ (R = Y or La; 0 ≤ х ≤ 1) samples were synthesized by the Pechini method [28]. The chemicals used were at least of chemically pure grade (Table 1): Y₂O₃, LaCl₃·7H₂O, TiOCl₂, NH₄H₂PO₄, aqueous NH₃, HCl, and HNO₃, citric acid С₆Н₈О₇, and ethylene glycol С₂Н₆О₂. The titanium oxychloride TiOCl₂ solution used in the synthesis was prepared by oxidizing a 15% aqueous solution of chemically pure grade TiCl₃ by a mixture of concentrated HCl and HNO₃ under air, followed by the gravimetric determination of its concentration [29]. Before use in the synthesis, weighed Y₂O₃ samples were dissolved in nitric acid and LaCl₃·7H₂O and NH₄H₂PO₄ weights were dissolved in distilled water.

Table 1.   Characteristics of the chemicals used for the synthesis

During the synthesis, the solutions of metal salts were mixed in stoichiometric ratios, then citric acid was added to them (in the ratio acid : metal ions = 15 : 1 mol/mol) and dissolved while heated to 60°C. Afterwards, to the thus-prepared mixture of metal citrate complexes, slowly added were ethylene glycol (in the ratio 4 : 1 to the metal ions in the solution) and, simultaneously, ammonium dihydrophosphate to provide a homogeneous gel; the gel was then dried stagewise at 90 (for 48 h), 130 (for 48 h), and 350°С (for 24 h). The precipitate was dispersed, annealed at 500, 550, and 600°С for 72 h at each stage and then at 650 (for 48 h) and 700°С (for 48 h). Each annealing stage was preceded by trituration of the powder mixture with ethanol (as a surfactant) to achieve monodispersity and for mechanical activation to accelerate the synthesis of nanocrystalline materials [30]. The phase composition of the samples after annealing was monitored by X-ray powder diffraction (XRD).

X-ray diffraction experiments were carried out on a Shimadzu XRD-6000 diffractometer. The X-ray diffraction powder patterns were recorded using filtered CuK_α radiation (λ = 1.54178 Å) over the 2θ angle range from 10° to 60° at a scan rate of 1 deg/min with 0.02° scan steps. The hkl indices of the synthesized phosphates were determined and their unit cell parameters were calculated based on the structural similarity of the studied compounds to those known from the literature, namely, (NaTi₂(PO₄)₃ [18] and La_0.33Ti₂(PO₄)₃ [26]), and by analytical indexing of X-ray diffraction patterns.

The X-ray diffraction pattern of a Na_0.5Y_0.165Ti₂(PO₄)₃ sample for structural studies was recorded in the 2θ angle range from 13° to 110° in 0.02° steps with an exposure time per point of 12 s. The X‑ray diffraction patterns were processed and the phosphate structures refined by the Rietveld method [25] in the Rietan-97 software [26]. The peak profiles were fitted to the modified pseudo-Voigt function (Mod-TCH pV).

To assess the thermal expansion and distortion of the structure upon heating, Na_0.5R_0.165Ti₂(PO₄)₃ (R = Y or La; х = 0.5) phosphates were studied by XRD in the temperature range from 25 to 200°С using an Anton Paar TTK 450 attachment. Linear coefficients of thermal expansion were calculated as a relative increment of the unit cell parameter а or с in response to the one-degree rise in temperature (T): ${{\alpha }_{a}} = \frac{1}{a}\frac{{da}}{{dT}},$ ${{\alpha }_{c}} = \frac{1}{c}\frac{{dc}}{{dT}}.$ The average linear coefficients of thermal expansion were estimated as α_avg = (2α_a + α_c)/3.

The surface morphology of samples was studied using a Jeol JSM-7600F scanning electron microscope with a thermal-field electron gun (Schottky cathode) at magnifications in the range from ×25 to ×1 × 10⁶. The microscope was equipped with a microanalysis system, namely, an Oxford X-Max 80 (Premium) energy-dispersive X-ray spectrometer with a semiconductor silicon drift detector. The uncertainty of electron microprobe analysis in the determination of elemental compositions was 0.5–2.5 mol %.

The IR spectra of the compounds synthesized were recorded on a FTIR-8400 spectrophotometer equipped with an ATR attachment at room temperature in the wavenumber range 1400−400 cm^–1. The test samples were phosphate and KBr mixtures compacted to semitransparent disks.

RESULTS AND DISCUSSION

The X-ray diffraction patterns, IR spectra, and electron microprobe analysis imply that the Na_1 ‒ xR_0.33xTi₂(PO₄)₃ (R = Y or La) systems form NZP phosphates. Single-phase phosphates were obtained upon long-term anneals at temperatures from 550 (for х = 1) to 700°С (for х = 0). Higher annealing temperatures gave rise to the thermal degradation of the NZP phase (above all, in samples with near-unity х values).

Figure 1 shows the X-ray diffraction patterns of the synthesized NZP phosphates. The 0 ≤ х ≤ 0.7 (R = Y) and 0 ≤ х ≤ 0.4 (La) samples were indexed in terms of space group R$\bar {3}$c, the 0.8 ≤ х ≤ 1.0 (Y) and 0.5 ≤ х ≤ 1.0 (La) samples were indexed in terms of space group R$\bar {3}.$ The phosphates of space group R$\bar {3}$ featured additional peaks in the X-ray diffraction patterns compared to the phosphate that crystallized in the higher symmetry of space group R$\bar {3}$c.

Fig. 1.

X-ray diffraction patterns of phosphates: (a) Na_{1 – x}Y_0.33xTi₂(PO₄)₃, where x = (1) 0, (2) 0.2, (3) 0.5, (4) 0.7, (5) 0.8, (6) 0.9, and (7) 1; and (b) Na_{1 – x}La_0.33xTi₂(PO₄)₃, where x = (1) 0, (2) 0.2, (3) 0.3, (4) 0.4, (5) 0.5, (6) 0.7, and (7) 1. The X-ray diffraction bar diagrams refer to NaTi₂(PO₄)₃ (x = 0) and La_0.33Ti₂(PO₄)₃ (x = 1).

Electron microscopy and microprobe data verified the homogeneity of the as-synthesized samples and their correspondence to the respective as-batch compositions within the uncertainty of the method.

The IR spectra (Fig. 2) of the synthesized phosphates verify their belonging to orthophosphates with the NZP structure and the absence of X-ray amorphous impurities in the samples.

Fig. 2.

IR spectra of phosphates: (a) Na_{1 – x}Y_0.33xTi₂(PO₄)₃, where x = (1) 0, (2) 0.2, (3) 0.5, (4) 0.7, (5) 0.8, (6) 0.9, and (7) 1; and (b) Na_{1 – x}La_0.33xTi₂(PO₄)₃, where x = (1) 0, (2) 0.2, (3) 0.3, (4) 0.4, (5) 0.5, (6) 0.7, and (7) 1.

Factor-group analysis implies that the IR spectra of NZP phosphates with space group R$\bar {3}$c can feature five bands due to asymmetric stretching vibrations (at 1250–1020 cm^–1), one band due to symmetric stretching vibrations (at 1020–950 cm^–1), five bands due the asymmetric bending vibrations (at 650–500 cm^–1), and two symmetric bending vibrations (at <500 cm^–1). In the IR spectra of the phosphates with space group R$\bar {3}$, selection rules allow to appear six bands of asymmetric stretching vibrations, six bands of asymmetric bending vibrations, two bands of symmetric stretching vibrations, and four bands of symmetric bending vibrations. Thus, the number of the allowed and observed bands of the stretching and bending vibrations was also greater in the spectra of phosphates having a lower symmetry (Fig. 2).

In order to verify the miscibility of sodium and REE in the interstices of the NZP structure in the studied series, we carried out a Rietveld structural study of a Na_0.5Y_0.165Ti₂(PO₄)₃ (х = 0.5) sample using X-ray powder diffraction data. An analysis of the X‑ray diffraction patterns of this sample according to the absence laws showed that the phosphate crystallized in space group R$\bar {3}$c; therefore, the atomic coordinates in the NaTi₂(PO₄)₃ structure served as the initial model [18]. Experimental details and selected refinement results are found in Tables 2–4. Figure 3 shows a good agreement between the measured and calculated X-ray diffraction patterns of the sample.

Table 2. Details of the experiment, unit cell parameters, and results of crystal structure refinement for Na_0.5Y_0.165Ti₂(PO₄)₃

Table 3.   Coordinates and isotropic thermal parameters of atoms in the Na_0.5Y_0.165Ti₂(PO₄)₃ structure

Table 4. Selected interatomic distances in the structure-forming polyhedra of Na_0.5Y_0.165Ti₂(PO₄)₃

Fig. 3.

Fragments of (1) measured, (2) calculated, and (3) difference X-ray diffraction patterns of Na_0.5Y_0.165Ti₂(PO₄)₃; and (4) the bar diagram showing Bragg reflection positions.

Figure 4 shows a fragment of the Na_0.5Y_0.165Ti₂(PO₄)₃ crystal structure. The basis of its structure is the {[Ti₂(PO₄)₃]^–}_3∞ framework, in which the Ti atoms are coordinated by six oxygen atoms of РО₄ tetrahedra. The tetrahedra each share two corners with two octahedra to form a column, and the other two corners use to link to adjacent columns. The octahedrally coordinated positions inside polyhedral columns are shared by Na⁺ and Y³⁺ ions, which is evidence of their isomorphic miscibility.

Fig. 4.

Fragment of the Na_0.5Y_0.165Ti₂(PO₄)₃ crystal structure.

Thus, the results of phase formation studies indicate that isodimorphism is observed in the series of synthesized phosphates Na_{1 – x}R_0.33xTi₂(PO₄)₃ (R = Y or La), where the terminal solid solutions based on the pure components (x = 0 and 1) crystallize in space groups R$\bar {3}$c and R$\bar {3}.$

A comparison of phase formation data in the systems studied (Table 5) illustrates a general trend of isomorphism: an ion of smaller radius enters the crystal structure as an isomorphic impurity more easily than an ion of a larger radius. For the NaTi₂(PO₄)₃-base solid solutions (space group R$\bar {3}$c), the variations of chemical composition only insignificantly influence the unit cell parameters (Fig. 5). For the solid solutions where cations are ordered in structural interstices (space group R$\bar {3}$), an even minor change in х gives rise to a considerable increase in unit cell height с and to a decrease in the parameter а due to the attendant distortions [1].

Table 5.   Existence areas of NZP solid solutions in the Na_{1 – x}R_х/3Ti₂(PO₄)₃ systems

Fig. 5.

Unit cell parameters of phosphates versus chemical composition (х) for (a) Na_{1 – x}Y_0.33xTi₂(PO₄)₃ and (b) Na_1 ‒ xLa_0.33xTi₂(PO₄)₃.

The temperature-dependent unit cell parameters of Na_0.5R_0.165Ti₂(PO₄)₃ (R = Y or La; х = 0.5) phosphates are plotted in Fig. 6. The increase in interatomic distances in response to rising temperature gives rise to an increase in unit cell height с in both samples. The phosphate containing larger interstitial ions (La³⁺) also has its unit cell parameter а increasing as temperature rises. In the case of Na_0.5Y_0.165Ti₂(PO₄)₃ (with smaller interstitial ions), the heating-induced correlated rotation of framework polyhedra induces a decrease in parameter а, which is also typical of the NZP compounds [1]. Studies of the thermal expansion of Na_0.5Y_0.165Ti₂(PO₄)₃ and Na_0.5La_0.165Ti₂(PO₄)₃ showed that, because of the negative value of the linear expansion coefficient α_a measured normal to the major third-order axis of a crystal, the value of α_avg, the average linear coefficient of thermal expansion, allows the yttrium-containing phosphate to be classified as a material with medium expandability (Table 6). The lanthanum-containing phosphate has a higher α_аvg value.

Fig. 6.

Unit cell parameters versus temperature for (a) Na_0.5Y_0.165Ti₂(PO₄)₃ and (b) Na_0.5La_0.165Ti₂(PO₄)₃.

Table 6. Na_0.5R_0.165Ti₂(PO₄)₃ thermal expansion coefficients in the temperature range from 25 to 200°C

CONCLUSIONS

The phase formation of titanium-containing NZP phosphates where rare-earth elements (Y or La) reside in structural cavities has been studied. Isodimorphism has been shown to occur in the series of the synthesized phosphates Na_{1 – x}R_0.33xTi₂(PO₄)₃ (R = Y or La) to form terminal solid solutions whose structures differ from one another in the extent of cation ordering in interstices. Structural data has verified the results of our study. Thermal expansion studies of Na_0.5Y_0.165Ti₂(PO₄)₃ and Na_0.5La_0.165Ti₂(PO₄)₃ have shown that the distortions of their unit cells upon heating are largely determined by the sizes of cations in the interstices.