Abstract
The phosphates M0.5+x M′x Zr2−x (PO4)3 (M–Ca, Mn, Co, Sr, Cd, Ba, Pb; M′–Mg, Mn, Co) were synthesized by sol-gel method with the following thermal treatment of reaction mixtures. X-ray diffraction, IR spectroscopy and electron microprobe analysis showed that the obtained phosphates crystallized in Sc2(WO4)3 (SW) and NaZr2(PO4)3 (NZP) structural types. Both types of crystal structures are based on a framework comprised of octahedra and tetrahedra, the difference between them is fragments orientation. Thermal expansion of the phosphates was studied in the temperature range 20–800°C. Some compounds were found to belong to low-expanding materials (αav ~2·10−6°C−1).
Introduction
Phosphates Mn L2(PO4)3 (0≤n≤4), with {[L2(PO4)3]p−}3∞ frameworks (L is an octahedrally coordinated cation, p is a framework charge) have attracted attention of researchers due to their unique set of properties. Mostly, framework phosphates are characterized as highly resistant to the extreme environmental conditions [1], such as high temperatures, wet or aggressive media and various emissions. The majority of discussed options of practical applications of these substances are connected with their controlled, often low thermal expansion [1, 2]. Low-expanding ceramic materials made of phosphates are widely proposed as solid ionic conductors (NASICON) that may well be exploited in a wide range of temperatures [3], thermal shock resistant ceramics, host matrices for nuclear waste immobilization [4], catalysts or supports [5].
These materials are characterized by flexible crystal structure formed by PO4 tetrahedra sharing corners with LO6 octahedra. Many variations in linking between PO4 and LO6 fragments lead to formation of varied interstitial sites that are intrinsic features of certain structural types [6], such as well-known types of NaZr2(PO4)3 (NZP/NASICON, mineral analog is kosnarite KZr2(PO4)3, sp. gr. R3̅c) and Sc2(WO4)3 (SW, sp. gr. P21/n) [7, 8] (Fig. 1). Two types of framework cavity sites are inherent for NZP structure, being M1, surrounded by six nearest O atoms, and M2, surrounded by eight nearest O atoms (Fig. 1a). When it comes to SW structure, there are several (depending on the distortion of structure) cavity sites, where cations are surrounded by four nearest O atoms, in the framework cavities.
Linking of L2(PO4)3 fragments in different structural types: (a) NZP (M1 and M2–cavitiy sites); (b) scandium tungstate.
It is known [9], that M0.5Zr2(PO4)3 compounds containing large M2+ cations (Cd2+, Ca2+, Sr2+, Pb2+, Ba2+) in the structural cavities are characterized as NZP type structure (trigonal symmetry). Substitution of large M2+ ions for small 3d-transition metal cations (Ni2+, Cu2+, Zn2+, Co2+, Mn2+) or Mg2+ leads to lowering of symmetry and turning structural type into SW (monoclinic symmetry). The data related to phosphates containing metals in oxidation state +2 in the framework sites, such as [M1/3Nb5/3(PO4)3] (M–Ni, Mg, Cu, Zn, Co, Mn) [10], Na1+2x [Mgx Zr2−x (PO4)3] (0≤x≤1) [11], Na3[MZr(PO4)3] (M–Ni, Mg, Zn, Mn) [12, 13] (frameworks are shown within [brackets]) is scarce. Isomorphic substitution of Zr4+ for metal in oxidation state +2 in the octahedrally coordinated framework sites is possible on condition that M2+ ionic radii is comparable with Zr4+.
In keeping with the crystal-chemical data (Table 1) [14], we made the prognosis that there is a high probability of the wide range of solid solutions to be acquired in the M0.5+x [M′x Zr2−x (PO4)3] systems with framework cations M′–Ni2+, Mg2+, Cu2+, Zn2+, Co2+, Mn2+. We have already shown that MgNi0.5Zr1.5(PO4)3 has SW structure and MNi0.5Zr1.5(PO4)3 (M–Ca, Sr), Ca0.5+x [Znx E2−x (PO4)3] (E–Ti, Zr; 0≤x≤0.5) crystallize in NZP structure [15, 16]. Within systematic investigation of these zirconium containing systems it was of interest to study phosphates containing cations with Δr/rmin varying (compared with Zr4+) from 0 (Mg2+) to 15% (Mn2+) in the framework sites. To compare the solid solution regions, we chose the systems with small, medium and large cations in the structural cavities.
Geometrical parameter of ability of Zr4+ for M2+ isomorphic substitution in octahedrally coordinated sites (r(Zr4+)=0.72 Å).
| M2+ | Be | Ni | Mg | Cu | Zn | Co | Mn | Cd | Ca | Sr | Pb | Ba |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| r (Å) | 0.45 | 0.69 | 0.72 | 0.73 | 0.74 | 0.745 | 0.83 | 0.95 | 1.00 | 1.18 | 1.19 | 1.35 |
| Δr/rmin (%) | 60 | 4 | 0 | 1 | 3 | 3 | 15 | 32 | 39 | 64 | 65 | 88 |
The aim of this work was to investigate experimentally the possibility of incorporation of metals in oxidation state +2 into the framework and cavities’ sites of M0.5+x M′x Zr2−x (PO4)3 (M–Ca, Mn, Co, Sr, Cd, Ba, Pb; M′–Mg, Mn, Co; 0≤x≤2.0) phosphates. By virtue of possible practical applications of the framework substances, thermal expansion of the phosphates’ representatives was examined in the temperature range from 20 to 800°C.
Experimental
M0.5+x M′x Zr2−x (PO4)3 (M–Ca, Mn, Co, Sr, Cd, Ba, Pb; M′–Mg, Mn, Co; 0≤x≤2.0; Table 2) phosphates were synthesized by sol-gel method. The initial reactants were the following: aqueous solutions of metal chlorides or acetates, ZrOCl2·8H2O, H3PO4 and NH4H2PO4 (all reagents were chemically pure). The solutions, taken in keeping with the stoichiometry of the phosphate, were poured together under continuous stirring. Afterwards, desiccation of reaction mixture at 90°C and heating in unconfirmed air access at 600–900°C with a step interval of 50–100°C for 24–48 h at each stage ensued.
Specific values of x, that were achieved during synthesis of M0.5+x M′x Zr2−x (PO4)3 compounds.
| System | x |
|---|---|
| Co0.5+x [Mgx Zr2−x (PO4)3] | 0, 0.25, 0.5, 0.75, 1.0, 1.1, 1.2, 1.5, 2.0 |
| Co0.5+x [Cox Zr2−x (PO4)3] | 0, 0.1, 0.25, 0.5, 1.0, 1.5, 2.0 |
| Co0.5Mnx [Cox Zr2−x (PO4)3] | 0, 0.1, 0.25, 0.5, 0.6, 0.75, 1.0, 1.5, 2.0 |
| Mn0.5+x [Mgx Zr2−x (PO4)3] | 0, 0.25, 0.5, 0.75, 1.0, 1.1, 1.2, 1.5, 2.0 |
| Mn0.5+x [Cox Zr2−x (PO4)3] | 0, 0.1, 0.25, 0.5, 1.0, 1.5, 2.0 |
| Mn0.5+x [Mnx Zr2−x (PO4)3] | 0, 0.1, 0.25, 0.5, 0.6, 0.75, 1.0, 1.5, 2.0 |
| Cd0.5Mgx [Cdx Zr2−x (PO4)3] | 0, 0.5, 0.6, 0.7, 0.8, 1.0, 1.5, 2.0 |
| Cd0.5+x [M′x Zr2−x (PO4)3] M′−Co, Mn | 0, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 1.5, 2.0 |
| Ca0.5+x [M′x Zr2−x (PO4)3] M′–Mg, Co, Mn | 0, 0.25, 0.5, 0.75, 1.0, 1.1, 1.2, 1.5, 2.0 |
| Sr0.5+x [M′x Zr2−x (PO4)3] M′–Mg, Co | 0, 0.3, 0.4, 0.5, 0.6, 0.7, 1.0, 1.5, 2.0 |
| Sr0.5+x [Mnx Zr2−x (PO4)3] | 0, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 1.5, 2.0 |
| Pb0.5+x [Mgx Zr2−x (PO4)3] | 0, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.5, 2.0 |
| Pb0.5+x [M′x Zr2−x (PO4)3] M′–Co, Mn | 0, 0.5, 0.6, 0.7, 0.8, 1.0, 1.5, 2.0 |
| Ba0.5+x [M′x Zr2−x (PO4)3] M′–Mg, Co, Mn | 0, 0.1, 0.25, 0.5, 0.75, 1.0, 1.5, 2.0 |
The obtained samples were white or colored polycrystalline powders. Their chemical composition and homogeneity were checked with the aid of a CamScan MV-2300 microprobe with a Link INCA ENERGY 200C energy-dispersion detector. The uncertainty of the chemical composition determination was within 2–2.5% mol.
Phase purity was confirmed with powder X-ray diffraction (XRD) at Shimadzu XRD-6000 diffractometer (CuKα radiation). Unit cell parameters of the compounds were determined at a room temperature from the corresponding diffraction patterns indexed within 2θ range 10–60 deg. with step being 0.02 deg. and the rate of 1 deg./min. Data for the structure refinement were collected within 2θ range 10–90(110) deg. with step being 0.02 deg. and preset time in every point 12 s. Structure refinement was performed by Rietveld method [17] via Rietan-97 program [18]. X-ray patterns for thermal expansion study were obtained in the 2θ interval 10–60 deg. with step being 0.02 deg. and the rate of 1 deg./min, at the temperature range from 20 to 800°C.
Functional composition of the samples was confirmed by IR spectroscopy on Shimadzu FTIR 8400S spectrometer within range 1400–400 cm−1.
Thermal behavior of the synthesized samples was investigated by differential thermal analysis (DTA) on Labsys TG-DTA/DSC thermoanalyzer in a continuous heating regime. Experiment was undertaken in the interval 25–950°C, with the heating and cooling rates being 10°C/min.
Results and discussion
Synthesis
Phase formation of the complex phosphates was studied with the use of X-ray powder diffraction of the reaction mixtures from the consequent temperature stages. For example, the results of the phase analysis of the Mn0.5+x [Mnx Zr2−x (PO4)3] system are shown in Fig. 2a. Solid solutions were formed in the composition array 0≤x≤0.25. Phase mixtures of the boundary solid solution Mn0.75[Mn0.25Zr1.75(PO4)3], ZrP2O7, ZrO2, Mn2P2O7 were observed in the samples with x>0.25.
Results of X-ray study of phase formation in the Mn0.5+x Mnx Zr2−x (PO4)3 system: (a) Intensity of the main diffraction peaks for the phases in the XRD pattern vs. sample composition (temperature of thermal treatment is 650°C); (b) Intensity of the main diffraction peaks for the phases in the XRD pattern vs. temperature of the sample treatment (time of every isothermal stage was 24 h).
Influence of temperature on the formation of solid solution is shown in Fig. 2b. The target product Mn0.75[Mn0.25Zr1.75(PO4)3] was formed in the sample treated at the temperatures from 600 to 750°C. Its crystallinity built up with the increase of temperature, but at T>750°C decomposition of the phosphate started.
Characterization of the samples
XRD results showed, that the phosphates with the common formula M0.5+x M′x Zr2−x (PO4)3 (M–Mn, Co; M′–Mg, Mn, Co) crystallize in the SW structural type.
Incorporation of Mg2+ (close to Zr4+ by ionic radius) into the framework sites leaded to comparatively wide solid solutions regions in the system Mn0.5+x Mgx Zr2−x (PO4)3 (0≤x≤1.0). Single phase compounds were synthesized at 650–700°C and remained intact up to 850°C. Samples with x>1.0 consisted of phosphate with x=1, Mg3(PO4)2 and Mn3(PO4)2 (Fig. 3a).
Characterization of the Mn0.5+x Mgx Zr2−x (PO4)3 samples: (a) X-ray patterns; (b) IR spectra; (c) lattice parameters.
The powder XRD data of the single-phase samples showed a similar distribution of Bragg reflections in the patterns of Mn0.5+x Mgx Zr2−x (PO4)3 (0≤x≤1.0) with smooth changing in their relative intensities and 2θ values across the studied compositions’ range. The phosphates were indexed with space group P21/n.
The results of electron microprobe analysis confirmed the homogeneity of the samples and indicated that their compositions were close to the theoretical ones.
The IR spectra of the synthesized single-phase samples (Fig. 3b) were typical for the orthophosphates that crystallize in SW structure. Factor-group analysis predicted the maximum amount of characteristic PO4 vibrations, being six asymmetric stretching ν1, 12 symmetric stretching ν2, 18 asymmetric ν3 and 18 symmetric bending ν4. The spectra showed less experimental number of absorption bands: the broad absorption bands at 1260–1020 cm−1, ν3; 1000–900 cm−1, ν1; 650–550 cm−1, ν4. Shift and intensity redistribution of absorption bands were observed in the spectral patterns with changing of phosphates’ composition.
XRD results allowed us to calculate unit cell parameters of the obtained phosphates. Incorporation of Mn2+ cations in the structural cavities leaded to escalation of the cell distortion (raise of monoclinic angle β difference from 90°) with corresponding monotonous increase of a parameter, decrease of b and c parameters (Fig. 3c).
Composition (x) regions of SW structure for other systems containing manganese were more shrunk, than in case of Mn0.5+x Mgx Zr2−x (PO4)3: 0≤x≤0.25 for Mn0.5+x Mnx Zr2−x (PO4)3, x=0 for Mn0.5+x Cox Zr2−x (PO4)3. Single phase compounds were synthesized at 650°C and remained stable up to 900–1000°C. Other samples of these rows were found to be mixtures of SW structure phases with Mn3(PO4)2 (or Co3(PO4)2).
The results of the study of M0.5+x M′x Zr2−x (PO4)3 (M–Ca, Sr, Cd, Pb; M′–Mg, Mn, Co) phosphates showed formation of limited solid solutions of NZP type.
In particular, the NZP solid solutions formed in the region 0≤x≤1.0 for the Ca0.5+x Cox Zr2−x (PO4)3 system (Fig. 4). X-ray patterns of the phosphates were indexed in the space group R3̅. Synthesis temperature was 700–800°C. The compounds were stable up to 850–1100°C.
Characterization of the Ca0.5+x Cox Zr2−x (PO4)3 samples: (a) X-ray patterns; (b) IR spectra; (c) lattice parameters.
IR spectra confirmed that investigated compounds belong to the NZP orthophosphates class (Fig. 4b). It may be deduced from the factor group analysis that maximum amount of characteristic PO4 vibrations is 6 ν3, 6 ν4, 4 ν2 and 2 ν1. The obtained spectra were presented by 4–6 ν3 absorption bands and characteristic (typical for NZP phosphates, R3̅ space group) triplet of ν4 bands. Bands of ν1 and ν2 vibrations were observed at 645–540 cm−1 and ≤500 cm−1, respectively.
Unit cell a and c parameters of the studied solid solutions (Fig. 4c) depend on several factors. The a parameter is the function of width of (Co or Zr)O6 and PO4 polyhedra columns, while the c parameter is related to structural fragments height. The difference in Co2+ and Zr4+ ionic radii is not big, so this factor had weak influence on the a and c parameters. Incorporation of Ca2+ ions in the structural cavities (with x growth) leaded to the increase of polyhedra columns height. Corresponding polyhedra deformation caused a parameter to decrease, in line with x growth (as described in [1]).
It is known [19], that polymorphic phase transitions were observed for CaM′0.5Zr1.5(PO4)3 (M′–Mg, Mn). These compounds crystallized in the NZP structure during synthesis at 700°C, whereas they underwent irreversible transition into orthorhombic symmetry phase (space group Pnma) at higher temperatures, in which Mn2+ and Ca2+ form polyhedra with coordination number seven.
To investigate CaCo0.5Zr1.5(PO4)3 thermal behavior, DTA study of the sample (obtained at 700°C) was carried out (Fig. 5a). The exothermic effect 859–876°C was observed on the DTA curve of the phosphate. X-ray pattern of the CaCo0.5Zr1.5(PO4)3 sample, treated at 900°C for 24 h (Fig. 5b), showed that the sample comprised of NZP phosphate and orthorhombic phase, similar to described in [19].
CaCo0.5Zr1.5(PO4)3 phase transition study: (a) DSC curve; (b) X-ray pattern of the sample treated at 900°C for 24 h.
Overall, XRD and IR spectroscopy data were in a good agreement. There was resemblance among the obtained data on the structural types, unit cell parameters and IR spectra of phosphates with x=0 and the literature ones [9]. The study of triple phosphates revealed the existence of solid solutions in the M0.5+x M′x Zr2−x (PO4)3 (M–Ca, Mn, Co, Sr, Cd, Pb; M′–Mg, Mn, Co) systems. Phosphates with M–Ba were exception to the pattern: NZP phase in these systems was obtained only for x=0, compounds with x=0.5 crystallized in the yavapaiite (KFe(SO4)2 [20]) structure, and there were no solid solutions observed within any significant regions.
Structural investigation
With the aim of making distribution of metals M, M′ in the crystallographic sites (framework and cavities) clear, we refined the crystal structures of triple phosphates with x=0.5 of the compositions MnMg0.5Zr1.5(PO4)3 (SW), CoMn0.5Zr1.5(PO4)3 (SW) и CaCo0.5Zr1.5(PO4)3 (NZP) by the Rietveld method using X-ray powder diffraction data.
For the initial fractional coordinates, that are indispensable for Rietveld analysis, we used those of Ni0.5Zr2(PO4)3 (SW, space group P21/n) [21] and Cd0.5Zr2(PO4)3 (NZP, space group R3̅) [22]. The experimental and refinement conditions, lattice parameters and R factors are presented in Table 3.
Conditions of X-ray diffraction experiment and refinement results for MM′0.5Zr1.5(PO4)3 phosphates.
| Compound | Mn[Mg0.5Zr1.5(PO4)3] | Co0.5Mn0.5[Co0.5Zr1.5(PO4)3] | Ca[Co0.5Zr1.5(PO4)3] |
|---|---|---|---|
| Space group | P21/n | P21/n | R3̅ |
| Z | 4 | 4 | 6 |
| 2θ range (deg.) | 10–90 | 10–90 | 10–110 |
| Unit cell parameters | |||
| a (Å) | 8.847(3) | 8.8317(14) | 8.7677(3) |
| b (Å) | 8.970(3) | 8.9664(11) | – |
| c (Å) | 12.453(4) | 12.441(2) | 22.715(2) |
| β (°) | 90.47(2) | 90.20(2) | – |
| V (Å 3) | 988.2(5) | 985.2(2) | 1512.23(4) |
| Number of reflections | 795 | 792 | 429 |
| Variables | |||
| Structural | 76 | 76 | 26 |
| Other | 22 | 22 | 20 |
| Reliabililty factors (%) | |||
| R wp ; Rp | 7.33; 5.18 | 2.17; 1.52 | 3.13; 2.19 |
There are two types of octahedrally coordinated framework sites (2×4e) in the SW structure. It was established, that one type of sites is filled by Zr4+ cations, while another one is occupied by statistically distributed Mg2+ (or Co2+) and Zr4+ cations of comparable sizes. Cavity sites (4e) are occupied by cations of larger average sizes than those of the framework ones: Mn2+ in MnMg0.5Zr1.5(PO4)3, Mn2+ together with Co2+ in CoMn0.5Zr1.5(PO4)3 structure.
There are also two types of framework-forming cationic sites (2×6c) in the CaCo0.5Zr1.5(PO4)3 (NZP) structure. These positions are occupied by Zr4+ and Co2+/Zr4+ ions, correspondingly. The peculiarity of space group R3̅ is that M1 cavities sites are divided into two types (3a and 3b, surrounded by two types of the framework octahedra), and that Ca2+ cations fill both types of these sites within the octahedral-tetrahedral columns. Varying occupancy of M2 sites showed that the sites remain vacant.
The observed, calculated and difference XRD patterns are shown in Fig. 6a. Obviously, the observed patterns were in a good agreement with the calculated ones. The final fractional coordinates and isotropic atomic displacement parameters are listed in Tables 4–6.
Results of Rietveld structure refinement: (a) Portions of the observed (1), calculated (2), difference (3) Rietveld refinement profiles and Bragg reflections (4) for XRD patterns of the studied phosphates; (b) Fragments of their crystal structures.
Fractional coordinates and isotropic atomic displacement parameters for Mn[Mg0.5Zr1.5(PO4)3].
| Atom | Site | g | x | y | z | B (Å2) |
|---|---|---|---|---|---|---|
| Mn(1) | 4e | 1 | 0.1694(4) | 0.2023(5) | 0.7147(5) | 2.8(4) |
| Zr(1) | 4e | 1 | 0.1160(3) | 0.0227(5) | 0.2518(4) | 2.4(5) |
| Zr(2)/Mg(2) | 4e | 1 | 0.3878(5) | 0.0407(3) | 0.7464(4) | 2.2(5) |
| P(1) | 4e | 1 | 0.5054(5) | 0.2548(3) | 0.4630(4) | 0.78(9) |
| P(2) | 4e | 1 | 0.1479(4) | 0.3764(2) | 0.3990(5) | 1.5(5) |
| P(3) | 4e | 1 | 0.3569(4) | 0.3972(4) | 0.8886(3) | 2.0(6) |
| O(1) | 4e | 1 | 0.585(2) | 0.335(1) | 0.562(1) | 1.7(4) |
| O(2) | 4e | 1 | 0.572(1) | 0.171(2) | 0.351(2) | 2.6(5) |
| O(3) | 4e | 1 | 0.424(3) | 0.353(1) | 0.383(2) | 1.4(7) |
| O(4) | 4e | 1 | 0.431(2) | 0.143(1) | 0.556(1) | 2.0(5) |
| O(5) | 4e | 1 | 0.175(2) | 0.219(1) | 0.373(2) | 1.6(2) |
| O(6) | 4e | 1 | 0.036(1) | 0.427(1) | 0.338(2) | 0.65(7) |
| O(7) | 4e | 1 | 0.164(2) | 0.408(1) | 0.565(1) | 0.48(5) |
| O(8) | 4e | 1 | 0.225(1) | 0.484(2) | 0.315(2) | 3.0(7) |
| O(9) | 4e | 1 | 0.332(2) | 0.240(1) | 0.829(1) | 2.9(7) |
| O(10) | 4e | 1 | 0.364(1) | 0.396(2) | 0.051(2) | 1.6(7) |
| O(11) | 4e | 1 | 0.464(2) | 0.445(1) | 0.817(3) | 2.9(8) |
| O(12) | 4e | 1 | 0.271(1) | 0.513(1) | 0.833(2) | 2.9(6) |
Fractional coordinates and isotropic atomic displacement parameters for Co0.5Mn0.5[Co0.5Zr1.5(PO4)3].
| Atom | Site | g | x | y | z | B (Å2) |
|---|---|---|---|---|---|---|
| Mn(1)/Co(1) | 4e | 1 | 0.7096(6) | 0.2206(7) | 0.1701(3) | 2.7(8) |
| Zr(1) | 4e | 1 | 0.7459(19) | 0.0275(9) | 0.3868(7) | 1.9(7) |
| Zr(2)/Co(2) | 4e | 1 | 0.253(2) | 0.0317(11) | 0.1222(9) | 1.2(5) |
| P(1) | 4e | 1 | 0.4601(3) | 0.2555(8) | 0.5029(3) | 2.9(2) |
| P(2) | 4e | 1 | 0.3971(5) | 0.3795(6) | 0.1431(7) | 2.5(5) |
| P(3) | 4e | 1 | 0.8872(4) | 0.3964(7) | 0.3559(8) | 2.0(6) |
| O(1) | 4e | 1 | 0.5709(8) | 0.3312(8) | 0.5764(7) | 2.9(9) |
| O(2) | 4e | 1 | 0.3538(3) | 0.1723(3) | 0.5687(5) | 2.4(4) |
| O(3) | 4e | 1 | 0.3822(8) | 0.3570(7) | 0.4258(6) | 1.9(8) |
| O(4) | 4e | 1 | 0.5592(4) | 0.1480(4) | 0.4344(3) | 2.2(2) |
| O(5) | 4e | 1 | 0.3758(2) | 0.2094(8) | 0.1684(2) | 1.0(4) |
| O(6) | 4e | 1 | 0.3463(2) | 0.4246(6) | 0.0320(5) | 1.6(7) |
| O(7) | 4e | 1 | 0.5598(7) | 0.4034(5) | 0.1628(5) | 2.3(2) |
| O(8) | 4e | 1 | 0.3157(3) | 0.4829(5) | 0.2273(6) | 1.8(4) |
| O(9) | 4e | 1 | 0.8282(4) | 0.2418(2) | 0.3359(6) | 1.4(5) |
| O(10) | 4e | 1 | 0.0516(5) | 0.3909(2) | 0.3685(4) | 1.8(4) |
| O(11) | 4e | 1 | 0.8105(7) | 0.4482(5) | 0.4610(4) | 1.9(7) |
| O(12) | 4e | 1 | 0.8336(2) | 0.5162(4) | 0.2648(2) | 1.2(4) |
Fractional coordinates and isotropic atomic displacement parameters for Ca[Co0.5Zr1.5(PO4)3].
| Atom | Site | g | x | y | z | B (Å2) |
|---|---|---|---|---|---|---|
| Ca(1) | 3a | 1 | 0 | 0 | 0 | 2.1(4) |
| Ca(2) | 3b | 1 | 0 | 0 | 0.5 | 3.4(2) |
| Zr(1)/Co(1) | 6c | 1 | 0 | 0 | 0.1496(5) | 0.49(7) |
| Zr(2) | 6c | 1 | 0 | 0 | 0.6412(4) | 0.96(3) |
| P(1) | 18f | 1 | 0.2853(2) | 0.0012(4) | 0.2503(3) | 0.31(2) |
| O(1) | 18f | 1 | 0.1852(2) | −0.0107(7) | 0.1891(9) | 0.53(4) |
| O(2) | 18f | 1 | 0.0647(3) | 0.8456(3) | 0.6941(4) | 0.97(5) |
| O(3) | 18f | 1 | 0.1838(2) | 0.1870(6) | 0.0890(2) | 0.91(6) |
| O(4) | 18f | 1 | 0.2103(3) | 0.0655(5) | 0.5970(5) | 0.97(9) |
In the chosen models all B parameters were positive and reasonable. The bond lengths in the cavities’ and framework-forming polyhedra in the studied structures (Table 7) were in agreement with the corresponding data for other NZP and SW phosphates [14, 21, 22, 23]. The corresponding average distances were close to each other for the one type of phosphates (SW), in spite of large scale dispersions of interatomic distances within the polyhedra. Larger NZP cavities sizes compared with the SW ones may be clearly observed.
Bond lengths in structure-forming polyhedra of the studied phosphates.
| Polyhedron | Interval of bond lengths (dmin–dmax), Å | Average bond length <d>, Å |
|---|---|---|
| Mn[Mg0.5Zr1.5(PO4)3] | ||
| MnO4 | 1.902–2.281 | 2.13 |
| ZrO6 | 1.994–2.186 | 2.09 |
| (Zr/Mg)O6 | 1.996–2.153 | 2.08 |
| PO4 | 1.499–1.598 | 1.53 |
| 1.470–1.558 | 1.52 | |
| 1.439–1.567 | 1.52 | |
| Co0.5Mn0.5[Co0.5Zr1.5(PO4)3] | ||
| (Mn/Co)O4 | 2.036–2.318 | 2.13 |
| ZrO6 | 2.013–2.151 | 2.08 |
| (Zr/Co)O6 | 2.010–2.141 | 2.07 |
| PO4 | 1.454–1.558 | 1.50 |
| 1.473–1.575 | 1.53 | |
| 1.461–1.631 | 1.53 | |
| Ca[Co0.5Zr1.5(PO4)3] | ||
| CaO6 | 2.594 | 2.59 |
| 2.743 | 2.74 | |
| ZrO6 | 1.918–2.090 | 2.00 |
| (Zr/Co)O6 | 1.898–2.130 | 2.01 |
| PO4 | 1.560–1.621 | 1.60 |
The fragments of the studied phosphates structures are shown in Fig. 6b. The framework fragments of three corner-sharing PO4-tetrahedra and two types of octahedra were in the basis of all presented structures. These fragments formed columns parallel to one another in NZP compound CaCo0.5Zr1.5(PO4)3 and columns oriented in two alternating directions in SW phosphates MnMg0.5Zr1.5(PO4)3 and CoMn0.5Zr1.5(PO4)3.
Different packing of the same fragments leaded to the variation in size, form, location and amount of framework cavities. In our case, Mn2+ (r=0.66 Å) and Co2+ (r=0.58 Å) occupied relatively small tetrahedrally coordinated cavities of SW structures, while Ca2+ (r=1.00 Å) ions occupied comparatively large cavities of NZP structure. Thus, the structural framework adapted to the size of located M2+ cations in its cavities.
Unusual cations distribution was found by ourselves [24] for the CdMg0.5Zr1.5(PO4)3 structure earlier. The results of Rietveld refinement of cations’ occupations within structural sites indicated that one type of the framework sites was filled with Zr4+ ions, whereas Cd2+ (r=0.95 Å) and Zr4+ (r=0.72 Å) were statistically distributed in another type. Interatomic distances showed that combined framework octahedra were far more distorted [(dmin–dmax)=1.862–2.237 Å] than Zr-based ones (2.006–2.187 Å). Cavities within the polyhedra columns were occupied by alternating Cd2+ and Mg2+ ions. Isomorphic substitution of Zr4+ by Cd2+ (not Mg2+, which is close to Zr4+ by ionic radius r=0.72 Å) may probably be explained by the proximity of Zr (4d25s2) electronic structure to Cd (4d105s2) one. In this case it exerted greater effect than size factor.
Basing on the results of this study, we made comparison between the concentration regions of NZP and SW-type phases existence in the M0.5+x M′x Zr2−x (PO4)3 (M–Ca, Mn, Co, Sr, Cd, Ba, Pb; M′–Mg, Mn, Co; 0≤x≤2.0) systems (Table 8).
Concentration limits of M0.5+x M′x Zr2−x (PO4)3 solid solutions existence.
| System | Limits x | Structure type |
|---|---|---|
| Co0.5+x [Mgx Zr2−x (PO4)3] | 0≤x≤1.0 | SW |
| Co0.5+x [Cox Zr2−x (PO4)3] | x=0 | SW |
| Co0.5Mnx [Cox Zr2−x (PO4)3] | 0≤x≤0.5 | SW |
| Mn0.5+x [Mgx Zr2−x (PO4)3] | 0≤x≤1.0 | SW |
| Mn0.5+x [Cox Zr2−x (PO4)3] | x=0 | SW |
| Mn0.5+x [Mnx Zr2−x (PO4)3] | 0≤x≤0.25 | SW |
| Cd0.5Mgx [Cdx Zr2−x (PO4)3] | 0≤x≤0.6 | NZP |
| Cd0.5+x [M′x Zr2−x (PO4)3] M′−Co, Mn | 0≤x≤0.3 | NZP |
| Ca0.5+x [M′x Zr2−x (PO4)3] M′–Mg, Co, Mn | 0≤x≤1.0 | NZP |
| Sr0.5+x [M′x Zr2−x (PO4)3] M′–Mg, Co | 0≤x≤0.5 | NZP |
| Sr0.5+x [Mnx Zr2−x (PO4)3] | 0≤x≤0.3 | NZP |
| Pb0.5+x [Mgx Zr2−x (PO4)3] | 0≤x≤0.7 | NZP |
| Pb0.5+x [M′x Zr2−x (PO4)3] M′–Co, Mn | 0≤x≤0.6 | NZP |
| Ba0.5+x [M′x Zr2−x (PO4)3] M′–Mg, Co, Mn | x=0 x=0.5 |
NZP yavapaiite |
On the whole, phase formation of the compounds was influenced by cations sizes ratio and isomorphic substitution of Zr4+ for M2+. Optimal conditions for the formation of SW structure were implemented in case, when the average ionic radius of cavities cations is comparable with those of framework-forming cations. If the size discrepancy is more significant, NZP structure formed.
Ability of the M2+ cation to the isomorphic substitution of Zr4+ is clearly observed in the SW systems. The widest solid solutions regions were formed in the Mg-containing phosphates rows that may be explained by the closeness of Mg2+ and Zr4+ ionic radii.
The widest solid solution limits in the NZP systems were obtained for phosphates, containing Ca2+ ions, which ionic radius is supposed to be optimal for NZP interstitial sites.
Thermal expansion
The obtained solid solutions limits may be useful for crystal chemical design of materials with smoothly changing properties. As possible fields of application of the framework phosphates imply using them in the conditions of high temperatures or thermal shocks, their thermal expansion is needed to be known.
Thermal expansion of ceramic characterizes structure deformation caused by growth of temperature increase. That depends on many factors, in particular on the occupation of structural sites by cations. NZP phosphates M0.5Zr2(PO4)3 (x=0) with half-occupied cavities are well-known in literature, and their thermal expansion was widely studied [1]. The majority of studied phosphates are characterized by low average expansion coefficients, albeit high expansion anisotropy.
In the present investigation we studied thermal expansion of the representatives of triple NZP phosphates with occupied cavities (x=0.5): Cd0.5Mg0.5[Cd0.5Zr1.5(PO4)3], Ca[Mn0.5Zr1.5(PO4)3], Pb[Mg0.5Zr1.5(PO4)3]. The phosphates were investigated by X-ray powder diffraction in the temperature interval 20–800°C (Fig. 7).
![Fig. 7:
Temperature dependences of lattice parameters of the phosphates: (a) Cd0.5Mg0.5[Cd0.5Zr1.5(PO4)3]; (b) Ca[Mn0.5Zr1.5(PO4)3]; (c) Pb[Mg0.5Zr1.5(PO4)3].](/document/doi/10.1515/pac-2016-1005/asset/graphic/j_pac-2016-1005_fig_014.jpg)
Temperature dependences of lattice parameters of the phosphates: (a) Cd0.5Mg0.5[Cd0.5Zr1.5(PO4)3]; (b) Ca[Mn0.5Zr1.5(PO4)3]; (c) Pb[Mg0.5Zr1.5(PO4)3].
The compounds showed a certain pattern and also belonged to materials with low average thermal expansion (Table 9). The average expansion coefficient became lower with the increase of average radius of the cation that occupies the cavities of the structure. The least average thermal expansion coefficient and least expansion anisotropy were observed for the Pb[Mg0.5Zr1.5(PO4)3] phosphate that may be connected with high deformation of its structure at room temperature because of relatively large Pb2+ ions in its cavities. So structure deformations are depressed with the temperature increase. The same tendency was observed earlier for other NZP compounds with occupied cavities (for example, alkali metals containing AZr2(PO4)3 (A–Na, K Rb, Cs) [1] and seems to be a general trend for this class of substances.
Thermal expansion of the triple NZP phosphates with x=0.5.
| Phosphate | r cavity ion/rframework ion | Linear coefficients of thermal expansion, α 106 °C−1 |
Anisotropy ∣αa–αc ∣·106°C−1 | ||
|---|---|---|---|---|---|
| αa | αc | αav | |||
| Cd0.5Mg0.5[Cd0.5Zr1.5(PO4)3] | 1.1 | −3.2 | 1.4 | 2.5 | 4.6 |
| Ca[Mn0.5Zr1.5(PO4)3] | 1.3 | −0.3 | 7.7 | 2.3 | 8.0 |
| Pb[Mg0.5Zr1.5(PO4)3] | 1.6 | 1.9 | 2.6 | 2.2 | 0.7 |
On the whole, low thermal expansion of the NZP compounds combined with their high temperature stability allows us to propose NZP ceramics as perspective thermal-shock resistant materials and hope to succeed in extending their application scope.
Conclusions
Overall, isomorphism regularities in the SW and NZP-type phosphate systems containing cations in oxidation state +2 have been studied.
The structural data illustrating the preferable distribution of M2+ (M′2+) cations in crystallographic framework and cavities sites expand the whole scope of isomorphic ability of the structures based on {[L2(PO4)3]p−}3∞ frameworks.
The present data demonstrate that thermal expansion of NZP materials can be adjusted to suit materials’ need by proper ionic substitutions and opting for certain techniques of materials’ production. Some NZP materials have very low thermal expansion and expansion anisotropy and exhibit high thermal shock resistance.
Article note
A collection of invited papers based on presentations at the 12th Conference on Solid State Chemistry (SSC-2016), Prague, Czech Republic, 18–23 September 2016.
Acknowledgments
This work was supported by the Russian Foundation for Basic Research, Projects Nos.15-03-00716, 16-33-00888.
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