J. Alamo

Chemistry and properties of solids with the [NZP] skeleton

Abstract Solids with an [NZP] skeleton in their structure from a very broad family presenting very different properties with varying chemical composition. These properties range from radioactive nuclide immobilization and insulation to ionic and/or electronic conduction, extending to the field of very low thermal expansion among others. The open [NZP] skeleton provides the stability and flexibility necessary to form continuous ranges of solid solution, allowing the tailoring of the best composition-property-structure material. Chemical activity may become particularly high at the surface and absorption reactions and catalytic properties have the highest interest at present. In this paper, chemistry and properties of [NZP] solids are presented as the result of the interactions between the [NZP] skeleton and the counter-ions. Properties of ion transport, anisotropic thermal expansion, ion exchange, isomorphism, polymorphism, phase transitions, exchange and reactivity at low temperatures are discussed.

Thermal expansion of NaTi2(PO4)3 studied by rietveld method from X-ray diffraction data

Abstract Previous disagreement about the thermal expansion of the rhombohedral compound, NZP-type, NaTi 2 (PO 4 ) 3 has been clarified. It is shown that thermal stresses affect the thermal expansion, but they relax after some time of storage. Its anisotropic thermal expansion, has been calculated from high temperature X-ray diffraction, and it is linear in the range from room temperature up to 800°C. Coefficients are α (a)=−4.4×10 −6 °C −1 and α (c)=20×10 −6 °C −1 . The predictability of thermal expansion and the tailoring of the composition of NZP ceramics require to check whether the thermal effect on the rotations and distortions of the atomic polyhedra in this structure is the responsible for the high anisotropy in the thermal expansion. This effect has been determined experimentally by solving the chemical structure at five different temperatures, applying the Rietveld method to deconvolute the powder X-ray diffraction profiles. The Rietveld computer program was modified to include and refine a coefficient relating to the eccentricity of the sample in the X-ray high-temperature camera.

Phase transition in NaSn2(PO4)3 and thermal expansion of NaMIV2 (PO43; MIV = Ti, Sn, Zr

Abstract NaSn2(PO4)3 presents a fast and reversible second order phase transition about 575°C. Both phases above and below the transition point are rhombohedral. The high temperature phase is isostructural with NaTi2(PO4)3 and NaZr2(PO4)3, NZP structure. The lattice thermal expansion of these three compounds has been determined from x-ray diffraction data at different temperatures ranging from room temperature up to 1000°C. Differences in behaviour are discussed in relation to the structure.

High temperature neutron diffraction study of sodium di-tin tri-phosphate

Abstract High temperature high resolution neutron diffraction study of the crystal chemistry of NaSn 2 (PO 4 ) 3 has been completed, at four different temperatures, applying the Rietveld method to the experimental neutron diffraction profile of a synthetic crystalline powder. Below the phase transition (∼860 K) the space group has been determined to be R3 which changes to R3c in the high temperature phase. No chemical bond breaks through the transition. The change in temperature makes the SnO 6 polyhedra rotate around the three fold axis in both structures. The PO 4 polyhedra rotate strictly around the two fold axis in the high temperature phase, but around the [0001] direction in the low temperature form.

High temperature neutron diffraction study of CaZr4(PO4)6

Abstract The lattice thermal expansion of rhombohedral CaZr4(PO4)6 has been determined to be anisotropic with a negative expansion coefficient along the directions in the plane (0001). This behaviour is anomalous if compared to the rest of the compounds in the set MIIMIV4 (PO4)6; MII=Mg, Ca, Sr, Ba; MIV=Ti, Sn, Zr and to the low temperature form of MISn2(PO 4)3 MI=Na, K, Rb. High temperature high resolution neutron diffraction shows that the rotations of ZrO6 and PO4 polyhedra are opposite to those in the other cases.

Thermal expansion of LiZr2(PO4)3: Water inclusion influence

Abstract Lattice thermal expansion has been measured on three samples of composition LiZr 2 (PO 4 ) 3 , prepared by (i) a ceramic method, (ii) a gel-route and (iii) a nonstoichiometric ceramic. The first sample is monoclinic (?), with a transition at 50°C to rhombohedral. The second is clearly monoclinic with lattice parameters depending on the calcination temperature (700 to 1200°C. The third kind of synthesis yielded a new PO 4 -deficitary rhombohedral structure. X-ray diffraction measurements in a high temperature camera have been made from room temperature up to 1100°C. Lattice parameters, as well as their dependence on temperature are different for the three samples. While thermal expansion of the ceramic sample shows a regular trend, in the gel-route sample there is a sharp transition between 155 and 200°C.

Ordered mesoporous materials: composition and topology control through chemistry

Abstract The atrane route constitutes a very versatile technique to obtain ordered mesoporous materials. A wide diversity of silica and silica-doped materials can be prepared by bringing into play fundamental synthesis parameters (like temperature, concentration and pH) which, in turn, allow modulation of the resulting material topology.

Synthesis and structural study of NaTi2(PO4)3-NaSn2(PO4)3 solid solutions. I. The effect of composition on lattice parameters

Abstract Compounds NaM2IV(PO4)3 with NZP-type structure present a different behavior depending on the nature of MIV. For MIV = Ti and Zr the structure shows the space group R3c, whereas for MIV = Sn the space group is R3. Differences in behavior of NaTi2(PO4)3 - NaSn2(PO4)3 solid solutions are discussed in relation to the composition. The variation of the lattice parameters with composition in NaTi2−xSnx(PO4)3 (0 1. The structure of the compound with x = 1 (NaSnTi(PO4)3) has been determined applying the Rietveld method to deconvolute the powder x-ray diffraction profile.

Stabilization of the rhombohedral phase in LiZr2(PO4)3 by thermal quenching

Abstract Two LiZr2(PO4)3 samples, which show rhombohedral and monoclinic symmetries, have been studied by powder XRD and MAS-NMR (31P and 7Li) techniques. In the monoclinic sample phosphorus occupies three crystallographic sites and lithium is preferentially placed in M2 environment. When the samples are subjected to successive heating-quenching treatments, the rhombohedral phase is not appreciably modified while the monoclinic phase is transformed into the rhombohedral one. In the quenched samples only one site for phosphorus has been detected and occupancy of lithium in the more symmetric M1 site is favoured. After the thermal treatments, when the samples are stored at room temperature for one year, the framework does not relax to that of the monoclinic phase but distribution of lithium changes towards a statistical occupancy of M1 and M2 sites.

Synthesis and structural study of sodium titanium phosphate-sodium tin phosphate solid solutions. II. Thermal expansion

Abstract The structure of NaTi 2 (PO 4 ) 3 shows the space group R3c, whereas that of NaSn 2 (PO 4 ) 3 presents the space group R3 at room temperature and it undergoes a second order phase transition at 575°C from this structure to another with the space group R3c. Evolution of the structure of NaTi 2 (PO 4 ) 3 NaSn 2 (PO 4 ) 3 solid solutions with temperature has been studied and temperature phase transition established for the compositions studied. Lattice thermal expansion of NaTi 2−x Sn x (PO 4 ) 3 solid solutions with x = 1, 1.2, 1.5 has been determined from x-ray diffraction data at temperatures ranging from 26 to 1000°C. DSC and dilatometric measurements have been also carried out on these samples. The composition with x = 1.5, NaTi 0.5 Sn 1.5 (PO 4 ) 3 , presents a second order phase transition at 355°C, from the phase with R3 space group to the phase with R3c space group, similar to that of the compound with x = 2, NaSn 2 (PO 4 ) 3 . For the solid solution with x = 1.2, NaTi 0.8 Sn 1.2 (PO 4 ) 3 , this transition seems to occur at a temperature closed to the room temperature, whereas the composition with x = 1, NaTiSn(PO 4 ) 3 , does not show any phase transition. Thermal expansion of these solid solutions for x 2 (PO 4 ) 3 whereas for x > 1.2 is analogous to that of NaSn 2 (PO 4 ) 3 .