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The apatite La10-x□x(Ge5.5Al0.5O24)O2.75-1.5x (10 − x = 9.80, 9.75, 9.67, 9.60, 9.50, and 9.40) series has been prepared and the single phase existence range has been established, 9.75 ≥ 10 − x ≥ 9.45. The hexagonal crystal structures of La9.5□0.5(Ge5.5Al0.5O24)O2 have been determined at room temperature, 500 °C, and 900 °C from neutron powder diffraction data using the Rietveld method. The room-temperature unit cell parameters were a = 9.9206(4) Å, c = 7.2893(3) Å, V = 621.29(6) ų, and Z = 1, and this refinement converged to RWP = 3.03 and RF = 1.30%. The most important structural result is the presence of interstitial oxygen ion associated with vacancies at the apatite oxide anions channels. Oxide ion conductivities have been measured by impedance spectroscopy. La9.5□0.5(Ge5.5Al0.5O24)O2 shows very high oxide conductivity, 0.16(1) S·cm^-1 at 800 °C, with negligible electronic contribution. The ionic transport number, obtained by combination of impedance and ion-blocking data, is higher than 0.99 in the studied oxygen partial pressure range, 0.21 to 10^-20 atm.

Two Li1+xAlxGeyTi2-x-y(PO4)3 (0.2 ≤ x ≤ 0.8; y = 0.8, 1.0) solid solutions have been prepared as polycrystalline powders. These compounds crystallize in the NASICON-type structure, R&thremacr;c space group, and the crystal structures have been characterized by the Rietveld method with laboratory X-ray powder diffraction data. The cell parameters evolution along the two series agrees with the substitution of larger Ti⁴⁺ by smaller Ge⁴⁺ and Al³⁺ cations. The electrical properties have been characterized by an impedance study. Bulk conductivity values at room temperature are close to 10^-3 S·cm^-1 with low activation energies (≈0.35 eV). The trajectories of the Li⁺ cations have been simulated from the bond valence sum calculation. Structural keys, which justify the high ionic conductivity and the low activation energy, are discussed.

La5/8-xPrxCa3/8MnO3 (x = 0, 0.1, 0.2, 0.25, 0.3, 0.35, 0.375, 0.4, 0.412, 0.425, 0.45, 0.5, ⁵/8) series has been synthesized by the ceramic method. All manganites are single phase crystallizing in the Pnma space group. The evolution of cell parameters along the series indicates that the orthorhombic perovskite superstructure distortion subtype switches from O‘ ‘ (x = 0; c > a > b/√2) to O‘ (x = 5/8; a > c > b/√2). This change is not abrupt and there is a wide compositional range (0.25≤ x ≤ 0.35) where the distortion subtype cannot be defined. We report high-resolution RT synchrotron and neutron powder diffraction data for four selected compositions (x = 0, 0.2, 0.35, and 5/8). Atomic parameters determined from the joint Rietveld refinement indicate that the tilting scheme slightly changes along the series. The tilting angle of the MnO6 octahedra around the b long-axis is larger than that around [101] for the O‘ ‘ subtype but is smaller than that for the O‘ subtype. For the x = 0.35 sample, where phase separation has been observed at low temperatures, the average tilting angles are the same within the small errors of the measurements. The microstructures of the samples have been investigated through Williamson−Hall plots for the synchrotron patterns. The sample broadening is markedly anisotropic and strongly dominated by microstrains with average values of the Δd/d term of about 10 × 10^-4. The sample broadening for Pr5/8Ca3/8MnO3 is larger than that for La5/8Ca3/8MnO3 although the Pr³⁺ radius is more similar to that of Ca²⁺. This indicates that other factors, in addition to A-cation radii mismatch and variance, such as distortion type, are important to justify the broadening commonly observed in manganites. The monotonic evolution in cell parameters, the magnetization, and resistivity curves indicate the good quality of the samples.

Rhombohedral Na1+xZr2-xInx(PO4)3 (x = 0, 0.2, 0.4, 0.8, 1.0, 1.2, 1.6, and 1.8) NASICON materials have been studied by XRPD, variable-temperature NPD, ³¹P and variable-temperature ²³Na MAS NMR, and impedance spectroscopies. Relative ³¹P MAS NMR peak intensities of the five detected signals, attributed to the environments [P(OZr)4-n(OIn)n (n = 0−4)], are close to those expected for a random distribution of octahedral cations. This local probe allows us to rule out the existence of segregated metal-rich nanoregions. Combined NPD and XRPD Rietveld studies showed that the occupation of M2 sites by the extra Na⁺ cations produces a slight distortion of the structure. Low temperatures freeze Na mobilities and permit the assignment of the resulting NMR bands to Na⁺ in the M1 and M2 sites. The mobility of Na at room temperature increases with the Na content. For samples with x < 0.8, Na ions are relatively localized; however, for x ≥ 0.8, Na mobility increases, yielding a unique signal in the ²³Na NMR spectra. The increase of Na mobility causes spatial disorder at the M1 site as deduced from the variable-temperature NPD study. At low temperatures, the activation energies do not change significantly. So, the observed increase in bulk conductivity by 2 orders of magnitude in the low-temperature regime is ascribed to an increase of mobile carrier, Na⁺, concentration. However, at higher temperatures a new regime has been identified by NPD, ²³Na MAS NMR, and Arrhenius plots of fmax data in Z‘ ‘ spectra and is associated with more extended sodium motion. The onset temperature for the curvature in the Z‘ ‘ fmax Arrhenius plots, when reaching the high-temperature regime, depends on sodium content.

The structures and electrical properties of four NASICON compositions, Na1.4M1.6In0.4(PO4)3 (M = Ti, Sn, Hf, Zr), have been determined and compared. Rietveld refinement of powder X-ray diffraction data confirmed the basic rhombohedral NASICON structure with random occupancy of the octahedral In/M sites, full occupancy of the Na(1) sites and partial occupancy of the Na(2) sites. For three compositions, M = Zr, Sn, and Hf, the ³¹P MAS NMR peak intensities of the four detected signals, attributed to four different phosphorus environments [P(OM)4-n(OIn)n (n = 0−3)], were close to the ratios expected for a random distribution of In/M. For M = Ti, some departures from statistical occupancy were apparent. ²³Na MAS NMR data gave evidence for two Na⁺ positions at room temperature for M = Ti, Sn, attributable to occupation of Na(1) and Na(2) sites. For M = Hf, Zr, only a single signal could be resolved at room temperature, which splits into two signals on cooling to − 50 °C, indicating high Na mobility at room temperature. Impedance data obtained on pressed sintered pellets over the range 25−300 °C showed that bulk ionic conductivities increased and activation energies decreased in the sequence Ti, Sn, Hf, Zr. The geometry of the M1M2 bottleneck has been determined from structural data, and a direct correlation found between activation energy for ion conduction and the bottleneck size.

Crystalline single-phase LiHf2(PO4)3 has been synthesized. It shows a phase transition at 235 K that has been characterized through DSC, low-temperature X-ray diffractometry, and ionic conductivity measurements. The crystal structures of the two phases have been established by neutron powder diffraction at 175 and 320 K. The room-temperature phase is rhombohedral R&thremacr;c, Z = 6, a = 8.8306(1) Å and c = 22.0270(5) Å and the structure has been refined by the Rietveld method to Rwp = 6.29% and RF = 4.68%. The framework is similar to that shown by NASICON materials with the Li cations disordered out of the M1 position and close to the M1−M2 bottleneck site. The crystallographically refined composition was Li0.87Hf2.032(PO4)3 in agreement with the chemical analysis. The low-temperature phase is triclinic C&onemacr;, Z = 4, a = 15.2680(4) Å, b = 8.6946(3), c = 9.0722(2) Å, α = 89.323(2)°, β = 123.740(2)°, and γ = 90.666(2)°, and the structure has been refined to Rwp = 4.61% and RF = 2.25%. This is a common reversible order−disorder phase transition with some subtle structural changes. These structural changes and the consequences in the ionic conductivity properties will be discussed. This is the first structural report on a low-temperature phase in the Li−NASICON system.