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Sodium hafnium germanate, Na3Hf1.7Ge3O10(OH)1.8, with garnet-type of structure was synthesized as a single phase, nanoparticles (100–600 nm) at mild hydrothermal conditions (230 °C) and autogenous pressure. The structure was solved ab initio from powder X-ray diffraction data and subsequently refined [Rexp: 6.10, Rwp: 7.26, Rp: 5.39, GOF: 1.19, RB: 1.50] by the method of Rietveld. The solid crystallizes in cubic space group Ia-3d (230), with a = 12.6819(2) A, V = 2039.65(1) A3 and Z = 8. The structure is built up of rare combination of two tetravalent (Ge4+ and Hf4+) and one monovalent (Na+) cation alternating in form corner-sharing GeO4 tetrahedra and HfO6 octahedra whose negative charge is compensated by Na+ residing in dodecahedral coordination. Unlike the typical tetrahedral site disorder in hydrogarnets (hydrogrossular, hibschite, and katoite) we found that the octahedral position of the heavy Hf4+ cation is partly vacant which reveals alternative mechanism of charge unbalance leading to incorporation of hydroxyl group. Furthermore, the synthesized material is the first hafnium germanate with garnet-type of structure and the second known sodium hafnium germanate.

The mild hydrothermal synthesis, crystal structure, photoluminescence properties and emission quantum yield of a new sodium zirconium germanate are reported. This material and the method for its preparation represent for the first time a germanium garnet-type material synthesized at autogenous pressure and temperature at 230 °C. The crystal structure was determined by starting from the crystallographic parameters of a common garnet structure and it represents not only a new chemical combination of atoms but also combination of oxidation states in garnet structure. The framework consists of interconnected corner sharing GeO4 tetrahedra and ZrO6 octahedra which form small cavities that accommodate charge compensation Na+ cations. In the course of synthesis the structure can be functionalized by in situ doping with different percentage of Eu3+ ions. This structural flexibility is used to explore the photoluminescent behavior of the active centers embedded in garnet-type host. The materials display the Eu3+ 5D0→7F0–4 transitions under excitation via intra-4f6 excitation levels and through the O2−→Eu3+ ligand-to-metal charge transfer. A maximum 5D0 quantum efficiency and emission quantum yield values (ca. 0.28 and 0.04±0.01, respectively) were found for the low Eu3+-containing sample, suggesting the presence of concentration quenching effects at higher Eu3+-content (5%).

Rare earth silicates with the structure of apatite are attracting considerable interest since they show oxygen ion conductivities higher than that of yttria-stabilized zirconia (YSZ) at moderate temperature. Based on the hydrothermal synthesis we presented a simple one step process for the direct preparation of the pure and the high crystalline nanosized rare earth silicates with the structure of apatite under a mild condition (230 °C). Since the preparation of the high crystalline silicon based rare earth apatites is performed at high temperature previously and accompanied by subsequent process of grinding, results of this work provide a promising alternative of the existing methodology. Furthermore, due to the relatively low temperature of the preparation of these materials, high doping of monovalent cation can be done, which was not achieved before.