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Oxyapatites A10(GeO4)6O2 (A – alkaline, rare‐earth elements) are considered promising materials for applications in solid‐state lighting and non‐contact thermometry. The temperature dynamics of the oxygen sublattice have a crucial influence on the luminescent properties of these optical hosts. Here, the structural evolution with temperature of germanates with the general formula ALa9(GeO4)6O2 (A – Li, Na, K, Rb) is investigated, using X‐ray and neutron powder diffraction methods. These compounds crystallize in space‐group P63/m. NPD experiments helped to determine the oxygen and alkaline metal positions. Thermally‐induced reentrant structural transitions relating to the oxygen in the apatite channel are discovered for the first time. In the example of Eu‐doped RbLa9(GeO4)6O2 and KLa9(GeO4)6O2, it is demonstrated that the luminescence of the Eu3+ ion is influenced by small changes in the crystal structure. [ABSTRACT FROM AUTHOR]

A new series of BaRE6(Ge2O7)2(Ge3O10) (RE = Tm, Yb, Lu) germanates and activated phases BaYb6(Ge2O7)2(Ge3O10):xTm3+ and BaLu6(Ge2O7)2(Ge3O10):12yYb3+,yTm3+ have been prepared using a solid-state reaction. An XRPD study has revealed that the compounds crystallize in the monoclinic system (space group P21/m, Z = 2). The crystal lattice consists of zigzag chains of edge-sharing distorted REO6 octahedra, bowed trigermanate [Ge3O10] units, [Ge2O7] groups, and eight-coordinated Ba atoms. The density functional theory calculations have confirmed a high thermodynamic stability of the synthesized solid solutions. According to the results of vibrational spectroscopy studies and diffuse reflectance measurements, the BaRE6(Ge2O7)2(Ge3O10) germanates are promising compounds for the creation of efficient lanthanide ion activated phosphors. Under 980 nm laser diode excitation, the BaYb6(Ge2O7)2(Ge3O10):xTm3+ and BaLu6(Ge2O7)2(Ge3O10):12yYb3+,yTm3+ samples exhibit upconversion luminescence corresponding to the characteristic 1G4 → 3H6 (455–500 nm), 1G4 → 3F4 (645–673 nm) and 3H4 → 3H6 (750–850 nm) transitions in Tm3+ ions. Heating of the BaLu6(Ge2O7)2(Ge3O10):12yYb3+,yTm3+ phosphor with the optimal composition up to 498 K leads to the enhancement of a broad band at 673–730 nm, caused by 3F2,3 → 3H6 transitions. It has been revealed that the fluorescence intensity ratio between this band and the band at 750–850 nm may be used for temperature sensing. The absolute and relative sensitivities in the studied temperature range reach 0.021% K−1 and 1.94% K−1, respectively. [ABSTRACT FROM AUTHOR]

A new series of Sr3RE2(Ge3O9)2 (RE = La, Pr, Nd, Sm, Eu, Gd) cyclogermanates has been prepared using an ethylenediaminetetraacetic acid assisted route. The powder XRD study reveals that the compounds crystallize in the monoclinic space group C2/c, Z = 4. A decrease in the crystal radius of RE3+ ions in Sr3RE2(Ge3O9)2 (RE = La–Gd) causes a morphotropic phase transition followed by a step-like change of the lattice parameters. The transition is accompanied by internal reorganization, including the redistribution of rare earth and strontium ions among three symmetry independent Sr/RE sites, a conformation change of [Ge3O9]6− rings and a variation of Sr/RE(2) atom coordination from seven-fold to nine-fold. Density functional theory calculations reveal that Sr3RE2(Ge3O9)2 compounds are indirect bandgap materials with a band gap around 3.0 eV. The magnetic property measurements indicate that Sr3RE2(Ge3O9)2 (RE = Pr, Sm, Gd) are Curie–Weiss paramagnets up to 2 K. For Sr3Eu2(Ge3O9)2 an anomaly of magnetic susceptibility is found at about 60 K. A photoluminescence study of the Sr3Sm2(Ge3O9)2 and Sr3Eu2(Ge3O9)2 samples reveals an emission in the visible spectral range, corresponding to the characteristic 4f–4f transitions in rare earth ions. [ABSTRACT FROM AUTHOR]