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.

A new series of Bi3+/Sm3+ and Bi3+/Eu3+ codoped Ba2Gd2Ge4O13 phosphors crystallizing in the monoclinic system (S.G. C2/c) have been prepared. The studied germanates are promising materials for non-contact temperature measurements.

Tetragermanates Ba2Gd2 – xSmxGe4O13 (x = 0.025–0.8) have been synthesized by the solid-phase method. Solid solutions crystallize in the monoclinic crystal system (space group С2/с, Z = 4) and are members of a small family of inorganic compounds containing [Ge4O13]10– anions. The photoluminescence properties of germanates upon excitation by radiation with λex = 275 nm have been studied. The spectra of the compounds show a broad band with a maximum at 313 nm and a set of lines in the range of 525–730 nm, corresponding to intraconfigurational 4f–4f transitions in Gd3+ and Sm3+ ions. It has been found that germanate Ba2Gd1.95Sm0.05Ge4O13 has the maximum luminescence intensity. For this sample, the color characteristics and the temperature dependences of the intensity ratios of the main luminescence bands upon heating to 498 K have been studied. It has been concluded that Ba2Gd1.95Sm0.05Ge4O13 can be used as a material for non-contact temperature sensing and light emitting diodes.

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.

Tetragermanates Ba 2 Gd 2 – x Sm x Ge 4 O 13 ( x = 0.025–0.8) have been synthesized by the solid-phase method. Solid solutions crystallize in the monoclinic crystal system (space group С 2/ с , Z = 4) and are members of a small family of inorganic compounds containing [Ge 4 O 13 ] 10– anions. The photoluminescence properties of germanates upon excitation by radiation with λ ex = 275 nm have been studied. The spectra of the compounds show a broad band with a maximum at 313 nm and a set of lines in the range of 525–730 nm, corresponding to intraconfigurational 4 f –4 f transitions in Gd 3+ and Sm 3+ ions. It has been found that germanate Ba 2 Gd 1.95 Sm 0.05 Ge 4 O 13 has the maximum luminescence intensity. For this sample, the color characteristics and the temperature dependences of the intensity ratios of the main luminescence bands upon heating to 498 K have been studied. It has been concluded that Ba 2 Gd 1.95 Sm 0.05 Ge 4 O 13 can be used as a material for non-contact temperature sensing and light emitting diodes.

A new series of Ba2RE2Ge4O13 (RE = Pr, Nd, Gd, Dy) germanates and Ba2Gd2-xEuxGe4O13 (x = 0.1-0.8) solid solutions have been synthesized using the solid-state reaction technique and characterized by X-ray powder diffraction. All compounds crystallize in the monoclinic system, space group C2/c, Z = 4. The crystal lattice consists of RE2O12 dimers, zigzag C2-symmetric [Ge4O13]10- tetramers, and ten-coordinated Ba atoms located in voids between polyhedra. The density-functional theory (DFT) calculations performed on a rich set of Ba2RE2Ge4O13 compounds have confirmed the high thermodynamic stability of monoclinic modification. Under ultraviolet (UV) light excitation Ba2Gd2-xEuxGe4O13 phosphors exhibit an orange-red emission corresponding to the characteristic f-f transitions in Eu3+ ions. The highest intensity of lines at 580 nm (5D0→7F0), 582-602 nm (5D0→7F1), 602-640 nm (5D0→7F2), 648-660 nm (5D0→7F3), and 680-715 nm (5D0→7F4) is observed for the samples with x = 0.4-0.6. The possibility of their application has been assessed by studying their color characteristics, quantum efficiency, and thermal stability. The obtained data indicate that Ba2Gd2-xEuxGe4O13 solids can be considered as promising materials for UV-excited phosphor-converted light-emitting diodes (LEDs) if an aluminum nitride substrate (λex = 255 nm) is used as a semiconductor chip.

A new series of CaY2–10xYb9xErxGe3O10 trigermanates (space group P21/c, Z=4) has been synthesized using an EDTA-assisted method. Under 980nm laser diode excitation, the samples exhibit intense upconversion luminescence corresponding to the characteristic transitions 2H9/2→4I15/2 (400–420nm), 2H11/2,4S3/2→4I15/2 (500–600nm) and 4F9/2→4I15/2 (625–725nm) in Er3+ ions. The CaY1.5Yb0.45Er0.05Ge3O10 phosphor can be used as a good temperature sensor with high absolute and relative sensitivities Sa(max)=0.0051 K−1 and Sr(max)=0.0109 K−1.

Triorthogermanates BaYb2 – xErxGe3O10 (x = 0.1–0.3) and BaY2 – 10yYb9yEryGe3O10 (y = 0.015–0.15) are synthesized by the solid phase method. According to X-ray powder diffraction data, the compounds crystallize in a monoclinic system, space group P21/m, Z = 2. The concentration and power dependences of upconversion luminescence that occurs in the range of 510–720 nm upon excitation by radiation with a wavelength of 980 nm are studied. The mechanism of energy transfer between optical sites is proposed and the optimal composition of the phosphor is determined. The temperature dependence of the ratio of the luminescence band intensities with maxima at 521 nm and 552 nm (the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions in Er3+) is studied, and conclusions are drawn about the possible application of triorthogermanates as materials for fluorescent temperature sensors.

The Ca(Ba)RE2Ge3O10 germanates are currently considered as a promising class of active optical media emitting in the visible and infrared spectral region. Herein, a new series of BaY2-x-yTmxDyyGe3O10 phosphors was synthesized using the solid-state reaction and characterized by X-ray diffraction, scanning electron microscopy, diffuse reflectance and luminescence spectroscopy. All the studied compounds crystallize in the monoclinic system, space group P21/m, Z = 2. Under 808 nm excitation, BaY2-xTmxGe3O10 germanates demonstrate a broad emission in 1.3–2.2 µm range owing to a series of cascade 3H4 → 3F4 and 3F4 → 3H6 transitions in Tm3+ ions. The BaY2-xTmxGe3O10 phosphors exhibit a high thermal stability over a wide temperature range and belong to promising infrared luminescence materials. The intensity of these emission bands decreases with an increase in the dysprosium content in the BaY1.97-yTm0.03DyyGe3O10 germanates. The magnetic properties measurements have been also carried out since Dy3+ ions exhibit a large magnetic anisotropy and non-collinearity of the magnetization easy axes. The magnetization curve of Tm3+-doped BaY2Ge3O10 shows the effects of saturation typical of paramagnetics, while the magnetization of germanates doped with Dy3+ ions occurs with hysteresis which is observed for single-ion magnets.

Triorthogermanates BaYb2-xErxGe3O10 (x = 0.1–0.3) and BaY2-10yYb9yEryGe3O10 (y = 0.015–0.15) have been synthesized by the solid-state method. According to X-ray powder diffraction data, the compounds crystallize in the monoclinic system, S.G. P21/m, Z = 2. The concentration and power pump dependences studies have been carried out for the lines in the 510–720 nm spectral range under 980 nm excitation. The mechanisms of energy transfer between optical centers have been also proposed and the optimal composition of the phosphor has been determined. The influence of temperature on the intensity ratio of the luminescence bands with maxima at 521 nm and 552 nm (2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions in Er3+) has been investigated, and conclusions about the possibility of using the germanates as materials for optical temperature sensors have been drawn.

Triorthogermanates BaYb 2 – x Er x Ge 3 O 10 ( x = 0.1–0.3) and BaY 2 – 10 y Yb 9 y Er y Ge 3 O 10 ( y = 0.015–0.15) are synthesized by the solid phase method. According to X-ray powder diffraction data, the compounds crystallize in a monoclinic system, space group P 2 1 / m , Z = 2. The concentration and power dependences of upconversion luminescence that occurs in the range of 510–720 nm upon excitation by radiation with a wavelength of 980 nm are studied. The mechanism of energy transfer between optical sites is proposed and the optimal composition of the phosphor is determined. The temperature dependence of the ratio of the luminescence band intensities with maxima at 521 nm and 552 nm (the 2 H 11/2 → 4 I 15/2 and 4 S 3/2 → 4 I 15/2 transitions in Er 3+ ) is studied, and conclusions are drawn about the possible application of triorthogermanates as materials for fluorescent temperature sensors.

The luminescence properties of reddish-orange emitting BaY2−xEuxGe3O10 phosphors crystallizing in the monoclinic system (S.G. P21/m) have been discussed in detail. The studied germanates are appropriate for applications in high-powered pc-LEDs.

For the first time, the Sr2La8 – xTmx(GeO4)6O2 (x = 0.1–1.0) solid solution with the apatite structure were synthesized by the solid-phase method, and their spectral–luminescence properties were studied. The prospects of using these compounds as phosphors in the visible and short-wave infrared ranges were demonstrated. The luminescence of Sr2La8 – xTmx(GeO4)6O2 germanates, which occurs under ultraviolet radiation, is characterized by the high purity of blue color; the chromaticity coordinates are close to commercially available phosphors. The Sr2La8 – xTmx(GeO4)6O2 compounds efficiently convert 808 nm laser radiation into a series of emission lines in the 1.3–2.2 μm spectral range, caused by sequential 3H4 → 3F4 and 3F4 → 3H6 transitions in Tm3+ ions. Germanate Sr2La7.6Tm0.4(GeO4)6O2 with a maximum emission intensity in the short-wave infrared region shows high thermal stability of luminescence in the 30–220°C range.

New germanate ALa9(GeO4)6O2 (A = Li, Na, K, Rb, Cs) oxyapatites, showing promise as host lattices for phosphors with emission in the visible and infrared spectral ranges, have been obtained for the first time by ultrasonic spray pyrolysis. The discussion is focused on native defectiveness and optical properties of these compounds, depending on the nature of alkaline ions. Spray pyrolysis method was consciously used to obtain the compounds with intrinsic defects as confirmed by electron paramagnetic resonance. Reviewing available EPR data, the registered EPR response was assigned to the paramagnetic centers related to germanium tetrahedra embracing the oxygen vacancies. According to quantum-chemical calculations the oxygen-deficient centers in ALa9(GeO4)6O2 are presumably stabilized by antisite defects in the cation sublattice. Diffuse reflectance and pulse cathodoluminescence spectra of the compounds were investigated in detail, establishing a distinct dependence of optical properties on the type of alkaline ion. The observed optical absorption maxima at 275 and 370 nm were attributed to the F-like centers. The inverse dependence of luminescence intensity on the degree of defectiveness was interpreted on the assumption that these centers were involved in non-radiative energy relaxation.

For the first time, the Sr 2 La 8 – x Tm x (GeO 4 ) 6 O 2 ( x = 0.1–1.0) solid solution with the apatite structure were synthesized by the solid-phase method, and their spectral–luminescence properties were studied. The prospects of using these compounds as phosphors in the visible and short-wave infrared ranges were demonstrated. The luminescence of Sr 2 La 8 – x Tm x (GeO 4 ) 6 O 2 germanates, which occurs under ultraviolet radiation, is characterized by the high purity of blue color; the chromaticity coordinates are close to commercially available phosphors. The Sr 2 La 8 – x Tm x (GeO 4 ) 6 O 2 compounds efficiently convert 808 nm laser radiation into a series of emission lines in the 1.3–2.2 μm spectral range, caused by sequential 3 H 4 → 3 F 4 and 3 F 4 → 3 H 6 transitions in Tm 3+ ions. Germanate Sr 2 La 7.6 Tm 0.4 (GeO 4 ) 6 O 2 with a maximum emission intensity in the short-wave infrared region shows high thermal stability of luminescence in the 30–220°C range.

The Sr2La8-xTmx(GeO4)6O2 (x = 0.1–1.0) solid solutions with the apatite structure were synthesized by the solid-state method for the first time, the spectral-luminescence properties were studied and the prospects of these compounds as phosphors of the visible and short-wave infrared ranges were demonstrated. The luminescence of germanates Sr2La8-xTmx(GeO4)6O2 under ultraviolet excitation is characterized by high blue color purity and color coordinates close to commercially available phosphors. In addition, it was shown that Sr2La8-xTmx(GeO4)6O2 compounds efficiently convert 808 nm laser radiation into a series of emission lines in the spectral range of 1.3–2.2 µm due to successive 3H4 → 3F4 and 3F4 →3H6 transitions in Tm3+ ions. The Sr2La7.6Tm0.4(GeO4)6O2 germanate with a maximum emission intensity in the short-wave infrared region demonstrates high thermal stability of luminescence in the range of 30–220 °С.

New apatite-type NaLa9-xTmx(GeO4)(6)O-2 (x = 0.025-0.8) germanates (space group P6(3)/m, Z = 1) have been prepared using both the citrate technique and the conventional solid state method. The samples were characterized by powder X-ray diffraction, scanning electron microscopy, diffuse reflectance and photoluminescence spectroscopy. Photoluminescence measurements of NaLa9-xTmx(GeO4)(6)O-2 compounds were carried out both in the 425-850 nm range under 356 nm excitation and in the infrared region under 808 nm excitation. The most intensive lines at 453 nm, 478 nm and in 625-850 nm wavelength range are associated with the D-1(2) -> F-3(4), (1)G(4) -> H-3(6) and a series of (1)G(4) -> F-3(4), F-3(2) -> H-3(6), H-3(4) -> H-3(6) transitions in Tm3+ ions. The spectra measured in the infrared spectral range consist of two broad emission bands centered at 1440 nm and 1810 nm, which correspond to the cascade( 3)H(4)degrees ->degrees F-3(4) and F-3(4)->degrees H-3(6) transitions in Tm3+. The influence of dopant content and temperature on possible mechanisms promoting the population of Tm3+ excited states has been studied.

A series of Sr3La2-xEux(Ge3O9)(2) (x = 0.1-1.8) cyclogermanates has been prepared by the citrate method. The powder XRD study has shown that the compounds crystallize in the monoclinic space group C2/c, Z = 4. An increase in the dopant content causes a morphotropic phase transition: at x = 0.1-0.7 the samples are isostructural to Sr3La2(Ge3O9)(2), while at x = 0.9-1.8 the germanates have a crystal structure of Sr3Eu2(Ge3O9)(2). In all compounds, strontium and rare earth ions occupy three symmetry independent cation sites and form layers alternating along the [10 (1) over bar] direction. The relationships between the crystal structure and the luminescence characteristics: the energy of the charge transfer transition, the values of 4f-4f/CTB and D-5(0) -> F-7(2)/D-5(0) -> F-7(1) intensity ratios have been revealed. CIE chromaticity coordinates for Sr3La1.4Eu0.6(Ge3O9)(2) and Sr3La0.8Eu1.2(Ge3O9)(2) phosphors under 393 nm excitation have been measured.

A new series of BaRE2Ge3O10 trigermanates crystallizing in the monoclinic system (S.G. P21/m) has been characterized by various methods.

The tetragermanate Ba2Eu2Ge4O13 was synthesized for the first time, and its structural and luminescent properties were examined. It crystallizes in a monoclinic system (space group C2/c, Z = 4) and its crystal structure incorporates a unique linear [Ge4O13]10− anion. The photoluminescence spectrum of the sample (λex = 280 nm) contains several intense peaks in the orange-red spectral range corresponding to the 4f–4f transitions in Eu3+ ions.