The optically active dopant distribution in a Tm3+–Yb3+ doped silica based glass ceramic sample has been investigated. A systematic analysis of the upconversion fluorescence of the Tm3+–Yb3+ codoped glass and glass ceramic has been performed at room temperature. Tm3+ and Yb3+ single doped glass and glass ceramics have also been included in the study. Upon infrared excitation at 790 nm into the H43 level of the Tm3+ ions a blue upconversion emission is observed, which is drastically increased in the Yb3+ codoped samples. A rate equation model confirmed the energy transfer upconversion mechanism. Based on these results, the temporal dynamic curves of the levels involved in the upconversion process, H43, F5/22, and G41 were interpreted in the glass ceramic samples. The contribution of the optically active Tm3+ and Yb3+ ions in the crystalline and in the vitreous phase of the glass ceramic was distinguished and the ratio of Tm3+ ions in the crystalline phase could be quantified for the 1 mol % Tm3+–2.5 mol % Yb3+ glass ceramic. A surprising result was obtained for that concentration: the main contribution to the upconversion emission of the glass ceramic is due to Tm3+–Yb3+ ions in the vitreous phase.

Lanthanum tungstates with a La/W atomic ratio between 6 and 4.8 have been synthesized as polycrystalline materials using the freeze-drying wet-chemical precursor method. Our results show that a single phase material is obtained when the La/W ratio is between 5.3 and 5.7 (T = 1500 °C). Outside this compositional range, segregation of either La₂O₃ (La/W ≥ 5.8) or La₆W₂O₁₅ (La/W ≤ 5.2) are found. We have solved the crystal structure for the composition with a La/W nominal atomic ratio of 5.6 by combining powder X-ray and powder neutron diffraction techniques. This structure substantially differs from that previously reported for Ln₆WO₁₂ (Ln = Y, Ho). The main differences between the two structure types are the crystal symmetry, the different coordination environment of the cations and the formula unit. The formula unit can be written as La_6.63W_1.17O_13.43 (Z = 4; calculated density = 6.395 g/cm³), well in accordance with the diffraction techniques, He-pycnometry and electron probe microanalysis. These materials can be described as a face centred cubic structure with space group F3m. Lattice parameters vary between 11.173 and 11.188 Å, depending on composition. Dense ceramic materials are obtained at 1400 °C, and microanalyses measurements indicate that no significant tungsten evaporation occurs compared to the nominal values. Compositions with La₂O₃ segregation show similar conductivity values as the single phase ones, but those containing segregation of W-rich phases show a considerable drop in conductivity with increasing content of the secondary phase.