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The work presented is mainly focused on synthesis and study of structural and optical properties of microcrystalline Nd3+-doped monoclinic dilanthanum dimolybdate at both room and cyrogenic temperatures (4K and 77 K). These compounds might be useful for application in the future as optical materials and also as transparent ceramics when the structure is cubic. The Nd3+-doped phases with monoclinic structure (α-form, space group P21, unit cell parameters a = 7:1426, b = 7:1544, c = 7:1618 Å and β = 89:538°) were observed for a concentration of the optically active ions equal to 5%. When the concentration of the Nd3+ ions is higher than 15%, a cubic structure is formed (β-form, space group P213, with the lattice parameter a = 7:155±0:005 Å). A series of Nd3+- doped La2Mo2O9 phases with different concentration of Nd3+ were prepared using conventional solid-state reactions. The formation of phase-pure Nd3+-doped La2Mo2O9 has been monitored by powder X-ray diffraction, DSC, SEM, Raman, and FT-IR absorption techniques. High-resolution absorption and emission spectra, as well as the dynamics of the Nd3+ excited states characterized by decay time measurements were recorded from room temperature to 4 K. At least two slightly different crystallographic sites are available for the Nd3+ ions. First results show that this new Nd3+-doped monoclinic La2Mo2O9 molybdate phosphor is promising for applications of ultra-short pulse lasers.

Yb3+-doped mixed molybdato-tungstate powders of chemical solution La2–x Yb x MoWO9, which crystallize in the cubic system with the space group P213 (No. 198), were studied so that in further research they could serve to the fabrication of transparent optical ceramics. The powders were activated by Yb3+ ions with the concentration of (0.1, 1, 3, 10, 20 mol %) and were prepared by three various techniques: high-temperature solid-state reaction, Pechini, and combustion methods. The main goal of this Part 1 is primarily concerned with the structural characterizations of the cubic structure by XRD analysis, FT-IR, and Raman spectroscopies and of the morphology by SEM and TEM techniques, as well as the calculation of the optical band gap by diffuse reflectance spectroscopy. We have also checked the thermal stability of the samples prepared by combustion and solid-state methods. The TEM images indicated that via combustion method at 600 °C we succeeded to obtain the smallest nanocrystallites with average diameters between 60 and 80 nm. Obviously, the grain sizes increased with the rise of the annealing temperature applied during various types of synthesis. The morphology also differed according to combustion, Pechini, and solid-state methods used for fabrication. The first synthesis of translucent microceramics are also shown.

Investigations of spectroscopic properties were performed for Yb3+-doped solid solutions of chemical formulas based on mixed La2MoWO9 molybdato-tungstate powders that crystallize in the cubic system with the space group P213 (No. 198) and from which it was also possible to prepare the first translucent optical ceramics. Samples activated by the Yb3+ ion in a wide concentration range were synthesized by three various techniques: high-temperature solid-state reaction, the Pechini method, and the combustion method. The microcrystalline solid solutions obtained by the high-temperature solid-state reaction characterized by intense luminescence are useful for detailed fundamental analysis. The direct excitation of Yb3+ into 2F7/2 → 2F5/2 absorption at 940–980 nm leads to reversed 2F5/2 → 2F7/2 transitions giving Yb3+ emission lines in the 970–1100 nm range. The absorption and emission 0-phonon lines of Yb3+ ions were also used as structural probes at a low temperature, and the conjugation with SEM and TEM techniques was particularly useful here. The multisite character of Yb3+ was confirmed in high-resolution site-selective emission spectra. In the case of microcrystalline ceramics, the grains are characterized by a wide 0-phonon line around 976 nm and a high number of multisites and white points by another sharper line around 968 nm. Based on the absorption and emission spectra, the Yb3+ electronic energy level diagram was proposed for the main site. The effect of dopant concentration as well as the grain size influence on the luminescent properties and the decay times were analyzed in order to attempt to understand the concentration quenching mechanism and estimate the parameters useful for a theoretical approach to laser potential first with cubic single crystals and then with cubic transparent ceramics. This second part is related to the spectroscopic properties of powders and microceramic samples analyzed in the first part.

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