Miron B. Kosmyna

Comparative characteristics of spontaneous and stimulated Raman scattering in Pb(MoO₄)_1−x(WO₄)_x single crystals (x = 0, 0.5, 0.8, and 1.0) including both the high (ν₁) and low (ν₂) frequency internal anionic group vibrations have been investigated. It was found that among these crystals, Pb(MoO₄)_0.2(WO₄)_0.8 is the most suitable for multi-wavelength Raman laser in several Raman modes simultaneously. This is caused by the optimal relative Mo/W content corresponding to the most efficient coherent combination of the (MoO₄)²⁻ and (WO₄)²⁻ vibrations enhancing the output radiation characteristics of the synchronously pumped Pb(MoO₄)_0.2(WO₄)_0.8 Raman laser. Oscillation of up to twelve, closely spaced SRS components in a range of 1128–1360 nm and the strongest pulse shortening down to 1.16 ps in comparison with not only PbMoO₄ and PbWO₄ but also with all the earlier investigated nominally pure scheelite-like tungstate and molybdate crystals has been achieved.

The structures of polycrystalline Ca ₃ RE ₂ (BO ₃ ) ₄ (RE = La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Y; space group Pnma ) orthoborates were determined using powder X-ray diffraction. Trends in the unit-cell dimensions and yet unreported trends in other structural properties (interatomic distances and the fractional occupation of three Ca/RE sites) for these compounds are demonstrated as a function of RE ionic radius. The unit-cell volume and a unit-cell parameter present a linear dependence, while the b and c unit-cell parameters change in a nonlinear manner. For the whole series, the RE atoms are present at all three cationic sites (labelled as M 1, M 2 and M 3), but the fractional occupancies depend on the RE ionic radius. The small rare-earth atoms tend to enter mainly the M 3 site; for the larger rare earths, the occupancy of this site decreases sharply. The occupancy of the M 1 site by RE atoms is around 0.5 and tends to increase with increasing RE ionic radius. The M 2 site is the least preferentially occupied by RE ions, but the occupancy discernibly increases with rising radius as well. These findings are assembled with properties of isostructural strontium and barium borates, allowing prediction of occupancy schemes for not yet investigated compounds from the A ₃ RE ₂ (BO ₃ ) ₄ ( A = Ca, Ba, Sr).

The structure of calcium europium orthoborate, Ca₃Eu₂(BO₃)₄, was determined using high-resolution powder X-ray diffraction data collected at the ID22 beamline (ESRF) under ambient conditions, as well as at high temperature. Rietveld refinement allowed determination of the lattice constants and structural details, including the Ca/Eu ratios at the three cationic sites and their evolution with temperature. Clear thermal expansion anisotropy was found, and slope changes of lattice-constant dependencies on temperature were observed at 923 K. Above this temperature the changes in occupation of the Ca/Eu sites occur, exhibiting a tendency towards a more uniform Eu distribution over the three Ca/Eu sites. Possible structural origins of the observed thermal expansion anisotropy are discussed.

Crystal growth and thermal properties of binary borates, Ca₃RE₂(BO₃)₄ (RE = Y, Gd, Nd), are considered promising crystals for laser applications. These single crystals were grown by the Czochralski method. The crystal and defect structure were characterized. Volumetric chemical methods without prior separation of the components were developed and applied for the determination of the dependence of chemical compositions of the crystals on the growth conditions. The thermal conductivity was investigated in the 50–300 K range. The character of the temperature dependence of thermal conductivity was found to be similar to that of glass. The possible reasons of the observed features of the thermal conductivity were analyzed.

Large-volume (V ≅ 350 cm³) CdWO₄ single crystals of high optical quality have been grown by the Czochralski method. A scintillation detection unit based on a large CdWO₄ crystal has been produced and its characteristics have been studied.