Nd pronounced anharmonicity in IR spectra of CaWO4 single crystals

CaWO4 and Nd:CaWO4 (0.8 at. % Nd) single crystals were grown from the melt by the Czochralski technique in air. The crystal growth parameters dc and ?c were calculated by equations from the hydrodynamics of the melt, whereas the rate of crystal growth was experimentally obtained. The infrared reflectance spectra of Nd:CaWO4 and CaWO4 single crystals were recorded in the range 50-5000 cm-1 at room and liquid nitrogen temperatures. Two oxygen Eu modes showed a splitting. That especially concerns one of them where the splitting is even more pronounced with the Nd doping. Instead of the earlier attempts based on a symmetry breaking or an activation of forbidden modes, we suggest that a strong anharmonicity is the source of this effect.