Site Occupancy of Ti4+-Doped ZrGeO4 and HfGeO4 Probed by Raman Spectroscopy

Isostructural HfGeO 4 and ZrGeO 4, activated with Ti 4+, yield blue luminescence when irradiated with X-rays. An adequate understanding of the observed blue luminescence requires the determination of the site occupancy and the local Ti 4+ environment; i.e., the site symmetry. Raman spectroscopy has been used to probe the chemical bonding and crystal structure of metal germanates. The group vibrational modes of MGeO 4 (M = Zr, Hf) are assigned through shifts of the Raman bands in a Hf 1 - x Zr x GeO 4 solid solution series and through the application of the Raman polarization selection rules to micro-Raman spectra of single crystals. Macro-Raman spectra of undoped and titanium-activated ZrGeO 4 and HfGeO 4 powder samples and micro-Raman spectra of single crystals of ZrGeO 4:Ti 4+ confirm that Ti 4+ occupies the Ge 4+ site in ZrGeO 4 and in HfGeO 4 . In addition, not all of the GeO 4 4 - modes are strongly affected by Ti 4+ doping, even though all of the modes most affected by Ti 4+ doping arise from the GeO 4 4 - group. It is observed that Ti 4+ substitution for Ge 4+ significantly affects Raman bands arising from GeO 4 4 - rotational and bending motions about the c -axis, within the ab -plane.