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

Abstract

Isostructural HfGeO4 and ZrGeO4, activated with Ti4+, 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 Ti4+ 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 MGeO4 (M = Zr, Hf) are assigned through shifts of the Raman bands in a Hf1-xZrxGeO4 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 ZrGeO4 and HfGeO4 powder samples and micro-Raman spectra of single crystals of ZrGeO4:Ti4+ confirm that Ti4+ occupies the Ge4+ site in ZrGeO4 and in HfGeO4. In addition, not all of the GeO44- modes are strongly affected by Ti4+ doping, even though all of the modes most affected by Ti4+ doping arise from the GeO44- group. It is observed that Ti4+ substitution for Ge4+ significantly affects Raman bands arising from GeO44- rotational and bending motions about the c-axis, within the ab-plane.