EPR Investigations of Er3+, Yb3+, and Gd3+ in Zircon-Structure Silicates

The EPR spectra of Er3+, Yb3+, and Gd3+ have been observed in the host single crystals ZrSiO4, HfSiO4, and ThSiO4 which were grown by a flux-gradient technique. For most systems, spectra were observed which exhibited both tetragonal and orthorhombic symmetry. The tetragonal spectra were characterized by a principal axis system which coincided with the principal crystallographic axes. The observed orthorhombic spectra were of two types. For Gd3+ in ZrSiO4 and HfSiO4, orthorhombic spectra due to eight inequivalent sites were observed and had principal axis systems which were not simply related to the host crystallographic axes. All other orthorhombic spectra resulted from four inequivalent sites whose z and x axes lay in {110} planes with equal angles between the crystal tetragonal symmetry axis (c axis) and each z axis. For both types of orthorhombic spectra, the corresponding EPR transitions from the different sites were super-imposed with the applied magnetic field parallel to the c axis. Spin-Hamiltonian parameters have been determined for the observed tetragonal and orthorhombic spectra. Powder pattern analysis was employed for the Gd3+ orthorhombic spectra, due to the complexity which resulted from the large number of inequivalent sites. The implications of these results regarding the nature of the various sites are examined in conjunction with the results obtained previously by others for impurities in zircon-structure and scheelite-structure hosts.