169Tm Mossbauer investigation of thulium pyrogermanate

Abstract

A 169Tm Mossbauer investigation of thulium pyrogermanate is reported. All spectra recorded over the temperature range 4.2 K-740 K are pure quadrupole doublets. This observation and the detailed temperature dependence of the quadrupole interaction are inconsistent with recent claims of a fivefold local symmetry at the Tm3+ site. Initial attempts at analysing the data in terms of the full crystal-field Hamiltonian appropriate for C1 symmetry suggest that the electronic ground state consists of a pair of non-magnetic singlets well isolated from the next state.