Local properties of Tm3+ and Yb3+ in TmAsO4 and ordering in YbAsO4 from 169Tm and 170Yb Mössbauer measurements

Abstract
Mössbauer measurements are reported on 169Tm in TmAsO4 and on 170Yb diluted into TmAsO4 and in YbAsO 4. For TmAsO4 we establish a set of crystal field parameters for Tm3+ in the tetragonal phase, above the Jahn-Teller transition temperature (TD = 6.0 K). The same parameters also account for the essential features of the observed ground state g-values for Yb 3+ in TmAsO4 both above and below TD. From the Tm3+ line shape analysis we find that the degeneracy of the lowest non Kramers doublet in the tetragonal phase is lifted by local distortions. Magnetic ordering is observed in YbAsO4 below about 0.3 K