Spin-phonon and lattice contributions to the ground-state splitting of Gd3+ and Eu2+ in scheelite crystals

The EPR spectra of Gd3+ in CaWO4 single crystals have been studied at temperatures T = 1.8, 4.2, and 114–300 K, and the temperature dependence of the parameters b n m (T) of the spin Hamiltonian has been found. The behavior of b 2 0 (T) has been analyzed. The spin-phonon and static lattice contributions b 2 0 (F) and b 2 0 (L) to b 2 0 (T) have been revealed. For this purpose, the variation of b 2 0 (L) has been calculated taking into account the thermal shifts of oxygen ions in CaWO4. Similar analysis has been carried out for CaWO4: Eu2+ based on the EPR data of other authors (Bronstein, Voterra and Harvey, Kiefte). It has been shown that at b 2 0 (F) > 0, the variation of b 2 0 (F) as a function of T for these impurity centers is described well by the Pfister model and a sign change of b 2 0 (T) for Eu2+ is determined by thermal expansion of the lattice.