Temperature dependences of the parameters of the initial splitting of Gd$^{3+}$ in CaMoO$_{4}$ (lattice and spin–phonon contributions)

The electron paramagnetic resonance (EPR) spectra of Gd$^{3+}$ impurity centers in CaMoO$_{4}$ single crystals have been investigated at temperatures $T$ = 1.8, 4.2, and 99–300 K. The temperature dependences of the spin Hamiltonian parameters $b_{n}^{m}(T)$ ($n$ = 2, 4, $m$ = 0, 4) have been determined, and their analysis has been carried out. The temperature contributions from the static lattice, $b_{2}$(L) and $b_{4m}$(L), to the spin Hamiltonian parameters $b_{n}^{m}(T)$ have been separated. For this purpose, the changes of the static lattice contributions $b_{2}$(L) and $b_{4m}$(L) have been determined taking into account the temperature shifts of the oxygen ions nearest to the Gd$^{3+}$ ions in CaMoO$_{4}$ single crystals. The differences $b_{nm}(F) =b_{n}^{m}-b_{nm}$(L) have been attributed to the spin–phonon contribution. The analysis of the results obtained has demonstrated that the parameters $b_{20}(F)$ and $b_{4m}(F)$ are positive, and the dependence of $b_{20}(F)$ on the temperature $T$ is well described in the model of local vibrations (G. Pfister). However, the temperature behavior of the parameter $b_{4m}(F)$ could not be described in the framework of the well-known models of the spin–phonon interaction.