Hyperfine structure of EPR spectra of Gd$^{3+}$ odd isotopes in PbMoO$_4$, Pb$_5$Ge$_3$O$_{11}$, YVO$_4$, and quadrupole interaction (temperature dependence)

A hyperfine structure of EPR signals of odd isotopes Gd$^{3+}$ in Pb$_5$Ge$_3$O$_{11}$, PbMoO$_4$, and YVO$_4$ single crystals has been investigated at different temperatures. The observation of forbidden (with the nuclear spin flip) transitions has made it possible to determine quadrupole interaction $P^0_2$ associated with the gradient of the electric field of ligands at the impurity. It has been shown for the first time that, under the condition $|P^0_2|\ge|A_{x,y}|$ ($A_i$ are the tensor components), not only the magnitudes of splitting but also the observed asymmetry in a hyperfine structure (in perpendicular orientations of the magnetic field) depends on mutual signs of parameters of initial splitting $b^0_2$ and $P^0_2$. Results of studying the spectra have demonstrated that $|b^0_2(T)|/|P^0_2(T)|\sim\operatorname{const}$ for a concrete single crystal, which assumes the similarity of physical mechanisms determining these parameters.