EPR Study of VO$\text{}^{2+}$ Center in Fast Proton Conductor K$\text{}_{3}$H(SO$\text{}_{4}$)$\text{}_{2}$

The pretransitional phenomena of superprotonic phase transition (T$\text{}_{sp}$=471 K) were studied in detail by X-band continuous wave EPR spectra of K$\text{}_{3}$H(SO$\text{}_{4}$)$\text{}_{2}$ crystal doped with VO$\text{}^{2+}$ ions. Three kinds of VO$\text{}^{2+}$ complexes (magnetically equivalent but structurally non-equivalent) denoted as VO$\text{}^{2+}$(I), VO$\text{}^{2+}$(II), and VO$\text{}^{2+}$(III) were found. "Smearing out" of the superhyperfine structure was observed as a result of interbond proton motion. VO$\text{}^{2+}$ impurity replaces K$\text{}^{+}$ ion and experiences the same average crystal field gradient at T$\text{}_{sp}$=471 K for I- and II-type complexes. The increase in interbond proton frequency hopping above 360 K is a reason of significant line broadening.