Electron paramagnetic resonance studies of VO²⁺ in nitrate single crystals. II NH₄NO₃, NaNO₃, and Ba(NO₃)₂

Abstract

The electron paramagnetic resonance studies of VO²⁺ doped in NH₄NO₃, NaNO₃, and Ba(NO₃)₂ single crystals have been performed over a wide temperature range. At room temperature an isotropic octet exhibiting mI dependence of linewidths is obtained in all the three crystals, which is thought to be due to a very fast readjustment of VO²⁺ ions which brings the system to an effective liquid with low viscosity. As the temperature is lowered below −20^° C an anistropic spectrum is obtained in VO²⁺-doped NH₄NO₃ which is presumably due to hindered rotation of VO²⁺ ions. The hindered motion of vanadyl ion below −20^°C, in spite of volume expansion associated with a phase transition at that temperature, is attributed to the effect of oxygens of NO₃⁻ ions on VO²⁺ ions. The anisotropic spectrum is observed at and below −80^°C in Ba(NO₃)₂, and at −196^°C in NaNO₃. A comparison of the present results with those of VO²⁺ in KNO₃ and CsNO₃ reported earlier by us is made, and the results are examined in relation to the recently reported low-temperature phase transitions in these crystals. A probable phase transition around −110^°C in CsNO₃ is suggested. The m_I dependence of linewidths for the room-temperature spectra is studied using Kivelson's formulism.