Electron Paramagnetic Resonance Studies of VO2+ in Nitrate Single Crystals. II NH4NO3, NaNO3, and Ba(NO3)2

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Abstract
The electron paramagnetic resonance studies of VO2+doped in NH4NO3, NaNO3, and Ba(NO3)2single crystals have been performed over a wide temperature range. At room temperature an isotropic octet exhibiting m I 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.