EPR of CrO Radicals in High Temperature Phases of (NH₄)₃H(SeO₄)₂ Crystal

Abstract

EPR X-band studies of Cr0:-radicals substituting SeO, groups in (NH,),H(SeO,), crystals are performed in the temperature range 280 to 400 K, where transitions to two superionic phases with protonic conductivity appear at 304 and 330 K. In the room temperature ferroelastic phase four types of Cr0:-radicals with different g-factors and direction cosines are identified. Three of them (with 40% of the total spectrum intensity) are identified as Halperin-Varma-type defects. These defects vanish in superionic phases. The electronic structure of the Cr0:-radical is different from that, predicted by the geometry of the host SeO, groups, and is described by the d,,-,2 ground state, characteristic of an elongated tetrahedron of distorted D,, symmetry. Only part of the CrO:-radicals show the dynamics expected for SeO, groups and follow their behaviour at the phase transitions. The transition to the superionic phase I1 at 304 K is observed as a rapid EPR line narrowing due to an increase in the crystal lattice dynamics and an increase in the SeO, reorientation rate. In the phase transition to the superionic phase I (330 K) a shift and broadening of the lines are observed. It allows to suggest that the crystal structure of phase I is dynamically disordered with two equally populated orientations of the SeO, group.