EPR OF CRO43- OF RADICALS IN HIGH-TEMPERATURE PHASES OF (NH4)(3)H(SEO4)(2) CRYSTAL

EPR X-band studies of CrO43- radicals substituting SeO4 groups in (NH4 )(3)H(SeO4)(2) crystals are performed in the temperature range 280 to 400 K, where transitions to two superionic phases with protonic conduc tivity appear at 304 and 330 K. In the room temperature ferroelastic p hase four types of CrO43- radicals with different g-factors and direct ion cosines are identified. Three of them (with 40% of the total spect rum intensity) are identified as Halperin-Varma-type defects. These de fects vanish in superionic phases. The electronic structure of the CrO 43- radical is different from that, predicted by the geometry of the h ost SeO4 groups, and is described by the d(x2-y2) ground state, charac teristic of an elongated tetrahedron of distorted D-2d symmetry. Only part of the CrO43- radicals show the dynamics expected for SeO4 groups and follow their behaviour at the phase transitions. The transition t o the superionic phase II at 304 K is observed as a rapid EPR line nar rowing due to an increase in the crystal lattice dynamics and an incre ase in the SeO4 reorientation rate. In the phase transition to the sup erionic phase I (330 K) a shift and broadening of the lines are observ ed. It allows to suggest that the crystal structure of phase I is dyna mically disordered with two equally populated orientations of the SeO4 group.