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EPR of CrO 4 3− Radicals in High Temperature Phases of (NH 4 ) 3 H(SeO 4 ) 2 Crystal
Author(s) -
Aucustyniak M. A.,
Hoffmann S. K.
Publication year - 1995
Publication title -
physica status solidi (b)
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.51
H-Index - 109
eISSN - 1521-3951
pISSN - 0370-1972
DOI - 10.1002/pssb.2221900124
Subject(s) - electron paramagnetic resonance , phase transition , crystallography , radical , crystal structure , crystal (programming language) , chemistry , atmospheric temperature range , phase (matter) , condensed matter physics , materials science , nuclear magnetic resonance , physics , thermodynamics , organic chemistry , computer science , programming language
Abstract EPR X‐band studies of CrO 4 3−radicals substituting SeO 4 , groups in (NH 4 ) 3 H(SeO 4 ) 2 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 CrO 4 3‐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 CrO 3‐ 4 radical is different from that, predicted by the geometry of the host SeO, groups, and is described by the d x 2 – y 2ground state, characteristic of an elongated tetrahedron of distorted D 2d symmetry. Only part of the CrO 3‐ 4 radicals show the dynamics expected for SeO 4 groups and follow their behaviour at the phase transitions. The transition to the superionic phase II 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 4 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 4 group.