EPR of the CuBr 4(NH 3) 22− paramagnetic centre in ammonium bromide

Abstract

Ammonium bromide crystals doped with Cu 2+ ions and grown from a basic solution have been studied at temperatures ranging from 4.2 to 300 K with Q-band EPR. The NH 4Br : Cu 2+ crystals grown under these conditions contain CuBr 4(NH 3) 2 2− paramagnetic centres showing sensibility to NH 4Br phase transitions. A detailed EPR study of this centre was conducted to determine some spin Hamiltonian parameters for the first time, as the NH 4Br : Cu 2+ crystals undergo phase transitions below room temperature. This study showed that the paramagnetic centre exhibited tetragonal symmetry at room temperature and below 78 K (a vestige of orthorhombic symmetry remained in this region), while at 78 K < T < 235 K the tetragonal and orthorhombic symmetries coexist. Models are mentioned for these two simultaneous symmetries.