EPR of Mn2+in NH4Br Single Crystals

The EPR of Mn 2+ in NH 4 Br single crystals has been studied at X-band in the temperature range from 573 K to 77 K. At room temperature, an isotropic spectrum (spectrum-I) comprising a sextet superimposed on a broad line is observed. On heating the crystal up to 400 K the sextet disappears due to explulsion of Mn 2+ impurity from the crystal. On lowering the temperature, the isotropic spectrum-I disappears about at 230 K and a new spectrum (spectrum-II) appears below this temperature. The spectrum-II is highly anistropic and unusual and is believed to be due to very large zero field splitting of Mn 2+ . Spectrum-I is attributed to Mn 2+ ions substituting for NH 4 + with charge compensating NH 4 + vacancies far away from Mn 2+ . The transformation of spectrum-I into spectrum-II is attributed to the structural transformation of NH 4 Br.