Electron paramagnetic resonance and electron nuclear double resonance of NaCl:Rh2+: A comparison between similarly doped Bridgman- and solution-grown single crystals

Abstract

In a recent electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) study several new rhodium related centres in a Bridgman-grown crystal were reported. For two centres a detailed microscopic model was established. The primary Rh2+ centre was identified as a [RhCl6]4− complex with a single charge compensating Na+ vacancy in a next nearest neighbour position. A second centre was identified as a rhodium dimer. Two other Rh2+ centres, with apparent axial symmetry, remained unidentified. In the present study a detailed comparison between Rh2+ defects in Bridgman- and solution-grown NaCl crystals is presented. One of the previously unidentified Rh2+ centres is shown not to be axial but to have orthorhombic-I symmetry and is identified as a cis-[RhCl6]4− complex with two next nearest neighbour Na+ vacancies. For the primary centre additional ENDOR interactions confirm the model mentioned above. Finally a comparison is made between the temperature stability of the primary and orthorhombic-I centre in Bridgman-grown and similarly doped solution-grown NaCl crystals.