Investigations on the spin Hamiltonian parameters and the local structures for the irradiation induced Rh2+ and Rh0 centres in NH4Cl

The spin Hamiltonian parameters (SHPs) (i.e., g factors, hyperfine structure constants and superhyperfine parameters) are theoretically investigated for the irradiation induced Rh2+ and Rh0 centres in NH4Cl by use of the perturbation calculations. The tetragonally compressed Rh2+ centre in the form of [RhCl4(H2O)2]2‾ group originates from interstitial Rh2+ (reduced from original Rh3+ of doped RhCl3 by capturing one electron during gamma irradiation) associated with two adjacent axial NH4+ cations substituted by two H2O molecules due to charge compensation. The orthorhombically compressed Rh0 centre in the form of [RhCl4(H2O)2]4‾ group arises from interstitial Rh0 (reduced from Rh3+ by capturing three electrons during gamma irradiation) with two adjacent axial NH4+ replaced by two H2O molecules as charge compensators and the planar Rh0-Cl‾ bond length variation of about 0.042 Ǻ along [011] and [01¯1] axes due to the Jahn-Teller effect. The calculated SHPs are in good agreement with the observed values. The EPR spectra and local structures are discussed for both centres.