Single-crystal EPR investigation of Mn(II)-doped biomineral cobalt ammonium phosphate hexahydrate: a case of multiple substitutions

Single-crystal electron paramagnetic resonance (EPR) study of Mn(II)-doped cobalt ammonium phosphate hexahydrate has been carried out at room temperature. The impurity shows more than 30 line pattern EPR spectra along the three crystallographic axes, suggesting the existence of more than one type of impurity ion in the host lattice. The spin Hamiltonian parameters, estimated from the three mutually orthogonal crystal rotations, are: site 1: g _xx =1.989, g _yy =1.994, g _zz =1.999; A _xx =?8.97, A _yy =?9.52, A _zz =?9.71 mT; D _xx =?8.09 mT, D _yy =?6.05 mT, D _zz =14.14 mT; site 2: g _xx =1.988, g _yy =2.009, g _zz =2.019; A _xx =?9.11 mT, A _yy =?9.58 mT, A _zz =?9.93 mT; D _xx =?6.61 mT, D _yy =?6.11 mT, D _zz =12.72 mT. The angular variation studies further reveal that the Mn(II) impurities enter the lattice substitutionally. The other Mn(II) sites which are at interstitial locations are difficult to follow due to their low intensity. The variation of zero-field splitting parameter with temperature indicates no phase transition. The observation of well-resolved Mn(II) spectrum at room temperature has been interpreted in terms of ‘host spin-lattice relaxation narrowing’ mechanism.