Electron paramagnetic resonance and polarized optical absorption spectra of Ni²⁺ in synthetic forsterite

Abstract

The investigated Ni doped forsterite was grown with the floating zone technique. The EPR spectra were taken at room temperature using both 9.5 and 35 GHz. All specimens show EPR signals resulting from Mn²⁺ at M2 and Fe³⁺ at M1, M2, and Si positions. Ni²⁺ EPR signals are observed at 35 GHz but not at 9.5 GHz. The Ni²⁺ spectra are described by the spin Hamiltonian

$$H = \beta SgB + D\left( {S_{\text{z}}^{\text{2}} - \left( {S + 1} \right)S/3} \right) + E\left( {S_{\text{x}}^{\text{2}} - S_{\text{y}}^{\text{2}} } \right)$$

with S=1. The EPR parameters obtained are: D=−39.56 cm⁻¹, E=−0.58 cm⁻¹, g _x=2.194, g _y=2.160, g _z=2.188. Polarized optical absorption spectra of Ni²⁺ were taken at room and liquid nitrogen temperature covering the range from 5500 to 42000 cm⁻¹. The spectra are nearly independent of the polarization of light. Both EPR and optical spectra reveal that Ni²⁺ is ordered into M1. The local symmetry of the Ni(M1) complex is described by the point group D _2h with the main symmetry axis along a. Heating experiments in air up to 1000° C and three days indicated no change in the Ni ordering in this synthetic sample.