Stress and Temperature Dependence of the Paramagnetic Resonance Spectrum of Nickel Fluosilicate

The crystalline field splitting of the ground state spin triplet of divalent nickel in the fluosilicate, NiSi${\mathrm{F}}_6$·6 ${\mathrm{H}}_2$O, has been measured at room temperature as a function of hydrostatic pressure to 10 000 kg/${\mathrm{cm}}^2$ and uniaxial stress to 150 kg/${\mathrm{cm}}^2$. The anisotropic compressibility and thermal expansion of this trigonal crystal have also been determined. Combining these data with the known variation of the splitting with temperature, its dependence on isothermal unit cell geometry and on temperature at constant unit cell dimensions is calculated. The splitting is found to be independent of volume within experimental error but proves to be quite sensitive to unit cell shape. The deduced explicit temperature dependence is three times larger than that measured at atmospheric pressure. The magnitude and geometrical variation of the crystalline field splitting may be qualitatively understood using a static, ionic model of the ${\left(\mathrm{N}\mathrm{i}\mathrm{·}6{\mathrm{H}}_2\mathrm{O}\right)}^{2+}$ octahedral complex. A rather general analysis of the explicit temperature dependence indicates, however, that low-frequency lattice vibrations play a dominant role in determining the observed value of the splitting.