Electron paramagnetic resonance of divalent nickel in the low temperature phases of magnesium fluosilicate and magnesium fluotitanate

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Abstract
The EPR spectra of Ni2+ in single crystals of MgSiF6⋅6H2O and MgTiF6⋅6H2O have been studied at and above 77 °K in the frequency ranges 12–18 and 23–25 GHz. Phase transitions occur on warming the crystals above room temperature at (302±1) °K in MgSiF6⋅6H2O and at (318±1) °K in MgTiF6⋅6H2O, and on cooling at (301±1) °K for both crystals. The EPR spectra at 77 °K were fitted to the spin Hamiltonian H=gβH⋅S+DSz2 +E (Sx2−Sy2) with g=2.250±0.005, ‖D‖= (2.16±0.01) cm−1 and E= (0.210±0.001) cm−1 for MgSiF6⋅6H2O and g=2.245±0.005, ‖D‖= (2.3±0.1) cm−1 and E= (0.149±0.001) cm−1 for MgTiF6⋅6H2O. In MgSiF6⋅6H2O, the spectrum in the perpendicular plane showed a sixfold rotational symmetry and consisted of two sets of three lines related by a rotation of (22±1) ° about the c axis. In MgTiF6⋅6H2O, a single three line spectrum with a sixfold rotational symmetry was observed in the perpendicular plane. In both crystals, the parameter E increased appreciably with temperature. In MgSiF6⋅6H2O, the temperature gradient ∂E/∂T was approximately 3.3×10−4 cm−1/°K in the range 77 to 240 °K and then decreased with increasing temperature above 240 °K. In MgTiF6⋅6H2O, ∂E/∂T was approximately 4.7×10−4 cm−1/°K in the range 120 to 290 °K and then decreased with decreasing temperature below 120 °K.