Zero-field splitting in NiSiF6−6H2O and its pressure, stress and temperature dependences: A supplementary study

The zero-field splitting in NiSiF₆?6H₂O was studied as a function, of hydrostatic pressure and uniaxial stress at temperatures above and below 220 K, at which the crystal exhibits a secondorder phase transition. The anisotropy of the thermal expansion coefficients parallel and perpendicular to the trigonal symmetry axis was measured in a wide range of temperatures. On the basis of the obtained data the spin-phonon interaction has been separated from the thermal expansion of the crystal. At high and low temperatures the two contributions have opposite signs and exhibit much higher values above 220 K than below this temperature. This is explained by the high anisotropy of the thermal expansion of the crystal characteristic of the high-temperature phase.