Magnetic and Other Studies of a Reversible Phase Change in CoSiF6·6H2O

Abstract

Magnetic, optical, and thermal expansion studies reveal a reversible phase transition in CoSiF₆·6H₂O at a temperature, T_c, which was found to be 246°±2°K during cooling and 259°±1°K during heating. This thermal hysteresis appears to be real as T_c is independent of the heating or cooling rate. The magnetic symmetry of the crystal is uniaxial above T_c but orthorhombic below it. The sharp change in color from pink to orange‐yellow at T_c is accompanied by spectral changes with the appearance of a new band, presumably indicating the effect of changes in the closed‐packed structure on the symmetry and shape of the Co(H₂O)₆ clusters. Crystals diluted with the similar zinc salt have an approximately uniaxial magnetic ellipsoid, a feeble anisotropy appearing in the ``symmetry'' plane at low temperatures. From these and the available resonance data the g values for the undiluted crystal have been estimated which resemble more closely those for a typical orthorhombic case than for a trigonal.