Anomalous thermal expansion of cobalt olivine, Co₂SiO₄, at low temperatures

The temperature variation of the Co₂SiO₄ structural parameters has been investigated by means of both neutron powder and single-crystal diffraction. Measurements were performed in a broad temperature range from 5 to 500 K. There is clear evidence of an anomalous thermal expansion related to the magnetic phase transition at 50 K. Below this temperature an abrupt elongation and contraction were observed along the c and b axes (space group Pnma), respectively, whereas the lattice parameter a remains constant in the range between 5 and 50 K. The resulting volume of the unit cell is contracted below 50 K. This phenomenon is attributed to magnetostriction effects. A model based on the specific heat capacity data was applied for the quantitative description of the anomalous thermal expansion behaviour. On the other hand, no anomalies can be observed in the temperature dependency of the interatomic distances and angles.