Thermal-expansion study of the displacive phase transitions in ${\mathrm{K}}_2$Re${\mathrm{Cl}}_6$ and ${\mathrm{K}}_2$Os${\mathrm{Cl}}_6$

Measurements of the thermal-expansion coefficient ${\alpha }_{〈111〉}$ in the pseudocubic direction in single crystals of ${\mathrm{K}}_2$Re${\mathrm{Cl}}_6$ and over a wide range of temperature are reported. Pronounced anomalies are noted in the vicinity of the purely displacive phase transitions. It is concluded that it is essential to include the effects of strain interactions in a comprehensive description of these phase transitions.