Phase transitions and thermal expansion in langbeinite type compounds

We report thermal expansion measurements using capacitance dilatometric methods on several polar and non-polar compounds of the langbeinite family, from liquid helium to room temperature. In K ₂Mn ₂(SO ₄) ₃ and Tl ₂Cd ₂(SO ₄) ₃ new thermal expansion anomalies indicative of phase transitions were detected. Similarly, two phase transitions were detected for the first time at 227.8 K and 330.8 K respectively in Tl ₂Mg ₂(SO ₄) ₃. In (NH ₄) ₂Mg ₂(SO ₄) ₃ two phase transitions were discovered at 220 K and 241 K with decreasing temperature, although with increasing temperature only one phase transition could be detected.