Low-temperature phase transitions of leonite-type compounds, K2Me2+(SO4)2 · 4H2O (Me = Mg, Mn, Fe), are investigated by temperature dependent measurements of single-crystal X-ray reflection intensities
and lattice parameters. The transition temperatures and the progress of the transitions are determined by birefringence data
and differential scanning calorimetry. The cause for the phase transitions of leonite-type compounds is a dynamic disorder
of sulphate groups at room temperature (C2/m), that freezes in to an ordered structure (I2/a) at −4(1) °C in leonite, K2Mg(SO4)2 · 4H2O. At −153(1) °C the crystal structure switches to another ordered phase (P21/a). The Mn analogue shows the same succession with transition temperatures at −68(1) °C and −104(1) °C. The disordered room
temperature structure of the isotypic mineral mereiterite, K2Fe(SO4)2 · 4H2O, transforms directly to the ordered P21/a structure at 3(2) °C.
Analysis of X-ray intensities and of excess birefringence reveals that the displacive I2/a ⇔ P21/a phase transition of leonite and Mn-leonite is first order. According to Landau theory the C2/m ⇔ I2/a (leonite, Mn-leonite) and C2/m ⇔ P21/a (mereiterite) order-disorder transitions are almost tricritical.
Received March 7, 2001; revised version accepted June 27, 2001