Multiple phase transitions of leonite-type compounds: Optical, calorimetric, and X-ray data

Autor(en)

Birgit Hertweck, Thomas Armbruster, Eugen Libowitzky

Abstrakt

Low-temperature phase transitions of leonite-type compounds, K2Me2+(SO4)2 ž 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 ž 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 ž 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.

Organisation(en)

Institut für Mineralogie und Kristallographie

Externe Organisation(en)

Universität Wien, Universität Bern

Journal

Mineralogy and Petrology

Band

75

Seiten

245-259

Anzahl der Seiten

15

ISSN

0930-0708

DOI

https://doi.org/10.1007/s007100200027

Publikationsdatum

2002

Peer-reviewed

Ja

ÖFOS 2012

1051 Geologie, Mineralogie

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/6d01d703-de89-4e3d-92c4-5b3d0f687326