Phase transitions and low-temperature structures of leonite-type minerals and synthetic compounds

Research project P 13728 Phase transitions in leonite-type minerals Eugen LIBOWITZKY 28.06.1999 Phase transitions change important physical properties of geological material and technically applied crystals. In the proposed project phase transitions and low-temperature structures of leonite-type minerals and synthetic compounds will be studied. The mineral leonite, K2Mg[SO4]2 . 4H2O, belongs to a wide group of minerals and synthetic compounds with the general formula A2B[ZO4]2 . 4H2O with A = K, Rb, Cs, NH4, TI; B = Mg, Mn, Fe, Co, Ni, Cu, Zn; Z = S, Se. Crystals of the compounds can be synthesised from aqueous solutions of the respective compounds. The leonite structure is composed of ZO4 tetrahedra and BO6 octahedra interconnected by the large A cations in 8- to 10- coordination and hydrogen bonds of the water molecules. As indicated by half occupancy of split oxygen atom sites, one of the Z04 groups is disordered. This disorder involves also the protons and H bonds of the H20 molecules. In a preliminary investigation, the birefringence versus temperature of leonite and Mn-leonite was determined. The resulting curves show non-linearities and discontinuities at low temperatures, which indicate at least one (may be two) reversible phase transition(s), probably changing from a structure with a dynamically disordered ZO4 group to a low-temperature structure with an ordered arrangement of sulphate groups. During the project, which is part of a continued work on hydrous species and phase transitions in minerals, phase transitions and low-temperature phases of leonite-type minerals and compounds will be studied, structurally refined, and characterised by different physical properties. Synthesis of as many as possible leonite-type compounds as well as Na and D substituted material will deliver the experimental starting material. Structural investigations using single-crystal X-ray diffraction at ambient and lowtemperature conditions will reveal the structural changes of the low-temperature phases below the transition temperatures. Vibrational spectroscopy (infrared and Raman) will reveal information on the dynamics of the phase transitions and constrain the hydrogen bond systems in terms of bond lengths and 0-H vector orientations (using also deuterated samples). Various physical properties shall be measured versus temperature (e.g. birefringence, differential scanning calorimetry, lattice parameters) to reveal the type of phase transition and to determine the exact transition temperatures. Elasticity measurements and neutron diffraction will be performed in cooperation with external institutions. The results of the different techniques may be reconciled in this well studied system and will help to understand the temperature dependent dynamic effects. The expected transitions to acentric space groups may be technically important (piezoelectrics,...). Finally, the role of hydrogen bonds and chemical (Na/K) and isotopic (D/H) substitution during the transitions will be better understood.

In the course of the project P13728-GEO phase transitions in leonite-type minerals and synthetic compounds were investigated at low temperatures. Minerals occur frequently in nature and as inorganic, natural, crystalline matter they are ideal substances in the search for new materials with specific technological applications. Variations of the environment, in terms of temperature, pressure, chemical composition, etc., cause changes of important physical properties of minerals over geological time. If these changes occcur instantaneously at discrete temperatures and pressures this is called phase transitions. The mineral leonite, K2 Mg(SO 4 ) 2 4H2 O, gives the name for a group of isotypic sulphate minerals and synthetic compounds. In addition to leonite, crystals of two further members of the group have been synthesized up to 3 cm in size (mereiterite, K2 Fe(SO4 ) 2 4H2 O, and Mn-leonite, K2 Mn(SO 4 ) 2 4H2 O). These crystals were investigated at room and low temperatures down to -190C by X-ray and neutron diffraction as well as by optical, calorimetric and spectroscopic methods. All three compounds show reversible phase transitions at low temperatures caused by a dynamic disorder of sulphate groups at room temperature, that freeze to one or two successive ordered structures. During the project, which is part of a continued work on hydrous species and phase transitions in minerals at the Institut für Mineralogie und Kristallographie (Universität Wien), the crystal structures of the three leonite-type compounds have been solved and refined, and the phase transitions have been characterized by several physical properties. Structural investigations using single-crystal X-ray diffraction at ambient and low temperatures revealed the structural changes of the low-temperature phases below the transition temperatures. With this structural background the phase transition temperatures and the behaviour of the transitions were determined by optical and physical methods, revealing the type of the phase transitions. According to a thermodynamic classification the first transition in the leonite-type compounds is tricritical whereas the second phase transition at lower temperatures is a first order type. Vibrational spectroscopy provided information on the dynamics of the phase transitions and constrained the hydrogen bond system of the leonite-type compounds. A special method of procession of spectroscopic data facilitated the interpretation of the spectroscopic results in agreement with thermodynamic data. Different experimental techniques such as X-ray and neutron diffraction, vibrational spectroscopy, optical and calorimetric methods were used during the project, each pointing out new aspects of the phase transitions. Realising the complementary character of the different techniques and reconciling the results within a certain thermodynamic concept were the major achievement. The knowledge gained by this research project is a contribution to the understanding of phase transitions in minerals.