Phase Transitions in Lawsonite-type Minerals

Lawsonite, CaAl2 (Si2 O7 )(OH) 2 H2 O, is an orthorhombic high-pressure silicate mineral that shows two phase transitions from dynamic disorder of H2 O and OH groups at ambient temperature to structural order and an enhanced hydrogen bond system at low temperatures. In recent years, a number of isotypic minerals and synthetic compounds with Sr, Ba, and Pb substituting for Ca, and Mn 3+ for Al (partially forming solid-solution series), have been discovered. Only hennomartinite, SrMn 2 (Si2 O7 )(OH) 2 H2 O, has been confirmed to show similar order- disorder phase transitions (at higher temperatures due to larger structure dimensions), the other compounds remained unexplored. Thus, the present project aims at an investigation of the phase transitions in endmembers and solid-solution series of the lawsonite group, to understand the influence of changing composition, lattice parameters, bond distances, and structural distortions on critical parameters of the transitions (temperatures, evolution of the order parameter, etc.). To accomplish these tasks, a number of methods will be combined. (i) In a first step, samples will be synthesized in high-pressure equipment such as piston-cylinder / multi-anvil devices at GFZ Potsdam, Germany (unavailable in Vienna). (ii) The obtained crystals will be inspected by scanning electron microscopy and characterized by chemical micro-analysis using energy-dispersive spectroscopy. (iii) X-ray structure analysis on single-crystals and/or powders (depending on grain size) will reveal lattice parameters and information on the bulk structure (long- range order). Vibrational spectroscopy techniques such as (iv) confocal micro-Raman and (v) Fourier-transform infrared spectroscopy will provide details of short-range order, structure dynamics, and hydrogen bonding. (vi) Measurement of various physical parameters vs. temperature such as heat flow, birefringence, etc. will yield the exact transition temperatures and the energetic evolution of the phase transitions and their order parameters. (vii) As an option, e.g. if additional information can be gained or if difficult samples require sophisticated techniques, diffraction and spectroscopy at international large-scale facilities (neutron and synchrotron beam lines) are considered. The project has been designed for a period of 3 years. Due to the difficult sample material that has first to be synthesized in high-pressure experiments at GFZ Potsdam, Germany, and the multiple complex techniques to investigate the crystal structures towards long- and short-range order, dynamics, and phase transitions, a postdoc position (DV) is requested from the FWF. With other costs for material and travel (synthesis-related), a total budget of ~ 215000,- Euro is applied for.

Lawsonite, CaAl2[Si2O7](OH)2 H2O, is an important high-pressure mineral, which is also responsible for the transport of water to the Earth mantle, and which shows two phase transitions at low temperatures. In project P 23108-N19 phase transitions of identical ("isotypic") structures with different mineral chemistry (e.g. Sr, Ba, Pb for Ca or Mn3+, Fe3+, Cr3+ for Al) were pursued, i.e. synthesized at <12 GPa and <1000 K in piston-cylinder presses and investigated by diffraction and spectroscopy techniques. Indeed, a distinct phase transition of Pb-lawsonite, PbAl2[Si2O7](OH)2 H2O, could be found at 450 K. Above 450 K this phase shows the same space group symmetry (Cmcm) as lawsonite above 273 K. In contrast, the low-temperature phase of Pb-lawsonite (Pbnm) is differently distorted than low-temperature lawsonite (Pmcn and P21cn). Similar to lawsonite a reversible, tricritical behavior of the phase transition can be observed with clear indications on coupling of two order parameters in Pb-lawsonite. Thus, the phase transition is characterized by both lattice distortions and order-disorder processes at the hydrogen sites. These may cause also other non-linear changes of physical parameters at ~ 350 and 150 K.