Ordering and elasticity associated with low-temperature phase transitions in lawsonite
- Author(s)
- Peter Sondergeld, Wilfried Schranz, Andreas Tröster, Thomas Armbruster, Gerald Giester, Andriy Kityk, Michael Allan Carpenter
- Abstract
The two low-temperature phase transitions of lawsonite have been studied using single-crystal X-ray diffraction from 86 to 318 K and a single-crystal high-frequency continuous-wave resonance technique from 323 to 102 K. While recently published data of the variations of strains, birefringence, and IR line widths are consistent with the (271 K) Cmcm-Pmcn transition being simply tricritical, our investigation of critical X-ray reflections and the six diagonal elastic constants of lawsonite reveals, consistently, a more complex crossover pattern in the temperature range of 205-225 K. Below 205 K the overall pattern is again in good agreement with a tricritical solution of the Cmcm-Pmcn transition and a second-order behavior of the (120 K) Pmcn-P2 1cn transition. The structure determination from single-crystal X-ray data at 215 K reveals a possible orientational disorder of some of the hydroxyl groups in the Pmcn phase. From this and a recent strain analysis of deuterated and hydrogenated lawsonite we conclude that down to 205 K the Cmcm-Pmcn transition is driven by a displacive component, as observed in strain and birefringence data, plus an order/disorder component or dynamical effects associated with proton ordering. Below 205 K only the displacive component plays a role, and the (120 K) Pmcn-P21cn transition is driven by a single order parameter. The remarkable elastic softening of C66 ahead of the Cmcm-Pmcn transition indicates another orthorhombic-monoclinic transition, which is suppressed on cooling through the low-temperature phase sequence Cmcm-Pmcn-P21cn, but can be observed on applying pressure to the mineral.
- Organisation(s)
- Department of Mineralogy and Crystallography
- External organisation(s)
- University of Cambridge, Universität Bern, Czestochowa University of Technology (CUT)
- Journal
- American Mineralogist: an international journal of earth and planetary materials
- Volume
- 90
- Pages
- 448-456
- No. of pages
- 9
- ISSN
- 0003-004X
- DOI
- https://doi.org/10.2138/am.2005.1243
- Publication date
- 2005
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 103008 Experimental physics
- Portal url
- https://ucrisportal.univie.ac.at/en/publications/f4989cce-b6c4-404b-a923-ac8f75da58c7