T-induced displacive phase transition of end-member Pb-lawsonite

Author(s)

Martin Ende, Bernd Wunder, Monika Koch-Müller, Thomas Pippinger, Gernot Buth, Gerald Giester, Christian Lengauer, Eugen Libowitzky

Abstract

Pb-lawsonite, PbAl

₂[(OH)

₂|Si

₂O

₇]·H

₂O, space group Pbnm, was synthesized as crystals up to 15 μm x 5 μm x 5 μm in size by a piston cylinder technique at a pressure of ∼4 GPa and a temperature of 873 ± 10 K. Temperature-dependent powder and single-crystal X-ray diffraction (XRD) analyses partly using synchrotron radiation as well as Raman spectroscopic investigations reveal a phase transition around 445 K resulting in the Cmcm high-temperature structure. The transformation temperature is considerably higher than that of lawsonite around 273 K, which is characterized predominantly by proton order/disorder. The transition is confirmed using principal component analysis and subsequent hierarchical cluster analysis on both the powder XRD patterns and the Raman spectra. Furthermore, a non-uniform change is observed around 355 K, which is not as pronounced as the 445 K transition and apparently comes from enhanced hydrogen bonding, which stops the atom shifts in Pb-lawsonite. These are the same bonds that mainly characterize the phase transition in lawsonite around 273 K. In contrast, the structural transition of Pb-lawsonite at 445 K seems to originate from the interaction of the SiO

₄ tetrahedra and AlO

₆ octahedra framework with the Pb

²⁺ cation. The structural environment of Pb

²⁺ can be described by a 12-fold coordination above 445 K, which changes towards irregular ten-fold coordination below this temperature. An assignment of the O-H stretching Raman bands confirms moderately strong H bonds in Pb-lawsonite, whereas both strong and weak H bonds exist in lawsonite. Therefore, a further phase transition of Pb-lawsonite, similar to that of lawsonite around 273 K, is not expected.

Organisation(s)

Department of Mineralogy and Crystallography

External organisation(s)

Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum, Karlsruher Institut für Technologie

Journal

Mineralogical Magazine

Volume

80

Pages

249-267

No. of pages

19

ISSN

0026-461X

DOI

https://doi.org/10.1180/minmag.2016.080.015

Publication date

04-2016

Peer reviewed

Yes

Austrian Fields of Science 2012

104026 Spectroscopy, 105116 Mineralogy, 105113 Crystallography

Keywords

ASJC Scopus subject areas

Geochemistry and Petrology

Portal url

https://ucrisportal.univie.ac.at/en/publications/tinduced-displacive-phase-transition-of-endmember-pblawsonite(49d73f2f-0777-43d4-b99e-1d7283464eb5).html