First evidence of P2{sub 1}/n to P2{sub 1}/c structural transformation in pyroxene-type LiAlGe{sub 2}O{sub 6} under high-pressure conditions
- Department of Geosciences, University of Padua, Via Gradenigo 6, I-35131 Padova (Italy)
- Institut für Mineralogie und Kristallographie, Universität Wien, Althanstrasse 14, A-1090 Wien (Austria)
- Fachbereich Materialforschung und Physik, Universität Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg (Austria)
The high-pressure behavior of the pyroxene-type compound LiAlGe{sub 2}O{sub 6}, the unique representative of a P2{sub 1}/n-pyroxene, was investigated by in-situ X-ray diffraction and Raman spectroscopy on single-crystal samples hydrostatically pressurized in a diamond-anvil cell. The structure was found to undergo a first-order phase transition on compression, with a critical transition at 5.23±0.02 GPa. Together with a strong volume discontinuity of −ΔV/V{sub 0}=−4.1% the transition shows a remarkable hysteresis loop over at least 0.70 GPa pressure interval. The bulk modulus of the low- and high-pressure polymorphs corresponds to K{sub 0}=114(1) and 123(2) GPa, respectively, as described by a 2{sup nd} order Birch–Murnaghan equation of state. Based on the systematic extinctions the transition was identified as a P2{sub 1}/n-to-P2{sub 1}/c transformation. The mechanism of structural transformation was identified to be controlled by the stereochemistry of the Li atoms at the M2 site, which changes its coordination number from 5 to 6. The formation of new Li–O bonds involves the co-operative folding of Ge{sub 2}O{sub 6}-chains, which explains the anisotropy of axial elasticities and the spontaneous strain across the transformation. Simultaneously the distortion correction of AlO{sub 6} units associated with the transition further explains the preference of the P2{sub 1}/c structure under pressure. - Graphical abstract: Detail of the crystal structures of LP-LAG at 3.48 GPa (left) and HP-LAG at 4.51 GPa (right). Li on M2 are green; AlO{sub 6} octahedra (Al at M1) are orange; tetrahedra sites in LP-LAG: T1 (yellow) and T2 (blue); tetrahedra sites in HP-LAG: A-chains (yellow), B-chains (blue). - Highlights: • A new high-pressure phase transition P2{sub 1}/n–P2{sub 1}/c was found for LaAlGe{sub 2}O{sub 6}. • The transition was also followed by micro-Raman spectroscopy. • The high-pressure polymorph shows a higher bulk modulus. • The structural evolution of the two symmetries was determined.
- OSTI ID:
- 22486745
- Journal Information:
- Journal of Solid State Chemistry, Vol. 228; Other Information: Copyright (c) 2015 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; Country of input: International Atomic Energy Agency (IAEA); ISSN 0022-4596
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ALUMINIUM COMPOUNDS
ANISOTROPY
CORRECTIONS
DIAMONDS
ELASTICITY
EQUATIONS OF STATE
GERMANATES
HYSTERESIS
LITHIUM COMPOUNDS
MONOCLINIC LATTICES
MONOCRYSTALS
PHASE TRANSFORMATIONS
PRESSURE DEPENDENCE
RAMAN SPECTROSCOPY
SILICATE MINERALS
STEREOCHEMISTRY
STRAINS
X-RAY DIFFRACTION