title: Incommensurate structure of Ca2Al2O5 at high temperatures--structure investigation and Raman spectroscopy.
abstract: A high-temperature X-ray diffraction study revealed that brownmillerite-type Ca(2)Al(2)O(5) transforms to an incommensurately modulated structure at elevated temperatures. Single crystals of Ca(2)Al(2)O(5) were synthesized in an end-loaded piston cylinder press at 2.5 GPa and 1273 K. The diffraction pattern observed at 1090 (10) K by in situ single-crystal diffraction experiments can be indexed by an I-centred orthorhombic cell and a modulation wavevector of q = 0.595 (1)c(*). A (3 + 1)-dimensional model in superspace group Imma(00gamma)s00 was used to refine the modulated structure. The structure is assembled from two building units: (i) layers of corner-sharing [AlO(6)] octahedra, stacked along b alternate with (ii) layers of zweier single chains of [AlO(4)] tetrahedra running along a. The modulated structure arises from an aperiodic sequence of two different configurations of the chains within the tetrahedral layers. The modulated high-temperature phase of Ca(2)Al(2)O(5) is isotypic to the modulated high-temperature modification of Ca(2)Fe(2)O(5). A large hysteresis was found in the phase-transition temperature. On heating, the transition occurs at ca 1075 (10) K; on cooling, satellite reflections can be observed down to 975 (10) K. The characterization of Ca(2)Al(2)O(5) is completed by Raman spectroscopy, including a partial interpretation of the spectra.
authors.affiliation: Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria. biljana.lazic@uibk.ac.at
authors.forename: Biljana
authors.initials: B
authors.lastname: Lazic
authors.forename: Hannes
authors.initials: H
authors.lastname: Krüger
authors.forename: Volker
authors.initials: V
authors.lastname: Kahlenberg
authors.forename: Jürgen
authors.initials: J
authors.lastname: Konzett
authors.forename: Reinhard
authors.initials: R
authors.lastname: Kaindl