Incommensurate structure of Ca2Al2O5 at high temperatures - structure investigation and Raman spectroscopy

• Published in: Acta crystallographica. Section B, Structural science Structural science, 2008-08, Vol.64 (4), p.417-425
• Main Authors: Lazic, Biljana, Krüger, Hannes, Kahlenberg, Volker, Konzett, Jürgen, Kaindl, Reinhard
• Format: Article
• Language: English
• Subjects: brownmillerite; Crystal structure; Crystallography; high temperature; incommensurate structure; Raman spectroscopy; Spectrum analysis; Temperature; X-rays
• ISSN: 0108-7681; 2052-5192; 1600-5740; 2052-5206
• DOI: 10.1107/S0108768108016029

A high‐temperature X‐ray diffraction study revealed that brownmillerite‐type Ca2Al2O5 transforms to an incommensurately modulated structure at elevated temperatures. Single crystals of Ca2Al2O5 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(00γ)s00 was used to refine the modulated structure. The structure is assembled from two building units: (i) layers of corner‐sharing [AlO6] octahedra, stacked along b alternate with (ii) layers of zweier single chains of [AlO4] 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 Ca2Al2O5 is isotypic to the modulated high‐temperature modification of Ca2Fe2O5. 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 Ca2Al2O5 is completed by Raman spectroscopy, including a partial interpretation of the spectra.