In the frame of research activities to identify novel materials for microwave applications, a systematic research of the phase relations in a ternary phase diagram system of CaO-La₂O₃-TiO₂ led, among others, to the preparation of single-phase ceramics based on CaLa₄Ti₅O₁₇, CaLa₈Ti₉O₃₁ and Ca₂La₄Ti₆O₂₀. Preliminary studies and structure predictions of these compounds, including the determination of their unit cells, were carried out much earlier [Nanot et al., J.Solid State Chem. 1974, 11, 272-284], but their detailed structures have not yet been published or reported in the ICSD database, resulting in a motivation to perform the structural analysis.

The three materials listed above were synthesized using a solid-state reaction method by mixing different molar ratios of CaCO₃, La₂O₃ and TiO₂. Their X-ray powder diffraction patterns have been collected on a PANalytical X’Pert PRO MPD diffractometer using CuKα₁ radiation. The diffraction patterns were compared with the entries in the PDF2 database.

The structure of CaLa₄Ti₅O₁₇ was successfully identified using Rietveld refinement of the isostructural models found in the database by the criterion of pattern matching. The most suitable model is orthorhombic having the Pmnn space group. For the CaLa₈Ti₉O₃₁ and Ca₂La₄Ti₆O₂₀ no suitable initial model was found. The structures were solved ab initio using the Fox programme [Favre-Nicolin and Cerny, J. Appl. Cryst. 2002, A58]. Reasonable structural models were obtained; however Rietveld refinements were less successful due to poor convergence of the positions of oxygen atoms. Among the possible reasons for this are possibly incorrect space groups and fitting errors, originating from a large set of fitted parameters which is due to a large asymmetric unit. Nevertheless, all the structures obtained so far are in a fair qualitative agreement with the initial predictions proposed by Nanot.

Details on the structure determinations of the listed compounds will be given and all the relevant aspect will be critically discussed.

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