Structural phase transitions in synthetic analogues of the titanite structure type

The aim of the work presented here is to contribute to the understanding of the driving forces behind displacive phase transitions in transition metal oxides. Towards this aim, synthetic derivatives of the mineral titanite, CaTiOSiO4, are used as model compounds. The study is focusing on the occurrence of displacive phase transitions in compounds with the 2:4:4 cation charge combination: SrTiOGeO4, CaTiOGeO4, CaZrOGeO4 and the binary solid solutions CaTiO(GexSi1-x)O4, (CaxSr1-x)TiOGeO4 and Ca(TixZr1-x)OGeO4. All experiments have been performed on synthetic materials obtained by solid state synthesis from the respective oxides. In the case of the end-member composition CaTiOGeO4 single crystals have been grown from the melt. As the phase transitions encountered in these compounds are of a displacive nature and therefore non-quenchable their observation requires in situ measurements at elevated temperatures. This applies to electron microscopy as well as to X-ray diffraction. In the case of CaTiOGeO4, single crystals have been studied in order to examine the distribution and temperature dependence of diffuse scattering associated with the displacive phase transitions in this material. X-ray powder diffraction measurements at ambient and at elevated temperatures have been used to characterize the temperature evolution of the crystal lattices in the solid solutions as well as in the endmembers. The observed structural phase transitions occur both as a function of temperature and of composition and are identified based on the determination of spontaneous strain. CaTiOGeO4 and SrTiOGeO4 behave in analogy to the well known P21/a – A2/a transition observed in titanite, CaTiOSiO4. While the transition temperature across (CaxSr1-x)TiOGeO4 stays constant near Tc = 590 K, it increases linearly from Tc = 487 K in titanite, to Tc = 588 K in CaTiOGeO4. Phase transitions across the solid solution Ca(TixZr1-x)OGeO4 additionally occur as a function of composition. The transition P21/a – A2/a is not observable above ambient temperature in samples with extrapolated Zr concentration in excess of 18 %. The aristotype structure of titanite (space group symmetry A2/a) is observed for intermediate compounds. CaZrOGeO4 and compounds with high Zr content exhibit a distorted titanite structure of space group symmetry A . This triclinic form transforms to the monoclinic titanite aristotype structure at elevated temperature and with increasing Ti content. The critical temperature is 488 K for the Zr-endmember and the critical composition at ambient temperature is at about 30% Ti-concentration.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Heidelberg

Year

2005

Author dc:creator

• Ellemann-Olesen, Rikke

Contributors dc:contributor

• Miletich, Ronald

Repository record source_url

http://www.ub.uni-heidelberg.de/archiv/5414

OAI identifier oai:identifier

oai:archiv.ub.uni-heidelberg.de:5414