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Elastic anomalies and acoustic dissipation associated with phase transitions in minerals

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In the lower mantle of the earth, minerals are mainly perovskites and oxides, e.g. CaSiO3, (Mg,Fe)(Si,Al)O3 and (Mg,Fe)O. They undergo displacive phase transitions and spin state transitions at high pressure and high temperature. For instance, (Mg, Fe)SiO3 has the orthorhombic Pnma structure and can change to orthorhombic Imma, tetragonal I4/mcm, or cubic Pm3m. CaSiO3 has the cubic Pm3m structure and can change to tetragonal I4/mcm or orthorhombic Pnma. Fe2+ in (Mg,Fe)O and (Mg,Fe)(Si,Al)O3 changes from high spin state to low spin state or intermediate spin state with increasing pressure. These phase transitions give rise to acoustic dissipation and changes in bulk and shear moduli, which could have significant implications for the physical and chemical properties of the lower mantle. It is very difficult to investigate them directly at high pressure. Therefore, analogue materials have been made use of. The transitions in BaCeO3, i.e. Pnma – Imma – R3c – Pm3m, and the transitions in Sr(Zr,Ti)O3, i.e. Pnma – Imma – I4/mcm – Pm3m cover a wider variety of transitions than has been investigated in other analogue materials. Co3+ is isoelectronic with Fe2+ and shows analogous high spin/ low spin behaviour at ambient pressure in a temperature range which is easily accessible for in-situ investigations. In the present study, the elastic anomalies and acoustic dissipation associated with displacive phase transitions in BaCeO3 [1] and Sr(Zr,Ti)O3 [2], and with spin state transitions in LaCoO3, NdCoO3, GdCoO3, and Co3O4 have been studied by resonant ultrasound spectroscopy (RUS) at high frequencies around 1 MHz, and by dynamic mechanical analysis (DMA) at low frequencies around1 Hz, as well as by strain analysis based on Landau theory. There is a diversity of pinning and relaxation processes for transformation twin walls and interfaces in different perovskites with I4/mcm, Imma and Pnma structures. Cation size changes due to spin state transitions are an important factor influencing the strain related to octahedral tilting. Anelasticity maps are very useful for displaying different thermally activated relaxation processes on the basis of their activation energies and attempt frequencies.

[1] Z. Zhang, J. Koppensteiner, W. Schranz, J. B. Betts, A. Migliori, and M. A. Carpenter, Phys. Rev. B 82 , 014113 (2010).

[2] Z. Zhang, J. Koppensteiner, W. Schranz, and M. A. Carpenter, J. Phys.: Condens. Matter 22, 295401 (2010).

This talk is part of the Mineral Sciences Seminars series.

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