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SUMMARY:Elastic anomalies and acoustic dissipation associated with phase t
 ransitions in minerals - Zhiying Zhang (Department of Earth Sciences)
DTSTART:20110214T160000Z
DTEND:20110214T170000Z
UID:TALK28916@talks.cam.ac.uk
CONTACT:Ian Farnan
DESCRIPTION:In the lower mantle of the earth\, minerals are mainly perovsk
 ites and oxides\, e.g. CaSiO3\, (Mg\,Fe)(Si\,Al)O3 and (Mg\,Fe)O. They und
 ergo displacive phase transitions and spin state transitions at high press
 ure and high temperature. For instance\, (Mg\, Fe)SiO3 has the orthorhombi
 c Pnma structure and can change to orthorhombic Imma\, tetragonal I4/mcm\,
  or cubic Pm3m. CaSiO3 has the cubic Pm3m structure and can change to tetr
 agonal 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 sta
 te with increasing pressure. These phase transitions give rise to acoustic
  dissipation and changes in bulk and shear moduli\, which could have signi
 ficant 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 transition
 s than has been investigated in other analogue materials. Co3+ is isoelect
 ronic with Fe2+ and shows analogous high spin/ low spin behaviour at ambie
 nt 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\, G
 dCoO3\, and Co3O4 have been studied by resonant ultrasound spectroscopy (R
 US) 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 wit
 h I4/mcm\, Imma and Pnma structures. Cation size changes due to spin state
  transitions are an important factor influencing the strain related to oct
 ahedral tilting. Anelasticity maps are very useful for displaying differen
 t thermally activated relaxation processes on the basis of their activatio
 n energies and attempt frequencies.\n\n[1] Z. Zhang\, J. Koppensteiner\, W
 . Schranz\, J. B. Betts\, A. Migliori\, and M. A. Carpenter\, Phys. Rev. B
  82\, 014113 (2010).\n\n[2] Z. Zhang\, J. Koppensteiner\, W. Schranz\, and
  M. A. Carpenter\, J. Phys.: Condens. Matter 22\, 295401 (2010).\n
LOCATION:Harker 2 lecture room\, Department of Earth Sciences\, Downing Si
 te
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