Title page for ETD etd-05082012-144621


Type of Document Dissertation
Author Wang, Di
Author's Email Address wangdi@vt.edu
URN etd-05082012-144621
Title Some Aspects of the Crystal Chemistry of Perovskites under High Pressures
Degree PhD
Department Geosciences
Advisory Committee
Advisor Name Title
Ross, Nancy L. Committee Chair
Angel, Ross J. Committee Member
Gibbs, Gerald V. Committee Member
King, Scott D. Committee Member
Wentzcovitch, Renata M. Committee Member
Keywords
  • DFT
  • YAlO3
  • LaAlO3
  • phonon
  • elasticity
  • phase transition
  • high pressure
  • symmetry-adapted mode
  • perovskite
Date of Defense 2012-04-26
Availability restricted
Abstract
This thesis makes contributions to the methodology of quantitative description of the tilting systems of perovskite structures and theoretical analysis of high-pressure phase transitions of representative perovskites.

Chapter 1 and 2 introduce the perovskite structures, tilting classification and descriptions. The structures in each of the 15 tilt systems have been decomposed in to the amplitudes of symmetry-adapted modes in order to provide a clear and unambiguous definition of the tilt angles. A general expression in terms of tilt angles for the ratio of the volumes of the two polyhedra within the perovskite structure is derived.

Chapter 3 uses the first-principles plane-wave pseudopotential calculations to investigate the high-pressure to phase transition and elasticity of LaAlO3 perovskite. This second order transition is determined to occur at ~14 GPa by the pressure variation of the squared frequency of the soft R-point mode in the structure. Elastic moduli are inverted from the calculated stress-strain data by the singular value decomposition method. The Landau parameters for this phase transition are calibrated from the calculation results.

Chapter 4 uses the same method to investigate the high-pressure phase transitions and elasticity of YAlO3 perovskite. The Pnma, Imma, I4/mcm, , perovskite structures and the NH4CdCl3-, Gd2S3-, U2S3-, CaIrO3-type structures are considered. A continuous Pnma to Imma transition occurs at ~89 GPa, determined from the soft Z-point mode of the Imma structure. Then, a discontinuous Imma to I4/mcm transition occurs at ~100 GPa, suggested by the relative enthalpies and phonon dispersions. The elasticities of the Pnma, Imma and I4/mcm structures show mechanical stabilities compatible with the phase transitions. The NH4CdCl3- and CaIrO3-type structures are dynamically stable although not energetically favorable. The relative A/B site polyhedral volume ratios are found to qualitatively reproduce the relative enthalpies of the perovskite structures.

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