Smith, A.G.H.; (2012) Structural and defect properties of strontium titanate. Doctoral thesis , UCL (University College London).

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Abstract

Strontium titanate is a material of considerable interest, with many applications. Though it has been extensively studied experimentally and computationally, there are unresolved issues regarding the structure and defect properties. Experimental examination is challenging due to the propensity of the material to form small domains. Even when single crystals are available, the observed behaviour is an average of these local domains. In this thesis we undertook an extensive computational investigation of strontium titanate. The material is known to undergo a second order phase transition from cubic (Pm-3m) to tetragonal (I4/mcm) at ~105 K. We began the investigation by examining the tetragonal phase. Using plane-wave DFT with the LDA, PBE and PBEsol density functionals, we mapped out in three dimensions the I4/mcm space group and fitted the resultant potential energy surfaces to polynomials. Extensive analysis was performed and expectation values for the 0 K octahedral rotation angle were calculated as being between 4.6° and 6.1° for the three functionals. We continued by carrying out an extensive study, where we examined many different low symmetry saddle points, and minima using the PBEsol functional (which was found to have produced the best results). A total of 38 structures (many of which are unique) were examined in detail and the vibrational and imaginary modes involved in the transitions between higher and lower symmetry structures were identified and described. We identified the lowest energy structure as being monoclinic, with a rhombohedral structure at slightly higher energy. We also note that, as the energy differences are so small between these lower symmetry structures (~0.1 meV per formula unit), it is unlikely that these phases will appear in nature. In the final results chapter we present three new sets of classical pair potentials for modelling strontium titanate. We applied these potentials to model vacancy type defects and investigated the transition pathway for oxygen and strontium migration between adjacent sites. We calculated migration barriers of between 0.96 and 1.35 eV for oxygen and between 3.17 and 3.20 eV for strontium, which are improved estimates over previous pair potential results.

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