Main field of study: 010900 Applied Mathematics and Physics
Area of specialization: Optics of Semiconductor Nanostructures and Nanotechnologies
Department of solid state physics
Scientific advisor: Dr. M.B. Smirnov
Reviewer: candidate PhD, senior researcher Roginskii E.M.

Rigid Unit Modes and incommensurate structural instabilities in cristobalite
Y.M. Kireenko

This dissertation is devoted to the analysis of Rigid Unit Modes (RUM) in the framework crystal lattice of β-cristobalite and low-symmetry structures that originate from condensation of various combinations of these modes. We have found that in the lattice of β-cristobalite there are RUMs localized in separate chains of tetrahedra that extend along certain crystallographic directions, denoted as Chain-RUM (CRUM). We then show that all the distorted structures of β-cristobalite known from the literature can be represented as a result of condensation of the combination of CRUMs. We also predict existence of several new cristobalite structures and show that condensation of CRUMs makes possible existence of the incommensurate phases of cristobalite.