Analiza zmian strukturalnych monokryształów niobianu strontowo - barowego w obszarze przejścia fazowego

Abstract

The aim of this thesis was to characterize a structural phase transition. The high quality single crystals of SrxBa1-xNb2O6 (0.35 < x < 0.72) solid solutions obtained by the Czochralski method were investigated. Using X-ray Bond's method lattice parameters of this crystals are determined with high accuracy in the order of Δd/d ≈ 10-5. The accurate values of lattice parameters were educed from absolute shift of the reflex position which was due to systematic uncertainty. Registered temperature changes a and c parameters have permitted accurate determination of the phase transition temperature TC. The occurrence of anisotropy of thermal expansion below TC was affirmed. The c parameters exhibit a negative coefficient of thermal expansion along [001] direction while the a parameters exhibit a positive one along [100] direction for all studied compositions. A conventional analysis of lattice parameter data in terms of spontaneous strain and strain/order parameter coupling shows that a normal structural phase transition does occur. The ferroelectric systems (SBN35, SBN40 and SBN42) display a nearly tricritical behavior, β ≈ 0.20. While the relaxor ones (SBN59 and SBN72) complies with the two-dimensional Ising-model-like criticality, β ≈ 0.17. Additionally, analysis of recorded diffraction patterns obtained using the four circle diffractometer, allowed to observe several signs of crystal structure modulation.