Investigation of structural phase transitions in inorganic ferroic systems

Abstract

The thesis entitled "Investigation of Structural Phase Transitions in Inorganic Ferroic Systems" consists of six chapters. Structural changes exhibited by ferroelectric and ferroelastic materials are studied as a function of temperature and composition. The changes in the crystal system resulting in subtle changes in the complex polyhedra have been analyzed using single crystal and powder X-ray diffraction. The results provide new insights into the structural changes during the phase transition.

Chapter 1 is a review describing briefly the basic features of structural phase transitions. Types of phase transitions and their mechanism are reviewed and some of the key characterization techniques employed in structural determination are discussed with special reference to single crystal, powder, and ab initio X-ray diffraction techniques. Chapters 2 to 4 report studies using single crystal X-ray diffraction, while chapters 5 and 6 report studies using powder X-ray diffraction.

Chapter 2 deals with RbHSO?, which undergoes a paraelectric to ferroelectric phase transition at 265 K. The crystal structure of the compound has been determined both at room temperature and at 200 K. A transformation from a monoclinic centric space group P2?/n to the noncentric Pn is observed at 200 K. The present study shows that the sulfate tetrahedron has no disorder at 293 K, contrary to earlier reports. However, there is a rotation of the sulfate tetrahedra, resulting in a significant displacement of rubidium atoms, leading to changes in the coordination polyhedra in the ferroelectric phase (200 K). High-temperature powder X-ray diffraction patterns indicate a possible reduction in symmetry above 393 K, suggesting a phase transition.

Chapter 3 reports the variable temperature X-ray crystal structure analysis of a type I langbeinite, Rb?Cd?(SO?)?. The structure displays three different phases: cubic at 293 K, monoclinic at 120 K, and orthorhombic at 85 K, respectively. Single crystal analyses of these phases indicate a distortion initially from cubic to monoclinic on cooling, followed by a significant reorientation of the SO? tetrahedra, resulting in an orthorhombic symmetry on further cooling. There is no formation of an intermediate triclinic phase or any lattice disorder, as suggested in earlier reports on other type I langbeinites. The bond valence sum analyses of the coordination around the Rb sites indicate asymmetry in the bond strengths, which is expected to be the driving force for the ferroelectricity.

In Chapter 4, an investigation of the super-subgroup phase transition in Rb?LiH?(SO?)? and K?LiH?(SO?)? is presented. At 293 K, Rb?LiH?(SO?)? crystallizes in the tetragonal space group P4?, while K?LiH?(SO?)? crystallizes in P4?. Single crystal X-ray diffraction data on Rb?LiH?(SO?)? indicate the change in the space group to P2? at 90 K, with distortions in the sulfate moieties as a key feature. K?LiH?(SO?)? does not undergo any structural transition at 100 K. However, an interesting feature in K?LiH?(SO?)? is the switching of the covalent nature from one hydrogen bond to another on cooling.

Chapter 5 deals with the crystal structure of three n=4 Aurivillius type of oxides at 298 K using high-resolution powder X-ray diffraction data. Pattern decomposition and peak extraction methods were used to derive starting models for SrBi?Ti?O?? and PbBi?Ti?O??. The zigzag arrangement of the distorted TiO? octahedra was observed. The Ba/Sr/Pb cations are distributed in the A sites as well as the [Bi?O?]²? sites. SrBi?Ti?O?? shows a structural transition to the space group I4/mmm at 803 K. It is also confirmed that the ferroelectric to paraelectric phase transition in BaBi?Ti?O?? and PbBi?Ti?O?? is not accompanied by any structural change.

Chapter 6 describes the structural features of the n=2 series of La-substituted n=2 Aurivillius phases as a function of composition. The crystal structures of the solid solutions of Bi??xLaxTiNbO? (0 < x < 1) have been analyzed by powder X-ray diffraction and selected area electron diffraction. The structure of the x=1 phase, solved by ab initio powder diffraction, is centrosymmetric (space group Pmcb). The intermediate compositions are refined in space group A2am by the Rietveld method. Occupancy refinements reveal that the La³? ion is disordered only in the A site and not in the [Bi?O?]²? layer. A decrease in the orthorhombic distortion is observed as x increases, and the effect of cation size manifests on the extent of tilting of the BO? octahedra.