EPR study of the phase transitions in LIKSO4 and certain similar salts.

Abstract

This thesis represents basically an attempt to investigate the low temperature phase transitions of LiKSO^ (LKS) through Electron Paramagnetic Resonance (EPR). There have been of course umpteen publications on the phase transitions in LKS by various techniques. Earlier work has been surveyed fairly exhaustively in Chapter 1. Magnetic Resonance as a tool for studying phase transitions is extremely valuable, since it probes very subtle changes at specific lattice sites in a crystal undergoing phase transitions. EPR has also been used by many workers to study the phase transitions in LKS. Most of these workers have employed SO^ or SeO^ as paramagnetic probes. SO^, a very good paramagnetic centre, has also drawbacks discussed in detail in Sec. 4.1 of this thesis. The same section also deals with + the reasons why a different probe, was employed in this investigation. At the time of writing this thesis there has + been one EPR investigation on LKS using NH^ [Yu Tsu et al 1986] this investigation has however interpreted satellites of EPR main signals as arising out of two chemically inequivalent K^ sites. These satellites admit of a more natural explanation in terms of ^Li spin-flips. This matter has been discussed in Sec. 4.2. We therefore undertook a detailed investigation of the low temperature phase transitions in LKS using NH^ as paramagnetic probe. Our results have not always agreed with those of many earlier workers as described and discussed in Chapters 3 and 4 respectively. We could investigate a fourth phase (PIV) below 135 K referred to sketchily, if at all, by earlier workers. We also tried Cu^^ as a paramagnetic centre in LKS. This however gave entirely different, unexpected but reproducible results. There was a single transition at ~247 K. The g and A tensors were isotropic above this temperature, while below this temperature they had axial symmetry. Both above and below this transition temperature of ~247 K the spectra were independent of magnetic field orientation, while the samples were single crystals -- as checked under the polarizing microscope and by the presence of the expected EPR spectra of NH^+ employed as a parallel probe. Furthermore^ Cu^"^ in LiNaSO^ and LiNH^SO^ - with crystal structures differing from LKS ---- gave spectra identical to those in LKS and with practically the same single transition temperature. This part of the investigation is reported and discussed in Part II (Chapter 5) of this thesis. As discussed there, the only interpretation of the transition at ~247 K seen by Cu^ -- a well knownJahn-Teller ion -- was as a transition from a dynamic Jahn-Teller state to a Jahn-Teller glass. The above description is a bird's eye view of the material presented in this thesis. Relevant introductions have also been included at appropriate places. Chapter 1 reviews earlier work on the low temperature phase transitions of LKS with some emphasis on EPR investigations. Chapter 2 gives a treatment of the general theory of Electron Paramagnetic Resonance and the concept of the spin- Hamiltonian. Chapter 2 on the general thepry of EPR. The emphasis however is on vibronic doublet states of relevance to this work