Investigation of Structural Phase Transitions in Minerals and Analogue Systems by High-temperature Magic-angle-spinning Nuclear Magnetic Resonance Spectroscopy PDF Download

Author: Brian L. Phillips
Publisher:
ISBN:
Category :
Languages : en
Pages : 190

Get Book Here

Book Description
This work comprises studies of structural phase transitions (SPT's) in three different phases (anorthite CaAl$sb2$Si$sb2$O$sb8$, Sr$sb2$SiO$sb4$, and AlPO$sb4$-cristobalite) via high-resolution nuclear magnetic resonance spectroscopy using mostly magic-angle spinning (MAS) techniques. The results provide important insight into each of these SPT's and illustrate the types of information that can be obtained about SPT's using MAS NMR spectroscopy. Insight regarding structural changes for all three phases was obtained from the temperature dependence of the chemical shift. In Chapter 1 the temperature variation of the modulation wave in the incommensurate phase of Sr$sb2$SiO$sb4$ is determined from $sp{29}$Si MAS NMR spectra. In Chapter 2 changes in the $sp{29}$Si chemical shifts of anorthite show that its alumino-silicate framework transforms displacively at the P1-I1 transition. In Chapter 3 comparison of the $sp{31}$P and $sp{27}$Al chemical shifts of the $alpha$ and $beta$ phases of AlPO$sb4$-cristobalite indicates that the $beta$ phase is dynamically disordered. For anorthite and Sr$sb2$SiO$sb4$ the chemical shifts also provide a measure of the temperature evolution of the order parameter within the context of Landau theory. The ability of these methods to provide dynamical information from chemical shifts is demonstrated for $beta$-AlPO$sb4$-cristobalite for which time-averaging of chemical shifts produces values different than expected from the crystal structure, and for Sr$sb2$SiO$sb4$, for which the results show that the modulation wave is pinned up to 200$spcirc$C. For anorthite and AlPO$sb4$-cristobalite the chemical shift results are supplemented by spin-lattice relaxation measurements ($sp{29}$Si in anorthite, $sp{31}$P and $sp{27}$Al in AlPO$sb4$-cristobalite). For AlPO$sb4$-cristobalite, a discontinuous increase of both the $sp{31}$P and $sp{27}$Al relaxation rates at the $alpha-beta$ transition and a logarithmic divergence of the $sp{27}$Al relaxation rates in the $beta$ phase support a dynamical order-disorder model for this transition. On the other hand, the lack of such an increase in the $sp{29}$Si relaxation rates in anorthite across the P1-I1 transition support the conclusion that this transition is displacive.