The role of disorder in determining the physical properties of negative thermal expansion materials

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Gallington, Leighanne C.
Wilkinson, Angus P.
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In this dissertation, the role of disorder in determining the physical properties of materials in the AM2O8 and AM2O7 families was investigated. A link was established between orientational disorder and the thermoelastic properties of these materials. It was also demonstrated that phase transition temperatures in these materials are noticeably pressure-dependent. In Chapters 3 and 4, it was shown that compression-induced orientational disorder was correlated with temperature-dependent bulk moduli and pressure-dependent CTEs in the orientationally ordered phases of ZrW2O8 and HfW2O8 . No changes in orientational order were observed in ZrMo2O8 or the orientationally disordered phases of ZrW2O8 and HfW2O8 upon compression, and the bulk moduli and CTEs were relatively temperature- and pressure-independent respectively. The pressure-sensitivity of the CTE has implications for the use of these materials in controlled thermal expansion composites, since internal stresses on par with the pressure range examined can be induced by mismatches in thermal expansion between components of the composite. In Chapter 5, it was demonstrated that the CTE of the orientationally ordered rhombohedral phase of SnMo2O8 was pressure-sensitive, while that of the orientationally disordered cubic phase was not. Additionally, at temperatures near the ambient pressure rhombohedral → cubic transition, it was possible to interconvert between these two phases upon compression and decompression. The phase transition pressure was also found to be elevated significantly by slight increases in temperature. Both phases were significantly softer than all phases of ZrW2O8, ZrMo2O8, and HfW2O8. In Chapter 6, it was demonstrated both the supercell → incommensurate and incommensurate → subcell transition temperatures of ZrV2O7 and HfV2O7 are extremely pressure-sensitive, to the point where NTE was not observed below 513K at pressures above 155 MPa. Additionally, the CTEs of both the low temperature and high temperature phases were strongly-pressure-dependent. This is due in part to the close proximity of the supercell → incommensurate and incommensurate → subcell transitions, which are both associated with large volume changes. The high temperature phase was found to be much stiffer than the low temperature phase. In Chapter 7, it was demonstrated that ZrAs2O7 and HfAs2O7 exhibit PTE between 100K and 500K. Their thermal expansion behavior was found to be more similar to that of ZrP2O7 than that of ZrV2O7. The crystal structure of ZrAs2O7and HfAs2O7 could not be determined from high resolution XRD data; however, these phases were determined to be of lower symmetry than previously reported.
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