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Gurak, Mary Catherine

Oxide compositions of equimolar YO1.5 and TaO2.5 in the Y–Ta–Zr–O system have attractive properties for high-temperature applications, including as thermal barrier coatings. The effect of zirconia concentration, from 0 to 20 mol.% cation, on the tetragonal-to-monoclinic phase transition has been studied using high-temperature X-ray diffraction, Raman spectroscopy and electron microscopy. The transformation is reversible and the temperature variation of an order parameter based on the spontaneous strain is consistent with the transformation being ferroelastic, a critical feature for toughening at high temperatures. The presence of twin domains further supports this conclusion. Additionally, stabilization of the tetragonal phase with increasing ZrO2 is evident from the amount of partially retained tetragonal phase at room temperature.

Ba6−xSrxNb10O30 solid solution with 0 x 6 forms the filled tetragonal tungsten bronze (TTB) structure. The Ba-end member crystallizes in the highest symmetry P4/mbm space group (a = b = 12.5842(18)A and ˚ c = 3.9995(8)A) and so do all the compositions with 0 ˚ x 5. The Sr-end member of the solid solution crystallizes in the tentatively assigned Amam space group (a∗ = 17.506(4)A, ˚ b∗ = 34.932(7)A, and ˚ c∗ = 7.7777(2)A). ˚ The latter space group is related to the parent P4/mbm TTB structure as a∗ ≈ √2a, b∗ ≈ 2 √2a, c∗ = 2c. Low-temperature specific heat measurements indicate that the Ba-rich compositions with x 2 are conventional BCS superconductors with TC 1.6 K and superconducting energy gaps of 0.38 meV. The values of the TC in the cation-filled Nb-based TTBs reported here are comparable with those of the unfilled KxWO3 and NaxWO3 TTBs having large alkali ion deficiency. As the unit cell volume decreases with increasing x, an unexpected metal-insulator transition (MIT) in Ba6−xSrxNb10O30 occurs at x 3. We discuss the possible origins of the MIT in terms of the carrier concentration, symmetry break, and Anderson localization. DOI: 1