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We present a model that provides a description of the microwave dielectric loss in oxides. The dielectric loss ͑tan ␦͒ in single crystal and polycrystalline MgO and Al 2 O 3 is measured over the temperature range 70-300 K. We are able to model the dielectric loss in terms of a two-phonon difference model. There are two key parameters in this model: The third derivative, 3 , of the lattice potential and the linewidth, ␥, of the thermal phonons. In polycrystalline samples, rather than considering the different mechanisms of extrinsic loss, it is assumed that the main effect of extrinsic factors is a modification of the linewidth of the thermal phonons. By varying ␥(T), it is shown that the model can describe the loss in both single crystals and polycrystallines materials. In single crystal and polycrystalline MgO, we use ␥ as a fitting parameter. In single crystal and polycrystalline Al 2 O 3 , we obtain ␥(T) by Raman spectroscopy. The theory gives the right order of magnitude of the measured loss.

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Microstructural and magnetic characterization were undertaken on high-rate, high-temperature grown YBa 2 Cu 3 O 7-␦ (YBCO) films. The films were of approximately 1 m thickness and were grown by pulsed laser deposition on (100) SrTiO 3 using a high-power industrial laser at growth temperatures between 750°C and 870°C and at growth rates of up to 4 m/min. Two YBCO layers with different c-lattice parameters were observed in the films, the higher c value occurring near the substrate interface and arising from cation disorder and oxygen nonstoichiometry, and the lower one near the film surface arising from cation disorder alone. BaCuO 2 precipitates were present near the surface of the films, indicative of partial melting during growth. The amount of BaCuO 2 increased with growth temperature. Epitaxial Y 2 O 3 also formed in increasing amounts suggestive of a different partial melting reaction in the films compared to bulk YBCO, where Y 2 BaCuO 5 coexists with liquid. Around 1 MA/cm 2 values of high critical current density (J c ) were observed in the films, and the in-field J c improved with growth temperature despite the fact that the superconducting transition width increased significantly.

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