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We have studied the microstructures of epitaxial YbBa2Cu3O7 superconducting films grown on SrTiO3  and  substrates by a new liquid-gas-solidification processing technique. Films grown on SrTiO3  and  substrates are epitaxially oriented with  and  axes normal to the film surface. Twinned domains of ~200 Å in size are observed in the  oriented films. Most defects observed can be attributed to the presence of stacking faults along the c axis. The presence of Cu-O bilayer defects is also observed. Rapid oxygen diffusion in the liquid phase is found to play an important role in the microstructure of the thin films.