Growth of Single Crystals of RTa sub 3 O sub 9.

The RTa sub 3 O sub 9 compounds offer the crystal structure and lattice parameters which could serve as the substrate for thin film epitaxial growth of several of the oxide superconducting materials such as Bi sub 2 SrCa sub 2 Cu sub 2 O sub 8 or YBa sub 2 Cu sub 3 O sub 7. The Czochralski growth of single crystals of several of these compounds has been studied. Differential scanning calorimetry has indicated several of the RTa sub 3 O sub 9 compounds have phase transformations between the melting point and room temperature. Trials at the growth of single crystals of these compunds from a melt were performed to evaluate if the phase transformations would introduce sufficient strain to result in fracturing of the crystal while cooling to room temperature. All samples grown were extensively fractured due to the stresses induced by these phase transformations. In addition this crystal system exhibits a strong tendency towards axis wander during the Czochralski growth process. The phase transformation observed for RTa sub 3 O sub 9 compounds results in these materials being unusable as single crystal substrates for the epitaxial growth of superconducting oxides such as Bi sub 2 SrCa sub 2 Cu sub 2 O sub 8 or YBa sub 2 Cu sub 3 O sub 7.