MIXED-PEROVSKITE SUBSTRATES FOR HIGH-TC SUPERCONDUCTORS

Mixed perovskites of high quality have been grown from multicomponent melts using the Czochralski method. The formation of mixed crystals is accomplished by coupled substitution. Rare-earth aluminium perovskites are used as host lattices. The cations of the host lattice are partially replaced by the guest ions Sr2+, Ca2+, Ta5+ and Nb5+. The distribution coefficients of the cations are adjusted to unity by appropriate choice of the melt compositions. Single crystals up to 30 mm in diameter and 80 mm in length have been obtained. Depending on the composition the crystallographic structure of the mixed perovskites is cubic, tetragonal or orthorhombic. The cubic crystals are twin free. The lattice constants can be varied in the range 0.378 less-than-or-equal-to a0 less-than-or-equal-to 0.388 nm. The dielectric constants (13 less-than-or-equal-to epsilon less-than-or-equal-to 21) and the dielectric losses (tan delta less-than-or-equal-to 1 x 10(-3) are resonably low. The thermal expansion coefficient alpha of the cubic perovskites is about 1 x 10(-5)-degrees-C-1 in the temperature range between 100 and 900-degrees-C.