CA-SR-GA-NB MIXED-OXIDE SYSTEM FOR HIGH-TEMPERATURE SUPERCONDUCTOR SUBSTRATE APPLICATIONS

Twin-free crystals with relatively low melting temperatures are desirable as substrates for high temperature superconductor (HTSC) oxide substrate materials. In the selection of new oxide substrate compositions, special requirements (e.g. suitable dielectric properties for microwave application and perovskite structure with good lattice matching with YBa2Cu3O7-delta) were considered. In this study the calcium-strontium gallate-niobate (CSGN) system has been investigated in both ceramic and single crystal samples, focusing on their crystal growth. The CSGN compositions were formed by mixing orthorhombic calcium gallate-niobate (CGN) and cubic strontium gallate-niobate (SGN) complex perovskites. CGN-excess mixtures do not create a single phase, as opposed to SGN-excess compositions, where single phase solid solutions are formed. The SGN-excess compositions can produce twin-free CSGN single crystals, which have excellent room temperature dielectric loss (tan delta less-than-or-equal-to 3 x 10(-4) and reasonably low dielectric constant (K = 41) values at 100 kHz. The growth temperature for a 50 mol% SGN starting composition (CSGN1) was about 1600-degrees-C, but for a 65 mol% SGN concentration (CSGN2) it increased to 1750-degrees-C. A lattice parameter a = 3.939 angstrom was obtained for the CSGN1 crystal increasing with SGN concentration. A significant drawback was observed for the Ca-Sr-Ga-Nb oxide system: the effective distribution coefficient of Ca2+ is very small (k(eff)(Ca2+) almost-equal-to 0.4). This can cause crystal growth difficulties due to constitutional supercooling phenomena.