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

Twin-free crystals with relatively low melting temperatures are desira ble as substrates for high temperature superconductor (HTSC) oxide sub strate materials. In the selection of new oxide substrate compositions , special requirements (e.g. suitable dielectric properties for microw ave application and perovskite structure with good lattice matching wi th YBa2Cu3O7-delta) were considered. In this study the calcium-stronti um gallate-niobate (CSGN) system has been investigated in both ceramic and single crystal samples, focusing on their crystal growth. The CSG N compositions were formed by mixing orthorhombic calcium gallate-niob ate (CGN) and cubic strontium gallate-niobate (SGN) complex perovskite s. CGN-excess mixtures do not create a single phase, as opposed to SGN -excess compositions, where single phase solid solutions are formed. T he 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 sta rting composition (CSGN1) was about 1600-degrees-C, but for a 65 mol% SGN concentration (CSGN2) it increased to 1750-degrees-C. A lattice pa rameter a = 3.939 angstrom was obtained for the CSGN1 crystal increasi ng with SGN concentration. A significant drawback was observed for the Ca-Sr-Ga-Nb oxide system: the effective distribution coefficient of C a2+ is very small (k(eff)(Ca2+) almost-equal-to 0.4). This can cause c rystal growth difficulties due to constitutional supercooling phenomen a.