Phase Equilibria and Dielectric Behavior in the CaO:Al2O3:Nb2O5 System

Subsolidus phase equilibria in the CaO:Al2O3:Nb2O5 system at 1325°C in air have been determined. One ternary phase forms, Ca2AlNbO6, which exhibits a perovskite-related structure with 1:1 or NaCl-type ordering of Al3+ and Nb5+ on the B sites. Indexed X-ray powder diffraction data for this monoclinic compound are given (P21/n (No. 11); a=5.3780 (1), b=5.4154(1), c=7.6248(2) ?, beta=89.968(2)°). The subsystem CaO-Nb2O5 was reexamined at CaO contents above 70 mol% to clarify inconsistencies in the literature. Two phases were confirmed to form in this region: the polymorphic-ordered perovskite Ca4Nb2O9, with solid solution ranging from approximately 17 to 20.5 mol% Nb2O5, and the compound referred to as Ca3Nb2O8, which was shown here to occur as essentially a point compound at the composition 75.25:24.75 CaO:Nb2O5. The perovskite-related structure of the Ca3Nb2O8-type phase was shown to be noncubic, and further studies are in progress. Capacitance methods at 1 MHz were used to determine the dielectric constants and associated temperature coefficients for eleven compounds in the CaO:Al2O3:Nb2O5 system. Ca2AlNbO6 and Ca3Nb2O8 coexist in equilibria and were found to exhibit temperature coefficients of permittivity with opposite signs. Five compositions in the xCa2AlNbO6:(1-x) Ca3Nb2O8 system were prepared and their dielectric properties measured by dielectric resonator methods at 5-7 GHz. The relative permittivities and temperature coefficients of resonant frequency obtained for the endmembers Ca2AlNbO6 and Ca3Nb2O8 were 30, -88 ppm/°C, and 45, +113 ppm/°C, respectively. Temperature compensation of the resonant frequency was obtained near x=0.67 with a permittivity of 32; no solid solution was detected by X-ray powder diffraction.