Subsolidus phase relations and dielectric properties in the SrO–Al2O3–Nb2O5 system

Abstract

Subsolidus phase equilibria in the SrO–Al2O3–Nb2O5 system were determined by synthesis of 75 compositions in air in the temperature range 1200–1600°C. Phase assemblages were determined by X-ray powder diffraction at room temperature. Two new ternary compounds, Sr4AlNbO8 and Sr5.7Al0.7Nb9.3O30, were observed to form in addition to the known double perovskite, Sr2AlNbO6 (Fm3̄m, a=7.7791(1) Å). Sr4AlNbO8 crystallizes with a monoclinic unit cell (P21/c; a=7.1728(2), b=5.8024(2), c=19.733(1) Å; β=97.332(3)°) determined by electron diffraction studies; the lattice parameters were refined using X-ray powder diffraction data, which are given. This compound decomposes above 1525°C; attempts to grow single crystals from neat partial melts, or using a strontium borate flux, were unsuccessful. The phase Sr5.7Al0.7Nb9.3O30 (Sr6−xAl1−xNb9+xO30, x=0.3) forms with the tetragonal tungsten bronze structure (P4bm; a=12.374(1), c=3.8785(1) Å), melts incongruently near 1425°C, and occurs essentially as a point compound, with little or no range of x-values; indexed X-ray powder diffraction data are given. The tungsten bronze structure exhibits a narrow region of stability in the SrO–Al2O3–Nb2O5 system, which is probably related to the small size of Al3+. The existence of an extensive cryolite-type solid solution, Sr3(Sr1+xNb2−x)O9−3/2x, occurring between Sr4Nb2O9 (x=0) and Sr6Nb2O11 (x=0.5), was confirmed, with cubic lattice parameters ranging from 8.268(2) to 8.303(1) Å, respectively. The dielectric properties of the three ternary compounds occurring in the system were measured using the specimen as a TE011 or TE0γδ dielectric resonator: Sr2AlNbO6: εr=25, τf=−3 ppm/°C, tan δ=1.9×10−3 (7.7 GHz); Sr4AlNbO8: εr=27, tan δ=2.8×10−3 (10.5 GHz); Sr5.7Al0.7Nb9.3O30: εr=168, tan δ=3.8×10−2 (3.1 GHz). Sr2AlNbO6, when sintered in 1 atm oxygen, exhibited a reduced permittivity (εr=21) and a significantly improved dielectric loss tangent (tan δ=5.2×10−4, 8.3 GHz), resulting in a four-fold increase in Q×f as compared to the specimen sintered in air.