Subsolidus Phase Relations and Dielectric Properties in the SrO-Al2O3-Nb2O5 System

Published

February 1, 2000

Author(s)

Julia Y. Chan, Igor Levin, Terrell A. Vanderah, Richard G. Geyer, Robert S. Roth

Abstract

Subsolidus phase equilibria in the SrO-Al₂O₃-Nb₂O₅ system were determined by synthesis of 75 compositions in air in the temperaturerange 1200-1600 C. Phase assemblages were determined by X-ray powder diffraction at room temperature. Two new ternary compounds, Sr₄AlNbO₈ and Sr_5.7Al_0.7Nb_9.3O₃₀, were observed to form in addition to the known double perovskite, Sr₂AlNbO₆ (Fm 3 m, a =7.7791(1) ). Sr₄AlNbO8 crystallizes with a monoclinic unit cell (P2 1/c; a=7.1728(2), b=5.8024(2), c=19.733(1) ; 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 Sr_5.7Al_0.7Nb_9.3O₃₀ (Sr_6-xAl_1-xNb_9+xO₃₀, x=0.3) forms with the tetragonal tungsten bronze structure (P4bm; a=12.374(1), c=3.8785(1) ), 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(Sr_1+xNb_2-x)O_9-3/2x, 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) , respectively. The dielectric properties of the three ternary compounds occurring in the system were measured using the specimen as a TE₀₁₁ or TE_0γδ dielectric resonator: Sr₂AlNbO₆: ξ_r = 25, τ_f = -3 pppm/ C, tan δ = 1.9 x 10^-3 (7.7 GHz); Sr₄AlNbO₈: ξ_r = 27, tan δ = 2.8 x 10^-3 (10.5 GHz); Sr_5.7Al_0.7Nb_9.3O₃₀: ξ_r = 168, tan δ = 3.8 x 10^-2 (3.1 GHz. Sr₂A1NbO₆, when sintered in 1 atm oxygen, exhibited a reduced permittivity (ξ_r = 21) and a significantly improved dielectric loss tangent (tan δ= 5.2 x 10^-4, 8.3 GHz), resulting in a four-fold increase in Qxf as compared to the specimen sintered in air.

Citation

International Journal of Inorganic Materials

Volume

2

Issue

No. 1

Pub Type

Journals

Keywords

dielectric properties, Sr₂AINbO₆, Sr₄AINbO₈, Sr_5.7Al_0.7Nb_9.3O₃₀, Sr_6-xAl_1-xNb9+x^O₃₀, strontium aluminum niobates

Citation

Chan, J. , Levin, I. , Vanderah, T. , Geyer, R. and Roth, R. (2000), Subsolidus Phase Relations and Dielectric Properties in the SrO-Al₂O₃-Nb₂O₅ System, International Journal of Inorganic Materials (Accessed October 7, 2025)