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

Published

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 Sr5.7Al0.7Nb9.3O30, 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 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) ), 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) , 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: ξr = 25, τf = -3 pppm/ C, tan δ = 1.9 x 10-3 (7.7 GHz); Sr4AlNbO8: ξr = 27, tan δ = 2.8 x 10-3 (10.5 GHz); Sr5.7Al0.7Nb9.3O30: ξr = 168, tan δ = 3.8 x 10-2 (3.1 GHz. Sr2A1NbO6, when sintered in 1 atm oxygen, exhibited a reduced permittivity (ξr = 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

Keywords

dielectric properties, Sr<sub>2</sub>AINbO<sub>6</sub>, Sr<sub>4</sub>AINbO<sub>8</sub>, Sr<sub>5.7</sub>Al<sub>0.7</sub>Nb<sub>9.3</sub>O<sub>30</sub>, Sr<sub>6-x</sub>Al<sub>1-x</sub>Nb9+x^O<sub>30</sub>, 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-Al<sub>2</sub>O<sub>3</sub>-Nb<sub>2</sub>O<sub>5</sub> System, International Journal of Inorganic Materials (Accessed July 15, 2024)

Issues

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Created January 31, 2000, Updated October 12, 2021