Phase Relations, Crystal Chemistry, and Dielectric Properties in Sections of the La2O3-CaO- MgO-TiO2 System

Published

June 1, 2004

Author(s)

Terrell A. Vanderah, V L. Miller, Igor Levin, S M. Bell, T Negas

Abstract

Subsolidus phase equilibria, crystal chemistry, and dielectric behavior were studied for the La₂O₃ MgO TiO₂ system and for the ternary sections LaMg_1/2Ti_1/2O₃ CaTiO₃ La₂O₃ and LaMg_1/2Ti_1/2O₃ CaTiO₃ 0.5La_2/3TiO₃ in the quaternary La₂O₃ CaO MgO TiO₂ system. Phase relations were obtained by X-ray powder diffraction analysis of approximately 80 specimens prepared by solid-state reactions in air at approximately 1450 C. Six ternary phases were found to form in the La₂O₃ MgO TiO₂ system, including the three previously reported compounds LaMg_1/2Ti_1/2O₃, La₅Mg_0.5Ti_3.5O₁₅, and La₆MgTi₄O₁₈ ; and the new phases La₁₀MgTi₉O₃₄, La₉Mg_0.5Ti_8.5O₃₁, and a perovskite-type solid solution (1-x)LaMg_1/2Ti_1/2O₃ xLa_2/3TiO₃. The phase previously reported as La₆MgTi₄O₁₈ was found to form off- composition, apparently as a point compound, at La₆Mg_0.913Ti_4.04O₁₈. Indexed X-ray powder diffraction patterns are given for LaMg_1/2Ti_1/2O₃, La₅Mg_0.5Ti_3.5O₁₅, La₆Mg_0.913Ti_4.04O₁₈, La₁₀MgTi₉O₃₄, and La₉Mg_0.5Ti_8.5O₃₁. LaMg_1/2Ti_1/2O₃ exhibits a slightly distorted perovskite structure with ordered B-cations (P2₁/n; a=5.608(2) , c=7.8610(5) , Β=90.034(4) ). La₅Mg_0.5Ti_3.5O₁₅ (P3ml; a=5.5639(1) c=10.9928(5) ) and La₆Mg_0.913Ti_4.04O₁₈ (R3m; a=5.5665(1) , c=39.7354(9) ) are n=5 and n=6 members, respectively, of the (111) perovskite-slab series A_nB_n-1O_3n. The new phases La₁₀Mgti₉O₃₄ (Pnnm; a=5.5411(2) , b=31.3039(9) , c=3.9167(1) and La₉Mg_0.5Ti_8.5O₃₁ (a=5.5431(2) , b=57.055(1) , c=3.9123(1) ) are n=5 and n=4.5 members, respectively, of the orthorhombic (110) perovskite-slab series A_nB_nO_3n+2. Extensive perovskite-type solid solutions form in the ternary sections LaMg_1/2Ti_1/2O₃-CaTiO₃-La₂O₃ and LaMg_1/2O₃-CaTiO₃-0.5La_2/3TiO₃. Dielectric properties (relative permittivity and temperature coefficient of resonant frequency, Π_f)were measured at frequencies between 5 GHz and 10 GHz and mapped onto the phase equilibria relations to reveal the compositions of temperature-stable (Π_f=0) compounds and mixtures. The La₂O₃-MgO-TiO₂ system contains two regions of temperature-stable compositions. The quaternary La₂O₃-CaO-MgO-TiO₂ system contains an extensive single- phase perovskite-type volume through which passes a surface of temperature-stable compositions with permittivites projected to be in the 40-50 range. Traces of this surface occur as lines of Π_f=0 perovskite-type compositions in the ternary sections LaMg_1/2Ti_1/2O₃-CaTiO₃ and LaMg_1/2O₃-CaTiO₃-0.5La_2/3TiO₃.

Citation

Journal of Solid State Chemistry

Volume

177

Issue

No. 6

Pub Type

Journals

Keywords

dielectric properties, La203-MgO-TiO2, La2MgTiO6, LaMg1/2Ti1/203-CaTiO3-La2/3TiO3, LaMg1/2Ti1/203- CaTiO3-La203, lanthanum magnesium titanates, temperature-stable ceramics

Citation

Vanderah, T. , Miller, V. , Levin, I. , Bell, S. and Negas, T. (2004), Phase Relations, Crystal Chemistry, and Dielectric Properties in Sections of the La₂O₃-CaO- MgO-TiO₂ System, Journal of Solid State Chemistry (Accessed April 23, 2024)