Subsolidus phase equilibria, crystal chemistry, and dielectric behavior were studied for the La2O3 MgO TiO2 system and for the ternary sections LaMg1/2Ti1/2O3 CaTiO3 La2O3 and LaMg1/2Ti1/2O3 CaTiO3 0.5La2/3TiO3 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 LaMg1/2Ti1/2O3, La5Mg0.5Ti3.5O15, and La6MgTi4O18 ; and the new phases La10MgTi9O34, La9Mg0.5Ti8.5O31, and a perovskite-type solid solution (1-x)LaMg1/2Ti1/2O3 xLa2/3TiO3. The phase previously reported as La6MgTi4O18 was found to form off-composition, apparently as a point compound, at La6Mg0.913Ti4.04O18. Indexed X-ray powder diffraction patterns are given for LaMg1/2Ti1/2O3, La5Mg0.5Ti3.5O15, La6Mg0.913Ti4.04O18, La10MgTi9O34, and La9Mg0.5Ti8.5O31. LaMg1/2Ti1/2O3 exhibits a slightly distorted perovskite structure with ordered B-cations (P21/n; a=5.608(2) , c=7.8610(5) , Β=90.034(4) ). La5Mg0.5Ti3.5O15 (P3ml; a=5.5639(1) c=10.9928(5) ) and La6Mg0.913Ti4.04O18 (R3m; a=5.5665(1) , c=39.7354(9) ) are n=5 and n=6 members, respectively, of the (111) perovskite-slab series AnBn-1O3n. The new phases La10Mgti9O34 (Pnnm; a=5.5411(2) , b=31.3039(9) , c=3.9167(1) and La9Mg0.5Ti8.5O31 (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 AnBnO3n+2. Extensive perovskite-type solid solutions form in the ternary sections LaMg1/2Ti1/2O3-CaTiO3-La2O3 and LaMg1/2O3-CaTiO3-0.5La2/3TiO3. Dielectric properties (relative permittivity and temperature coefficient of resonant frequency, Πf)were measured at frequencies between 5 GHz and 10 GHz and mapped onto the phase equilibria relations to reveal the compositions of temperature-stable (Πf=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 Πf=0 perovskite-type compositions in the ternary sections LaMg1/2Ti1/2O3-CaTiO3 and LaMg1/2O3-CaTiO3-0.5La2/3TiO3.
Citation: Journal of Solid State Chemistry
Issue: No. 6
Pub Type: Journals
dielectric properties, La203-MgO-TiO2, La2MgTiO6, LaMg1/2Ti1/203-CaTiO3-La2/3TiO3, LaMg1/2Ti1/203-CaTiO3-La203, lanthanum magnesium titanates, temperature-stable ceramics