Phase relations, crystal chemistry, and dielectric properties in sections of the La2O3–CaO–MgO–TiO2 system
| Autor: | V L. Miller, T Negas, Terrell A. Vanderah, Igor Levin, S M. Bell |
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| Rok vydání: | 2004 |
| Předmět: |
Materials science
Crystal chemistry Crystal structure Condensed Matter Physics Electronic Optical and Magnetic Materials Inorganic Chemistry Crystallography Materials Chemistry Ceramics and Composites Orthorhombic crystal system Physical and Theoretical Chemistry Ternary operation Powder diffraction Phase diagram Monoclinic crystal system Solid solution |
| Zdroj: | Journal of Solid State Chemistry. 177:2023-2038 |
| ISSN: | 0022-4596 |
| Popis: | Subsolidus phase equilibria and crystal chemistry were studied for the La2O3–MgO–TiO2 system and for the ternary sections LaMg1/2Ti1/2O3–CaTiO3–La2O3 and LaMg1/2Ti1/2O3–CaTiO3–La0.833Mg0.25Ti0.75O3 in the quaternary La2O3–CaO–MgO–TiO2 system. Dielectric properties (relative permittivity and temperature coefficient of resonant frequency, tf ) were measured at 5–10 GHz and mapped onto the phase equilibria relations to reveal the compositions of temperature-stable (tf ¼ 0) compounds and mixtures. Phase equilibria relations were obtained by X-ray powder diffraction analysis of approximately 80 specimens prepared by solid-state reactions in air at B1450 � C. Six ternary phases were found to form in the La2O3–MgO–TiO2 system, including the three previously reported compounds LaMg1/2Ti1/2O3 ,L a 5Mg0.5Ti3.5O15, and ‘‘La6MgTi4O18’’; and the new phases La10MgTi9O34, La9Mg0:5Ti8:5O31; and a perovskite-type solid solution (1 � x)LaMg1/2Ti1/2O3–xLa2/3TiO3 (0pxp0.5). The phase previously reported as ‘‘La6MgTi4O18’’ was found to form off-composition, apparently as a point compound, at La6Mg0.913Ti4.04O18. Indexed experimental X-ray powder diffraction patterns are given for LaMg1/2Ti1/2O3 ,L a 5Mg0.5Ti3.5O15 ,L a 6Mg0.913Ti4.04O18, La10MgTi9O34, and La9Mg0.5Ti8.5O31. LaMg1/2Ti1/2O3 exhibits a slightly distorted perovskite structure with ordered B-cations (P21=n; a=5.5608(2) A ˚ , b=5.5749(3) A ˚ , c=7.8610(5) A ˚ , b ¼ 90:034ð4Þ � ). La5Mg0.5Ti3.5O15 (P%m1; a=5.5639(1), c=10.9928(5) A ˚ ) and La6Mg0.913Ti4.04O18 (R3m; a=5.5665(1), c=39.7354(9) A ˚ ) are n ¼ 5 and n ¼ 6 members, respectively, of the (111) perovskiteslab series AnBn� 1O3n. The new phases La10MgTi9O34 (a=5.5411(2), b=31.3039(9), c=3.9167(1) A ˚ ) and La9Mg0.5Ti8.5O31 (a=5.5431(2), b=57.055(1), c=3.9123(1) A ˚ ) are n ¼ 5 and n ¼ 4:5 members, respectively, of the (110) perovskite-slab series AnBnO3n+2, which exhibit orthorhombic subcells; electron diffraction revealed monoclinic superlattices with doubled c-parameters for both compounds. Extensive perovskite-type solid solutions form in the ternary sections LaMg1/2Ti1/2O3–CaTiO3–La2O3 and LaMg1/2Ti1/2O3–CaTiO3–La0.833Mg0.25Ti0.75O3. The La2O3–MgO–TiO2 system contains two regions of temperature-stable (tf ¼ 0) 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 permittivities projected to be in the 40–50 range. Traces of this surface occur as lines of tf ¼ 0 perovskite-type phases in the ternary sections LaMg1/2Ti1/2O3–CaTiO3–La2O3 and LaMg1/2Ti1/2O3– CaTiO3–La0.833Mg0.25Ti0.75O3. |
| Databáze: | OpenAIRE |
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