Phase relations, crystal chemistry, and dielectric properties in sections of the La2O3-CaO-MgO-TiO2 system

Subsolidus phase equilibria and crystal chemistry 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-La_0.833Mg_0.25Ti_0.75O₃ in the quaternary La₂O₃-CaO-MgO-TiO₂ system. Dielectric properties (relative permittivity and temperature coefficient of resonant frequency, τ_f) were measured at 5-10 GHz and mapped onto the phase equilibria relations to reveal the compositions of temperature-stable (τ_f=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 ∼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 (0?x?0.5). The phase previously reported as “La₆MgTi₄O₁₈” was found to form off-composition, apparently as a point compound, at La₆Mg_0.913Ti_4.04O₁₈. Indexed experimental 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=5.5608(2) Å, b=5.5749(3) Å, c=7.8610(5) Å, β=90.034(4)°). La₅Mg_0.5Ti_3.5O₁₅ (Pm1; a=5.5639(1), c=10.9928(5) Å) and La₆Mg_0.913Ti_4.04O₁₈ (R3m; a=5.5665(1), c=39.7354(9) Å) are n=5 and n=6 members, respectively, of the (111) perovskite-slab series A_nB_n−1O_3n. The new phases La₁₀MgTi₉O₃₄ (a=5.5411(2), b=31.3039(9), c=3.9167(1) Å) and La₉Mg_0.5Ti_8.5O₃₁ (a=5.5431(2), b=57.055(1), c=3.9123(1) Å) are n=5 and n=4.5 members, respectively, of the (110) perovskite-slab series A_nB_nO_3n+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 LaMg_1/2Ti_1/2O₃-CaTiO₃-La₂O₃ and LaMg_1/2Ti_1/2O₃-CaTiO₃-La_0.833Mg_0.25Ti_0.75O₃. The La₂O₃-MgO-TiO₂ system contains two regions of temperature-stable (τ_f=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 τ_f=0 perovskite-type phases in the ternary sections LaMg_1/2Ti_1/2O₃-CaTiO₃-La₂O₃ and LaMg_1/2Ti_1/2O₃-CaTiO₃-La_0.833Mg_0.25Ti_0.75O₃.