Investigations of crystal structure, phase compositions and intrinsic dielectric properties of novel Ba2RE2Si4O13 ceramics by bond theory and infrared spectroscopy

Novel Ba₂RE₂Si₄O₁₃ (RE = La, Nd, Sm, Eu, Gd, Ho, Er and Yb) ceramics were prepared by traditional solid reaction methods. The phase compositions of Ba₂RE₂Si₄O₁₃ ceramics were explored. The triclinic structure with P$\overline{1}$ space group of Ba₂RE₂Si₄O₁₃ ceramics was confirmed by TEM and Rietveld refinement analyses, and the decrease in the ionic radius of RE³⁺ induced the phase transition from low symmetry (triclinic) to high symmetry (monoclinic) between Ba₂Sm₂Si₄O₁₃ and Ba₂Eu₂Si₄O₁₃. ε_r-exp of the Ba₂RE₂Si₄O₁₃ ceramics was significantly affected by ionic polarisability. The ‘rattling and compressing effects’ of cations also affected the ε_r-exp of Ba₂RE₂Si₄O₁₃ ceramics. The intrinsic dielectric loss of Ba₂RE₂Si₄O₁₃ ceramics were evaluated by the far-IR reflectivity spectrum, and high Q × f values of the Ba₂Nd₂Si₄O₁₃ and Ba₂Eu₂Si₄O₁₃ ceramics were attributed to their large total lattice energy and activation energy. The average bond valence of RE³⁺ played an important role in controlling the τ_f values of the Ba₂RE₂Si₄O₁₃ single-phase ceramics, and the high average bond valence of RE³⁺ corresponded with the small negative τ_f values. Great microwave dielectric properties (ε_r = 11.52, Q × f = 33,600 GHz at 11.80 GHz and τ_f = ‒25.6 ppm/ °C) were obtained in the Ba₂Nd₂Si₄O₁₃ single-phase ceramic.