Abstract
K20 microwave dielectric ceramics SrLnAlO4 (Ln = Eu, Gd, and Ho) were prepared at 1440–1520 °C. SrEuAlO4 and SrGdAlO4 crystallized into tetragonal K2NiF4-type structures. While SrHoAlO4 was a mixture (SrHo2Al2O7, Ho2O3, and Sr3HoAl2O7.5) due to the too-small radius of Ho3+ to bond in [HoO9] dodecahedron. SrEuAlO4 and SrGdAlO4 exhibited near-zero τf values of +1.7 and +4.3 ppm/°C and low εr (19.37 and 19.14) along with contrasting Q×f values (70,030 and 5560 GHz). Compared with the modified molecular polarizability (αCorr) based on the bond valence deviation, the αobs values of SrEuAlO4 and SrGdAlO4 are much higher, indicating that their dielectric polarizabilities were underestimated, and this phenomenon was mainly influenced by rattling cations, which also causes their τf with near-zero values. Due to the magnetic subsystem adversely affecting microwave dielectric losses, the worsened Q×f value of SrGdAlO4 compared to that of SrEuAlO4 is due to the high magnetic moment of Gd3+ (7.94 μB).
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