Rattling and compressed cations resulting in distinct microwave dielectric characteristics of SrPrBO4 (B = Ga, Al) with K2NiF4 structure

In this work, tetragonal K2NiF4 structured SrPrBO4 (B = Ga, Al) ceramics with I4/mmm space group were synthesized via the conventional solid-phase method. High relative density (>95 %) and excellent microwave dielectric properties of epsilon(r) = 18.15, Qxf = 36,643 GHz, tau(f) = -20.20 ppm/degrees C for SrPrAlO4, and epsilon(r) = 22.02, Qxf = 27,475 GHz, tau(f) = +19.11 ppm/degrees C for SrPrGaO4 were obtained when sintered at 1500 degrees C and 1375 degrees C, respectively. Analyses of the bond valences show that the epsilon(r) and tau(f) values are closely related to the rattling and compressing effects of the A-site cation. The strong compressed effect of Sr2+ in SrPrAlO4 leads to its epsilon(r) value less than the epsilon(r(C-M)); while, the strong rattling effect of Pr3+ in SrPrGaO4 leads to its epsilon(r) value higher than the epsilon(r(C-M)), and thus to a positive tau(f). The Al/Ga-O bonds play a crucial role in the influence of the dielectric loss according to the P-V-L chemical bonding theory analysis. The E-dc of SrPrAlO4 (0.99 eV) and SrPrGaO4 (0.52 eV) indicates that the formation mechanisms of oxygen vacancies in both ceramics are different.