High-entropy Mg1.8R0.2Al4Si5O18 (R = Ni, Co, Zn, Cu, Mn) cordierite ceramics: Influence of octahedral distortion and electronegativity mismatch on the microwave dielectric properties

In this study, high-entropy Mg1.8R0.2Al4Si5O18 (RNi, Ni1/2Co1/2, Ni1/3Co1/3Zn1/3, Ni1/4Co1/4Zn1/4Cu1/4, Ni1/5Co1/5Zn1/5Cu1/5Mn1/5) cordierite ceramics based on equimolar ratios were designed and synthesized through the solid-state reaction technique at high temperatures. The effect of various R-ionic substitutions at the octahedral site and their relationship with ionic polarization, chemical bonding, and configurational entropy were explored. These factors have greatly affected the microwave dielectric properties of Mg1.8R0.2Al4Si5O18 ceramics. The presence of cation radius mismatch and electronegativity mismatch at [MgO6]-octahedron lattice sites revealed lattice distortion resulting from multiple cation substitutions. As the entropy values increased, the temperature stability of microwave dielectric properties of Mg1.8R0.2Al4Si5O18 ceramics improved. For RNi1/2Co1/2, a higher Q×f of approximately 73,658 GHz @ 13 GHz, with ɛr ∼4.44 and τf ∼ −23.9 ppm/°C was obtained. In the case of RNi1/5Co1/5Zn1/5Cu1/5Mn1/5, a near zero τf (−10.7 ppm/°C), ɛr = 4.68 and Q×f = 35,478 GHz @13 GHz were achieved, making it potentially suitable for practical applications.