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 Mg 1.8 R 0.2 Al 4 Si 5 O 18 (R Ni, Ni 1/2 Co 1/2 , Ni 1/3 Co 1/3 Zn 1/3 , Ni 1/4 Co 1/4 Zn 1/4 Cu 1/4 , Ni 1/5 Co 1/5 Zn 1/5 Cu 1/5 Mn 1/5 ) 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 Mg 1.8 R 0.2 Al 4 Si 5 O 18 ceramics. The presence of cation radius mismatch and electronegativity mismatch at [MgO 6 ]-octahedron lattice sites revealed lattice distortion resulting from multiple cation substitutions. As the entropy values increased, the temperature stability of microwave dielectric properties of Mg 1.8 R 0.2 Al 4 Si 5 O 18 ceramics improved. For R Ni 1/2 Co 1/2 , a higher Q×f of approximately 73,658 GHz @ 13 GHz, with ɛ r ∼4.44 and τ f  ∼ −23.9 ppm/°C was obtained. In the case of R Ni 1/5 Co 1/5 Zn 1/5 Cu 1/5 Mn 1/5 , a near zero τ f (−10.7 ppm/°C), ɛ r  = 4.68 and Q×f  = 35,478 GHz @13 GHz were achieved, making it potentially suitable for practical applications.