Ionic Substitution,Chemical Bond Characteristics And Microwave Dielectric Properties Of REVO₄ Ceramics

Microwave dielectric ceramics are widely used in electronic components（resonators,filters and dielectric substrates,etc.）and are key materials for 5G/6G communication technology.Based on the low dielectric constant（ε_r）and moderately high quality factor（Q×f）of ABO₄-type microwave dielectric ceramics,a series of low dielectric constant vanadium-based microwave dielectric ceramics were prepared by traditional solid-phase reaction method,and the effects of A and B site substitutions in rare-earth vanadates on the composition of material phase,crystal structure,microscopic morphology,bonding properties and microwave dielectric properties of the ceramics were discussed in depth,and the main factors affecting the values ofε_r,Q×f andτ_f in each system were also elucidated.The details are as follows:（1）A series of zircon-type La_1-xYb_xVO₄（x=1/2,2/3,3/4）ceramics with space group I4₁/amd were prepared by gradual substitution of Yb³⁺for La³⁺in the A-site of LaVO₄ ceramics,which were sintered densely at 1350,1350 and 1225℃and obtained the best microwave dielectric properties:ε_r=11.19,10.93 and 10.71,Q×f=23,500,20,300 and 19,400 GHz,τ_f=-34.6,-29.2 and-26.1 ppm/℃,respectively.With the increase of Yb³⁺substitution,the A-site bond valence and relative bond valence deviation of La_1-xYb_xVO₄ are decreasing,and the whole A-site ion(La³⁺/Yb³⁺)is in the compressed state,indicating that compression effect decreases in turn,and theτ_f values also change in the positive direction.The packing fractions（60.24%,59.34%and 58.63%）and lattice energies（31564,31373 and 31126 k J/mol）are positively correlated with the variation of Q×f values,indicating that the Q×f values of La_1-xYb_xVO₄ are correlated with the packing fraction and lattice energy.The ionicities of A-site ions of La_1-xYb_xVO₄ are strong,and the average ionicities of the ceramics decrease sequentially with increasing Yb³⁺content,leading to a sequential decrease in theε_r.（2）A series of tetragonal zircon-type La_1-xHo_xVO₄（x=1/2,2/3,3/4）ceramics with space group I4₁/amd were prepared by progressive substitution of Ho³⁺for La³⁺at the A-position in LaVO₄ ceramics,and all were sintered densely at 1350℃and obtained the best performance:ε_r=11.94,11.81 and 11.68,Q×f=18,100,17,500 and15,800 GHz,τ_f=-33.9,-27.6 and-25.1 ppm/℃,respectively.With the increase of Ho³⁺content,the bond valence and relative bond valence deviation of La_1-xHo_xVO₄ keep decreasing,and the compression effect of La_1-xHo_xVO₄ is weaker than that of La_1-xYb_xVO₄,hence theτ_f values are closer to zero.The packing fractions and lattice energies of La_1-xHo_xVO₄ are lower than those of La_1-xYb_xVO₄,and the Raman full width at half maxima of La_1-xHo_xVO₄ are wider,hence the Q×f values are relatively lower.In addition,the average ionicities of La_1-xHo_xVO₄ are stronger,soε_r are higher than that of La_1-xYb_xVO₄.（3）A series of Yb(V_1-xNb_x)O₄（0.1≤x≤0.9）ceramics were prepared by Nb⁵⁺substituting for V⁵⁺at the B-position in Yb VO₄ ceramics with microwave dielectric properties ofε_r=11.6～20.3,Q×f=21,130～62,630 GHz,τ_f=-39.5～-22.1 ppm/℃.The ceramic coexists in the tetragonal zircon and monoclinic fergusonite phases at 0.1≤x<0.8 and transforms to monoclinic fergusonite phase at 0.8≤x≤0.9,and the maximumε_r and Q×f values are obtained at x=0.8（20.3和62,630 GHz）.The values of Q×f is positively correlated with the packing fractions.