| Microwave dielectric ceramics are the key materials for 5G/6G communication technology and are mainly used to manufacture electronic components such as resonators,filters,and dielectric substrates.ABO4-type zircon-structured vanadiumbased microwave dielectric ceramics have attracted much attention due to their lower sintering temperature,low dielectric constant(εr)and high quality factor(Q×f),but the large negative temperature coefficient of resonant frequency(τf)limits their practical application.In this paper,a series of Eu1-xBiVO4(x=1/4,1/3,1/2),Y1-xLaxVO4(x=1/4,1/3,1/2)and YV1xNbxO4(0.1≤x≤0.9)ceramics were prepared by a solid-state reaction method using tetragonal AVO4-type(A=Eu,Y)zircon ceramics with substitution at the A and B sites,respectively.The phase composition,crystal structure,microstructure,vibrational spectrum,complex chemical bond characteristics and microwave dielectric properties were investigated using X-ray diffraction,Rietveld refinement,scanning electron microscopy,Raman spectrum,vector microwave network analyzer and other test and analysis methods.The factors affecting εr,τf,and Q×fvalues were elucidated from the perspectives of ion polarizability,packing fraction,bond length,bond valence,ionicity,lattice energy and bond energy,which provide the experimental and theoretical support for properties regulation of AVO4-type zirconstructured materials.The details are as follows:(1)A series of Eu1-xBixVO4(x=1/4,1/3,1/2)microwave dielectric ceramics were prepared by a solid-state reaction method using Bi3+to substitute Eu3+in the A-site.The relative densities of Eu3BiV4O16(x=1/4),Eu2BiV3O12(x=1/3)and EuBiV2O8(x=1/2)ceramics,all of which have a tetragonal zircon structure with space group of I41/amd,were greater than 96%sintered at optimum of 900℃;both Bi3+ and Eu3+ at the A-site are "rattling" cations and exhibit an overall "rattling" state.With increasing substitution,their εr gradually increases(14.2,19.9 and 28.7),Q×f gradually reduces(18,100,16,800 and 14,000 GHz)and τf rapidly increases(+34.9,+56.1 and+75.7 ppm/℃).As cell parameters gradually increase,the A-O bond length increases,and the overall "rattling" effect gradually increases,leading to the rise in the ion polarizability and εr values,with a consequent increase in the rf value.The full width at half maxima(FWHM)of the Big Raman mode gradually increases and the Q×f value decreases.Theoretical calculations of complex chemical bonds show that the average ionicity of Eu1-xBixVO4(x=1/4,1/3,1/2)ceramics gradually increases and is positively correlated with the change in εr values,Eu3BiV4O16 ceramic has the largest bond energy and a more close to zero τf value.(2)A series of Y1-xLaxVO4(x=1/4,1/3,1/2)microwave dielectric ceramics were prepared by a solid-state reaction method using La3+ to substitute Y3+ in the A-site.The relative densities of Y3LaV4O16(x=1/4),Y2LaV3O12(x=1/3)and YLaV2O8(x=1/2)ceramics,all of which have a tetragonal zircon structure with space group of I41/amd,were greater than 95%sintered at optimum of 1300℃;the A-site Y3+is a "rattling"cation,while La3+ is a "compressed" cation with a large deviation from the ideal value,resulting in an overall "compressed" state at the A-site.With increasing substitution,their gradually reduces(12.6,12.3 and 12.0),Q×f gradually increases(18,800,21,300 and 23,700 GHz)and τf gradually reduces(-44.1,-52.8 and-60.2 ppm/℃).The overall "compressed" effect gradually increases,causing a decrease in the ion polarizability,resulting in a decrease in the εr value and a negative shift in the τf value.The packing fraction increases,the FWHM of the B1g Raman mode decreases and the Q×f value increases.Theoretical calculations of complex chemical bonds show that the average ionicity of Y1-xLaxVO4(x=1/4,1/3,1/2)ceramics gradually decreases,which is positively correlated with the change in εr values,the lattice energy gradually increases,which is positively correlated with the change in Q×f values;Y3LaV4O16 ceramic has the largest bond energy and a more close to zero value of rf value.(3)A series of YV1-NbxO4(0.1≤x≤0.9)microwave dielectric ceramics were prepared by a solid-state reaction method with Nb5+ selected to substitute V5+in the Bsite,the tetragonal zircon phase was found to coexist with the monoclinic fergusonite phase at x≤0.7,which transformed into a monoclinic fergusonite pure phase at 0.8≤x≤0.9.The microwave dielectric properties in the range of x=0.1～0.8 were:εr=11.4～16.0,Q×f=21,100～49,900 GHz,τf=-55.8～-44.7 ppm/℃,the sample with x=0.8 has the largest εr and Q×f values;τf varies in an approximately linear relationship with εr,indicating that the tuning mechanism of YV1-xNbxO4 ceramics are the dilution of ion polarizability and Q×f values positively correlated with packing fraction. |
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