Structure,Microwave Dielectric Properties And Their Relationship Of Aeschynite-type LaTiNbO₆ Ceramics

A series of RETi(Nb,Ta)O6(RE:rare-earth ions)materials with excellent microwave dielectric properties,usually crystalize in orthorhombic structure(O).However,when RE is La,the ceramic would be monoclinic structure(M),which shows different microwave dielectric properties from those materials of the same type but with other lanthanide elements.The structure-property relationship of this particular case is very important to comprehensively understand the structural evolution and excellent performance of the whole system.This thesis took LaTiNbO6 as the research object with the method of controlling the heat treatment process,improving the sintering behavior,introducing A-site or B-site ion substitution together with the spectroscopy analysis.The phase structure,sintering behavior,morphology and microwave dielectric properties of LaTiNbO6 ceramics were systematically studied.A special focus was on the relationship between the two structures and microwave dielectric properties of LaTiNbO6 ceramics.An M-O phase transition was induced by annealing the well-sintered LaTiNbO6 ceramics.A single O-type LaTiNbO6 ceramic was then achieved and its microwave dielectric properties were reported for the first time.M and O phases exhibited opposite-sign temperature coefficient of resonant frequency.By controlling the annealing time,the relative content of M and O phase gradually changed in the ceramic,meanwhile a series of new self-composite ceramics with continuously changed microwave dielectric properties were formed.Moreover,the special grain refinement appeared in the above-mentioned thermal induced phase transition was explained by a novel self-healing mechanism.The substitution of a small amount of Zn2+ for La3+ was found to effectively reduce the sintering temperature below the phase transition temperature,and a dense O-phase LaTiNbO6 ceramic was successfully obtained.This method overcame the deterioration in the density of O-phase LaTiNbO6 ceramics caused by the inner stress during the above annealing process.The corresponding sintering mechanism was also proposed by means of comparative studies on La-deficient LaTiNbO6 ceramics and the same amount of ZnO added LaTiNbO6 ceramics.Three typical isovalent cations(or complex ion)were used to replace the A-or B-site ions of LaTiNbO6 ceramics.It was found that the A-site substitution of lanthanides Ce3+and Sm3+,larger Bi3+ and smaller Al3+ all induced the M-O phase transition in well-sintered LaTiNbO6 ceramic matrix at relatively high temperatures.M phase was more prone to losing its stability with the appearance of O phase if the A-site substitution ions were smaller.At B sites,the substitution of larger Zr4+ for Ti4+ induced the M-O phase transition in well-sintered LaTiNbO6 matrix while the ceramics remained M phase with the substitution of equally-sized Ta5+ and smaller(W0.5Ti0.5)5+for Nb5+.Subsequently,in combination with the above A-and B-site substitution experimental results,the structure stability of M or O phase was found to mainly depend on the octahedral distortion in each phase,irrespective of the substitution ionic radius.In addition,the completely different microwave dielectric properties of M and O phase LaTiNbO6 essentially originated from the different connecting ways of the same double octahedral structure unit and the obviously different grain morphology.Besides,the relationship among the average radius of rare earth ions,phase structure and microwave dielectric properties in the whole RETiNbO6 system was also summarized.Finally,the typical A-and B-site ion-substituted samples were selected according to the two structures of LaTiNbO6 based on the above experimental results.Their Raman spectra and infrared reflectance spectra revealed that the dielectric constant of current system stemmed primarily from the contribution of A-BO6 external vibration in far-infrared band.The intrinsic loss was mainly related to BO6 octahedral distortion and grain boundary effect while the temperature coefficient of resonance frequency was closely related to the structural symmetry,BO6 octahedral tilt,and dielectric constant.Moreover,the effects of substitution ions at A-and B-site on the microwave dielectric properties of M or O phase LaTiNbO6 ceramics were made by changing the average bond valence of A-site ion,the BO6 octahedral distortion,polarizability,unit cell volume,and packing fraction of the matrix.In addition,a series of microwave dielectric ceramics with a near-zero temperature coefficient of resonant frequency and excellent performances was obtained in current system.