Study On The Structure And Dielectric Properties Of Ba₈B₇O₂₄Perovskites Synthesized By Conventional And High Pressure Methods

Microwave dielectric ceramics,as new functional electronic materials developed in recent several decades,have the characteristics of high dielectric constant,low dielectric loss,and near-zero temperature coefficient of resonant frequency,which make them the key materials as resonators and filters for modern telecommunication.As a branch of microwave dielectric ceramic materials,Ba₈B₇O₂₄ type eight-layer hexagonal perovskite which with B-site defect has attracted much attention due to its advantages of two different structure types（shifted and twinned）,various preparation methods,excellent microwave dielectric properties and adjustable resonant frequency temperature coefficient with different B-site ion compositions.In this work,we have improved the problem of high temperature coefficient of resonant frequency of Ba₈B₇O₂₄ type eight-layer hexagonal perovskite which with B-site defect,study the transition mechanism of two different structural types of shifted and twinned and the influence of extreme physical conditions of high pressure on the structure of eight-layer hexagonal perovskite,and have tried to synthesise new eight-layer hexagonal perovskites which are difficult to obtain by conventional methods.The results of our work and research are as follows:（1）The low cost d¹⁰ Sb⁵⁺was used to replace the d⁰ Nb⁵⁺in Ba₈Co Nb₆O₂₄structure,and a new eight-layer hexagonal perovskite microwave dielectric ceramic Ba₈Co Nb_6-xSb_xO₂₄（x=0-1.8）was synthesised by the traditional high temperature solid phase method.Due to the different bonding preferences between d¹⁰ Sb⁵⁺cations and d⁰ Nb⁵⁺cations,the second-order Jahn-Teller effect between d¹⁰ Sb⁵⁺and d⁰ Nb⁵⁺is also different,which enables the transformation of Ba₈Co Nb_6-xSb_xO₂₄ structure from shifted to twinned at a very low doping amount（～15%）.At the same time,because the polarizability of Sb⁵⁺is lower than that of Nb⁵⁺,the dielectric constantε_r of the material is reduced,the resonant frequency temperature coefficientτ_f is changed by decreasing the dielectric constantε_r of the material.Finally,a new shifted structure perovskite ceramics Ba₈Co Nb_5.4Sb_0.6O₂₄ with a near-zero temperature coefficient of resonant frequency（τ_f～0.66 ppm/°C）was obtained.（2）Ba₈NiNb₆O₂₄ with twinned structure was treated by means of high temperature and high pressure in an environment of 1200°C and 10 GP for 30 minutes.As a result,the distance between the face-sharing octahedral（FSO）block of the twin structure decreases,and the electrostatic repulsion of the central cation increases,resulting in the less stable twin structure.Moreover,Nb⁵⁺cation radius decreases more under high pressure,making the size ratio of the originally small Ni²⁺ and Nb⁵⁺ion radius gradually increase.Combined with high temperature,the ions are rearranged,and eventually the structure of Ba₈NiNb₆O₂₄ is transformed from twin type to displacement type,which is irreversible under normal pressure.The shifted structure Ba₈NiNb₆O₂₄ has strong thermal stability and low electrical conductivity under normal pressure and high temperature.（3）Eight-layer hexagonal perovskite Ba₈MgTa₆O₂₄ and Ba₈MgNb₆O₂₄ were tried to synthesize by high temperature and high pressure method,at the pressure of 10GPa and the temperature of 1500°C-1700°C,the pressure and temperature was kept for 30minutes.The final products obtained were all mixtures of Ba₃MgM₂O₉and Ba₅M₄O₁₅（M=Ta,Nb）,no single phase of Ba₈MgTa₆O₂₄ or Ba₈MgNb₆O₂₄was obtained.