Synthesis,Structure And Electrical Properties Of The New Twinned Ba₈B_8-xO₂₄Hexagonal Perovskites

Among the reported hexagonal microwave dielectric materials,8-layer twinned hexagonal perovskites receive growing attention due the excellent microwave properties.In this work,we aim to search for new twinned hexagonal perovskites,and study their crystal structure and properties.Here,Ba₈Zr_x Ti_4-xTa₄O₂₄（BZTT）,Ba₈Zr_x Ti_4-xNb₄O₂₄（BZTN）,Ba₈Cr_4-xTa_4+0.6xO₂₄（BCT）,Ba₈Cr_4-x-x Nb_4+0.6x+0.6x O₂₄（BCN）and Ba₈Al_4-x-x Ta_4+0.6xO₂₄（BAT）are studied with their structure,phase competition and stabilization mechanism,electrical properties and effects that structure has on properties.The main study contents are below:First,the aimed ceramic system is Ba₈Zr_x Ti_4-xTa₄O₂₄（BZTT）.Ti⁴⁺in the 8-layer Ba₈Ti₃Ta₄O₂₄ is substituted by Zr⁴⁺to study how the cationic radium affects the phase formation,structure stabilization and properties.Single 10-layer twinned structure is isolated in x=0.1-0.8 at 1300°C.However,8-layer twinned hexagonal structure is isolated at a broader range（x=0-0.1）,and single 10-layer twinned hexagonal structure forms at a narrower range of x=0.2-0.4 when the samples are synthesized at 1400°C.Thus,the phase competition between 8-layer and 10-layer twinned structures is indicated in x=0.1 composition when the firing temperature is changed.But,this phase competition is suppressed when Zr increases,temperature is also considered to contribute to the phase preference.The Zr⁴⁺substitution for Ti⁴⁺has little influence on microwave and conducting properties according to the results of measurement on both 8-layer and 10-layer pure phases.The semicircle of the bulk grain shows two capacitance plateaus indicating the electrically heterogeneous microstructure.Second,Ti⁴⁺in Ba₈Ti₃Nb₄O₂₄ is substituted by Zr⁴⁺as performed in BZTT,thus Ba₈Zr_x Ti_4-xNb₄O₂₄（BZTN）ceramics are obtained.Both 8-layer and 10-layer twinned hexagonal structures are isolated,and the same solid solution range x=0-0.1of8-layer structure is formed compared to BZTT fired at 1400°C,but a much narrower10-layer phase solid solution range（only x=0.2）is indicated.No 10-layer to8-layer phase transformation was observed in this system.The pellet shows the similar plate-like morphology of crystals as in BZTT.The Zr-doped 8-layer and10-layer phases show lower Qf¹0 000 GHz,similar bulk conductivity¹0^-8-10^-5S/cm and activation energies¹.3-1.5eV compared with BZTT.The Zr⁴⁺substitution for Ti⁴⁺is also inoperative in enhancing microwave and conducting properties.Two capacitance plateaus in the semicircles of bulk grains of x=0,0.1,0.3 are observed again.The Ti reduction at a higher temperature during the complex impedance measurement or partially ordered and unordered crystallites may contribute to the heterogeneity in bulk grains.The exacted factors are not clear because TEM was not performed on BZTN composition,thus a further study is expected to be executed.Third,new 8-layer hexagonal perovskites were explored in Ba₈Cr_4-xTa（Nb）_4+0.6xO₂₄（BCT and BCN）and Ba₈Al_4-xTa_4+0.6xO₂₄（BAT）which contain trivalent cations.In the three ceramic systems,trivalent cations are substituted by Ta⁵⁺,thus introducing B-site cationic vacancies.In BCT,pure simple 8-layer twinned structure analogous to Ba₂CrTaO₆ was isolated in x=0-2.2,and 8-layer twinned superstructure and trace of Ba₅Ta₄O₁₅ grow in x=2.4,2.6,2.8.The parent material shows the highest conductivity up to 0.04 S/cm.Conductivity of Ta⁵⁺substituted x=1,2,2.4 compositions decreases but reaching 0.003 S/cm at high measuring temperature.Compositions x=2.6,2.8 isolated with 8-layer superstructure show comparable conductivity,but lower than that of compositions x<2.6.Qf values are not improved a lot after enhancing the pellet density to⁹0%,but showing a continuous increase.Theτ_f spans in-8-10 ppm/°C ensuring a slight shift of working frequency with a changed environment temperature.Relatively low permittivityε_r¹7-22 are obtained.Due to the limited time,BCN was only studied with a solid solution range of 8-layer twinned structure in x=0-1.2 which is much narrower than that of BCT.High content of 8-layer twinned hexagonal structure accompanied with a spot of BaAl₂O₄ forms in BAT,however,mixed Ba₅Ta₄O₁₅ and BaAl₂O₄ phases or mixed 10-layer,Ba₅Ta₄O₁₅ and Ba₃MgTa₂O₉-analogous phases were obtained in different repeating work giving a low reproducibility and indicating a metastable 8-layer twinned hexagonal perovskite in BAT.