Microstructure Modulation And Dielectric Behaviors Of Perovskite LiCuNb₃O₉-based Ceramics

Dielectric materials are important and basic for manufacturing electronic components.Many non-ferroelectric transition-metal oxides have been found to have colossal permittivity in a wide temperature and frequency range,which meet the requirement of miniaturization and high performance for electronic devices.However,the involved mechanisms for colossal permittivity in such materials are diverse and controversial,resulting in a lack of clarity in improving and optimizing the performances of the oxides to achieve better practical application.For example,in some famous classes of colossal permittivity materials,e.g.CaCu₃Ti₄O₁₂,doped TiO₂ and doped SrTiO₃,the mechanisms of semiconducting grains,barrier layer capacitance,electron-pinned defect dipoles and local charge hopping were put forward.The reason for this situation is that the correlations among defects,local structures and dielectric response models in transition-metal oxides are not clear.Bearing all of these in mind,to address the aforementioned issues,we chose a novel A-site deficient quadruple perovskite LiCuNb₃O₉(□(Li_4/9Cu_4/9□_1/9)₃Nb₄O₁₂),with similar composition and crystal structure to quadruple perovskite CaCu₃Ti₄O₁₂,as a prototype compound.By using the conventional solid-state method,different elements are doped in the A′and B sites of LiCuNb₃O₉.Combining with X-ray diffraction（XRD）,bond valence sum（BVS）,X-ray photoelectron spectroscopy（XPS）,electron paramagnetic resonance（EPR）,temperature/frequency/magnetic field-dependent dielectric properties and magnetic behavior measurements,the effects of A′-site mixed valence states and A′/B-site local structures on the dielectric behavior and related mechanisms in the A-site deficient quadruple perovskite LiCuNb₃O₉ have been studied.This thesis provides a reference for understanding the correlations among defects,local structures and dielectric response models in this kind of non-ferroelectric transition-metal oxides.The main contents of this thesis are as follows:（1）The correlations among mixed valence ions,local structures and colossal permittivity in LiCuNb₃O₉ are verified.The LiCuNb₃O₉ ceramic shows room-temperature colossal permittivity（ε′>10⁴,1 k Hz）but quickly decreases down to around100 combining with a significant dielectric relaxation.It originates from the electrons in the A′-site mixed valences Cu⁺/Cu²⁺,i.e.a freezing/activation process for the localized electron.Under an electric field,the localized electrons will hop to other sites through the distorted NbO₆ octahedra.This polaron transport obeys the Mott variable range hopping mechanism,which induces a low-temperature dielectric relaxation process and room-temperature colossal permittivity.Besides,under a magnetic field,the dielectric relaxation in paramagnetic LiCuNb₃O₉ will shift to a higher temperature,and the hopping activation energy and hopping distance also be affected.The colossal permittivity behavior in LiCuNb₃O₉ comes from the A′-site mixed valences Cu⁺/Cu²⁺and B-site distorted NbO₆ octahedra related hopping polarons,which involves complex transport and interaction between local structures,electrons and/or lattices.（2）The contributions of A′-site polyvalent Cu ions on the colossal permittivity behaviors of LiCuNb₃O₉-based ceramics are clarified.The A′-site Ca²⁺substitution for Li⁺in Li_1-xCa_xCu Nb₃O₉（x=0.01,0.02,0.05）induces polyvalent Cu cations,i.e.Cu⁺/Cu²⁺/Cu³⁺,which brings two dielectric relaxations(T_R1 and T_R2).Comprehensive dielectric response and bulk dc conductivity analysis suggest that the low-temperature T_R1 relaxation follows the variable range hopping mechanism,in which the electron hopping between the mixed Cu⁺/Cu²⁺bridges via B-site NbO₆ octahedra with forming polarons,similar to the low-temperature dielectric relaxation in LiCuNb₃O₉ ceramic,while the high-temperature T_R2 relaxation mainly contributes from the Cu³⁺nearest neighbor hopping.Therefore,the dielectric relaxations come from the A′-site polyvalent Cu ions,but the A′-site alterations have little influence on the original low-temperature dielectric relaxation related colossal permittivity behavior in LiCuNb₃O₉ ceramic.（3）The impact of the B-site octahedral bridge on the low-temperature dielectric relaxation is verified.On the one hand,in LiCuNb_2.95M_0.05O₉（M=Mg,Ti）ceramics,different acceptor dopants substitution for Nb⁵⁺at B-site effectively causes multivalent states of A′-site Cu cations,more distorted NbO₆ octahedra and drives the charge transfer between the A′-site and B-site ions.On the other hand,upon Mg/Ti substitution,an overall increase of permittivity in the whole temperature range is achieved and polaron hopping is activated at much lower temperatures.There are two low-temperature dielectric relaxations and both of them obey the Mott-VRH model.Besides,the dielectric relaxation behaviors between the equivalent substituted LiCuNb_2.95Ta_0.05O₉ and LiCuNb₃O₉ceramics have fewer differences,which come from the A′-site Cu⁺/Cu²⁺ions and B-site distorted octahedra.Tuning the B-site octahedral bridges in the polaron hopping path can directly regulate the low-temperature dielectric relaxations,hopping activation energy and hopping distance.（4）The impact of the reduced distortion at B-site octahedra on the colossal permittivity of LiCuNb₃O₉ is shown,and multiple physical characteristics in A-site deficient quadruple perovskite LiCuTa₃O₉are detected.There are also mixed valence Cu⁺/Cu²⁺ions at the A′sites of LiCuNb_3-xTa_xO₉（x=1,1.5,2,3）.However,the distortion of B-site octahedra was reduced gradually with the increase of the substitution level x,resulting in the disappearance of room-temperature colossal permittivity,which further proves that the distorted B-site NbO₆ octahedra play an important role in the hopping polarons obeying Mott-VRH model.In the A-site deficient quadruple perovskite LiCuTa₃O₉,a phase transition with the ordered A′-site Cu²⁺cations may occur at low temperature,which induced the magnetic field controlled dielectric peaks,incipient ferroelectric behavior and low-temperature antiferromagnetic Cu-O-Ta-O-Cu super-exchange interaction.