Structure And Diffusion Phase Transition Behavior Of Strontium Niobate Based Tungsten Bronze Ferroelectric Ceramics

Tungsten-bronze type ferroelectricity,[（A1）₂（A2）₄（B1）₂（B2）₈（C）₄O₃₀],is a popular and envirorment-friendly dielectric material.They exhibit unique ferroelectric,piezoelectric,and pyroelectric properties,which attract the great attention of scientists.However,the mechanism of electrical properties changes caused by structure manipulation need to be further explored for tungsten-bronze type ferroelectricity has complex structures.Therefore,this paper focuses on strontium niobate based tungsten-bronze type structural ceramics.The novel Sr₅BiTi₃Nb₇O₃₀ ceramic,(Ba_xSr_1-x)₄Sm₂Ti₄Nb₆O₃₀ ceramics,and Sr₄Sm₂Ti₄(Nb_1-xTa_x)₆O₃₀ ceramics were synthesized by standard solid-state reaction technique.The effects of ions substitution/occupation upon the structure,the diffusion phase transition,and the dielectric response were discussed and analyzed.An average structure with the centrosymmetric space group P4/mbm and a local non-centrosymmetric structure were confirmed for the tungsten-bronze Sr₅BiTi₃Nb₇O₃₀ceramic at room temperature.A broad permittivity peak with strong frequency dispersion was observed around 260 K,which followed well the Vogel-Fulcher relationship,and its maximum dielectric constant is about 800.The second harmonic measurement showed that the Sr₅BiTi₃Nb₇O₃₀ ceramic was of ferroelectricity at room temperature.The dielectric anomaly in Sr₅BiTi₃Nb₇O₃₀ ceramic,which explained by macroscopic and phenomenological statistical model,was associated with disorder on the A and B sites.The polar nanomicroregions（PNRs）are related to dielectric relaxation behavior.The magnitude of PNRs in Sr₅BiTi₃Nb₇O₃₀ ceramic is about 1.01 nm.The A2 sites of tungsten bronze Sr₄Sm₂Ti₄Nb₆O₃₀ ceramic was replaced by Ba²⁺with a larger radius.The dielectric response and high-temperature conductance behavior of(Ba_xSr_1-x)₄Sm₂Ti₄Nb₆O₃₀ ceramics were studyed over a broad temperature and frequency range.The tetragonal tungsten bronze structure in space group P4bm was confirmed for(Ba_xSr_1-x)₄Sm₂Ti₄Nb₆O₃₀ ceramics.The Dielectric anomalies were associated with the A2 site substitution.The variable-temperature Raman spectra indicated that the material undergoes a diffuse-ferroelectric phase transition when Sr²⁺is completely replaced by Ba²⁺.Raman spectra at room temperature showed that the dielectric relaxation behavior was related to the deformation of oxygen octahedral.The dielectric relaxation behavior was well described by Vogel-Fulcher law.Activation energy E_a associated with dipole response is between 0.039 e V and 0.064 e V,which reflects that the coupling between dipolar regions is weak.A phenomenological statistical model indicateed that the origin of the low-temperature dielectric relaxor was related to the size and number of PNRs.The magnitude of PNRs at lower temperatures are between 1.29 nm and 4.49 nm,while the magnitude of PNRs at higher temperatures are between 5.33 nm and 7.03 nm.The impact of ferroelectric active ions with the same B-site radius,various electron configurations,and polarizability on dielectric characteristics was discussed in Sr₄Sm₂Ti₄(Nb_1-xTa_x)₆O₃₀ ceramics.Two dielectric relaxation behaviors were found in Sr₄Sm₂Ti₄(Nb_1-xTa_x)₆O₃₀ ceramics at low temperatures,which is mainly caused by the disorder at B-sites and the distored octahedron.Raman spectra at room temperature indicated that interaction in BO₆ octahedral was increasing with the the decrease of Ta content.Not only does the doping of Ta⁵⁺weaken the ferroelectricity in Sr₄Sm₂Ti₄(Nb_1-xTa_x)₆O₃₀ ceramics,but it also weakens the coupling between the dipoles.Semi-quantitative parameters for dielectric dispersion were provided by a phenomenological statistical model.The magnitude of PNRs was positively correlated to the activation energy E_b,and it was related to the dielectric relaxation behavior.