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Li- and Mg-codoped bismuth niobate pyrochlores: Synthesis, structure, electrical properties
摘要： The structure and conductivity of the new Li- and Mg-codoped bismuth niobates Bi1.5Mg1?xLixNb1.5O7?δ (0 ≤ x ≤ 0.50) with the pyrochlore structure have been investigated. The samples were synthesized by the method of organic-inorganic precursors combustion. A structural characterization was performed using 7Li nuclear magnetic resonance (NMR) spectroscopy in combination with the ?tting of X-ray di?raction patterns. The Li+ cations dynamics were studied by temperature-dependent 7Li NMR lineshape analysis. The measurements have shown that the Li+ cations are distributed in one of two possible sublattices in the structure and are not mobile up to 120 °C. According to the results of structural analysis of the Bi1.5Mg1?xLixNb1.5O7?δ (x = 0.25; 0.50) pyrochlores the lithium atoms are distributed in the bismuth sites. The electrical properties were investigated by impedance spectroscopy method in the air, oxygen and “wet” atmospheres in the 25–750 °C temperature range. The activation energy value of dc conductivity is about 1.2–1.3 eV for the all samples and corresponds to the ionic (oxygen) conductivity at T > 400 °C. Electronic (p-type) conductivity was determined at T < 360 °C. 1H MAS NMR data and the results of the comparison of the conductivity of Li- and Mg-codoped bismuth niobates in dry and “wet” atmospheres point to the proton conductivity up to 500 °C. The dielectric permittivity ε′ values increase with lithium content from 86 (x = 0) to 143 (x = 0.5) at the same dielectric loss tanδ = 0.002 (1 MHz, 25 °C), TCC values vary from ?590 to ?530 ppm/°C in the 25–280 °C temperature range.
关键词： Conductivity,Dielectric properties,Pyrochlore,Dopant distribution,NMR spectroscopy,Bismuth niobate
更新于2025-09-23 15:23:52
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Pyrochlore Ca-doped Gd2Zr2O7 solid state electrolyte type sensor coupled with ZnO sensing electrode for sensitive detection of HCHO
摘要： Solid state electrolyte type formaldehyde (HCHO) gas sensors based on a novel pyrochlore structure Gd2Zr2O7 solid electrolyte coupled with rod-shaped ZnO sensing electrode (SE) were initially designed and fabricated in this paper. The incorporation of alkaline-earth metal Ca can signi?cantly improve sensing performance of the Gd2Zr2O7 based sensors to HCHO. The response value (ΔV) of the sensors based on Gd2-xCaxZr2O7 (x = 0, 0.02, 0.05 and 0.1) varied approximately linear with the logarithm of HCHO concentration in the range of 1?100 ppm at 600 °C, with sensitivities of -25, -30, -15 and -5 mV/decade, respectively. The optimal sensor based on Gd1.98Ca0.02Zr2O7 (GCZ(0.02)) exhibited the excellent sensing characteristics with maximum response value (-61.3 mV) and extremely short response time (3 s) to 100 ppm HCHO. In addition, the GCZ(0.02) sensor also showed good selectivity and stability within 20-day period of continuous high-temperature aging under high-humidity condition. Conclusively, the sensing behavior and mechanism based on mixed potential were investigated and demonstrated by the mearsurement of the polarization curves.
关键词： Ca-doped Gd2Zr2O7,Rod-shaped ZnO,HCHO sensor,Pyrochlore solid electrolyte
更新于2025-09-19 17:13:59
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Surfaces of axion insulators
摘要： Axion insulators are magnetic topological insulators in which the nontrivial Z2 index is protected by inversion symmetry instead of time-reversal symmetry. The naturally gapped surfaces of axion insulators give rise to a half-quantized surface anomalous Hall conductivity (AHC), but the sign of the surface AHC cannot be determined from topological arguments. In this paper, we consider topological phenomena at the surface of an axion insulator. To be explicit, we construct a minimal tight-binding model on the pyrochlore lattice and investigate the all-in-all-out (AIAO) and ferromagnetic (FM) spin configurations. We also implement a recently proposed approach for calculating the surface AHC directly, which allows us to explore how the interplay between surface termination and magnetic ordering determines the sign of the half-quantized surface AHC. In the case of AIAO ordering, we show that it is possible to construct a topological state with no protected metallic states on boundaries of any dimension (surfaces, hinges, or corners), although chiral hinge modes do occur for many surface configurations. In the FM case, rotation of the magnetization by an external field offers promising means of control of chiral hinge modes, which can also appear on surface steps or where bulk domain walls emerge at the surface.
关键词： pyrochlore lattice,all-in-all-out,topological insulators,axion insulators,ferromagnetic,anomalous Hall conductivity
更新于2025-09-04 15:30:14