Synthesis and High-Pressure Regulation of Hexagonal ReO3
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摘要: ReO3具有A位缺失的立方钙钛矿结构，在压力下会经历系列结构相变。近期，通过高压低温电阻测试，发现其高压R-Ⅰ相（空间群为R
$ \overline{3} $ c）具有高达 17 K的超导转变温度。为探索新型Re氧化物超导体，采用ReO3为前驱体，在10 GPa和600 ℃的高压高温条件下制备了具有六方对称性（空间群为P6322）的亚稳相，并在常压下对其晶体结构、磁性和电输运性质进行了表征。实验发现，六方亚稳相的电阻在常压下250 K附近出现明显异常，温度低至2 K时仍未出现超导现象。高压电阻测试表明：常压下250 K附近ReO3的电阻异常迅速消失，亚稳相表现出典型的金属行为；在62 GPa的高压条件下，温度低至1.5 K时仍未出现超导电性。Abstract: ReO3 with A-site-vacant perovskite structure undergoes sequential pressure-driven structural transformations. Recently, we found that its high-pressure rhombohedral R-Ⅰ phase (space group R$ \overline{3} $ c) is superconducting with an optimal superconducting transition temperature (Tc) of 17 K via high-pressure resistance measurements. To explore new superconductors among Re oxides, in this work we prepared a metastable hexagonal phase of ReO3 (space group P6322) by treating the ReO3 precursor under 10 GPa and 600 ℃, and characterized its crystal structure, magnetic and electrical transport properties. The results show that P6322 phase is not a superconductor down to 2 K at ambient pressure, but displays an anomaly around 250 K in resistivity. High-pressure resistance measurements show that the anomaly at about 250 K in ambient pressure disappears quickly upon compression, and P6322 phase shows typical metallic behavior in the whole temperature range without showing any signature of superconductivity down to 1.5 K under pressures up to 62 GPa. In the future, comparative theoretical studies of the hexagonal P6322 phase and the R-Ⅰ phase of ReO3 will help to understand the mechanism of superconductivity in this system. -
图 1 ReO3在高温高压处理前后的粉末XRD谱对比
Figure 1. Comparison of powder XRD patterns of ReO3 before and after high-pressure and high-temperature (HPHT) treatment
图 2 (a) 高温高压处理后ReO3粉末的XRD谱和Rietveld精修结果，(b) ReO3高压相的晶体结构和 (c) 氧原子晶格结构
Figure 2. (a) Powder XRD pattern and Rietveld refinement results of ReO3 after HPHT treatment;(b) crystal structure for the P6322 phase of ReO3 and (c) the structure of oxygen atoms
图 3 P6322相ReO3的磁性特征：(a) 在0.1 T外加磁场下，2～300 K温区的零场冷（zero-field-cooling, ZFC）和场冷（field cooling, FC）变温磁化率曲线χ(T)和磁化率倒数曲线χ−1(T)（粉色实线CW fitting为居里-外斯拟合曲线）；(b)不同温度下，−7 ～7 T磁场范围内的磁化曲线
Figure 3. Characterization of magnetic property for the P6322 phase of ReO3: (a) temperature dependence of magnetic susceptibility χ(T) and its inverse χ−1(T), measured in the zero-field-cooling (ZFC) and field cooling (FC) modes under external field of 0.1 T at 2−300 K (The Curie-Weiss (CW) fitting curve is shown by the pink solid lines.);(b) isothermal magnetization M(H) curves measured between −7−7 T at various temperatures
图 4 P6322相在常压下的变温电阻率曲线
Figure 4. Temperature dependence of resistivity for the P6322 phase at ambient pressure
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