Oxidation State of Uranium in A6Cu12U2S15 (A = K, Rb, Cs) Compounds

Black single crystals of A6Cu12U2S15 (A = K, Rb, Cs) have been synthesized
by the reactive
flux method. These isostructural compounds crystallize in the cubic
space group Ia3̅d at room
temperature. The structure comprises a three-dimensional framework
built from US6 octahedra and CuS3 trigonal planar
units with A cations residing in the cavities. There are no S–S
bonds in the structure. To elucidate the oxidation state of U in these
compounds, various physical property measurements and characterization
methods were carried out. Temperature-dependent electrical resistivity
measurement on a single crystal of K6Cu12U2S15 showed it to be a semiconductor. These three
A6Cu12U2S15 (A = K, Rb,
Cs) compounds all exhibit small effective magnetic moments, < 0.58
μB/U and band gaps of about 0.55(2) eV in their optical
absorption spectra. From X-ray absorption near edge spectroscopy (XANES),
the absorption edge of A6Cu12U2S15 is very close to that of UO3. Electronic band
structure calculations at the density functional theory (DFT) level
indicate a strong degree of covalency between U and S atoms, but theory
was not conclusive about the formal oxidation state of U. All experimental
data suggest that the A6Cu12U2S15 family is best described as an intermediate U5+/U6+ sulfide system of (A+)6(Cu+)12(U5+)2(S2–)13(S–)2 and (A+)6(Cu+)12(U6+)2(S2–)15.

通过反应熔盐法合成了A6Cu12U2S15（A为K、Rb、Cs）的黑色单晶。这些同构化合物在室温下以立方空间群Ia3̅d结晶。其结构由US6八面体和CuS3三角平面单元构成的立体框架构成，A阳离子位于空腔中。该结构中不存在S-S键。为阐明这些化合物中U的氧化态，进行了多种物理性质测量和表征方法。对K6Cu12U2S15单晶的室温电导率测量表明，它是一种半导体。这三个A6Cu12U2S15（A = K、Rb、Cs）化合物均表现出微小的有效磁矩，小于0.58 μB/U，并且在它们的可见光吸收光谱中具有约0.55(2) eV的带隙。通过X射线吸收近边光谱（XANES）分析，A6Cu12U2S15的吸收边与UO3非常接近。在密度泛函理论（DFT）水平上的电子能带结构计算表明，U和S原子之间具有强烈的共价性，但理论并未明确U的正式氧化态。所有实验数据均表明，A6Cu12U2S15系列最佳描述为(A+)6(Cu+)12(U5+)2(S2–)13(S–)2和(A+)6(Cu+)12(U6+)2(S2–)15的U5+/U6+硫化物中间体系。