EXAFS光谱和反向蒙特卡罗模拟研究NiWO $_4$和NiWO $_4$ - ZnWO $_4$固溶体中镍离子二聚化的证据
Evidence of nickel ions dimerization in NiWO$_4$ and NiWO$_4$-ZnWO$_4$ solid solutions probed by EXAFS spectroscopy and reverse Monte Carlo simulations
摘要：
通过X射线吸收光谱结合反向蒙特卡罗（RMC）模拟，发现在高镍含量的钨型NiWO $_4$和Zn $_c$ Ni $_{1-c}$ WO $_4$固溶体中存在交换耦合的$^{2+}$离子--所谓的磁性二聚体.在微晶NiWO $_4$、Zn $_c$、Ni $_{1-c}$ WO $_4$和ZnWO $_4$的Ni K边、Zn K边和W L $_3$边测量了温度（10- 300 K）和成分依赖的X射线吸收光谱。结构模型，获得同时分析的扩展X射线吸收精细结构（EXAFS）光谱在三个金属吸收边，使用RMC模拟。不同原子对的径向分布函数使得详细追踪金属离子局域环境的变化和热无序的影响成为可能。在整个研究的温度范围内，在NiWO $_4$中证明了[NiO $_6$ ]八面体的准一维锯齿形链内的Ni $^{2+}$离子的二聚化。它表现为Ni-Ni径向分布函数分裂成两个单独的峰。该效果进一步保存在固溶体Zn $_c$ Ni $_{1-c}$ WO $_4$中用于$c \leq 0.6$，这与在相邻位置找到两个Ni $^{2+}$离子的概率有关。
英文摘要：
The existence of exchange-coupled Ni$^{2+}$ ions -- the so-called magnetic dimers -- in wolframite-type NiWO$_4$ and Zn$_c$Ni$_{1-c}$WO$_4$ solid solutions with high nickel content was discovered by X-ray absorption spectroscopy combined with reverse Monte Carlo (RMC) simulations. Temperature- (10--300 K) and composition-dependent x-ray absorption spectra were measured at the Ni K-edge, Zn K-edge, and W L$_3$-edge of microcrystalline NiWO$_4$, Zn$_c$Ni$_{1-c}$WO$_4$ and ZnWO$_4$. Structural models were obtained from simultaneous analysis of the extended x-ray absorption fine structure (EXAFS) spectra at three metal absorption edges using RMC simulations. The obtained radial distribution functions for different atomic pairs made it possible to trace in detail the changes in the local environment of metal ions and the effect of thermal disorder. Dimerization of Ni$^{2+}$ ions within quasi-one-dimensional zigzag chains of [NiO$_6$] octahedra was evidenced in NiWO$_4$ in the whole studied temperature range. It manifests itself as the splitting of the Ni-Ni radial distribution function into two separate peaks. The effect is further preserved in solid solutions Zn$_c$Ni$_{1-c}$WO$_4$ for $c \leq 0.6$, which is related to the probability to find two Ni$^{2+}$ ions in neighboring positions.
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