用近边和扩展X射线吸收精细结构研究Sr $_{2-x}$ La $_x$ CoNbO $_6$的电子和局域结构
Probing the electronic and local structure of Sr$_{2-x}$La$_x$CoNbO$_6$ using near-edge and extended x-ray absorption fine structures
摘要：
利用X射线吸收近边结构（XANES）和扩展X射线吸收精细结构（EXAFS）对双钙钛矿结构Sr $_{2-x}$ La $_x$ CoNbO $_6$（$x=$ 0- 1）的电子和局域结构进行了研究。$initio$ $initio$模拟和对Nb和Co的$K$边XANES谱的详细分析表明，观察到的边前特征是由1个$s$电子跃迁到混合的$p-d$杂化态引起的.我们揭示了一个$z$ -出扬-泰勒（JT）失真的CoO $_6$八面体，这是单调下降，由于增强JT非活性的Co $^{2+}$离子与$x$。另一方面，在NbO $_6$八面体中的$z$ -in畸变保持不变直到$x=$ 0.4，然后随着$x$的进一步增加而减小。发现Nb原子周围局部配位的突然变化是$x \geqslant$ 0.6样品中反铁磁相互作用演化的原因。此外，我们还建立了八面体畸变程度与XANES谱中白线特征强度之间的相关性，并讨论了可能的原因。更有趣的是，我们观察到的签名KN $_1$双电子激发的Sr $K$边缘EXAFS光谱的所有样品，这是发现在很好的协议与$Z$ +1近似。此外，Co L $_{2,3}$边缘显示出晶体场强度的降低，并因此显示出电荷转移能（$Δ_{ct}$）随着La取代而增加。
英文摘要：
We report the electronic and local structural investigation of double pervoskites Sr$_{2-x}$La$_x$CoNbO$_6$ ($x=$ 0--1) using x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structures (EXAFS) at the Nb, Co, and Sr $K$-edges. The $ab$ $initio$ simulations and detailed analysis of the Nb and Co $K$-edge XANES spectra demonstrate that the observed pre-edge features arise from the transition of 1$s$ electrons to the mixed $p-d$ hybridized states. We reveal a $z$-out Jahn-Teller (JT) distortion in the CoO$_6$ octahedra, which decreases monotonically due to an enhancement in the JT inactive Co$^{2+}$ ions with $x$ . On the other hand, the $z$-in distortion in NbO$_6$ octahedra remains unaltered up to $x=$ 0.4 and then decreases with further increase in $x$. This sudden change in the local coordination around Nb atoms is found to be responsible for the evolution of the antiferromagnetic interactions in $x \geqslant$ 0.6 samples. Also, we establish a correlation between the degree of octahedral distortion and intensity of the white line feature in the XANES spectra and possible reason for this are discussed. More interestingly, we observe the signature of KN$_1$ double electron excitation in the Sr $K$-edge EXAFS spectra for all the samples, which is found to be in good agreement with the $Z$+1 approximation. Further, the Co L$_{2,3}$ edge shows the reduction in the crystal field strength and hence an increase in the charge transfer energy ($Δ_{ct}$) with the La substitution.
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