## The Complexity of the CaF2:Yb System: A Huge, Reversible, X‑rayInduced Valence Reduction

CaF2中，Ce3+到Yb3+掺杂的晶体在γ-ray的辐照下都能被partially reduced。Yb3+掺杂CaF2能作为激光晶体，但是Yb2+掺杂的不能，因为存在anomalous emission(宽的发射峰，大的斯托克斯位移)。

Yb2+掺杂CaF2的发光研究：
(1) 1980s开始人们就将奇异发光归因于impurity trapped exciton model；
(2) 2014年的JCP，发现ab initio calculations could not support the ITE hypothesis and showed that the AE can be identified with an Yb2+/Yb3+ intervalence charge transfer (IVCT) luminescence instead.
(3) XANES和UV−vis emission spectra的实验表明发光强度is far from propotional to the Yb2+浓度。
(4) 最近的研究表明可能Yb2+-to-Yb3+ electron transfer is more efficient when the Yb3+ centers are nonlocally compensated and hence cubic. 而低掺杂Yb下会有更多比例的cubic Yb3+

• 掺杂浓度为 0.1%的样品，Yb主要是Yb3+，因此需要a charge compensating defect such as interstitial Fi or O2−，作者认为他的样品是O-free environment with a slight excess of F, so no O2− present。
• 存在不同的Yb3+ defects，有着不同的对称性，depending on the type and location of these charge-compensating defects.
• 在低掺杂浓度下(<0.1%)，只有少于30%的Yb是Yb2+，但是Yb2+的含量占比会随着总的掺杂浓度的减小而增大。
• 实验过程：The effects that X-ray exposure at different temperatures and subsequent thermal treatment have on the 2+/3+ valence conversion, the dopant local structures, and the Yb anomalous emission in dilute CaF2:Yb single crystals are studied using a combination of XANES, X-ray absorption fine structure (EXAFS), and UV−vis absorption and emission spectra plus ab initio calculations of the redox reaction$$\mathrm{F}_{i}^{-}+\mathrm{Yb}^{3+} \underset{T}{\stackrel{\mathrm{X} \text {-ray }}{\rightleftharpoons}} \mathrm{F}_{i}^{0}+\mathrm{Yb}^{2+}$$
• 实验现象：0.01%的样品，X射线辐照(温度200 K)后的还原为Yb2+的转化率最高，from f(2+) = 0.3 to f(2+) = 0.7，即使辐照200分钟后，依然有更多的Yb2+缓慢生成。辐照得到的Yb2+在200K下很稳定，但是如果放到室温下，几个小时候就恢复到原来的比例，如果放在320 K的环境下，只需要几分钟。
• 实验现象-奇异发光：在 as-grown的样品中，可以观察到奇异发光，但是lost after the X-ray exposure and throughout the thermal reoxidation process, and, surprisingly, is only recovered after annealing at high temperature (900 K), even though annealing does not change the Yb2+ concentration or the 2+/3+ fraction.
• 实验结果-理论计算：the existence of a critical distance between the dopant and the interstitial fluorine ions below which the $$\mathrm{F}_{i}^{0}-\mathrm{Yb}^{2+}$$ hole−electron recombine spontaneously and above which the $$\mathrm{F}_{i}^{0}-\mathrm{Yb}^{2+}$$ pair is metastable. This central feature, combined with temperature-dependent $$\mathrm{F}_{i}^{0}$$ and $$\mathrm{F}_{i}^{-}$$ mobilities, provides a model for interpreting the complex experimental observations

(1)XANES和EXAFS

• Further investigation showed that a large valence reduction only occurs near 200 K and is extremely unusual. (X射线辐照)
• 第一张图是200 K for 0.01% Yb样品，不同X射线辐照时间。
插图-a，表示的是200 K下辐照还原了，然后放到320 K的环境下，价态又回到了初始的状态；
插图b，showed that valence reduction is also achieved at 7940 eV, 1 keV below the Yb LIII edge; i.e., no excitation of the Yb ions is required.
• 第二张图，用a sum of two pseudo-Voigt peaks for the 2+ and 3+ lines plus a broadened step function for the main edge拟合出Yb2+和Yb3+的相对含量。两个曲线表示测了两次。
插图-a，Yb2++Yb3+的XANES峰的总面积为纵轴，Yb2+的比例为横轴。
插图-b，某一次拟合的示意图。
• EXAFS的作用是measurements probe the differences in local structure around Yb2+ and Yb3+ configurations。Yb占据的是Ca的格位，8 nearestneighbor F, 12 second neighbor Ca, and 24 third neighbor F atoms about the Yb atoms.
• 第三张图是EXAFS测试的结果，a large increase in nearest-neighbor Yb−F bond length for Yb2+ (red; reduced sample) compared to Yb3+ (black; fresh sample).
• 第四/五张图是拟合EXAFS的结果，分别对应fresh sample和valence reduced sample。
1. the closest Yb−F pair is contracted for Yb3+ and slightly expanded for Yb2+ relative to the Ca−F pair in the host material.
2. the second neighbor pair (Yb−Ca) has almost the same distance for both valence states