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)。

(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



  • 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。
  • 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

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