Recently, MacKeen et al.(1) published a paper dedicated to an important issue: CaF₂:Yb crystals are widely used for generating radiation in the near-infrared part of the spectrum range with the use of Yb(III) ions. Low “quantum defect” value for Yb(III) ion lasing under diode pumping provides a significant advantage to these crystals, but CaF₂:Yb is prone to Yb(III) reduction down to Yb(II). The authors(1) mention that their study became possible only after they developed their protocol to determine Yb(II) concentration in CaF₂:Yb crystals in the background of Yb(III) ions.(2,3) However, said method has been known since earlier times, when Shcheulin et al.(4) published their technique of calculating Yb(II) concentration by determining cross sections of Yb(II) absorption bands in CaF₂:Yb matrices. Development of this technique was based on conversion of all Yb(III) ions in the standard sample to Yb(II) by reduction with calcium metal vapor. Experiments by Shcheulin et al.(4) unequivocally proved that cross-section values for Yb(II) f → d permitted transitions were about 10,000 times higher than the values of Yb(III) f → f forbidden transitions (technique accuracy is about 0.0014 × 10¹⁸ cm^–3). The technique of Shcheulin et al.(4) is simple and available for everyone utilizing polished monocrystalline or optical ceramics plates. The lack of reference to the Shcheulin et al.(4) method in the MacKeen et al. paper(1) is surprising. Also we would like to note that the MacKeen et al.(1) statement “Our samples are grown in an O free environment with a slight excess of F, so no O^2– present” is incorrect from the chemical point of view. It is very hard to get rid of oxygen impurities in fluorite. Use of an inert atmosphere/vacuum along with fluorinating agents (e.g., gaseous CF₄) or oxygen scavengers (e.g., PbF₂) does not allow eliminating oxygen impurities in CaF₂ crystals below 0.001 wt %.(5) The latter value is comparable with the level of ytterbium doping in ref (1). Therefore, the authors(1) needed a complex oxygen impurity analysis in their precision studies. It is quite obvious that, due to its (−2) formal charge, oxygen should be located closer to Yb(III) ions, thus forming Yb³⁺–O^2– dipoles in the crystal lattice.(6) Perhaps, the latter would require correction to the model suggested in ref (1). Also, it would be highly desirable to carry out a complete analysis of all contaminants in the specimens when such precision studies as the ones in ref (1) are endeavored. The authors declare no competing financial interest. This article references 6 other publications.

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关于“ CaF ₂：Yb系统的复杂性：巨大的，可逆的，X射线诱导的化合价降低”的评论

最近，MacKeen等人（1）发表了一篇致力于重要问题的论文：CaF ₂：Yb晶体被广泛用于利用Yb（III）离子在光谱范围的近红外部分产生辐射。二极管泵浦下Yb（III）离子激光的低“量子缺陷”值为这些晶体提供了显着优势，但CaF ₂：Yb易于将Yb（III）还原至Yb（II）。作者（1）提到，只有在他们制定了确定CaF _2中Yb（II）浓度的方案后，他们的研究才有可能：Yb（III）离子背景下的Yb晶体。（2,3）但是，这种方法早在Shcheulin等人（4）公布了通过测定交叉离子来计算Yb（II）浓度的技术以来就已为人所知。 CaF ₂：Yb矩阵中Yb（II）吸收带的截面。这项技术的开发基于标准样品中所有的Yb（III）离子通过用钙金属蒸气还原而转化为Yb（II）。Shcheulin等人（4）的实验明确证明，Yb（II）f→d允许跃迁的横截面值比Yb（III）f→f禁止跃迁的横截面值高约10,000倍（技术精度约为0.0014 ×10 ¹⁸厘米^–3）。Shcheulin等人（4）的技术很简单，适用于使用抛光的单晶或光学陶瓷板的每个人。缺乏对MacKeen等人的Shcheulin等人（4）方法的参考。paper（1）令人惊讶。我们还想指出，MacKeen等人（1）的说法“从化学角度来看，我们的样品是在不含F的无氧环境中生长的，因此不存在O ² ”。很难除去萤石中的氧杂质。惰性气氛/真空以及氟化剂（例如气态CF ₄）或除氧剂（例如PbF ₂）的使用不能消除CaF _2中的氧杂质低于0.001 wt％的晶体。（5）后者的值与参考文献（1）中的do掺杂水平相当。因此，作者（1）在其精密度研究中需要进行复杂的氧杂质分析。很明显，由于其（-2）形式电荷，氧应位于更靠近Yb（III）离子的位置，从而在晶格中形成Yb ³⁺ –O ^2–偶极子。（6）也许是后者将需要对参考文献（1）中建议的模型进行校正。同样，当需要进行如参考文献（1）所述的精密研究时，非常需要对样品中的所有污染物进行完整的分析。作者宣称没有竞争性的经济利益。本文引用了其他6个出版物。