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Comment on “The Complexity of the CaF:Yb System: A Huge, Reversible, X-ray Induced Valence Reduction” Pavel P. Fedorov, and Alexandr I. Ryskin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12458 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018
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The Journal of Physical Chemistry
Commnet on “The Complexity of the CaF2:Yb System: A Huge, Reversible, X-ray Induced Valence Reduction” A.I. Ryskin*, P.P. Fedorov** *ITMO University, St-Petersburg, Russia ** Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow 119991, Russia E-mail:
[email protected] 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). Authors1 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. Shcheulin et al.4 experiments unequivocally proved that cross-section values for Yb(II) fd permitted transitions were about 10,000 times higher than the values of Yb(III) ff forbidden transitions (technique accuracy is about 0.0014*1018 cm-3). Shcheulin et al.4 technique 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 MacKeen et al. paper1 is surprising. Also we would like to note that MacKeen et al.1 statement “Our samples are grown in an O free environment with a slight excess of F, so no O2− present” is incorrect from the chemical point of view. It is very hard to get rid of oxygen impurities in fluorite. Use of 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 1. Therefore, 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+ – O2- dipoles in the crystal lattice 6. Perhaps, the latter would require correction to the model suggested in 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 1 are endeavored. References. ACS Paragon Plus Environment
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(1). MacKeen, C.; Bridges, F.; Seijo, L.; Barandiaran Z.; Kozina M.; Mehta, A.; Reid, M.F.; Wells, J.-P.R. The Complexity of the CaF2:Yb System: A Huge, Reversible, X-ray Induced Valence Reduction J. Phys. Chem. 2017, 121 28435– 28442. (2) Hughes-Currie, R. B.; Ivanovskikh, K. V.; Wells, J.-P. R.; Reid, M. F.; Gordon, R. A. The Determination of Dopant Ion Valence Distributions in Insulating Crystals Using XANES Measurements. J. Phys.: Condens. Matter. 2016, 28, 135502. (3) MacKeen, C.; Bridges, F.; Kozina, M.; Mehta, A.; Reid, M. F.; Wells, J.-P. R.; Barandiaran, Z. Evidence That the Anomalous Emission from CaF2:Yb2+ Is Not Described by the Impurity Trapped Exciton Model. J. Phys. Chem. Lett. 2017, 8, 3313–3316. (4) Shcheulin, A.S.; Angervaks, A.E.; Semenova, T.S.; Koryakina, L.F.; Petrova, M.A.; Fedorov, P.P.; Reiterov, V.M.; Garibin, E.A.; Ryskin, A.I. Additive Coloring of CaF2:Yb Crystals: Determination of Yb2+ Concentration in CaF2:Yb Crystals and Ceramics. Applied Physics B. Lasers and Optics. 2013, 111, 551-557. (5) Fedorov, P.P.; Osiko, V.V. Crystal Growth of Fluorides // In: Bulk Crystal Growth of Electronic, Optical and Optoelectronic Materials. Ed. P. Capper. Wiley Series in Materials for Electronic and Optoelectronic Applications. John Wiley & Son, Ltd. Chichester, UK. 2005. P. 339-356. (6) Jacobs, P.W.M.; Ong, S.H. Thermal depolarization in Crystals of Calcium Fluoride Doped with Oxygen. J. Phys. Chem. Solids. 1980, 41, 437-441.
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