X-ray Excitation Triggers Ytterbium Anomalous Emission in CaF2:Yb

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Letter

X-ray Excitation Triggers Ytterbium Anomalous Emission in CaF :Yb but Not in SrF:Yb. 2

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Rosa B. Hughes-Currie, Konstantin V. Ivanovskikh, Jon-Paul R. Wells, Michael F Reid, Robert A. Gordon, Luis Seijo, and Zoila Barandiaran J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00262 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 25, 2017

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The Journal of Physical Chemistry Letters

X-ray Excitation Triggers Ytterbium Anomalous Emission in CaF2:Yb but not in SrF2:Yb. Rosa B. Hughes-Currie,† Konstantin V. Ivanovskikh,†, Jon-Paul R. Wells,‡ Michael F. Reid,‡ Robert A. Gordon,¶ Luis Seijo,§ and Zoila Barandiarán∗,§ †Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8140, New Zealand ‡Dodd-Walls Centre for Quantum and Photonic Technologies, Department of Physics and Astronomy, University of Canterbury, PB4800, Christchurch 8140, New Zealand ¶Moyie Institute and Deptartment of Physics, Simon Fraser University, Burnaby, BC, Canada §Departamento de Química, Instituto Universitario de Ciencia de Materiales Nicolás Cabrera, and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain Institute of Physics and Technology, Ural Federal University, 620002 Ekaterinburg, Russia E-mail: [email protected]

Abstract

Graphical TOC Entry

Materials that luminesce after excitation with ionizing radiation are extensively applied in physics, medicine, security, and industry. Lanthanide dopants are known to trigger crystals scintillation through their fast d-f emissions; the same is true for other important applications as lasers or phosphors for lighting. However, this ability can be seriously compromised by unwanted anomalous emissions often found with the most common lanthanide activators. We report high resolution X-ray excited optical (IR to UV) luminescence spectra of CaF2 :Yb and SrF2 :Yb samples excited at 8949 eV and 80 K. Ionizing radiation excites the known anomalous emission of ytterbium in the CaF2 host but not in the SrF2 host. Wavefunction-based ab initio calculations of host-to-dopant electron transfer and Yb2+ /Yb3+ intervalence charge transfer explain the difference. The model also explains the lack of anomalous emission in Ybdoped SrF2 excited by VUV radiation.

Keywords Radioluminescence, X-ray excited optical luminescence, anomalous emission, ab initio, MMCT, IVCT, electron transfer, Yb, CaF2 , SrF2 , fluorites.

ACS Paragon Plus Environment

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centration of Yb2+ . However, this is not incompatible with the IVCT luminescence interpretation of the anomalous emission of Ref. 8: The proposed mechanism for the UV excitation includes a non-radiative intermediate step where an electron is transferred from the 4f 13 5d excited Yb2+ to an Yb3+ . Such non-radiative electron transfer should be more likely towards uncompensated Yb3+ acceptor ions (i.e. remotely compensated by distant interstitial fluorine ions F− i ) as suggested by Welber in an experimental study of electron transfer from divalent to trivalent lanthanides. 10 Such special cubic sites are more probable as the concentration is lowered. In this work we present high resolution Xray excited optical luminescence spectra (IR to UV) of Yb-doped CaF2 and SrF2 crystals where samples are excited with 8949 eV radiation at 80 K. The spectra show that the ionizing radiation excites the anomalous emission of ytterbium in the CaF2 host but not in the SrF2 host. This difference is observed regardless of the Yb concentration. This is consistent with VUV experiments in these materials, where anomalous emission is observed in Yb-doped CaF2 on excitation into the host free exciton levels, but not in Yb-doped SrF2 (unpublished results). 11 Calculations of diabatic potential energy surfaces and configurational energy diagrams for metal-to-metal charge transfer (MMCT) in Ybdoped CaF2 and SrF2 have been previously used to explain photoconductivity in these materials. 12 Here we use those calculations to model emission following host electron capture. Electron capture by Ca2+ after thermalization gives Ca+ . Subsequent charge transfer Yb3+ + Ca+ → Yb2+ + Ca2+ leaves the system in an excited state that can radiate by intervalence charge transfer between the dopant ions, Yb2+ + Yb3+ → Yb3+ + Yb2+ . In the case of SrF2 , non-radiative decay makes this emission impossible. The high resolution X-ray excited luminescence spectra of CaF2 :Yb and SrF2 :Yb crystals are shown in Fig. 1. In all spectra, the broad bands peaking around 35000 cm−1 can be associated with the host self-trapped exciton (STE) emissions; 13 dips are observed in the STE emissions of SrF2 at 28000 cm−1 , 36800 cm−1 , and

The anomalous emission of Yb2+ in CaF2 and SrF2 , detected for the first time in the 50’s, 1 is characterized by a very broad emission band whose peak energy is much lower than the UV 4f 14 → 4f 13 5d absorption used to excite it, so that an anomalous Stokes shift is observed. Anomalous emissions are not infrequent and are most abundant in Ce3+ , Pr3+ , Eu2+ , and Yb2+ doped crystals. 1–5 McClure and collaborators proposed the impurity-trapped exciton (ITE) model to explain the anomalous emission: 3,6 In this model the excited state responsible for the anomalous emission is a bound electron-hole pair with the electron delocalized outside the YbF8 volume, presumably over the nearest Ca or Sr neighbors, and the hole localized at the Yb site. This allows for a large shrinkage of the Yb–F bond length upon the 4f 13 5d →ITE relaxation and explains the band width of the emission. The ITE model has been widely accepted in the literature for over 30 years. 3,4,6 Ab initio studies of the interplay between regular 5d → 4f and anomalous emissions in Cedoped and Yb-doped crystals do not support the ITE model. Rather, they suggest that the anomalous emission is an intervalence charge transfer (IVCT) luminescence which involves electron transfer from Yb2+ to Yb3+ (or Ce3+ to Ce4+ ). 7,8 The large local relaxations associated with the charge transfer explain the very large band widths and Stokes shifts of the emissions. Recent XANES experiments in Yb-doped CaF2 and SrF2 have shown that divalent and trivalent Yb coexist and the 2+/3+ ratio grows as the nominal Yb concentration decreases. 9 Furthermore, combined X-ray absorption spectroscopy and UV-vis studies have demonstrated the breakdown of the ITE model in the CaF2 :Yb system experimentally (unpublished results): When the Yb concentration is lowered from 0.1 to 0.01 % Yb, the 2+/3+ ratio grows from 0.21 to 0.35, hence the concentration of Yb2+ decreases from 0.021 to 0.0035 %; however, the UV excited anomalous emission increases dramatically (unpublished results). This is difficult to reconcile with the ITE model, which predicts that the anomalous emission should be proportional to the con-


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38400 cm−1 , in close correspondence with intra Yb2+ f -d absorptions. 6 The sharp peaks around 10000 cm−1 correspond to Yb3+ emissions. 14 The relative intensity of the host STE emission is shown to decrease with increasing Yb doping.

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only in CaF2 (unpublished results). 11 This is consistent with the X-ray measurements reported here. In the case of Yb-doped CaF2 we explained the anomalous emission as energy transfer from the STE states to Yb2+ . However, this model was not quantitative, and did not explain why the anomalous emission is absent in Yb-doped SrF2 (unpublished results). We now present an explanation of the presence or absence of anomalous Yb emission based on the assumption that a conductionband electron, excited by the X-rays, becomes localized in the lowest 3d or 4d orbital of a Ca+ or Sr+ ion. Yb3+ ions are subsequently reduced by charge transfer, leaving an excited Yb2+ and Ca2+ or Sr2+ in the ground state. The necessary calculations were performed in previous work, where they were used to investigate the mechanisms of dopant-to-host electron transfer in photoconductivity experiments. 12 Metal-to-metal charge transfer (MMCT) configuration diagrams of Yb/Ca pairs in CaF2 and Yb/Sr pairs in SrF2 are presented in Fig. 2. Details of the calculations are given in Ref. 12. Here, only the features/processes relevant to interpret the new X-ray excited luminescence experiments are discussed. These diagrams are diabatic, meaning that they ignore electronic interactions between the Ca and Yb or Sr and Yb ions. These interactions would have only a small effect on the parts of the diagrams that are of importance to our model. According to the calculations, the hostto-dopant electron transfer that excites the anomalous emission of Yb in the CaF2 host can occur as illustrated by the red dashed arrow in the leftmost graph of Fig. 2: (i) After ionizing excitation, a host electron relaxes to the lowest host state found in the MMCT calculations: the [Yb3+ 4f 13 (2 F7/2 ) – Ca+ 3deg ] state (lowest of Yb3+ -Ca+ energy curves, in cyan in Fig. 2, left). At the minimum of this state, both Yb3+ and Ca+ centers are structurally and electronically relaxed in their ground states; yet, the MMCT energy diagram clearly shows it is unstable to metal-to-metal electron transfer. (ii) Non-radiative electron transfer can occur at the MMCT crossing (at the ori-

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Figure 1: X-ray excited optical luminescence of CaF2 :Yb and SrF2 :Yb at a sample temperature of 80 K excited at 8949 eV. In Fig. 1, the broad emission bands peaking around 18000 cm−1 correspond to the characteristic yellow-green anomalous emission of Yb2+ in CaF2 . 1,2,6,15 The analogous IR-red anomalous emission of Yb2+ in SrF2 1,3,6,15 does not appear in the emission spectra of Fig. 1. Each incident X-ray photon can induce hundreds of free electrons. 16 These electrons will non-radiatively decay to the lowest conductionband states, which are largely formed from 3d and 4d orbitals of Ca and Sr respectively. From there, it is well-known that there can be radiative relaxation via self-trapped exciton transitions. 13 In previous work we showed that VUV direct excitation into the host exciton levels in Yb-doped CaF2 and SrF2 gave STE emission in both crystals, but anomalous Yb emission


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Figure 2: Calculated diabatic MMCT configuration coordinate diagrams for Yb/Ca or Yb/Sr electron transfer in CaF2 :Yb and SrF2 :Yb crystals, respectively. The red dashed arrows connect the MMCT crossing, where an electron is transferred from Ca+ or Sr+ to Yb3+ , with relaxed Yb2+ – Ca2+ or Yb2+ –Sr2+ states. The IVCT luminescent level 4f 13 (2 F5/2 )5deg –2A1u of Yb2+ is reached in this process in the CaF2 host only. Levels 1T1u and 1Eu of the 4f 13 (2 F7/2 )5deg configuration are reached in the SrF2 host instead. Colors of energy curves: violet, blue, green, and marroon energy curves combine states of the Yb2+ 4f 14 , 4f 13 5deg , 4f 13 5dt2g , and interacting 4f 13 5d– 4f 13 aY1gbT E configurations, respectively, with the ground state of Ca2+ or Sr2+ ; cyan, red, orange, and black energy curves combine states of the Yb3+ 4f 13 (2 F7/2 ) and 4f 13 (2 F5/2 ) configurations with states of the Ca+ 3deg , 4s, 3dt2g , and 4p configurations, or Sr+ 4deg , 5s, 4dt2g , and 5p configurations, respectively. See text for details. gin of the red dashed arrow). After the 3+ + 2+ 2+ Yb +Ca →Yb +Ca transfer, both sites are structurally and electronically stressed: the Yb–F distance around the resulting Yb2+ is too short, the Ca–F distance around the resulting Ca2+ is too long. The Ca2+ center is in its electronic 1 A1g ground state, but the Yb2+ moiety is electronically very excited, at the top of the 4f 13 5deg manifold whose 4f 13 subshell is in the 2 F5/2 higher multiplet (higher manifold of states in blue in Fig. 2, left). (iii) Non-radiative relaxation will therefore occur, leading to the metastable state indicated by the red arrow tip in Fig. 2, left: [Yb2+ 4f 13 (2 F5/2 )5deg -2A1u , Ca2+ 1 A1g ]. Here, both moieties are structurally relaxed, the Ca2+ center is always in its ground state, and the Yb2+ center has reached the bottom of the excited 4f 13 (2 F5/2 )5deg manifold, in the 2A1u

state. Note that below this MMCT state is an energy gap of 5700 cm−1 , which hinders further non-radiative relaxation. (iv) Finally, there is the possibility of radiative relaxation. According to the theoretical study of Ref. 8, the anomalous emission is a radiative intervalence charge transfer from the very excited 2A1u state of an Yb2+ donor to the ground state of an Yb3+ acceptor ion (cf. final step V in the mechanism discussed in Ref. 8). Hence, once the 2A1u state has been reached after the host-to-dopant electron and energy transfer process (i) to (iii), the anomalous emission takes place as the following [Yb2+ 2A1u ,Yb3+ 1Γ7u ]→[Yb3+2 F7/2 ,Yb2+ 1A1g ] IVCT luminescence illustrated by the green arrow in the CaF2 :Yb2+ /Yb3+ IVCT configuration energy diagram shown in the leftmost graph of Fig. 3. This means an electron trans-


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fer Yb2+ 5d →Yb3+ 4f plus a reorganization in the 4f 13 inner shell of Yb2+ . Now we consider the SrF2 :Yb system. The SrF2 :Yb/Sr MMCT configuration coordinate diagram (rightmost graph in Fig. 2) indicates that the Yb3+ +Sr+ →Yb2+ +Sr2+ transfer (origin of the red dashed arrow in the rightmost graph of Fig. 2) leads now to much lower 4f 13 5deg states of Yb2+ , whose 4f 13 subshell is in the lowest 2 F7/2 multiplet, bypassing the IVCT luminescent 2A1u state. So, nonradiative relaxation following the host electron capture ultimately leads to the lowest excited states of Yb2+ : 1T1u (red arrow tip of Fig. 2, right) and 1Eu below. This means that the IVCT luminescence is not excited by the ionizing radiation, explaining the absence of the IR-red anomalous emission in the X-ray excited spectra of SrF2 :Yb in Fig. 1. Rather, the regular intra-Yb2+ 5d → 4f blue emission 1Eu →1A1g could be excited (see blue arrow in Fig. 3, right). This electric dipole forbidden blue emission has been seldomly observed. It was reported by Moine et al. around 24000 cm−1 ; 17 so, it would coincide with the low energy edge of the strong STE emission band of SrF2 (Fig. 1, right). Alternatively, nonradiative relaxation could lead to excitation of the Yb3+ IR 2 F5/2 →2 F7/2 emission, as illustrated by the red arrow tips and solid red arrow in Fig. 3. This type of Yb2+ 5d → Yb3+ 4f IVCT excitation of the Yb3+ emission has been discussed in Ref. 8. In summary, the IVCT luminescent state cannot be excited in SrF2 :Yb because electron transfer from the host does not leave Yb2+ in the 4f 13 (2 F5/2 )5deg manifold. This is due to two facts: First, the lowest Yb3+ -Sr+ state of SrF2 :Yb is 6600 cm−1 lower in energy than the lowest Yb3+ -Sr+ state of CaF2 :Yb (cf. Fig. 2). Second, both 4f 13 5deg manifolds: the highest 4f 13 (2 F5/2 )5deg , which includes the IVCT luminescent level, and the lowest 4f 13 (2 F7/2 )5deg , are shifted by only some +1000 cm−1 in the SrF2 host. 12 Altogether, X-ray excited optical luminescence spectra have proven that ytterbium is not capable of activating the SrF2 host in the visible spectral range whereas it shines in the

yellow-green in the CaF2 host. This distinct scintillating behaviour suggests significant differences in the mechanism of host-to-dopant electron transfer after ionizing radiation, which have been analyzed using ab initio quantum chemical calculations of host-to-dopant (MMCT) and dopant-to-dopant (IVCT) charge transfer configuration coordinate energy diagrams. The analyses show that whether the host electron capture delivers the host energy to the higher 4f 13 (2 F5/2 )5deg or to the lower 4f 13 (2 F7/2 )5deg manifold, determines whether or not the anomalous yellow-green/IR-red luminescence of ytterbium is observed.

Experimental methods CaF2 and SrF2 crystals doped with 0.5 mol% and 0.05 mol% of ytterbium were grown at the University of Canterbury as described in Ref. 9. To study optical luminescence properties, the fluoride crystals were excited at the Yb3+ core 2p3/2 level at 8949 eV. The sample temperature of 80 K. The emission spectra were collected at beamline 20BM of the Advanced Photon source (Argonne National Lab, US). X-rays from a double-crystal Si(111) monochromator (with second crystal detuned 20% from maximum signal) were focused to a 500 um by 250 um spot size using a toroidal mirror. For additional rejection of harmonic energies (beyond detuning), a Rh-coated flat mirror was also used. Incident photon flux was monitored using a 15 cm, N2-filled transmission ion chamber. Luminescence emission spectra were recorded using an Avantes AvaSpec2 CCD detector with a grating giving a resolution of approximately 2.4 nm. Low temperature measurements were performed using an MMR Technologies JouleThompson cooler under vacuum with a silica optical window. Acknowledgement This work was partially supported by the Marsden fund of the Royal Society of New Zealand via grant 09-UOC080 and by the Ministerio de Economía y Competitividad, Spain (Dirección General de Investigación y Gestión del Plan Nacional de I+D+i, MAT2014-54395-P). R.B.H.C. ac-


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Figure 3: Calculated diabatic IVCT energy diagrams for Yb2+ /Yb3+ pairs in CaF2 (left) and SrF2 (right). Green arrow: IVCT luminescence corresponding to the green anomalous emission of Yb in CaF2 . Blue and red arrows: Yb2+ regular blue emission and Yb3+ IR emission, respectively. Colors of energy curves: black and blue energy curves combine Yb2+ 4f 14 and 4f 13 5deg states, respectively, with Yb3+ 4f 13 (2 F7/2 ); red and orange energy curves combine them with Yb3+ 4f 13 (2 F5/2 ). See text for details. Ionic Crystals. Phys. Rev. B 1985, 32, 8465–8468.

knowledges support from the University of Canterbury via a doctoral scholarship. L.S. and Z.B. acknowledge support from the University of Canterbury Erskine Fund while in New Zealand. Use of Sector 20 at the APS, supported by US DoE and the Canadian Light Source, is also acknowledged.

(4) Dorenbos, P. Anomalous Luminescence of Eu2+ and Yb2+ in Inorganic Compounds. J. Phys.: Condens. Matter 2003, 15, 2645–2665.

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(16) Weber, M. J. Inorganic scintillators: today and tomorrow. J. Lumin. 2002, 100, 35–45. (17) Moine, B.; Pédrini, C.; McClure, D. S.; Bill, H. Fluorescence and photoionization processes of divalent Yb ions in SrF2 . J. Lumin. 1988, 40&41, 299–300.

(9) 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, 135502–1–135502– 4. (10) Welber, B. Reversible Phototransfer of Electrons between Rare-Earth Ions in CaF2 . J. Chem. Phys. 1965, 42, 4262– 4264. (11) Hughes-Currie, R. B.; Salkeld, A. J.; Ivanovskikh, K. V.; Reid, M. F.; Wells, J. P. R.; Reeves, R. J. Excitons and interconfigurational transitions in CaF2 :Yb2+ crystals. J. Luminescence 2015, 158, 197– 202. (12) Barandiarán, Z.; Seijo, L. Metal-to-Metal Charge Transfer Between Dopant and Host Ions: Photoconductivity of YbDoped CaF2 and SrF2 crystals. J. Chem. Phys. 2015, 143, 144702–1–144702–13. (13) R. T. Williams, M. N. Kabler, W. Hayes,; J. P. Stott, Time-resolve spectroscopy of self-trapped excitons in fluorite crystals. Phys. Rev. B 1976, 14, 725–740. (14) Kirton, J.; A. M. White, Zeeman Effect in the Absorption Spectra of Trivalent Ytterbium Ions in Different Site Symmetries in Calcium Fluoride. Phys. Rev. 1969, 178, 543–547. (15) Reid, M. F.; Senanayake, P. S.; Wells, J.-P. R.; Berden, G.; Meijerink, A.; Salkeld, A. J.; Duan, C.-K.; Reeves, R. J. Transient Photoluminescence Enhancement as Probe of the Structure of Impurity-Trapped Excitons in CaF2 :Yb2+ . Phys. Rev. B 2011, 84, 113110–1–113110–4.


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X-ray Excitation Triggers Ytterbium Anomalous Emission in CaF2:Yb

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