Combinatorial characterization of metastable luminous silver cations

Thermodynamically metastable glasses that can contain metastable species are important functional materials. X-ray absorption near-edge structure (XANES) spectroscopy is an effective technique for determining the valence states of cations, especially for the doping element in phosphors. Herein, we first confirm the valence change of silver cations from monovalent to trivalent in aluminophosphate glasses by X-ray irradiation using a combination of Ag L3-edge XANES, electron spin resonance, and simulated XANES spectra based on first-principles calculations. The absorption edge of the experimental and simulated XANES spectra demonstrate the spectral features of Ag(III), confirming that AgO exists as Ag(I)Ag(III)O2. A part of Ag(I) changes to Ag(III) by X-ray irradiation, and the generation of Ag(III) is saturated after high irradiation doses, in good agreement with conventional radiophotoluminescence (RPL) behaviour. The structural modelling based on a combination of quantum beam analysis suggests that the local coordination of Ag cations is similar to that of Ag(III), which is confirmed by density functional theory calculations. This demonstration of Ag(III) in glass overturns the conventional understanding of the RPL mechanism of silver cations, redefining the science of silver-related materials.


Supplementary Note 1: Nature of FD-7 glass
The FD-7 glass, whose chemical composition is shown in Table S1, was provided by Chiyoda Technol Corporation.The glass has been used as a glass badge for personal monitoring.Figure S1 presents the physical and structural data.

Table S1.
Chemical composition and glass transition temperatures (T g ) of FD-7 glass 1 .

Chemical composition (mol%)
Tg    From the image, the beam size for the XANES measurement is estimated to be 0.4  5.8 mm 2 , which is too small to measure the irradiated part of the optical absorption spectrum.

Supplementary Note 3: X-ray-induced change in FD-7 glass
The X-ray-induced change in the FD-7 glass increased with increasing irradiation dose and then appeared to saturate after a long irradiation time (Fig. S5).These changes were detected by optical absorption (Fig. S6), XANES (Fig. S7), and ESR (Fig. S8) measurements.

Fig. S6
Representative optical absorption spectra of FD-7 glass after different X-ray irradiation doses.

Supplementary Note 4: Structural modelling of FD-7 glass by RMC modelling
Recently, we reported on the atomic configuration of oxide glasses derived from reverse Monte Carlo (RMC) modelling based on a combination of 31 P MAS NMR, Zn K-edge extended EXAFS, and X-ray and neutron diffraction data [2][3][4] .Several datasets from different measurement techniques are used as constraints and are essential in modelling reliable structures.A reliable structural model of FD-7 was created using this approach.
As the raw data of Ag K-edge EXAFS were not suitable for fitting because of the noise of the signal derived from the low concentration, back-Fourier transformed data were used (Fig. S9). Figure S10 presents the cation-oxygen coordination number distributions in the FD-7 glass and the partial pair correlation functions gij(r).Only two Ag cations were present in the RMC model containing 6,000 atoms.Ag coordination in the FD-7 glasses is illustrated in Fig. S11 (and another is shown in Fig. 5e).

Figures
Figures S2-S4 show the conditions under which experiments were conducted at the Kyushu Synchrotron Light Research Center.

Fig. S3 Defect
Fig. S3 Defect formation in FD-7 glass by X-ray irradiation.(a) Photograph of the sample holder for XANES measurement.(b) Optical micrograph of the GAFCHROMIC film after X-ray irradiation.
Fig. S4Decay of electrical current during irradiation for 1 d at SAGA-LS.From the decay curve, we evaluated the average current per day to be 170 mAꞏh.

Fig. S5 Comparison
Fig. S5 Comparison of Ag L 3 -XANES spectra.Generated absorption peak intensity at 3.349 keV for commercial FD-7 glass as a function of electrical current multiplied by time.Inset: Ag L3-edge XANES spectra of FD-7 glass after different X-ray irradiation doses (118 and 536 mAꞏh).
Fig. S7 Change in absorption as a function of irradiation dose.Differential XANES spectra of FD-7 glass obtained by subtracting the absorbance of FD-7 glass after 46.4 mA•h of irradiation from that of the other FD-7 glass sample.The absorption spectrum of Ag2O is shown for comparison.

Fig. S8 Assignment
Fig. S8 Assignment of peak observed at 325 mT in ESR measurement.ESR spectra of a silica tube used for ESR measurements and non-irradiated FD-7 glass in the silica tube.The peak at g = 2.007 is assigned as the oxygen hole centre for SiO2.
RMC modelling is affected by coordination number constraints.Because two sets of oxygen coordination are forced on the Ag cations, which correspond to the local structure in Ag2O, the reliability of the RMC-generated atomistic model can be validated.We compared the experimental data with three RMC models: (1) an RMC model without an Ag-O constraint, (2) an RMC model with a constraint of NAg-O = 2, and (3) an RMC model without EXAFS fitting.As Fig. S12 shows, the RMC modelling without the constraint of Ag-O coordination was most consistent with the experimental EXAFS oscillation data.

Fig. S9 Comparison
Fig. S9 Comparison of Ag K-edge EXAFS data of FD-7 glass.Raw data (solid line) and back-Fourier transformed data (dotted line).
Fig. S11 Network structure of FD-7 glass obtained from RMC modelling.Ag coordination in FD-7 glasses.From the chemical composition, atomic ratios of P:Na:Al:Ag:O = 59.04:27.73:13.14:0.09:209 to 656:308:146:1: 2,320 1 .In the RMC model using 6,000 atoms, only two Ag cations are present, namely, the Ag shown in this figure and that in Fig. 5e.The P=O bonds are expected to capture electrons to compensate for the charge valance when the valence state of Ag + cations is increased.

Fig. S12 Coordination
Fig. S12 Coordination numbers and comparison between neutron/X-ray data and RMC models for DF-7 glass with/without Ag coordination constraint.The coordination number distribution of oxygen around (a) Ag, (b) Na, and (c) Al in the RMC model with a structural constraint for Ag2O coordination (NAg-O = 2).(d) Neutron total structure factor S N (Q), (e) X-ray total structure factor S X (Q), and (f) Ag K-edge EXAFS spectra of FD-7 glass with and without the structural constraint.Black, red, cyan, and blue lines show the experimental data and the RMC models without an Ag-O constraint, with a constraint of NAg-O = 2, and without EXAFS fitting, respectively.
AgO with space group P21/c, contributions of the A site (6-fold) and B site (4-fold) are shown as solid and dotted lines, respectively.

Fig. S14 Comparison
Fig. S14 Comparison of the simulated XANES spectra of Ag oxides and Ag fluorides.The absorption energy E0 values of Ag fluorides are less than those of Ag oxides.The white-line intensities of Ag 3+ are higher in both oxides and fluorides.