Lead-related centers of UV, visible and near-IR luminescence in SiO2 glass

Journal of Non-Crystalline Solids 452 (2016) 176–186

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Lead-related centers of UV, visible and near-IR luminescence in SiO2 glass V.O. Sokolov a,⁎, A.V. Kharakhordin a, A.Yu. Laptev b, V.G. Plotnichenko a, A.N. Guryanov b, E.M. Dianov a a b

Fiber Optics Research Center of the Russian Academy of Sciences, 38 Vavilov Street, Moscow, 119333, Russia Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences, 49 Tropinin Street, Nizhny Novgorod, 603600, Russia

a r t i c l e

i n f o

Article history: Received 10 June 2016 Received in revised form 15 August 2016 Accepted 22 August 2016 Available online 5 September 2016 Keywords: Lead-doped silica glass Near-IR luminescence Computer modeling

a b s t r a c t Absorption, luminescence and luminescence excitation spectra of MCVD-prepared SiO2:Pb glasses with Pb content up to 0.35 wt% were studied. UV and visible luminescence was observed in the 0.3–0.45 and 0.5–0.6 μm ranges. IR luminescence was found in the 0.85–0.95, 1.1–1.2 and 1.3–1.4 μm ranges. Computer modeling of SiO2:Pb glass was performed. The main possible forms of Pb in the SiO2 glass network were found to be threefold coordinated Pb atom, diatomic Pb center, twofold coordinated Pb atom, and possibly fourfold coordinated Pb atom. Structure, electronic properties, absorption and luminescence of the centers corresponding to these forms and of possible lead-related centers responsible for the IR luminescence were calculated. The results of the experiments and modeling suggest that (1) the 0.3–0.45 μm luminescence band is caused mainly by twofold coordinated Pb atoms and partly by threefold coordinated Pb atoms, (2) the 0.5–0.6 μm luminescence band is caused by twofold coordinated Pb atoms, (3) 1.1–1.2 μm IR luminescence band is caused by Pb+ interstitial ions, and (4) 0.85–0.95 and 1.3–1.4 μm IR luminescence bands are caused by complexes formed by interstitial Pb0 atom and a pair of ≡Si\\Si≡ oxygen vacancies. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Bismuth-doped glasses represent a class of novel optically active materials with a wide luminescence bands in the near IR range [1,2]. These provide a means for lasers and optical amplifiers operating in this wavelength range and attract considerable attention to their uses in optical communication lines [3]. It is agreed that the luminescent properties of bismuth-related centers in such glasses result from unique features of heavy p-elements. It would appear reasonable to assume that doping glass other 6p elements may both provide additional information about the origin of the centers of IR luminescence and offer the prospect of further developing novel active materials for fiber optics applications. Lead-doped glasses are to be studied first. IR luminescence in the 1.1–1.2 μm range and near 1.4 μm was observed in MCVD-prepared optical fibers with silica and germanate-silicate Pb-doped glass cores [4–6]. Absorption properties of the fibers were studied in the visible and near IR ranges. Other observations of the IR luminescence in the 1.1–1.2 μm range in Pb-doped glasses were performed in GeO2:Pb glasses [7] and in 7GeO2 − AlF3 : Pb glass [8]. However no justified models of any lead-related centers of IR luminescence were suggested ever. On the other hand, IR luminescence caused by lead-related centers was observed in a several alkaline earth fluoride crystals [9,10]. The origin of the IR luminescence in these lead-related centers is believed [9, ⁎ Corresponding author. E-mail address: [email protected] (V.O. Sokolov).

http://dx.doi.org/10.1016/j.jnoncrysol.2016.08.033 0022-3093/© 2016 Elsevier B.V. All rights reserved.

10] to be similar to that of Tl0 (1) thallium center in alkali halide crystals [11,12]. Models of bismuth-related centers of IR luminescence in glasses were advanced [13] basing primarily on the analogy with the lead-related centers in alkaline earth fluoride crystals and, in part, with the Tl0 (1) center. More recently these models have been severely strengthened in both experimental [14] and theoretical [15] studies. Such models would be expected to be valid as well for lead-related centers of IR luminescence in Pb-doped glasses. The prime object of this work is to determine the structure of leadrelated centers in network of SiO2 glass doped with Pb at low content. In particular, the centers responsible for the near-IR luminescence discovered and explored in [4–6], are of the most interest. 2. Experimental 2.1. Preforms preparation Plane-parallel 2 mm-thick plates from the fiber preforms prepared by MCVD-method were cut and polished for spectroscopic measurements. Lead was introduced in the preforms by impregnation of the porous core layers on the inner surface of a SiO2 substrate tube with a solution of lead nitrate, Pb(NO3)2, in nitric acid. Then the porous layers were dried, sintered at ~ 1900 ∘ C and the silica tube was collapsed at about 2100 ∘ C into a rod. In the case of the 1LSO sample all the operations were carried out in the presence of oxygen. For the 2LSHe and 4LSN samples the impregnation, porous layer drying and the subsequent collapsing were carried

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Table 1 Investigated samples. Sample number

Sample mark

Pb content, wt%

MCVD preform collapsed in

1 2 3 4 5 6

1LSO 2LSHe 3LSO 4LSN Z 6LSH

0.0045 0.005 0 0.36 b0.07 0.01

O2 He O2 N2 Ar H2

out in an inert atmosphere of helium and nitrogen, respectively. For the 6LSH sample the impregnation and porous layer drying was performed in helium atmosphere and during both vitrification and collapsing hydrogen was added in helium atmosphere. Lead concentration in the core of preforms was measured by the electrothermal atomic absorption spectrometry (ETAAS) method. Table 1 represents the designation of preform samples under investigation, Pb content in the preform core and atmosphere used during the preform drying and collapsing. For reference purposes we used the 3LSO sample prepared without lead doping and the Z sample previously studied in [5] courteously provided by the authors of that work. 2.2. Measurement of absorption and luminescence spectra of bulk samples Fig. 1. Absorption of the samples listed in Table 1.

For the purposes of spectroscopic measurement polished 2 mmthick plane-parallel plates were cut of the preforms. Transmittance spectra in the preform core area were measured with Perkin Elmer Lambda 900 spectrophotometer in the 0.186–3.300 μm wavelength range with both wavelength step and resolution of 4 nm. The transmission spectra then were transformed in an absorption spectra with Fresnel reflection taken into account. In all the samples under study the absorption bands were observed only in the wavelength range beginning from the short-wave absorption edge (Fig. 1) to approximately 0.350 μm. In certain samples the absorption band near 2.7 μm were found as well. This absorption is known to correspond to the overtone of the stretching vibration of hydroxyl groups in SiO2 host. From Fig. 1 it will be obvious that the doping of SiO2 by lead results in a shift of the short-wave absorption edge and in the appearance of strongly overlapping absorption bands in the 0.24– 0.30 μm (5.2–4.5 eV) range. The luminescence spectra and luminescence excitation spectra of the samples were measured with Edinburgh Instruments FLSP920 fluorescence spectrometer. As an excitation source 250 W xenon lamp was used, which allows to measure the luminescence excitation spectra in the 0.24–0.85 μm wavelength range and luminescence spectra in the 0.24–1.65 μm range with both the resolution and wavelength step of 1–10 nm. 2.3. Luminescence spectra 2.3.1. UV and visible range As one might expect reasoning from the absorption spectra (Fig. 1), neither UV and visible luminescence nor IR luminescence was observed under UV excitation in the 3LSO reference sample and in 1LSO and 2LSHe samples. Under nearly 0.25 μm excitation luminescence was observed in the band near 0.36 μm in the 4LSN sample and in the bands near 0.29 μm and near 0.40 μm in the 6LSH sample (Fig. 2).1 When excited in the 0.25–0.38 μm range, luminescence in the 0.5–0.6 μm range was observed in 4SLN and 6LSH samples and in Z sample. Two bands near 0.29 and 0.40 μm are clearly defined in the luminescence spectra of the 6LSH sample (Fig. 2(a)). The luminescence band 1

In Figs. 2–5 and 9 the luminescence intensity is normalized to unity.

near 0.40 μm is undoubtedly related to the Pb dopant. The position of the maximum of the luminescence band near 0.29 μm and the shape of the excitation spectrum of this band are in good agreement with the well-known data on the short-wavelength luminescence band (0.28 μm or 4.45 eV) of SiODC-II oxygen deficient center (=Si) characteristic of SiO2 glass [31–33]. The long-wavelength luminescence band (0.46 μm or 2.7 eV) of SiODC-II is known to be by an order of magnitude less intense. We have failed to detect such a luminescence band in the 6LSH sample since it falls at the edge of intensive lead-related 0.40 μm luminescence band. In the luminescence spectra of the 4LSN sample (Fig. 2 (b)) two bands are distinguished, one near 0.38 μm and another in the 0.50– 0.65 μm range. One may assume the presence of two excitation bands near 0.26 and 0.29 μm for the first luminescence band and one excitation band near 0.31 μm for the second luminescence band. Most likely, the UV and visible luminescence in this sample is caused by centers of several types with mutually overlapping excitation and luminescence bands. 2.3.2. Near IR range In the work [5] luminescence bands near 0.9, 1.15 and 1.4 μm were observed in optical fibers with Pb-doped silica glass core. The bands were assigned to luminescence centers of two types without any assumption on their origin. Shown in Fig. 3 is the luminescence spectra of the 6LSH and 4LSN samples. In both samples an intense band near 1.15 μm is clearly visible and two substantially weaker bands near 0.9 and 1.35 μm are distinguished. The excitation spectra of the luminescence in the 0.9 and 1.35 μm bands are similar to each other and differ from the excitation spectrum of the 1.15 μm luminescence band. This is consistent with the conclusions of [5]. Namely, the luminescence near 1.15 μm is excited in the absorption bands in 0.29–0.32, 0.41–0.46 and ≲0.25 μm ranges (the latter is not shown in Fig. 3), while the luminescence near 0.9 and 1.35 μm is excited in the absorption bands near 0.32 μm and in the ≲0.28 μm range (see Fig. 9 as well). The most intense IR luminescence among all the samples studied was observed in the 6LSH sample, although Pb content in the 6LSH sample (0.01 wt%) was considerably lower than e.g. in the 4LSN sample (0.36 wt%). This may suggest a significantly higher probability of IR

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Fig. 2. UV and visible luminescence excited in the 0.22–0.29 μm range in the (a) 6LSH and (b) 4LSN samples.

luminescence centers formation in presence of hydrogen. In addition to that it should be remarked that no luminescence in the IR range was detected in the 3LSO reference sample. In the Z sample we observed the only IR luminescence band near 1.15 μm excited in 0.45 μm band, in accordance with [5]. 2.3.3. Influence of γ irradiation of UV, visible, and IR luminescence The spectra of luminescence and luminescence excitation of the samples are found to change significantly both in the UV and visible ranges (Figs. 4–7) and in the IR range (Figs. 8, 9) under γ irradiation.

Fig. 3. IR luminescence excited in the 0.30–0.40 μm range in the (a) 6LSH and (b) 4LSN samples.

The irradiation was carried out using 60Co source with intensity 7.25 Gy⋅s −1 for 3 h. The γ radiation dose was approximately 78 kGy. Main changes in the luminescence spectrum in the UV and visible range turn out to be as follows. In the 6LSH sample γ irradiation results in a significant (by 2.5–3 times) increase of intensity of the luminescence in the 0.29 and 0.40 μm bands (Fig. 4 and the a, b spectra in Fig. 6). In addition, the luminescence near 0.38 μm and in the 0.50– 0.65 μm characteristic of 4LSN sample and almost indistinguishable in non-irradiated 6LSH sample becomes noticeable (Fig. 5 and the a, b spectra in Fig. 7). In the 4LSN sample γ irradiation results mainly in

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Fig. 6. UV and visible luminescence excited at 0.25 μm in 6LSH sample (a) before and (b) after γ irradiation and in 4LSN sample (c) before and (d) after γ irradiation. Fig. 4. UV and visible luminescence in γ irradiated 6LSH sample.

the appearance of the luminescence band of SiODC-II centers near 0.28 μm characteristic for 6LSH sample but absent in the non-irradiated 4LSN sample (Fig. 5 and the c, d spectra in Fig. 6, to be compared with Fig. 2). There are no considerable changes in other areas of the luminescence spectra. In the IR range γ irradiation leads to a significant (more than by an order of magnitude) decrease of the intensity of the 1.15 μm luminescence band, while the intensity of the 0.9 and 1.35 μm luminescence bands changes marginally. These changes are especially pronounced in the 6LSH sample (Figs. 8, 9). In the γ irradiated 6LSH sample the

Fig. 5. UV and visible luminescence in γ irradiated 4LSN sample.

above-mentioned similarity of the 0.9 and 1.35 μm luminescence and its distinction from the 1.15 μm luminescence become particularly striking. 3. Modeling of lead-related centers in SiO2 glass: Results and discussion 3.1. Modeling approaches The modeling of lead-related centers in SiO2 glass network was performed using periodical network models. 2 ×2 × 2 supercell of α quartz lattice (24 SiO2 groups with 72 atoms in total) was chosen as a model of initial perfect SiO2 network. Using ab initio molecular dynamics (MD) the system formed by the supercells previously melted at high

Fig. 7. UV and visible luminescence excited at 0.29 μm in 6LSH sample (a) before and (b) after γ irradiation and in 4LSN sample (c) before and (d) after γ irradiation.

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Fig. 8. IR luminescence excited at 0.30 μm in the 6LSH sample (a) before and (b) after γ irradiation.

temperature (about 3500 K or slightly higher) was maintained at certain intermediate temperature (about 1500 K) until the equilibrium atom velocities distribution was reached and then cooled to 300 K. The intermediate temperature exposure was needed to ensure that all dangling-bond-type defects were removed. Notice that the only purpose of the MD calculation was to get a reasonable initial model of the glass network and not to describe adequately any thermal properties. Periodical model of SiO2 network based on final supercell configuration was applied to study the lead-related centers. Oxygen vacancies, ≡Si\\Si≡, were formed by a removal of proper O atoms. Charged centers were modeled by changing the total number of electrons in the supercell. Equilibrium configurations of the lead-related centers were

found by a subsequent ab initio MD and complete optimization of the supercell parameters and atomic positions by the gradient method. All such calculations were performed using Quantum-Espresso package [16] in the plane wave basis in generalized gradient approximation (GGA) of density functional theory (DFT) with ultra-soft projector augmented-wave pseudopotentials using PerdewBurkeErnzerhof density functional for solid state (PBEsol). The pseudopotential sources were taken from the pseudopotential library [17] (v. 1.0 and v. 0.3). Configurations of lead-related centers obtained by this means were used to calculate the electron localization functions using the programs from Quantum-Espresso package, to calculate and analyze the electron density distribution and effective charges of atoms by Bader's method (bader v. 0.28 code [18]), and to calculate the absorption spectra of the centers by Bethe-Salpeter equation (BSE) method based on DFT pseudopotentials plane wave approach or on all-electron full-potential linearized augmented-plane wave approach. In the first case the BSE calculations were performed using YAMBO code [19]. Wavefunctions were obtained using Quantum-Espresso code in GGA with PBEsol norm-conserving pseudopotentials built on the basis of the sources taken from the library [17]. GW quasi-particle approach was used in transition energy calculation. In the second case the BSE calculations were performed using Elk code [20] in the local spin density approximation (LSDA) with Perdew-Wang-Ceperley-Alder (PW-CA) density functional. Scissor correction was used to calculate transition energies. The scissor value was calculated using modified Becke-Johnson exchangecorrelation potential. Spin-orbit interaction essential for systems containing lead atoms was taken into account in both cases. Further details and corresponding references may be found in [15]. Configurational coordinate curves of lead-related centers were calculated in a simple model restricted to the lowest excited states basing on absorption spectra dependence on Pb atom(s) displacement along three mutual orthogonal directions. In spite of the fact that the model is inherently oversimplified, it turns out to be reasonable enough to estimate Stokes shifts and the IR luminescence wavelengths in the bismuth-related centers modeled in [15]. On the contrary to these centers, the Stokes shifts corresponding to transitions between the first or second excited state and the ground one turns out to be large in all the lead-related centers under investigation. So the luminescence wavelengths given in what follows should be regarded as rough estimations. As for absorption intensities or excited states lifetimes calculations, significantly larger supercells and k points sets are required. We do not mean to examine this problem in the present work.

3.2. Main forms of lead in SiO2 glass According to the results of our simulation of lead substitutional centers in SiO2 glass, single Pb atoms built in the network (i.e. bonded with the surrounding Si atoms with bridging O atoms) can form centers of several types. It should be emphasized that nominal valence of Pb is equal to two in all these centers, which is confirmed by the calculations of the effective charges of Pb atoms (see below). Among monoatomic centers are single threefold coordinated lead atom,

, bonded with three Si atoms by

two bridging O atoms and one threefold coordinated O atom, and twofold coordinated lead atom,

Fig. 9. IR luminescence in γ irradiated 6LSH sample.

bonded to two Si atoms by

two bridging O atoms. Fourfold coordinated Pb atom in a bipyramidal configuration bonded with four Si atoms by four bridging O atoms is not improbable to occur in SiO2 network. The possibility of formation of three- and fourfold coordinated Pb atoms is conditioned by threefold coordinated O atoms in the network. Each threefold coordinated O atom makes possible formation of one extra bond of Pb atom. In the case of fourfold coordinated Pb atom such O atoms may occur anywhere in the network (not necessarily bonded directly with the Pb atom).

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Calculations show that the single fourfold coordinated Pb atom is not stable in SiO2 network and can easily turn into the threefold coordinated Pb atom. In addition to these monoatomic centers, diatomic centers formed by two threefold coordinated Pb atoms may occur in the SiO2 network. In such centers each Pb atom is bonded with the second Pb atom and with two Si atoms by two threefold coordinated O atoms and with the third Si atom by bridging O atom. Such centers are proved to occur in the lead silicate glass e.g. by the NMR and X-ray photoelectron spectroscopy data [21,22]. Provided that PbO content is high enough, these centers are bonded together forming lead oxide network [21,22]. The structure of this network is similar to that of characteristic lead silicate crystals lattice (see e.g. [23,24]). All these coordination forms of Pb atoms found in the calculations have been observed experimentally in lead oxides crystals (see e.g. [23–25]). 3.3. Lead-related centers of visible luminescence in SiO2 glass 3.3.1. Threefold coordinated Pb atom The calculated configuration of the center in SiO2 network is shown in Fig. 10. The Pb\\O distances in ≡ Pb\\O\\Si linkages are found to be ≈ 0.230 nm (to be compared with ≈0.162 nm SiO\\ distance in ≡Si\\O\\Si≡ linkage in SiO2 glass) and the for the threefold coordinated O atom the Pb\\O distance is ≈0.257 nm. The O\\Pb\\O angle between two bridging O atoms and between the bridging and the threefold coordinated O atoms are ≈89∘ and ≈118∘, respectively. Calculation of electron density distribution by Bader's method proved the effective charge of the threefold coordinated Pb atom to be ≈ + 1.4 | e |. Effective charges of the bridging O atoms are ≈ − 1.9 | e |, and effective charge of the threefold coordinated O atom is ≈− 1.2|e|. Since effective charge of O atoms in SiO2 is ≈− 1.9|e|, the total effective charge localized in vicinity of the threefold coordinated Pb atom turned out to be ≈ +2|e|. This proves lead to be divalent in this center.

Fig. 10. Threefold coordinated lead atom in SiO2.

181

According to the calculation results, the main absorption bands of this center lie at 3.87, 5.05 and 5.55 eV (0.32, 0.25 and 0.22 μm, respectively) and are related with transitions between the ground state, 1S0, and three spin-orbit components of the first excited state, 3P1, of Pb2 + ion (see e.g. [26]). The absorption in these bands may give rise to the luminescence corresponding to the transition from the first excited state to the ground one. Our estimations proves the Stokes shift for the transition between these two states to be not small. Unfortunately, center

the structural peculiarities of the

(threefold coordination of Pb atom and the presence of threefold coordinated O atoms) do not allow any accurate estimation of the luminescence wavelength. Therefore we can only assume basing on the analogy with the experimental data available, that in common with other different Pb-containing glasses [27–30] luminescence of the centers may be observed in 0.35– 0.45 μm range under excitation in the above-mentioned absorption bands.

3.3.2. Diatomic lead center (paired threefold coordinated Pb atoms) The calculated configuration of the diatomic center in SiO2 network is shown in Fig. 11. The Pb\\Pb distance between two threefold coordinated Pb atoms is ≈0.309 nm. The Pb\\O distances are ≈0.215 nm for the ≡Pb\\O\\Si linkages and ≈0.229 nm for the threefold coordinated O atoms. The O\\Pb\\O angles between bridging and threefold coordinated O atoms and between two threefold coordinated O atoms are ≈85∘ and ≈92∘, respectively. Calculation of electron density distribution by Bader's method proves the effective charge of each threefold coordinated Pb atom in the diatomic center to be ≈ + 1.4|e|. Effective charges of the bridging O atoms are ≈ − 1.6|e|, and effective charge of the threefold coordinated O atoms are ≈ − 1.5|e|. With the above-mentioned effective charge of O atoms in SiO2 taken into account the total effective charge localized in vicinity of Pb atoms in the diatomic lead center turned out to be ≈+ 4|e|. This proves each lead atom to be divalent in this center. Levels and transitions schemes of the diatomic Pb center in SiO2 are given in Fig. 12(a).

Fig. 11. Diatomic lead center in SiO2.

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Fig. 12. Calculated level and transition schemes of lead-related centers of visible and IR luminescence in SiO2: (a) diatomic lead center, (b) twofold coordinated lead atom, (c) interstitial Pb+ ion, (d)

lead center. Level energies are given in eV, transition wavelengths in μm.

According to the calculation results, the main absorption bands of this center are at 4.44, 4.77 and 5.67 eV (0.28, 0.26 and 0.22 μm, respectively). As a rough approximation one may consider the ground state and the three lowest excited states of the center to a large degree as symmetric and antisymmetric combinations of the ground-state terms, 1S0, of two Pb2+ ions. In addition the three excited states contain significant admixture of three spin-orbit components of the first excited state, 3P1, of two Pb2+ ions. The antisymmetric combination is split in three levels both due to this admixture and under influence of the atomic environment of two Pb2 + ions. The absorption in the mentioned bands may give rise to the luminescence corresponding to the transition from the first and/or second excited states to the ground one. The Stokes shift for these transitions is found to be not small but again the luminescence wavelengths cannot be estimated accurately. Basing on the analogy with such diatomic centers as ≡Si\\Si≡, ≡Si\\Ge≡ and ≡ Ge\\Ge≡ oxygen vacancies in SiO2 and GeO2 hosts, we may suggest that luminescence of the diatomic lead centers may be observed in 0.4–0.5 μm range under excitation in the above-mentioned absorption bands. 3.3.3. Twofold coordinated Pb atom Twofold coordinated atoms, both intrinsic (Si) and impurity (Ge, Sn), are known to be characteristic oxygen-deficient centers in SiO2

glass [31–35] forming an isoelectronic series of twofold coordinated atoms Si, Ge, and Sn. Our modeling showed that twofold coordinated lead atom,

, in SiO2 glass may be considered as an oxy-

gen-deficient center belonging to this series. center in SiO2

The calculated configuration of the

network is shown in Fig. 13. The Pb\\O and Si\\O distances in ≡Pb\\O\\Si linkages were found to be ≈0.216 nm and ≈0.160 nm, respectively. O\\Pb\\O angle was ≈76∘. Calculation of electron density distribution by Bader's method proves that the effective charges of the twofold coordinated Pb atom and of the O atoms in the `Si\\O\\Pb are ≈ + 1.4|e|, and ≈ + 1.6|e|, respectively. With the effective charge of O atoms in SiO2 network taken into account the total effective charge localized in vicinity of the twofold coordinated Pb atom turned out to be ≈ + 2 | e | and hence lead is divalent in the

center.

Levels and transitions schemes of the

center in

SiO2 are given in Fig. 12(b). As distinct from twofold coordinated Si and Ge atoms in SiO2, the electronic states structure and spectral

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Fig. 14. Interstitial Pb+ interstitial ion in SiO2.

Fig. 13. Twofold coordinated lead atom in SiO2.

properties of the twofold coordinated Pb atom are governed to a large degree by strong intra-atomic spin-orbit interaction (spin-orbit coupling constant A ≈ 1.22 eV or 9874 cm−1 for Pb2+ ion [36]). Owing to the spin-orbit coupling the first excited singlet and triplet states are strongly mutually mixed. The transition between the ground (singlet) state and the first excited triplet state, which is spin-forbidden in the absence of spin-orbit coupling (as in twofold coordinated Si, Ge, and, to a lesser degree, Sn atoms), turns out to be practically allowed. Therefore intensities of absorption or luminescence corresponding to the transitions between the ground state and these lowest excited states approach each other in

center. So this center has two

main absorption bands, 4.15 and 5.33 eV (0.23 and 0.30 μm). Luminescence in two bands is excited due to this absorption (Fig. 12(b)). As in the case of twofold coordinated Si and Ge atoms in SiO2 glass [31–33, 35], considerable Stokes shift is characteristic of the twofold coordinated Pb atom. The Stokes shift is estimated to be ~ 1.8 … 1.9 eV for both 0.23 μm and 0.30 μm absorption bands. Respectively, the luminescence wavelengths may be estimated as ~0.3 … 0.4 and ~ 0.5 … 0.6 μm (~ 3.5 and ~2.3 eV).

3.4. Lead-related centers of IR luminescence in SiO2 glass 3.4.1. Interstitial Pb+ ion According to the calculations, only Pb+ ion can occur as an interstitial center in six-member ring interstitial site in SiO2 host. Both Pb0 atom and Pb− ion turn out to be unstable in this site. Owing to electron(s) transfer to the glass network, Pb0 atom or Pb− ion are found either to turn into interstitial Pb+ ion, or to built in the surrounding glass network forming usually threefold coordinated Pb atom. The calculated configuration of the interstitial Pb+ ion in SiO2 network is shown in Fig. 14. Calculation of electron density distribution by Bader's method proves the effective charge of the Pb interstitial species to be +0.87|e| in the center under consideration. Hence this center actually may be regarded as an interstitial Pb+ ion. Levels and transitions schemes of Pb+ interstitial ion in SiO2 are given in Fig. 12(c). The Pb+ interstitial ion turn out to interact only weakly with the surrounding host atoms and do not form any bond with them. So the influence of the host on electronic states and spectral properties of the interstitial ion consists mainly in crystal field effect. The origin of IR

luminescence in the Pb+ center may be understood in a model similar to that of Pb+(1) center in alkaline earth fluorides [9] (or Tl0(1) center in LiCl [11,12]). A weak axial crystal field is caused by O and Si ligands of the SiO4 six-member rings surrounding the interstitial center. The electronic configuration of Pb+ ion is known to be [Xe]4f145d106s26p1. The ground and first excited states, 2P1/2 and 2P3/2, respectively, arise from the 2P atomic state split by a strong spin-orbit interaction (spinorbit constant A ≈ 1.16 eV or 9387 cm−1) by 1.75 eV (14,081 cm−1 ) [36,37]. The higher excited states arise from the 4P and 2P atomic states corresponding to 6s16p2 and 6s27s1 electronic configurations, respectively. The ground state of Pb+ ion, 2P1/2, is not split by the crystal field. The axial crystal field spits the first excited state, 2P3/2 in two components and mixes them forming two excited states of the Pb+ center (approximately 2.5 and 1.5 eV or 22,000 and 12,100 cm−1). Electric dipole transitions between three spin-orbit components of 2P state forbidden in a free Pb+ ion become allowed due to state mixing under the influence of the crystal field giving rise to absorption at approximately 0.5 and 0.8 μm, respectively. Transition from the lowest excited state to the ground one give rise to IR luminescence. As in the case of the Pb+(1) center in alkaline earth fluorides, and as distinct from similar bismuthrelated center [15], Stokes shift is found to be not small and is estimated as ~1.4 and ~0.4 eV for the absorption bands at 2.5 and 1.5 eV, respectively. Accordingly (Fig. 12(c)), IR luminescence wavelength may be estimated as ~1.0 … 1.2 μm or (~1.2 … 1.0 eV). It is known [9] that in the alkaline earth fluorides a simple crystal field model describes unsatisfactorily the origin of the third and the next excited states of the Pb+(1) center, since splitting of the 4P and 2 P states of Pb+ ion and their mixing, both mutual and with the lower states is too strong and leads to unrealistic values of the model parameters. Our calculations proves this conclusion to be valid for the Pb+ center in SiO2 as well: the excited states wavefunctions are distantly related with those of free Pb+ ion and contain significant contribution both of the lower states of the ion and the host electronic states. 3.4.2. Interstitial Pb0 atom and ≡Si\\Si≡ vacancies Recently we have showed [15] that bismuth-related IR luminescence in bismuth-doped SiO2 glass may be caused by a complex center formed by an interstitial Bi atom and a ≡Si\\Si≡ oxygen vacancy. The latter is known to be intrinsic defect characteristic of the vitreous SiO2 (see e.g. [33]). However, our present calculation of similar lead center prove a complex formed by an interstitial Pb0 atom and ≡Si\\Si≡ vacancy to be charge-unstable, in contrast to the case of bismuth. As for the single interstitial Pb0 atom, either an electron is transferred from the atom into the glass network to form a charged complex of an interstitial Pb+ ion with a ≡Si\\Si≡ vacancy, or the glass network undergo certain local transformation resulting in twofold coordinated Pb atom formed.

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On the other hand our calculation show that interstitial Pb+ ion can form stable complex with ≡ Si\\Si≡ vacancy. The ≡Si\\Si≡ oxygen vacancy in SiO2 have been modeled in our previous work [15] using the same models and calculation approach as described above. In the single ≡Si\\Si≡ vacancy the distance between Si atoms is found to be 0.244 nm and a covalent bond between two atoms is formed, somewhat relaxed in comparison with crystalline Si. In the ≡Si\\Si≡ vacancy the effective charges of Si atoms calculated by Bader's method are found to be ≈ + 3.0 | e |. When interstitial Pb+ ion forms a complex with single ≡ Si\\Si ≡ vacancy, the distance between Si atoms in the vacancy increases to 0.344 nm. The distances between the Pb atom and Si atoms turn out to be 0.267 nm. Bader's analysis of electron density proves the effective charge of Pb atom and each of Si atoms in the vacancy to be ≈ + 1.7 | e | and ≈ + 3.1 | e |, respectively. So an extra charge ≈ + 0.9 | e | is localized in Pb atom and Si atoms of ≡ Si\\Si ≡ vacancy. With the complex being formed, the electron density is shifted from PbO ion into the area between these Pb and Si atoms and, to a lesser extent, into the area between two Si atoms. Hence lead occurs in nearly divalent state in such a complex. As a rough approximation one can consider such a center as a complex of an interstitial Pb2+ ion and a negatively charged ≡Si\\Si≡ vacancy. The lowest excited states of Pb2+ ion are known to fall into the 7.5– 9.8 eV range and the absorption caused by transitions from the ground state to these excited ones occur in the 0.16–0.13 μm range [36]. The influence of the charged vacancy on the ion states consists mainly in crystal field effect. The electronic configuration of Pb2+ ion is known to be [Xe]4f145d106s2, and the lowest excited states arise from 3P multiplet corresponding to 6s6p configuration. Under the influence of the nearly axial perturbation caused by the charged vacancy the excited multiplet energy is somewhat decreased. The lower state of the multiplet, 3P0, is not split in the axial field. As a result, the absorption bands of the complex turn out to fall into 0.20–0.15 μm (6–8 eV) range. Hence such a center cannot be related to IR luminescence. Our modeling show that the Pb0 atom residing in the network interstitial formed by six-member rings of SiO4 tetrahedra can form a stable complex with two ≡Si\\Si≡ oxygen vacancies located in opposite parts of the six-member ring. The calculated configuration of such a complex, , is shown in Fig. 15. In this complex the distance between Si atoms in the vacancy turn out to increase up to 0.475 nm, to be compared with 0.244 nm in a single ≡ Si\\Si ≡ vacancy and ≈0.31 nm in ≡Si\\O\\Si≡ linkage in SiO2 glass. The distances between the Pb atom and Si atoms are found to be 0.261 nm. So with the complex formed the six-member ring is broken into two fragments linked together by the Pb atom.

Calculation of electron density distribution by Bader's method proves the effective charge of Pb atom and each of four Si atoms in the center to be and ≈− 0.05|e| and ≈ +3.01|e|, respectively. Hence as a rough approximation this center may be considered as an interstitial Pb0 atom forming a complex with two neutral ≡ Si\\Si ≡ oxygen vacancies. Nevertheless with the complex being formed the electron density is shifted slightly from the space between two Si atoms in each vacancy towards the Pb atom and very weak bonding arises among the Pb atom and Si atoms of the vacancies. Coordination-type multi-center bond would be expected to occur in such a complex. Thus, a group of mutually bound Pb atom and four Si atoms is formed instead of two pairs of covalently bonded Si atoms. Since this bonding it weak, the influence of the host on electronic states and spectral properties of the interstitial Pb atom consists mainly in crystal field effect. The origin of IR luminescence in the center may be described qualitatively in a model similar to that of the Pb0(2) center in alkaline earth fluorides [9,10]. The electronic states of the interstitial Pb atom is perturbed by weak axial crystal field. The crystal field is constituted a superposition of two nearly axial fields, one caused by two ≡Si − Si≡ vacancies in the opposite sides of the SiO4 six-member ring surrounding the interstitial Pb atom and another caused by O and Si ligands of the six-member ring itself. The electronic configuration of Pb0 atom is known to be [Xe]4f14510 d 6s26p2. The ground state, 3P0, and two lowest excited states, 3P1 (0.97 eV or 7819 cm−1) and 3P2 (1.32 eV or 10,650 cm−1), are formed of the 2P atomic state due to a strong spin-orbit splitting (spin-orbit coupling constant A ≈ 1.00 eV or 8076 cm−1) [36,38,39]. The next two excited states, 1D2 (2.66 eV or 21,458 cm− 1) and 1S0 (3.65 eV or 29,467 cm−1), arise from the 1D and 2P atomic states corresponding to the same electronic configuration. In an axial crystal field the ground state of Pb0 atom, 3P0, is not split. The first excited state, 3P1, is split by the axial crystal field in two levels, approximately 1.06 and 1.66 eV (8550 and 13,390 cm‐1). The second excited state, 3P2, is split in three levels, approximately 1.99, 2.91, and 3.76 eV (16,050, 23,470, and 30,326 cm−1). These levels are shifted considerably to higher energies in comparison with the unperturbed SiO2 state owing to interaction between this state and the 1D2 one. Higher excited states at 4.54 eV (36,618 A ≈ 1.22), 4.93 eV (39,765 cm−1), and others arises from 1D2 and 1S0 atomic states. Electric dipole transitions between the split spin-orbit components of 3P state forbidden in a free Pb0 atom become allowed due to state mixing in the crystal field. The transitions from the ground state correspond to several absorption bands near 1.2, 0.45 and 0.35 μm and in the 0.6–0.8 and ≲0.3 μm ranges. The absorption in the bands near 1.2 μm and in the 0.6–0.8 μm range is significantly lower than in others. Transitions from the first and the second excited states to the ground one correspond to two bands of IR luminescence. As in the case of the Pb0(2) center in alkaline earth fluorides [9,10] and as distinct from the bismuth center formed by an interstitial Bi atom and ≡Si\\Si≡ vacancy [15], the Stokes shift is not small for these transitions in the

center. Accord-

ing to our estimation the Stokes shift is ~ 0.2 eV for the transition between the ground state and the first excited state, and ~0.4 eV for the transition between the ground state and the second excited state. Accordingly, the wavelength of the two bands of IR luminescence can be estimated as ~1.2 … 1.5 μm and ~0.8…1.1 μm (Fig. 11). 4. Discussion

Fig. 15.

lead center in SiO2.

In the present work we observed UV and visible luminescence in SiO2:Pb glasses in the 0.50–0.60 and 0.30–0.45 μm (~2.3 and ~ 3.5 eV) ranges excited in the absorption bands in the 0.23–0.26 and 0.29– 0.30 μm (~4.1 and ~5.3 eV).

V.O. Sokolov et al. / Journal of Non-Crystalline Solids 452 (2016) 176–186

Single luminescence band in the 0.3–0.4 μm range excited in the absorption band near 0.25 μm have been observed repeatedly in Pb-doped silicate glasses (e.g. in Na2O\\SiO2 [27] and in K2O\\PbO\\SiO2 [28,29] glasses). It seems to be similar to one observed in the present work in the 6LSH sample. Broadband luminescence in 2.5–3.7 eV (0.50–0.34 μm) range with weakly pronounced maxima near 2.8, 3.2 and 3.5 eV (0.45, 0.40 and 0.35 μm) excited in two bands near 5.2 and 6.1 eV (0.23 and 0.20 μm) was discovered in sodium silicate glasses containing about 0.05 wt% of PbO [40]. On the whole, this is consistent with our observations for the 4LSN sample. Luminescence in 1.6–3.2 eV (0.75–0.39 μm) range consisting of three components with peaks near 1.8, 2.0 and 2.55 eV (0.7, 0.6 and 0.5 μm) was observed in lead silicate glasses containing from 20 to 75 mol% of PbO [41] and the 2.55 eV component, intensity of which decreases with PbO content increasing, was suggested to be caused mainly by the transition between 6p and 6s states of Pb atom. With a relatively low PbO content, a pronounced maximum near 5.2 eV (0.23 μm) was observed in the luminescence excitation spectrum. This luminescence is probably similar to that observed in the present study in the 0.5–0.6 μm range. The results of our modeling of the lead-related centers corresponding to the main forms of Pb in SiO2 host are in good agreement with the experimental results. In particular, the main absorption bands of center correspond to the absorption

the

spectrum (Fig. 1) and the absorption in these bands increases with Pb content in the sample (e.g. the strongest absorption is observed in the 4LSN center with the highest Pb content, 0.36 wt%) Furthermore, the recenter allow us to explain the

sults of modeling of the origin

of

the

0.29–0.30

μm

absorption

and

band.

For

both

centers the calcu-

lation predicts UV (0.30–0.45 μm) and visible (0.50–0.60 μm) luminescence and the lack of luminescence in the 0.70–0.80 μm range, in agreement with experiment. The differences in the UV and visible luminescence spectra of the 4LSN ~ 0.36 μm) and 6LSH (~ 0.40 μm) samples under excitation near 0.25 μm can be explained on the assumption that centers of two different types with similar absorption bands occur in the samples. It follows from this assumption that depending on the preparing conditions (primarily on the atmosphere used) the and

centers are formed in various concentrations.

Although

center as lead-related analog of SiODC-II. formation

in a wide band with a pronounced maxima near 4.2 and 5.3 eV (0.29 and 0.23 μm) was observed [40]. Such a luminescence may be caused by just the diatomic centers which should occur in significant concentration in such glasses (see e.g. [21,22]). Thus, it seems reasonable to assume that in SiO2:Pb glasses − the absorption band in the 0.23–0.26 μm (5.4–4.8 eV) range is caused by threefold coordinated Pb atoms, , and twofold coordinated Pb atoms, ; − the absorption band at 0.29–0.30 μm (4.3–4.1 eV) is caused by twofold coordinated Pb atoms; − the luminescence in the 0.30–0.45 μm (4.1–2.8 eV) range excited in both above-mentioned absorption bands is caused by both threeand twofold coordinated Pb atoms; − luminescence in the 0.50–0.60 μm (2.5–2.1 eV) range excited in both above-mentioned absorption bands is caused by twofold coordinated Pb atoms. (see Table 2). In the glasses under investigation in the present work the luminescence in the 0.30–0.45 μm (4.1–2.8 eV) range is caused mainly by twofold coordinated Pb atoms and may contain only a relatively small contribution caused by threefold coordinated Pb atoms. IR luminescence recently observed in FV glasses [4–6] was detected as well in the present study in the 0.85–0.95, 1.0–1.2 and 1.3–1.4 μm (1.4–1.3, 1.2–1.0 and 0.95–0.9 eV) ranges. The luminescence near 0.9 μm and near 1.35 μm was excited in the absorption bands near ~0.32 μm and in the ≲0.28 μm range (~3.9 and ≲4.5 eV), The luminescence near 1.1 μm was excited in the 0.41–0.46, 0.29–0.32 and ≲0.25 m (3.0–2.7, 4.3–3.9 and ≲5.0 eV) ranges. The difference between the excitation spectra of these luminescence bands and changes in the IR luminescence spectra under γ irradiation suggest that the luminescence in the 1.0–1.2 μm range and one in the 0.85–0.95 and 1.3– 1.4 μm ranges are caused by centers of two types. Results of our modeling allow us to suggest models of the IR luminescence centers in SiO2:Pb glass. The luminescence in 1.0–1.2 μm range may be caused by an interstitial Pb+ ions. According to the calculations, in the Pb+ center such a luminescence is excited in absorption bands in the ≲~0.25 μm range and near 0.35, 0.5 and 0.8 μm. These results agree well with the experimental data. The luminescence in 0.85–0.95 and 1.3–1.4 μm ranges may be caused by centers. The calculations show that the lumines-

It should be noted that in both samples under 0.25 μm excitation we observed the luminescence near 0.28 μm caused by SiODC-II (_Si) oxygen-deficient centers [31–33]. We failed to distinguish the luminescence near 0.46 μm (characteristic of SiODC-II) against the background of intense luminescence in the 0.36 μm (4LSN), 0.40 μm (6LSH) and 0.55 μm bands. The growth of intensity of the luminescence near 0.28 μm under γ irradiation is known to be caused by SiODC-II formation. Simultaneous increase of intensity of the luminescence near 0.40 μm and in the 0.50–0.60 μm range supports a model of oxygen-deficient

185

of diatomic centers in

SiO2:Pb glasses under investigation seems to be unlikely due to low Pb concentration, it is no reason to exclude completely the contribution of such centers in the luminescence in 0.45–0.55 μm range observed in the 4LSN sample. In lead silicate SiO2\\PbO glasses containing about 40 mol% PbO a luminescence band near 2.7 eV (0.46 μm) excited

cence in two bands in the 0.8–1.1 and 1.2–1.5 μm ranges is excited due to absorption in 0.6–0.8, 0.3–0.5 and ≲0.3 μm in this center. The IR luminescence spectra and corresponding excitation spectra in the b0.5 μm range are explained adequately by the suggested model of the luminescence center. Lack of IR luminescence under 0.50–0.85 μm excitation may be explained by low excitation efficiency owing the above-mentioned weakness of absorption in this range. It should be emphasized that the Pb+ and centers can be regarded as sufficiently close analogues of, respectively, Pb+ (1) and Pb+ (2) centers in alkaline earth fluorides [9,10,13]. As noted above, in the samples prepared using a hydrogen atmosphere the IR luminescence turned out to be the most intense, whereas in the samples prepared in an oxygen atmosphere, no IR luminescence was found. These facts agree well with the properties of two defects suggested as models of the IR luminescence centers since lead turns out to be subvalent (reduced) in these centers (Pb+ and Pb0 , respectively). So it may safely enough suggested that in SiO2:Pb glass − interstitial Pb+ ions are responsible for the luminescence in the 1.0– 1.2 μm range;

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Table 2 Luminescence bands and possible centers: summary. Luminescence band, μm

Excitation band(s), μm

0.30–0.45

0.23–0.26 0.29–0.30 0.23–0.26 0.29–0.30 ~0.32 ≲0.28 0.41–0.46 0.29–0.32 ≲0.25 ~0.32 ≲0.28

0.50–0.60 0.85–0.95 1.0–1.2

1.3–1.4

−

centers are responsible for the luminescence in the 0.85–0.95 μm and 1.3–1.4 μm ranges.

(see Table 2).

5. Conclusion Our experimental study of the absorption (transmission), luminescence and luminescence excitation spectra of SiO2:Pb glasses showed that at a relatively low PbO content (≲10−1 wt%) UV and visible luminescence in the 0.30–0.45 and 0.5–0.6 μm bands and IR luminescence in the 0.85–0.95, 1.0–1.2, and 1.3–1.4 μm bands are observed in these glasses. The most intensive IR luminescence was found in the glass samples prepared using hydrogen atmosphere. Modeling of SiO2:Pb glass performed by computational solid state physics approaches in a periodic model of the glass network proved the main possible forms of lead in the SiO2 glass network to be twofold coordinated,

,

and

threefold

coordinated,

lead atoms, diatomic lead centers,

, and possibly fourfold coordinated lead atoms. The results of the calculation of structure, electronic properties, absorption and luminescence properties of the leadrelated centers in SiO2:Pb glass corresponding to these forms suggest that in the glasses studied in our experiments the 0.3–0.45 μm luminescence band is caused mainly by twofold coordinated Pb atoms and possibly in part by threefold coordinated Pb atoms, and the 0.5–0.6 μm one is caused by twofold coordinated Pb atoms. Basing on the results of the calculation of structure, electronic properties, absorption and luminescence properties of the possible lead-related centers in SiO2 glass responsible for the IR luminescence in SiO2\\PbO glass, we suggest that the 1.0–1.2 μm IR luminescence band is caused by interstitial Pb+ ions, and both 0.85–0.95 and 1.3–1.4 μm IR luminescence bands are caused by

centers, complexes formed by the in-

terstitial Pb0 atom and two `Si\\Si` oxygen vacancies. Acknowledgments The authors are grateful to S.V. Firstov for his assistance in luminescence measurements and to V.M. Mashinsky and A.S. Zlenko for valuable discussions. This work is supported in part by Basic Research Program of the Presidium of the Russian Academy of Sciences.

Luminescence center(s) suggested

Pb+

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