Multiple optical features in binary-transition-metal borate glasses

Glasses doped with transition metal (TM) ions exhibit rich optical transitions spanning the entire ultraviolet, visible, and infrared spectral regimes. Here we utilize the melt-quenching technique to synthesis binary-TM-doped alkali-borate glasses of composition xCuO-(75-x)B2O3-24.7Li2O-0.3Cr2O3, with variable copper ion concentrations (x = 0, 0.2, 1.0 and 2.0 mol %) and a fixed amount of chromium ions (0.3 mol %). We identify several optical transitions from Cr3+, Cr6+, and Cu2+ ions, the latter manifests exclusively at longer wavelengths and gains noticeably higher intensity with Cu additives. Despite the Cr concentration being fixed, the Cr6+ peaks are quenched and Cr3+ signals are barely attenuated. This behavior rules out possible phase separation and suggests a non-trivial interplay between the two types of TM ions and their ligand, as supported by probing their oxidation states from electron spin resonance. The crystal field and Racah parameters followed an opposite behavior, while the optical band gap was reduced upon doping. These changes are correlated with the structural modifications introduced by Cu additives, where we anticipate homogenous and preferential proximity of Cu-Cr ions within the network.


Introduction
Transition metal (TM) ions offer unique structural probes for the local environments within amorphous glasses, due to their distinguished wide-radial-distribution of the outer d-shell electronic configurations, which renders TM ions highly sensitive to minimal changes in compositions and/or structures of the surrounding ligands (Ebrahimi et al. 2018;Hassan et al. 2012;Samir et al. 2019;Wen et al. 2015;Hassan et al. 2017). The interaction between TM ions and the non-spherically symmetric ligands induces energy splitting of the otherwise degenerate d-electrons, which shows up as spectral peaks in conventional optical spectroscopies. Many such optical transitions often take place in the visible spectral region, thereby imparting glasses with distinct colors which are tunable through the type and concentration of the TM ion and the overall composition of the glass host (Weyl 2016). Chromium (Cr), among other TM ions, is enormously important for the development of tunable solid state lasers, optical fibers, and as active component in luminescence materials, as it can exist in different oxidation states, the most abundant of which are Cr 3+ and Cr 6+ (Rao et al. 2013;Othman et al. 2021;Morshidy et al. 2021;Hassan et al. 2018). Moreover, Cr ions also act as glass modifiers, so that these distinctive oxidation states within the structural units constituting the glass could be regulated by suitable selection of the glass modifiers/formers, by the size and field strength of the ions, and eventually by the mobility of the modifier cations (Hassan et al. 2018;Rao and Veeraiah. 2002;Hassan et al. 2013;Aktas et al. 2019;Hassan et al. 2019). Optical transitions solely stemming from Cr ions can span the ultraviolet (Cr 6+ ) and visible (Cr 3+ ) spectral regimes, the former could be readily identified for diluted Cr ion concentrations .
Likewise, TM copper (Cu) ions with 3d 9 electronic configuration, exhibit interesting electrical and optical properties which make them appealing candidates for superionic conductors and solid state lasers (Rao et al. 2009). They can also be loaded with higher concentrations without disrupting the amorphous nature of the glass host (Samir et al. 2019;Hassan and Hogarth. 1988;Abokhadra et al. 2017). Analogous to the split-d-bands in Cr, the ground state of free Cu ions ( 2 D) splits in an octahedral crystal field into 2 E g and 2 T 2g . Such a splitting manifests in the optical spectra as a broad resonance in the red side of the electromagnetic spectrum, near the infrared (Samir et al. 2019;Hassan and Hogarth. 1988;Abokhadra et al. 2017;Lever 1984;Ballhausen 1962). Additional splitting of 2 E g and 2 T 2g states, in a tetragonally-distorted octahedral coordination with six oxygen ligands, into ( 2 B 1 and 2 A 1 ) and ( 2 E and 2 B 2 ) takes place due to Jahn-Teller effect (Samir et al. 2019;Lever 1984;Ballhausen 1962;Bae and Weinberg. 1994;Wen and Tanner. 2016), which could lead to more optical transitions in the ultraviolet regime. Information regarding levels splitting just discussed for Cr and Cu TM ions could readily be inferred from optical and electron spin resonance (ESR) spectroscopies, where the latter identifies the oxidation states of TM ions exclusively with unpaired electrons.
Among different TM glass hosts, borate glasses are appealing given their various wonderful properties, such as low melting point, moisture resistance, good thermal stability, and fantastic optical properties (Ahmad 2014;Ravikumar et al. 2003). They consist of a complex three-dimensional network of boron and oxygen, which in the present of lowlevel alkali oxide such as lithium oxide, develop the most stable BO 4 derived from the conversion of triangular borate structural units (BO 3 ). Such alkali-oxide borate glasses can accommodate various types and concentrations of TM ions.
Here, we utilize an alkali-borate glass host to explore the optical properties and oxidation states imparted by a combination of binary TM ions, namely Cr and Cu. The two TM ions coherently mix into a rather homogenous glass, where the measured optical spectra clearly exclude possible phase separation. Specifically, we find a complete quenching of Cr 6+ features and attenuated signals from Cr 3+ , while Cu 2+ resonances become dominant at higher Cu-doping levels.

Experimental techniques
Hybrid TM-doped alkali-borate glass system with composition xCuO-(75-x)B 2 O 3 -24.7Li 2 O-0.3Cr 2 O 3 (x = 0, 0.2, 1.0, and 2.0 mol%) was prepared by melt quenching technique. The respective amounts of the powder oxides were melted in an electrical furnace, under ordinary atmospheric conditions, at 1100 °C for one hour until a bubble-free liquid was formed. The molten were quenched rapidly to room temperature between two polished copper plates. Green-blue colored glasses with high optical quality and transparency were obtained. The as-quenched samples take the shape of irregular solid flakes of lateral area ~ 4.0 cm 2 and thickness of ~ 2.0 mm. Optical absorption spectra of the solid glass samples were recorded using JenWay-6405-type UV-VIS spectrophotometer in the 190-1100 nm spectral range. Electron spin resonance (ESR) data were collected using EMX Bruker type spectrometer, operating at the X-band frequency with 100 kHz field modulation and 10 mW microwave power. The magnetic field was varied between 100 to 4000 G. For a reasonable quantitative spin analysis, the weight of the powder is kept fixed for all samples. Infrared absorption spectra were recorded in the range 400-1600 cm-1 using FTIR Nicolet 6700 spectrometer, on powdered samples pressed into disks after mixing with high purity KBr.

Optical absorption spectroscopy
The optical absorption spectra of two reference samples are presented in Fig. 1(a). These are Cu-free sample containing 0.3 mol % Cr (red), and Cr-free sample with 1.0 mol % Cu (black), so that we easily identify optical contributions from individual TM ions. The Cu-free sample (x = 0.0) exhibits multiple optical transitions in the ultraviolet and visible spectral regimes which are indicated by arrows and are, respectively, assigned to Cr 6+ and Cr 3+ (Rao et al. 2013;Hassan et al. 2018;Terczynska-Madej et al. 2010). The Cr-free sample with Cu concentration of (x = 1.0) is characterized by a broad absorption spanning the wide spectral region > 500 nm, and belongs to optical transition from Cu 2+ ions (Samir et al. 2019;Abokhadra et al. 2017). These two reference samples are characterized by distinct colors (see insets in a) derived from Cu 2+ (bluish) and exclusively Cr 3+ transition at ~ 620 nm (greenish). The mathematical summation of the two spectra in (a) yields the yellow spectrum in Fig. 1(b), which obviously contains contributions from Cr 6+ , Cr 3+ , and Cu 2+ ions. This spectrum is compared to the green spectrum in (b) recorded for a prepared sample consisting of the respective Cu and Cr amounts, i.e., 1.0 mol % Cu and 0.3 mol% Cr. We note that, for such hybrid TM sample, Cr 3+ transitions are only slightly attenuated while Cr 6+ are entirely suppressed, despite the Cr amount is fixed to the sum of the two samples in (a). This sharp contrast between the two spectra in (b) confirms the homogeneity of the two TM ions within the samples, so that no phase separation between areas with single type TM ions is taking place. Given that Cr 3+ transition at ~ 620 nm, which is responsible for the color, is only minimally affected, the final glass color of this TM hybrid system (see insets in b) is the simple summation of the two colors in (a). In order to follow these spectral changes in such homogenous and binary TM-doped systems, we present in Fig. 2 the optical absorption spectra for a series of Cu-doped samples (x = 0, 0.2, 1.0, 2.0 mol %) with fixed Cr concentration (0.3 mol %).
The spectra exhibit clear suppression of Cr 6+ transitions at (~ 339 and ~ 370 nm) and slight attenuation of the spin-allowed 4 A 2 (F) → 4 T 1g Cr 3+ transition around 418 nm. In fact, the two Cr 6+ transitions, which originate from 4 A 2g g → 4 T 1 (P) and 4 A 2g → 4 A 1g , is only barely recognized for the least Cu-doped sample (x = 0.2 mol%). On the other hand, the broad spectral transition in the region (530-1100 nm), which belongs to Cu 2+ 2 E g → 2 T 2g transition (Samir et al. 2019;Wen and Tanner. 2015;Rao et al. 2009;Hassan and Hogarth 1988;Abokhadra et al. 2017;Lever 1984;Ballhausen 1962), gains noticeably larger intensity with Cu additives. Indeed, the center of gravity of this broad transition, which is located near ~ 790 nm ( Fig. 1(a)) for the Cr-free sample, progressively shifts towards shorter wavelengths, thereby mixing with the band position of the nearby spin-allowed 4 A 2g → 4 T 2g Cr 3+ transition at ~ 623 nm. Notice that the intensity of this Cr 3+ signal, which is responsible of the greenish color of the samples (Padlyak et al. 2012;Singh et al. 2014;Flower et al. 2007;Koepke et al. 2001;Durga and  The tunable Cr optical transitions, here reported for fixed Cr concentration, clearly demonstrate that Cu additives modify the ligand environment around Cr ions within the glass host. Such information can be coined within a set of ligand field parameters, namely the crystal field (10Dq) and Racah parameters (B & C). Their determination for a given TM ion demand knowledge of the position of all its absorption peaks, which could be precisely extracted via a deconvolution process, as exemplified at the inset of Fig. 2. The magnitude of 10Dq is then determined from the simple one-to-one relation (Hassan et al. 2018, Ismail et al. 2021aPisarski et al. 2009;Ismail et al. 2021b; Ahmad and Nabhan 2019), while the Racah parameters (B & C) are given by the following equations: where υ 1 , υ 2 , and υ 3 refer to the peak position of the high/low energy Cr 3+ transitions and the average of Cr 6+ peaks, respectively. The estimated values of 10Dq and both B & C (1) 10Dq = 1 , parameters are plotted in Fig. 3(a-b) as a function of the Cu content. While 10Dq parameter (a) clearly decreases with Cu additives, both Racah parameters (b) follow the opposite behavior. These results suggest strong electron localization at Cr 3+ ions and, therefore, the d shell inter-electronic repulsion (encoded in B & C) becomes comparatively more intense, leading to chemical bonds of more ionic character between Cr 3+ ions and ligands (Wen and Tanner 2015;Hassan. 2017;Vicente et al. 2014;Calas et al. 2006).
With regard to the crystal field of Cu ions, it is well known that Cu 2+ (3d 9 ) in octahedral crystal field loses its degeneracy and splits into 2 E g and 2 T 2g with 2 E g being the lower level. Generally speaking, Cu 2+ ions are often found in octahedral sites, and the 2 E g state splits due to Jahn-Teller effect. Therefore, the here observed red shift of the broad Cu 2+ optical transition (~ 800 nm) indicates that Cu ions occupy tetragonally-distorted octahedral symmetry as discussed later from ESR results (Samir et al. 2019;Hassan and Hogarth 1988;Abokhadra et al. 2017;Lever 1984;Ballhausen 1962). The consequence of such a distortion on the shielding of the atomic charges affects the energy involved in the 2 E g → 2 T 2g transition, which become lower as the inner shell electron density is increased. Finally, we shed the light into a physical quantity which encompasses valuable information about the momentum-integrated band structure of the whole glass system, namely the optical band gap (E g ). This is often determined from absorption spectra by utilizing Tauc's routine applied to the linear part of the absorption edge, as depicted in Fig. 3(c). The estimated band gap, presented in Fig. 3(d) as a function of Cu content, was found to decrease by ~ 0.32 eV at the highest Cu-doing level (2.0 mol %). This behavior of the band gap can be traced back to additional defect states introduced by CuO into the glass matrix, but also relates to the increased concentration of BO 4 structural units and the creation of bridging oxygen (BO), as we discuss later through the FITR section (Hassan et al. 2012;Hassan et al. 2018;Varshneya 1994). We state that for a comparable doping level as used here, the optical band gap of several borate glasses and semiconductors was found to change by a similar amount (i.e., few hundreds of meV) when doped with transition metals and rare earths (Morshidy et al. 2021;Hassan et al. 2018;Dubey and Singh 2017; Mote 2012).

Electron spin resonance spectroscopy
The ESR spectra of all samples (x = 0, 0.2, 1.0, and 2.0 mol %) are shown in Fig. 4. We initiate the analysis and discussion here by first exploring the spectra of two reference samples, namely Cr-free (x = 1.0, Cr = 0) and Cu-free (x = 0, Cr = 0.3), depicted at the upper The upper inset contains the spectra of the two references, namely Cr-and Cu-free samples, while lower inset shows a close-up view for the low field Cr 3+ signal. Hyperfine splitting form Cu 2+ at g || ≈ 2.4 is indicated by dashed lines inset. We utilized this strategy to distinguish features originating due to Cr ions from those resulting from Cu ions. It is clear that there are three signals in the Cu-free sample (x = 0), two of which are located at the low field side with effective g values 4.82 and 4.08, and are often attributed to the presence of Cr 3+ (Wen and Tanner 2015;Hassan et al. 2018, Hassan et al. 2018Rao and Veeraiah 2002;Hassan et al. 2013;Aktas et al. 2019;Hassan et al. 2019;Padlyak et al. 2012). The absorption at g = 2.25 is due to exchange coupled pairs or large Cr 3+ clusters, while the resonance located at high field with an effective g value of 1.93 is mainly due to Cr 6+ (Hassan et al. 2018, Singh et al. 2014. For the Crfree and (x = 1) sample, strong signals show up in the range 2800-3600 Gauss, with a well-defined resonance at g = 2.05 which obviously belong to Cu 2+ (3d 9 ) ions in axially distorted octahedral symmetric sites (Samir et al. 2019;Abokhadra et al. 2017;Funabiki et al. 2011;Yao et al. 2016;Andronenko et al. 2004). Additional weak signal from Cu 2+ is present at g = 4.08, perfectly coinciding with one Cr 3+ resonance. The incorporation of Cu ions onto the Cr-doped borate glasses has pronounced influence on all observed signals as evident from Fig. 4. The intensity of the two signals with effective g values 4.82 and 4.08 were attenuated by rising Cu ions (see lower inset), while the Cr 6+ signal with g value 1.93 was entirely vanished, in perfect agreement with the Uv-Vis results. The intensities of the signals in the field range 2800-3600 Gauss become noticeably stronger with increasing the Cu concentration, and exhibit hyperfine interaction (indicated by the four dashed lines) clearly distinguished for the x = 2.0 sample. Earlier literature (Funabiki et al. 2011;Yao et al. 2016;Andronenko et al. 2004) reported that Cu ions reside heavily in tetragonallydistorted octahedral field. Based on Jahn-Teller theorem, the distortion of octahedral symmetry will remove the degeneracies of the ground terms of Cu ions. The distortion is probably elongated or compressed, which in turn will determine the relative positions of g || and g ⟂ . Here g || ≈ 2.4 and g ⟂ = 2.03, i.e., g || ˃ g ⟂ , so that it is reasonable to claim that Cu ions reside in elongated octahedral distortion (Funabiki et al. 2011;Yao et al. 2016).

Infrared spectroscopy
The previously discussed modifications, particularly, in Cr optical and ESR spectradespite its concentration being fixed-reflects structural rearrangement of ligands surrounding chromium ions, introduced upon Cu additives. Relevant structural information could be inferred from the FTIR spectra, presented in Fig. 5, for all prepared samples. The spectra mainly consist of three dominant absorption bands often present in alkali-borate glasses (Ismail et al. 2021b). The high wavenumber band (1600-1165 cm −1 ) is frequently assigned to asymmetric stretching vibration mode of B-O within the triangular (BO 3 ) structural unit (Ibrahim et al. 2018). The nearby band spanning the range (1165-760 cm −1 ) most likely originates from B-O stretching vibration within the more stable tetrahedral (BO 4 ) structural groups (Armenak et al. 2019). The last main band in the range (760-590 cm −1 ) is often ascribed to B-O-B bending vibration of BO 3 groups (Mahesh et al. 2017). Additional weak band at the low wavenumber side (< 500 cm −1 ) is discernable, which is predicted to correlate with vibrations from Cr 6+ in CrO 2− 4 structural units (Hassan et al. 2018). It was also equally assigned to ionic vibration from Li ions, B-O-B bending vibrations, and to borate ring deformations (Yao et al. 2016;Hverhoef andDen-Hartog 1995, Kamitsos et al. 1993).
The two absorption shoulders identified at 1250 and 900 cm −1 are assigned to asymmetric stretching vibrations of NBO of B-O-B in tetrahedral and triangular structural units, respectively. The estimated number of NBO and the BO 4 fractions (i.e., the N 4 ratio) were found to barley increase (by ~ 2%) for the highest Cu-doped sample, indication insignificant structural changes of the main borate structural group. However, the low wavenumber band, which contains partial contribution from Cr 6+ , is lowered by ~ 5%.
In order to correlate these minimal structural changes with the noticeable variations in optical and ESR spectra, particularly the complete quenching of Cr 6+ , we discuss possible TM environments through the structural model presented in Fig. 6. In Fig. 6 we envision a structural model containing isolated, adjacent, and shared Cr and Cu octahedrons within alkali-borate network. The model is obtained using Avogadro's software (Avogadro 1.2; Hanwell et al. 2012) utilizing the UFF algorithm for forces minimization. The glass network consists mainly of BO 3 structural units (red shaded planner triangle), in addition to NBO (blue atoms) and BO 4 structural groups (red shaded tetrahedron) produced by Li additives.
For diluted concentrations of TM ions, such as those considered here (≤ 2.3 mol%), the borate structural units are practically unaffected upon doping, so that the ratios NBO, BO 3 , and BO 4 are mainly determined by the fixed alkaline (Li) percentage, as concluded from FTIR results. Although this valid for the three Cr-Cu environments depicted in the structural model, the Cr ligand environment is distinct in each case. At the top-left of the model, isolated Cr and Cu octahedrons are expected to exhibit the average sum of the optical and ESR properties of the reference samples, which we have excluded this case in Fig. 1(b). In contrast, the proximity of Cr and Cu octahedrons at bottom-left and to the right, should facilitate mutual interactions, beside the shared oxygen, which effectively quenches Cr 6+ oxidation state, being consistent with optical and ESR results. Additional sign of adjacent/ shared Cr-Cu interactions, is the shift of the center of gravity of Cu 2+ absorption band (Fig. 2) towards the nearby Cr 3+ peak position. While at the high preparation temperature and rapid quenching required for glass formation many such structural models are possible, the occurrence of adjacent and shared Cr-Cu octahedrons appeared to be preferential.

Conclusion
Binary-TM doped borate glasses of composition xCuO-(75-x)B 2 O 3 -24.7Li 2 O-0.3Cr 2 O 3 , were prepared using the melt-quenching techniques. Distinct optical transitions from Cr 3+ , Cr 6+ , and Cu 2+ ions were identified. The intensity of these Cr transitions was found to exhibit strong intensity modulation, although the Cr concentration is fixed. Specifically, the Cr 6+ peaks are strongly suppressed, while those of Cr 3+ are barely attenuated after Cu doping. These optical modifications are supported by probing the oxidation states of Cr and Cu ions using electron spin resonance spectroscopy. The crystal field (10Dq), Racah parameters (B, C), and the optical band gap were determined, where the latter decreases by ~ 0.32 eV at the highest Cu content. These findings are complemented by utilizing FTIR spectroscopy to map the structural modifications introduced by Cu additives, and a structural model assuming homogenous and preferential proximity of Cu and Cr ions within the network was proposed.