3.1. 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 Cr6+ and Cr3+ [7, 10, 24]. The sample with Cu concentration of (x = 1.0) but containing no Cr ions, is characterized by a broad absorption spanning the wide spectral region > 500 nm, and belongs to optical transition from Cu2+ ions [3, 17]. These two reference samples are characterized by distinct colors (see insets in a) derived from Cu2+ (bluish) and exclusively Cr3+ 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 Cr6+, Cr3+, and Cu2+ ions. This is compared to a prepared sample (green spectrum in (b)) 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, Cr3+ transitions are only slightly attenuated while Cr6+ are entirely suppressed, despite the Cr amount is fixed for both 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 Cr3+ transition, 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 (b). 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 Cr6+ transitions at (~ 339 and ~ 370 nm) and slight attenuation of the spin-allowed 4A2(F) → 4T1g Cr3+ transition around 418 nm. In fact, the two Cr6+ transitions, which originate from 4A2g g→ 4T1(P) and 4A2g → 4A1g, 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 Cu2+ 2Eg → 2T2g transition [3, 4, 15–19], 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 4A2g → 4T2g Cr3+ transition at ~ 623 nm. Notice that the intensity of this Cr3+ signal, which is responsible of the greenish color of the samples [10, 29–34], is likely barley affected, since the overall hybrid glass samples exhibit an intermediate color between the greenish-bluish extremes (see insets in Fig. 1).
The tunable Cr optical transitions, here reported for fixed Cr concentration, clearly demonstrates 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 [10, 35–38],
10Dq = υ1, (1)
while the Racah parameters (B & C) are given by the following equations:
B= (2υ1- υ2) (υ2- υ1)/ (27 υ1-15 υ2), (2)
C= (υ3-4B- υ1)/3, (3)
where υ1, υ2, and υ3 refer to the peak position of the high/low energy Cr3+ transitions and the average of Cr6+ peaks, respectively. The estimated values of 10Dq and both B & C 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 Cr3+ 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 Cr3+ ions and ligands [4, 5, 39, 40].
With regard to the crystal field of Cu ions, it is well known that Cu2+ (3d9) in octahedral crystal field loses its degeneracy and splits into 2Eg and 2T2g with 2Eg being the lower level. Generally speaking, Cu2+ ions are often found in octahedral sites, and the 2Eg state splits due to Jahn–Teller effect. Therefore, the here observed red shift of the broad Cu2+ optical transition (~ 800 nm) indicates that Cu ions occupy tetragonally-distorted octahedral symmetry as discussed later from ESR results [3, 16–19]. The consequent of such a distortion on the shielding of the atomic charges affects the energy involved in the 2Eg→ 2T2g 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 (Eg). 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 BO4 structural units and the creation of bridging oxygen (BO), as we discuss later through the FITR section [2, 10, 41].
3.2. 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 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 Cr3+ [4, 10–14, 27]. The absorption at g = 2.25 is due to exchange coupled pairs or large Cr3+ clusters, while the resonance located at high field with an effective g value of 1.93 is mainly due to Cr6+ [10, 28]. For the Cr-free 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 Cu2+ (3d9) ions in axially distorted octahedral symmetric sites [3, 17, 42–44]. Additional weak signal from Cu2+ is present at g = 4.08, perfectly coinciding with one Cr3+ 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 Cr6+ 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 [42 − 34] reported that Cu ions reside heavily in tetragonally-distorted 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 [42, 43].
3.3. Infrared Spectroscopy:
The previously discussed modifications, particularly, in Cr optical and ESR spectra- despite 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 . The high wavenumber band (1600–1165 cm− 1) is frequently assigned to asymmetric stretching vibration mode of B–O within the triangular (BO3) structural unit . The nearby band spanning the range (1165–760 cm− 1) most likely originates from B–O stretching vibration within the more stable tetrahedral (BO4) structural groups . The last main band in the range (760 − 590 cm− 1) is often ascribed to B-O-B bending vibration of BO3 groups . Additional weak band at the low wavenumber side (< 500 cm− 1) is discernable, which is predicted to correlate with vibrations from Cr6+ in structural units . It was also equally assigned to ionic vibration from Li ions, B–O–B bending vibrations, and to borate ring deformations [43, 48, 49].
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 BO4 fractions (i.e., the N4 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 Cr6+, 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 Cr6+, 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 [50, 51] utilizing the UFF algorithm for forces minimization. The glass network consists mainly of BO3 structural units (red shaded planner triangle), in addition to NBO (blue atoms) and BO4 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, BO3, and BO4 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 Cr6+ 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 Cu2+ absorption band (Fig. 2) towards the nearby Cr3+ 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.