Spectroscopic properties of the V-doped borate glasses

Spectroscopic properties of the V-doped borate glasses

Journal of Non-Crystalline Solids 528 (2020) 119741 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: ww...

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Journal of Non-Crystalline Solids 528 (2020) 119741

Contents lists available at ScienceDirect

Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol

Spectroscopic properties of the V-doped borate glasses B.V. Padlyak a b

a,b,⁎

, T.B. Padlyak

T

b

Vlokh Institute of Physical Optics, Department of Optical Materials, 23 Dragomanov Str., 79-005 Lviv, Ukraine University of Zielona Góra, Institute of Physics, Division of Spectroscopy of Functional Materials, 4a Szafrana Str., 65-516 Zielona Góra, Poland

A R T I C LE I N FO

A B S T R A C T

Keywords: Borate glasses Vanadyl (VO2+) Epr Spin hamiltonian parameters Optical absorption Crystal field parameters

Electron paramagnetic resonance (EPR) and optical absorption spectra in borate glasses of Li2B4O7:V, LiKB4O7:V, CaB4O7:V, and LiCaBO3:V compositions with added 0.5 and 1.0 mol.% V2O5, were investigated. The EPR spectroscopy clearly shows that the V impurity is incorporated into the network of studied glasses, mainly, as isolated vanadyl (VO2+) molecular complex centres. Spin Hamiltonian parameters of axial symmetry (g||, g⊥, A||, A⊥), dipolar hyperfine coupling (P) and Fermi contact interaction (K) constants of the VO2+ centres in octahedral sites with a tetragonal compression (C4v local symmetry) for all investigated glasses were determined from their experimental EPR spectra. Optical absorption bands, observed in the investigated glasses have been identified and interpreted. Crystal field parameters (Dq, Ds, Dt) and molecular orbitals (bonding) coefficients (β⁎2, επ⁎2) for VO2+ centres in the studied glasses were calculated and compared with referenced data for other V-doped borate glasses with similar compositions.

1. Introduction The borate crystals and glasses, both pure and doped with transition and rare-earth elements are characterised by attractive spectroscopic, optical, and luminescence properties and represent very promising materials for solid-state lasers and harmonic transformations [1–4], scintillators and thermoluminescent dosimeters [5–8], gamma and neutron detectors [9,10] and other applications. Wide possibilities of technical applications of borate crystals and glasses stimulate search of new borate functional materials and the research their structure and properties by different structural and spectroscopic methods, including electron paramagnetic resonance (EPR) and optical spectroscopy. From technological point of view, the borate glasses are more perspective for practical applications in comparison with corresponding single crystals, which are produced by very expensive and complicated crystals growth technology. Furthermore, a very low velocity of the borate crystal growth and a high viscosity of the melt leads to problems with doping of borate single crystals by transitional and rare-earth elements. Borate glasses with compositions, which are similar to their well-known crystalline analogies (Li2B4O7, LiKB4O7, CaB4O7, LiCaBO3 etc.), doped with rare-earth and transitional elements in a wide concentration range, including vanadium, can be easily obtained using standard glass technology. The above-mentioned borate glasses activated by transitional and rare earth ions widely were investigated during last decade and published in our articles [11–31] as promising



luminescent and optoelectronic materials. The V impurity can be incorporated into glass network in different valence states that leads to wide range of interesting spectroscopic properties of the V-doped glasses. However, up to now the spectroscopic properties of the Vdoped Li2B4O7, LiKB4O7, CaB4O7, and LiCaBO3 glasses were reported only on conferences [32,33] without their detailed description, analysis, and interpretation. The vanadium neutral atom (V0) contains 23 electrons and the structure of V0 electronic shells can be presented as 1s22s22p63s23p63d54s2. Electronic configuration and ground state of the neutral free vanadium atom can be presented also as [Ar] 3d54s2 and 4 F3/2, respectively. The V free ions can exist in the following electronic configurations and corresponding ground states: V+ (3d4, 5D0), V2+ (3d3, 4F3/2), V3+ (3d2, 3F2), V4+ (3d1, 2D3/2), and V5+ (3d0, 1S0). The ground state and splitting of excited 3d-electron states of the V+, V2+, V3+, and V4+ impurity ions in solids depend on the symmetry of their local environment that reveals in EPR, optical absorption, and luminescence spectra of these ions. The paramagnetic V2+ (electron spin S = 3/2) and V4+ (S = 1/2) Kramers ions can be easily observed by EPR in a number of oxide crystals and glasses even at room and liquid nitrogen temperatures [34,35]. Available published data show that the V+ and V3+ impurity ions rarely are observed in oxide crystals and glasses, because for observation of EPR spectra of the V+ (S = 2) and V3+ (S = 1) non-Kramers ions needs special extraordinary experimental conditions (low temperatures, high frequencies and magnetic

Corresponding author at: Sector of Spectroscopy, Dragomanov Str. 23, 79-005 Lviv, Ukraine. E-mail addresses: [email protected], [email protected] (B.V. Padlyak).

https://doi.org/10.1016/j.jnoncrysol.2019.119741 Received 17 August 2019; Received in revised form 10 October 2019; Accepted 17 October 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.

Journal of Non-Crystalline Solids 528 (2020) 119741

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coefficients (β∗2, επ∗2) for VO2+ ions in the SrB4O7:V [46] and MB4O7 (M = Zn, Cd) [47] glasses have been calculated and confirm the moderate covalence of the VeO chemical bonds. Similar results of the EPR and optical absorption spectroscopy for VO2+ ions were obtained in other borate glasses with different and more complicated chemical compositions [44,45,48–50]. Based on the considered above referenced data one to conclude that the results of detailed EPR and optical spectroscopy of the V-doped Li2B4O7, LiKB4O7, CaB4O7, and LiCaBO3 glasses were not satisfactory analysed and published yet. In this paper are presented and discussed results of our systematically studies of the EPR and optical absorption spectra in a series of glasses of the Li2B4O7, LiKB4O7, CaB4O7, and LiCaBO3 basic compositions with added V2O5 impurity in amounts 0.5 and 1.0 mol.%.

fields) [34,35]. The pentavalent vanadium ions (V5+) has no electrons in d-shell and no reveal electronic d – d transitions in EPR and optical (absorption and luminescence) spectra. The V+, V3+, and V5+ ions as well as neutral V0 atoms and their local structure in disordered solids, including glasses, can be studied using X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques [36]. The V3+ ions in oxide compounds also can be registered by luminescence spectroscopy in near-infrared range, which was clearly demonstrated for A12O3:V and YP3O9:V crystals in [37]. The V impurity, generally, is incorporated into the structure of oxide glasses as paramagnetic V4+ ions and more often as stable oxo-vanadium molecular (VO2+ or vanadyl) ions, which were widely investigated by EPR spectroscopy in a variety of host systems [34,35,38], including V-doped borate glasses with different chemical compositions and vanadium content [39–50]. According to published data the V4+ ions usually are located in tetrahedral coordination by oxygen and are characterised by short spin-lattice relaxation time (T1), therefore their EPR spectra, generally, can be observed at low temperatures (T ≤ 77 K) [38]. The VO2+ complex ions usually are located in a pyramidal or distorted octahedral coordination and have a relatively long spin-lattice relaxation time, so their EPR spectra can be easily observed even at room temperature (RT) [34,35,38–50]. It should be noted that the V4+ and VO2+ ions also can be registered by optical absorption spectroscopy, because three absorption bands reveal in the visible – UV spectral range, which correspond to the 2B2g→ 2Eg, 2B2g→ 2B1g, and 2B2g→ 2 A1g transitions of the V4+ (VO2+) ions, located in the tetragonally distorted octahedral sites of the V-doped crystals and glasses [51,52]. Let us consider some articles, which describe the EPR and optical absorption spectra of some V-doped borate glasses with chemical compositions, which are close to compositions of the investigated glasses. The EPR spectra of VO2+ complex ions in the 30Li2O-70B2O3 [39] and Li2O-Na2O-B2O3 [40] glasses were investigated and their spin Hamiltonian parameters (g||, g⊥, A||, A⊥) were determined. Particularly, in [40] it was shown that the replacement of Li+ by Na+ ions is responsible for large local distortion around the VO2+ ions that giving rise to higher g-values, especially for g||. The X-band EPR spectra of VO2+ ions in glasses with xLi2O⋅yBaO (100–x–y)B2O3 (0 ≤ x ≤ 35 and 0 ≤ y ≤ 35) compositions were investigated at T = 295 K in [41]. Analysis of the spin Hamiltonian (g||, g⊥, A||, A⊥), dipolar hyperfine coupling (P) and Fermi contact interaction (K) parameters calculated for investigated glasses shows that a decrease of the B2O3 content from 85 to 65 mol.% leads to improvement in the octahedral symmetry of sites, occupy by V4+ ions. In [42] were investigated EPR and optical absorption spectra of the VO2+ ions, localised in octahedral site with a tetragonal compression in alkali-calcium borate glasses. Particularly, in [42] have been evaluated spin Hamiltonian parameters as well as bonding coefficient (β∗2), Fermi contact interaction term (K) and crystal field (Dq, Ds, and Dt) parameters of the VO2+ ions. Decreasing of the vanadyl EPR signal at high concentration of V impurity in [42] has been attributed to the change of oxidation state from V4+ to V5+. In the xV2O5(100−x)[2B2O3•Li2O] (0.5 mol.% ≤ x ≤ 50 mol.%) glasses characteristic infrared bands were identified and it was shown that at high concentration (x > 20 mol.%) the V2O5 becomes a former of the glass network together with B2O3 [43]. The EPR spectra of xV2O5(100−x)[2B2O3•Li2O] glasses with small contents of V2O5 (x ≤ 5 mol.%) show well-resolved hyperfine structure of the isolated vanadyl ions in sites with C4v symmetry, whereas the glasses with x >20 mol.% reveal in EPR spectra additional broad un-structured signal at g ≅ 1.96 belonging to associated (clustered) V4+ ions, coupled by dipole-dipole interaction [43]. The EPR and optical absorption spectra of the V-doped SrB4O7 [46] and MB4O7 (M = Zn, Cd) [47] tetraborate glasses were investigated and analysed, where it was shown that the V impurity is incorporated as VO2+ ions in tetragonally compressed octahedron (g⊥ > g∥) with site symmetry C4V. Spin Hamiltonian, dipolar hyperfine coupling and Fermi contact coupling parameters as well as crystal field parameters (Dq, Ds, Dt) and molecular orbitals (bonding)

2. Experimental details The V-doped borate glasses of high optical quality and chemical purity with Li2B4O7 (or Li2O–2B2O3), LiKB4O7 (or 0.5 Li2O–0.5 K2O–2B2O3), CaB4O7 (or CaO–2B2O3), and LiCaBO3 (or 0.5 Li2O–CaO–0.5 B2O3) basic compositions were obtained from corresponding polycrystalline compounds using standard glass technology and conditions, described firstly in [32,33,53]. For solid-state synthesis of the Li2B4O7:V, LiKB4O7:V, CaB4O7:V, and LiCaBO3:V polycrystalline compounds were used the Li2CO3 and CaCO3 carbonates and boric acid (H3BO3) of high chemical purity (99.999%, Aldrich). The vanadium impurity was added to the raw materials as V2O5 oxide compound of chemical purity (99.99%) in amounts of 0.5 and 1.0 mol.%. Solid-state synthesis of the corresponding polycrystalline borates was carried out using multi-step heating chemical reactions [53], which can be described for Li2B4O7, LiKB4O7, CaB4O7, and LiCaBO3 compounds using the following equations: H3BO3 = α-НВО2 + H2O↑ (170 °C),

(1)

2α-НВО2 = B2O3 + H2O↑ (250 °C),

(2)

Li2CO3 + 2B2O3 = Li2B4O7 + CO2↑ (800 °C),

(3)

Li2CO3 + K2CO3 + 4B2O3 = 2LiKB4O7 + 2CO2↑ (720 °C)

(4)

CaCO3 + 2B2O3 = CaB4O7 + CO2↑ (800 °C)

(5)

Li2CO3 + 2CaCO3 + В2О3 = 2LiCaBO3 + СО2↑ (700 °C).

(6)

Samples of the Li2B4O7:V, LiKB4O7:V, CaB4O7:V, and LiCaBO3:V glasses were obtained in corundum crucibles by rapid cooling of the corresponding melts, which were heated on 100 K higher that their melting temperatures (Tmelt = 917 °C (1190 K), 807 °C (1080 K), 980 °C (1253 K), and 777 °C (1050 K)) for Li2B4O7, LiKB4O7, CaB4O7, and LiCaBO3 compounds, respectively) for excluding crystallisation process [53]. The obtained V-doped materials show typical glassy-like X-ray diffraction (XRD) patterns without any discrete sharp peaks, similar to XRD patterns for Sm-doped [20,21] and Ce-doped [23] borate glasses with the same basic compositions that confirms their disordered structure. Polarimetry showed that the obtained glass samples are characterised by significant mechanical stresses. Thermal annealing in air atmosphere in the temperature range of 680–730 K eliminates the mechanical stresses in the borate glasses [53]. The V-doped borate glasses were almost uncoloured and characterised by a high optical quality. The nominal V dopant concentration in the V-doped glasses were analytically proved by EDS technique that was shown the following average values: 0.47, 0.85, 0.37, and 0.28% respectively for samples Li2B4O7:V, LiKB4O7:V, CaB4O7:V, and LiCaBO3:V with added 1.0 mol.% V2O5. It should be noted that the EDS results show different coefficient of incorporation as well as inhomogeneous distribution of the V impurity in the network of investigated borate glasses. The V-doped glass samples for EPR studies were cut to an 2

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B.V. Padlyak and T.B. Padlyak

approximate size of 4 × 3 × 2 mm3. The paramagnetic impurities in the obtained glasses with Li2B4O7:V, CaB4O7:V, LiKB4O7:V, and LiCaBO3:V compositions were registered by EPR technique using modernised commercial X-band radiospectrometers of the SE/X-2013 and SE/X2544 types (“RADIOPAN” corporation, Poznań, Poland), operating in the high-frequency (100 kHz) modulation mode of magnetic field at room temperature (RT). Microwave frequency were measured with a Hewlett Packard (model 5350 B) microwave frequency counter and DPPH g-marker (g = 2.0036 ± 0.0001). The optical absorption spectra of the Li2B4O7:V, CaB4O7:V, LiKB4O7:V, and LiCaBO3:V glasses were recorded at RT with usage a VARIAN (model 5E UV–Vis–NIR) and a SHIMADZU (model UV-2600) spectrophotometers. For registration of optical absorption spectra the glass samples were cut and polished to an approximate size of 5 × 4 × 2 mm3. 3. Results and discussion 3.1. EPR spectra of the V-doped borate glasses and their analysis In the investigated glasses at RT were observed characteristic EPR spectra, which are presented in Figs. 1–4. One can notice that intense EPR signal with geff ≅ 4.29 is observed in all investigated borate glasses (see Figs. 1–4 (A)). This signal is typical for glassy compounds and

Fig. 2. The X-band EPR spectra of the CaB4O7:V glasses with added 0.5 (a) and 1.0 mol.% (b) V2O5, registered at T = 300 K in the wide (A) and narrow (B) scan ranges of magnetic field.

according to [11,12,14–17] was assigned to the Fe3+ (3d5, 6S5/2) ions of non-controlled iron impurity. Besides the Fe3+ single-line EPR signal in all investigated glasses is observed complex EPR signal that especially good reveals in the Li2B4O7:V and CaB4O7:V glasses (see Figs. 1 (A, B) and 2 (A, B)). Analysis of the observed spectra with usage corresponding referenced data [38–50] shows that this complex EPR signal consists of 16 lines, which correspond to 8 parallel and 8 perpendicular components of the axially-symmetric hyperfine structure (HFS), caused by interaction of unpaired 3d1 - electron of the V4+ ions (electron spin S = 1/2) with nucleus of the 51V isotope (nuclear spin I = 7/2, natural content – 99.76%). This 16-component EPR signal (Figs. 1–4) is typical for other V-doped borate glasses [39–50] and belongs to the VO2+ (vanadyl) molecular complex ions, because it is observed at RT [38]. Observed EPR spectra of the VO2+ centres have been satisfactory described by spin Hamiltonian of axial symmetry, written in the following form:

^ = g βB S^ + g β (B S^ + B S^ ) + A S^ I^ + A (S^ I^ + S^ I^ ), H ∥ z z ⊥ z z x x y y x x y y ∥ ⊥

(7)

where β is the Bohr magneton, g|| and g| are the principal values of the axial g-tensor describing electron Zeeman interaction, A|| and A⊥are principal values of the axial A-tensor describing hyperfine structure caused by 51V isotope, Bx, By, and Bz are projections of the magnetic field induction, S^x , S^y , S^z and I^x , I^y , I^z are components of the spin operators for electron spin and nuclear spin, respectively.

Fig. 1. The X-band EPR spectra of the Li2B4O7:V glasses with added 0.5 (a) and 1.0 mol.% (b) V2O5, registered at T = 300 K in the wide (A) and narrow (B) scan ranges of magnetic field. 3

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Fig. 3. The X-band EPR spectra of the LiKB4O7:V glasses with added 0.5 (a) and 1.0 mol.% (b) V2O5, registered at T = 300 K in the wide (A) and narrow (B) scan ranges of magnetic field.

Fig. 4. The X-band EPR spectra of the LiCaBO3:V glasses with added 0.5 (a) and 1.0 mol.% (b) V2O5, registered at T = 300 K in the wide (A) and narrow (B) scan ranges of magnetic field.

Solutions of spin-Hamiltonian (7) in the second order approximation for positions of the hyperfine lines corresponding to parallel and perpendicular orientations are given by the following relations [34,35]:

their measurements and calculations (see Table 1). The spin Hamiltonian parameters of VO2+ centres were verified using EPR spectra simulation program “Simfonia” (Bruker). For simulation of EPR spectra of the VO2+ centres were used experimental spin Hamiltonian parameters of the VO2+, obtained for Li2B4O7:V and CaB4O7:V glasses with added 1.0 mol.% V2O5, because only in these glasses is well observed “pure” EPR spectrum of the VO2+ centres. During simulations were used also corresponding linewidths and lineshapes. Simulated EPR spectra of the VO2+ axial centres in the Li2B4O7:V and CaB4O7:V glasses show good agreement with the corresponding experimental spectra (see Figs. 1 and 2 (B), curves (b) and Figs. 5(A) and (B). For comparison in Table 1 also are presented spin Hamiltonian parameters of the VO2+ centres in some borate glasses with compositions similar to compositions of studied glasses, which have been published earlier by different authors. The spin Hamiltonian parameters of VO2+ centres in the investigated glasses show satisfactory correlation with corresponding spin Hamiltonian parameters for VO2+ centres in other V-doped borate glasses, especially in glasses with closely similar compositions to our glasses (see Table 1). Some differences in the spin Hamiltonian parameters of the VO2+ centres in Li2B4O7:V glasses, obtained in this article and in [39,43,54], can be explained by different technological conditions of glasses synthesis as well as by precision of EPR spectra measurements and calculations of their parameters. Thus, the EPR data show that V4+ ions in the network of all

B∥ (m) = B∥ (0) − mA∥ −

2

{ 634 − m } 2BA (0) 2



(8)



B⊥ (m) = B⊥ (0) − mA⊥ −

2

2 ∥

{ 634 − m } A4B+(0)A 2





(9)

where m is the magnetic quantum number that takes values ± 7/ 2, ± 5/2, ± 3/2, and ± 1/2 for 51V nucleus, B∥ (0) = hν / g∥ and B⊥ (0) = hν / g⊥ , where h is the Planck's constant, v is the working frequency of EPR spectrometer. Measured B||(0) and B⊥(0) positions in the observed EPR spectra correspond to maxima in the registered first derivatives of absorption lines for parallel and perpendicular HFS components for a given value of magnetic quantum number (m). The spin Hamiltonian (7) parameters for VO2+ centres in all investigated borate glasses were calculated using relations (8) and (9) as well as measured positions of resonance lines and working microwave frequencies of the radiospectrometer. Calculated spin Hamiltonian parameters of the VO2+ centres for all investigated borate glasses are presented in Table 1. The obtained spin Hamiltonian parameters of the VO2+ centres slightly depend on the basic glass composition and practically independent of the V2O5 content within the uncertainties of 4

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Table 1 The spin Hamiltonian, dipolar hyperfine coupling, and Fermi contact interaction parameters for VO2+ centres in the investigated glasses and some borate glasses with close compositions, obtained by other authors. The experimental errors and calculation uncertainties for obtained in this article g||, g| and A||, A| values equal ± 0.001 and ± 0.5 × 10−4 cm−1, respectively. Glass, reference

V2O5 content [mol.%]

g||

g|

A||, [10−4 сm−1]

A|, [10−4 сm−1]

P, [10−4 сm−1]

K

Li2B4O7:V, this article Li2B4O7:V, this article LiKB4O7:V, this article LiKB4O7:V, this paper CaB4O7:V, this article CaB4O7:V, this article LiCaBO3:V, this article LiCaBO3:V, this article 30Li2O–70B2O3, [38] Li2O–2B2O3, [42] Li2O–2B2O3 [42] Li2B4O7:V [53] K2B4O7:V [53] SrB4O7:V, [45] ZnB4O7:V, [46] CdB4O7:V, [46]

0.5 1.0 0.5 1.0 0.5 1.0 0.5 1.0 1.0 0.5 1.0

1.946 1.946 1.946 1.952 1.950 1.943 1.942 1.947 1.946 1.941 1.941 1.948 1.944 1.9359(8) 1.9370 1.9370

1.977 1.979 1.978 1.988 1.986 1.974 1.976 1.976 1.980 1.997 1.998 1.966 1.972 1.9967(8) 1.9802 1.9770

168.3 168.5 164.6 168.3 168.2 168.4 168.1 165.5 170.8 170.7 173.5 158 165 172(4) 176 176

62.7 65.5 61.7 62.7 65.3 65.6 61.8 61.6 61.0 61.4 63.0 55 50 56(4) 57 61

–116.8 –113.5 –113.7 –117.0 –113.8 –113.1 –117.4 –114.9 121.3 – – – – –126 –130 –129

0.92 0.85 0.81 0.81 0.85 0.84 0.79 0.80 0.77 0.77 0.77 – – 0.73 0.7063 0.7147

0.1 0.1 0.1

CaB4O7:V glasses with added 0.5 and 1.0 mol.% V2O5 the broadband EPR signals are very weak and practically were not reveal. The experimental spectral parameters of the broadband EPR signals in the studied borate glasses are presented in Table 2. The broadband signals in the V-doped borate glasses can be assigned to the V4+–V4+ pair centres, coupled, generally, by magnetic dipole-dipole interaction. Analysis of the broadband EPR signals and their parameters (Table 2) shows that their effective g-factor (geff) and peak-to-peak linewidth (ΔBpp) values show tendency to decreasing with increasing of the V2O5 content in the glass composition (see Table 2). The observed tendency indicates the influence of exchange interaction between the nearestneighbouring paramagnetic centres, including their small clusters. The proposed interpretation of observed broadband EPR signals correlates with published data for lithium tetraborate glasses with xV2O5(100 – x) [2B2O3–Li2O] composition [43], where for samples with x >20 mol.% the VO2+ isolated centres and magnetically-coupled V4+–V4+ pair centres were observed by EPR technique. Asymmetry of the broadband EPR signals allows to suggest the presence of V4+–Fe3+ and Fe3+–Fe3+ pair centres in the studied glasses, besides the V4+ – V4+ pairs. This suggestion is based on the reference EPR data for other glasses [58,59]. For example, asymmetric broad bands of the Mn2+–V4+ and Cu2+ – V4+ pair centres were observed in EPR spectra of phosphate glasses with x(MnO, V2O5)(100–x) [2P2O5–Na2O] and x(CuO, V2O5)(100 – x)[2P2O5 – Na2O] compositions at high (x >10 mol.%) MnO, V2O5, and CuO content [58]. The Cu2+ – V4+ pair centres were investigated also in phosphate glasses of the BaO–V2O5–CuO (V2O5–60 mol.%, CuO–5÷20 mol.%) and P2O5–V2O5–CaO–CuO (V2O5–55 mol.%, CuO – 1÷15 mol.%) systems [59]. Paired paramagnetic centres in [43,58,59] were observed at high content of transition metals ions, whereas in the investigated by us borate glasses the V4+ – V4+, V4+–Fe3+, and Fe3+–Fe3+ pair centres reveal at relatively low (0.5 and 1.0 mol.%) content of the paramagnetic impurities. It should be noted that tendency of pairing and clustering of the paramagnetic centres earlier were observed in EPR spectra of our borate glasses with low content of transitional and rareearth impurities [11,15,16,20,22,23,25–27]. Structural [52] and spectroscopic [11,15,25] data show close localisation of transitional impurity ions in the same (Li, K, Ca) cationic sites of the network of studied glasses that confirms the possibility of formation in them the V4+–V4+, V4+–Fe3+, Fe3+–Fe3+ pair centres and their clusters at presence of oxygen vacancies between them. The EDS analysis that shows inhomogeneous distribution of the V impurity in the network of investigated borate glasses also confirm the possibility of formation in them pair centres and small clusters.

investigated borate glasses are located in the six-coordinated by oxygen (octahedral) sites and form the VO2+ complex ions with a tetragonal compression (local group of symmetry C4V) because g∥ < g⊥ < ge = 2.0023 and A|| > A⊥, where ge = 2.0023 is the g-factor of free electrons (see Table 1). The obtained spin Hamiltonian parameters of the VO2+ centres in the investigated glasses were used for evaluation dipolar hyperfine coupling constant (P) and Fermi contact interaction term (K). According to [55,56] the components of axial hyperfine structure (A|| and A⊥) of the axial A-tensor can be presented by the following relations:

A∥ = −P [K + (4/7) − Δg∥−(3/7)Δg⊥],

(10)

A⊥ = −P [K − (2/7) − (11/14)Δg⊥],

(11)

〈r −3〉

is dipolar hyperfine coupling constant with where P = 2γββN average value 〈r −3〉 for the V4+ 3d orbitals, and K is the Fermi contact interaction dimensionless parameter that represents the amount of unpaired 3d1 - electron density at the position of 51V nucleus, Δg∥ = g∥ − ge , and Δg⊥ = g⊥ − ge . The calculated dipolar hyperfine coupling and Fermi contact interaction parameters for VO2+ centres in all investigated glasses as well as in some borate glasses with close compositions are presented in Table 1. The calculated P and K parameters in the investigated glasses show relatively weak dependencies on glass composition and V2O5 content. The P value varies from –117.4•10−4 сm−1 for LiCaBO3:V (V2O5 – 0.5 mol.%) glass to –113.1•10−4 сm−1 for CaB4O7:V (V26O5–1.0 mol.%) glass, whereas the K value varies from 0.79 for LiCaBO3:V (V2O5–0.5 mol.%) glass to 0.92 for Li2B4O7:V (V2O5–0.5 mol.%) glass (see Table 1). The calculated P and K parameters in the studied glasses are similar to the corresponding parameters, obtained for VO2+ centres in the 30Li2O–70B2O3 [39], Li2O–2B2O3 [43] glasses with close compositions and show some difference in glasses with following compositions: SrB4O7:V [46], ZnB4O7:V, and CdB4O7:V [47] (see Table 1). It should be noted that the Fermi contact interaction parameter (K) for transitional ions is positive [57] and relatively high K values show considerably contribution of the s-electron to the hyperfine constant. According to [44] negative PK values, obtained from relations (10) and (11) is due to the s-character of electron spin of the V4+ ions. Additional asymmetric broadband EPR signals clearly are observed in the Li2B4O7:V, LiKB4O7:V, and LiCaBO3:V glasses (see Figs. 1–4). Therefore, the central part of EPR spectra in Li2B4O7:V (0.5 mol.% V2O5), LiCaBO3:V (0.5 and 1.0 mol.% V2O5), and LiKB4O7:V (0.5 and 1.0 mol.% V2O5) glasses are superposition of the VO2+ and broadband signals. In the Li2B4O7:V glass with added 1.0 mol.% V2O5 as well as in 5

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Fig. 5. Simulated X-band EPR spectra of the VO2+ centres in the Li2B4O7:V (A) and CaB4O7:V (B) glasses with added 1.0 mol.% V2O5.

3.2. Optical absorption spectra, crystal field and bonding parameters of the VO2+ centres

Table 2 Spectral parameters for broad EPR signals, observed in the investigated borate glasses. The experimental errors and calculation uncertainties for obtained geff and ΔBpp values are equal ± 0.01 and ± 1 mT, respectively. Glass composition

V2O5 content [mol.%]

geff

ΔBpp, [mT]

Li2B4O7:V Li2B4O7:V LiKB4O7:V LiKB4O7:V LiCaBO3:V LiCaBO3:V

0.5 1.0 0.5 1.0 0.5 1.0

2.01 – 2.10 2.02 2.23 2.01

75 – 84 75 113 110

The obtained V-doped glasses were detailed investigated by optical absorption method that is a useful complementary to the EPR technique in the framework of spectroscopic research of the impurity ions in crystals and non-crystalline solids, including glasses. Optical absorption spectra of the Li2B4O7:V, LiKB4O7:V, CaB4O7:V and LiCaBO3:V glasses are virtually similar. As an example in Fig. 6 are presented optical absorption spectra of the Li2B4O7:V and LiKB4O7:V glasses with added 1.0 mol.% V2O5, registered at RT in the 400÷–2500 nm spectral range. The optical absorption spectra clearly show two relatively broad bands with maxima near 650 nm and 800 nm as well as third band that only weakly manifested near 450 nm on the fundamental absorption edge of 6

Journal of Non-Crystalline Solids 528 (2020) 119741

B.V. Padlyak and T.B. Padlyak

for 2B2g →2 Eg transition: [−4Dq − Ds + 4Dt − (−4Dq + 2Ds − Dt )] (12)

= −3Ds + 5Dt , for 2B2g →2 E1g transition: [6Dq + 2Ds − Dt − (−4Dq + 2Ds − Dt )] = 10Dq,

(13)

for 2B2g →2 A1g transition: [6Dq − 2Ds − 6Dt − (−4Dq + 2Ds − Dt )] = 10Dq − 4Ds − 5Dt .

(14)

In the above relations (12–14), Dq is the octahedral crystal field parameter, Ds and Dt are the tetragonal crystal field parameters. The crystal field parameters (Dq, Ds, and Dt) for VO2+ centres in the investigated glasses with added 1.0 mol.% V2O5, which were calculated using relations (12–14), are presented in Table 3. Opposite sign of the obtained Dq and Ds parameters indicates tetragonal compression of the VO2+ centres in the network of studied glasses that correlates with the abovementioned EPR data. As we can see from Table 3 the obtained crystal field parameters for VO2+ centres in the investigated glasses show some differences related to different distortion of their local environment. The obtained crystal field parameters have been compared with the corresponding parameters for VO2+ centres in other V-doped borate glasses with similar compositions, which also are presented in Table 3. It should be noted good correlation of the calculated by us crystal field parameters (Dq, Ds, and Dt) with the corresponding parameters for VO2+ centres in borate glasses with similar chemical compositions. Using obtained g-factor values and energy of the optical absorption bands the molecular orbitals coefficients (β⁎2, επ⁎2) for VO2+ centres in the investigated borate glasses were calculated from the following relations [55,61]:

Fig. 6. Optical absorption spectra of the Li2B4O7:V and LiKB4O7:V glasses with added 0.5 (a) and 1.0 mol.% (b) V2O5, registered at T = 300 K.

the glass host (see Fig. 6). It should be noted that in optical absorption spectra of all investigated glasses with added 0.5 mol.% V2O5 the absorption bands weakly reveal only. Optical absorption bands, observed in the investigated V-doped glasses, have been identified and interpreted in the framework of ligands field theory for the VO2+ complex molecular centres [51,56,60]. The 2T2g ground state arises from single d-electron of the V4+ ion that occupies the t2g orbital in the octahedral crystal field. Excited d-electron of the V4+ ion occupies the eg upper orbital that gives the 2Eg excited state. As a result, in crystal field of an ideal octahedral symmetry (group of symmetry – Oh) can be observed only one absorption band corresponding to the 2T2g → 2Eg transition. The ligands field leads to nonsymmetrical distortion of the V ] O bond in the VO2+ molecular complex ions and to lowering their local symmetry to the tetragonal (C4v) or even lower (C2v) group. At tetragonal (C4v) local symmetry, the 2 T2g and 2Eg levels are splitted into the 2B2g, 2Eg and 2B1g, 2A1g sublevels, respectively, with the following ordering of splitted levels in the energy scale: 2B2g < 2Eg < 2B1g < 2A1g. Diagram of electronic energy levels of the VO2+ centres in octahedral sites with tetragonal distortion (group of local symmetry – C4v) is presented in Fig. 7. According to diagram of energy levels for the VO2+ molecular orbitals in the ligand field of tetragonal (C4v) symmetry (Fig. 7) the optical absorption bands in the studied glasses, were assigned to the 2 B2g → 2Eg (dxy → dzx,yz), 2B2g → 2B1g (dxy → dx2−y2), and 2B2g → 2A1g (dxy → dz2) electronic transitions [51]. The optical absorption transitions of the VO2+ centres in octahedral sites with tetragonal (C4v) distortion are described by the following relations [51,61]:

g∥ = ge −

8λβ *2 ΔExy

g⊥ = ge −

2λεπ*2 , ΔExz

(15)

(16)

where ΔExy is the energy of B2g → B1g transition, ΔExz is the energy of 2 B2g → 2Eg transition, and λ is the spin-orbit coupling constant that was accepted 170 cm−1 according to [51,55]. The calculated molecular orbitals (bonding) coefficients (β⁎2 and επ⁎2) for VO2+ centres in the investigated borate glasses are presented in Table 3. Comparison of the calculated β⁎2 and επ⁎2 values show good agreement with the corresponding bonding coefficients, obtained for other borate glasses with similar chemical compositions, excluding the επ⁎2 in the SrB4O7:V glass [46] (see Table 3). In particular, the molecular orbitals coefficients show a moderate covalence degree of the Ve–O in-plane σ-bonds (β⁎2) and π-bonds with the vanadyl oxygen (επ⁎2) for VO2+ centres in the borate glasses, presented in Table 3. Hence, the optical absorption spectroscopy data clearly correlate with the obtained EPR results, because shows typical absorption bands belonging to the VO2+ molecular centres, located in the tetragonally compressed (C4v) octahedral sites of the investigated glasses. At last it should be noted that besides the isolated vanadyl (VO2+) complex ions and V4+–V4+ pair centres, which reveal in the EPR and optical absorption spectra, in the investigated glasses also clearly have been observed characteristic luminescence spectra of the V3+ centres in the near infrared spectral range. Detailed investigation of the V3+ luminescence centres in a series of borate glasses with the Li2B4O7:V, LiKB4O7:V, CaB4O7:V, and LiCaBO3:V compositions will be a subject of a separate our work. 2

2

4. Conclusions Fig. 7. Energy levels diagram indicating assignments of the electronic absorption transitions for vanadyl (VO2+) ions in the tetragonally compressed octahedral sites (C4v local symmetry).

The borate glasses of the Li2B4O7:V, LiKB4O7:V, CaB4O7:V, and LiCaBO3:V compositions with added 0.5 and 1.0 mol.% V2O5 have been 7

Journal of Non-Crystalline Solids 528 (2020) 119741

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Table 3 Energies of the optical absorption transitions, crystal field parameters (Dq, Ds, Dt), and molecular orbitals (bonding) coefficients (β⁎2, επ⁎2) for VO2+ centres in the investigated glasses and some borate glasses with similar compositions. Glass composition

V2O5 content [mol.%]

Transition energies from 2B2g [cm−1] 2 2 Eg B1g A1g

Dq [cm−1]

Ds [cm−1]

Dt [cm−1]

β⁎2

επ⁎2

11,580 11,594 11,739 11,683 15,744 12,267 12,497

1661 1679 1652 1671 1801(4) 1470 1428

−2365 −2375 −2391 −2395 −2740(5) −3513 −3605

897 894 913 900 1504(5) 345 336

0.688 0.621 0.720 0.679 0.88 0.7027 0.6857

0.794 0.488 0.977 0.904 0.26 0.8045 0.9299

2

Li2B4O7:V LiKB4O7:V CaB4O7:V LiCaBO3:V SrB4O7:V [45] ZnB4O7:V [46] CdB4O7:V [46]

1.0 1.0 1.0 1.0 0.1 0.1 0.1

16,611 16,794 16,516 16,708 18,013 14,702 14,282

21,635 21,824 21,516 21,789 21,453 27,020 27,020

Institute of Physical Optics (Lviv, Ukraine) for glass samples preparation and Dr A. Drzewiecki from Institute of Physics of the University of Zielona Góra (Poland) for registration of EPR spectra.

detailed investigated by conventional EPR and optical absorption spectroscopy T = 300 K. Based on the obtained experimental results and their analysis, supported by available reference data, its was shown the following:

References

• The vanadium impurity is incorporated into the network of Li B O , 2 4 2+







7

LiKB4O7, CaB4O7, and LiCaBO3 glasses mainly as the VO (vanadyl) complex paramagnetic ions. The spin Hamiltonian parameters of EPR spectra as well as constants of dipolar hyperfine coupling and Fermi contact interaction for VO2+ centres in all investigated borate glasses are determined. The obtained spinHamiltonian parameters for VO2+ centres in the Li2B4O7:V and CaB4O7:V glasses have been verified by computer simulation. The simulated EPR spectra show satisfactory agreement with the corresponding experimental spectra. The obtained spin-Hamiltonian parameters for VO2+ centres in all investigated glasses indicate that the VO2+ ions are located in octahedral sites with a tetragonal compression (C4v local symmetry). The spectral parameters of additional broadband EPR signals in the Li2B4O7:V, LiCaBO3:V, and LiKB4O7:V glasses have been determined. It was suggested that the observed complex asymmetric EPR signals represent a superposition of bands belonging to the (V4+–V4+), (V4+–Fe3+), and (Fe3+–Fe3+) pair centres and their small clusters, coupled by magnetic dipolar and exchange interactions. The observed three optical absorption bands in the investigated borate glasses are satisfactory described in the framework of ligands field theory. The crystal field parameters (Dq, Ds, and Dt) for VO2+ centres in the investigated glasses with added 1.0 mol.% V2O5 have been calculated. Analysis of the optical absorption spectra confirms localisation of the VO2+ centres in octahedral sites with a tetragonal compression (C4v local symmetry group). The evaluated molecular orbitals parameters in-plane V–O σ-bonds (β⁎2) and π-bonds with the vanadyl oxygen (επ⁎2) indicate moderate covalence degree for VO2+ centres in the investigated glasses. Comparison of the spin-Hamiltonian, crystal field, and bonding parameters, obtained for VO2+ centres in the studied glasses shows good correlation with corresponding parameters for VO2+ centres in other borate glasses with similar chemical compositions.

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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was partly supported by the Vlokh Institute of Physical Optics (Lviv, Ukraine) within the research project No. 0119U100357 of the Ministry of Education and Science of Ukraine. The authors are thankful to Dr.Sc. V.T. Adamiv and M.Sc. I.M. Teslyuk from Vlokh 8

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