Elsevier

Materials Research Bulletin

Volume 83, November 2016, Pages 400-407
Materials Research Bulletin

Optical properties of 3dN transition metal ion-doped lead borate glasses

https://doi.org/10.1016/j.materresbull.2016.06.032Get rights and content

Highlights

  • Synthesis and characterization of 3dN transition metal glasses.

  • Identification of doped ion oxidation states (O.S.) by XANES.

  • Identification of electronic transitions and O.S. by electronic spectra.

  • Vacuum ultraviolet spectra of glasses.

Abstract

Glasses from the lead borate system of the composition 50%PbO-50%B2O3 together with samples containing different concentrations of 3d-transition metal (TM) species from V to Zn were prepared by the melting-quenching technique and their optical properties were investigated. The X-ray absorption near edge spectra of V, Cr, and Mn-doped glasses indicates that the oxidation states of V(IV/V), Cr(III, VI) and Mn(II, III/IV) exist in the studied glasses. The oxidation states revealed from the diffuse reflectance spectra, absorption spectra, excitation and emission spectra of the glasses are V(IV), Cr(III, VI), Mn(II, III), Fe(II), Co(II), Ni(II), and Cu(II). The undoped lead borate glass exhibits strong UV absorption which corresponds to interconfigurational transitions of Pb2+ and the host absorption, and intrinsic broad host emission at 470 nm using excitation at 350 nm. The 3d-TM ions from V to Cu doped at high concentration (0.5 or 2 mol%) show additional distinctive strong absorption bands and a red-shifted absorption edge. The Mn2+ emission at 625 nm due to the 4T1g(4G) ⿿ 6A1g(6S) spin-forbidden transition of octahedrally-coordinated Mn2+ was observed under excitation at 412 nm or 369 nm.

Graphical abstract

Transition metal ion-doped (TMC)x(PbO)50⿿x(B2O3)50 glass samples (upper photographs, x = 0.01; lower photographs, x = 0.5).

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Introduction

This work is devoted to the investigation of the UV⿿vis-NIR absorption and the luminescence properties of first row transition metal (TM) ion (from V through Zn)⿿doped lead borate glass. Some additional studies have been made with the use of synchrotron radiation. The motivation for the study is that this investigation is a pre-requisite to further studies involving co-doping the TM-lead borate glass with lanthanide ions (Ln3+). The TM ion could act as photon absorber and transfer energy to Ln3+, with the subsequent emission having applications for downconversion, such as for optical detection or solar energy conversion. It is therefore necessary to fully understand the behavior of the glass systems in the absence of Ln3+. The following introduction describes the glass host and provides spectroscopic information concerning some of the TM systems.

The reasons for choosing the glass system PbO-B2O3 are that it exhibits a low rate of crystallization, it is moisture resistant, stable and transparent. It also has the advantages of high refractive index and high density. Lead oxide (PbO) has a unique influence on the structure of glasses [1], acting as a network modifier to give three-dimensional structures [2], with PbO6 structural units if Pb-O is ionic, and as a glass network former [3] with PbO4 structural units if Pb-O is covalent. The formation of covalent Pb-O linkages in the glass network is due to the strong interaction of Pb2+ ions with the highly polarizable O2⿿ ion [4].

B2O3 acts as glass network forming oxide and the boron atoms can coordinate with oxygen to form BO4 tetrahedral units and also BO3 triangular units. These structural units may further link together through common oxygen atoms, allowing the formation of isolated rings and cages, or through polymerization into infinite chains, sheets and networks [5].

Vanadium could be present in the 3+, 4+ and 5+ oxidation states in glasses. Charge transfer transitions of the 3d0 V5+ ion in various glass and crystal hosts occur in or around the UV region [6], [7], [8]. Vanadium 4+ of the 3d1 electronic configuration is expected to produce absorption bands due to d⿿d transitions. This electron configuration has the 2D ground state which splits into 2T2g and 2Eg states in an octahedral crystal field, while a tetragonal distortion further splits the former into 2Eg and 2B2g states and the latter into 2A1g and 2B1g states, with 2B2g being the electronic ground state. Thus, V4+ in simple octahedral or tetrahedral coordination only yields a single broad absorption band. Johnston [8] and Ballhausen and Gray [9] discussed in general that tetravalent vanadium may exist in glass as VO2+ and the vanadyl ions are expected to give characteristic bands in the regions 1000⿿880 nm, 710⿿590 nm and 430⿿370 nm, corresponding to the transitions from the ground state to the 2Eg, 2B1g and 2A1g states, respectively [10], [11]. VO2+ ions are present in tetragonally-compressed octahedral sites in zinc lead borate glass and exhibit an intense absorption band at 407 nm due to the 2B2g ⿿ 2A1g transition [10]. With the inspection of the Tanabe-Sugano diagram for a 3d2 ion [12], V3+ is expected to have three spin-allowed absorption transitions in both tetrahedral and octahedral coordination. Tetrahedral V3+ in Y3Al5O12 has three absorption bands, centered at 600 nm, 800 nm and 1320 nm. A weak and narrow absorption at 1140 nm is attributed to a spin-forbidden transition [13]. According to Kakabadse and Vassiliou [14], the presence of V3+ in oxide glasses produces absorption bands around 690 nm and 444 nm corresponding to transitions from 3T1g(F) to the 3T2g and 3T1g(P) states, respectively. Similarly, octahedral V3+ in Na2O·2SiO2 glass displays two absorption peaks, at 690 nm and 450 nm [8].

Mn2+ ion has the 3d5 electronic configuration, and in octahedral symmetry the ground state of Mn2+ is the spherically non-degenerate 6A1g state. In a cubic crystalline field of low to moderate strength, the five d electrons of the Mn2+ ion are distributed in the t2g and eg orbitals, with the ground state configuration t2g3eg2 [15]. The energy levels of Mn2+ in an octahedral environment comprise 6A1g(6S), 4T1g, 4T2g, 4Eg-4A1g(4G) and 4T2g, 4Eg(4D) and a number of doublet states. The crystal field has less influence on the 4Eg levels, compared to other levels, which means that relatively sharp lines corresponding to the 6A1g(6S) ⿿ 4Eg, 4A1g(4G) and 6A1g(6S) ⿿ 4Eg(4D) transitions can be expected in the absorption or excitation spectrum. The transitions to the 4T1g(4G) and 4T2g(4G) states are expected to be broad since the configuration changes from t2g3eg2 to t2g4eg1 [16]. Since all the excited states of Mn2+ ion are either quartets or doublets, all transitions are spin-forbidden and of low intensity [17]. The Mn3+ ion has the 3d4 electron configuration. According to the Tanabe-Sugano diagram, in octahedral symmetry, Mn3+ exhibits a single spin-allowed transition 5Eg ⿿ 5T2g. The Mn4+ ion has the 3d3 configuration. The 4A2 ⿿ 4T2 transition has a band at 490 nm, and 4A2 ⿿ 4T1 at 400 nm.

Co2+ has the 3d7 electronic configuration. The electronic spectra of Co2+ in octahedral and tetrahedral environments are relatively well known [18], [19]. The crystal field splitting of the energy levels of Co2+ in a tetrahedral field is similar to that of a 3d3 electronic configuration in an octahedral field, while the value of the crystal field parameter Dq is significantly smaller in the tetrahedral case [20]. An octahedral environment splits the (Co2+) free ion ground state 4F into 4T1, 4T2 and 4A2 states with the 4T1 state lowest. Consequently, the octahedrally-coordinated Co2+ ion has three absorption bands associated with the spin-allowed transitions 4T1(F) ⿿ 4T2(F), 4A2(F), 4T1(P). The second of these is expected only with low intensity [15], [19] and the absorption band near 540 nm is due to the 4T1g(F) ⿿ 4T1g(P) transition [18]. In tetrahedral geometry, the ground state 4F of Co2+ ion splits into 4T1, 4T2 and 4A2 states, with the latter lying lowest so that two transitions 4A2(4F) ⿿ 4T1(P), 4T1(4F) are expected [15]. The high spectral intensity in tetrahedral coordination results from the mixing of the 3d-orbitals with 4p-orbitals and ligand orbitals [15], [20].

The ion Co3+ has the 3d6 configuration. In an octahedral environment with a weak crystal field and with the 5T2(5D) ground state, the absorption spectrum would consist of one band associated with the 5T2(5D) ⿿ 5E(5D) transition. In an octahedral environment of Co3+, with a strong crystal field and the non-magnetic 1A1g(5D) ground state, two visible absorption bands are associated with the 1A1g(5D) ⿿1T1g(1I), 1T2g(1I) transitions [21].

It is generally accepted that only the 3d8 Ni2+ oxidation state is stable in glass under normal atmospheric conditions [15], [22], [23]. These Ni2+ ions can exist in a glass with different forms: an ⿿undulatory⿿ form in which Ni2+ is four coordinated or a ⿿brown⿿ form in which the Ni2+ ion is octahedrally coordinated [24]. Several studies [22], [23] have reported that Ni2+ strongly favors the octahedral over the tetrahedral coordination in a glass, and Ni2+ has an exceptionally large crystal field stabilization energy, particularly in an octahedral environment. High alkali borate and borosilicate glasses favor the existence of tetrahedral Ni2+, while low alkali borate and borosilicate glasses prefer the presence of octahedral Ni2+, and these are pink and green-blue in color, respectively. The electronic ground state of Ni2+ in octahedral symmetry is 3A2g(3F), with the excited states 3T2g(3F), 1Eg(1D), 3T1g(3F), 1T2g(1D), 3T1g(3P), so that the absorption spectrum will contain the spin-allowed transitions 3A2g(3F) ⿿3T2g(3F), 3T1g(3F), 3T1g(3P), and the spin-forbidden transition 3A2g(3F) ⿿ 1Eg(1D). The Tanabe-Sugano diagram predicts that the spectrum of Ni2+ in tetrahedral symmetry will comprise the following possible spin-allowed transitions 3T1(3F) ⿿ 3T2(3F), 3A2(3F), 3T1(3P) and spin-forbidden transitions 3T1(3F) ⿿ 1E(1D), 1T2(1D). Several authors have reported broad band near infrared emission from Ni2+-doped glasses [25], [26], [27].

Cu2+ has the 3d9 electron configuration. An octahedral crystal field splits the ground free ion state 2D into 2Eg and 2T2g with the former being the lower level and generally split due to the Jahn-Teller effect. In a tetragonally distorted octahedral coordination with six oxygen ligands, the 2Eg state splits into 2A1 and 2B1, and 2T2g splits into 2E and 2B2, with the ground state being 2B1 [28], [29].

The Zn2+ ion has the 3d10 electronic configuration and although 3d10⿿3d10 transitions are not possible, various emission bands have been reported for Zn2+ in glasses, attributed to trapped and free exciton transitions [30], [31], [32].

Section snippets

Materials

The following chemicals were used in the preparation of TM ion-doped lead borate glass: PbO (99.9 +%), V2O3 (98%), CoCl2 (97%), NiO (99.99%) and H3BO3 (99.5%) (Sigma-Aldrich), Cr2O3, CuO, ZnO and Fe2O3 (all 99.999%) (Strem), MnCO3 (99.9%) (International Laboratory, USA).

Preparation of doped glasses

The glass samples had the following initial compositions (in mol%): (TMC)x(PbO)50-x(B2O3)50 (x = 0.5 and 0.01), where TMC (referring to transition metal compound) is 0.5 V2O3, 0.5Cr2O3, MnCO3, 0.5Fe2O3, CoCl2, NiO, CuO and ZnO; and

Spectral data of undoped (PbO)50(B2O3)50 glass

Pb2+ ions with 6s2 configuration absorb strongly in the ultraviolet region with an absorption band centered at 310 nm [33], due to the parity allowed 6s2-6sp transition [34]. The studied undoped glass (PbO)50(B2O3)50 is pale yellow in color, and reveals an absorption spectrum consisting of strong ultraviolet absorption bands between 349 nm and 237 nm but showing no visible region bands, as shown in Fig. 1(a). Compared with the excitation spectrum of host glass with excitation peak at 350 nm, as

Conclusions

The undoped lead borate glass is transparent in the visible spectral region. A weak, broad emission band peaking at 470 nm is observed under UV excitation at room temperature. The major conclusions from the study of the TM ion-doped lead borate glass are listed below:

  • 1.

    The XANES results indicate that the oxidation states of V(IV/V), Cr(III, VI) and Mn(II, III/IV) exist in the studied lead borate glasses.

  • 2.

    The oxidation states revealed from the DRS, absorption spectra, excitation and emission spectra

Acknowledgements

This study was supported by the Guangdong Natural Science Foundation (No. 2014A030310132) and Guangdong University Students Science and Technology Innovation Cultivation Special Fund (Grant No. 400160013), Guangdong University of Technology through the One-Hundred Young Talents Program start-up grant (Grant No. 220413505) and University Students' Innovation and Entrepreneurship Training Program (Grant No. yj201611845021). We are indebted to Dr Ting Shan for recording the XANES at NSRRC, Hsinchu

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