Spectroscopy of transition metal ions in inorganic glasses

Transition metal (TM) ions have been used as colouring agents in the glass industry for a long time. Recently, great attention has been paid to the TM ion doped glasses for the development of new lasers or luminescence materials. The absorption spectra of TM ions in different kinds of glasses have been studied extensively, but little work has been done for fluorescence and relaxation spectra. In this paper emphasis is laid on analysing the influence of chemical bond characteristics of the base glass on the spectra and the site structure of transition metal ions in glasses. Recent experimental results about the luminescence characteristics of low valence ions (such as Ti³⁺, Cr³⁺, V²⁺, Mn²⁺, Cu⁺, Mo³⁺) in glasses are also reported.     

Structure and properties of MoO3-containing zinc borophosphate glasses

ZnO–B₂O₃–P₂O₅ glasses doped with MoO₃ were investigated in the series (100?x)[0.5ZnO–0.1B₂O₃–0.4P₂O₅]–xMoO₃, where bulk glasses were obtained by slow cooling in air within the compositional region of 0 ? x ? 60 mol% MoO₃. The incorporation of MoO₃ into the parent zinc borophosphate glass results in a weakening of bond strength in the structural network, which induces a decrease in chemical durability and glass transition temperature. Raman spectra reflect the incorporation of molybdate groups into the glass network of the studied glasses by the presence of the polarized vibrational band at ≈976 cm^?1 ascribed to the MO_x symmetric stretching vibrations and the depolarized band at ≈878 cm^?1 ascribed to the Mo–O–Mo stretching vibration. The incorporation of molybdate units into the glass network results in the depolymerization of phosphate chains and the formation of P–O–Mo bonds, as reflected in Raman and ³¹P NMR spectra. According to the ¹¹B MAS NMR spectra, tetrahedral B(OP)_4?x(OMo)_x units are formed in the glasses, whereas only a small amount of BO₄ units is converted to BO₃ units in the MoO₃-rich glasses.     

Structure and optical properties of chalcohalide glasses doped with Pr and Yb ions

Glasses of systems 100-y((GeS₂)₈₀(Sb₂S₃)_20−x(PbI₂)_x)yPr₂S₃, x = 0; 2; 5, 8; y = 0; 0.01; 0.1; 0.5 and 99.9-z((GeS₂)₈₀(Sb₂S₃)₁₈(PbI₂)₂)0.1Pr₂S₃zYb₂S₃, z = 0.05; 0.1; 0.15) were synthesized in high purity. Optically well transparent glasses were obtained for x  5 mol.% PbI₂, for y  0.1 mol.% Pr₂S₃ and for z  0.15 mol.% Yb₂S₃. The glasses were stable and homogeneous, as confirmed by X-ray diffraction and electron microscopy, with high optical transmittivity from visible (red) region up to infrared region (900 cm⁻¹). The density of the glasses was 3.26–3.33 gcm⁻³ for PbI₂ containing glasses. The glass transition temperature, T_g, was 320–336 °C. The optical absorption bands in rare-earth doped glasses corresponded to ³H₄–³F₄, ³H₄–³F₃, ³H₄–(³F₂ + ³H₆) f–f electron transitions of Pr³⁺ ions and to ²F_7/2–²F_5/2 f–f electron transitions of Yb³⁺ ions. Strong luminescence band with maximum near 1340 nm (electron transition ¹G₄–³H₅) was found in Pr₂S₃ doped glasses. The intensity of this band was rising with doping by Yb³⁺ ions. The possible mechanism of the luminescence enhancement is suggested.     

Multiple acoustic and optical phonon-assisted hopping in oxide glasses containing transition metal ions

Oxide glasses containing transition metal ions usually exhibit electrical conductivity due to polaron hopping. An interpretation of d/c conductivity data in 50P₂O₅50V₂O₅, 40P₂O₅60V₂O₅, 50P₂O₅50FeO, and 50TeO₂40V₂O₅10Fe₂O₃ semiconducting oxide glasses is attempted by fitting the Gorhan–Bergeron and Emin model parameters. Their model is much more general than those usually employed to interpret experimental electrical conductivity data and takes into account multiphonon interactions with optical as well as acoustic phonons. The Holstein criterion of non-adiabatic hopping is verified as well.     

Structure and properties of Sb2O3-containing zinc borophosphate glasses

ZnO–B₂O₃–P₂O₅ glasses formulated with Sb₂O₃ were investigated in the series 50ZnO–10B₂O₃–40P₂O₅ + xSb₂O₃ (x = 0–70 mol%). With increasing Sb₂O₃ content, the values of glass transition temperature decrease from 492 °C down to 394 °C. The dissolution rate of the glasses reveals a maximum for the glass with x = 15 mol% Sb₂O₃. Raman spectra with increasing Sb₂O₃ content reflect the depolymerisation of phosphate chains. Antimony at low Sb₂O₃ content forms individual SbO₃ pyramids manifested in the Raman spectra by a broad vibrational band at ∼520–690 cm⁻¹. In the glasses with a higher Sb₂O₃ content SbO₃ units link into chains and clusters with Sb–O–Sb bridges manifested in the Raman spectra by a strong broad band at 380–520 cm⁻¹. The ³¹P MAS NMR spectra with increasing Sb₂O₃ content reflect the depolymerisation of phosphate chains at low Sb₂O₃ content and only small changes in the PO₄ coordination at a high Sb₂O₃ content. ¹¹B MAS NMR spectra reveal a steady transformation of B(OP)₄ units into B(OP)_4−x(OSb)_x units, accompanied by the transformation of BO₄ into BO₃ units with increasing Sb₂O₃ content.     

Optical properties of pure and metal ions doped ammonium sulfate single crystals

A study of the optical properties of pure‐and some metal ions doped ammonium sulfate crystals (AS) were made. Optical constants of AS crystals were calculated at room temperature. The optical absorption coefficient (α ) was analyzed and interpreted to be in the allowed direct transition. The introduction of Rb⁺ or Cs⁺ ions gives rise to an intense charge transfer band with a maximum at λ= 310 nm in the optical spectrum. In case of Cr³⁺ ‐doping, the absorption shows a shoulder just before the onset band to band transition. The values of the allowed direct energy gap E_g for undoped and doped crystals were calculated. It was found that E_g values were decreased with metal ions doping. The refractive index, the extinction coefficient and both the real and imaginary parts of the dielectric permittivity were calculated as a function of photon energy. The validity of Cauchy‐Sellimeier equation was checked in the wavelength range 4.9 ‐ 5.6 eV and its parameters were calculated. Applying the Single‐Effective‐Oscillator model, the moments of ε (E ) could be estimated. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Correlation between physical and structural properties of Co doped mixed alkali zinc borate glasses

The physical and structural properties of Co²⁺ doped 20ZnO + xLi₂O + (30 − x)Na₂O + 50B₂O₃ (5 ≤ × ≤ 25) (ZLNB) glasses have been studied and correlated. The physical and structural parameters of all the glasses are evaluated and a non-linear behavior is observed. No sharp peaks are observed in XRD patterns of the glass samples which confirm the amorphous nature. FT-IR spectra of ZLNB glasses reveal diborate units in borate network. The optical absorption spectra suggest the site symmetry of Co²⁺ in the glasses is near octahedral. Crystal field and inter-electronic repulsion parameters are also evaluated. The optical band gap and Urbach energies exhibit the mixed alkali effect. All the samples are found to be strong and stable in structure with low values of Urbach energy which lie between 0.027 eV and 0.039 eV. The correlation between densities and Urbach energies of Co²⁺ doped ZLNB glasses with respect to Li₂O content suggest a changeover conduction mechanism from electronic to ionic, with a diffusivity crossover point at x = 15 mol%.     

DC conductivity of rare earth ions doped vanado-tellurite glasses

Two novel sets of vanado-tellurite glasses doped with rare earth ions (La₂O and CeO₂) have been investigated for density and dc electrical conductivity in the temperature range 300–500 K. The conductivity data has been analyzed in the light of Mott’s small polaron hopping (SPH), variable range hopping (VRH) and Greave’s VRH models. The electrical conduction was confirmed to be due to non-adiabatic small polaron hopping in nature. High temperature conductivity behavior was found to be in agreement with Mott’s SPH model. The low temperature conductivity behaved as per Mott’s VRH model. Various physical and polaronic parameters, such as radius, bandwidth and coupling constant, have been determined and discussed.     

Optical and magnetic properties of first-row transition metal ions in lead-silicate glass

First-row transition metal ions in lead-silicate glasses of composition: 37.9% mol PbO, 61.8% mol SiO₂ have been studied with regard to the oxidation state and the coordination by means of optical and magnetic measurements. Optical spectra have been recorded at 300 and 77 K. All the observed bands are assigned as being due to d-d transitions in T_d and/or O_h fields. Magnetic moments are of the order of the spin-only values. By ESR measurements we reveal and quantify titanium, vanadium and manganese oxidation states not detectable by the other techniques.It turns out that the various metal ions have the following oxidation states: Ti³⁺ and Ti⁴⁺, V⁴⁺ and V⁵⁺, Cr³⁺ and Cr⁶⁺, Mn²⁺ and Mn³⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺. The coordination is 4-fold for Ti⁴⁺ and V⁵⁺ and 6-fold for Mn³⁺ and Cu²⁺. Moreover it has been found that Fe³⁺, Co²⁺ and Ni²⁺ are both 4-fold and 6-fold coordinated and that Cr³⁺ and Cr⁶⁺ oxidation states coexist in the same sample.     

Composition and structure dependence of the properties of lithium borophosphate glasses showing boron anomaly

This work presents a study on the structure, microstructure and properties of 50Li₂O·xB₂O₃·(50 ? x)P₂O₅ glasses. The structure has been studied through NMR spectroscopy and the microstructure by TEM. The properties of the glasses are discussed according to their structure and microstructural features. The introduction of boron produces new linkages between phosphate chains through P–O–B bonds, whose amount increases with boron incorporation; at the same time, a depolymerisation of the phosphate chains into Q¹-type phosphate units takes place. The introduction of boron produces an increase in T_g together with a decrease in the molar volume. The room temperature electrical conductivity increases with boron content as well. However, B₂O₃ contents higher than 20 mol% lead to crystallisation of lithium orthophosphate which contributed to hinder ionic conduction of the glasses.     

Optical properties of porous glasses doped with ZnS

The paper presents a new preparation method to form semiconductor nanoclusters inside silica porous glasses as matrices and describes investigations of the luminescence properties of the samples loaded repeatedly with ZnS nanoparticles. Silica porous glasses were obtained by leaching the two-phase sodium borosilicate glasses in acid solution. The samples consisting of two types of glasses with different pore sizes and immobilized nanosized ZnS in these matrices by a chemical deposition process were used. EDS measurements confirmed purity of the samples. The recorded absorption and photoluminescence spectra exhibit the quantum size effect due to the increasing number of the chemical deposition cycles.     

Formation of oxygen-deficient type structural defects and state of ions in SiO2 glasses implanted with transition metal ions

The properties and concentrations of oxygen-deficient type structural defects in type III SiO₂ glasses implanted with Ti⁺, Cr⁺, Mn⁺, Fe⁺, or Cu⁺ to doses from 0.5×10¹⁶ to 6×100¹⁶ ions/cm² at an energy of 160 keV have been measured by using vacuum UV and EPR spectroscopies. An intense absorption band centered around 7.5 eV is observed in all the samples except for Cu-implanted ones and is attributed primarily to Si---Si homo-bonds with the bond distance close to that of the Si₂H₆ molecule. The homo-bond and implanted ion concentrations are of the same order of magnitude in the implanted layers. An E′ type center associated with the homo-bond is observed in all the samples except for Cu-implanted ones. Anomalous behaviors of the Cu-implanted samples are attributed to the formation of Cu-colloids. An enhanced formation of metallic particles or colloids is suggested for the samples implanted with Cr, Mn or Fe to doses higher than 3×10¹⁶ ions/cm².     

Electrical conductivity studies in single and mixed alkali doped cobalt-borate glasses

The glasses of the type (Li₂O)_x-(CoO)_0.2-(B₂O₃)_0.8−x and (Li₂O)_0.2-(K₂O)_x-(CoO)_0.2-(B₂O₃)_0.6−x were prepared by melt quench technique and their non-crystallinity has been established by XRD studies. The glasses were investigated for room temperature density and dc electrical conductivity in the temperature range 300-550 K. Molar volumes were estimated from density data. Composition dependence of density and molar volume in both the sets of glasses has been discussed. Conductivity data has been analyzed in the light of Mott’s Small Polaron Hopping (SPH) Model and activation energies were determined. Variation of conductivity and activation energy with Li₂O content in single alkali glasses indicated change over conduction mechanism from predominantly electronic to ionic, at 0.4 mole fraction of Li₂O. In mixed alkali glasses, the conductivity has passed through minimum and activation energy has passed through maximum at x = 0.2. This has been attributed to the mixed alkali effect. It is for the first time that a change over of predominant conduction mechanism in lithium-cobalt-borate glasses and mixed alkali effect in lithium-potassium-cobalt-borate glasses has been observed. Various physical and polaron hopping parameters such as polaron hopping distance, polaron radius, polaron binding energy, polaron band width, polaron coupling constant, effective dielectric constant, density of states at Fermi level have been determined and discussed.     

Coloured coatings containing mixed transition metal oxides

Glass coloured coatings of M_nO_m---N_pO_q---SiO₂ (M, N = Fe, Co, Ni, Mn, Cr) prepared on soda-lime-silica flat glasses by the sol-gel route using the dip-coating technique.

The optical properties of these films were investigated by spectrophotometry and the optical parameters obtained allowed the colouration mechanism acting in each case to be determined as well as a comparison to be made with the parameters on the single TMO coloured films. Colour coordinates were calculated in every case.

On the other hand coatings have been prepared consisting of two different single TMO films. The corresponding mixed coatings were compared with these double films, showing different behaviours and widening the colour spectrum of glass TMO coatings.     

The effect of composition on the structure of sodium borophosphate glasses

Glasses in the system 40(P₂O₅)-x(B₂O₃)-(60 − x)(Na₂O) (10 ? x ? 30 mol%) have been prepared by the melt-quenching technique. Thermal properties were studied using differential thermal analysis and the relationship between composition and thermal stability was obtained. Structural characterization was achieved by a combination of experimental data (infrared and Raman spectroscopy, ¹¹B and ³¹P solid state NMR). In particular, variations in the phosphate network structure upon addition of B₂O₃ and Na₂O were investigated. Analysis of the data indicates that with increasing B₂O₃ content and decreasing Na₂O, the glass network shows increasing levels of cross-linking between phosphate and borate units. Evidence of direct B-O-P bonds was observed. In the compositional range investigated, borate groups contain boron almost exclusively in four-fold coordination.     

The application of small polaron theory to transition metal oxide glasses

A review of the experimental data on a range of transition metal oxide glasses shows a strong relationship between activation energy and site spacing for glasses based on V₂O₅. The behaviour confirms that transport in amorphous V₂O₅ and vanadate glasses is by adiabatic hopping with an appreciable polaron bandwidth, while non-adiabatic hopping between isolated centres is apropriate for glasses based on most other transition metal oxides. The behaviour of vanadium is linked to the retention of a well-defined VO^5? structural unit in the glass network and to optimisation of the packing of this unit within the glass.