Redox ratio and optical absorption of polyvalent ions in industrial glasses

The changes in glass structure and redox ratio, R (reduced ion to oxidized ion) of Mn²⁺-Mn³⁺, Cu⁺-Cu²⁺, Cr³⁺-Cr⁶⁺, Ni²⁺-Ni³⁺ and Co²⁺-Co³⁺ couples and optical absorption due to Mn³⁺, Cu²⁺, Cr³⁺, Ni²⁺ and Co²⁺ ions in industrial soda-lime-silica glass were investigated as a function of Na₂O concentration in the range 11–19 mol%. With increasing Na₂O concentration in the experimental glasses, the basicity, expressed as calculated basicity, Λ_cal, increased. ²⁹Si NMR and X-ray diffraction were used to investigate the structural change in glasses. The NMR spectra showed high non-bridging oxygens (NBOs) when the basicity of glass was increased. The results were interpreted to be due to the tetrahedral networks; Q ⁴ species were depolymerized by replacing the bridging oxygens (BOs) with NBOs to Q ³ species. These results confirmed the shift of broadening peaks of XRD patterns. The redox reactions of the Mn²⁺-Mn³⁺, Cu⁺-Cu²⁺ and Cr³⁺-Cr⁶⁺ couples shifted more toward their oxidized ions due to the oxygen partial pressure, p(O₂), during melting and the oxide ion activity, a _O2−, increased with increasing glass basicity. These changes caused the redox ratio of these ion couples to decrease. The Ni²⁺-Ni³⁺ and Co²⁺-Co³⁺ couples were assumed to be present only in the Ni²⁺ and Co²⁺ ions in these glasses, respectively. The optical absorption bands due to Mn³⁺, Cu²⁺, Cr³⁺, Ni²⁺ and Co²⁺ ions were also investigated. Their spectra occurred at constant wavelengths with different optical densities or intensities as a function of glass basicity. The increase in the intensities of the absorption bands of these absorbing ions, except for Cr³⁺ ion, at the maximum wavelength, depends not only on the ion concentration but also on the increase of polarizability of oxide (−II) species, α _oxide(−II), surrounding the ions. This value affected directly the extinction coefficients of the ions, ɛ _ion. The increase of ɛ _ion caused the colour of glasses appearing in high intensity. In the case of Cr³⁺ ion, the results were reversed such that the lower the concentration, the higher the intensities of colour.     

Luminescence of 60B2O3-32CaF2-8Bi2O3 glasses codoped with chromium and neodymium

60B₂O₃-32CaF₂-8Bi₂O₃ glasses codoped with chromium and neodymium have been prepared for the first time, and their luminescence properties have been studied. The glasses have been shown to contain chromium in two oxidation states. We have obtained luminescence spectra of Cr³⁺ in octahedral coordination and Cr⁴⁺ in tetrahedral coordination. Energy transfer from Bi³⁺ to Nd³⁺ and Cr³⁺ has been demonstrated.     

Effect of heavy mixing lithium with doping of manganese on structural, EPR and dielectric spectroscopic properties of Layered Na2Ti3O7

Sintered ceramic samples of Na₂Ti₃O₇ with 50 % (1.0 Molar Percentage of Li₂CO₃ i. e. 50 % Lithium) with different doping molar percentages MnO₂ (0.0 < X<0.1) have been prepared through solid state reaction route. The microstructure, EPR, dielectric properties and ac conductivity of (NaLi)Ti₃O₇ with Mn [0.0 ≤ X≤0.1] have been investigated. The X-ray diffraction patterns of pure and doped layered ceramics suggest the crystals are orthorhombic in phase. The room temperature electron paramagnetic resonance spectra reveal that that for lower percentage of doping manganese ions occupies Ti ⁴⁺ site with oxidation state Mn³⁺, while higher percentage of manganese ions doping leads to oxidation state Mn²⁺ in interlayer mixed (Na Li) site. In both cases the charge compensation mechanism should operate to maintain the overall charge neutrality of the lattice. For all pure and doped layered ceramics, ferroelectric transition having high transition temperature has been identifying at 648 K. Manganese ion doping decreases dielectric loss and increases dielectric constant due to inhibition of domain wall motion. It is apparent that ionic conduction becomes difficult due to presence of more lithium ions with sodium ions in interlayer space which shrinking the wide space of interlayer channels.     

Electron paramagnetic resonance and Mössbauer spectroscopy of transition metal ion doped mullite

Appreciable difference in the properties of undoped and oxide-doped mullite are observed. The oxidation state of cation, its concentration and the position of the mullite lattice occupied by it appear to be the responsible factors. Mullite has, therefore, been doped with four transition metal ions, Mn, Fe, Cr and Ti. With the help of EPR and Mössbauer spectroscopy (supplimented by X-ray diffractometry) the oxidation states of these ions and the mullite lattice sites where they enter has been investigated. It was observed that Mn ion was present in Mn²⁺ and Mn³⁺ states, the former remained as clusters and the latter occupied the octahedral sites in the mullite lattice. Only Fe³⁺ ion was detected and conclusive evidence was obtained for the entry of Fe³⁺ in the octahedral lattice position of mullite from the analysis of Mössbauer spectra with the help of a specially written computer programme. The Cr ion entered the mullite structure only in the Cr³⁺ state. The change in lattice parameters of Cr doped mullite were measured by the XRD technique. The results showed that the expansion of b-axis was more than that of the a-axis which supported the presence of Cr³⁺ ion in the octahedral site of mullite lattice. The absence of signal in the EPR spectra of Ti doped mullite suggested the presence of only Ti⁴⁺ (3d⁰) ion. Very low electrical resistivity of Ti doped mullite and close similarity between mullite and Al₂TiO₅ structures stood as evidence for incorporation of Ti⁴⁺ ion in the octahedral site of mullite lattice by replacing Al³⁺ ion.     

Local Environment of Fe Ions in a 40ZnO · 60B<Subscript>2</Subscript>O<Subscript>3</Subscript> Glass Matrix

The valence state and coordination environment of the Fe ions in a number of Fe-containing zinc borate glasses with the general formula xFe₂O₃–(100–x)[40ZnO · 60B₂O₃] (x = 2.5–10 mol %) have been studied by Mössbauer spectroscopy. The results indicate that all of the glasses contain both Fe²⁺ and Fe³⁺ ions. The percentage of trivalent ions considerably exceeds that of Fe²⁺ ions and the relationship between the Fe²⁺ and Fe³⁺ concentrations is essentially independent of the Fe₂O₃ content in the zinc borate matrix. Most of the Fe³⁺ ions are in tetrahedral coordination, but raising the Fe₂O₃ content to above 5 mol % leads to a gradual increase in the percentage of Fe ions in octahedral coordination.     

Crystal structure,energy gap and photoluminescence investigation of Mn2+/Cr3+-doped ZnS nanostructures by precipitation method

Mn added ZnS (Zn_0.97Mn_0.03S) and Mn–Cr-doped ZnS (Zn_0.95Mn_0.03Cr_0.02S) nanostructures were synthesized by co-precipitation process. XRD pattern confirmed the cubic phase with highest intensity along (111) orientation. The shrinkage of crystallite size from 36 Å (Zn_0.97Mn_0.03S) to 26 Å (Zn_0.95Mn_0.03Cr_0.02S) and the influence of Cr/Mn on microstructural, optical and photoluminescence properties in ZnS were investigated. The substitution of Cr in Zn_0.97Mn_0.03S lattice not only diminished the crystallite size and also produced more defect-associated luminescent activation centres. The elevated micro-strain from 9.71?×?10^–3 (Zn_0.97Mn_0.03S) to 13.11?×?10^–3 (Zn_0.95Mn_0.03Cr_0.02S) by Cr substitution is due to the decrease of size and the higher micro-strain at Cr?=?2% is owing to the drop off of activation energy which is originated from higher electro-negativity of Cr ions than Zn²⁺ ions. The enhanced lattice parameters by Cr doping may be due to the coexistence of both Cr³⁺ ions and Cr²⁺ ions where the existence of Cr²⁺ ions is higher than Cr³⁺ ions and substitute Zn²⁺ basic ions with the ionic radius of 0.74 Å in the Zn–Mn–S host lattice. The presence of Zn²⁺, Mn²⁺ and Cr³⁺ ions in Zn–Mn–Cr–S lattice was confirmed by XPS spectra. SEM/TEM micrographs explored the microstructure and confirmed the sized reduction by Cr doping. The elevation in band gap from 3.50 eV (Zn_0.97Mn_0.03S) to 3.63 eV (Zn_0.95Mn_0.03Cr_0.02S, ?E_g?~?0.13 eV) by Cr addition was explained by Burstein–Moss effect and reduced crystallite size. The tuning of band gap and crystallite size of basic ZnS nanostructure by Mn/Cr substitution encourages these materials for modern electronic applications. FTIR spectra established the occurrence of Mn/Cr in Zn–S lattice by their characteristic bondings. The elevated yellowish-orange emission at 594 nm in Mn/Cr substituted ZnS is due to the exchange communication among the s–p electron states of Cr³⁺, Mn²⁺ and Zn²⁺ ions in Zn–S lattice. The inclusion of Mn /Cr provides an efficient control over modification of various emissions which suggests their applications in organic LED materials.

     

EPR and optical absorption of Cr3+ in binary Na2O-B2O3 glasses

The optical absorption and EPR spectra of Cr³⁺ in binary sodium borate glasses have been studied as functions of chromium concentration and Na₂O/B₂O₃ ratio in the glass; the ligand field and EPR parameters have been calculated and were found to be independent of Cr₃₊ concentration in any particular glass. In low-alkali borate glasses (Na₂O = 11 or 14 mol %) a single symmetrical EPR line was observed with g _¦=g =1.984±0.001 corresponding to perfect octahedral symmetry of the Cr³⁺ ion in these glasses. With increasing Na₂O content of the glass, the EPR line becomes more asymmetric (characteristic two-peaked pattern); this has been explained as being due to axial elongation of the six co-ordinated Cr³⁺-complex in these glasses.     

Electrical conductivity of zinc ferrites near stoichiometry and manganese-zinc ferrites under vacuum or in the presence of oxygen

The electrical conductivity of zinc ferrites near stoichiometry and of manganese-zinc ferrites has been investigated as a function of temperature under vacuum and in the presence of oxygen. Under vacuum, the conductivity of these ferrites with iron excess is explained by the hopping mechanism, and with ZnO excess by the development of vacancies in octahedral sites of cation-deficient spinel. Activation energies and the transition temperatures are presented. During the oxidation in oxygen of Mn-Zn ferrites, the profile of the log σ= f(T) curves shows that the mechanism of electrical conduction in the temperature range 100 to 350 °C can be explained in terms of the oxidation of Fe²⁺ to Fe³⁺ ions at octahedral sites. For the temperature range 300 to 450 °C, the conductivity involves the hopping of electrons from octahedral sites of Mn³⁺ ions to octahedral sites of Mn⁴⁺ ions. Above 550 °C the oxidation of Mn²⁺ ions leads to a marked change in conductivity with the generation of new phases.     

Comparative investigation on the effect of alkaline earth oxides on the intensity of absorption bands due to Cu<Superscript>2+</Superscript>, Mn<Superscript>3+</Superscript> and Cr<Superscript>3+</Superscript> ions in ternary silicate glasses

Absorption characteristics of Cu²⁺, Mn³⁺ and Cr³⁺ ions in ternary silicate (20Na₂O·10RO·70SiO₂, where R=Ca, Sr, Ba) glasses were investigated. The intensities of absorption bands due to Cu²⁺ ion was found to increase with increasing ionic radii of the alkaline earth ions whereas it was found to decrease in case of Mn³⁺ and Cr³⁺ ions with increasing ionic radii of the alkaline earth ions. The results were discussed in the light of relation between linear extinction coefficients of these ions and coulombic force of alkaline earth ions. The change in intensities of Cu²⁺, Mn³⁺ and Cr³⁺ ion is attributed due to change in silicate glass compositions.     

EPR Study of MnO and MnO2 Doped Barium Aluminoborate Glasses

An off-equilibrium redox condition study of Mn²⁺ and Mn⁴⁺ ions added to the batch is reported for the case of the preparation of the barium aluminoborate glasses, by considering that the out of equilibrium reactions started already on the melt. It is shown that for lower dopant concentrations the relative concentrations of the oxidation states of the manganese are found out of equilibrium up to the added amount of 0.3 mol% of MnO₂, but on doping with MnO, the equilibrium state is achieved at the MnO added amount of 0.07 mol%. The change of the redox condition of the manganese ions toward the equilibrium was studied by tracking the Mn²⁺ amount as a function of the doping concentration in the two cases of MnO and MnO₂.     

A study of interaction between copper and manganese in a soda-borate glass by ESR

The ESR spectra of 30 Na₂O-70 B₂O₃ glasses, containing very low amounts of copper and manganese singly and in mixed proportions, have been studied. The copper glass shows that Cu²⁺ is present in a distorted octahedral environment. The manganese glass shows that Mn²⁺ is also present in an octahedral site possibly with some distortion. The mixed copper-manganese glasses show that both Cu²⁺ and Mn²⁺ are present with Cu⁺ and Mn³⁺, and that there is a considerable interaction between these two ions giving rise to the exchange coupled Cu²⁺-Mn²⁺ pairs. The covalency of Cu²⁺-O bonding decreases in the mixed glasses. The possibility of spin diffusion and nuclear relaxation in the Mn²⁺ site is proposed to account for the absence of the Mn²⁺ hyperfine lines in the spectra of mixed glasses. The possibility of nuclear relaxation of the Cu²⁺ site is not completely ruled out from the broadening of the hyperfine lines in the mixed glasses, as compared to that in the copper glass.     

The influence of different alkaline earth oxides on the structural and optical properties of undoped,Ce-doped,Sm-doped,and Sm/Ce co-doped lithium alumino-phosphate glasses

Undoped, singly Sm doped, Ce doped, and Sm/Ce co-doped lithium alumino-phosphate glasses with different alkaline earth modifiers were prepared by melt quenching. The structure of the prepared glasses was investigated by FT-IR and Raman, as well as by optical spectroscopy. The effect of the optical basicity of the host glass matrix on the added active dopants was studied, as was the effect doping had on the phosphate structural units. The optical edge shifts toward higher wavelengths with an increase in the optical basicity due to the increased polarizability of the glass matrix, but also with increasing CeO₂ concentration as a result of Ce³⁺/Ce⁴⁺ inter valence charge transfer (IV-CT) absorption. The optical band gap for direct and indirect allowed transitions was calculated for the undoped glasses. The glass sample containing Mg²⁺ modifier ions is found to have the highest value (4.16 eV) for the optical band gap while Ba²⁺ has the lowest value (3.61 eV). The change in the optical band gap arises from the structural changes and the overall polarizability (optical basicity). Refractive index, molar refractivity R_m and molar polarizability α_m values increase with increasing optical basicity of the glasses. The characteristic absorption peaks of Sm³⁺ were also investigated. For Sm/Ce co-doped glasses, especially at high concentration of CeO₂, the absorption of Ce³⁺ hinders the high energy absorption of Sm³⁺ and this effect becomes more obvious with increasing optical basicity.     

EPR and optical absorption studies of Cu ions in alkaline earth alumino borate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectra of Cu²⁺ ions in alkaline earth alumino borate glasses doped with different concentrations of CuO have been studied. The EPR spectra of all the glasses exhibit the resonance signals, characteristic of Cu²⁺ ions present in axially elongated octahedral sites. The number of spins participating in the resonance has been calculated as a function of temperature for calcium alumino borate (CaAB) glass doped with 0.1 mol% of CuO. From the EPR data, the paramagnetic susceptibility (χ) was calculated at different temperatures (T) and from the 1/χ-T graph, the Curie temperature of the glass has been evaluated. The optical absorption spectra of all the glasses show a single broad band, which has been assigned to the ²B_1g → ²B_2g transition of the Cu²⁺ ions. The variation in the intensity of optical absorption with the ionic radius of the alkaline earth ion has been explained based on the Coulombic forces. By correlating the EPR and optical absorption spectral data, the nature of the in-plane σ bonding between Cu²⁺ ion and the ligands is estimated. From the fundamental ultraviolet absorption edges of the glasses, the optical energy gap (E_opt) and the Urbach energy (ΔE) are evaluated. The variation in E_opt and ΔE is explained based on the number of defect centers in the glass.     

Spectroscopic characterization and optical waveguide fabrication in Ce, Tb and Ce/Tb doped zinc–sodium–aluminosilicate glasses

A spectroscopy investigation of Ce³⁺ and Tb³⁺ ions in sodium–zinc–aluminosilicate glasses is performed using the photoluminescence technique. Blue–white light, with x = 0.24 and y = 0.24 CIE chromaticity coordinates, is obtained for the Tb³⁺ singly-doped glass excited at 351 nm. When the sodium–zinc–aluminosilicate glass is co-doped with Ce³⁺ and Tb³⁺ a non-radiative energy transfer from Ce³⁺ to Tb³⁺ ions is observed upon 320 nm excitation. From an analysis of the cerium emission decay curve, the Ce³⁺ → Tb³⁺ energy transfer microscopic parameter and efficiency are obtained. Different concentrations of Ce³⁺ and Tb³⁺ ions in the glass host gives rise to blue and blue–green emissions, with different CIE coordinates. Optical waveguides were produced in the samples by Ag⁺–Na⁺ ion-exchange, and their characterization is presented.     

Synthesis and luminescence properties of RMgPO<Subscript>4</Subscript>:Mn<Superscript>2+</Superscript> (R = Li,Na, and K) red phosphor

RMgPO₄:Mn²⁺ (R?=?Li, Na, and K) phosphors are synthesized by a solid-state reaction method in air. The crystal structures and luminescence properties are investigated. A broad emission band peaking at 638 nm of RMgPO₄:Mn²⁺ (R?=?Li and Na) phosphors with excitation 408 nm is observed in the range of 550–800 nm owing to the ⁴T₁(G) → ⁶A₁ transition of the Mn²⁺ ion. KMgPO₄:Mn²⁺ phosphor with excitation 416 nm shows a broad emission band peaking at 658 nm in the range of 550–800 nm owing to the ⁴T₁(G) → ⁶A₁ transition of the Mn²⁺ ion. The optimal Mn²⁺ ion concentration in RMgPO₄:Mn²⁺ (R?=?Li, Na, and K) phosphors is about 5 mol%. The lifetimes of RMg_0.95PO₄:0.05Mn²⁺ (R?=?Li, Na, and K) phosphors is ~6.21, 6.03, and 6.28 ms, respectively. The luminous mechanism is explained by Tanabe-Sugano diagram of Mn²⁺ ion.     

Optical and esr spectra of titanium (III) in Na2O-B2O3 and Na2O-P2O5 glasses

The optical absorption and the esr spectra of titanium(III) in binary Na₂O-B₂O₃ and Na₂O-P₂O₅ glasses have been studied. Titanium(III) produces two optical absorption bands around 20 000 and 14 000 cm^–1 which are assigned to the²B_2g ²B_1g and²B_2g ²A_1g transitions respectively of Ti³⁺ in a tetragonally distorted octahedral environment. The absorption bands in phosphate glasses are narrower and absorption coefficients higher than those in borate glasses. The esr spectrum of titanium(III) in all the glasses consists of a broad asymmetric line withg 1.94 in borate glasses, andg 1.92 in phosphate glasses; no hyperfine structure has been observed.     

Matrix effect on local structure surrounding Yb3+and spontaneous emission probability in oxide glasses

Extended X-ray absorption fine structure measurements have been performed on Yb³⁺ in silicate, borate, phosphate, and gallate glasses in order to investigate the local structures surrounding Yb³⁺. The local structures of Yb³⁺ ions in silicate, borate, and phosphate glasses were similar to those of Er³⁺ ions. Yb³⁺ ions do not occupy the network structural sites, but sit around the terminal region of the network or the region between the networks in these glasses. On the other hand, Yb³⁺ ions substitute the Ga³⁺ sites in complex anion structural units of the K₂O-Ga₂O₃-Nb₂O₅ glasses. We classified the local structures surrounding Yb³⁺ ions in oxide glasses into two types: the former and the latter are interstitial and substitutive types, respectively. The relationship between the spontaneous emission probability for ²F_5/2 – ²F_7/2 transition of Yb³⁺ and the local structure of Yb³⁺ in oxide glasses are discussed in terms of these two types.     

Electrical conductivity study of oxidation process in manganese-substituted magnetites

During oxidation in air of finely-grained manganese-substituted magnetites (Mn _0.8x ²⁺ Fe _1–0.8x ³⁺ )_A– (Fe _1+0.6x ³⁺ Fe _1–0.8x ²⁺ Mn _0.2x ³⁺ )_BO ₄ ^2– (A=tetrahedral, B=octahedral) the temperature dependence of the electrical conductivity over a temperature range of 100 to 700° C was investigated. Below 500° C the evolution of electrical conductivity might be closely associated with the position and nature of cations in the spinel lattice. The profile of the =f(t) curves show that the mechanism of electrical conduction in the temperature range 150 to 300° C can be explained in terms of the oxidation of Fe²⁺ to Fe³⁺ ions at octahedral sites. For the temperature range 300 to 400° C the conductivity involves the hopping of electrons from tetrahedral-site Mn²⁺ ions to tetrahedral-site Mn³⁺ ions. Above 500° C the oxidation of Mn²⁺ ions leads to an increase in conductivity with the generation of new phases of -Fe₂O₃, Mn₂O₃ and -(MnFe)₂O₃.     

Malayaite ceramic pigments: A combined optical spectroscopy and neutron/X-ray diffraction study

Ceramic pigments based on the Cr-doped malayaite structure were synthesized by solid state reaction and characterized by optical spectroscopy and combined X-ray and neutron powder diffraction in order to elucidate the still unclear chromium substitution mechanisms. The results show that coloration is actually due to simultaneous occurrence of Cr⁴⁺ and Cr³⁺ ions in the crystal lattice. Spectroscopy data confirm that Cr⁴⁺ is replacing Sn⁴⁺ in the octahedral site and, in minor amount, Si⁴⁺ in the tetrahedral site. In addition, neutron powder diffraction data suggest that Cr³⁺ substitution for octahedral Sn⁴⁺ is charge balanced by the formation of oxygen vacancies with no preference over the different oxygen sites. Upon incorporation of Cr ion, the SnO₆ octahedra exhibit an off-centre displacement of central cation which in turn induces a rearrangement of both the octahedral and tetrahedral coordination shells.