Electrical and interface state density properties of polyaniline–poly-3-methyl thiophene blend/p-Si Schottky barrier diode

We have formed conjugated polymeric aniline–thiophene organic material on p-Si substrate by adding polyaniline–poly-3-methyl thiophene blend solution in acetonitrile on top of a p-Si substrate and then evaporating the solvent. It has been seen that the forward bias current–voltage (I–V) characteristics of polyaniline–poly-3-methyl thiophene blend/p-Si/Al with a barrier height value of 0.60 eV and an ideality factor value of 3.37 showed rectifying behaviour at room temperature. The polyaniline–poly-3-methyl thiophene blend/p-Si/Al Schottky barrier diode showed non-ideal I–V behaviour with the value of ideality factor greater than unity that could be ascribed to the interfacial layer, interface states and series resistance. Furthermore, Cheung's functions and modified Norde's function were used to extract the diode parameters including ideality factor, barrier height and series resistance. It has been seen that there is a good agreement between the barrier height values from all methods. However, the values of series resistance obtained from Cheung's functions is higher than the values obtained from Norde's functions. The energy distribution of interface states density, determined from forward bias current–voltage (I–V) characteristic technique at room temperature, increases exponentially with bias from 2.81 × 10¹⁶ cm^?2 eV^?1 in (0.73–E_v) eV to 1.14 × 10¹⁷ cm^?2 eV^?1 in (0.48–E_v) eV.     

Impedance spectroscopy of V2O5–Bi2O3–BaTiO3 glass–ceramics

The glasses within composition as: (80 − x)V₂O₅/20Bi₂O₃/xBaTiO₃ with x = 2.5, 5, 7.5 and 10 mol% have been prepared. The glass transition (T_g) increases with increasing BaTiO₃ content. Synthesized glasses ceramic containing BaTi₄O₉, Ba₃TiV₄O₁₅ nanoparticles of the order of 25–35 nm and 30–46 nm, respectively were estimated using XRD. The dielectric properties over wide ranges of frequencies and temperatures were investigated as a function of BaTiO₃ content by impedance spectroscopy measurements. The hopping frequency, ω_h, dielectric constant, ε′, activation energies for the DC conduction, E_σ, the relaxation process, E_c, and stretched exponential parameter β of the glasses samples have been estimated. The, ω_h, β, decrease from 51.63 to 0.31 × 10⁶ (s⁻¹), 0.84 to 0.79 with increasing BaTiO₃ respectively. Otherwise, the E_σ, increase from 0.279 to 0.306 eV with increasing BaTiO₃. The value of dielectric constant equal 9.5·10³ for the 2.5BaTiO₃/77.5V₂O₅/20Bi₂O₃ glasses-ceramic at 330 K for 1 KHz which is ten times larger than that of same glasses composition. Finally the relaxation properties of the investigated glasses are presented in the electric modulus formalism, where the relaxation time and the respective activation energy were determined.     

Lithium conductivity in an Li-bearing double-ring silicate mineral, sogdianite

The crystal structure of an Li-bearing double-ring silicate mineral, sogdianite ((Zr_1.18Fe³⁺_0.55Ti_0.24Al_0.03)(?_1.64,Na_0.36)K_0.85[Li₃Si₁₂O₃₀], P6/mcc, a≈10.06 Å, c≈14.30 Å, Z=2), was investigated by neutron powder diffraction from 300 up to 1273 K. Rietveld refinements of displacement parameters revealed high anisotropic Li motions perpendicular to the crystallographic c-axis, indicating an exchange process between tetrahedral T2 and octahedral A sites. AC impedance spectra of a sogdianite single crystal (0.04×0.09×0.25 cm³) show that the material is an ionic conductor with conductivity values of σ=4.1×10⁻⁵ S cm⁻¹ at 923 K and 1.2×10⁻³ S cm⁻¹ at 1219 K perpendicular to the c-axis, involving two relaxation processes with activation energies of 1.26(3) and 1.08(3) eV, respectively.     

Electrochemical,magnetic, catalytic and DNA cleavage studies of binuclear copper(II) complexes derived from pendant substituted tetraaza macrobicyclic compartmental ligands

A series of macrobicyclic unsymmetrical binuclear copper(II) complexes of compartmental ligands were synthesized from the Schiff base condensation of 1,8[N,N′-bis{(3-formyl-2-hydroxy-5-methyl)benzyl}]-1,4,8,11- tetraaza-5,5,7,12,12,14-hexa methylcyclotetradecane with diamines like 1,2-diamino ethane, 1,3-diamino propane, 1,4-diaminobutane, 1,2-diaminobenzene and 1,8-diaminonaphthalene. The complexes were characterized by elemental and spectral analysis. Electrochemical studies of the copper(II) complexes show two irreversible one-electron reduction processes around E¹_pc = −0.70 to −1.10 V and E²_pc = −0.98 to −1.36 V. ESR spectra of the binuclear copper(II) complexes show a broad signal at g = 2.10 and μ_eff values in the range 1.46–1.59 BM, which convey the presence of antiferromagnetic coupling. Cryomagnetic investigation of the binuclear complexes [Cu₂L³(ClO₄)](ClO₄) and [Cu₂L⁴(ClO₄)](ClO₄) show that the observed −2J values are 144 and 216 cm⁻¹, respectively. The observed initial rate (V_in) for the catalytic hydrolysis of p-nitrophenyl phosphate by the binuclear copper(II) complexes were in the range 1.8 × 10⁻⁵ to 2.1 × 10⁻⁵ Ms⁻¹. The initial rate (V_in) for the catalytic oxidation of catechol to o-quinone by the binuclear copper(II) complexes were in the range 2.7 × 10⁻⁵ to 3.5 × 10⁻⁵ Ms⁻¹. The copper(II) complexes have been found to promote cleavage of plasmid pBR 322 DNA from the supercoiled form I to the open circular form II.     

Temperature effects on the relaxation parameters of the 1-methylnaphthalene excimer

The excimer lifetime τ_D and the excimer fluorescence efficiency for 1-methylnaphthalene in ethanol have been determined between −30 and 60°C. Expressing the rate constant for excimer deactivation. k_D, in terms of radiative k_FD) and nonradiative (k_ID) processes as k_D = k_FD + k_ID it is found that k_FD is independent of tempeature and k_ID = 5 × 10⁶ + 3.2 × 10¹¹ exp (-ΔE/RT) sec⁻¹, where ΔE = 6.7 kcal mole⁻¹. The behaviour of k_FD and k_ID with temperature and the appearance of isoemissive points in a limited region of temperature are discussed.     

Adsorption of a protein-porphyrin complex at a liquid-liquid interface studied by total internal reflection synchronous fluorescence spectroscopy

Interfacial analysis has attracted more and more attention owing to its fundamental and biological importance. Total internal reflection fluorescence (TIRF) spectroscopy is a useful method to study interfacial properties. The synchronous scanning fluorescence technique provides a selective tool to analyze a specific component in a complex system. The interaction and adsorption of bovine serum albumin (BSA) and meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) at toluene-water interface were studied successfully by the coupling technique of total internal reflection synchronous fluorescence (TIRSF). New methods are provided for the determination of the critical micelle concentration (cmc), apparent adsorption equilibrium constant (K_ad) and maximum amount of adsorption (f_max) at the liquid-liquid interface. The results indicated that BSA could adsorb onto the toluene-water interface as a complex of BSA-TPPS in a ratio of 1:1 ratio based on Langmuir adsorption isothermal model. The cmc, apparent K_ad and f_max for BSA at pH 3.1 were determined to be 1.0 × 10⁻⁴ mol L⁻¹, 1.15 × 10⁵ L mol⁻¹ and 1.14 × 10⁻⁹ mol cm⁻², respectively.     

Influence of hydrofluoric acid on extraction of thorium using a commercially available extraction chromatographic resin

The dependence of Th recovery on hydrofluoric acid (HF) concentration in nitric acid (HNO₃) solutions (1–5 mol/dm³) containing 1 × 10⁻⁶ mol/dm³ of Th and various concentrations of HF and the elution behavior were studied using a commercially available UTEVA (for uranium and tetravalent actinide) resin column. Thorium recovery decreased with an increase in HF concentration in the sample solutions. The concentration of HF at which Th recovery started to decrease was ∼1 × 10⁻⁴ mol/dm³ in 1 mol/dm³ HNO₃ solution, ∼1 × 10⁻³ mol/dm³ in 3 mol/dm³ HNO₃ solution, and ∼1 × 10⁻² mol/dm³ in 5 mol/dm³ HNO₃ solution. When Al(NO₃)₃ (0.2 mol/dm³) or Fe(NO₃)₃ (0.6 mol/dm³) was added as a masking agent for F⁻ to the Th solution containing 1 × 10⁻¹ mol/dm³ HF and 1 mol/dm³ HNO₃, Th recovery improved from 1.4 ± 0.3% to 95 ± 5% or 93 ± 3%. Effective extraction of Th using UTEVA resin was achieved by selecting the concentration of HNO₃ and/or adding masking agents such as Al(NO₃)₃ according to the concentration of HF in the sample solution.     

A theoretical approach for exploration of non-linear optical amplification of fused azacycle donor based thiophene polymer functionalized chromophores

The quantum chemical calculations are executed for a series of designed carbazole-based oligothiophene systems (CPTR1 and CPTD2-CPTD8) having D₁-π₁-D₂-π₂-A architecture. The effect of addition of π-linkers on designed architecture for the electronic and non-linear optical response was examined at M06/6-311G(d,p) level of theory. The frontier molecular orbitals (FMOs), density of states (DOS), natural population analysis (NPA), UV–Vis and transition density matrix (TDM) and non-linear optical (NLO) analyses were utilized in order to comprehend key electronic and non-linear optical response. All the designed molecules exhibited a lower energy gap (E_LUMO-E_HOMO) as 2.434–2.780 eV, as compared to the CPTR1 (2.875 eV). Among all the derivatives, CPTD8 exhibited the highest dipole polarizability 〈α〉 and second hyperpolarizability (γ_tot) as 2.946 × 10^-22 esu and 41.372 × 10^-33 esu, respectively. Dipole moment (µ) and first hyperpolarizability (β_tot) of CPTD8 were found to be as 3.478 D and 118.886 × 10^-29 esu, correspondingly. The second hyperpolarizability (γ_tot) of CPTD8 was observed to be ∼6.4 ∼4.0 ∼2.5 ∼1.8 ∼1.4 ∼1.3 and ∼1.1 times higher in comparison to CPTR1 and CPTD2-CPTD7, respectively. It is concluded that carbazole-based oligothiophene might be used as a potential material in optoelectronic devices.     

The interface behavior of hemoglobin at carbon nanotube and the detection for H(2)O(2)

Direct electrochemistry of hemoglobin (Hb) is observed at carbon nanotube (CNT) interface. The adsorbing Hb can transfer electron directly at CNT interface compared with common carbon material. The heterogeneous electron transfer rate constant k of Hb can be calculated as 0.062 s⁻¹, the transfer coefficient α is 0.21 and the average surface coverage of Hb on CNT surface is 3.58 × 10⁻⁹ ± 2.7 × 10⁻¹⁰ mol/cm². It is found that the adsorbing Hb still keeps its catalytic activity to H₂O₂. This sensor was used to detect H₂O₂. The apparent Michaelis-Menten constant is calculated as 6.75 × 10⁻⁴ mol L⁻¹.     

Theoretical study of intra- and inter-chain magnetic interactions in [Ni(chxn)2Br]Br2

The intra- and the inter-chain magnetic interactions in [Ni(chxn)₂Br]Br₂, which is one of the typical one-dimensional (1-D) MX complexes are examined by using an unrestricted hybrid DFT (UB3LYP) method. Calculated effective exchange integral (J) value along the 1-D chain is 2J_Intra = −4016 K and is close to an experimental result (−3600 K). On the other hand, a very weak anti-ferromagnetic inter-chain interaction through Br⁻ ions is observed. The value is estimated to be 2J_Inter = −2 to −6 K. In addition to the J values, transfer integral (t), on-site Coulomb repulsion (U) and charge transfer energy (E^CT) values along the 1-D chain are also estimated to be 0.46, 2.46 and 0.6 eV, respectively.     

Kinetics study on phase transformation from titania polymorph brookite to rutile

TiO₂ is a polymorphic material of great scientific interest due to its semiconductor properties and uses in heterogeneous photocatalysis. Understanding the stability of the polymorphs is important for designing TiO₂-based photocatalysts and solar cells. Although the phase transformation of anatase→rutile has been well studied, there is only one published work on brookite→rutile to date. The brookite→rutile transformation has been studied in this work using natural material from the Magnet Cove igneous complex mechanically processed to several micrometers in size. The pure phase brookite is annealed from 800 to 900 °C without detection of the anatase polymorph. The transformation kinetics are described by both the standard first-order model, with an activation energy of E_a=411.91 kJ/mol, and the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, with an activation energy of E_a=492.13 kJ/mol. The rate parameter of the first-order model for the phase transformation is expressed as k=6.85×10¹⁴ exp(−49,451/T) s⁻¹ for the first-order model and k=4.19×10¹⁸ exp(−59,189/T) s⁻¹ using the JMAK model. The obtained activation energy is higher than that of brookite nano-crystals. Our results show that the JMAK model fits the kinetics data better than other models.     

Structure and oxide anion conductivity in Ln2(TO4)O (Ln=La, Nd; T=Ge, Si)

Oxy-silicate and oxy-germanate, Ln₂(TO₄)O (Ln=La and Nd, T=Ge and Si) compounds have been prepared. Oxy-germanates can be readily obtained as highly crystalline single phases, while, the oxy-silicates are difficult to prepare as pure phases. The crystal structure of Nd₂(SiO₄)O has been studied from a joint Rietveld refinement of neutron and laboratory X-ray powder diffraction data. The electrochemical characterisation indicates that these compounds display oxide anion conductivity with p-type electronic contribution under oxidising conditions. The apparent activation energies under dry flowing nitrogen, where p-type contribution is minimised, are 0.97(1), 1.05(3) and 1.17(4) eV, for Nd₂(SiO₄)O, La₂(GeO₄)O and Nd₂(GeO₄)O, respectively. The overall conductivities at 1173 K range from 1.2×10⁻⁴ S cm⁻¹ for Nd₂(SiO₄)O to 1.3×10⁻⁶ S cm⁻¹ for La₂(GeO₄)O. Finally, the stability of these compounds under very reducing conditions has been studied and partial degradation is reported.     

Enhanced electrochemical performance at screen-printed carbon electrodes by a new pretreating procedure

A new method for the pretreatment of screen-printed carbon electrodes (SPCEs) by two successive steps was proposed. In step one, fresh SPCEs were soaked into NaOH with high concentration (e.g. 3 M) for tens to hundreds of minutes, and the resulted electrodes were called as SPCE-I. In step two, SPCE-I were pre-anodized in low concentration of NaOH, which were designated as SPCE-II. The pretreated electrodes showed remarkable enhancement in heterogeneous electron transfer rate constant (k⁰) increased from 1.6 × 10⁻⁴ cm s⁻¹ at the fresh SPCE to 1.1 × 10⁻² cm s⁻¹ at SPCE-I for Fe(CN)₆^3−/4− couple. The peak to peak separation (ΔE_p) in cyclic voltammetry was reduced from ca. 480 to 84 mV, indicating that the electrochemical reversibility was greatly promoted, possibly due to the removing of polymers/oil binder from the electrode surfaces. The electroactive area (A_ea) of the electrode was increased by a factor of 17 after pretreatment in step one. Further analysis by the electrochemical impedance method showed that the electron transfer resistance (R_ct) decreased from ca. 2100 to 1.4 Ω. These pretreated electrodes, especially SPCE-II, exhibited excellent electrocatalytic behavior for the redox of dopamine (DA). Interference from ascorbic acid (AA) in the detection of DA at SPCE-II could be effectively eliminated due to the anodic peak separation (190 mV) between DA and AA, which resulted from the functionalization of the electrode surface in the pretreatment of step two. Under optimum conditions, current responses to DA were linearly changed in two concentration intervals, one was from 3.0 × 10⁻⁷ to 9.8 × 10⁻⁶ M, and the other was from 9.8 × 10⁻⁶ to 3.3 × 10⁻⁴ M. The detection limit for DA was down to 1.0 × 10⁻⁷ M.     

A pH responsive electrochemical switch sensor based on Fe(notpH3) [notpH6 = 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylene-phosphonic acid)

The electrochemistry of a macrocyclic metal complex Fe(notpH₃) [notpH₆ = 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylene-phosphonic acid)] reveals that the protonation/deprotonation of the non-coordinated P-OH groups in Fe(notpH₃) affects its formal potential value (E⁰′) considerably. Plotting E⁰′ as function of solution pH gives a straight line with a slope of −585 mV pH⁻¹ in the pH range of 3.4-4.0, which is about ten times larger than the theoretical value of −58 mV pH⁻¹ for a reversible proton-coupled single-electron transfer at 20 °C. A sensitive pH responsive electrochemical switch sensor is thus developed based on Fe(notpH₃) which shows an “on/off” switching at pH ∼ 4.0.     

Thermodynamic Studies on NdFeO3(s)

The enthalpy increments and the standard molar Gibbs energy of formation of NdFeO₃(s) have been measured using a high-temperature Calvet microcalorimeter and a solid oxide galvanic cell, respectively. A λ-type transition, related to magnetic order-disorder transformation (antiferromagnetic to paramagnetic), is apparent from the heat capacity data at ∼687 K. Enthalpy increments, except in the vicinity of transition, can be represented by a polynomial expression: {H°_m(T)−H°_m(298.15 K)}/J·mol⁻¹ (±0.7%)=−53625.6+146.0(T/K) +1.150×10⁻⁴(T/K)² +3.007×10⁶(T/K)⁻¹; (298.15≤T/K ≤1000). The heat capacity, the first differential of {H°_m(T)−H°_m(298.15 K)} with respect to temperature, is given by C^o_{p, m}/J·K⁻¹·mol⁻¹=146.0+2.30×10⁻⁴(T/K)−3.007×10⁶(T/K)⁻². The reversible emf's of the cell, (−) Pt/{NdFeO₃(s) +Nd₂O₃(s)+Fe(s)}//YDT/CSZ//{Fe(s)‘FeO’(s)}/Pt(+), were measured in the temperature range from 1004 to 1208 K. It can be represented within experimental error by a linear equation: E/V:(0.1418±0.0003)−(3.890±0.023)×10⁻⁵(T/K). The Gibbs energy of formation of solid NdFeO₃ calculated by the least-squares regression analysis of the data obtained in the present study, and data for Fe_0.95O and Nd₂O₃ from the literature, is given by Δ_fG°_m(NdFeO₃, s)/kJ·mol⁻¹(±2.0)=−1345.9+0.2542(T/K); (1000≤T/K ≤1650). The error in Δ_fG°_m(NdFeO₃, s, T) includes the standard deviation in emf and the uncertainty in the data taken from the literature. Values of Δ_fH°_m(NdFeO₃, s, 298.15 K) and S°_m(NdFeO₃, s, 298.15 K) calculated by the second law method are −1362.5 (±6) kJ·mol⁻¹ and 123.9 (±2.5) J·K⁻¹·mol⁻¹, respectively. Based on the thermodynamic information, an oxygen potential diagram for the system Nd-Fe-O was developed at 1350 K.     

Neutron powder diffraction study of tetragonal Li7La3Hf2O12 with the garnet-related type structure

We have successfully synthesized a polycrystalline sample of tetragonal garnet-related Li-ion conductor Li₇La₃Hf₂O₁₂ by solid state reaction. The crystal structure is analyzed by the Rietveld method using neutron powder diffraction data. The structure analysis identifies that tetragonal Li₇La₃Hf₂O₁₂ has the garnet-related type structure with a space group of I4₁/acd (no. 142). The lattice constants are a=13.106(2) Å and c=12.630(2) Å with a cell ratio of c/a=0.9637. The crystal structure of tetragonal Li₇La₃Hf₂O₁₂ has the garnet-type framework structure composed of dodecahedral La(1)O₈, La(2)O₈ and octahedral HfO₆. Li atoms occupy three types of crystallographic site in the interstices of this framework structure, where Li(1) atom is located at the tetrahedral 8a site, and Li(2) and Li(3) atoms are located at the distorted octahedral 16f and 32g sites, respectively. These Li sites are filled with the Li atom. The present tetragonal Li₇La₃Hf₂O₁₂ sample exhibits bulk Li-ion conductivity of σ_b=9.85×10⁻⁷ S cm⁻¹ and grain-boundary Li-ion conductivity of σ_gb=4.45×10⁻⁷ S cm⁻¹ at 300 K. The activation energy is estimated to be E_a=0.53 eV in the temperature range of 300-580 K.     

Anchored thiol smectite clay—kinetic and thermodynamic studies of divalent copper and cobalt adsorption

A natural smectite clay sample from Serra de Maicuru, Pará State, Brazil, had aluminum and zirconium polyoxycations inserted within the interlayer space. The precursor and pillarized smectites were organofunctionalized with the silyating agent 3-mercaptopropyltrimethoxysilane. The basal spacing of 1.47 nm for natural clay increased to 2.58 and 2.63 nm, for pillared aluminum, S_Al/SH, and zirconium, S_Zr/SH, and increases in the surface area from 44 to 583 and 585 m² g⁻¹, respectively. These chemically immobilized clay samples adsorb divalent copper and cobalt cations from aqueous solutions of pH 5.0 at 298±1 K. The Langmuir, Redlich-Peterson and Toth adsorption isotherm models have been applied to fit the experimental data with a nonlinear approach. From the cation/basic center interactions for each smectite at the solid-liquid interface, by using van’t Hoff methodology, the equilibrium constant and exothermic thermal effects were calculated. By considering the net interactive number of moles for each cation and the equilibrium constant, the enthalpy, Δ_intH⁰ (−9.2±0.2 to −10.2±0.2 kJ mol⁻¹) and negative Gibbs free energy, Δ_intG⁰ (−23.9±0.1 to −28.7±0.1 kJ mol⁻¹) were calculated. These values enabled the positive entropy, Δ_intS⁰ (51.3±0.3 to 55.0±0.3 JK⁻¹ mol⁻¹) determination. The cation-sulfur interactive process is spontaneous in nature, reflecting the favorable enthalpic and entropic results. The kinetics of adsorption demonstrated that the fit is in agreement with a second-order model reaction with rate constant k₂, varying from 4.8×10⁻² to 15.0×10⁻² and 3.9×10⁻² to 12.2×10⁻² mmol⁻¹ min⁻¹ for copper and cobalt, respectively.     

NMR study of the order-disorder phase transitions of KHCO3 and KDCO3 single crystals

KHCO₃ and its deuterated analogue KDCO₃ are typical materials that undergo order-disorder phase transitions at 318 and 353 K, respectively. The spin-lattice relaxation times, T₁, spin-spin relaxation times, T₂, and the number of resonance lines for the ¹H, ²D, and ³⁹K nuclei of these crystals were investigated using NMR spectrometer. These materials are known to exhibit anomalous decreases in T₁ near T_C, which have been attributed to a structural phase transition. Additionally, changes in the symmetry of the (HCO₃)₂²⁻ (or (DCO₃)₂²⁻) dimers in these materials are associated with large changes in T₁, T₂, and the number of resonance lines. Here we found that the resonance lines for ¹H, ²D, and ³⁹K nuclei decrease in number as the temperature is increased up to T_C, indicating that the orientations of the (HCO₃)₂²⁻ (or (DCO₃)₂²⁻) dimers and the environments of the K ions change at T_C. Moreover, based on number of resonance lines, the results further indicate that the (HCO₃)₂²⁻ (or (DCO₃)₂²⁻) dimers reorientate to approximately parallel to the directions of the hydrogen bonds (or deuteron bonds) and the direction of the a-axis. The transitions at 318 and 345 K of the two crystals are of the order-disorder type. The present results therefore indicate that the orientations of the (HCO₃)₂²⁻ and (DCO₃)₂²⁻ dimers and the environment of the K ion play a significant role in these phase transitions.     

Synthesis, optical properties and electronic structures of polyoxometalates K3P(Mo1−xWx)12O40 (0?x?1)

Various compositions of solid solutions K₃P(Mo_1−xW_x)₁₂O₄₀ (0?x?1) were prepared using two solid state synthetic routes. The crystallite size was determined by linewidth refinements of X-ray diffraction patterns using the Warren-Averbach method, and the grain size distribution by laser scattering experiments. Optical properties were determined by diffuse reflectance measurements in the UV-visible range. The optical gap E_g was found to increase exponentially from ∼2.5 to ∼3.30 eV with increasing x, and is systematically shifted to a higher energy when the grain size decreases. The relation between E_g and x was analyzed by calculating the HOMO-LUMO gaps of the [P(Mo_1−xW_x)₁₂O₄₀]³⁻ anions on the basis of tight-binding electronic structure calculations.