Optical absorption edge in rare earth sesquisulfides

Optical absorption edge spectra have been measured by diffuse reflectance spectroscopy on a complete suite of rare earth sesquisulfide compounds of varying structure type. Optical band gaps were extracted from these data. The observed band gaps tend to vary with crystal structure type but many compounds with the same structure have similar band gaps independently of the particular rare earth ion. The ordered α-structure has a substantially lower band gap than the defect γ-structure for all compounds examined except for La₂S₃.     

Optical and DSC studies of the Na2B4O7-Pb3O4 glass system

A series of glass samples were prepared from admixtures of Na₂B₄O₇ and Pb₃O₄ and results are reported for optical absorption, infrared absorption spectra and differential scanning calorimetry (DSC) as a function of lead content up to 40mol%. It is found that the addition of lead oxide decreases the optical energy gap and shifts the optical absorption edges towards lower energies in the range from 4.96 to 3.29 eV. The addition of lead oxide does not seem to introduce any new absorption band as compared with the infrared spectrum of pure sodium tetraborate glass.     

Radiative recombination in CdGa<Subscript>2</Subscript>S<Subscript>4</Subscript> single crystals

We have studied the effects of light polarization and temperature on the photoluminescence spectrum of CdGa₂S₄ single crystals at temperatures from 77 to 300 K. Based on analysis of the present experimental data in conjunction with earlier band structure, optical absorption, and photoconductivity data, we have identified the mechanism of radiative recombination in CdGa₂S₄.     

UV–VUV spectroscopy of rare earth doped persistent luminescence materials

The electronic and defect energy level structure of polycrystalline SrAl₂O₄:Eu²⁺,R³⁺ persistent luminescence materials were studied with thermoluminescence and UV–VUV synchrotron radiation emission and excitation spectroscopy. Theoretical calculations using the density functional theory (DFT) were carried out simultaneously with the experimental work. The experimental band gap energy (E_g) value of 6.6 eV agrees very well with the DFT value of 6.4 eV. The 4f⁷ → 4f⁶5d¹ excitation bands of Eu²⁺ were found rather similar irrespective of the R³⁺ co-dopant. The trap level energy distribution depended strongly on the R³⁺ co-dopant except for the shallowest trap energy above the room temperature remaining the same, however. The different processes in the mechanism of persistent luminescence from SrAl₂O₄:Eu²⁺,R³⁺ was constructed and discussed.     

UV–VUV spectroscopy of rare earth doped persistent luminescence materials

The electronic and defect energy level structure of polycrystalline SrAl₂O₄:Eu²⁺,R³⁺ persistent luminescence materials were studied with thermoluminescence and UV–VUV synchrotron radiation emission and excitation spectroscopy. Theoretical calculations using the density functional theory (DFT) were carried out simultaneously with the experimental work. The experimental band gap energy (E_g) value of 6.6 eV agrees very well with the DFT value of 6.4 eV. The 4f⁷ → 4f⁶5d¹ excitation bands of Eu²⁺ were found rather similar irrespective of the R³⁺ co-dopant. The trap level energy distribution depended strongly on the R³⁺ co-dopant except for the shallowest trap energy above the room temperature remaining the same, however. The different processes in the mechanism of persistent luminescence from SrAl₂O₄:Eu²⁺,R³⁺ was constructed and discussed.     

Some optical and differential scanning calorimetry studies of sodium tetraborate glasses containing vanadium oxide

A series of glass samples was prepared from mixtures of Na₂B₄O₇ and V₂O₅. Measurements were made for infrared absorption and optical absorption spectra as a function of V₂O₅ content up to 10 mol%. The addition of V₂O₅ did not introduce any new absorption band as compared with the infrared spectrum of pure sodium tetraborate glass. The addition of V₂O₅ shifted the optical absorption edges towards lower energies, and introduced a new absorption band as compared with the optical absorption of pure Na₂B₄O₇ glass. Differential scanning calorimetry as a function of V₂O₅ content was also measured.     

Structure and luminescence of the monoclinic LnWO4Cl-type rare earth halo tungstates

GdWO₄Cl is representative of a series of LnWO₄Cl compounds where Ln can also be: Eu, Tb, Dy, Ho, Er, Tm and Y. These compounds crystallize in monoclinic C2/m symmetry, and the structure has been refined to R = 5.0% for GdWO₄Cl. Luminescing ions such as Eu⁺³ and Tb⁺³ have been incorporated into GdWO₄Cl and a Raman spectrum was recorded.     

Standard free energies of formation of rare earth sesquisulphides

An e.m.f. technique is used for the measurement of the Gibbs' energy changes involved in the conversion of the rare earth oxysulphides to the corresponding sesquisulphides in the temperature range 870 to 1120 K. The cell arrangement is Au, Ag + Ag₂SR₂O₂S + R₂S₃ (Y₂O₃)ThO₂ Fe + Fe_0.95O, Au where R = La, Pr, Nd, Sm, Eu and Gd. Yttria-doped thoria is used as the solid electrolyte because it has predominant ionic conduction at the low oxygen potentials developed at the R₂O₂S + R₂S₃ electrode, especially for systems containing heavier rare earth elements. The sulphur potential at the left-hand electrode is established by the dissociation of Ag₂S to silver in a closed system. The changes in the standard free energy, enthalpy and entropy associated with the conversion of the rare earth oxysulphide to the sesquisulphide all show a smooth variation with rare earth atomic number. Based on these empirical trends, the corresponding thermodynamic functions for terbium, dysprosium and holmium systems are estimated. The measured free energy values are combined with recent data on the standard free energies of formation of the rare earth oxysulphides to obtain the Gibbs' energies of formation of the corresponding sesquisulphides.     

Position of the optical absorption edge of alkaline earth borates

Based on the results of luminescent measurements on MB₂O₄ (M = Ca, Sr), Ca₂B₅O₉Cl, SrB₆O₁₀, SrAl₂B₂O₇ and the literature data, the influence of the crystal structure of alkaline earth borates on the position of their optical absorption edge is discussed. It is found that the optical absorption edge tends to shift to higher energy from β-BaB₂O₄ (6.4 eV) to M₃(BO₃)₂, MB₂O₄ (6.9–7.2 eV) to SrB₆O₁₀ (7.7 eV), SrB₄O₇ (>7.8 eV). It is shown that this tendency can be interpreted as a result of (a) the removal of M²⁺ states from the top of the valence band; (b) a more efficient stabilization of the O 2p states by B–O covalent bonding with increasing condensation degree of the borate anions.     

A comparative study of the properties of thermally evaporated CuIn2n + 1S3n + 2 (n = 0, 1, 2 and 3) thin films

CuInS₂, CuIn₃S₅, CuIn₅S₈ and CuIn₇S₁₁ compounds were synthesized by the horizontal Bridgman method using high-purity copper, indium and sulphur elements. Crushed powders of these ingots were used as raw materials for the vacuum thermal evaporation. So, CuIn_2n + 1S_3n + 2 (n = 0, 1, 2, and 3) thin films were deposited by single source vacuum thermal evaporation onto glass substrates heated at 150 °C. The structural, compositional, morphological, electrical and optical properties of the deposited films were studied using X-ray diffraction (XRD), energy dispersive X-ray, atomic force microscopy and optical measurement techniques. XRD results revealed that all the films are polycrystalline. However, CuInS₂ and CuIn₃S₅ films had a chalcopyrite structure with preferred orientation along 112 while CuIn₅S₈ and CuIn₇S₁₁ films exhibit a spinel structure with preferred orientation along 311. The absorption coefficients of the all CuIn_2n + 1S_3n + 2 films are in the range of 10⁻⁴ and 10⁻⁵ cm⁻¹. The direct optical band gaps of CuIn_2n + 1S_3n + 2 layers are found to be 1.56, 1.78, 1.75 and 1.30 eV for n = 0, 1, 2, and 3, respectively. CuIn₃S₅ and CuIn₅S₈ films are p type with electrical resistivities of 4 and 12 Ω cm whereas CuInS₂ and CuIn₇S₁₁ are highly compensated with resistivities of 1470 and 1176 Ω cm, respectively.     

Structural study and electronic band structure investigations of the solid solution NaxCu1 − xIn5S8 and its impact on the Cu(In,Ga)Se2/In2S3 interface of solar cells

The present work reports investigations on the new In₂S₃ containing Cu and/or Na compounds, which are expected to be formed at the Cu(In,Ga)Se₂/In₂S₃ interface. The knowledge of these materials properties is very important in order to better understand the operation of the devices based on these junction partners.It has been observed that a solid solution Na_xCu_1 − xIn₅S₈ exists from CuIn₅S₈ (x = 0) to NaIn₅S₈ (x = 1) with a spinel-like structure. The single crystal structure determination shows that indium, copper and sodium atoms are statistically distributed on the tetrahedral sites.XPS investigations on the CuIn₅S₈, Na_0.5Cu_0.5In₅S₈ and NaIn₅S₈ compounds combined with the band gap changes reported in a previous work show that these variations are mainly due to valence band maximum shift; it is moved downward when x increases from 0 to 1. These observations are confirmed by the electron structure calculations based on the density functional theory, which additionally demonstrate that the pure sodium compound has direct gap whereas the copper-containing compounds have indirect gaps.     

Effect of rare earth dopants on the morphologies and photocatalytic activities of BiFeO3 microcrystallites

Pure and rare earth elements (La, Yb)-doped BiFeO₃ microcrystallites were synthesized by a typical hydrothermal process. The crystal structure, morphologies and photocatalytic activities of these microcrystallites were investigated. The results and analysis revealed that substitution of La and Yb not only changed the energy band gap of BiFeO₃ system, but also affected the morphologies and dimensions of BiFeO₃ microcrystallites. Despite much smaller particle size compared with pure BiFeO₃, the Yb-doped BiFeO₃ microcrystallites exhibited lower photocatalytic efficiency due to much larger energy band gap, which suggests the energy band configuration intensely influences the photocatalytic activity rather than the particle size.     

Photoelectrochemical investigation on spray depositedn-CdIn2S4 thin films

Polycrystalline thin films ofn-CdIn₂S₄ have been spray deposited onto amorphous and fluorinedoped tin oxide (FTO) coated glass substrates at the optimized substrate temperature of 380°C. The films were characterized by X-ray diffraction (XRD) and optical absorption studies. XRD studies revealed that the films were polycrystalline with spinel cubic structure. The optical absorption studies showed the band gap energy to be 2·14 eV. Photoelectrochemical (PEC) investigations were carried out using cell configurationn-CdIn₂S₄/1 M NaOH+1 M Na₂S+1 M S/C. Using Butler model, the optical band gap and minority carrier diffusion length (L _P) were found to be 2·22 eV and 0·07 μm, respectively. Gartner’s model was used to calculate the minority carrier diffusion length and the donor concentration (N _D) for CdIn₂S₄ films at three different wavelengths.N _D was found to be of the order of 10¹⁶ cm⁻³.     

Precision lattice constants of the rare earth sulfofluorides of the type LnSF

All LnSF compounds for Ln=La to Lu (including Y) have been prepared by the interaction of H₂S and the appropriate LnF₃ rare earth fluoride between 900 and 1000°C. Only EuSF could not be prepared this way and the new Eu⁺² defect sulfofluoride of the type Euф_0.5S_0.5F was obtained. Using this technique the LnSF compounds to and including Ho are tetragonal, space group P4/nmm, whereas those of Er to Lu are hexagonal, space group P6₃22. Refined cell constants of all compounds are reported.     

Structural,thermodynamical and optical properties of Cu<Subscript>2</Subscript>-II-IV-VI<Subscript>4</Subscript> quaternary compounds

The Cu₂-II-IV-VI₄ compounds (II = Zn, Cd; IV = Si, Ge, Sn; VI = S, Se, Te) with a tetrahedral coordinated structure have been investigated, as compared with the results of the products synthesized from respective elemental mixtures. These crystal structures and melting points are determined, using differential thermal analysis and powder X-ray diffraction, respectively. The optical band gaps of these balk crystals grown by the horizontal gradient freezing or Iodine transport method are also measured by optical absorption. These melting points, lattice constants and band gaps are found to vary linearly with increasing of mean atomic weight, and can be established from the empirical equations.     

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.     

Band gaps and defect levels in functional oxides

Most ab-initio calculations of the electronic structure use the local density approximation, which gives good structural data but severely underestimates the band gaps of semiconductors and insulators. This paper presents calculations of the band structures and oxygen vacancy levels of some important oxide semiconductors and insulators, using density functional methods which do give more accurate band gaps. The materials SnO₂, Cu₂O, SrCu₂O₂, CuAlO₂, SrTiO₃, HfO₂, ZrO₂, La₂O₃, ZrSiO₄, and SiO₂ are covered.     

Electronic structure and photocatalytic water splitting of lanthanum-doped Bi2AlNbO7

Bi_2−xLa_xAlNbO₇ (0 ≤ x ≤ 0.5) photocatalysts were synthesized by the solid-state reaction method and characterized by powder X-ray diffraction (XRD), infrared (IR) spectra and ultraviolet-visible (UV-vis) spectrophotometer. The band gaps of the photocatalysts were estimated from absorption edge of diffuse reflectance spectra, which were increased by the doping of lanthanum. It was found from the electronic band structure study that orbitals of La 5d, Bi 6p and Nb 4d formed a conduction band at a more positive level than Bi 6p and Nb 4d orbitals, which results in increasing the band gap. Photocatalytic activity for water splitting of Bi_1.8La_0.2AlNbO₇ was about 2 times higher than that of nondoped Bi₂AlNbO₇. The increased photocatalytic activity of La-doped Bi₂AlNbO₇ was discussed in relation to the band structure and the strong absorption of OH groups at the surface of the catalyst.     

Crystal field of rare earth impurities in LaF3

The crystal field parameters of 13 trivalent lanthanide ions substituted for La in LaF₃ were calculated using the combination of the band structure and Wannier function calculations. Performing an atomic exact diagonalization with thus obtained crystal-field parameters we compute the crystal-field splitting of atomic multiplets. The calculation is compared with the available experimental results and a good agreement is found.     

Luminescence properties of Ce3+ and Tb3+ in a new family of boron-rich alkaline earth rare earth borates

A novel family of borates formulated MLnB₉O₁₆ (M = SrorBa, Ln = rareearth), is reported. Under UV excitation BaLaB₉O₁₆:Ce³⁺ shows an emission band peaking at 350 nm. Its quantum efficiency is almost independent of Ce concentration. Energy transfer from Ce³⁺ to Tb³⁺ is efficient for the composition BaCe_1−xTb_xB₉O₁₆ when x exceeds 0.3, owing to the overlap of the Ce³⁺ emission band with the most intense f-f Tb³⁺ absorption lines. The Tb³⁺ emission sensitized by Ce³⁺ shows a very intense green color; its stability in a mercury vapor lamp has been checked.