Substitution of Li for Cu in Cu2ZnSnS4: Toward Wide Band Gap Absorbers with Low Cation Disorder for Thin Film Solar Cells.

The substitution of lithium for copper in Cu2ZnSnS4 (CZTS) has been experimentally and theoretically investigated. Formally, the (Cu1-xLix)ZnSnS4 system exhibits two well-defined solid solutions. Indeed, single crystal structural analyses demonstrate that the low (x < 0.4) and high (x > 0.6) lithium-content compounds adopt the kesterite structure and the wurtz-kesterite structure, respectively. For x between 0.4 and 0.6, the two aforementioned structure types coexist. Moreover, 119Sn NMR analyses carried out on a (Cu0.7Li0.3)2ZnSnS4 sample clearly indicate that lithium replaces copper preferentially on two of the three available 2-fold crystallographic sites commonly occupied by Cu and Zn in disordered kesterite. Furthermore, the observed individual lines in the NMR spectrum suggest that the propensity of Cu and Zn atoms to be randomly distributed over the 2c and 2d crystallographic sites is lowered when lithium is partially substituted for copper. Additionally, the first-principles calculations provide insights into the arrangement of Li atoms as a function of the Cu/Zn disorder and its effect on the structural (lattice parameters) and optical properties of CZTS (band gap evolution). Those calculations agree with the experimental observations and account for the evolutions of the unit cell parameters as well as for the increase of band gap when the Li-content increases. The calculation of the formation enthalpy of point defect unambiguously indicates that Li modifies the Cu/Zn disorder in a manner similar to the change of Cu/Zn disorder induced by Ag alloying. Overall, it was found that Li alloying is a versatile way of tuning the optoelectronic properties of CZTS making it a good candidate as wide band gap materials for the top cells of tandem solar cells.

Structure flexibility of the Cu2ZnSnS4 absorber in low-cost photovoltaic cells: from the stoichiometric to the copper-poor compounds.

Here we present for the very first time a single-crystal investigation of the Cu-poor Zn-rich derivative of Cu(2)ZnSnS(4). Nowadays, this composition is considered as the one that delivers the best photovoltaic performances in the specific domain of Cu(2)ZnSnS(4)-based thin-film solar cells. The existence of this nonstoichiometric phase is definitely demonstrated here in an explicit and unequivocal manner on the basis of powder and single-crystal X-ray diffraction analyses coupled with electron microprobe analyses. Crystals are tetragonal, space group I ̅4, Z = 2, with a = 5.43440(15) Šand c = 10.8382(6) Šfor Cu(2)ZnSnS(4) and a = 5.43006(5) Šand c = 10.8222(2) Šfor Cu(1.71)Zn(1.18)Sn(0.99)S(4).

Bis(butane-1,4-diammonium) di-µ-oxido-bis[trifluoridooxidomolybdate(V)] monohydrate.

The title compound, (C4H14N2)2[Mo2O4F6]·H2O, was obtained by solvothermal reaction at 443 K for 72 h from a mixture of MoO3, HF, 1,4-diamino-butane (dab), water and ethyl-ene glycol. The structure consists of [Mo2O4F6](4-) anionic dimers containing strongly distorted MoO3F3 octa-hedra (with twofold symmetry), diprotonated dab cations and water mol-ecules (twofold symmetry) in the ratio 1:2:1. The cohesion of the three-dimensional structure is ensured through N-H⋯O, N-H⋯F and O-H⋯F inter-actions.

A new organic-inorganic hybrid oxyfluorotitanate [Hgua]2·(Ti5O5F12) as a transparent UV filter.

A new generation UV absorber is obtained by microwave-heating-assisted hydrothermal synthesis: [Hgua](2)·(Ti(5)O(5)F(12)). The structure of this hybrid titanium(IV) oxyfluoride is ab initio determined from powder X-ray data by combining a direct space method, Rietveld refinement [orthorhombic, Cmm2, a = 22.410(1) Å, b = 11.191(1) Å, c = 3.802(1) Å], and density functional theory geometry optimization. The three-dimensional network is built up from infinite inorganic layers (∞)(Ti(5)O(5)F(12)) separated by guanidinium cations. The theoretical optical gap (3.2 eV) estimated from density of state calculations is in good agreement with the experimental gap (3.3 eV) obtained by UV-vis diffuse reflectivity. The optical absorption is mainly due to O(2p) → Ti(3d) and F(2p) → Ti(3d) transitions at higher energies. The refraction index is low in the visible range (n ≈ 1.9) compared to that of TiO(2) and, consequently, [Hgua](2)·(Ti(5)O(5)F(12)) shows a good transparency adapted to UV shielding. Under UV irradiation at 254 nm for 40 h, the white microcrystalline powder turns to light purple-gray. This color change is caused by the reduction of Ti(IV) to Ti(III), confirmed by magnetic measurements.

Investigation of the first-order phase transition in the Co(1-x)Mg(x)MoO4 solid solution and discussion of the associated thermochromic behavior.

A series of compounds of Co(1-x)Mg(x)MoO(4) compositions has been prepared by a conventional ceramic route. The members of the whole solid solution exhibit a reversible first-order phase transition which was probed by using thermal expansion and low-temperature reflectivity techniques. Whereas the α → ß transition temperature evolves linearly on warming from 435 to 200 °C with x going from 0 to 0.9, the ß â†’ α transition temperature variation falls down on cooling from -40 °C to -140 °C going from CoMoO(4) to Co(0.1)Mg(0.9)MoO(4) with an asymptotic evolution. The phase transition temperatures have been explained on the basis of a crystal polarization effect under substitution of Mg for Co. Thus, from an applicative point of view, new thermochromic pigments with tunable transition temperatures are here proposed.

Short-range magnetic order and temperature-dependent properties of cupric oxide.

The temperature dependence of the optical and magnetic properties of CuO were examined by means of hybrid density functional theory calculations. Our work shows that the spin exchange interactions in CuO are neither fully one-dimensional nor fully three-dimensional. The temperature dependence of the optical band gap and the (63)Cu nuclear quadrupole resonance frequency of CuO originate from the combined effect of a strong coupling between the spin order and the electronic structure and the progressive appearance of short-range order with temperature.

Characterization of the piezochromic behavior of some members of the CuMo1-xWxO4 series.

The members of the CuMo(1- x)WxO4 series (0 < or = x < 0.1) undergo a first-order phase transition that can be induced by pressure application; the thermochromic properties of such a series have already been reported. The two polymorphic forms exhibit two distinguishable colors: green for the low pressure form (alpha) and brownish-red for the high pressure one (gamma). These oxides can open up a new market for friendly pressure indicators, particularly for the compositions (0.07 < or = x < or = 0.1) for which the two polymorphs are stable at room temperature, that is, for which the color transition via pressure application is nonreversible. Within the CuMo(1- x)WxO4 solid solution domain, the dependence of the transition pressure versus tungsten content, temperature of measurement, and sample thermal-pressure history was studied. A large control of the transition pressure (from 5 to several 100 MPa) was brought to the fore. The transition was then studied using X-ray diffraction and transmission electron microscopy-energy dispersive X-ray analyses. This first-order transition, occurring by atomic migration inside the cell, seems to be preceded by an atomic disordering; moreover, transition temperatures may be modified by W segregation at the surface of the grains.

Adaptable thermochromism in the CuMo1-xWxO4 series (0

The members of the CuMo1-xWxO4 series (0alpha transition can occur between 260 and 360 K, and the alpha-->gamma transition between 175 and 275 K as a function of x. The control of the alpha/gamma transition temperatures with x is related to the larger propensity of tungsten compared to molybdenum, to adopt a tetrahedral environment.

Analysis of the spin exchange interactions and the ordered magnetic structures of lithium transition metal phosphates LiMPO4 (M = Mn, Fe, Co, Ni) with the olivine structure.

The olivine-type compounds LiMPO4 (M = Mn, Fe, Co, Ni) consist of MO4 layers made up of corner-sharing MO6 octahedra of high-spin M2+ ions. To gain insight into the magnetic properties of these phosphates, their spin exchange interactions were estimated by spin dimer analysis using tight binding calculations and by electronic band structure analysis using first principles density functional theory calculations. Three spin exchange interactions were found to be important for LiMPO4, namely, the intralayer superexchange J1, the intralayer super-superexchange Jb along the b-direction, and the interlayer super-superexchange J2 along the b-direction. The magnetic ground state of LiMPO4 was determined in terms of these spin exchange interactions by calculating the total spin exchange interaction energy under the classical spin approximation. In the spin lattice of LiMPO4, the two-dimensional antiferromagnetic planes defined by the spin exchange J1 are antiferromagnetically coupled by the spin exchange J2, in agreement with available experimental data.

Experimental and theoretical investigation on the relative stability of the PdS2- and pyrite-type structures of PdSe2.

Under ambient condition PdSe2 has the PdS2-type structure. The crystal structure of PdSe2 under pressure (up to 30 GPa) was investigated at room temperature by X-ray diffraction in an energy-dispersive configuration using a diamond anvil cell with a mixture of water/ethanol/methanol as a pressure transmitting medium. A reversible structural transition from the PdS2-type to the pyrite-type structure occurs around 10 GPa, and the applied pressure reduces the spacing between adjacent 2/proportional to [PdSe2] layers of the PdS2-type structure to form the three-dimensional lattice of the pyrite-type structure. First principles and extended Hückel electronic band structure calculations were carried out to confirm the observed pressure-induced structural changes. We also examined why the isoelectronic analogues NiSe2 and PtSe2 adopt structures different from the PdS2-type structure on the basis of qualitative electronic structure considerations.

On the origin of the square root of (7) x square root of (7) superstructure and the anomalous magnetic and transport properties of the layered compound Sr(6)V(9)S(22)O(2).

We examined why the 1T-VS(2) layer of the layered compound Sr(6)V(9)S(22)O(2) has the x superstructure in terms of electronic band structure calculations and metal-metal bonding across the shared edges of adjacent VS(6) octahedra. On the basis of this analysis we explored how the anomalous magnetic and transport properties of Sr(6)V(9)S(22)O(2) can be explained. Our work shows that the x superstructure is not caused by a charge density wave instability associated with Fermi surface nesting but by the metal-metal bonding through the shared edges of adjacent VS(6) octahedra. The weak and strong electron localizations observed for Sr(6)V(9)S(22)O(2) were discussed in terms of three-center two-electron and two-center two-electron V-V bonds in the 1T-VS(2) layers.

Synthesis and crystal structure determination of a new pressure-induced iridium ditelluride phase, m-IrTe2, and comparison of the crystal structures and relative stabilities of various IrTe2 polymorphs.

The new monoclinic IrTe2 phase m-IrTe2 was synthesized under pressure, and its structure was determined by X-ray powder diffraction. The relative stabilities of the three known and three hypothetical IrTe2 polymorphs were discussed on the basis of tight binding electronic band structure calculations. m-IrTe2 exhibits structural features of both CdI2- and pyrite-type IrTe2 phases and is expected to be nearly as stable as that of the CdI2-type IrTe2. The hypothetical IrS2- and ramsdellite-type IrTe2 phases are predicted to be more stable than the CdI2-type IrTe2.

K(9)CeP(4)S(16), a new thiophosphate of cerium with discrete [Ce(PS(4))(4)](9-) anions.

The structure of nonapotassium cerium tetraphosphorus hexadecasulfide, a zero-dimensional material isostructural with Rb(9)CeP(4)Se(16), is reported.