Search

Different compounds in the Cu{sub 2}ZnSnS{sub 4}–Cu{sub 2}ZnSiS{sub 4} system have been prepared via ceramic route and structurally characterized via X-ray diffraction on powders and single crystals. Two solid solutions were identified along the Cu{sub 2}Zn(Sn,Si)S{sub 4} series. Namely, materials with Si-content x=Si/(Sn+Si) lower than 0.5 crystallize with the Cu{sub 2}ZnSnS{sub 4} kesterite structure type while materials with x higher than 0.8 adopt the Cu{sub 2}ZnSiS{sub 4} enargite structure type. In between, a miscibility gap occurs where the Cu{sub 2}ZnSn{sub 0.5}Si{sub 0.5}S{sub 4} and Cu{sub 2}ZnSn{sub 0.2}Si{sub 0.8}S{sub 4} compounds co-exist. The optical bandgap increases continuously with the Si content in the whole series. This opens up the possibility to fine tune the absorption threshold and to adjust it to 1.7 eV for x∼0.5, the optimum value for the top cell of tandem solar devises to achieve high photovoltaic conversion efficiency. - Graphical abstract: Two solid solutions have been pointed out in the Cu{sub 2}Zn(Sn{sub 1−x}Si{sub x})S{sub 4} series with the kesterite and the enargite type structures. - Highlights: • New compounds in Cu{sub 2}Zn(Sn,Si)S{sub 4} series have been prepared. • Two solid solutions Cu{sub 2}ZnSn{sub 1−x}Si{sub x}S{sub 4} were identified for x≤0.5 and x≥0.8. • In the miscibility gapmore » 2 phases co-exist with kesterite and enargite structure types. • The optical bandgap increases continuously with the Si-content in the whole series. • These materials could be envisioned as absorber in thin-film solar cells.« less

Two new compounds, La5Ti2MS5O7 (M=Cu, Ag) were synthesized and their structures solved from single crystal X-ray data. Both compounds are isotypic. They crystallize in the orthorhombic system (space group Pnma, Z=4) with lattice constants a=19.423(1) A, b=3.9793(2) A, c=18.1191(9) A for La5Ti2CuS5O7, and a=19.593(2) A, b=3.9963(1) A, and c=18.2973(15) A for La5Ti2AgS5O7. The structure of these compounds is built from fragments of the rock-salt, perovskite and fluorite types and a clear anionic segregation of the anions appears in the structure. La5Ti2CuS5O7 and La5Ti2AgS5O7 exhibit an orange-yellow color and measurement of their optical band gap gave 2.02 and 2.17 eV, respectively.

A new phase Sm2Ti2S2O4.5 was synthesized and its crystal structure was solved by single-crystal X-ray diffraction. This compound crystallizes in the monoclinic system (C2/m) with lattice constants a=17.9987(11) A, b=3.71607(14) A, c=12.6172(8) A and β=133.645(4)°. The structure is built up from double chains of Ti-centered octahedra between which Sm-polyhedra develop. In spite of very close formulations, the structure of Sm2Ti2S2O4.5 differs completely from that of the defect Ruddelsden–Popper phase Sm2Ti2S2O5 previously reported. The title compound presents a mixed valence for titanium with Ti(III) (d1) and Ti(IV) (d0) located in different crystallographic sites. However, conductivity measurements show that this compound is non-metallic.

Crystal structures of two new misfit compounds, [SrGd 0.5 S 1.5 ] 1.16 NbS 2 and [Sr(Fe,Nb) 0.5 S 1.5 ] 1.13 NbS 2 , were determined through the composite approach, i.e., by refining each subpart ( Q , H -parts, and the common part) of these composite materials, separately. The Q -part is a three-atom-thick layer, with the NaCl-type structure, where external SrS planes enclose the inner GdS or (Fe,Nb)S plane; the structural difference between these two compounds lies in the central layer within the Q -part: Gd and S atoms are in special positions (octahedral coordination), while Fe and S atoms are statistically distributed on split (×4) positions (tetrahedral coordination) around a central unique site (=special position occupied by Nb). The H -part is a sandwich of sulfur planes enclosing the inner Nb plane as observed for the structure of the binary compound NbS 2 itself. The Sr–Gd derivative shows a paramagnetic behavior in the whole studied temperature range (2–300 K). On the other hand, antiferromagnetic interactions occur in the Sr–Fe derivative; the complex magnetic behavior of this compound is related to the statistical distribution of Fe atoms which leads to frustration of the magnetic interactions. At room temperature, experimental values obtained from Mossbauer spectrum correspond to Fe 3+ in tetrahedral sulfur environment: isomer shift δ=0.32 mm s −1 , and quadrupole splitting Δ E =0.48 mm s −1 .

A new compound Gd 4 TiSe 4 O 4 has been obtained by a solid state reaction of Gd 2 O 3 , TiO 2 , and TiSe 2 , at T =1000°C, with the addition of iodine to favor crystallization. Single-crystal X-ray diffraction measurements show that Gd 4 TiSe 4 O 4 crystallizes in space group C 2/ m (No. 12) of the monoclinic system with two formula units in a cell of dimensions a =15.7878(4) A, b =3.7610(2) A, c =9.6639(3) A, and β =117.569(2)°. The structure contains two crystallographically independent Gd atoms with a bicapped trigonal prismatic coordination. Association of four adjacent Gd-centered polyhedra constitutes the elemental building unit, which develops into ribbons along b . The ribbons are interconnected by infinite chains of edge-sharing Ti-centered octahedra. Magnetic susceptibility of the title compound has been measured from 1.8 to 300 K, and from B =1.10 −3 to 0.1 T for the ZFC and FC cooling modes. The linear variation of the inverse susceptibility versus T can be modeled with a Curie–Weiss law in agreement with a Gd 3+ free ion behavior. The maximum of the χ (T) curve at about 2.5 K could be attributed to both an antiferromagnetic transition and the saturation of the weak ferromagnetic component.

The new layered misfit sulfide of the 1.5Q/1H homologue type, (Pb(Mn, Nb)0.5S1.5)1.15NbS2, has been synthesized. Electron probe microanalysis gives the chemical formula (Pb1.02Mn0.33Nb0.16 S1.50)1.15 NbS2, indicating Nb incorporation in the rock-salt-type part (Q layer). Its crystal structure has been solved through a superspace approach: orthorhombic symmetry, superspace group Cm2m(α00), cell parameters a=3.326(1), b=5.788(1), c=14.326(3) A, q vector (0.5744(2),0,0); R=2.27% for 688 reflections and 36 variables. Nb-for-Mn substitution in the central part of the Q layer is clearly proved. Bond valence calculation on this compound, compared to some other misfit compounds, makes it possible to evaluate the charge transfer that governs the stability of such composite materials. This calculation, which takes into account the modulation of Pb–SH bondings at the Q/H interface through the superspace approach, shows that the transfer depends on the nature of cations in the Q layer. This leads to a formal oxidation state for Nb, within the H layer, varying between ∼3 and 3.6.

The distribution of the Pb and Sb atoms in the (Pb, Sb)S layers of the misfit compound [(Pb, Sb)S]2.28NbS2 was investigated by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). This compound has NaCl-type MS (M=Pb, Sb) layers (Q-layers) alternating with NbS2 layers (H-layers) in which the Nb atoms are located in slightly distorted trigonal prism sites. Both the H- and Q-layers of [(Pb, Sb)S]2.28NbS2 were imaged by STM and AFM. The atomic-scale STM and AFM images were analyzed in terms of the partial and total electron density plots calculated for the Q- and H-layers. The STM images of the Q-layers allowed us to detect all the constitutive atoms of the Q-layers with a short-range ordering of pure Pb and Sb rows in the aQ modulation direction.

The complete structure of the misfit layer compound [(EuS) 1.5 ] 1.15 NbS 2 is described as a (3+1)-dimensional intergrowth compound with two modulated subsystems ( ν =1, 2, for NbS 2 and EuS, respectively) which have the (a * ν 2 , a * ν 3 ) reciprocal-lattice plane in common. A superspace embedding follows from four reciprocal-lattice vectors given by a * 1 =a * 11 , a * 2 =a * 12 , a * 3 =a * 23 , and a * 4 =a * 21 . The symmetry is given by the superspace group G s = Fm 2 m ( α 00) with modulation wave vector q=a * 21 = α a * 11 ( α =0.5768(1)). The structure of this composite compound is built up of an alternated sequence of two different types of slabs (i.e., NbS 2 ( H -part) and EuS ( Q -part)), stacked along the c direction. The Q -part is a three-atom-thick layer (Eu2S2-Eu1S1-Eu2S2). The NbS 2 subsystem has cell parameters a 11 =3.3241(4) A, a 12 =5.7841(5) A, and a 13 =14.816(2) A, with space group Cm 2 m . The EuS subsystem has lattice parameters a 21 =5.7628(5) A, a 22 =5.7841(5) A, and a 23 =29.632(4) A, with space group Fm 2 m . Refinements on 1721 unique reflections converged to R =3.58% ( R w =4.75%) on a modulated structure model with 50 parameters. The largest modulation amplitudes occur on the outer Eu2 atoms. A bond-valence calculation using results from this structure determination shows that Eu atoms on the exterior sides (Eu2) of the EuS slab are in the +II oxidation state while inner Eu atoms (Eu1) are at the +III state. Mossbauer and magnetic measurements confirm the mixed-valence (Eu 2+ /Eu 3+ ) state of this compound. A ferromagnetic long-range order appears below 7 K.

A crystal structure study of the layered misfit compound (Pb 2 FeS 3 ) 0.58 NbS 2 has been performed using the composite approach. The structure can be described on the basis of a regular alternation of [Pb 2 FeS 3 ] and [NbS 2 ] slabs stacked along the c direction. The [Pb 2 FeS 3 ] part has a thickness of three layers, the central ∣FeS∣ layer being sandwiched between the two external ∣PbS∣ layers. Thus, Pb atoms on the exterior sides of this part are in a squared-pyramidal sulfur coordination, while Fe atoms in the central zone are octahedrally and tetrahedrally surrounded by sulfur atoms. From bond valence calculations, Fe atoms are found in the +III oxidation state whatever their environment. This is corroborated by a Mossbauer study. Magnetic and electrical properties complete this study.

New compounds La M S 3 with NH 4 CdCl 3 -type crystal structure could be prepared by high-pressure synthesis where M =Ti, V, Cr. The high-pressure compounds were obtained above 2.5 GPa at 1375 K for M =Cr, above 3.5 GPa at 1375 K for M =Ti, and above 5 GPa at 1475 K for M =V, respectively. The crystal structure of LaCrS 3 was refined as NH 4 CdCl 3 -type structure. It has an orthorhombic crystal lattice ( Pnma ) with a =0.787(1), b =0.3851(7), and c =1.334(2) nm. The polycrystalline product of LaCrS 3 was semiconducting and antiferromagnetic with T N =195 K and μ eff =3.83 μ B .

Sm 3 NbSe 3 O 4 has been synthesized and its structure determined. The following crystal data were obtained: M r =844.84 g·mol −1 , orthorhombic, Pnma , a =6.8943(4) A, b =7.7529(8) A, c =14.7644(12) A, V =789.2(1) A 3 , Z =4, D x =7.111 g·cm −3 , Mo Kα , λ =0.71073 A, μ =372 cm −1 , F (000)=1444, T =295 K, R =0.035 for 1466 unique reflections with F o >4 σ ( F o ), 62 variables, GoF=0.818. The structure of Sm 3 NbSe 3 O 4 was determined by single-crystal X-ray diffraction. Two distinct types of polyhedra can be distinguished: a very distorted one with eight surrounding atoms (four O and four Se atoms) around Sm1 and Nb, and a bicapped trigonal prismatic one around Sm2. The crystal structure of this new compound can be described on the basis of corrugated planes perpendicular to the c axis. These planes are built up from edge-sharing trigonal prismatic polyhedra (Sm2); connection between successive planes is achieved through the distorted polyhedra which surround atoms Sm1 and Nb.

The structure of Sm2/3Cr2S4has been solved from single-crystal X-ray diffraction data. This compound presents a 3 ×asuperstructure alongaand an incommensurate superstructure alongbclose to 3 ×b. The basic structure is described in the space groupPmnbwitha= 3.5030(3) A,b= 10.9199(8) A,c= 12.7987(8) A,Z= 4,R= 0.046 for 916 observed reflections. The modulated structure refinement, taking into account the two superstructures, was carried out in a (3 + 2)D superspace group and leads toR= 0.063 for 1706 reflections. This structure consists of staggered double columns of [CrS6] octahedra connected by samarium atoms in a trigonal prismatic bicapped environment. This structure is reminiscent of that found in the lillianite mineral family. The statistical occupancy of samarium atoms (1/3), in agreement with the charge equilibrium (Sm3+, Cr3+, S2−), is related to the superstructure.

The crystal structure of the new misfit layer compound (NdS)[sub 1.18]NbS[sub 2] has been determined by a composite approach which leads to average structures and their relative arrangement. (NdS)[sub 1.18]NbS[sub 2] is built of [vert bar]NdS[vert bar] double layers (approximately a [l brace]100[r brace] slice of NaCl type) alternating with [vert bar]NbS[sub 2][vert bar] sandwiches in which an Nb atom is surrounded by a trigonal prismatic arrangement of S atoms. Both subsystems ([vert bar]NdS[vert bar] and [vert bar]NbS[sub 2][vert bar]) are F centered (orthorhombic space group Fm2m). Resistivity measurements (300-5 K) indicate metallic behavior attributed to the presence of an almost filled 4dz[sup 2] (Nb) band of the [vert bar]NbS[sub 2][vert bar] part. The inverse magnetic susceptibility as a function of temperature shows linear variation over the whole temperature range (300-4.2 K). An effective moment of [mu][sub exp] = 3.7 [mu][sub B] is deduced, very close to the theoretical Nd[sup 3+] free ion value ([mu][sub theor] = 3.62 [mu][sub B]). A large magnetic anisotropy is observed, as made evident by the magnetization curves measured at 1.85 K with a magnetic field applied parallel and perpendicular to the crystal plane ((a, b) plane). 13 refs., 5 figs., 5 tabs.