M. Evain

Comparative study of some rare earth sulfides: doped γ-[A]M2S3 (M = La, Ce and Nd, A = Na, K and Ca) and undoped γ-M2S3 (M = La, Ce and Nd)

Abstract The crystal structure determination of γ-M2S3 compounds (M = La, Ce, and Nd) has been carried out for the first time from single crystals obtained through high-temperature melting under sulfur pressure. The three phase structures do not depart from the cubic Th3P4 structural type, with a statistical filling of the dodecahedral sites by the cations. The γ-Na0.5Ce2.5S4-doped phase structure has also been determined from a powder neutron diffraction study. Na+ was found to be located at the dodecahedral site, in agreement with the composition limit of Na/Ce = 0.20 as determined by cell parameter variation versus composition. A powder X-ray diffraction study of the potassium- and calcium-doped derivatives (γ-K0.46C2.54S4 and γ-Ca0.89Ce2.07S4) confirmed the results obtained for the sodium-doped phase. In no case, at least in the phases studied, does the alkali or alkaline earth metal occupy the inter-dodecahedral tetrahedral sites. The electronic band structures of Ce2S3 and of Ce3−xS4 ( 0 1 3 ) indicate an insulating behavior for the former compound and a metallic behavior for the latter, assuming in this case a rigid band model. In Ce3S4, the electronic conductivity takes place along the CeCe bonds. No SS bonding was found in the two binaries. It seems possible to assign the color of some γ-M2S3 materials to electronic transitions to the conduction band from (i) the valence band (La2S3), (ii) the 4f level (Ce2S3) and (iii) the 4f and valence band (Nd2S3).

X-ray resonant single-crystal diffraction technique, a powerful tool to investigate the kesterite structure of the photovoltaic Cu2ZnSnS4 compound

Cu/Zn disorder in the kesterite Cu2ZnSnS4 derivatives used for thin film based solar cells is an important issue for photovoltaic performances. Unfortunately, Cu and Zn cannot be distinguished by conventional laboratory X-ray diffraction. This paper reports on a resonant diffraction investigation of a Cu2ZnSnS4 single crystal from a quenched powdered sample. The full disorder of Cu and Zn in the z = 1/4 atomic plane is shown. The structure, namely disordered kesterite, is then described in the I42m space group.

Determination of the modulated structure of Sr14/11CoO3 through a (3 + 1)-dimensional space description and using non-harmonic ADPs.

Sr(14/11)CoO(3) (i.e. Sr(14)Co(11)O(33), tetradecastrontium undecacobalt tritriacontaoxide), a new phase in the hexagonal perovskite Sr(x)CoO(3) system, has been prepared and its structure solved from single-crystal X-ray data within the (3 + 1)-dimensional formalism. Sr(14/11)CoO(3) crystallizes in the trigonal symmetry, R3;m(00gamma)0s superspace group with the following lattice parameters: a(s) = 9.508 (2), c(s) = 2.5343 (7) Å, q = 0.63646 (11)c(*) and V(s) = 198.40 (13) Å(3). With the commensurate versus incommensurate test not being conclusive, the structure was considered as commensurate (P32 three-dimensional space group), but refined within the (3 + 1)-dimensional formalism to a residual factor R = 0.0351 for 47 parameters and 1169 independent reflections. Crenel functions were used for the oxygen and cobalt description and a Gram-Charlier expansion up to the third order of the atomic displacement parameter was employed for one Co atom. The structure is similar to that of Sr(6/5)CoO(3), but with a different sequence of the octahedra and trigonal prism polyhedra along the [CoO(3)] chains. An interesting feature evidenced by the non-harmonic expansion is the displacement of the prismatic Co atoms from the site center, towards the prism rectangular faces.

Ta4P4S29: A new tunnel structure with inserted polymeric sulfur

Abstract Ta 4 P 4 S 29 was prepared from the elements heated together in stoichiometric proportions in an evacuated Pyrex tube for 10 days at 500°C. The crystal symmetry is tetragonal, space group P 4 3 2 1 2, with the cell parameters: a = b = 15.5711(7) A, c = 13.6516(8) A, V = 3309.9(5) A 3 , and Z = 4. The structure calculations were conducted from 2335 reflections and 146 variables, leading to R = 0.033. The structure basic framework, corresponding to the chemical composition [TaPS 6 ], is made of biprismatic bicapped [Ta 2 S 12 ] units (average d TaS = 2.539 A), including sulfur pairs (average d SS = 2.039 A), bonded to each other through [PS 4 ] tetrahedral groups (average d PS = 2.044 A) sharing sulfurs. This framework leaves large tunnels running along the c axis of the cell and in which (S 10 ) ∞ sulfur chains are found to be inserted (average d SS = 2.052 A and SSS = 105.75°). Diamagnetic and semiconducting Ta 4 P 4 S 29 can be formulated: Ta V 4 P V 4 (S −II ) 16 (S −II 2 ) 4 (S 0 5 ).

Structures and phase transitions of the A7PSe6 (A = Ag, Cu) argyrodite-type ionic conductors. III. α-Cu7PSe6

The crystal structure of the third polymorph of the Cu(7)PSe(6) argyrodite compound, alpha-Cu(7)PSe(6), heptacopper phosphorus hexaselenide, is determined by means of single-crystal diffraction from twinned crystals and X-ray powder diffraction, with the help of extensive NMR measurements. In the low-temperature form, i.e. below the last phase transition, alpha-Cu(7)PSe(6) crystallizes in orthorhombic symmetry, space group Pna2(1), with a = 14.3179 (4), b = 7.1112 (2), c = 10.1023 (3) A, V = 1028.590 (9) A(3) (deduced from powder data, T = 173 K) and Z = 4. Taking into account a twinning by reticular merohedry, the refinement of the alpha-Cu(7)PSe(6) structure leads to the residual factors R = 0.0466 and wR = 0.0486 for 127 parameters and 3714 observed, independent reflections (single-crystal data, T = 173 K). A full localization of the Cu(+)d(10) element is reached with one twofold-, one threefold- and five fourfold-coordinated Cu atoms. The observation of two phase transitions for Cu(7)PSe(6), to be compared with only one for Ag(7)PSe(6), is attributed to the d(10) element stability in a low coordination environment, copper being less prone to lower coordination sites than silver, especially at low temperature.

Temperature dependence of the silver distribution in the crystal structure of natural pearceite, (Ag,Cu)16(As,Sb)2S11.

The crystal structure of the mineral pearceite, (Ag,Cu)16(As,Sb)2S11, has been solved and refined at 300, 120 and 15 K. At room temperature pearceite crystallizes with trigonal symmetry, space group P3m1; the refinement of the structure leads to a residual factor of R = 0.0464 for 1109 independent observed reflections and 92 variables. The crystal structure consists of sheets stacked along the c axis. The As atoms form isolated (As,Sb)S3 pyramids, which typically occur in sulfosalts, copper cations link two S atoms in a linear coordination, and the silver cations are found in a fully occupied position and in various sites corresponding to the most pronounced probability density function locations (modes) of diffusion-like paths. These positions correspond to low-coordination (2, 3 and 4) sites, in agreement with the preference of silver for such environments. d10 silver-ion distribution has been determined by means of a combination of a Gram-Charlier description of the atomic displacement factors and a split-atom model. To analyse the crystal chemical behaviour of the silver cations as a function of temperature, a structural study was carried out at 120 K (R = 0.0450). The refinement indicates that the mineral exhibits the same structural arrangement as the room-temperature structure (space group P3m1) and shows that the silver cations are still highly disordered. In order to investigate a possible ordering scheme for the silver cations, a data collection at ultra-low temperature (15 K) was performed. The structural skeleton was found to be similar to that of the room-temperature and 120 K atomic structures, but the best solution was achieved with a fully split-atom model of five silver positions, giving an R value of 0.0449 for 651 observed reflections and 78 parameters. Although the silver cation densities condense into better defined modes, the joint probability density function still exhibits a strong overlapping of neighbouring sites.

Structural and electronic properties of transition metal thiophosphates

From the viewpoint of metal coordination the authors examine the structural characteristics of several new members of transition metal thiophosphates (i.e., M-P-S phases with M = V, Nb, Ta), in which various ligands such as S/sup 2 -/, S/sup 2 -//sub 2/, and phosphorus-sulfur polyanions P/sub n/S/sub m/sup x-/ (1 less than or equal to n less than or equal to4; 3 less than or equal to m less than or equal to 13; 2 less than or equal to x less than or equal to 6) provide either an octahedral or a bicapped prismatic coordination of the metal. Tight-binding band electronic structure calculations show that the low-lying acceptore orbitals responsible for lithium intercalation of thiophosphates are their d-block bands. This prediction is confirmed by our electrochemical lithium intercalation study which reveals that the reduction sites of thiophosphates are their metal cations. Molecular orbital calculations are carried out on vanadium compounds with extremely short interligand S...S contacts. The occurrence of such short contact distances is not caused by covalent bonding in the S...S contacts but by the small size of vanadium cations which forces its surrounding sulfur ligands to squeeze one another.

Structural complexity in minerals : twinning, polytypism and disorder in the crystal structure of polybasite, (Ag,Cu)16(Sb,As)2S11

The crystal structures of 222- and 221-polybasite [(Ag,Cu)(16)(Sb,As)(2)S(11)] crystals have been solved and refined by means of X-ray diffraction data (collected at 100 and 120 K, respectively) from twinned crystals. Both structures consist of the stacking of [(Ag,Cu)(6)Sb(2)S(7)](2-) and [Ag(9)CuS(4)](2+) module layers in which Sb forms isolated SbS(3) pyramids typically occurring in sulfosalts; copper links two S atoms in a linear coordination and silver occupies sites with coordination ranging from quasi-linear to almost tetrahedral. An Ag --> Cu substitution in the [(Ag,Cu)(6)Sb(2)S(7)](2-) module layer is observed in both structures, the substitution amount being larger in the 221- than in the 222-polybasite. A pattern of the possible mechanism regulating the type of unit cell that is stabilized is proposed: starting from the hypothetical stoichiometric and fully ordered Ag(15)CuSb(2)S(11) 222-polybasite structure, with a low C2/c monoclinic symmetry and a large 222 supercell, the disorder introduced by the substitution of Cu for Ag increases the symmetry with a cell reduction along the c axis yielding the 221 supercell and a trigonal crystal system. A further increase of the substitution gives rise to a folding of the cell along the a and b axes and the 111-pearceite structure, space group P(bar)3m1.

Charge balance in some BixSey phases through atomic structure determination and band structure calculations.

The atomic structure of BiSe was redetermined to ascertain the structural features of this layered material and to carry out band structure analyses on the BixSey phases. BiSe crystallizes in trigonal symmetry (space group P3 ml) with a = 4.212(1)A, c = 22.942(8)A, V = 352.5(2)A3, and Z = 6. The structure determination conducted from 191 reflections [I ≥ 3 σ(I)] and 22 variables led to a reliability factor of R = 2.42% and Rw = 2.50%. Some Bi↔Se substitutional disorder is revealed and some clues for similar disorder in BiTe phase are given. Apart from the disorder, BiSe structural features are in good agreement with those previously reported. In particular, atomic distances match those found in parent Bi2Se3 and Bi4Se3 phases. The BiSe structure is built up from two Se-Bi-Se-Bi-Se (5) and one Bi-Bi (2) slabs. Van der Waals contacts only exist between adjacent (5) slabs, thus making BiSe a true 2D material (like Bi2Se3 and contrary to Bi4Se3). Band structure calculations confirm the absence of anionic bonding between selenium atoms and predict good conductivity within the layers. From a Mulliken population analysis, an oxidation state balance could be proposed for all BixSey phases and for the BixTey compounds alike, e.g. Bi0(BiIII)2(Se−II)3 and (Bi0)2(BiIII)2(BiIII)2(Se−II)3 for BiSe and Bi4Se3, respectively. Starting with BiIII and Se−II ions in Bi2Se3, the adding of zerovalent Bi-Bi slabs does not change the pristine charge balance and, thus, explains the formation of all the BixSey compounds observed to date.

The pearceite-polybasite group of minerals : Crystal chemistry and new nomenclature rules

Abstract The present paper reports changes to the existing nomenclature for minerals belonging to the pearceite-polybasite group. Thirty-one samples of minerals in this group from different localities, with variable chemical composition, and showing the 111, 221, and 222 unit-cell types, were studied by means of X-ray single-crystal diffraction and electron microprobe. The unit-cell parameters were modeled using a multiple regression method as a function of the Ag, Sb, and Se contents. The determination of the crystal structures for all the members of the group permits them to be considered as a family of polytypes and for all members to be named pearceite or polybasite. The main reason for doubling the unit-cell parameters is linked to the ordering of silver. The distinction between pearceite and polybasite is easily done with an electron microprobe analysis (As/Sb ratio). A hyphenated italic suffix indicating the crystal system and the cell-type symbol should be added, if crystallographic data are available. Given this designation, the old names antimonpearceite and arsenpolybasite are abandoned here and the old names pearceite and polybasite, previously defined on a structural basis (i.e., 111 and 222), are redefined on a chemical basis. The old name pearceite will be replaced by pearceite-Tac, antimonpearceite by polybasite-Tac, arsenpolybasite-221 by pearceite-T2ac, arsenpolybasite-222 by pearceite-M2a2b2c, polybasite-221 by polybasite-T2ac, and polybasite-222 by polybasite-M2a2b2c. Since all polytypes are composed of two different layers stacked along [001]: layer A, with general composition [(Ag,Cu)6(As,Sb)2S7]2-, and layer B, with general composition [Ag9CuS4]2+, the chemical formulae of pearceite and polybasite should be written as [Ag9CuS4][(Ag,Cu)6(As,Sb)2S7] and [Ag9CuS4][(Ag,Cu)6(Sb,As)2S7], respectively, instead of (Ag,Cu)16(As,Sb)2S11 and (Ag,Cu)16(Sb,As)2S11, as is currently accepted. The new nomenclature rules were approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association.