ABSTRACT
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.
ABSTRACT
1,2-Ethylenediphosphonic acid reacts with gallium nitrate in water to give new pillared layered gallium phosphonates, the structure of which depends on the experimental conditions used for their preparation. Thus, Ga(4)(O(3)PC(2)H(4)PO(3))(3) (1) and Ga(2)(OH)(2)(O(3)PC(2)H(4)PO(3)) (2) were isolated and the structure of 1 was solved by single-crystal X-ray diffraction, while 2 was shown to have the same metal/PO(3) arrangement as the previously described Ga(OH)(O(3)PCH(3)). Ga(4)(O(3)PC(2)H(4)PO(3))(3) (1) is triclinic, with space group P1 with Z = 2, a = 5.1480(4), b = 8.0354(7), and c = 12.383(1) A; alpha = 91.34(1), beta = 101.40(1), and gamma = 90.86(1) degrees; V = 501.9(1) A(3). The structure of 1 is unusual with (i) mixed GaO(4) and GaO(5) sites while a 6-fold coordination is observed for gallium in (2) and (ii) the presence of the organic moiety within both the layers and the interlayer space. The two compounds were fully characterized using (31)P and (71)Ga solid-state NMR.
ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
The crystal structures of two of the three polymorphic forms of the Cu7PSe6 argyrodite compound are determined by means of single-crystal X-ray diffraction. In the high-temperature form, at 353 K, i.e. 33 K above the first phase transition, gamma-Cu7PSe6 crystallizes in cubic symmetry, space group F43m. The full-matrix least-squares refinement of the structure leads to the residual factors R = 0.0201 and wR = 0.0245 for 31 parameters and 300 observed independent reflections. In the intermediate form, at room temperature, beta-Cu7PSe6 crystallizes again in cubic symmetry, but with space group P2(1)3. Taking into account a merohedric twinning, the refinement of the beta-Cu7PSe6 structure leads to the residual factors R = 0.0297 and wR = 0.0317 for 70 parameters and 874 observed, independent reflections. The combination of a Gram-Charlier development of the Debye-Waller factor and a split model for copper cations reveals the possible diffusion paths of the d10 species in the gamma-Cu7PSe6 ionic conducting phase. The partial ordering of the Cu+ d10 element at the phase transition is found in concordance with the highest probability density sites of the high-temperature phase diffusion paths. A comparison between the two Cu7PSe6 and Ag7PSe6 analogues is carried out, stressing the different mobility of Cu+ and Ag+ and their relative stability in low-coordination chalcogenide environments.
ABSTRACT
Sr(9/8)TiS3, strontium titanium sulfide, a new phase in the hexagonal perovskite-like Sr(x)TiS3 system, has been prepared and its structure solved from single-crystal X-ray data within the (3 + 1)-dimensional [(3 + 1)D] formalism. Sr(9/8)TiS3 crystallizes with trigonal symmetry [R3m(00gamma)0s superspace group], with the following lattice parameters: a(s) = 11.482 (3), c(s) = 2.9843 (8) A, q = 0.56247 (7)c* and Vs = 340.7 (3) A3. The structure was considered as commensurate [R3c three-dimensional (3D) space group], but refined within the (3 + 1)D formalism to a residual factor R = 2.79% for 64 parameters and 1084 independent reflections. Original crenel functions were used for the sulfur and strontium description. The structure is different from that of the hexagonal perovskite-like oxide counterparts. The main difference is related to the presence of a new type of polyhedron in the [MS3] transition metal chains, intermediate between the octahedra classically found in such chains and the trigonal prismatic sites encountered in the oxides.
ABSTRACT
The structure of nonapotassium cerium tetraphosphorus hexadecasulfide, a zero-dimensional material isostructural with Rb(9)CeP(4)Se(16), is reported.
ABSTRACT
A new triclinic polymorphic form of N,N'-diphenyl-1,4-phenylenediamine (C(6)H(5)NHC(6)H(4)NHC(6)H(5)) has been obtained through appropriate recrystallization of the orthorhombic form. It crystallized in the centrosymmetric space group P$\overline 1$, with two half molecules as the asymmetric unit.
ABSTRACT
The incommensurately modulated structure of a square Te-net, namely that of K1/3Ba2/3AgTe2, is determined from single-crystal X-ray diffraction data within a (3+1)D higher dimension formalism. The phase is shown to crystallize in the monoclinic symmetry, P2(1)(alpha 0 gamma) superspace group with the following lattice parameters: a = 4.6441(10) A, b = 4.6292(12) A, c = 23.765(9) A, and beta = 101.28(2) degrees with q = 0.3248(6)a* -0.071(8)c*, that is, in a symmetry different from that reported for the average structure (tetragonal) or that assumed from electron diffraction measurements (orthorhombic). After the introduction of a crenel function for the Te displacive description, the refinement converged to a residual factor R = 0.033 for 2583 observed reflections and 115 parameters (R = 0.024 and 0.101 for 1925 main reflections and 658 first-order satellites, respectively). The [Ag2-Te2] and the Ba/K layers are found to be only weakly modulated. The modulation of the square Te-net is, however, both substantial and unique. Namely, it results in two different units: a "V"-shaped Te3 trimer and a "W"-shaped Te5 pentamer. To examine both unit types, which are segregated in domains that aperiodically alternate within the Te layers, first principles electronic band structure calculations were carried out for three model commensurate structures using the tight-binding linear-muffin-tin-orbital method (LMTO). The calculations show that the distorted structures of V-pattern (model 2) and W-pattern (model 3) are more stable than the average structure (model 1) and that the V-pattern distortion provides a slightly larger stabilization than does the W-pattern distortion. The Fermi surface calculated for the average structure shows nesting vectors that are consistent with the occurrence of the V- and W-pattern distortions in the Te layers. However, these vectors do not predict the observed modulation vector of the incommensurately distorted structure, because the stabilization energy associated with the distortion is not mainly dominated by the energy lowering of the occupied band levels near the Fermi level.
ABSTRACT
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.
