THE CRYSTAL STRUCTURE OF SYNTHETIC KUTINAITE, Cu14Ag6As7

Abstract

The crystal structure of synthetic kutinaite, Cu 14 Ag 6 As 7 , was determined from powder-diffraction data recorded on a Guinier diffractometer and refined by Rietveld method with the program FULLPROF. The final refinement (⌬/ max < 0.2) with 31 (9 profile and 23 structural) parameters using 4600 data points and 226 reflections yielded the following agreement indices: R wp = 18.2%, R p = 13.4%, R exp = 14.7%, S = 1.54, R(B) = 1.78%. The cubic structure has a equal to 11.78079(2) Å (standard uncertainties are underestimated by a score-factor of 2.4686) and space group Pm3m, Z = 4. Apart from partially ordered Cu vacancies, the arrangement of atoms is very close to that in Fm3m. The Pm3m structure of kutinaite is a result of the frozen-in partial order of Cu vacancies which, in a presumed high-temperature Fm3m structure, should be completely disordered. The atoms Cu2 and Cu3 are tetrahedrally coordinated by As, the atoms Cu1 and Cu4 (with partial occupancies) have trigonal planar As coordination. Atoms of Ag form octahedral Ag 6 clusters in octahedral cavities of the Cu-As framework. On the one hand, the structure of kutinaite contains a multitude of short metal-metal and metal-semimetal contacts, resulting in high coordination-numbers and indicating direct metal-metal interactions. The presence of metal-metal bonding contacts is supported by an electron-density analysis, which gives the following bonding contact limits: 2.70 Å (Cu-Cu), and 3.05 Å (Ag-Cu). On the other hand, a network of important Cu-As bonds forms a three-dimensional complex with a nominal net negative charge balanced by nominal positive charges of localized Ag 6 complexes. This situation results in two complementary descriptions of the structure of kutinaite: kutinaite as an icosahedral alloy and kutinaite as an arsenide with metal clusters.Keywords: kutinaite, X-ray diffraction, powder method, crystal structure, icosahedral alloy, arsenide, metal clusters. SOMMAIRENous avons établi la structure cristalline de la kutinaïte synthétique, Cu 14 Ag 6 As 7 , à partir de données en diffraction X obtenues sur poudre, enregistrées avec un diffractomètre Guinier et affinées par la méthode de Rietveld avec le logiciel FULLPROF. L'affinement final (⌬/ max < 0.2), avec 31 paramètres (9 portant sur le profil et 23 sur la structure), repose sur 4600 données et 226 réflexions, et ont mené aux indices de concordance suivants: R wp = 18.2%, R p = 13.4%, R exp = 14.7%, S = 1.54, R(B) = 1.78%. Le paramètre a de la structure cubique est égal à 11.78079(2) Å (l'écart-type est sous-estimé par un facteur de 2.4686); le groupe spatial est Pm3m, Z = 4. Mis à part les lacunes aux sites du Cu, partiellement ordonnées, l'agencement des atomes se rapproche de celui du groupe spatial Fm3m. La structure Pm3m de la kutinaïte résulterait de la mise en ordre partielle des lacunes aux sites Cu; dans un précurseur de haute température, dont le groupe spatial est Fm3m, ces lacunes devraient être complètement désordonnées. Les atomes Cu2 et Cu3 ont une coordinence tétraédri...

Crystal structure and chemistry of natural kutinaite from Černý Důl, Krkonoše, Czech Republic

“…We can observe that the Me-Me distances are mostly shorter to the cations participating in other configurations than to those participating in the 'own' one, giving an alloy character to the examined compound. In agreement with this, one can still find the icosahedral Cu-As-Ag configurations around As1, Ag and Cu1A as defined in the synthetic phase by Karanovićet al (2002), as well as the cube of 8 Cu atoms around As, and other features defined for the synthetic phase, in the natural phase studied here.…”

“…We find the latter description mode most suitable for the natural kutinaite structure. Atom notation (Table 3) in the present refinement is directly related to that used for the cubic synthetic phase by Karanovićet al (2002).…”

“…From the experimental point of view, studied the phase relations of the Cu-Ag-As system in the temperature range 350-500°C and showed that the synthetic compound Cu 14 Ag 6 As 7 , corresponding to the formula of natural kutinaite as known at the time, appears in the As-richer parts of the system at 400°C, being involved in the β-domeykite-kutinaite-arsenic, β-domeykite-kutinaite-silver and arsenic-kutinaite-silver ternary fields. By using the synthetic Cu 14 Ag 6 As 7 material produced in these experiments , Karanovićet al (2002) determined the crystal structure of the synthetic analogue of kutinaite by means of direct methods and subsequent Rietveld refinement and demonstrated that it exhibits a cubic structure with a = 11.78079(2) Å and space group Pm3m. These authors showed that the Cu 14 Ag 6 As 7 structure is a result of the frozen-in partial order of Cu vacancies which, in a presumed high-temperature Fm3m structure, should be completely disordered.…”

Modular Crystal Chemistry of Thallium Sulfosalts

“…This is an alloy full of metal-metal bonds; however, As still plays a role of anion in it. Unlike synthetic 'kutinaite' Cu 14 Ag 6 As 7 [77,78] which is cubic, Pm-3m and full of partly occupied atom sites without cavities, natural kutinaite turned out to be tetragonal (a 11.789 Å, c 11.766 Å), space group P4/mmm. In this structure, clusters of 8 edge-sharing tetrahedra of copper alternate in a three-dimensional chess-board pattern with octahedral clusters of six silver atoms, surrounded by triangularly coordinated copper atoms, situated in eight faces of a cuboctahedron.…”

The crystal structure of sarmientite, Fe23+ (AsO4)(SO4)(OH)·5H2O, solved ab initio from laboratory powder diffraction data

“…In the next section we briefly review how to solve a crystal structure from powder diffraction data with special emphasis on the recently introduced clusterbased variant of Patterson-function direct methods (hereafter denoted cPDM) that has been employed for solving the crystal structure of the title mineral sarmientite (Rius, 2011). Some previous noteworthy examples of minerals whose structures have been solved by ab initio powder diffraction methods are priceite (Wallwork et al, 2002), kutinaite (Karanović et al, 2002), kingite (Wallwork et al, 2003), souzalite-gormanite (Le Bail et al, 2003), aerinite (Rius et al, 2004), parascorodite (Perchiazzi et al, 2004), anthoinite (Grey et al, 2010), sanjuanite (Colombo et al, 2011), and cranswickite (Peterson, 2011). Detailed descriptions on the methodologies for solving and refining crystal structures from powder diffraction data can be found in David et al (2002) and in Young (1993), respectively.…”