THE CRYSTAL STRUCTURE OF SYNTHETIC KUTINAITE, Cu14Ag6As7

The crystal structure of synthetic kutinaite, Cu14Ag6As7, 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: Rwp = 18.2%, Rp = 13.4%, Rexp = 14.7%, S = 1.54, R(B) = 1.78%. The cubic structure has a equal to 11.78079(2) Å (standard uncertain-ties are underestimated by a score-factor of 2.4686) and space group Pm¯3m, Z = 4. Apart from partially ordered Cu vacancies, the arrangement of atoms is very close to that in Fm¯3m. The Pm¯3m structure of kutinaite is a result of the frozen-in partial order of Cu vacancies which, in a presumed high-temperature Fm¯3m 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 Ag6 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 Ag6 complexes. This situation results in two complementary descriptions of the structure of kutinaite: kutinait