The refinement of the crystal structure of skutterudite, CoAs3

Abstract: The crystal structure of skutterudite, cobalt triarsenide, has been re-refined with new data collected at 22 °C on a Picker automatic diffractometer (Mo Ke) out to a maximum sin 0/).=0.60, with the moving-crystal moving-counter technique. There were 224 unique reflections collected. Space group is Ira3 (TnS) with eight formulas per unit cell, with a= 8"195 + 0.003 A.. Each cobalt atom is surrounded octahedrally by six arsenic neighbors at a distance of 2.334+0.001 A. The arsenic atoms form rectangles with bond distances of 2.464 + 0.002 and 2.572 + 0.002 A,. Final R is 3.5 %.

The refinement of the structure of skutterudite, CoAsa, was undertaken to study the four-membered arsenic ring structure. A view of this structure is shown in Fig. 1. The structures of skutterudite-type phosphides have been studied by Rundqvist & Ersson (1968) who showed that the cobalt atoms have a trigonally distorted octahedral environment of six phosphorus atoms? The phosphorus atoms form rectangles in preference to squares which had been postulated by Oftedal (1928) in the original determination of the skutterudite structure. The single-crystal study on skutterudite by Ventriglia (1957) was the first to indicate a distorted cobalt octahedron and rectangles of arsenic atoms.

The crystal used i'n the present investigation was a truncated octahedron (0.23 mm average diameter) chipped from an ore sample from Bou-Azzer, Morocco. From elemental analysis the sample of skutterudite contained 67-5% As, 15.7% Co, 2.1% Fe, 2.6% Ni, and trace amounts of Ir, Pd, Rh, and Sb. This corresponds to the formula Co0.87Fe0.zzNi0.zaAs3.0. Some oxide was doubtless present, as well as some silicate matrix. The linear absorption coefficient is 447-6 cm -~, and due to the almost spherical nature of the crystal, a simple spherical absorption correction was applied to the data using the transmission factors from International Tables for X-ray Crystallography (1967). The space group was verified as lm3 (T s) from precession and Weissenberg photographs. The crystal was transferred to a Picker automatic diffractometer (k= 0.70926 A, Mo Ko0 where eight independent 20 values were used in least-squares refinement, which gave a lattice constant of a=8.195+0.003 A. Data were collected at 22°C out to a maximum sin 0/2=0.60 with the moving-crystal moving-counter technique. A scan speed of 2 o min_l, and a 20 sec background count were used. Of the 224 unique reflections, 7 were considered unobserved based on the intensities being less than 2-330- (1).

Starting coordinates for refinement were taken as Co in position 8(c), at (kkk), etc., and As in position 24(g), at (Oyz), etc., with y=0"35 and z=0.15, as given by Oftedal. The initial scale factor and overall isotropic temperature factor were calculated from Wilson statistics, and were used as a starting point for three cycles of isotropic least-squares refinement, during which R dropped from 25-5 to 6.5 %. Eight more cycles of isotropic least-squares refinement, using a scattering-factor table for the metal atom calculated by appropriately weighting the f values of Co, Fe, and Ni in International Tables for X-ray Crystallography (1968), gave an R of 3-8 %. Three cycles of anisotropic refinement reduced R to 3.5 %. Observed and calculated F values are presented. Final parameters and their variances are presented in Table 2. The thermal anisotropy, if real, is not very severe.

Three quantities are of particular interest in discussing the structure: the Co-As distance, the distortion of the As4 groups from a square, and the degree of distortion of the CoAs6 octahedron. For the two As-As distances equal, 2(y+ z)= 1. This trival equation has been called the Oftedal relation. For regular octahedral coordination about the Co atom, the parameters y and z for the As atoms must satisfy the relation y(2z-l)=z-~. Deviation from this relationship leads to a trigonal antiprismatic distortion from octahedral symmetry. The distances for the Oftedal structure are Co-As=2.35 A, and As-As=2.45/~. Ventriglia's distances are Co-As=2-23 A, As-As=2-55 and 4.09 ,~. Ventriglia stated that these distances agreed better with experimental data, but gave no observed or calculated structure factors. The distances found in the present study are Co-As-- 2.334_ 0.001 A, As-As= 2.572+_ 0.002 and 2"464+_0.002/~,. The two As-Co-As angles in the octahedron are 95"4 and 84.6 °. Fig. 2 shows the relation between y and z for regular octahedral coordination and for equal As-As distances. Note, as was also pointed out by Rundqvist & Ersson (1968), that both 'ideal' conditions cannot be realized simultaneously without a change in the structure type, as mentioned below.

Following the discussion of Rundqvist & Ersson (1968), an increasing trigonal distortion of the ideal octahedral symmetry of the ligands about the Co atoms presumably leads to a less favorable energetic situation as regards the Co-As bonds. On the other hand, a departure from the Oftedal relation, which means an elongation of two of the As-As bonds, probably leads to a less favorable bonding in the As4 rings. As seen in Fig. 2, Oftedal's relation and the condition for ideal octahedral symmetry cannot be simultaneously fulfilled, except for a drastic structural change to the ReO3-type structure. This condition occurs at the intersection of the extension of the two lines in Fig. 2, at y= z=?. Since the presence of As-As bonding is apparently essential for the stability of the compounds, a compromise is reached between the two opposing factors.

Note that in the skutterudite-type phosphides, the shorter P-P bond distance is virtually constant at 2-23 + 0"01/~ which corresponds well with the P-P distances of 2.21 /~, in P4 and 2.25A in P4S3 (Sutton, 1958). In skutterudite itself the shorter As-As distance of 2.46/~ is in good agreement with distances of 2.45 A in (AsCF3)4 (Mandel & Donohue, 1971), 2.43 A, in (AsCH3)5 (Burns & Waser, 1957), 2.46 A in (AsC6Hs)6 (Hedberg, Hughes & Waser, 1961), and of 2-44/~ in gaseous AS4 (Maxwell, Hendricks & Moseley, 1935).

References:

[1] N. MANDEL J. D. The refinement of the crystal structure of skutterudite, CoAs3[J]. Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry, 1971. DOI:10.1107/S0567740871005727.