Safflorite, (Co,Ni,Fe)As2, isomorphous with marcasite

Safflorite, a naturally occurring cobalt-nickel-iron diarsenide (Co,Ni,Fe)As2, possesses the marcasite-type structure, with cations (M = Co + Ni + Fe) at site symmetry 2/m and As anions at m. The MAs6 octahedra share two edges, forming chains parallel to c. The chemical formula for safflorite should be expressed as (Co,Ni,Fe)As2, rather than the end-member format CoAs2, as its structure stabilization requires the simultaneous interaction of the electronic states of Co, Ni, and Fe with As2 2− dianions.

Safflorite, a naturally occurring cobalt-nickel-iron diarsenide (Co,Ni,Fe)As 2 , possesses the marcasite-type structure, with cations (M = Co + Ni + Fe) at site symmetry 2/m and As anions at m. The MAs 6 octahedra share two edges, forming chains parallel to c. The chemical formula for safflorite should be expressed as (Co,Ni,Fe)As 2 , rather than the end-member format CoAs 2 , as its structure stabilization requires the simultaneous interaction of the electronic states of Co, Ni, and Fe with As 2 2À dianions.
Safflorite is isomorphous with marcasite. Each cation (M = Co, Ni, and Fe) at site symmetry 2/m is octahedrally coordinated by six anions (As) at site symmetry m and each anion is tetrahedrally bonded to another anion (forming As-As dianion units) plus three M cations. The MAs 6 octahedra share two edges, forming chains parallel to c, and two vertices with adjacent chains (Fig. 1). The average M-As bond distance (2.360 Å) is identical to that in clinosafflorite (Kjekshus, 1971), but slightly shorter than that in löllingite (2.379 Å) (Kjekshus et al., 1979;Lutz et al., 1987) or rammelsbergite (2.378 Å) (Kjekshus et al., 1979). Notably, as the d-orbital electrons in M cations increase from Fe (d = 6) in löllingite to Co (d = 7) in safflorite, and Ni (d = 8) in rammelsbergite, the M-M separation along the chain direction increases significantly from 2.882 to 3.134, and 3.545 Å, respectively, while the As-As edge length shared by the two M octahedra concomitantly decreases from 3.808 to 3.547, and 3.219 Å. The octahedral distortion, measured by the octahedral angle variance (OAV) and quadratic elongation (OQE) (Robinson et al., 1971) supplementary materials sup-2 The variation of the M-M separation with the number of d-orbital electrons in marcasite-type disulfides, diarsenides, and sulfarsenides has been a matter of discussion (see Vaughan & Rosso, 2006 for a thorough review). Theoretical calculations based on molecular orbital and band models predict that, due to the interaction between the 3dσ(e g ) orbitals of M 2+ and the π b orbitals of As 2 2-, the M-As-M angle subtending the M-M separation across the shared octahedral edge should be substantially smaller for FeAs 2 than for CoAs 2 and NiAs 2 , resulting in the so-called ''compressed marcasite-type'' structure (Tossell et al., 1981;Tossell, 1984). Indeed, this angle is 74° in FeAs 2 löllingite (Lutz et al., 1987), but 83° in (Co,Ni,Fe)As 2 safflorite and 96° in rammelsbergite (Kjekshus et al., 1979). It is intriguing to note that the end-member CoAs 2 has been found to only crystallize in the arsenopyrite-type structure (P2 1 /c) (Holmes, 1947;Swanson et al., 1966;Radcliffe & Berry, 1971), rather than the marcasite-type structure (Pnnm). This observation may be explained by the existence of an unpaired electron occupying one of the π b orbitals, which splits into a lower-energy filled band and a higher-energy empty band (Goodenough, 1967), thus resulting in the symmetry reduction from Pnnm to P2 1 /c. In other words, the presence of some Ni/Fe in place of Co appears to be an essential requirement for the CoAs 2 system to crystallize in the Pnnm symmetry. The pure system will otherwise be stabilized energetically in the clinosafflorite structure.
Another outstanding feature of the safflorite structure is the prominent anisotropic displacement ellipsoid of the M cation, the U 11 :U 22 :U 33 ratio being approximately 3:1:9, with the ellipsoid axial directions roughly parallel to the unit cell axes. This ratio can be compared to the differences of three unit-cell dimensions between FeAs 2 löllingite and NiAs 2 rammelsbergite (Kjekshus et al., 1974(Kjekshus et al., , 1979Lutz et al., 1987), which is about 3:1:8. Accordingly, the marked anisotropy of the displacement parameters of the M cation in safflorite is interpreted as a consequence of positional disorder with Fe and Ni occupying apparent different positions, which in turn results from the different interactions of their d-electrons with the As 2 2dianions.

Experimental
The safflorite specimen used in this study is from Timiskaming County, Ontario, Canada, and is in the collection of the

Refinement
Due to the similar X-ray scattering powers for Co, Ni, and Fe, all cations were assumed to be Co and their site occupancies were not determined during the refinement. All crystals examined were twinned, with {011} as twin plane. The structure refinements were performed based on X-ray diffraction data collected from a twinned crystal, which were processed with TWINABS (Sheldrick, 2007). The ratio of two twin components is 0.73:0.27. The highest residual peak in the difference Fourier maps was located at (0.133, 0.370, 0.256), 0.85 Å from atom As, and the deepest hole at (0.133, 0.473, 0), 0.69 Å from As. Fig. 1. Crystal structure of safflorite, with displacement ellipsoids drawn at the 99.9% probabiliy level. The M (=Co+Ni+Fe) cations (yellow spheres) are situated in octahedra coordinated by six As atoms (pink spheres).