Kristallstrukturen des Löllingits FeAs₂ und des Pyrits RuTe₂

Abstract

Die bisher nur aus Pulverdaten bekannten Strukturen der Verbindungen FeAs2 (Löllingit) und RuTe2 (Pyrit) wurden anhand von Einkristalldaten neu bestimmt. Die Darstellung der FeAs2‐Einkristalle (Kantenlänge bis zu 6 mm) erfolgte durch Gasphasentransport mit Iod, die Einkristalle des RuTe2 wurden aus einer Tellurschmelze erhalten. Die Verfeinerung der Strukturparameter konvergierte beim FeAs2 für 511 beobachtete Reflexe (I > 2σI) bei R = 3.9% und beim RuTe2 für 429(410) beobachtete Reflexe (I > 3σI) bei 3,8% (3,7%) (zwei Kristalle). Einige Reflexe des RuTe2 mit I > 2σI sind auf Mehrfachreflexionen zurückzführen.

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

“…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.…”

“…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 ratio can be compared to the differences of three unit-cell dimensions between et al, 1974, 1979Lutz et al, 1987), which is …”

“…Topologically, löllingite FeAs 2 (Kjekshus et al, 1979;Lutz et al, 1987;Ondrus et al, 2001) and rammelsbergite NiAs 2 (Kjekshus et al, 1974(Kjekshus et al, , 1979 possess the marcasite (FeS 2 )-type structure with space group Pnnm, whereas clinosafflorite CoAs 2 (Darmon & Wintenberger, 1966;Kjekshus, 1971) is isostructural with the modified marcasite-type structure of arsenopyrite (FeAsS) with space group P2 1 /c (Hem et al, 2001;Makovicky, 2006). From the unit-cell dimensions measured from X-ray diffraction, safflorite was assumed to be isotypic with marcasite (Holmes, 1947;Radcliffe & Berry, 1968.…”

“…For related literature, see: Anawar et al (2003); Carlon & Bleeker (1988); Darmon & Wintenberger (1966); Ennaciri et al (1995); Goodenough (1967); Grorud (1997); Hem et al (2001); Holmes (1947); King (2002); Kjekshus (1971); Kjekshus et al (1974Kjekshus et al ( , 1979; Lutz et al (1987); Makovicky (2006); O'Day (2006); Ondrus et al (2001); Palenik et al (2004); Petruk et al (1971); Radcliffe & Berry (1968; Reich et al (2005); Robinson et al (1971); Swanson et al (1966);Tossell (1984); Tossell et al (1981); Vaughan & Rosso (2006);Wagner & Lorenz (2002).…”