The puzzling structure of Cu5FeS4 (bornite) at low temperature

The crystal structure of Cu5FeS4 (bornite) has been investigated using synchrotron X-ray powder diffraction at temperatures between 10 and 275 K. Diffraction data confirm that bornite crystallizes in the orthorhombic space group Pbca at 275 K. The unit-cell volume decreases continuously on cooling, but undergoes an abrupt contraction below ∼65 K, where a first-order Pbca→Pca21 structural transition takes place. The primary active mode yielding the observed ordered structure corresponds to the irreducible representation Γ2 −, with wavevector (0,0,0). Pair distribution function analysis shows strong discrepancies between the local and the average structure. The average Fe—S bond length obtained through the EXAFS local probe is consistent with the values independently provided by X-ray powder diffraction data, strongly supporting the preferred location of Fe