Combined neutron and x-ray powder diffraction study of Fe 0.50 Co 0.48 V 0.02
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M. Karnowski Sandia National Laboratory, Albuquerque, New Mexico 87185
W. K. Warburton X-ray Instrumentation Associates, Menlo Park, California 94025 (Received 27 December 1989; accepted 14 February 1990)
The large number of constituent elements and high symmetry of the ordered Fe-Co-V alloys make a complete solution of its structure impossible from a single diffraction experiment. We have carried out a Rietveld refinement of the site occupancies for a partially ordered Fe-Co-V alloy using both neutron and anomalous dispersion x-ray powder diffraction data in order to obtain a complete crystallographic solution to its structure. Contrary to previous assumptions that V occupies both sites randomly or magnetic and Mossbauer studies that suggest that V has a site preference for the Fe sublattice, we find that the V dopant preferentially occupies the Co sites.
I. INTRODUCTION
Equiatomic FeCo is a famous example of an alloy exhibiting a /3-brass-type second-order phase transition: at 720 °C it undergoes an order-disorder transformation from the ordered, primitive cubic CsCl structure (space group Pm3m) to a disordered bcc structure (space group Imhrri). The ordered alloy is an excellent soft ferromagnet with a high saturation magnetization and low permeability and is consequently of great interest to the industrial community as a commercial magnet material. It is further known that the addition of a small amount of V greatly improves the mechanical properties of this phase, but the precise role of V in the crystal structure has never been unambiguously determined. While it is commonly assumed that the V substitutes randomly for Fe and Co, this has never been rigorously established and some contrary evidence has begun to accumulate. Primarily from studies of the magnetic behavior of dilute alloys, Mal'tsev et al.1 proposed that V should have a site preference for the Fe sublattice of the ordered structure; more recent Mossbauer studies2 also suggest that this may indeed be the case. While the technique of choice for the study of such a system would normally be x-ray diffraction, the proximity of Fe and Co in the periodic table makes the order-disorder transformation impossible to observe using conventional x-ray diffraction. Both neutron diffraction1'3'4 and anomalous scattering from the Fe using Co Ka x-rays5'6 have been applied to this problem. In all cases to date it has been possible to obtain only a single Deceased.
order parameter from diffraction experiments by observing the relative intensities of the (100) superlattice and (200) fundamental lines. It has not been possible to solve the crystallography of such a ternary alloy with high symmetry, and no information on the site preference of V is available. Since the ordered phase has two sites but three constituent elements, in principle a minimum of two linearly independent structure factors is required to provide complete information on the site occupancies. Since it is difficult in practice to measure absolute intensities with precision, a scale factor is u
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