Simulation of layer-faulting in Nb2Co7intermetallic compound using DIFFaX+

Abstract

The crystal structure and the fault structure of the intermetallic Nb2Co7 phase were determined using DIFFaX+ refinements on the basis of synchrotron powder-diffraction data. The crystal structure was confirmed to be of the monoclinic Zr2Ni7 type. The layer faulting can be understood as partial occurrence of structural features of the Yb2Ag7-type structure, which contains the same “triple layers” as the Zr2Ni7-type structure does, but stacked in a different fashion. Introduction The binary Nb-Co system [1] contains, among other phases, the intermetallic Nb2Co7 compound with the recently identified crystal structure described in ref. [2]. For an intermetallic, Nb2Co7 exhibits an unusually high plastic deformability [3, 4]. Consequently, the question arises, whether there is a relation between the actual crystal structure and the high deformability. The X-ray powder-diffraction patterns used of Nb2Co7 for structural studies exhibit strong line broadening, which can be ascribed to faulting with respect to the layered structure of Nb2Co7. Such defects are expected to play a role in the deformability of this phase, but the traditional Rietveld approach is not able to handle fully the diffraction phenomena associated with faulting. Therefore, DIFFaX+ refinements are provided here, which depart from the layered nature of the material and from previous qualitative considerations concerning the character of the faulting [2]. The results provide useful inside into the defect structure of the material. Crystal structure and faulting model Structure/microstructure analyses were conducted on a Nb2Co7 powder sample containing tiny amounts of the cubic C15-NbCo2 Laves phase (equilibrium phase with Nb2Co7 in a twophases region at the final annealing temperature of 900°C [1]). An X-ray powder-diffraction pattern recorded on the powder diffractometer installed at the ID31 beamline of the ESRF synchrotron (Grenoble, F) using a wavelength of 0.400094 A was available for the specimen. 424 European Powder Diffraction Conference, EPDIC 11 The average (defect free) crystal structure of Nb2Co7 was previously identified [2] to be similar to monoclinic Zr2Ni7 [5, 6], figure 1a. Within the crystal structure one can identify triple-layers parallel to (001) with a complex internal atomic structure: an inner Kagometype layer (KA) of “Co3” sandwiched by two outer “close-packed” layers (CL) “NbCo2” (with Nb-Co ordering), leading to a layer composition of Nb2Co7 corresponding also to the overall phase composition. The triple layers (CL-KA-CL) by themselves ideally exhibit an hexagonal layer symmetry, but in view of the final crystal structure, a centred rectangular unit cell with basis vectors a and b should be considered, identical to the lattice-basis vectors of the monoclinic crystal structure (figure 1b). (a) a tri pl e la ye r