Synthesis and crystal structure of a new Cu 3 Au-type ternary phase in the Au-In-Pd system: distribution of atoms over crystallographic positions.

A new Cu3 Au-type ternary phase (τ phase) is found in the AuPd-rich part of the Au-In-Pd system. It has a broad homogeneity range based on extensive (Pd,Au) and (In,Au) replacement, with the composition varying between Au17.7 In25.3 Pd57.0 and Au50.8 In16.2 Pd33.0 . The occupancies of the crystallographic positions were studied by single-crystal X-ray diffraction for three samples of different composition. The sites with m-3m symmetry are occupied by atoms with a smaller scattering power than the atoms located on 4/mmm sites. Two extreme structure models were refined. Within the first, the occupation type changes from (Au,In,Pd)3 (Pd,In) to (Au,Pd)3 (In,Pd,Au) with an increase in the Au gross content. For the second model, the occupation type (Au,In,Pd)3 (Pd,Au) remains essentially unchanged for all Au concentrations. Although the diffraction data do not allow the choice of one of these models, the latter model, where Au substitutes In on 4/mmm sites, seems to be preferable, since it agrees with the fact that the homogeneity range of the τ phase is inclined to the Au corner and provides the same occupation type for all the studied samples of different compositions.