The ternary system Au-Cr-Sn and the crystal structure of Au1.15Cr2Sn8.85

The phase relations in the ternary system Au-Cr-Sn have been studied by powder X-ray diffraction, metallography, electron microprobe analysis and thermal analysis. The condensed phases occurring, tie-lines and tie-triangles are presented for an isothermal section of the phase diagram at 300°C. Most of the binary phases show no significant solid solubility in the ternary dimension at 300°C. The present study has not been able to confirm or rule out the existence of the Au4Cr phase. The earlier reported Cr2Sn3 phase is shown to have the composition CrSn2. A new, genuine ternary phase, Au1.15Cr2Sn8.85 (Au0.540CrSn4.460Au0.690CrSn4.310), has been uncovered. Its unit cell is tetragonal (space group P4/mcc) with a=687.7(1) and c=566.0(2) pm at room temperature, and the crystal structure is determined by direct methods from single crystal data (150(2) K). There is a random distribution of 58% Au and 42% Sn in the 2b position, Cr holds the 2c position and Sn the 8m position with x= 0.1774(1) and y=0.3763(1). The Au1.15Cr2Sn8.85 structure is related to the AlCu2- and CoNb4Si-type structures and its atomic arrangement is discussed in relation to these prototypes. Au1.15Cr2Sn8.85 exhibits weak (Pauli-like) paramagnetism with a slight temperature dependence (5-300 K).