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 pow der X-ray diffraction, metallography, electron microprobe analysis and ther mal analysis. The condensed phases occurring, tie-lines and tie-triangles a re presented for an isothermal section of the phase diagram at 300 degrees C. Most of the binary phases show no significant solid solubility in the te rnary dimension at 300 degrees C. The present study has not been able to co nfirm or rule out the existence of the Au4Cr phase. The earlier reported Cr 2Sn3 phase is shown to have the composition CrSn2. A new, genuine ternary p hase, Au1.15Cr2Sn8.85 (Au0.540CrSn4.460-Au0.690CrSn4.310), has been uncover ed. 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 determine d 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 posit ion and Sn the 8m position with x = 0.1774(1) and y = 0.3763(1). The Au1.15 Cr2Sn8.85 structure is related to the AlCu2- and CoNb4Si-type structures an d its atomic arrangement is discussed in relation to these prototypes. Au1. 15Cr2Sn8.85 exhibits weak (Pauli-like) paramagnetism with a slight temperat ure dependence (5-300 K). (C) 2000 Elsevier Science S.A. All rights reserve d.