THE TERNARY-SYSTEM AU-CO-SN AND THE CRYSTAL-STRUCTURE OF AUCO2(1-MU)SN-4

The phase relations in the ternary system Au-Co-Sn have been studied b y powder X-ray diffraction, metallography, electron microprobe analysi s, and thermal analysis. The condensed phases occurring, tie-lines and tie-triangles are presented for an isothermal section of the phase di agram at 380 degrees C. A hitherto unknown genuine ternary phase AuCo2 (1-u)Sn-4 is found, with 0.167 less than or equal to u less than or eq ual to 0.180, and as indicated by the formula, the non-stoichiometry i s of the subtractional kind (confirmed by density measurements). The u nit cell is monoclinic (space group C2/m) with a = 1260.3(3), b = 421. 3(1), c = 522.9(1) pm and beta = 104.62(2)degrees for u = 0.167. Au in 2a; (0.833) Co, Sn(I) and Sn(II) in 4i with x = 0.2116(2), z = 0.3339 1(5) for Co, x = 0.41759(9), z = 0.6493(2) for Sn(I) and x = 0.18489(9 ), z = 0.8142(2) for Sn(II). The structure of AuCo2(1-u)Sn-4 is of the Ni3+vSn4 type and its atomic arrangement is discussed in relation to that of the prototype. AuCo2(1-u)Sn-4 behaves as a-virtually ideal Cur ie paramagnet with mu(P) = 1.191(3) mu(B) per Co atom.