PHASE-EQUILIBRIA IN THE GD-AU SYSTEM

The Gd-Au phase diagram was studied in the 0-100 at% Au composition ra nge by differential thermal analysis (DTA), X-ray diffraction (XRD), o ptical and scanning electron microscopy (SEM) and electron probe micro analysis (EPMA). Eight intermetallic compounds were identified and the ir melting behaviour was determined: Gd2Au (orthorhombic, oP12-Co2Si t ype, peritectic decomposition 935 degrees C), GdAu (L.T. form, orthorh ombic, oC8-CrB type, and H.T. form, cubic, cP2-CsC1 type, m.p. 1585 de grees C), Gd3Au4 (trigonal, hR42-Pu3Pd4 type, peritectic decomposition 1340 degrees C), Gd7Au10 (tI136-Gd7Au10, peritectic decomposition 116 0 degrees C), GdAu2 (tetragonal, tIG-MoSi2 type, m.p. 1180 degrees C), GdAu3 (orthorhombic, oP8-TiCu3, type, m.p. 1200 degrees C), Gd14Au51 (hexagonal, hP65-Gd14Ag51 type, m.p. 1165 degrees C) and GdAu6 (tetrag onal, tP56-SmAu6 type, peritectic decomposition 855 degrees C). Five e utectic reactions were observed to occur at 845 degrees C and 21.0 at% Au, at 1140 degrees C and 635 at% Au, at 1110 degrees C and 70.5 at% Au, at 1135 degrees C and 77.0 at% Au and at 800 degrees C and 89.0 at % Au, respectively. A catatectic decomposition reaction of the (beta-G d) phase was also found at 1220 degrees C and approximate to 3 at% Au. The crystal structures of the Gd3Au4 and Gd7Au10 phases were determin ed by single crystal methods. Gd3Au4 is isotypic with Pu3Pd4. The Gd7A u10 phase is of a new type and can be described as a sequence of layer s perpendicular to the c-axis in which three kinds of layers are recog nizable. The results are briefly discussed and compared with those of the Sm-Au system.