THE STRUCTURE OF PLATINUM-RICH ALUMINIUM PLATINUM ALLOYS/

Platinum-rich aluminium/platinum alloys have been synthesised via a co upled reaction between aluminium oxide and platinum in a stream of hyd rogen at temperatures in the region of 1200 degrees C. The phase riche st in aluminium that was prepared was the alloy Al1.16Pt2.84 which cry stallises in the AuCu3 structure type. In accordance with the composit ion, the platinum sites are statistically occupied to a slight extent by aluminium atoms. In a phase somewhat poorer in aluminium (Al1.05Pt2 .95), the edge-sharing octahedra consisting almost exclusively of plat inum atoms are twisted with respect to each other. The severity of the limited tetragonal distortion which results increases with further de crease in the aluminium content in the phase. A two phase region exist s between the limiting composition of this phase and that of the alumi nium-poor Pt/Al mixed crystal possessing a statistical atom distributi on. By heating the alloys with the slight tetragonal distortion, a sec ond order phase transition into the cubic phase occurs whilst the allo ys with a stronger tetragonal distortion undergo a corresponding phase transition which is first order. This result conforms with the idea t hat, in the former case, the distortion of the platinum octahedra is p rogressively relieved while in the second case this event results disc ontinuously. Consequently, both the tertagonal and the cubic phase are observed simultaneously in a distinct temperature region.