The structure of platinum-rich aluminium/platinum alloys

Platinum-rich aluminium/platinum alloys have been synthesised via a coupled reaction between aluminium oxide and platinum in a stream of hydrogen at temperatures in the region of 1200 degrees C. The phase richest in aluminium that was prepared was the alloy Al1.16Pt2.84 which crystallises in the AuCu3 structure type. In accordance with the composition, 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 platinum atoms are twisted with respect to each other. The severity of the limited tetragonal distortion which results increases with further decrease in the aluminium content in the phase. A two phase region exists between the limiting composition of this phase and that of the aluminium-poor Pt/Al mixed crystal possessing a statistical atom distribution. By heating the alloys with the slight tetragonal distortion, a second order phase transition into the cubic phase occurs whilst the alloys with a stronger tetragonal distortion undergo a corresponding phase transition which is first order. This result conforms with the idea that, in the former case, the distortion of the platinum octahedra is progressively relieved while in the second case this event results discontinuously. Consequently, both the tertagonal and the cubic phase are observed simultaneously in a distinct temperature region.