HIGH-TEMPERATURE PHASE-TRANSITIONS IN AL1-X(XPT3)

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. In accordance w ith the composition, the platinum sites are statistically occupied to a slight extent by aluminium atoms. Upon heating the alloys containing a slight tetragonal distortion, a second order phase transition into a cubic phase occurs whilst the alloys with a stronger tetragonal dist ortion undergo a corresponding phase transition which is first order. This result conforms with the idea that, in the former case, the disto rtion of the platinum octahedra is progressively relieved with increas ing temperature while in the second case this event results discontinu ously.