CRYSTAL-STRUCTURE OF UAL2 ABOVE 10 GPA AT 300 K

A high pressure X-ray diffraction study on UAl2 has been done up to ap proximate to 28 Gpa. It undergoes a structural transition at approxima te to 11 GPa and the structure of the high pressure phase has been ide ntified to be of the MgNi2 type with space group P6(3)/mmc. The struct ure of the ambient pressure phase is of the MgCu2 type with space grou p Fd3m. From the electron per atom ratio e/a, it is expected that it m ay transform back to the MgCu2 type structure at still higher pressure s. On the basis of similar arguments, it is expected that most of the AB(2) type Laves phase compounds of the f electron systems with suitab le e/a ratios may undergo the pressure induced structural transition i n the sequence MgCu2-->MgZn2(or MgNi2)-->MgCu2 due to increased deloca lization of their f electron states.