Pressure-induced structural phase transition in NdAl x Ga 2-x

High-pressure neutron-diffraction experiments on the pseudo-ternary compound NdAlxGa2-x revealed a pressure-induced isostructural phase transition for x=1.00 and 1.08 above about 1.5 GPa at room temperature. Within its hexagonal AlB2 structure the compound changes its lattice ratio c/a discontinuously from 0.97 to 0.84 in analogy to chemical pressure for x>1.12. Pressures up to 5.2 GPa did not result in a pure high-pressure phase but rather in a mixture of both phases. The intermetallic MN2 phases of the AlB2 structure are known to appear in two distinct groups with different c/a ratios. The observed isostructural transition is discussed within the framework of near-neighbor diagrams.