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Angle-resolved x-ray diffraction patterns of Xe to 127 GPa indicate that the fcc-to-hcp transition occurs martensitically between 3 and 70 GPa in diamond-anvil cells without an intermediate phase. These data also reveal that the transition occurs by the introduction of stacking disorder in the fcc lattice at low pressure, which grows into hcp domains with increasing pressure. The small energy difference between the hcp and the fcc structures may allow the two phases to coexist over a wide pressure range. Evidence of similar stacking disorder and incipient growth of an hcp phase are also observed in solid Kr.

We have measured the pressure-volume (P-V) relations for cubic iron-nickel alloys for three different compositions: Fe 0.64Ni (0.36), Fe 0.55Ni (0.45), and Fe 0.20Ni (0.80). It is observed that for a certain pressure range the bulk modulus does not change or can even decrease to some minimum value, after which it begins to increase under still higher pressure. In our experiment, we observe for the first time a new effect, namely, that the Fe-Ni alloys with high Ni concentrations, which show positive thermal expansion at ambient pressure, become Invar system upon compression over a certain pressure range.

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