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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.

The applicability of the coherent-potential approximation ͑CPA͒ for the description of electronic properties of completely random alloys is investigated. This is done by calculating the density of states and the total energy for different systems and by comparing the results with those obtained for large supercells consisting of up to 320 atoms in the framework of the order-N locally self-consistent Green's function method. Thereby it is found that in the framework of the CPA one obtains a reliable description of the electronic structure of random alloys. The total energy of a completely disordered alloy can also be reliably estimated provided an appropriate account is given for the electrostatic contribution to the one-electron potential and energy. Therefore, we conclude that the CPA can be safely applied to study the influence of disorder on various properties of metallic alloys for which the muffin-tin or atomic sphere approximation is sufficient and the chemical contribution to the total energy dominates. ͓S0163-1829͑98͒03122-1͔

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The crystal structure of CoSn represents a highly peculiar intermetallic structure type because of the presence of a large void and highly nonspherically coordinated tin atoms. Voids in crystal structures of intermetallic compounds are most unusual and always give rise to speculations whether the observed phase is the ground state or not. Here we examine this problem by means of ab initio calculations and show that the void structure of CoSn is indeed the stable phase. With pressure we predict the following sequence of structural transformations: CoSn type ! FeSi type ! CsCl type. The structural properties of CoSn are understood in terms of competition between covalent and metallic bonding in the system. [S0031-9007(97)03781-2]