Vibrational Infrared and Raman Spectra of Non-Metals

The coherent potential approximation has been employed to calculate the dielectric matrix in random binary alloys. The dielectric matrix so obtained has been examined to deduce the behaviour of the density-density response function, which has been used to infer the average and the random parts of the force constants that appear in the phonon Hamiltonian. The force constants between two sites have been shown to be proportional to the nuclear charges of the corresponding nuclei, within the proposed approximation. This result suggests that the fluctuations of the force constants in random binary alloys are small. The author thinks that the present calculation can give an explanation of the long phonon lifetime found experimentally in alloys with small mass disorder.

https://iopscience.iop.org/article/10.1088/0305-4608/8/5/007/pdf

Dielectric matrix and related properties in random binary alloys

Firstly we report here the results of a neutron scattering experiment on the phonon dispersion relations and phonon lifetimes in the alloy Pd0.84-Pt0.16 and compare them with those of an existing coherentpotential approximation calculation. We find good agreement in the frequency and shape of the resonance and remaining discrepancies are discussed. Then the results of existing or new CPA calculations for several other alloys are compared with experimental data on resonant and localized modes observed by other authors in the alloys. For these other systems in most cases a satisfactory agreement is found in the frequency position and shape of the resonance or of the localized mode, when the alloy characteristics are suited for a straightforward application of mass defect CPA. The relevance of these tests for CPA is also discussed.

Phonon modes in Pd-Pt and other substitutional alloys with large mass disorder

The lattice dynamics of the binary alloys Ni0.55Pd0.45, Pd0.84Pt0.16, Ni0.3Fe0.7 and Ni0.5Co0.5 has been studied within the framework of the transition metal model potential approach and the mean-crystal approximation. A short-range three-body interaction, incorporated in a phenomenological way in the study, leads to an improvement in the transverse branches of the phonon dispersion curves. The theoretically calculated phonon dispersion curves are found to be in good agreement with the experimental data.

Lattice dynamics of binary alloys

The phonon dispersion relations of Nb–Ta and Pd–Pt alloy systems are calculated in the framework of the coherent potential approximation. Though only a few experimental data are available to be compared with the theory, the satisfactory agreement obtained, indicates the necessity of further experimental and theoretical effort in the study of alloy phonon dispersion relations.



Die Phononendispersion–Beziehungen von Nb–Ta- und Pd–Pt-Legierungen werden im Rahmen der CPA berechnet. Obwohl derzeit nur wenige experimentelle Daten zum Vergleich mit der Theorie zur Verfugung stehen, zeigt die zufriedenstellende Ubereinstimmung die Notwendigkeit weiterer experimenteller und theoretischer Untersuchungen der Phononendispersion.

M. C. Spinelli

Papers

Phonon calculation in Nb-Ta and Pd-Pt alloys in the coherent potential approximation