Alloying-Driven Phase Stability in Group-VB Transition Metals under Compression
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Alloying-Driven Phase Stability in Group-VB Transition Metals under Compression*, ** Alexander Landa1 and Per Söderlind1 1 Condensed Matter and Materials Division, Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, L-045, 7000 East Avenue, Livermore, CA 94551-0808, U.S.A.
ABSTRACT The change in phase stability of Group-VB (V, Nb, and Ta) transition metals due to pressure and alloying is explored by means of first-principles electronic-structure calculations. It is shown that under compression stabilization or destabilization of the ground-state bodycentered cubic (bcc) phase of the metal is mainly dictated by the band-structure energy that correlates well with the position of the Kohn anomaly in the transverse acoustic phonon mode. The predicted position of the Kohn anomaly in V, Nb, and Ta is found to be in a good agreement with data from the inelastic x-ray or neutron scattering measurements. In the case of alloying the change in phase stability is defined by the interplay between the band-structure and Madelung energies. We show that band-structure effects determine phase stability when a particular GroupVB metal is alloyed with its nearest neighbors within the same d-transition series: the neighbor with less and more d electrons destabilize and stabilize the bcc phase, respectively. When V is alloyed with neighbors of a higher (4d- or 5d-) transition series, both electrostatic Madelung and band-structure energies stabilize the body-centered-cubic phase. The opposite effect (destabilization) happens when Nb or Ta is alloyed with neighbors of the 3d-transition series.
INTRODUCTION Vanadium metal has been the subject of numerous experimental and theoretical studies due to its high superconducting transition temperature. Ishizuka et al. [1] found that for V the superconducting transition temperature, Tc = 5.3 K, increases linearly with pressure reaching 17.2 K (the highest Tc among the elemental metals reported so far) at 1.2 Mbar. In order to explore the response of electron-phonon interaction to high pressures, Suzuki and Otani [2] performed first-principles calculations of the lattice dynamics of V in the pressure range up to 1.5 Mbar. They found that the frequencies of the TA mode [ 00] soften around = ¼ with increasing pressure and become imaginary (unstable) at pressures higher than 1.3 Mbar, indicating a structural phase transition. Later this result was confirmed by other ab-initio calculations [3]. On the experimental side, recently by using inelastic x-ray scattering technique Bosak et al. [4] discovered several anomalies in the phonon dispersion curve for V along highsymmetry directions. Among these anomalies was an upward bending of the transverse acoustic (TA) mode along the -H direction, [00], around = 0.24 – the anomaly that has been predicted theoretically in Refs. [2, 3]. The recent synchrotron x-ray diffraction measurements on V [5] reported a new rhombohedral (rh) phase around 630-690 kbar. Later theoretical studies confirm this finding and also suggest vanadi
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