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Brightness traces of the solidification of highly undercooled Ta and Re droplets processed in an ultrahigh vacuum drop tube give evidences of two successive phase transformations. First-principles calculations of the structural stability of these metals are developed in order to appreciate the possibilities of obtaining alternative phases. Some prototype transition metal crystal structures were examined and the fee phase for Re and the A 15 phase for Ta are possible candidates for the observed metastable phases. Good agreement is obtained between theoretical calculations and experimental measurements of their respective melting temperatures.PACS numbers: 64.60. My, 64.60.Qb, 71.25.Pi Crystal structures of elemental metals tend to have certain sequences when viewed as functions of atomic number. Using qualitative pictures based on simple tightbinding arguments, it has been shown that, for the transition metals, the structural energy differences are given by only the ^-band energy differences, assuming that the dominant term which minimizes the total energy of a given structure is associated with the shape and the filling of the canonical d bands [ll. The canonical hcp-bcchcp-fcc sequence of structures observed across the nonmagnetic Ad and Sd transition series can be understood from the fact that the topology of the lattice is reflected in powers of the Hamiltonian [2]. All these results are now well supported by detailed quantum-mechanical calculations [3]. For instance, Andersen et al. and Heine [4] showed the critical role played by the band contribution, demonstrating the cancellation of other contributions to the total energy, which are the electrostatic double counting and the exchange and correlation terms, provided that the electron potential is frozen from one structure to the other. This approach, referred to as Andersen's force theorem, was used by Skriver [5] to calculate the relative stability of all elements. Very recently, Paxton, Methfessel, and Polatoglou [6] have shown that non-self-consistent Harris-Foulkes calculations are remarkably successful in reproducing self-consistent calculations of structural properties in first-row transition metals. Similar transition metal structure sequences are expected to occur in individual transition metals with increasing pressure since compression leads to an increase in ^-electron population by transfer from the s band. This pressure-induced phase transformation sequence has received extensive experimental as well as theoretical attention [7]. However, there are many other ways of obtaining metastable phases. In this Letter, we present the first experimental evidence of metastable phases in Re and Ta metals brought on by undercooling and we predict these structures from first principles.High undercooling was recently obtained on some refractory metals by means of an ultrahigh vacuum drop tube [8]. For instance, undercoolings of 900 and 550 K were respectively achieved for rhenium and tungsten millimeter size droplets. As noticed by different investigato...

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