Band structure theory of the bcc to hcp Burgers distortion

The Burgers distortion is a two-stage transition between body centered cubic\n(BCC) and hexagonal close-packed (HCP) structures. Refractory metal elements\nfrom the Sc and Ti columns of the periodic table (BCC/HCP elements) form BCC\nstructures at high temperatures but transition to HCP at low temperatures via\nthe Burgers distortion. Elements of the V and Cr columns, in contrast, remain\nBCC at all temperatures. The energy landscape of BCC/HCP elements exhibits an\nalternating slide instability, while the normal BCC elements remain stable as\nBCC structures. This instability is verified by the presence of unstable\nelastic constants and vibrational modes for BCC/HCP elements, while those\nelastic constants and modes are stable in BCC elements. We show that a\npseudogap opening in the density of states at the Fermi level drives the\nBurgers distortion in BCC/HCP elements, suggesting the transition is of the\nJahn-Teller-Peierls type. The pseudogap lies below the Fermi level for regular\nBCC elements in the V and Cr columns of the periodic table. The wave vector\n$k_S$ when the gap opens relates to the reciprocal lattice vector G=(1\n$\\frac{1}{2}$ $\\frac{1}{2}$) of the distorted BCC structure as\n$k_S$=$\\frac{1}{2}$G. BCC binary alloys containing both BCC/HCP and BCC\nelements exhibit a similar instability but stabilize part way through the BCC\nto HCP transition.\n