The Breathing Mode of B aB i03: Electric Field Gradient and Total Energy Calculations*

BaBiOa is the parent compound of the new class of high Tc superconductors that contain no copper. By partially replacing Ba with K (or Bi with Pb) B a!_xK xBi0 3 becomes superconducting in a small range of x. The undoped material is a semiconductor in which the perovskite structure is unstable with re­ spect to a breathing-like distortion (Fig. 1) of the oxy­ gen octahedron (Bi disproportionates into two in equivalent Bi valences) and a rotation of these octahedra leads to a monoclinic structure [1]. Substitution removes these distortions and leads to an ideal per­ ovskite (simple cubic) structure and to superconduc­ tivity. Several ab initio band structure calculations are available, [2-5] but the stabilty of the simple cubic (sc) B aB i03 with respect to the breathing distortion dif­ fers, although the band structures are very similar. The results depend on the method used and the com­ putational details how the Bi and Ba semicore states are treated. The following results were found i) no breathing instability at all [3], ii) a small instability [5] of 0.1 mRy, iii) one of 3 mRy [4] or iv) a relative large instability [2] of about 6 mRy. In the present paper we investigate the total energy as a function of this