  Anomalous low temperature electronic and structural behaviour has been
discovered in PbRuO3. The structure (space group Pnma, a = 5.56314(1), b =
7.86468(1), c = 5.61430(1) A) and metallic conductivity at 290 K are similar to
those of SrRuO3 and other ruthenate perovskites, but a sharp metal-insulator
transition at which the resistivity increases by four orders of magnitude is
discovered at 90 K. This is accompanied by a first order structural transition
to an Imma phase (a = 5.56962(1), b = 7.74550(1), c = 5.66208(1) A at 25 K)
that shows a coupling of Ru4+ 4d orbital order to distortions from Pb2+ 6s6p
orbital hybridization. The Pnma to Imma transition is an unconventional
reversal of the group-subgroup symmetry relationship. No long range magnetic
order is evident down to 1.5 K. Electronic structure calculations show that
hybridization of Pb 6s6p and Ru 4d orbitals and strong spin-orbit coupling
stabilise this previously hidden ground state for ruthenate perovskites
