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
@article{arxiv.0811.3378,
title = {Metal-Insulator Transition and Orbital Order in PbRuO3},
author = {S. A. J. Kimber and J. A. Rodgers and H. Wu and C. A. Murray and D. N. Argyriou and A. N. Fitch and D. I. Khomskii and J. P. Attfield},
journal= {arXiv preprint arXiv:0811.3378},
year = {2009}
}