Transition from a weak ferromagnetic insulator to an exchange-enhanced paramagnetic metal in the BaIrO3 polytypes

As a follow-up of the high-pressure synthesis and the structural determination of two new BaIrO3 polytypes, we report in this paper a systematic study of the physical properties of all polytypes available to us through measurements of magnetic, electronic transport, thermodynamic, and low-temperature structural as well as pressure effects. With increasing fraction of the corner-to face-sharing octahedra in the sequence 9R (hhChhChhC) -> 5H(hChCC) -> 6H(hCChCC), the ground states of BaIrO3 evolve from a ferromagnetic insulator with T-c approximate to 180 K in the 9R phase to a ferromagnetic metal with T-c approximate to 50 K in the 5H phase, and finally to an exchange-enhanced paramagnetic metal near a quantum critical point (QCP) in the 6H phase. The experimental results for the 9R phase confirm that the ferromagnetic transition is accompanied by a lattice instability, presumably associated with the formation of a charge density wave. The evidence includes a sudden increase in resistivity and thermoelectric power, an anomaly in the thermal conductivity, an unusual expansion of the c axis, and an extraordinarily large pressure coefficient of T-c. In contrast, the ferromagnetic transition in the 5H BaIrO3 only gives rise to weak anomalies in the resistivity and specific heat near T-c, similar to SrRuO3; the 5H phase is the first weak ferromagnetic metal among the known oxide iridates. The 6H phase remains a paramagnetic metal to the lowest temperature. However, a strongly enhanced thermoelectric power and a non-Fermi-liquid behavior from the resistivity measurement at low temperature show that quantum critical fluctuations play a role in this exchange-enhanced paramagnetic phase. A positive thermoelectric power confirms the charge carriers are holelike for all polytypes, which is consistent with the electronic configuration of Ir(IV) (5d(5)) in the low-spin state. The low-temperature specific-heat coefficients and Sommerfeld-Wilson ratios are in agreement with the evolution of the ground states across a ferromagnetic to paramagnetic QCP.