Bandwidth-driven nature of the pressure-induced metal state of LaMnO3

Using X-ray absorption spectroscopy (XAS), we studied the local structure in LaMnO3 under applied pressure across and well above the insulator-to-metal (IM) transition. A hysteretic behavior points to the coexistence of two phases within a large pressure range (7 to 25GPa). The ambient phase with highly Jahn-Teller (JT) distorted MnO6 octahedra is progressively substituted by a new phase with less-distorted JT MnO6 units. The electronic delocalization leading to the IM transition is finger-printed from the pre-edge XAS structure around 30GPa. We observed that the phase transition takes place without any significant reduction of the JT distortion. This entails band overlap as the driving mechanism of the IM transition. Copyright c © EPLA, 2011 LaMnO3 is the parent compound of a family of doped manganates that exhibit a multitude of electronic phases and unusual properties with strong potential for new electronic devices [1]. The compound crystallizes in the orthorhombically distorted perovskite structure (space group Pbmn), in which every Mn ion with high-spin configuration t2ge 1 g is surrounded by an octahedron of six oxygen ligands. Under ambient conditions, the Jahn-Teller (JT) instability of the singly occupied eg orbitals gives rise to cooperative distortions of the MnO6 octahedra, which induce orbital ordering and may be responsible for the insulating behavior of LaMnO3. Although the groundstate of LaMnO3 can be explained both by cooperative JT distortion and orbital exchange interaction, their relative importance lead to different physics for the doped systems and is then an important issue for predicting and optimizing physical properties. However, the separation of the two effects is a very delicate issue [2–8] that calls for accurate experimental probes. At ambient pressure LaMnO3 undergoes at TIM = 710K an insulator-to-metal (IM) transition, structurally described as a transition from an ordered toward a (a)E-mail: [email protected] disordered array of JT distorted octahedra. The long-range structure is characterized by a strong cell symmetrization and the loss of the orbital order [9]. The local scale JT splitting persists essentially unaltered across the IM transition, which is marked by the symmetrization of the thermal fluctuations in the distorted MnO6 units [10,11] At ambient temperature, transition towards a metal state can also be attained by application of external hydrostatic pressure. High pressure reduces lattice parameters, favoring orbital overlap and concomitant enhancement of electron delocalization. It also forces an atomic rearrangement, tending to reduce lattice distortions (inter-octahedral and intra-octahedra rearrangements). In LaMnO3 the IM transition has been reported by Loa and coworkers at an applied pressure of 32GPa [12]. Using X-ray diffraction under pressure, these authors obtained refined atomic positions up to 11GPa [12]. From an extrapolation of their results, they predict that the local JT distortion should completely vanish around 18GPa. However, in a previous X-ray absorption spectroscopy (XAS) study under pressures up to 15GPa [13], our group observed a shortening of the long MnO bond lenght too limited to lead to a