Bandwidth-driven nature of the pressure-induced metal state of LaMnO ₃

Abstract

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 25 GPa). The ambient phase with highly Jahn-Teller (JT) distorted M nO6 octahedra is progressively substituted by a new phase with less-distorted JT M nO6 units. The electronic delocalization leading to the IM transition is finger-printed from the pre-edge XAS structure around 30 GPa. 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.PACS numbers: 61.50. Ks,71.70.Ej, 71.30.+h LaMnO 3 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 M n 3+ ion with high-spin configuration tis surrounded by an octahedron of six oxygen ligands. Under ambient conditions, the Jahn-Teller (JT ) instability of the singly occupied e g orbitals gives rise to cooperative distortions of the M nO 6 octahedra, which induce orbital ordering and may be responsible for the insulating behavior of LaMnO 3 . Although the ground-state of LaMnO 3 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 LaMnO 3 undergoes at T IM =710 K an insulator to metal (IM) transition, structurally described as a transition from an ordered toward a 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 M nO 6 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 LaMnO 3 the IM transition has been reported by Loa and coworkers at an applied pressure of 32 GPa 12 . Using X-ray diffraction under pressure, these authors obtained refined atomic positions up to 11 GPa 12 . From an extrapolation of their results, they predict that the local JT distortion should completely vanish around 18 GPa. However, in a previous X-ray ...

Uniaxial pressure-induced half-metallic ferromagnetic phase transition inLaMnO3

“…This pressure induces a phase separated (PS) state characterized by the coexistence of different magnetic and structural domains. The PS state along with half-metallic character have been suggested by several authors to be key for optimizing the CMR state in manganites [8][9][10][11][12][13][14][15].…”

Dynamic and structural properties of orthorhombic rare-earth manganites under high pressure

“…A first report by Loa et al [5] of the suppression of the Jahn-Teller distortion at 18 GPa followed by an insulator-to-metal transition at 32 GPa triggered a number of theoretical studies devoted to Jahn-Teller distortion under pressure and the mechanism responsible for the insulator-to-metal transition [6][7][8][9]. Experimentally, subsequent Raman and extended x-ray absorption fine structure (EXAFS) studies have given evidence for the persistence of the Jahn-Teller distortion over the entire range of the insulating phase at the local scale [10,11]. In contrast, only a couple of works have been reported on other manganites.…”

Strong coupling of Jahn-Teller distortion to oxygen-octahedron rotation and functional properties in epitaxially strained orthorhombic LaMnO3

“…Strain and pressure effects on magnetism and metal-insulator transition in LaMnO 3 (Refs. [39][40][41][42] and LaMnO 3 -based superlattices [43][44][45][46] have received considerable attention. In particular, previous first-principles studies of LaMnO 3 demonstrate electronic and magnetic phase transitions driven by uniaxial strain.…”