Stability of Jahn-Teller distortion inLaMnO3under pressure: An x-ray absorption study

Abstract

The local environment of manganese atoms in LaMnO3 under pressure up to 15.3 GPa has been studied by x-ray absorption spectroscopy. For pressures below 8 GPa, no change is detected within the MnO6 octahedra. Above this pressure a continuous reduction of the long Mn-O distance takes place, however the octahedral distortion persists over the whole pressure range. At 15.3 GPa the average Jahn-Teller splitting of the distances is reduced by about one third, indicating that a total removal of the local Jahn-Teller distortion would occur only for pressures around 30 GPa, where metallization is reported to take place. A hysteresis in the long distance reduction is observed down to ambient pressure, suggesting the coexistence of MnO6 distorted and undistorted units. The physics underlining the remarkable properties of the manganite LaMnO 3 and its doped alloys is rich and complex. The actual path followed by a given system towards ferromagnetism and/or metallization, through chemical substitution, thermal treatment or pressure involves modifications of a delicate balance between delocalization and carriers trapping not yet completely understood. In the ground state LaMnO 3 is an antiferromagnetic semiconductor crystallizing in an orthorhombic variant of the cubic perovskite structure space group P bnm. The MnO 6 octahedra are distorted due to the Jahn-Teller (JT ) effect of the Mn) and the M n − O distances are split into 4 short bonds and 2 long bonds. In the basal ab plane long and short M n − O bonds alternates. The apical and basal short bonds have different length, however this additional distortion is not resolved by local probes such as real space high resolution diffraction and x-ray absorption spectroscopy. The local radial distribution actually seen by these probes corresponds then to the JT splitting, with 4 oxygensat short distances (M n − O) s ≈ 1.94Å and 2 oxygens at the long distance (M n − O) l ≈ 2.15Å. LaMnO 3 undergoes a transition at T * ≈ 710-750 K from the JT distorted orthorhombic phase O to a high temperature nearly cubic O' phase 1 . The transition is accompanied by abrupt changes in the electrical resistivity and Weiss constant 2 . The cell distortion is nearly removed and the orbital ordering disappears in the O' phase, but the JT distortion of MnO 6 octahedra persists at the local scale 3,4,5 . The transition then happens as an order-disorder transition, in agreement with the thermodynamic calculations 6 . More recently Qiu and co-workers 7 reported on neutron powder diffraction measurements showing that the JT distortion of MnO 6 octahedra is maintained also in the high temperature rhomboedral phase (T ≥ 1010 K) and suggested the presence of ordered clusters with strong antiferrodistorsive coupling.New insights for the role of the JT distortion can be obtained by the exploration of its pressure dependence. In LaMnO 3 , the application of an external hydrostatic pressure produces a reduction of the lattice distortions and an enhancement of the carrier mobility 8,9 . The M n − O − M n angle -ti...

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

“…[1][2][3][4] As a result, there have been many experimental [5][6][7][8] and theoretical [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] studies of the end-member compound LaMnO 3 (LaMnO 3 ). The observed sequence of phases with decreasing temperature is as follows: 8,26 At very high temperature (above 1010 K), LaMnO 3 has a rhombohedral R3c structure, produced by rotation of the oxygen octahedron network of the ideal perovskite structure.…”

Structural and insulator-to-metal phase transition at 50 GPa in GdMnO3

“…Other authors have reported the persistence of the Jahn-Teller distortion over the entire stability range of the insulating phase of LaMnO 3 , suggesting a non classical Mott insulator. 3,6 No other orthorhombic manganite has attracted such an attention, although we note the pressure investigation of structural properties of the magnetoelectric manganites TbMnO 3 and DyMnO 3 or BiMnO 3 , or more complex solid solutions. [9][10][11] To the best of our knowledge, manganites have not yet been investigated in the very high-pressure regime around 50 GPa, despite promising studies on similar orthoferrites or BiFeO 3 , which have revealed intriguing insulator-to-metal or structural transitions in a similar pressure range.…”

“…A notable exception is the study of the crystal structure, Jahn-Teller distortion, orbital order, and pressureinduced insulator-metallic phase transition in LaMnO 3 , although the driving mechanism still remains controversial. [2][3][4][5][6][7][8] According to the pioneer work by Loa et al 2 , the Jahn-Teller effect and the concomitant orbital ordering are suppressed above 18 GPa. The system retains insulator behavior up to 32 GPa, undergoing a bandwidth driven insulator-metal phase transition.…”

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

“…The evolution δE of the edge threshold as a function of the applied pressure (inset fig. 1) is then essentially related to the specific reduction in this long bond 13 . The overall shift of δE = +1.5 eV corresponds to a reduction of 0.15Å in bond length.…”

“…In between the evolution of the distortion parameter is given by the open diamond shaped symbols in the figure 3. In the precedent XAS study 13 the data were limited in pressure and quality. By extrapolating the evolution of the JT distortion up to 30 GPa we guessed that the distortion could disappear.…”