The electronic structure and magnetism of Ir 5d5 states in nonmetallic, weakly ferromagnetic BaIrO3 are probed with x-ray absorption techniques. Contrary to expectation, the Ir 5d orbital moment is found to be ~1.5 times larger than the spin moment. This unusual, atomiclike nature of the 5d moment is driven by a strong spin-orbit interaction in heavy Ir ions, as confirmed by the nonstatistical large branching ratio at Ir L(2,3) absorption edges. As a consequence, orbital interactions cannot be neglected when addressing the nature of magnetic ordering in BaIrO3. The local moment behavior persists even as the metallic-paramagnetic phase boundary is approached with Sr doping or applied pressure.

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The pressure- and anion-dependent electronic structure of EuX (X=Te, Se, S, O) monochalcogenides is probed with element- and orbital-specific x-ray absorption spectroscopy in a diamond anvil cell. An isotropic lattice contraction enhances the ferromagnetic ordering temperature by inducing mixing of Eu 4f and 5d electronic orbitals. Anion substitution (Te-->O) enhances competing exchange pathways through spin-polarized anion p states, counteracting the effect of the concomitant lattice contraction. The results have strong implications for efforts aimed at enhancing FM exchange interactions in thin films through interfacial strain or chemical substitutions.

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...

Two intriguing unresolved issues of iridate physics are the avoided metallization under applied pressure of undoped Sr2IrO4 and related materials, and the apparent absence of superconductivity under electron doping despite the similarity of the fermiology of these materials with respect to cuprates. Here, we investigate the crystal structure and lattice vibrations of Sr2IrO4 by a combined phonon Raman scattering and x-ray powder diffraction experiment under pressures up to 66 GPa and room temperature. Density functional theory (DFT) and ab-initio lattice dynamics calculations were also carried out. A first-order structural phase transition associated with an 8 % collapse of the c-axis is observed at high pressures, with phase coexistence being observed between ∼ 40 and 55 GPa. At lower pressures and still within the high-symmetry tetragonal phase, a number of lattice and phonon anomalies were observed, reflecting crossovers between isostructural competing states. A critical pressure of P1 = 17 GPa is associated with the following anomalies: (i) a reduction of lattice volume compressibility and a change of behavior of the tetragonal c/a ratio take place above P1; (ii) a four-fold symmetry-breaking lattice strain associated with lattice disorder is observed above P1; (iii) two strong Raman active modes at ambient conditions (at ∼ 180 and ∼ 260 cm −1 ) are washed out at P1; and (iv) an asymmetric Fano lineshape is observed for the ∼ 390 cm −1 mode above P1, revealing a coupling of this phonon with electronic excitations. DFT indicates that the Ir 4+ in-plane canted magnetic moment is unstable against a volume compression, indicating that the phase above P1 is most likely non-magnetic. Exploring the similarities between iridate and cuprate physics, we argue that these observations are consistent with the emergence of a rotational symmetry-breaking electronic instability at P1, providing hints for the avoided metallization under pressure and supporting the hypothesis of possible competing orders that are detrimental to superconductivity in this family. Alternative scenarios for the transition at P1 are also suggested and critically discussed. Additional phonon and lattice anomalies in the tetragonal phase are observed at P2 = 30 and P3 = 40 GPa, indicating further competing phases that are stabilized at high pressures.