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The crystal lattice of Sr2IrO4 is investigated with synchrotron X-ray powder diffraction under hydrostatic pressures up to P = 43 GPa and temperatures down to 20 K. The tetragonal unit cell is maintained over the whole investigated pressure range, within our resolution and sensitivity. The c-axis compressibility κc(P, T ) ≡ −(1/c)(dc/dP ) presents an anomaly with pressure at P1 = 17 GPa at fixed T = 20 K that is not observed at T = 300 K, whereas κa(P, T ) is nearly temperatureindependent and shows a linear behavior with P . The anomaly in κc(P, T ) is associated with the onset of long-range magnetic order, as evidenced by an analysis of the temperature-dependence of the lattice parameters at fixed P = 13.7 ± 0.5 GPa. At fixed T = 20 K, the tetragonal elongation c/a(P, T ) shows a gradual increment with pressure and a depletion above P2 = 30 GPa that indicates an orbital transition and possibly marks the collapse of the J ef f = 1/2 spin-orbit-entangled state. Our results support pressure-induced phase transitions or crossovers between electronic ground states that are sensed, and therefore can be probed, by the crystal lattice at low temperatures in this prototype spin-orbit Mott insulator. arXiv:1912.07330v1 [cond-mat.str-el]

The series of intermetallic compounds RNiSi3 (R = rare earth) shows interesting magnetic properties evolving with R and metamagnetic transitions under applied magnetic field for some of the compounds. The microscopic magnetic structures must be determined to rationalize such rich behavior. Here, resonant x-ray magnetic diffraction experiments are performed on single crystals of GdNiSi3 and TbNiSi3 at zero field. The primitive magnetic unit cell matches the chemical cell below the Néel temperatures TN = 22.2 and 33.2 K, respectively. The magnetic structure is determined to be the same for both compounds (magnetic space group Cmmm ). It features ferromagnetic ac planes that are stacked in an antiferromagnetic + − +− pattern, with the rare-earth magnetic moments pointing along the a direction, which contrasts with the + − −+ stacking and moment direction along the b axis previously reported for YbNiSi3. This indicates a sign reversal of the coupling constant between second-neighbor R planes as R is varied from Gd and Tb to Yb. The long b lattice parameter of GdNiSi3 and TbNiSi3 shows a magnetoelastic expansion upon cooling below TN , pointing to the conclusion that the + − +− stacking is stabilized under lattice expansion. A competition between distinct magnetic stacking patterns with similar exchange energies tuned by the size of R sets the stage for the magnetic ground state instability observed along this series.

Charge ordering is prone to occur in crystalline materials with mixed-valence ions. It is presumably accompanied by a structural phase transition, with possible exceptions in compounds that already present more than one inequivalent site for the mixed-valence ions in the charge-disordered phase. In this work, we investigate the representative case of the homometallic Co ludwigite Co 2+ 2 Co 3+ O 2 BO 3 (Pbam space group) with four distinct Co crystallographic sites [M1-M4] surrounded by oxygen octahedra. The mixed-valent character of the Co ions up to at least T = 873 K is verified through x-ray absorption near-edge structure (XANES) experiments. Single crystal x-ray diffraction (XRD) and neutron powder diffraction (NPD) confirm that the Co ions at the M4 site are much smaller than the others at low temperatures, consistent with a Co 3+ oxidation state at M4 and Co 2+ at the remaining sites. The size difference between the Co ions in the M4 and M2 sites is continuously reduced upon warming above ≈370 K, indicating a gradual charge redistribution within the M4-M2-M4 (424) ladder in the average structure. Minor structural anomalies with no space group modification are observed near 475 and 495 K, where sharp phase transitions were previously revealed by calorimetry and electrical resistivity data. An increasing structural disorder, beyond a conventional thermal effect, is noted above ≈370 K, manifested by an anomalous increment of XRD Debye-Waller factors and broadened vibrational modes observed by Raman scattering. The local Co-O distance distribution, revealed by Co K-edge extended x-ray absorption fine structure (EXAFS) data and analyzed with an evolutionary algorithm method, is similar to that inferred from the XRD crystal structure below ≈370 K. At higher temperatures, the local Co-O distance distribution remains similar to that found at low temperatures, at variance with the average crystal structure obtained with XRD. We conclude that the oxidation states Co 2+ and Co 3+ are instantaneously well defined in a local atomic level at all temperatures, however the thermal energy promotes local defects in the charge-ordered configuration of the 424 ladders upon warming. These defects coalesce into a phase-segregated state within a narrow temperature interval (475 < T < 495 K). Finally, a transition at ≈500 K revealed by differential scanning calorimetry (DSC) in the iron ludwigite Fe 3 O 2 BO 3 is discussed.