First-order structural transition and pressure-induced lattice/phonon anomalies in Sr2IrO4

Abstract

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.

Anisotropic lattice compression and pressure-induced electronic phase transitions in Sr2IrO4

“…Since the accuracy of the Bragg intensities was limited by poor grain statistics (see ref. 21) and possible preferred orientation effects, the atomic positions and Debye Waller factors were kept fixed in the refinements at previously reported values at zero pressure (Ref. 12).…”

“…a) and4(b) show the a-and c-axes compressibilities κ a (P ) and κ c (P ) obtained from the refined lattice parameters at (a) 20 K and (b) room temperature, respectively, where the latter was extracted from the data shown Ref 21…”

Persistent insulating state at megabar pressures in strongly spin-orbit coupled Sr2IrO4

“…These emergent peaks become more pronounced with increasing P and are well indexed by an orthorhombic structure with space group Pbca, which requires both a rotation and tilt of the IrO6 octahedra of Sr2IrO4. We note that a pressure-induced phase transition in Sr2IrO4 was reported in an early study; however, broadly overlapping peaks of XRD patterns at high pressures prevented a refinement of the pressure-induced space group [34].It is clear that the crystal structure of Sr2IrO4 remains the ambient tetragonal phase below 37 GPa, and then assumes the orthorhombic Pbca phase at 40.6 GPa (Fig.3(c)). Further, when applied pressure is reduced from 74 GPa to 0.1 GPa, the ambient tetragonal phase is recovered, confirming the observed pressure-driven structural transition is intrinsic and reproducible (see the green line in Fig.2(a)).…”

Aberrant electronic and structural alterations in pressure tuned perovskite NaOsO3

“…T MIT is less affected by pressure, and the experimentally traced rate of T MIT decrease is quite slow~0.36 GPa/K. We note that such sluggish process of metallization during compression among 5d systems (especially iridates) is common with the presence of strong SOC, which usually sustains a rigid band gap [27][28][29][30][31] . In NaOsO 3 with the t 3 2g electronic configuration, the SOC formally should not play a dominant role because the resulting L eff = 0 state yields a vanishing orbital moment, yet it is found to be responsible for the weakening of the electron-electron correlation 7 , magnetic anisotropy, and large spin wave gap formation 32 .…”