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.

Low-dimensional metal halide compounds, usually described as low-dimensional perovskites, present exciting properties as functional materials for a broad range of optoelectronic applications. These compounds are characterized by intense photoluminescence (PL), a narrow emission line width, and a high exciton binding energy. In particular, the mechanism behind the strong green emission of the zero-dimensional compound Cs 4 PbBr 6 has been the subject of intense debate. As a propertytuning tool, hydrostatic pressure was used to investigate the structural and optical properties of bulk Cs 4 PbBr 6 through synchrotron X-ray diffraction combined with Raman and PL spectroscopies. As a result, two structural phase transitions at 3.2 and 4.6 GPa were identified, with the latter not observed in previous investigations performed on nanocrystals. Also, the pressure dependence of the PL emission was recorded and compared with the previous results on Cs 4 PbBr 6 and CsPbBr 3 nanocrystals. Under the ambient conditions, strong green emission exhibits a subtle redshift, followed by a blueshift under pressure, being associated first with an intensity enhancement and subsequent quenching above 3 GPa. These results support the CsPbBr 3 luminescent inclusions as the PL emission mechanism in Cs 4 PbBr 6 .

A remarkable hardening (∼ 30 cm −1 ) of the normal mode of vibration associated with the symmetric stretching of the oxygen octahedra for the Ba 2 FeReO 6 and Sr 2 CrReO 6 double perovskites is observed below the corresponding magnetic ordering temperatures. The very large magnitude of this effect and its absence for the anti-symmetric stretching mode provide evidence against a conventional spin-phonon coupling mechanism. Our observations are consistent with a collective excitation formed by the combination of the vibrational mode with oscillations of local 3d and 5d occupations and spin magnitudes.