In a methodical theoretical of the CLC transition reported the prediction of a generality of the effect in 5d transition metals 11, revealing a CLC transition for Ir at a reasonable low-pressure of 80 GPa. This pressure-induced transition has been associated with interactions between the core electrons that affect the pressure evolution of the lattice parameter ratio c/a in Os at 440 GPa. 10 have reported a new type of electronic transition, the so-called core-level crossing (CLC) transition in osmium (Os) metal at around 440 GPa. More recent studies have shown that 5d metals undergo pressure-induced peculiarities in their electronic structure. In subsequent experimental-theoretical studies, such a phase transition was not predicted 8, 9. The structure of such a superlattice corresponded to a 14-layer hexagonal closed packed structure with lattice parameters a = 2.60 Å and c = 29.68 Å. 7 reported the formation of a complex superlattice in iridium above 59 GPa in a high-pressure energy-dispersive x-ray diffraction (XRD) experiment. In this sense, the debate on the structural stability of iridium under pressure has taken years. The study of the 5d metals under pressure have attracted the attention of the scientific community since the beginning of the 21 st century because of the relevance of their behaviour under extreme conditions for improving the knowledge of planet interiors 6. Whereby, its use in numerous applications make the study of Ir metal of great interest from a fundamental standpoint. Furthermore, Ir is being currently used to synthesize Ir-based double perovskite compounds and iridium-based transition metal oxides or iridates, and its application fields stand from high-temperature superconductivity 3, magnetoresistance 4, to multiferroicity 5. On the other hand, Ir is used for the construction of thermocouples and encapsulators of nuclear-powered electrical generators in space technology 2. These properties make Ir useful for many technological applications for instance, as a high-pressure gasket or as a pressure calibrant in high temperature and high-pressure (HP) diamond-anvil cell (DAC) experiments. On top of that, Ir has an extremely high thermal stability, being able to preserve mechanical stability at temperatures above 2000 ☌ and it is not easily susceptible to corrosion. It is the second densest elemental metal having an ambient pressure density of 22.65 g/cc at T = 0 K and 22.56 g/cc at T = 293.15 K, and a shear modulus, G o = 210 GPa, comparable to that of osmium, G o = 220 GPa, at ambient conditions 1. Iridium (Ir), with electronic structure 4d 105s 25p 64f 145d 76s 2, is one of the most incompressible 5d transition metals with face-centered cubic (fcc) structure. The remarkable agreement observed between experimental and calculated spectra validates the reliability of theoretical predictions of the pressure dependence of the electronic structure of iridium in the studied interval of compressions. X-ray absorption spectroscopy, which probes the local structure and the empty density of electronic states above the Fermi level, was also utilized. The compressibility behaviour was characterized by an accurate determination of the pressure-volume equation of state, with a bulk modulus of 339(3) GPa and its derivative of 5.3(1). Synchrotron-based powder x-ray diffraction results highlight a large stability range (up to 1.4 Mbar) of the low-pressure phase. Here, we report an experimental structural characterization of iridium by x-ray probes sensitive to both long- and short-range order in matter. In particular, iridium metal has been proposed to exhibit a recently discovered pressure-induced electronic transition, the so-called core-level crossing transition at the lowest pressure among all the 5d transition metals. The 5d transition metals have attracted specific interest for high-pressure studies due to their extraordinary stability and intriguing electronic properties.
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