Advanced synchrotron radiation focusing down to a size of 300 nm has been used to visualize nanoscale phase separation in the K${}_{0.8}$Fe${}_{1.6}$Se${}_2$ superconducting system using scanning nanofocus single-crystal x-ray diffraction. The results show an intrinsic phase separation in K${}_{0.8}$Fe${}_{1.6}$Se${}_2$ single crystals at $T$ < 520 K, revealing the coexistence of (i) a magnetic phase characterized by an expanded lattice with superstructures due to Fe vacancy ordering and (ii) a nonmagnetic phase with an in-plane compressed lattice. The spatial distribution of the two phases at 300 K shows a frustrated or arrested nature of the phase separation. The space-resolved imaging of the phase separation permitted us to provide direct evidence of nanophase domains smaller than 300 nm and different micrometer-sized regions with percolating magnetic or nonmagnetic domains forming a multiscale complex network of the two phases.

Local structure of ${\text{FeSe}}_{1-x}{\text{Te}}_x$ has been studied by extended x-ray absorption fine-structure (EXAFS) measurements as a function of temperature. Combination of Se and $\text{Fe}K$ edge EXAFS has permitted to quantify the local interatomic distances and their mean-square relative displacements. The Fe-Se and Fe-Te bond lengths in the ternary system are found to be very different from the average crystallographic Fe-Se/Te distance, and almost identical to the Fe-Se and Fe-Te distances for the binary FeSe and FeTe systems, indicating distinct site occupation by the Se and Te atoms. The results provide a clear evidence of local inhomogeneities and coexisting electronic components in the ${\text{FeSe}}_{1-x}{\text{Te}}_x$, characterized by different local structural configurations, with direct implication on the fundamental electronic structure of these superconductors.

We report the structural, vibrational, and electrical transport properties up to $\sim 16\mathrm{GPa}$ of $1T\text{−}\mathrm{TiT}{\mathrm{e}}_2$, a prominent layered 2D system. We clearly show signatures of two isostructural transitions at $\sim 2\mathrm{GPa}$ and $\sim 4\mathrm{GPa}$ obtained from the minima in $c/a$ ratio concomitant with the phonon linewidth anomalies of $E_g$ and $A_{1g}$ modes around the same pressures, providing a strong indication of unusual electron-phonon coupling associated with these transitions. Resistance measurements present nonlinear behavior over similar pressure ranges shedding light on the electronic origin of these pressure-driven isostructural transitions. These multiple indirect signatures of an electronic transition at $\sim 2\mathrm{GPa}$ and $\sim 4\mathrm{GPa}$ are discussed in connection with the recent theoretical proposal for $1T\text{−}\mathrm{TiT}{\mathrm{e}}_2$ and also the possibility of an electronic topological transition from our electronic Fermi surface calculations. Between 4 GPa and $\sim 8\mathrm{GPa}$, the $c/a$ ratio shows a plateau suggesting a transformation from an anisotropic 2D layer to a quasi-3D crystal network. First-principles calculations suggest that the 2D to quasi-3D evolution without any structural phase transitions is mainly due to the increased interlayer Te-Te interactions (bridging) via the charge density overlap. In addition, we observed a first-order structural phase transition from the trigonal ($P\overline{3}m1$) to monoclinic ($C2/m$) phase at higher pressure regions. We estimate the start of this structural phase transition to be $\sim 8\mathrm{GPa}$ and also the coexistence of two phases [trigonal ($P\overline{3}m1$) and monoclinic ($C2/m$)] was observed from $\sim 8\mathrm{GPa}$ to $\sim 16\mathrm{GPa}$.

We have used scanning micro x-ray diffraction to characterize different phases in superconducting ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ as a function of temperature, unveiling the thermal evolution across the superconducting transition temperature ($T_c\sim 32$ K), phase separation temperature ($T_{ps}\sim 520$ K), and iron-vacancy order temperature ($T_{vo}\sim 580$ K). In addition to the iron-vacancy ordered tetragonal magnetic phase and orthorhombic metallic minority filamentary phase, we have found clear evidence of the interface phase with tetragonal symmetry. The metallic phase is surrounded by this interface phase below $\sim 300$ K, and is embedded in the insulating texture. The spatial distribution of coexisting phases as a function of temperature provides clear evidence of the formation of protected metallic percolative paths in the majority texture with large magnetic moment, required for the electronic coherence for the superconductivity. Furthermore, a clear reorganization of iron-vacancy order around the $T_{ps}$ and $T_c$ is found with the interface phase being mostly associated with a different iron-vacancy configuration, that may be important for protecting the percolative superconductivity in ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$.