Pressure-induced collapse of large-moment magnetic order and localized-to-itinerant electronic transition in the host-guest compound [Cs6Cl][Fe24Se26]

The magnetic properties of iron chalcogenides and pnictides have found much interest as magnetic fluctuations are suggested to drive the formation of Cooper pairs in iron-based superconductors. Here, we have studied the pressure dependence of the magnetic and electrical properties of the iron-selenide compound $[\mathrm{C}{\mathrm{s}}_{6}\mathrm{Cl}][\mathrm{F}{\mathrm{e}}_{24}\mathrm{S}{\mathrm{e}}_{26}]$ by energy-domain synchrotron M\"ossbauer spectroscopy up to 17 GPa and by resistivity measurements up to 45 GPa. Similar to the binary superconductor $\mathrm{F}{\mathrm{e}}_{1+x}\mathrm{Se}$, the host-guest-type crystal structure of $[\mathrm{C}{\mathrm{s}}_{6}\mathrm{Cl}][\mathrm{F}{\mathrm{e}}_{24}\mathrm{S}{\mathrm{e}}_{26}]$ contains edge-sharing $\mathrm{FeS}{\mathrm{e}}_{4}$ units, but its ground state is antiferromagnetically ordered. A complex hyperfine pattern suggests a nontrivial spin structure like a spin spiral with large magnetic moments in the range 2 to $3\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ at ambient pressure. High pressure drastically suppresses the N\'eel temperature from 220 K at ambient pressure to below 30 K at 12 GPa. Between 5 and 10 GPa the complex magnetic hyperfine pattern collapses and a low-moment magnetic ground state emerges at higher pressures. A concomitant decrease of the resistivity by several orders of magnitude indicates that the electronic system becomes more itinerant. Full metallization occurs near 25 GPa, but no sign of superconductivity down to 1.5 K was found at any pressure. It is suggested that the insulating ground state of $[\mathrm{C}{\mathrm{s}}_{6}\mathrm{Cl}][\mathrm{F}{\mathrm{e}}_{24}\mathrm{S}{\mathrm{e}}_{26}]$ corresponds to an orbitally selective Mott phase, which due to an increased bandwidth, is transformed into a Hund's metal state under pressure.