Cleavable quaternary oxychlorides with high magnetic ordering temperatures

Quaternary oxychlorides derived from Ruddlesden--Popper $3d$ transition metal oxides offer a route to cleavable crystals with bulk antiferromagnetic ordering temperatures reaching at least 550 K. Here, we study the magnetic, optical, and mechanical behavior of ${\mathrm{Sr}}_{2}{\mathrm{FeO}}_{3}\mathrm{Cl}$, ${\mathrm{Ca}}_{2}{\mathrm{FeO}}_{3}\mathrm{Cl}$, ${\mathrm{Ca}}_{3}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}{\mathrm{Cl}}_{2}$, and ${\mathrm{Sr}}_{3}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}{\mathrm{Cl}}_{2}$. Through optical absorption measurements, we show that these antiferromagnetic semiconductors have optical band gaps of $\ensuremath{\approx}2.1(1)\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. The magnetic ordering symmetries and temperatures were probed by neutron powder diffraction and M"ossbauer spectroscopy on polycrystalline samples, demonstrating N\'eel temperatures $({T}_{\mathrm{N}})$ near room temperature in the single layer ${\mathrm{Sr}}_{2}{\mathrm{FeO}}_{3}\mathrm{Cl}$ $({T}_{\mathrm{N}}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}311\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ and ${\mathrm{Ca}}_{2}{\mathrm{FeO}}_{3}\mathrm{Cl}$ $({T}_{\mathrm{N}}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}360\phantom{\rule{0.16em}{0ex}}\mathrm{K})$, and the double-layer compound ${\mathrm{Sr}}_{3}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}{\mathrm{Cl}}_{2}$ has ${T}_{\mathrm{N}}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}545\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The high-spin moments of ${\mathrm{Fe}}^{3+}$ lie within the basal plane and the magnetic structures are compensated within each magnetic layer and characterized by magnetic propagation vectors $\mathbit{k}=(\frac{1}{2}\frac{1}{2}0)$. Magnetization results demonstrate the quasi-2D nature of the magnetism, with a broad maximum in the susceptibility near $2{T}_{\mathrm{N}}$ for ${\mathrm{Sr}}_{2}{\mathrm{FeO}}_{3}\mathrm{Cl}$. Scotch tape tests and mechanical exfoliation onto ${\mathrm{SiO}}_{2}$ confirm the micaceous nature of these crystals with cleavage down to a single unit cell (two magnetic layers) achieved for ${\mathrm{Sr}}_{3}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}{\mathrm{Cl}}_{2}$. This paper highlights strong antiferromagnetic interactions, semiconducting band gaps, and cleavability of quaternary Fe-based oxychlorides and motivates future work on crystals and exfoliated flakes of these and related oxyhalide systems.