Frustrated magnetic planes with intricate interaction pathways in the mineral langite Cu${}_4$(OH)${}_6$SO${}_4\cdot 2$H${}_2$O

Magnetic and crystallographic properties of the mineral langite Cu${}_4$(OH)${}_6$SO${}_4\cdot 2$H${}_2$O are reported. Its layered crystal structure features a peculiar spatial arrangement of spin-$\frac{1}{2}$ Cu${}^{2+}$ ions that arises from a combination of corner- and edge-sharing chains. Experimentally, langite orders antiferromagnetically at $T_N\simeq 5.7$ K as revealed by magnetization and specific heat measurements. Despite this very low energy scale of the magnetic transition, langite features significantly stronger couplings on the order of 50-70 K. Half of the Cu${}^{2+}$ spins are weakly coupled and saturate around 12T, where the magnetization reaches 0.5${\mu }_B$/Cu. These findings are rationalized by density-functional band-structure calculations suggesting a complex interplay of frustrated exchange couplings in the magnetic planes. A simplified model of coupled magnetic sublattices explains the experimental features qualitatively. To start from reliable structural data, the crystal structure of langite in the 100-280 K temperature range has been determined by single-crystal x-ray diffraction, and the hydrogen positions were refined computationally.