Two-dimensional triangular-lattice Cu(OH)Cl, belloite, as a magnetodielectric system

Quantum spins on a triangular lattice may bring out intriguing and exotic quantum ground states. Here we report a magnetodielectric system of CuOHCl wherein $S=1/2\mathrm{C}{\mathrm{u}}^{2+}$ spins constitute a two-dimensional triangular lattice with the layers weakly coupled via Cl-H-O bonding. Despite strong magnetic interactions, as expected from the relatively high value of ${\ensuremath{\theta}}_{\mathrm{CW}}=\ensuremath{-}100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, antiferromagnetic transition occurred at ${T}_{N}=11\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, followed by an uprising turn of the magnetic susceptibility below \ensuremath{\sim}7 K. Neutron-diffraction experiment revealed a coplanar spin structure on the triangular lattice below the ${T}_{N}$, with each spin pointing toward the center of a triangle. Of the three spins on a triangle, two are antiparallel and the third one is angled ${120}^{\ensuremath{\circ}}$ to the antiparallel spins. A concerted effect of geometric frustration in the triangular lattice and superexchange interactions through a zig-zag path via double Cu-O-Cu and double Cu-Cl-Cu bridges counted for this spin arrangement. Further investigation using dielectric constant and heat capacity measurements, as well as a microscopic probe of muon spin rotation, revealed a magnetodielectric effect and the possibility of multiferroic transition at ${T}^{*}\ensuremath{\sim}5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is suspected to be in close relation to geometric frustration in this triangular lattice. The present paper presents a magnetodielectric system on a two-dimensional triangular lattice with chemical stoichiometry. It can also serve as a rare reference to the hotly debated quantum spin-orbital liquid compound $\mathrm{LiNi}{\mathrm{O}}_{2}$.