Helical order and multiferroicity in theS=12quasi-kagome systemKCu3As2O7(OD)3

Several ${\mathrm{Cu}}^{2+}$ hydroxide minerals have been recently identified as candidate realizations of the $S=1/2$ kagome Heisenberg model. In this context, we have studied the distorted system ${\mathrm{KCu}}_{3}{\mathrm{As}}_{2}{\mathrm{O}}_{7}$(OD)${}_{3}$ using neutron scattering and bulk measurements. Although the distortion favors magnetic order over a spin liquid ground state, refinement of the magnetic diffraction pattern below ${T}_{N1}=7.05(5)$ K yields a complex helical structure with $\mathbf{k}=(0.77,0,0.11)$. This structure, as well as the spin excitation spectrum, are well described by a classical Heisenberg model with ferromagnetic nearest neighbor couplings. Multiferroicity is observed below ${T}_{N1}$, with an unusual crossover between improper and pseudoproper behavior occurring at ${T}_{N2}=5.5$ K. The polarization at $T=2$ K is $P=1.5\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}$Cm${}^{\ensuremath{-}2}$. The properties of ${\mathrm{KCu}}_{3}{\mathrm{As}}_{2}{\mathrm{O}}_{7}$(OD)${}_{3}$ highlight the variety of physics which arise from the interplay of spin and orbital degrees of freedom in ${\mathrm{Cu}}^{2+}$ kagome systems.