Strong magnetic-dielectric-lattice coupling in transition metal hydroxyhalides and ferroelectric response in rhombohedral Co₂(OD) ₃X (X=Cl, Br)

Hydroxyl salts of the type M₂(OH)₃X, and M(OH)X, where M represents a transition metal ion and X represents a halogen ion, widely exist as minerals and were recently reported to be geometrically frustrated magnets. Here, we report the finding of ferroelectric response in them. First, we observed strong magnetic-lattice-dielectric couplings in all of them as witnessed during their magnetic transitions at low temperatures. Secondly, we identified apparent ferroelectric responses in the deuterated hydroxyl salts of high crystal symmetries, i.e., rhombohedral Co₂(OD)₃Cl and Co₂(OD)₃Br, at high temperatures of 220-230 K through an isotope effect. The present work shows that multiferroicity may be a potentially universal phenomenon in magnetic hydroxyl salts. Meanwhile, it provides the first link between magnetic geometric frustration and hydrogen-bonded soft-mode ferroelectrics.