M_3−<i>x</i>(NH₄)_<i>x</i>CrO₈ (M = Na, K, Rb, Cs): A New Family of Cr⁵⁺-Based Magnetic Ferroelectrics

Upon consideration of the hydrogen-bonding properties of the NH4+ cation, we synthesized a new class of compounds, M3−x(NH4)xCrO8 (M = Na, K, Rb, Cs). These magnetic compounds with the simple 3d1 ground state become ferroelectric. X-ray studies confirmed that the phase transition involves a symmetry change from I4̅2m to Cmc21 to P1. The transition temperature depends linearly on the composition variable x. The transitions are of the order−disorder type, with N−H···O bonding playing the central role in the mechanism. Extension of this idea to the introduction of ferroelectricity in several other classes of materials is suggested.