M_(3-x)(NH_4)_xCrO_8 (M = Na, K, Rb, Cs): A New Family of Cr~(5+)-Based Magnetic Ferroelectrics

摘要

Upon consideration of the hydrogen-bonding properties of the NH4 cation, we synthesized a new class of compounds, M_(3_x)(NH_4)_xCrO_8 (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 I42m to Cmc2_1 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.