M3−x(NH4)xCrO8 (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.

鉴于对NH4+阳离子氢键特性的考量，本研究合成了新型化合物系列M3−x(NH4)xCrO8（M = Na, K, Rb, Cs）。此类具有简单3d1基态的磁性化合物转变为铁电性。X射线研究证实，相变涉及从I4̅2m对称性转变为Cmc21至P1对称性的变化。转变温度与组成变量x呈线性关系。这些转变属于序-无序类型，其中N−H···O键在机制中扮演核心角色。将此概念扩展至引入其他几类材料中的铁电性亦被提出。