Deuterium ordering found in new ferroelectric compound Co2(OD)3Cl

A detailed temperature-dependent Raman spectroscopic study revealed a new type of deuterium-order ferroelectrics in a geometrically frustrated magnet Co-2(OD)(3)Cl at T-epsilon = 229 K. Significant changes in the parameters of the Raman vibration modes were observed near Te, suggesting a strong phonon-charge coupling. Additional asymmetric phonon bands appeared below around Te, which are consistently interpreted by phonon folding processes due to a small local structural change resulting from the ordering of deuterium. The wavenumber and intensity changes of the Ramanactive modes, as well as the normalized intensities of the additional bands, all follow a power-law fit Delta omega, Delta I, I alpha (1 - T/T-C)(2 beta), wherein T-C = 230 K similar to T-epsilon and beta = 0.35(2), clearly demonstrating an ordering process below T-epsilon. The critical exponent is reminiscent of a second order transition. Our study presents a rare and new type of multiferroic material with ferroelectricity arising from the deuterium ordering in geometrically frustrated magnets.