Magnetic and electric properties in the distorted tetrahedral spin chain   system Cu3Mo2O9
  We study the multiferroic properties in the distorted tetrahedral quasi-one dimensional spin system Cu$_3$Mo$_2$O$_9$, in which the effects of the low dimensionality and the magnetic frustration are expected to appear simultaneously. We clarify that the antiferromagnetic order is formed together with ferroelectric properties at $T_{\rm N}=7.9$ K under zero magnetic field and obtain the magnetic-field-temperature phase diagram by measuring dielectric constant and spontaneous electric polarization. It is found that the antiferromagnetic phase possesses a spontaneous electric polarization parallel to the c axis when the magnetic field $H$ is applied parallel to the a axis. On the other hand, there are three different ferroelectric phases in the antiferromagnetic phase for $H$ parallel to the c axis. 
T. Hosaka, S. Hachiuma, H. Kuroe, T. Sekine, M. Hase, K. Oka, T. Ito,   H. Eisaki, M. Fujisawa, S. Okubo, H. Ohta
cond-mat.str-el cond-mat.mtrl-sci