Unconventional multiferroicity induced by structural distortion and magnetostriction effect in the layered spin-1/2 ferrimagnet Bi2Cu5B4O14

Complex systems with strongly entangled microscopic degrees of freedom often host a plethora of exotic properties. Herein, we report the unconventional and interesting multiferroic behavior of a layered noncen-trosymmetric ferrimagnetic spin-1/2 system, Bi2Cu5B4O14. Neutron powder-diffraction study deciphers a robust ferrimagnetic (up arrow down arrow down arrow down arrow down arrow) ordering below a temperature TC = 25 K, which remains unaltered even under a high magnetic field of H = 10 T. Interestingly, two successive ferroelectric transitions are observed, one of which is triggered by the magnetostriction effect and emerges concomitantly with the ferrimagnetic ordering at TC = TE2 = 25 K. This is accompanied by a pronounced dielectric peak and a prominent magnetodielectric coupling at TC. By contrast, other ferroelectric transition commences in the paramagnetic state at TE1 = 32 K, which is solely caused by a prominent structural distortion. This strong structural distortion is revealed by the synchrotron x-ray-diffraction study, which is further supported by a concurrent, sharp, anomalous phonon softening observed in the Raman spectra.