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 noncentrosymmetric ferrimagnetic spin-1/2 system, ${\mathrm{Bi}}_{2}{\mathrm{Cu}}_{5}{\mathrm{B}}_{4}{\mathrm{O}}_{14}$. Neutron powder-diffraction study deciphers a robust ferrimagnetic ($\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$) ordering below a temperature ${T}_{C}=25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which remains unaltered even under a high magnetic field of $H=10\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. Interestingly, two successive ferroelectric transitions are observed, one of which is triggered by the magnetostriction effect and emerges concomitantly with the ferrimagnetic ordering at ${T}_{C}={T}_{E2}=25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. This is accompanied by a pronounced dielectric peak and a prominent magnetodielectric coupling at ${T}_{C}$. By contrast, other ferroelectric transition commences in the paramagnetic state at ${T}_{E1}=32\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, 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.