  Conventional perovskite-type ferroelectrics are based on octahedral units of
oxygen, and often comprise toxic Pb to achieve robust ferroelectricity. Here,
we report the ferroelectricity in a silicate-based compound, Bi2SiO5 (BSO),
induced by a structural instability of the corresponding silicate tetrahedral
chains. A low-energy phonon mode condenses at ~ 673 K to induce the proper
ferroelectric phase transition. Polarization switching was observed in a BSO
single crystal with a coercive field of 30 kV/cm and a spontaneous polarization
of 0.3 microC/cm2 along a direction normal to the cleavage plane. The in-plane
polarization was estimated by first principles calculations to be 23
microC/cm2. The present findings provide a new guideline for designing
ferroelectric materials based on SiO4 tetrahedral units, which is ubiquitously
found in natural minerals.
