Intrinsic polarization of ferroelectrics (FE) helps separate photon-generated charge carriers thus enhances photovoltaic effects. However, traditional FE with transition-metal cations (M) of d0 electron in MO6 network typically has a band gap (Eg) exceeding 3.0 eV. Although a smaller Eg (2.6 eV) can be obtained in multiferroic BiFeO3, the value is still too high for optimal solar energy applications. Computational “materials genome” searches have predicted several exotic MO6 FE with Eg < 2.0 eV, all thus far unconfirmed because of synthesis difficulties. Here we report a new FE compound with MO4 tetrahedral network, KBiFe2O5, which features narrow Eg (1.6 eV), high Curie temperature (Tc ~ 780 K) and robust magnetic and photoelectric activities. The high photovoltage (8.8 V) and photocurrent density (15 μA/cm2) were obtained, which is comparable to the reported BiFeO3. This finding may open a new avenue to discovering and designing optimal FE compounds for solar energy applications.

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Three bismuth borates, Bi[B4O6(OH)2]OH (I), BiB2O4F (II), and Bi3[B6O13(OH)] (III), were obtained in boric acid flux at low temperatures and their structures were determined by using single-crystal and powder X-ray diffraction techniques. All these three bismuth borates contain one-dimensional borate chains and crystallize in noncentrosymmetric space groups. I is a hydrated borate, crystallizing in the space group P1 with lattice constants a = 4.300(1) Å, b = 8.587(2) Å, c = 10.518(2) Å, α = 113.11(3)°, β = 100.50(3)°, and γ = 90.36(3)°, and the borate chain consists of a 3-ring (2BO3 + BO4) and a BO3 group. II is a fluoroborate, which crystallizes in the trigonal space group P32 with a = 6.7147(1) Å and c = 6.4688(1) Å, and the borate anion is a 3-fold helix chain formed by extensive linkage of three-membered borate rings (3BO4). III is also a hydrated borate with a structure that is closely related to II, in which one-sixth of the borate groups are in triangular geometry (BO3), which reduces the symmetry to P1, a = 6.6257(1) Å, b =6.6238(1) Å, c = 6.6541(1) Å, and α = 89.998(3)°, β = 89.982(2)°, and γ = 119.992(2)°. All these three bismuth borates exhibit nonlinear optical (NLO) properties.

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