New high Tc multiferroics KBiFe2O5 with narrow band gap and promising photovoltaic effect

Intrinsic polarization of ferroelectrics (FE) helps separate photon-generated charge carriers thus enhances photovoltaic effects. However, traditional FE with transition-metal cations (M) of d(0) electron in MO6 network typically has a band gap (E-g) exceeding 3.0 eV. Although a smaller E-g (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 E-g < 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 E-g (1.6 eV), high Curie temperature (T-c similar to 780 K) and robust magnetic and photoelectric activities. The high photovoltage (8.8 V) and photocurrent density (15 mu A/cm(2)) 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.