New high T(c) multiferroics KBiFe(2)O(5) 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 MO(6) 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 BiFeO(3), the value is still too high for optimal solar energy applications. Computational “materials genome” searches have predicted several exotic MO(6) FE with E(g) < 2.0 eV, all thus far unconfirmed because of synthesis difficulties. Here we report a new FE compound with MO(4) tetrahedral network, KBiFe(2)O(5), which features narrow E(g) (1.6 eV), high Curie temperature (T(c) ~ 780 K) and robust magnetic and photoelectric activities. The high photovoltage (8.8 V) and photocurrent density (15 μA/cm(2)) were obtained, which is comparable to the reported BiFeO(3). This finding may open a new avenue to discovering and designing optimal FE compounds for solar energy applications.