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 M O 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 M O 6 FE with E g < 2.0 eV, all thus far unconfirmed because of synthesis difficulties. Here we report a new FE compound with M O 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.