Strong room-temperature blue-violet photoluminescence of multiferroic BaMnF4

BaMnF4 microsheets have been prepared using a hydrothermal method. Strong room-temperature blue-violet photoluminescence has been observed (an absolute luminescence quantum yield of 67%) with two peaks located at 385 nm and 410 nm. More interestingly, photon self-absorption phenomenon has been observed, leading to an unusual abrupt decrease in the luminescence intensity at a wavelength of 400 nm. To understand the underlying mechanism of such emission, the electronic structure of BaMnF4 has been studied using first principles calculations. The observed two peaks are attributed to electron transitions between the upper-Hubbard bands of the Mn's t2g orbitals and the lower-Hubbard bands of the Mn's eg orbitals. The Mott gap mediated d-d orbital transitions may provide additional degrees of freedom to tune the photon generation and absorption in ferroelectrics.