  BaMnF$_4$ microsheets have been prepared by hydrothermal method. Strong
room-temperature blue-violet photoluminescence has been observed (absolute
luminescence quantum yield 67%), with two peaks located at 385 nm and 410 nm,
respectively. More interestingly, photon self-absorption phenomenon has been
observed, leading to unusual abrupt drop of luminescence intensity at
wavelength of 400 nm. To understand the underlying mechanism of such emitting,
the electronic structure of BaMnF$_4$ has been studied by first principles
calculations. The observed two peaks are attributed to electrons' transitions
between the upper-Hubbard bands of Mn's $t_{2g}$ orbitals and the lower-Hubbard
bands of Mn's $e_g$ orbitals. Those Mott gap mediated d-d orbital transitions
may provide additional degrees of freedom to tune the photon generation and
absorption in ferroelectrics.
