Luminescence and Charge Trapping in Cs2HfCl6 Single Crystals: Optical and Magnetic Resonance Spectroscopy Study

Single crystals of the undoped Cs2HfCl6 were grown using the vertical Bridgman method. Photoluminescence spectra and decays were measured in the extended temperature range of 8–500 K and approximated by a phenomenological model. Self-trapped exciton and defect-related emissions were clearly distinguished with the onset of thermal quenching above the room temperature. Electron paramagnetic resonance experiment revealed the formation of two configurations of Vk center (Cl2– molecular ion) at low temperatures. One of the Vk centers is created by a hole self-trapping at two neighboring Cl ions. In the temperature range 70–80 K, this state is thermally disintegrated and reappears in another configuration formed most probably by an interstitial Cl ion (an analog of the H-type center in alkali halides).