We report on the growth of NH4Pb2Br5 single crystals, crystallizing in two structural modifications: monoclinic and tetragonal. We develop a new method for growing monoclinic NH4Pb2Br5 single crystals, study their optical properties, and compare and contrast their spectral responses. We find that the appearance of the luminescence spectra of these crystals strongly depends on the excitation wavelength. When the monoclinic NH4Pb2Br5 is excited in a narrow energy range of 3.75–3.3 eV, the spectrum may exhibit emission with a maximum at 2.3 and 1.85 eV and the simultaneous presence of both bands. These luminescence bands correspond to luminescence excitation spectra that differs from each other in the position and shape of the bands. It was concluded that the presence of two emission bands in the luminescence spectra of monoclinic NH4Pb2Br5 is due to the presence of two sublattices with different dimensionalities in its crystal lattice. The corresponding excited states are close in energy and apparently do not communicate with each other. The independence of two exciting states in monoclinic NH4Pb2Br5 crystals provides an opportunity for practical applications in controlled alteration of the luminescence chromaticity and can be used to create an optically logical element.
Selivanov et al. (Thu,) studied this question.