Pressure-Induced Two-Color Photoluminescence and Phase Transition of Two-Dimensional Layered MnCl2

The transition metal ions are often used to enhance the photoluminescent efficiency in many compounds, which are always received as scintillators, optical sensors and memories, laser media, optoelectronic devices, etc. The photoluminescence of two-dimensional layered transition metal materials, which is associated with electron excitation, has long been the focus of applied materials studies. Most photoluminescence of materials was reported to exhibit redshift and quenched emission as pressure increases. Here, we report the pressure effect on photoluminescence and the correlations with structural phase transitions of manganese dichloride (MnCl2). A remarkable emission fluctuation at low pressure (below 12 GPa) is observed, which is ascribed to the competition between the shrinkage and tensile strain of the lattice along the c axis. In situ synchrotron Xray diffraction indicates that the MnCl2 underwent three structural phase transitions at 4, 21.7, and 47.2 GPa, respectively, which led to the two-color photoluminescence during compression. The structural transitions are further confirmed by theoretical calculations. Raman scattering experiments are consistent with XRD, supporting the occurrence of structural transitions. This study provides a comprehensive characterization of the correlation between structural and photoluminescence properties of MnCl2 at high pressure. The analysis of optical and X-ray diffraction data suggests the important clues for improving photoluminescence performance and an expansive research prospects for two-dimensional layered transition metal halide materials, toward realizing novel material designs.