Electronic Structure of LLM-105 Crystal under High Pressure and Low Temperature

The electronic structural evolution of LLM-105 crystal under high pressure and low temperature is investigated by photoluminescence, absorption, Raman spectra, and density functional theory (DFT) calculations. The result shows that the LLM-105 crystal possesses a large tunable indirect band gap from 2.45 eV at the ambient condition to 1.33 eV at 34.0 GPa. With increasing pressure at room temperature, the luminescence of LLM-105 crystal first increases in intensity owing to the raised absorption at excited wavelength and the enhanced hydrogenbond network; then, over 9.0 GPa, the increase of nonradiative transition probability causes the decrease of emission intensity due to the increase of phonon energy. In the cooling process under high pressure, the emission is enhanced due to the lower phonon energy and the pressure value at maximum emission can be adjusted up to 10.2 GPa at 200 K from 9 GPa at room temperature. The absorption evolution reveals that the band gap of LLM-105 crystal decreases as the pressure increases and an electronic structure phase transition occurred at about 10 GPa. The DFT calculation indicated that it is attributed to the electronic transfer abrupt change from the exposed oxygen atom to the amino groups at about 10 GPa, which also reduces the band gap red shift rate. A pressure-induced structure phase transition occurs at 26.5 GPa indicated by a sudden decrease of band gap. This material is expected to be metalized under further compression up to about 250 GPa.