Crestal Structure Prediction Of BeC₂Under High-Pressure

Recently, alkaline earth metal dicarbides have studied extensively due to their interesting structural properties when pressure is applied. To our knowledge, there are few studies on compressed BeC2. Therefore, we studied the ambient and high pressure properties of alkaline earth metal dicarbide BeC2by using first principle method. We first determined their crystal structures based on the crystal structure prediction method CALYPSO under ambient and high pressure; then we studied their physical properties by analyzing their electronic properties and lattice dynamics behaviors. Results are as follows:(1) We found the ambient pressure structure of BeC2with a trigonal.P3ml space group. As found in the phonon-dispersion curve, the α-BeC2structure is predicted to be dynamically stable. Band-structure calculations reveal that the a-BeC2shows a nonmetallic compound behavior with a band gap of1.005eV.(2) The pressure-induced structural transition of the BeC2occurs at2.75GPa from the a-BeC2to the β-BeC2. The crystal structure of β-BeC2has a trigonal with the space group of P3ml. This transition is identified as isostructural phase transition undergoes volume collapse of26.5%. The predicted β-BeC2structure is dynamically stable as found as in the phonon-dispersion curve. This phase has a metallic character. (3) The pressure-induced structural transition of BeC2occurs at38.5GPa from the β-BeC2to the y-BeC2. The crystal structure of y-BeC2is found to be monoclinic with space group C2/m. This transition is identified as the first-order with volume collapse of6.5%. The predicted y-BeC2structure is dynamically stable from the phonon-dispersion curve. Band-structure calculation reveals that the y-BeC2shows a insulator compound behavior with a band gap of2.829eV.(4) In order to explore the phase transformation mechanism, we have performed calculations of the phonon-dispersion curves at the phase transition pressures. The result shows that the pressure-induced softening of phonon frequencies is found in α-BeC2at the phase transition pressure. Moreover, it is not found in the β-BeC2at about the phase transition pressure.