Phase transitions of actinium dihydride: Pressure-induced charge transfer driving effect.

The crystal structures of actinium dihydride (AcH 2 ) in a wide pressure range of 0–200 GPa have been investigated by using an unbiased structure searching method coupling with first-principles density functional calculations. A series of pressure-induced phase transitions are predicted for AcH 2 , as Fm -3 m → P 4 2 / mmc → Imma → P 6/ mmm , and the calculated pressures of the phase transitions are 12, 27, and 68 GPa. Under pressure, the coordination number of H atom increases and the nature of chemical bonds changes. The electronic band structures show that all of the competitive phases are metallic. The pressure-induced charge transfer drives the phase transition. Phonon calculations show the competitive phases are thermodynamically stable within their favored pressure range. Our results elucidate the phase transition mechanisms of AcH 2 under pressure, and have major implications for the high-pressure behaviors of actinide and rare-earth metal dihydrides. [ABSTRACT FROM AUTHOR]