Elastic Properties Of CaCO₃ High Pressure Phases From First Principles

Recently,the new high-pressure structures of CaCO₃ were found by high temperature and high pressure experiments and theoretical calculations.It indicated that the sequences of phase transitions of CaCO₃ are calcite?aragonite?post-aragonite?P21/c-h-CaCO₃.CaCO₃ might keep stable at the lower mantle base.These results have important implication in understanding deep carbon cycle and seismic wave velocity structure in the lower mantle.However,after the P21/c-h-CaCO3 structures were found,elastic properties and structures of CaCO₃ at high pressures are rarely studied.In this study,we investigated the enthalpies,structures,equation of state and elastic properties of aragonite,post-aragonite and the P21/c-h-CaCO3 at high pressures up to 150 GPa with first-principles based on density functional theory,as follows:?1?The enthalpies of aragonite,post-aragonite and P21/c-h-CaCO3 structure at high pressure up to 150 GPa were calculated to investigate the phase transition pressure.Results indicate that aragonite transforms to post-aragonite at 40 GPa and post-aragonite transforms to P21/c-h-CaCO₃ at 65 GPa.P21/c-h-CaCO3 is stable at 65-150 GPa.The structures of three CaCO₃ phases were analyzed.Aragonite have Ca09 dodecahedron and C03 tetrahedron,post-aragonite exhibit CaO12 polyhedron and CO₃^2-plane,and P21/c-h-CaCO₃ show CaO10 polyhedron and CO44-tetrahedron.The equations of state parameters were also obtained.For aragonite,the parameters are V0= 232.50 A3,K0= 66.09 GPa,and K0'= 4.64.For post-aragonite,V0=106.44 A3,K0= 81.93 GPa,K0'= 4.49.For P21/c-h-CaCO₃,V0=218.02 A3,KO= 56.55 GPa,K0'= 5.40.?2?The elastic constants of three CaCO3 phases were calculated.The orders of calculated longitudinal elastic constants of aragonite,post-aragonite and the CaCO₃-P21/c-h were C11>C22>C33?C11<C22?C33?C11?C22?C33,reflecting that axial compression behavior of c>b>a?a>b>c?a?b?c.Furthermore,the densities,compressional and shear wave velocities were obtained.The density,compressional and shear wave velocity changes at 40 GPa between aragonite and post-aragonite are 5.39%,1.43%,and 7.08%,respectively.The huge density increase leads to smaller acoustic velocities of post-aragonite.And those between post-aragonite and P21/c-h-CaCO₃ at 65 GPa are 1.86%,5.11%and 10.66%,respectively.It may be caused by the polymerization of CO44-tetrahedron,leading a more compact structure,can increase the densities and acoustic velocities of CaCO3.?3?Aragonite,post-aragonite and the P21/c-h-CaCO₃ exhibit on high anisotropy.Aragonite,post-aragonite,P21/c-h-CaCO₃ exhibit 29.60%-44.71%,74.34%-104.30%,42.81%-48.00%Vs splitting anisotropy.Post-aragonite presents a sharp decrease at 40 GPa and it may be caused by the decreasing of layer distance.Compared with major minerals of the lower mantle,CaCO₃ high pressure polymorphs have lower isotropic wave velocity and higher wave velocity anisotropies.It may relate with the appearance of high anisotropies and lower velocities in the lower mantle.