Theoretical Study On The Properties Of Nanolaminate Ceramics Under Hydrostatic Pressure By The First Principle

Nanolaminate ceramics composites collaboratively combine many excellent properties from metal and ceramics and thus attract intensive interests from diverse fields.The geometries and properties of ceramic composites(Mn+1AXn)under hydrostatic pressure are theoretically calculated based on density functional theory,including Mo2GaC(211),Mo2Ga2C(221),Mo2TiAlC2(312),ferromagnetic Cr2TiAlC2(312)and Ti-Al-C system.This paper focuses on the structural evolution of nanolaminate ceramic composites under hydrostatic pressure and their relationships during structural changes.The structural evolution of Mo2GaC,without magnetic moment but with low superconductor temperature,was calculated under pressure up to 100 GPa,during which two abnormal expansion of c axis is observed.This is the first observation of this interesting phenomenon in this kind of compounds.The highly similar variation trend of Mo-Mo bond length with that of c axis indicates the crucial role of Mo-Mo bond in resistance of c-axis contraction.Elastic constants and phonon dispersion curves calculations demonstrate that the lattice is stable below 100 GPa.Positive and large value of Mo-Mo bond population means its strong bonding,small and negative value of Ga-Mo reflects its weak bond,these bonds behave even stronger and weaker under pressure,whereas the C-Mo bond varies least,meaning its relatively stable under compression.Considering the spin polarization effect of electrons,the structural evolution characteristics of Mo2Ga2C is studied.The axial compression of the c axis is smaller than that of a axis from 0 GPa to 100 GPa,which is consistent with the axial compressibility of Mo2GaC.The constantly increasing Mo-Mo bong length and angle of Mo-Mo with its horizontal projection line demonstrate that Mo-Mo is responsible for the stiff of c axis.The elastic constants calculations and phonon dispersion curves demonstrate that the lattice is unstable above about 48 GPa.However the phonon dispersion curves fail to predict the imaginary frequency once we neglect the influence of spin-polarized effect.Calculations on the bond population found that only C-Mo bond shows positive value,whereas the other bonds display negative values.Bond population of Mo-Mo increases with pressure but the other bonds show opposite variations,including C-Mo,Ga-Ga,Ga-Mo,respectively.Mo3AlC2 compound is unstable at ambient conditions,but substitution one Mo atom by Ti atom can form stable Mo2TiAlC2 compound.Elastic constants calculations demonstrate the stability of Mo2TiAlC2 in a large pressure range 0 GPa-100 GPa,which is consistent with the result of the phonon dispersion curve.Investigations to the density of states of the doped Ti atom,we observe that the value of Ti d states contribute less to the Fermi level,which is far smaller than that of Mo atom at the substituted site.This significant reduction of Ti states could effectively stabilize the lattice framework.This conclusion is also applicable to Cr2TiAlC2.We also study the thermodynamic properties using quasi-harmonic Debye model and find Debye temperature and Gruneisen parameter show less sensitivity to the external temperature,whereas the thermal expansion coefficients shows obvious sensitivity with temperature in particular at low temperature range.Without the influence of the on-site coulomb interaction of Cr d states during the calculation of Cr2TiAlC2,the obtained lattice has the lowest energy and most consistent parameters with that of experimental one.The interesting behavior is that once the magnetic moment is collapsed under pressure,the opposite axial compressibility occurs,behaving same order of axial compressibility with that of nonmagnetic one at 0 GPa.Comparison investigation between ferromagnetic Cr2TiAlC2 and nonmagnetic Mo2TiAlC2 observe that the negligible effect of spin-polarized to the thermal expansion coefficient,but the significant effect could be clearly seen in the other thermodynamic quantities.Pressure induces spin-up and spin-down density of states shifts towards opposite directions simultaneously,thus causing the magnetism disappear at about 50 GPa.TisAl2C3 could be viewed as the sum of the Ti2AlC and Ti3AIC2 from the structural point of view,therefore it is necessary to reveal the general tendency between the Ti5Al2C3 and the Ti2AlC and Ti3AlC2.Our extensive simulations find that TisAl2C3 is constructed by the alternative Ti2AIC and Ti3AlC2.Many quantities with their values including axial compressibility,bulk modulus,shear modulus,Young's modulus locate at the intermediate range of the corresponding components,namely Ti2AlC and Ti3AlC2.The relationships between the 312-like stacking in 523 phase and individual 312 phase,as well as between the 211 slabs in523 phase and individual 211 phase in electronic structure,bond overlap population,atom Mulliken charge and electron density difference from first principles are determined.