Research On Self-consistent Thermal Expansion Lattice Dynamics Method And Its Application In Uranium And Uranium-based Compounds

In this work,the theory and process of newly developed SCALD method were presented.Similar to the theory to the self-consistent lattice phonon method,this method take into account the anharmonicity effect caused by phonon-phonon interaction and thermal expansion.Firstly,we introduce the process of its calculated detail based on the python code.The STELD is close to the Born self-consistent lattice phonon method,while our method can be used to the interaction between phonons and the volumetric expansion induced anharmonic effect.Using this method,we calculated the phonon dispersion of body centered cubic(BCC)Ti and Zr at high temperature.Results indicated that the accurate degree of calculations is consistent with the similar calculated tool SCALD and we can get the high-temperature lattice dynamics of metals based on the present method.More importantly,the average displacements in the simulation of lattice can be rightly described in the framework of STELD.Also,our method can be applied to any symmetry lattice structure,i.e,14 Bravais lattices.Due to considering the effect of volumetric expansion induced by thermal expansion(including negative thermal expansion),our method calculated phonon dispersion is in good agreement with results from neutron inelastic scattering measure in experiments.For the face centered cubic(FCC)Al,we used the present method to calculate the thermal expansion and find the STELD calculated thermal expansion of A1 at range of high temperature is more close to the experiments,with respect to results from the quasi-harmonic approximation.Secondly,we investigate the equation of state,thermodynamics properties,and the anharmonic effect of a-U using the quasi-harmonic approximation and the STELD method.The four-order interatomic interactions are responsible for the dynamics unstability of the volumetric dependent ∑4 vibration model in a-U.In the framework of the harmonic approximation,the second-order interatomic interaction can not be used to rightly study the lattice vibration of a-U.The STELD calculated phonon dispersion indicated that the α-U is stable under the temperature of phase transition.The comparison from different methods(the quasi-harmonic approximation,Gruneisen theory,and STELD method)suggested that the thermal expansion anisotropy mainly derives from the interaction between the models of lattice vibration and the elastic anisotropy.Thirdly,using the STELD method,we study the stability of the U-based U2Mo intermetallic compound.Ab initio calculations indicated that the I4/mmm-U2Mo is a metastable phase and there exist the stable hexagonal phases at low temperature.We found the stable phase of U2Mo with Space group No.194.Its stable mechanism is the occupies of electrons are relatively less at the Fermi level,which may be responsible for the phase transition of body-centered tetragonal metastable structure under the effect of strain field.According to the lattice dynamics,this unstability may be from the bending-type force constant of the fourth nearest neighboring U-U bonding.The STELD calculations suggest that the strong anharmonic effects occur in I4/mmm-U2Mo and there exist no negative frequency in the phonon dispersions.The calculated thermal expansion coefficient is close to the experimental measure.The validity of STELD method is further estimated.Lastly,the volumetric unharmonicity and thermodynamic properties of partially ordered UZr2 alloy are discussed by using quasi harmonic approximation,then we give our conclusions and propose the possible fast converged method of STELD applied to the large-size atomic system with low-order and fully disorder.