Ordering in liquid and its heredity impact on phase transformation of Mg-Al-Ca alloys

It is a long-sought goal to achieve desired mechanical properties through tailoring phase formation in alloys,especially for complicated multi-phase alloys.In fact,unveiling nucleation of competitive crystalline phases during solidification hinges on the nature of liquid.Here we employ ab initio molecular dynamics simulations(AIMD)to reveal liquid configuration of the Mg-Al-Ca alloys and explore its effect on the transformation of Ca-containing Laves phase from Al2Ca to Mg_(2)Ca with increasing Ca/Al ratio(rCa/Al).There is structural similarity between liquid and crystalline phase in terms of the local arrangement environment,and the connection schemes of polyhedras.The forming signature of Mg_(2)Ca,as hinted by the topological and chemical short-range order originating from liquid,ascends monotonically with increasing rCa/Al.However,Al_(2)Ca crystal-like order increase at first and then decrease at the crossover of rCa/Al=0.74,corresponding to experimental composition of phase transition from Al_(2)Ca to Mg_(2)Ca.The origin of phase transformation across different compositions lies in the dense packing of atomic configurations and preferential bonding of chemical species in both liquid and solid.The present finding provides a feasible scenario for manipulating phase formation to achieve high performance alloys by tailoring the crystal-like order in liquid.

First-principles calculations of lattice dynamics and thermal properties of polar solids

Although the theory of lattice dynamics was established six decades ago,its accurate implementation for polar solids using the direct(or supercell,small displacement,frozen phonon)approach within the framework of density-function-theory-based first-principles calculations had been a challenge until recently.It arises from the fact that the vibration-induced polarization breaks the lattice periodicity,whereas periodic boundary conditions are required by typical first-principles calculations,leading to an artificial macroscopic electric field.The article reviews a mixed-space approach to treating the interactions between lattice vibration and polarization,its applications to accurately predicting the phonon and associated thermal properties,and its implementations in a number of existing phonon codes.

Searching for a route to synthesize in situ epitaxial Pr_(2)Ir_(2)O_(7) thin films with thermodynamic methods

In situ growth of pyrochlore iridate thin films has been a long-standing challenge due to the low reactivity of Ir at low temperatures and the vaporization of volatile gas species such as IrO_(3)(g)and IrO_(2)(g)at high temperatures and high PO_(2).To address this challenge,we combine thermodynamic analysis of the Pr-Ir-O_(2)system with experimental results from the conventional physical vapor deposition(PVD)technique of co-sputtering.Our results indicate that only high growth temperatures yield films with crystallinity sufficient for utilizing and tailoring the desired topological electronic properties and the in situ synthesis of Pr_(2)Ir_(2)O_(7)thin films is fettered by the inability to grow with PO_(2)on the order of 10 Torr at high temperatures,a limitation inherent to the PVD process.Thus,we suggest techniques capable of supplying high partial pressure of key species during deposition,in particular chemical vapor deposition(CVD),as a route to synthesis of Pr_(2)Ir_(2)O_(7).