First-principles Study Of Negative Thermal Expansions In Mental Oxides And Graphene-like Materials

Most substances demonstrate an expansion on heating, however a few are knownto contract, i.e. show negative thermal expansion (NTE). This naturally occurringphenomenon is found more often in solids, but probably the most familiar example ofNTE from everyday life is the increase in density of liquid water between0and4℃.NTE has been studied experimentally and theoretical for decades. Recent years, therehas been substantial renewed interest towardNTEfollowing the discovery that cubicZrW2O8exhibiting strong thermal contraction.The studies of NTE can contribute a lotto fundamental scientific understanding. And due to their abnormal thermal behavior,NTE materials have a widely potential application in electronics and aerospaceindustry.Interestingly, practical use has been made of the phenomenon in thedevelopment of low expansion materials, starting with the discovery of Invar in1897.The use of Li-Al-Si ceramics in the Coring glass ceramic process has beendescribed as one of the most significant breakthroughs in materials processing in50years; millions of casserole dishes, saucepans and stove-top are made of theseceramics each year. Good knowledge of the thermal properties is essential toimplement these applications. However, investigates of the thermal expansions inmany NTE materials are very limited, and there is still much to do. For example, weneed a better understanding of many individual materials, including the expansion offramework structures, thin films and microstructures. Here, first-principlescalculations have been adopted to study the thermal expansion properties of mentaloxides and monolayer semiconducting graphene-like transition metaldichalcogenides(STMDs). The paper includes two parts, and its main contents andresults are in the following:In the first part, the thermal properties for framework solids (cuprite Cu2OandAg2O, and perovskite ReO3) and crystals with tetrahedral coordination (wurtzite andzinc-blende ZnO)were investigated, especially for their negative thermal expansion(NTE) behavior.For the framework solids, NTE is observed over wide ranges of temperature. Their NTE can be attributed tothe network folding induced by lowfrequency rigid unit modes (RUMs). For the wurtzite and zinc-blende ZnO, NTE areobtained at T<95K and T<84K respectively. For the wurtzite structure, theexperimental thermal expansion coefficient is well reproduced by our calculation. Forboth phases,detailed study of mode Grüneisen parameters shows that maximumcontribution to NTE comes from transverse acoustic modes. Whereas the contributionof longitudinal acoustic and lowest-energy optical modes for wurtzite structure is notignorable. From analysis of theeigenvectors for many low-energy modeswith largenegative Grüneisen parameters,we found that NTE in ZnO is dominated by thetension effect.In the second part,temperature dependence of lattice constants was studied usingfirst-principles calculations to determine the effect of in-plane stiffness and chargetransfer on the thermal expansion of monolayer semiconducting transition metaldichalcogenides (STMDs). Unlike the corresponding bulk material, our simulationsshow that monolayer MX2(M=Mo and W; X=S, Se, and Te) exhibits a negativethermal expansion (NTE) at low temperatures arising from the bending modes.Transition from contraction to expansion at higher temperatures is observed.Interestingly, the thermal expansion can be tailored regularly by alteration of M or Xatom. Detailed analysis shows that the positive thermal expansion coefficient ismainly determined by the in-plane stiffness, which can be expressed by a simplerelationship. Essentially the regularity of this change can be attributed to thedifference in the charge transfers between the different elements. These findingsshould be applicable to other two-dimensional (2D) systems.