Several Typical Negative Thermal Expansion Materials: A First-principles Study

Negative thermal expansion (NTE) materials have abnormal thermal phenomena,i.e. volume contracts on heating. In the1990s, as an emerging interdisciplinary, theinvestigation of NTE materials has aroused wide interest and attention by scholarsfrom various countries. In the design and application stage of many fields, such aselectric, optics and mechanical, thermal expansion behavior of material has importantimplications. NTE materials provide an effective way to control the thermalexpansion coefficient and temperature range. Especially, for the matching of thermalexpansion coefficient and design of zero-expansion materials, NTE materials play animportant role.At present, there are few types and amount for NTE materials. Meanwhile, themechanisms that lead to NTE are not very clear, or the existing mechanistic model isnot perfect. To expand the NTE family and develop new functional materials, it isnecessary to explore more NTE materials and study the corresponding NTEmechanisms.Many experimental investigations on NTE materials, such as the explore of newNTE materials, the explanation of the NTE behavior, the preparation of composites,the control of thermal expansion coefficient and so on, have been reported. However,there is not much research of NTE materials based on theoretical calculations. In viewof this, it is very necessary to carry out the research of NTE materials byfirst-principles calculation. In this dissertation, our research contains four parts,whose main results are listed as follows:1. ZnF2is reported experimentally to exhibit NTE behavior at low temperaturesvery recently. In this chapter, we present the NTE properties of ZnF2by means offirst-principles calculations. Our results are in good agreement with experimentalmeasurements. It is shown that low-frequency optical phonons lead to the NTEbehavior. The lowest frequency rigid unit mode (RUM) of ZnF6causes a rotarycoupling between two adjacent octahedrons, which is most responsible for the NTE properties of ZnF2. In addition, some fluorides, as the new NTE materials, arepredicted. The ionic radius and electronegativity of these fluorides NTE materials areconcentrated in certain area. As a representative, rhombohedral TiF3is investigated indetail.2. In orthorhombic A2M3O12(A=transition metal, M=Mo, W) family, Y2Mo3O12exhibits the largest NTE behavior. In the past, the NTE behavior of A2M3O12wasattributed to the translational mode of the O bridge atom in A-O-M linkage. Here, theNTE properties and vibrational modes of Y2Mo3O12are investigated. Apart from thetranslational mode of the O bridge atom, we also find that YO6octahedra and MoO4tetrahedra distort unevenly. Based on this, we propose an extended model, i.e. thedumbbell-shaped YO6-MoO4model. This model presents us with a simultaneousdynamic process: the translational mode of the O bridge atom in the Y-O-Mo linkagemakes the YO6octahedra and MoO4tetrahedra closer, leading to the distance betweentwo polyhedron decreases; meanwhile, the YO6octahedra and MoO4tetrahedradistort unevenly due to the distinction of Y-O and Mo-O bonds. Both the movementand the distortion of polyhedron contribute to the NTE behavior. Thedumbbell-shaped AO6-MO4model may be applicable in other open frameworkstructures in A2M3O12family.3. The NTE behavior in functional materials may couple with ferroelectric,magnetic, superconducting properties. So far, in perovskite compounds, PbTiO3is theonly ferroelectric material which can exhibit the NTE behavior below the Curietemperature. However, the theoretical calculations on the NTE properties of PbTiO3are almost empty. Although related experiments indicate that spontaneouspolarization is strongly associated with volumetric contraction, the positive thermalexpansion of PbTiO3below room temperature can’t be explained. The thermalbehavior of ferroelectric PbTiO3can be affected by two aspects: phonon andspontaneous polarization. The role of phonon in thermal expansion behavior ofPbTiO3is investigated. Our work confirms the positive contribution of overallphonons to thermal expansion. The effect of phonon on the negative thermalexpansion behavior of PbTiO3is also analyzed. Our work confirms the positivecontribution of overall phonons to thermal expansion. Also, it can be indirect evidence for experimental conclusions. Moreover, we find that different phononmodes have different effects on thermal expansion. Based on this, we can solve theexperimental legacy issues.4. Within the framework of first-principles, the thermal expansion properties ofall II-VI semiconductors with zinc blende structure are investigated. We find that allII-VI semiconductors have NTE behavior at low temperatures. In addition, the NTEmechanism and other thermodynamic properties are also studied. Our results showthat the NTE behavior and related thermodynamic properties are closely associatedwith basic physical parameters, such as ionic radius, atomic mass andelectronegativity. Along with the increase of ionic radius and larger atomic mass,there is an increased electronegativity in IIB-group and decreased electronegativity inVI-group. In II-VI semiconductors, more covalent character in bonding nature willlead to weaker interatomic force constants and lower frequencies in lattice vibrations.Therefore, despite same vibrational modes, there will be a distinction in NTEbehavior and related thermodynamic properties.