First-principles Study Of Mechanism Of Several Typical Negative Thermal Expansion Compounds

With the emergence of the new technologies, new methods and new ways, more and more attentions are focused on the study of the negative thermal expansion（NTE） materials, which serves as the new favorite in the field of materials science. Most of the present NTE materials are insulator. Based on the doping and compositing with metal, etc, lots of works dedicated to the improving of the metallic character of NTE materials. However, severely restrictions are hindered the applications of NTE materials in electronic and thermodynamic fields. With a view to the conductivity of the negative thermal expansion materials, the anomaly thermal expansion mechanisms of several typical conductive NTE compounds are studied using first-principles methods, and comparing to the relevant insulating NTE materials, the connections of the conductivity and NTE phenomenon are analyzed and clarified.The contents and conclusions of the thesis are listed as follows:1. The cubic Re O₃ is reported experimentally that not only to be a NTE material, but the excellent electrical conductivity is as good as that of silver and copper. Based on the first-principles calculations and quasiharmonic approximation, the results show that the Grüneisen parameters reached the negative maximum at the point M and R in the first Brillouin zone. The corresponding low-frequencies optical branches lead to the coupling rotations of the Re O6 octahedra, thus, the volume contracts upon heating. The hybridization of the O-2p and Re-t2 g electronic states contributes to the excellent conductive properties. Having the same crystal structure, the insulating Sc F3 shows more powerfully NTE behavior, the coefficient of NTE is about ten times larger than that of Re O₃. The calculations show that the mechanisms are same; all can be contributed to the negative maximum of Grüneisen parameters at M and R, and consequently the coupling rotations of octahedra. The huge difference of the coefficients of NTE is attributed to the difference in electronic structure. The covalent Re-O bonds are stronger than the ionic Sc-F bonds, and displacing atoms according to the vibrational modes at M and R, the potential barrier of displacing O atoms is steeper than that of F atoms. These factors hinder the rotation of Re O6 octahedra, and resulting in a smaller coefficient of NTE.2. The cubic Re O₃ can be considered as A-vacant ABO₃ perovskite structure. The huge voids in the center makes the cubic Re O₃ unstable and be easy to phase transition. With the pressure improving, a series of phase transitions appear in the Re O₃. Whether the tetragonal P4/mbm exists or not, is still paradoxical. In this part, based on the first-principles calculations and quasiharmonic approximation, the phonon dispersion, the formation of enthalpy and energy barrier curves are calculated, and the possibility of the phase transition from Pm-3m to P4/mbm is clarified with a critical pressure of 5.0 kbar. Among the several crystal structures, the NTE behavior is found only in cubic phase. Analyzing the crystal structures and w-q dispersion curves under the different pressure, we find that a softening M3 mode appears at M in Brillouin zone in cubic Pm-3m phase at 5.0 kbar, and the structure is unstable. The softening M3 mode corresponding to the displacement of O atoms from x=0.2500 to x=0.2401. The phase transition leads to the disappearance of NTE phenomenon.3. Though possessing excellent mechanical properties, the transition metal lanthanide carbide is a potential superconducting material. The experiments reported that the tetragonal La C2 presents superconductivity in low temperature range, also shows anisotropic NTE behavior. In this part, we reproduced the NTE phenomenon of the La C2 over the temperature of superconductivity using first-principles method and quasiharmonic approximation. The calculated coefficient of NTE and temperature range along c-axis is in good agreement with the present experiment. According to the calculated results, we predict a tiny NTE behavior along a-axis, which is missed in poor-precision experimental instruments. From the calculated Grüneisen parameters and factor group analysis, the Eu and Eg vibrational modes in Brillouin zone center and the other three modes at M and Z in the boundary of Brillouin zone have the negative Grüneisen parameters, which corresponding to the C-C dimmer vibrating vertical to the La…La chain, and lead to NTE phenomenon.4. It is reported experimentally that during the triclinic to orthorhombic phase transition with improving temperature, a pronounced NTE behavior appeared in the perovskite Bi Ni O₃. Meanwhile, the insulating to metallic properties transition and antiferromagnetic to ferromagnetic transition are accompanied. These make the mechanism of NTE to be more complicated. To resolve the confusions, the first-principles calculations are adopted. The calculations show that the hybridization of Bi-6s and O-2p electronic states moves the electronic peaks around the Fermi level in high temperature orthorhombic phase, and displays conductive property. The magnetic moment is 1.732μB/Ni. Analyzing the electronic DOS, we found charge transfer between Ni and Bi ions occurred. The calculations of bond valence sums（BVSs） also confirmed that the oxidation states of the two crystal structures are Bi+3Ni+3O₃ and Bi+30.5 Bi+50.5Ni+2O₃. The results reveal that the low temperature triclinic phase is G-type antiferromagnetic phase. With the charge transfer between Bi and Ni ions, volume shrinkage appeared from big triclinic to small orthorhombic phase. Our calculations explained the mechanism of NTE in perovskite Bi Ni O₃ is attributed to the intermetallic charge transfer, which enrich the mechanisms of NTE behavior.