Theoretical Investigation Of The Negative Thermal Expansion Properties Of The Antiperovskite Manganese Nitrides Mn₃(A_1-xB_x)N

Since the discovery of the negative thermal expansion (NTE) in the antiperovskite manganese nitrides Mn3(Cu1-xGex)N in2005, these compounds have been paid much attention, because of their excellent NTE behaviors, such as:(1) the coefficient and working temperature of NTE can be tunable;(2) the negative coefficient can reach as large as-25x10-6K-1;(3) the negative thermal expansion is isotropic;(4) the compounds show good mechanical and metallic behaviors. Prompted by this pioneering research, many other groups have studied the doping effect on the NTE behaviors through doping Zn, Sn, Si or some other elements into the compound. At the same times, the origin of the NTE has been studied. Although many efforts have been done, the mechanism of the NTE properties is still a puzzle for the Mn3(Cu1-xGex)N compounds. Obviously, exploring the mechanism is not only important for understanding the fascinating property of such a class of compounds, but also of benefit to the synthesizing of advanced NTE materials in the future. So in this dissertation, we investigate the mechanism of the NTE behaviors and the effect of the doped Ge atoms on the NTE behaviors of the Mn3(Cu1-xxGex)N compounds.This dissertation contains five chapters. In the first chapter, we introduce the common NTE materials as well as their applications in brief. Then, the detailed NTE properties and its possible mechanism for Mn3(Cu1-xGex)N compounds are reviewed.Since the Mn ions in the compounds have the local magnetic moments, our dissertation will be relevant to the knowledge about the magnetism theory. So in the second chapter, the magnetism theory and the Monte Carlo method that is used to simulate the transition temperature are introduced. In this chapter, we also present the density functional theory and the VASP package used in our theoretical calculations.From the third chapter, we start to report our research on Mn3(Cu1-xGex)N compounds.In the third chapter, we report our study on the origin and the effect of the Ge atoms on the NTE behaviors in the compounds of Mn3(Cu1-xGex)N. By minimizing the energies of many different magnetic configurations (the arrangements of the local magnetic moments), we find that the antiferromagnetic (AFM) configuration Δ5g have the lowest energy and the largest lattice constant. So, when the compound transforms its magnetic state from the ground state to the paramagnetic (PM) state, the volume of the compound will be contracted. Analyzing the electronic structure of the compound indicates that the doped Ge atoms can easily donate more electrons into the conduction bands nearby the Fermi level of the compound via thermal excitation, enhancing the attractive interaction between the second neighboring Mn ions, as well as polarizing some of the local electrons to change the local magnetic moments of Mn ions, causing a gradual contraction of volume with increasing temperature.We also study the influence of the Ge content and the N vacancies on the NTE properties of Mn3(Cu1-xGex)N compounds, which is summarized in Chapter3too. Our results indicate that, as the Ge content and the N vacancies increase, the ground state configuration Γ5g become more stable, and the energies of the matestable configurations increase, meaning that the transition temperature of the compound is elevated. The different values of the volumes between the ground state and the PM state increase as the increment of the Ge content, suggesting that the Ge atoms could enlarge the volume contraction at the magnetic transition. The exchange interactions and the influence of these interactions on the volume of the compound are also extracted, from which we find that the interactions between the first and the second neighboring Mn ions dominate the magnetic interactions and make the configuration Γ5g more stable.In the fourth chapter, we study the elastic properties of the compounds of Mn3(Cu1-xGex)N. Through analyzing the results, we find that the compounds have high elastic anisotropy and the doped Ge atoms enhance the ductile character. Furthermore, when the Ge content increases from12.5%to50%, the bulk modulus and the Young’s modulus of the compound increase. This is in agreement with the experimental results. Then, we analyze the electronic structures of the compound and propose that these elastic features are essentially attributed to the valence electrons from the doped Ge.In the last chapter, we study the influence of different doping elements on the NTE properties of the compound Mn3(A0.5B0.5)N(A=Cu、Zn、Ag、Cd or Mg; B=Si、Ge or Sn). Our results indicate that (1) the doped elements Si, Ge or Sn could stabilize the configuration Γ5g, and make it be the ground state configuration;(2) the doped Si, Ge or Sn enlarge the volume difference between Γ5g and the PM state, which is beneficial to the NTE of the compound;(3) compared to the compound containing Ag or Cd, the NTE behaviors of the compound containing Cu, Zn or Mg are better, among which the compound containing Zn or Mg has the better NTE behaviors than the other one; (4) the transition temperature of the compound containing Ag or Cd is higher than that containing Mg, Cu or Zn, and among Mg, Cu and Zn, the compound containing Zn has a higher transition temperature.