First Principles Study On Tuning The Negative Thermal Expansion Property Of ScF₃ By Doping

Most solid materials exhibit thermal expansion, but some of the materials have shown thermal anomaly phenomenon:with the increase of temperature, volume contract,i.e. negative thermal expansion（NTE）. Since an isotropic large NTE behavior in ZrW₂O₈ at 0.3K to 1050 K in Science was reported in 1996, materials with NTE have been attracted much attention and has become a new research field in materials science at present. So far, a series of NTE materials have been found and put forward several physical mechanisms of NTE according to the different materials, but it is still need to work hard to find novel NTE materials with excellent mechanics and electronic properties and to tailor materials with proper thermal expansion properties as well as to reveal underlying mechanisms.Usually, the NTE property of materials can be tailored by two ways: The first way is that materials with different thermal expansion properties are composited to prepare near zero thermal expansion materials; the second is that the NTE behavior in a single NTE compound is tuning by means of element doping. Because the resulting compounds show uniform in structure and property, the second way is frequently used to tailor thermal expansion property of materials in experiments. For example, recently it was reported by Xing and Chen’s research group of Beijing University of Science and Technology that NTE property of ScF₃ can be greatly affected by co-doping Ga and Fe and near zero thermal expansion property can be achieved in a wide temperature range by co-doping with proper doping level. This result caused the people’s attention. In this paper, Ga, Ti and Fe doping effects in ScF₃ are studied by first principles calculations based on density functional theory（DFT）. The structural distortion and the change of electronic property and thermal expansion caused by doping are calculated. The physical mechanisms for the change of NTE property are discussed. This paper mainly includes the following two parts:In the first part, we calculated the electronic structure and thermal properties of pure ScF₃ system, including the phonon spectrum of the system and the Gruneisen parameters of different phonon modes. The phonon modes, which cause NTE of the system, are analyzed. From the electronic density of states（DOS）, the ScF₃ is a wide band gap insulator, so we can speculate that the effect of thermal excitation on the NTE can be neglected. On the basis of the quasi harmonic approximation（QHA）, we get each vibration phonon mode Gruneisen parameters, and find that the phonon modes between M^R points in first Brillouin zone（FBZ）（related to lateral vibration of fluorine atoms） contribute to the NTE. The transverse vibration of the fluorine atoms cause the rotation of the octahedra ScF6, resulting in the volume contraction. Thus, the NTE of the ScF₃ can be explained by the rigid unit mode coupling rotation. Interestingly, the cubic phase ScF₃ is very sensitive to the change of the lattice constants, and we find that the low frequency phonon modes between M^R are not stable when the system is compressed（imaginary frequencies）, mainly due to the reason that the structure phase transition occurs when the ScF₃ is slightly compressed.In the second part, the structure, electronic properties and thermal expansion properties of the doped systems(Sc_0.963Ga_0.037F₃, Sc_0.963Ti_0.037F₃ and Sc_0.963Fe_0.037F₃) were studied by using a 3×3×3 supercell of ScF₃. Due to the mismatch of the atomic size, the local distortion of the doped system is inevitable, and the phonon modes of the system should be affected. With Ga, Ti and Fe atoms replacing Sc, firstly the structural distortion of the doped systems is trivial due to our low doping level. Secondly, for Ga-doped system the change of electronic band structure, especially the band gap is hardly found. While for Ti- and Fe-doped systems new states can be found near Fermi level and in the gap. Besides, for Fe-doped system Sc_0.963Fe_0.037F₃, it exhibits ferromagnetic property with magnetic moment up to 5 per cell. On the basis of the QHA, we calculate thermal expansion curves of the doped systems. We find that for Sc_0.963Ga_0.037F₃, regulation of NTE is very weak, while for systems Sc_0.963Fe_0.037F₃ and Sc_0.963Ti_0.037F₃ the regulation are larger, especially for the Sc_0.963Fe_0.037F₃ system, which coefficient of NTE is decreased up to 10 times and temperature range for NTE is also reduced. This results are in agreement with the experimental reports. According to our study, we find that for the Ti- and Fe-doped systems, there exists strong coupling between the phonon modes. This coupling is manifested in the sharper potential curve as well as the drastic change of electronic property due to the vibrations related to the transverse displacement of F atoms.