| It is important to study high pressure-induced phase transition of the fluoride system. The Nickel Cobalt Manganese Iron difluorides are non-ferromagnetic rutile structure at room temperature. They all translate to orthorhombic CaCl2 structure under pressure. This second phase transition also appears in IV family oxides, for example GeO2, SnO2, PbO2. The rutile structure transform to calcium chloride structure is second phase transition, the volume continuous in the process, only F ions occupy the position from the original rutile structure (u, u,0) changed to (x, y,0) (x not equal y).When ambient reduce to Neel temperature, they all change to anti-ferromagnetic. It means the central metallic ion get the opposite magnetic moment to the point. While the system translate from rutile to CaCl2 the the arrangement of magnetic moment remain unchangedThe theoretical and experimental researches of this phase transition were carried out extensive, but physical mechanism has not been reported yet. In addition, the post-CaCl2 structure of four substances under pressure and the magnetic moment arrangement has not been set. In this paper, we study fluoride of manganese, iron, cobalt, nickel based on first principles density functional theory simulation.In order to explain this phase transition, we calculated the G point phonon of rutile structure with linear response theory, the Raman active mode optical B1g is getting to be softening with compression. Similar to MgF2 calculations, these types of substances Big model does not soften to zero at phase transition, but the need for further compression. For further study of the structure caused by the B1g mode instability, we conducted " frozen phonon" calculation:other structural parameters remain unchanged, according to B1g mode eigenvectors series of atomic movement, and calculate the total energy change of system. When the volume is getting smaller, we can clearly see the formation of energy "valley"; While energy lower by only a few meV/fu, it clearly shows that the rutile structure of is not stability with compression. This shows softening of Raman optical modes B1g promote to be one driving force of the phase transform.We use strain-stress method to study the elastic properties of the rutile structure. Rutile structure has six independent elastic constants:C11, C12, C13, C33, C44, C66; shear modulus, which is defined as Cs= 1/2 (C11-C12), describes the transverse acoustic phonon behavior of B1g symmetry along the [110] direction. We found that C11, C12, C13, C33, C44,C66 become larger with increasing pressure, indicating that they are stable, while the elastic modulus Cs is shown to be softening trend with the pressure. According to the soft membrane theory of phase transitions, multiple or single soft membrane between the membrane and can be coupled with other factors lead to phase transition. Therefore, we believe that the soft membrane and shear modulus softening of the coupling co-led the rutile-CaCl2 second phase transition of four anti-ferromagnetic fluorides.What kind of magnetic moment arrangement will be in post-CaCl2 structure? We takeα-PbO2 structure of NiF2 for example to explain its magnetic moment arrangement. Inα-PbO2, the metallic ions occupy 4c position:a, (0, y,1/4); b, (0,-y,3/4); c, (1/2,-y+1/2,3/4); d, (1/2,y+1/2,1/4).The four different position divide into two groups. In one group the two metallic ions gain the same magnetic moment, while the two groups gain the opposite magnetic moment. We calculated all three possibilities. While a, b get the same magnetic moment; c, d get the same magnetic moment; the two groups get the opposite magnetic moment, the structure is most steady. Thought discussing all the possible arrangement of anti-ferromagnetic structure, we find out way to get the most stable structure of other anti-ferromagnetic structure.Theoretical arithmetic confirm the Nickel Cobalt Manganese Iron difluorides keep anti-ferromagnetic till 40 GPa, which are different to MgF2, ZnF2.
|
PDF Full Text Request