Theoretical modelling of phase stability, magnetic and mechanical properties of compounds based on transition metals

Abstract

In this Doctoral thesis, ab initio calculations of electronic structure within the projector augmented wave method are employed to study physical properties of several compounds containing transition metals and manifesting magnetic ordering. In particular, three material problems were studied, namely the magnetic properties of complex A2BBO6 oxides in dependence of various A, B and B metallic elements; the mechanism twinning on atomic level and its application on NM martensite of Mn-rich Ni-Mn-Ga alloys and the phase transformation and stability in Fe2C. The results of first part show that to achieve the highest Curie temperature in A2BBO6 oxides, A element must have ionic radius as small as possible while the magnetic moments of B and B must be maximized. For deformation twinning of Ni-Mn-Ga alloys it is demonstrated that Mn excess leads to more difficult twinning due to increased phase stability of non-modulated martensite and magnetic ordering of excess Mn atoms. At last but not least, the formation of orthorhombic Fe2C from tetragonal rutile structure is explained from thermodynamical, mechanical and kinetic perspectives as a stabilization due to pseudo-Jahn-Teller effect.In this Doctoral thesis, ab initio calculations of electronic structure within the projector augmented wave method are employed to study physical properties of several compounds containing transition metals and manifesting magnetic ordering. In particular, three material problems were studied, namely the magnetic properties of complex A2BBO6 oxides in dependence of various A, B and B metallic elements; the mechanism twinning on atomic level and its application on NM martensite of Mn-rich Ni-Mn-Ga alloys and the phase transformation and stability in Fe2C. The results of first part show that to achieve the highest Curie temperature in A2BBO6 oxides, A element must have ionic radius as small as possible while the magnetic moments of B and B must be maximized. For deformation twinning of Ni-Mn-Ga alloys it is demonstrated that Mn excess leads to more difficult twinning due to increased phase stability of non-modulated martensite and magnetic ordering of excess Mn atoms. At last but not least, the formation of orthorhombic Fe2C from tetragonal rutile structure is explained from thermodynamical, mechanical and kinetic perspectives as a stabilization due to pseudo-Jahn-Teller effect.