Atomic disorder and phase transformation in intermetallic compounds of the typeT3X2(T=Ni,Fe,Mn;X=Sn,Ge) by mechanical milling

The structural development of the ordered intermetallic compounds ${\mathit{T}}_3$ ${\mathit{X}}_2$ (T=Ni,Fe,Mn; X=Sn,Ge) upon mechanical milling was investigated by x-ray diffraction, magnetic measurements, and subsequently by differential scanning calorimetry (DSC). It is found that the magnetization at 4.2 K increases continuously with increasing milling time in ferromagnetic ${\mathrm{Ni}}_3$ ${\mathrm{Sn}}_2$ and ${\mathrm{Fe}}_3$ ${\mathrm{Ge}}_2$. In contrast, in ferrimagnetic ${\mathrm{Mn}}_3$ ${\mathrm{Sn}}_2$ it decreases. The unit-cell volume of both ${\mathrm{Mn}}_3$ ${\mathrm{Sn}}_2$ and ${\mathrm{Fe}}_3$ ${\mathrm{Ge}}_2$ continuously increases. These results are explained well in terms of a special type of atomic disorder: redistribution of transition-metal atoms over two different types of transition-metal sites, induced by ball milling. Exothermic heat effects corresponding to atomic reordering are observed in the DSC scans of ${\mathrm{Mn}}_3$ ${\mathrm{Sn}}_2$ and ${\mathrm{Fe}}_3$ ${\mathrm{Ge}}_2$ as well as ${\mathrm{Ni}}_3$ ${\mathrm{Sn}}_2$ after various periods of milling. The heat evolved in the atomic reordering process increases gradually with milling time. After long-time milling all physical parameters tend to become constant. After prolonged periods of milling, a phase transformation in ${\mathrm{Ni}}_3$ ${\mathrm{Sn}}_2$ from the orthorhombic-structure low-temperature phase (LTP) to the hexagonal-structure high-temperature phase (HTP) is observed accompanied by a sharp increase in magnetization. ${\mathrm{Mn}}_3$ ${\mathrm{Sn}}_2$ and ${\mathrm{Fe}}_3$ ${\mathrm{Ge}}_2$ remain in the hexagonal structure.