Atomic disorder and phase transformation in intermetallic compounds of the type T3X2 (T=Ni,Fe,Mn; X=Sn,Ge) by mechanical milling.

Abstract

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.The exothermic heat effect corresponding to the phase restoration of the ball-milled metastable HTP to the original equilibrium LTP is evident from DSC scans of ${\mathrm{Ni}}_{3}$${\mathrm{Sn}}_{2}$ after long milling periods. The occurrence of the ball-milling-induced phase transformation in ${\mathrm{Ni}}_{3}$${\mathrm{Sn}}_{2}$ is also confirmed by a comparison of the ball-milled phase to the high-temperature phase obtained by rapid quenching. The excellent agreement of all experimental results obtained by different techniques proves that by mechanical milling well-defined metastable states are generated in these B8-like compounds and that atomic disorder is the main source of energy storage during ball milling of intermetallic compounds. The particular type of atomic disorder in these B8-like compounds cannot be obtained by rapid quenching from high temperatures.