ATOMIC DISORDER AND PHASE-TRANSFORMATION IN INTERMETALLIC COMPOUNDS OF THE TYPE T3X2 (T=NI, FE, MN, X=SN, GE) BY MECHANICAL MILLING

The structural development of the ordered intermetallic compounds T3X2 (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 Ni3Sn2 and Fe3Ge2. In contrast, in ferrimagnetic Mn3Sn2 it decreases. The unit-cell volume Of both Mn3Sn2 and Fe3Ge2 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 Mn3Sn2 and Fe3Ge2 as well as Ni3Sn2 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 Ni3Sn2 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. Mn3Sn2 and Fe3Ge2 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 Ni3Sn2 after long milling periods. The occurrence of the ball-milling-induced phase transformation in Ni3Sn2 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.