Phase formation and transformation in the R-Fe-T systems: (R=Nd, Gd, Tb, Dy, Er, Ho, Tm and Lu, T=Si, Ti and Zr)

A systematic study of the R-Fe-T (R = Nd, Gd, Tb, Dy, Ho, Er, T m and Lu, T = Si, Ti, Zr) systems has shown that besides the well-known rhombohedral 2:17 (2:17-R), hexagonal 2:17 (2:17-H) and tetragonal 1:12 (1:12-T) phases, several novel ternary Ferich rare-earth (R) transition metal (T) compounds, namely R3(Fe1-xSix)22 (3:22), R3(Fe1-xTix)29 (3:29), R(Fe1-xTix)10 (1:10), P1:12, orthorhombic 1:12 (1:12-0), R(Fe1-xTix)u (1:11) and (Nd1/3Zr2/3)Fe3 (1:3), also crystallize in these systems. Like the 2:17 and 1:12 phases, all the new phases have close structural relationships with the hexagonal CaCu5-type structure (1:5). The 3:22, 3:29 and 1:10 phases are intermediate phases between the 1:and 1:12 phases and their structures can be directly derived from the CaCu5 structure replacing a proportion of the large R (Ca) atoms with pairs of small transition metal atoms (dumb-bell) along the c-axis of the CaCu5 structure. This procedure can be facilitated by a simple equation R1-xT5+(T-T)x =\u3e RTy with the phase being given by 3:22 (x = 1/4 ), 2:17 (x = 1/3 ), 3:29 (x = 2/5), 1:10 (x=5/12) and 1:12 (x = 1/2 ). The structures of the P1:12 and orthorhombic 1:12 phases are closely related to the tetragonal 1:12 structure which, as shown above, is derived from the 1:5 structure by replacing 1/2 of the R atoms with dumb-bells. The structure of the orthorhombic 1:12 phase is a result of an extreme preferential site occupation by the Ti atoms in the 1:12-T structure. The novel pseudo-ternary (Nd1/3Zr2/3)Fe3 phase has a PuNi3-type(1:3) structure. Although the 1:3 phase forms in many binary R-T systems, it does not crystallize in the binary Nd-Fe system. By introducing the third element, Zr, into the Nd-Fe system the 1:3 phase is stabilized with a well defined stoichiometric composition (Nd1/3Zr2/3)Fe3 which suggests that all the crystallographic 6c and 3a R sites are exclusively occupied by Zr and R atoms, respectively, while all 18/z sites are occupied by Fe atoms. The extremely preferential occupation of the transition metal Zr atoms at rare-earth Nd sites instead of transition metal (T) sites is consistent with a fact that Zr has an atomic size (1.45A) closer to that of Nd (1.64A) than that of Fe (1.17A). This suggests that the atomic size plays a very important role in stabilizing the pseudo-ternary (Nd1/3Zr2/3)Fe3 phase. It has long been noticed that the structural similarities of the intermetalhc compounds formed between transition metals can be resolved by seeing the structures in the form of the coordination polyhedra of large atoms. Only limited types of coordination polyhedra that satisfy the closely-packed arraignment are abundantly transition metals, it was noted that the rare-earth (R) atoms in all the structures studied in observed in the real structures. In the study of the structures formed between the rare earths and the present work have 20 coordination neighbors and actually all these structures can be viewed as simple stackings of the 20-vertiex coordination polyhedra formed by the 20 coordination neighbors around the R atoms, which we refer to as the CPR (CPR stands for coordination polyhedron of R atom). To understand the structures and structural relationships of the novel phases identified in the present work, as well the 1:5, 2:17 and 1:12 compounds, we extended this by assuming that the R atoms in our new structures would reserve 20 coordination neighbors. From the CPR point of view, all the structures of the 1:5, 2:17, and 1:12 phases, as well as the novel 3:22, 3:29, 1:10 and (Nd1/3Zr2/3)Fe3 phases can be viewed as simple stackings of the 20-vertex CPRs along one direction. Although the direct relationships between the CaCu5 and BaCd11 structures and the CaCu5 and Nd2Fe14B structures are not obvious in the dumb-bell replacement model, their relationships are apparent from the CPR point of view. Both the BaCd11 and Nd2Fe14B structures are also simple stackings of the 20-vertex CPRs along one direction. From the CPR point of view, the only differences between these structures are the CPR structure as well as the CPR stacking sequences Considering our CPR model, we propose possible structures for our novel R3(Fe1-xSix)22 (3:22), R3(Fe1-xTix)29 (3:29) and R(Fe1-xTix)10 phases. R3(Fe1-xSix)22 is an intermediate phase between 2:17 and 1:12 and undergoes a structural change correlated with that undergone by the 2:17 phase as R changes from Gd and Tb to Dy. R3(Fe1-xSix)22 with R=Dy, Er, Ho, Tm and Lu has an intermediate structure between the hexagonal 2:17 and tetragonal 1:12 structures, which is an orthorhombic with a possible space group Pmm2 (No. 25). R3(Fei1-xTix)29 is an intermediate phase between the rhombohedral 2:17 and tetragonal 1:12 phases and has a monoclinic structure with a possible space group A2/m. R3(Fe1-xTix)10 is an intermediate phase between the hexagonal 2:17 and tetragonal 1:12 phases and has an orthorhombic structure with a possible space group Pmm2 (No. 47). Like the 1:12 phase, all the novel phases identified in the present work do not exist in binary R-Fe systems and a third element T (Si, Ti and Zr) is essential to stabilize them. The key to the role of the third element atoms in stabilizing these structures is size effect that can be well explained by our CPR model