Investigation of the Y2Te3–Cu2Te–PbTe system at 870 K and crystal structures of the Y7Cu3Te12 and YCu0.264Te2 compounds

The structural and magnetic properties of doubly substituted Nd₂Fe_17−x−yTi_xGa_y (0 ≤ x ≤ 1.0, 0 ≤ y ≤ 3) compounds have been investigated by a combined technique of X-ray diffraction, neutron diffraction and magnetic measurements. Rietveld refinements of the diffraction data indicate that all the samples crystallize in the rhombohedral Th₂Zn₁₇-type structure. For a given Ti content (x), the lattice parameters a and c, and unit cell volume V of Nd₂Fe_17−x−yTi_xGa_y all increase linearly with increasing Ga content. The addition of Ti initially has a considerably positive effect on the increasing rates of a, c, and unit cell volume V, but later the effect becomes very slight and even negative with the further increase of Ti content. The site occupancies of Ti and Ga in the crystallographic sites change a little compared to what is observed in the corresponding singly substituted compounds. The effects of Ti and Ga on the bond lengths of the doubly substituted compounds are quite different from that of the singly substituted compounds. Magnetic measurements show the T_C of Nd₂Fe_17−x−yTi_xGa_y increases with increasing Ti content for a low Ga content while it behaves conversely for a higher Ga content. The T_C of Nd₂Fe_17−x−yTi_xGa_y increases with increasing Ga content for a particular Ti content, while the addition of Ti results in a slower increase of T_C on the Ga content (y ≤ 3). For a given Ti content, the M_s of Nd₂Fe_17−x−yTi_xGa_y first increases a little and then decreases with the increase of Ga content, while for a given Ga content the M_s of Nd₂Fe_17−x−yTi_xGa_y decreases with the increase of Ti content.