Crystal and Magnetic Structures of the Ternary Ho2Ni0.8Si1.2 and Ho2Ni0.8Ge1.2 Compounds: An Example of Intermetallics Crystallizing with the Zr2Ni1{\textendash}{xP} Prototype

We report two new rare-earth (R) ternary intermetallic compounds-Ho2Ni0.8T1.2 with T = Si and Ge-that correspond to the R5Ni2T3 phase earlier reported to form in Dy-Ni-T and Ho-Ni-T ternary systems. The compounds crystallize in a filled version of the orthorhombic Zr2Ni1-xP-type structure with x = 0.52; their stoichiometry, determined from both single-crystal and powder X-ray diffraction data, is centered on Ho2Ni0.8T1.2 with a narrow solid solubility range for the silicide, while the germanide appears to be a line phase. In addition to R = Dy and Ho, R2Ni0.8T1.2 compounds also form for R = Y and Tb, representing the first examples of rare-earth-based compounds adopting the Zr2Ni1-xP structural prototype. Bulk magnetization data reveal the main transitions of the ferrimagnetic or ferromagnetic type at T-C = 38 K for Ho2Ni0.8Si1.2 and T-C = 37 K for Ho2Ni0.8Ge1.2, which are followed by subsequent magnetic reordering at lower temperatures. Neutron diffraction shows complex magnetic structures below T-C with both ferromagnetic and antiferromagnetic components and magnetic propagation vector kappa(1) = [0, 0, 0]. Below T-N congruent to 24 K (22 K) for the silicide (germanide), an additional antiferromagnetic coupling following an incommensurate magnetic propagation vector kappa(2) = [kappa(x), 0, 0] appears to coexist with the first magnetic structure.