X-ray and neutron structure determination and magnetic properties of new quaternary phases RE_0.67Ni₂Ga_5+n-xGe_x and RE_0.67Ni₂Ga_5+n-xSi_x (n = 0, 1; RE = Y, Sm, Gd, Tb, Dy, Ho, Er, Tm) synthesized in liquid Ga

Liquid Ga was used as a solvent to explore the phase formation between rare-earth metals (REs), Ni, and a tetrelide (Tt = Si, Ge). The reactions were performed in excess liquid Ga at 850 °C. Two new phases of general formulas RE_0.67Ni₂Ga_5-xTt_x and RE_0.67Ni₂Ga_6-xTt_x were found and structurally characterized. The Co analogues of the latter RE_0.67Co₂Ga_6-xGe_x (RE = Y, Gd) were also prepared. Single-crystal X-ray data: The first group of compounds RE_0.67Ni₂Ga_5-xTt_x crystallizes in the hexagonal space group P6₃/mmc with a structure related to the RE_2-xPt₄Ga_8+y type (Sm_0.53Ni₂Ga_5-xGe_x, a = 4.1748(7) Å, c = 16.007(4) Å, V = 241.61(8) ų, Z = 2; Y_0.59Ni₂Ga_5-xGe_x, a = 4.1344(11) Å, c = 15.887(6) Å, V = 235.18(12) ų, Z = 2; Tb_0.67Ni₂Ga_5-xSi_x, a = 4.1415(11) Å, c = 15.843(6) Å, V = 235.33(12) ų, Z = 2; Ho_0.67Ni₂Ga_5-xGe_x, a = 4.1491(4) Å, c = 15.877(2) Å, V = 236.71(5) ų, Z = 2). The second group RE_0.67Ni₂Ga_6-xTt_x crystallizes in P6m2 (Gd_0.67Ni₂Ga_6-xGe_x, a = 4.1856(10) Å, c = 9.167(3) Å, V = 139.08(7) ų, Z = 1; SM_0.67Ni₂Ga_6-xSi_x, a = 4.1976(8) Å, c = 9.159(3) Å, V = 139.76(5) ų, Z = 1) in a structure related to the ErNi₃Al₉ structure type with disorder in the RE-Ga plane. Dy_0.67Ni₂Ga_6-xGe_x crystallizes in space group P31c (a = 7.2536(8) Å, c = 18.308(3) Å, V = 834.21(2) ų, Z = 6) with partial disorder in the RE-Ga plane. The structures of these two groups of compounds are related to each other and contain similar building motifs, namely Ga_n[NiGa_2-x/2Ge_x/2]₂ slabs and RE_0.67Ga monatomic layers which alternate along the c-direction, forming a 3D structure. The parameter x and the position of the tetrelide in the structure were determined by single-crystal neutron diffraction. Complete or partial disorder of the RE and Ga atoms is observed in the RE-Ga plane. The origin of the disorder lies in the extensive and random stacking faults of ordered RE-Ga planes, which apparently slide easily in the ab plane and create an averaged disordered picture. Electrical conductivity and thermopower measurements indicate that these compounds are metallic conductors. The magnetic measurements show antiferromagnetic ordering at ∼3-4 K and Curie-Weiss behavior at higher temperatures with the values of μ_eff close to those of RE³⁺ free ions. Strong 2-fold crystal field anisotropy is observed for the heavy RE analogues. The anisotropy constants K₂^para calculated from the Weiss constant anisotropy for heavy RE analogues are reported.