Magnetic order of the La3NiGe2-type Tb3NiSi2

• At 135 K and 52 K the La3NiGe2-type Tb3NiSi2 shows a magnetic transition.
• Tb3NiSi2 shows four types of mixed F-AF ordering below ~134, 82, 66 and 53 K.
• At 1.5 K the terbium magnetic moment reaches the value 9.1(1) μB.
• The unit cell of Tb3NiSi2 undegoes isotropic compression down to ~138 K.
• The unit cell of Tb3NiSi2 shows anisotropic distortion below ~138 K.

The novel Tb3NiSi2 phase adopts the La3NiGe2-type structure (Pnma, N 62, oP24). According to the thermal dependence of the magnetization measurements in 100 Oe applied field, Tb3NiSi2 undergoes a ferromagnetic-like transition at 135 K and a spin-orientation transition at 53 K. Based on the neutron diffraction studies in a zero applied field, Tb3NiSi2 shows four types of mixed ferromagnetic–antiferromagnetic ordering below ~130 K, 82 K, 66 K and 53 K. The unit cell of Tb3NiSi2 undergoes an isotropic compression down to 143 K and an anisotropic one below the temperature of the ferromagnetic transition at ~130 K: the a cell parameter increases, whereas the b and c cell parameters and unit cell volume decrease with the decreasing temperature. At 1.5 K the terbium magnetic moment reaches the value 9.1(1) μB.