Mo2NiB2-type {Gd, Tb, Dy)2Ni2.35Si0.65 and La2Ni3-type {Dy, Ho}2Ni2.5Si0.5 compounds: Crystal structure and magnetic properties

• {Gd, Tb, Dy}2Ni2.35Si0.65 compounds crystallize in the Mo2NiB2-type structure.
• {Dy, Ho}2Ni2.5Si0.5 compounds crystallize in the La2Ni3-type structure.
• Gd2Ni2.35Si0.65 shows pure ferromagnetic type ordering.
• Tb2Ni2.35Si0.65 and Dy2Ni2.5Si0.5 show mixed ferro-antiferromagnetic ordering.
• Tb2Ni2.35Si0.65 and Dy2Ni2.5Si0.5 exhibit low-temperature metamagnetic behaviour.

The crystal structure of new Mo2NiB2-type {Gd, Tb, Dy}2Ni2.35Si0.65 (Immm, No. 71, oI10) and La2Ni3-type {Dy, Ho}2Ni2.5Si0.5 (Cmce No. 64, oC20) compounds has been established using powder X-ray diffraction studies.Magnetization measurements show that the Mo2NiB2-type Gd2Ni2.35Si0.65 undergoes a ferromagnetic transition at ~66 K, whereas isostructural Tb2Ni2.35Si0.65 shows an antiferromagnetic transition at ~52 K and a field-induced metamagnetic transition at low temperatures. Neutron diffraction study shows that, in zero applied field, Tb2Ni2.35Si0.65 exhibits c-axis antiferromagnetic order with propagation vector K=[1/2, 0, 1/2] below its magnetic ordering temperature and Tb magnetic moment reaches a value of 8.32(5) μB at 2 K.The La2Ni3-type Dy2Ni2.5Si0.5 exhibits ferromagnetic like transition at ~42 K with coexisting antiferromagnetic interactions and field induced metamagnetic transition below ~17 K.The magnetocaloric effect of Gd2Ni2.35Si0.65, Tb2Ni2.35Si0.65 and Dy2Ni2.5Si0.5 is calculated in terms of isothermal magnetic entropy change and it reaches a maximum value of −14.3 J/kg K, −5.3 J/kg K and −10.3 J/kg K for a field change of 50 kOe near 66 K, 52 K and 42 K, respectively. Low temperature magnetic ordering with enhanced anisotropic effects in Tb2Ni2.35Si0.65 and Dy2Ni2.35Si0.65 is accompanied by a positive magnetocaloric effect with isothermal magnetic entropy changes of +12.8 J/kg K and ~+9.9 J/kg K, respectively at 7 K for a field change of 50 kOe.

The {Gd, Tb, Dy}2Ni2.35Si0.65 supplement the series of Mo2NiB2-type rare earth compounds, whereas the {Dy, Ho}2Ni2.5Si0.5 supplement the series of La2Ni3-type rare earth compounds.The variation of alloy’s composition by ~3 at% i.e. from Dy2Ni2.35Si0.65 to Dy2Ni2.5Si0.5 leads to significant transformation of crystal structure of compound with different variant of distortion of Po-type rare earth sublattice, as in Gd–Co–Ga and Er–Ni–In systems: the Mo2NiB2-type Gd2Co2Ga and La2Ni3-type Gd2Co2.9Ga0.1, and Mo2FeB2-type Er2Ni1.78In and Mn2AlB2-type Er2Ni2In.Magnetization measurements indicate collinear ferromagnetic ordering of Mo2NiB2-type Gd2Ni2.35Si0.65 and a complex antiferromagnetic ordering with low-temperature metamagnetic nature for Mo2NiB2-type Tb2Ni2.35Si0.65 compounds. However, neutron diffraction study in zero applied field of Tb2Ni2.35Si0.65 reveals c-axis pure antiferromagnetic ordering of terbium sublattice with K=[1/2, 0, 1/2] propagation vector.Magnetization measurements indicate ferromagnetic order with coexisting antiferromagnetic interactions and low-temperature metamagnetic state for La2Ni3-type Dy2Ni2.5Si0.5.We suggest possible polymorphism in other Mo2FeB2-type, Mo2NiB2-type, La2Ni3-type and Mn2AlB2-type rare earth compounds with corresponding change in their magnetic properties.Figure optionsDownload as PowerPoint slide