Correlation between the structure and thermomagnetic properties of pseudo-binary (Tb,Er)Ni2 solid solutions

Highlights: • A good agreement was found between the Maxwell relation and theoretical calculations based on the Landau theory. • The Curie temperatures decrease from 29 K for Tb_0.75Er_0.25Ni₂ to 13.6 K for Tb_0.25Er_0.75Ni₂. • The formation of the superstructure in the Tb_1‑xEr_xNi₂ solid solutions reduces their magnetocaloric properties. • The crystal structure of the solid solutions corresponds to a space group with the lower translation symmetry. • Solid solutions show promise as materials for magnetic refrigerators operating within a specific low‑temperature range. -- Abstract: Magnetic and thermodynamic properties, the isothermal magnetic entropy change ΔS_mag is among them, of the polycrystalline pseudo-binary Tb_1-xEr_xNi₂ (x = 0.25, 0.5, and 0.75) solid solutions are studied in accordance with the erbium substitution for terbium, which causes changing the crystal structure. The Tb_1-xEr_xNi₂ solid solutions at room temperature were found to have the C15 Laves-phase superstructure rather than the C15 structure typical of the parent compounds, TbNi₂ and ErNi₂. Moreover, as was shown by magnetic measurements, the substitution of Er for Tb in Tb_1-xEr_xNi₂ results in the decrease in the ordering temperature of the pseudo-binary compounds, which corresponds to the second-order ferromagnet to paramagnet magnetic phase transition; in the case of x = 0.25 and x = 0.75, the decrease is from 29.0 to 13.6 K, respectively. The isothermal magnetic entropy change, which is estimated to characterize the magnetocaloric properties of the pseudo-binary compounds, was analyzed using the thermomagnetic Maxwell relation and in terms of the Landau theory of phase transitions. The maximum magnetic entropy change of the Tb_0·75Er_0·25Ni₂, Tb_0·5Er_0·5Ni₂ and for Tb_0·25Er_0·75Ni₂ compositions reaches 6.6 (12.7), 6.9 (13.8) and 7.5 (14.9) J/kgK for a magnetic field change of 2 (5) T, respectively. The relatively large reversible and close values of ΔS_mag allow us to state that these pseudo-binary solid solutions show promise as materials for magnetic refrigerators operating within a specific low-temperature range.