Gd5Ni0.96Sb2.04 and Gd5Ni0.71Bi2.29: Crystal structure, magnetic properties and magnetocaloric effect. Structural transformation and magnetic properties of hexagonal Gd5Bi3

Nickel was successfully introduced into the Gd₅Sb₃ and Gd₅Bi₃ binaries to yield the Gd₅Ni_0.96(1)Sb_2.04(1) and Gd₅Ni_0.71(1)Bi_2.29(1) phases. Both Ni-substituted compounds adopt the orthorhombic Yb₅Sb₃-type structure. While the Gd₅Ni_0.71Bi_2.29 phase is thermodynamically stable at 800 °C and decomposes at lower temperatures upon annealing, it can be easily quenched to room temperature by rapid cooling from 800 °C. The Gd₅Ni_0.96Sb_2.04 phase is found to be thermodynamically stable till room temperature. Through annealing at different temperatures, Gd₅Bi₃ was proven to undergo the Mn₅Si₃-type (LT)↔Yb₅Sb₃-type (HT) transformation reversibly, whereas Gd₅Sb₃ was found to adopt only the hexagonal Mn₅Si₃-type structure. Orthorhombic Gd₅Ni_0.96Sb_2.04 and Gd₅Ni_0.71Bi_2.29 and low-temperature hexagonal Gd₅Bi₃ order ferromagnetically at 115, 162 and 112 K, respectively. In Gd₅Bi₃, the ferromagnetic ordering is followed by spin reorientation below 64 K. Magnetocaloric effect in terms of ΔS was evaluated from the magnetization data and found to reach the maximum values of −7.7 J/kgK for Gd₅Ni_0.96Sb_2.04 and −5.6 J/kgK for Gd₅Ni_0.71Bi_2.29 around their Curie temperatures.