Intermetallic RE₆T₅Al₇ Phases (RE = Sc, Y, Ce–Nd, Sm, Gd–Lu; T = Ru, Ir): Diversity in their Magnetic, Magnetocaloric, and Critical Properties

Abstract

Over 20 new compounds of the RE₆T₅Al₇ series (RE = Sc, Y, Ce-Nd, Sm, Gd-Lu; T = Ru, Ir; Yb₆Ir₅Ga₇ type structure; superstructure of MgZn₂; fully ordered Nb_6.4Ir₄Al_7.6 type) have been synthesized. They crystallize in the hexagonal crystal system with space group <i>P</i>6₃/<i>mcm</i>. Their lattice parameters are in the ranges of <i>a</i> = 935-963 and <i>c</i> = 851-874 pm for the RE₆Ru₅Al₇ and <i>a</i> = 913-966 and <i>c</i> = 825-865 pm for the RE₆Ir₅Al₇ series. Four structures (Ho₆Ru₅Al₇, Yb₆Ru_4.68(1)Al_7.32(1), Sc₆Ir₅Al₇, and Ho₆Ir_4.53(1)Al_7.47(1)) have been refined from single-crystal data, indicating fully ordered structures for Ho₆Ru₅Al₇ and Sc₆Ir₅Al₇; however, T/Al mixing was observed on one crystallographic site for Ho₆Ir_4.53(1)Al_7.47(1) and on two sites for Yb₆Ru_4.68(1)Al_7.32(1). The Sc-, Y-, and Lu-containing compounds exhibit Pauli paramagnetism in line with a filled d band for Ru and Ir. Ce₆Ru₅Al₇ exhibits mixed-valent behavior, while Yb₆Ru₅Al₇ is solely trivalent. The other compounds exhibit paramagnetism and ferromagnetic phase transitions up to temperatures of <i>T</i>_C = 83.4(1) K for Gd₆Ru₅Al₇. In addition to the basic magnetic characterizations and studies of the electrical resistivity and heat capacity, the magnetocaloric properties of Gd₆Ru₅Al₇, Tb₆Ru₅Al₇, and Dy₆Ru₅Al₇ have been investigated, revealing magnetic entropy changes of -Δ<i>S</i>_M^max = 6.2(1), 7.7(1), and 5.4(1) J kg^-1 K^-1 (0 → 5 T) and relative cooling powers RCP = 242, 207, and 135 J kg^-1, respectively. For a deeper insight into the second-order magnetic phase transitions, the critical behavior was investigated according to the scaling hypothesis. The critical behavior of Gd₆Ru₅Al₇ is in accordance with the mean-field theory; the critical exponents of Tb₆Ru₅Al₇ and Dy₆Ru₅Al₇, however, deviate strongly from this universality class. For comparison of the structural and magnetic properties, the thus far unstudied members of the RERuAl series (RE = Sc, Y, Sm, Gd-Lu; MgZn₂ type) have additionally been prepared and characterized.