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 Nb6.4Ir4Al7.6 type) have been synthesized. They crystallize in the hexagonal crystal system with space group P63/mcm. Their lattice parameters are in the ranges of a = 935–963 and c = 851–874 pm for the RE6Ru5Al7 and a = 913–966 and c = 825–865 pm for the RE6Ir5Al7 series. Four structures (Ho6Ru5Al7, Yb6Ru4.68(1)Al7.32(1), Sc6Ir5Al7, and Ho6Ir4.53(1)Al7.47(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 Ho6Ir4.53(1)Al7.47(1) and on two sites for Yb6Ru4.68(1)Al7.32(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 T 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 −ΔS 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.

Laves phases: superstructures induced by coloring and distortions

“…Yb 6 Ir 5 Ga 7 [122] was the first example of that superstructure variant with a complete ordering on the tetrahedral substructure. This was meanwhile also observed for the rare earth‐based series RE 6 Ir 5 Ga 7 [127] and RE 6 T 5 Al 7 ( T =Ru, Ir) [92] . The reason for superstructure formation is the different coloring within the rows of face‐ and corner‐sharing tetrahedra.…”

“…The flexibility of this structure type can be demonstrated as also equiatomic compounds have been reported to adopt the MgZn 2 type structure and also various other solid solutions. [14,15,[88][89][90][91][92]…”

“…This was meanwhile also observed for the rare earth-based series RE 6 Ir 5 Ga 7 [127] and RE 6 T 5 Al 7 (T = Ru, Ir). [92] The reason for superstructure formation is the different coloring within the rows of face-and corner-sharing tetrahedra. For Yb 6 Ir 5 Ga 7 we observe rows of condensed IrGa 3 and Ir 3 Ga tetrahedra in 2 : 1 ratio, leading to the [Ir 5 Ga 7 ] substructure (Figure 19).…”

Searching for Laves Phase Superstructures: Structural and 27Al NMR Spectroscopic Investigations in the Hf–V–Al System

“…19 Finally, also some aluminum representatives of the Yb 6 Ir 5 Ga 7 type structure 20 (P6 3 /mcm, superstructure of MgZn 2 ) could be synthesized and characterized by our group. 21 Based on these and other recent results, we summarized the structural and crystal chemistry of Laves phases 22 and the Mo 3 Al 2 C type compounds 23 with a special focus on superstructure formation and discussed the crystal-chemical peculiarities based on group−subgroup relations. During the search in the Pearson database, 24 it became clear that the majority of the binary MAl 2 phases adopts the cubic MgCu 2 type structure (M = Ca−Ba, Sc, Y, La−Nd, Sm−Lu, Th, U−Pu), 17,25−34 while the hexagonal MgZn 2 type is only realized for ZrAl 2 and HfAl 2 .…”

On the Ytterbium Valence and the Physical Properties in Selected Intermetallic Phases

“…The superstructure formation leads to a small deviation. The correct model was solved for Yb 6 Ir 5 Ga 7 (space group P 6 3 / mcm ) and the series of gallides RE 6 Ir 5 Ga 7 . , Shortly later, also the aluminum containing RE 6 T 5 Al 7 phases ( RE = Sc, Y, Ce–Nd, Sm, Gd–Lu; T = Ru, Ir) have been structurally characterized. With respect to the physical properties, one must differentiate between the fully ordered 6–5–7 phases and solid solutions where mixed occupied sites occur.…”