Influence of the La/M Network on Magnetic Properties of Mn₄ Tetrahedra in Intermetallic Compounds La_21−δMn₈M₇C₁₂ (M = Ge, Sn, Sb, Te, Bi)

Abstract

Crystals of La 21-δ Mn 8 M 7 C 12 (M = main group elements Ge/Al, Sn, Sb, Te, Bi) were synthesized in La/Ni eutectic flux. The structure features tetrahedral manganese clusters edge-capped by carbon atoms, Mn 4 C 6 , embedded into a nonmagnetic La/M network. The structures were determined from single-crystal X-ray diffraction (La 21 Mn 8 -Ge 6.2 Al 0.8 C 12 , Fm3m; a = 16.2259(3) Å; Z = 4; R 1 = 0.022). While most of the La 21 Mn 8 M 7 C 12 analogues are completely ordered, the M = Te compound features partial occupancy of one of the La sites, and the M = Ge analogue exhibits mixed Ge/Al occupancy on one of the M sites. La 21-δ Mn 8 M 7 C 12 with M = Sn, Sb, Te, Bi are paramagnets, but the susceptibility of La 21 Mn 8 Ge 6.2 Al 0.8 C 12 exhibits ZFC-FC divergence at T 1 ≈ 6 K and an additional feature at T 2 ≈ 3 K. Such behavior is indicative of frustration of antiferromagnetically coupled manganese magnetic moments within the Mn 4 tetrahedra. AC susceptibility measurements confirm the presence of two cusps at T 1 and T 2 ; however, no frequency dependence of cusp maxima was found. This differentiates La 21 Mn 8 Ge 6.2 Al 0.8 C 12 from the Fe analogue, La 21 Fe 8 Sn 7 C 12 , where frequency dependence indicates spin-glass behavior. Electronic calculations on La 21-δ Mn 8 M 7 C 12 confirm that the vacancy on one of the La sites in the La 20 Mn 8 Te 7 C 12 analogue stabilizes the phase by shifting the Fermi level to a pseudogap in the density of states. 57 Fe and 119 Sn M€ ossbauer spectroscopy of spin-glass compound La 21 Fe 8 Sn 7 C 12 and paramagnetic La 21 Mn 8 Sn 7 C 12 supports the presence of spin glass behavior in the Fe analogue and not in the Mn analogue.

Synthesis and crystal structure of La21Cr8−2a Al b Ge7−b C12 [a = 0.22 (2) and b = 0.758 (19)]

“…1e form cages about their central La3 and Al2/Ge2 sites respectively. The cage-like nature of this configuration is clear from the large anisotropic displacement parameters U eq corresponding to these two central sites, as has been previously observed in isostructural materials (Benbow et al, 2009;Zaikina et al, 2011). These sites are likely characterized by strong rattling modes of the central loosely bound atom, such as is observed in skutterudite compounds (Sergueev et al, 2015).…”

“…Here too, geometric frustration has been observed to manifest itself in a spin glass ground state, as inferred from a frequency f-dependent cusp in the real part of measurements of ac magnetic susceptibility 0 near temperature T = 5 K (Benbow et al, 2009). On the other hand, if Fe is replaced with Mn as in isostructural La 21 Mn 8 Ge 6.2 Al 0.8 C 12 , ISSN 2056-9890 similar cusps occurring at T = 3 K and 6 K in 0 exhibit no such dependence, even over four orders of magnitude in f, suggesting that only local antiferromagnetic ordering within the Mn 4 C 6 cluster arises while the spin glass state remains absent down to T = 1.8 K (Zaikina et al, 2011). With the aim of unveiling a new avenue to explore frustrated states within this class of compounds, we present here the synthesis and crystal structure of a new Cr-based analog that is isostructural and likewise geometrically frustrated, La 21 Cr 8À2a Al b Ge 7Àb C 12 , [a = 0.22 (2), b = 0.758 (19)].…”

Structural relationships between new carbide La14Sn(MnC6)3 and fully ordered La11(MnC6)3

“…This leads to a higher yield of $20% of La 11 (MnC 6 ) 3 and larger crystal size (up to 0.5 cm, see Figure S1); the crystal of La 11 (MnC 6 ) 3 used for structure determination by X-ray diffraction was selected from these later reactions. Well-crystallized specimens of La 14 Sn(MnC 6 ) 3 (including the crystal for X-ray single-crystal determination) were found together with the cubic phase La 21 Mn 8 Sn 7 C 12 [16] as products of a reaction with the starting composition La:Mn: Sn:C¼21:8:7:12 in excess La/Ni flux. Modifying the reaction to produce only La 14 Sn(MnC 6 ) 3 was not possible due to the presence of all four elements in both La 14 Sn(MnC 6 ) 3 and La 21 Mn 8 Sn 7 C 12 ; changes in reaction ratio and heating profile did not prevent formation of the cubic phase.…”

“…Crystals of La 11 (MnC 6 ) 3 were initially found as a byproduct in reactions intended to produce La 21 Mn 8 M 7 C 12 [16] (M¼Sb or Bi) from La/Ni eutectic flux. Later syntheses of La 11 (MnC 6 ) 3 were carried out in La/Ni flux with optimized starting composition (2) 18f 1 0.15769(2) 0.24382(2) 0.41922(2) 0.00867(9) La (3) 18f 1 0.24245(2) 0.09037(2) 0.24906(2) 0.00706(9) La (4) 6c (3) 0.0820 (3) 0.0078(9) C(2) 18f 1 0.1896 (4) 0.2378 (4) 0.2489 (3) 0.0075(9) C (3) 18f 1 0.2517 (3) 0.0501 (3) 0.0841 (3) 0.0083(9) C(4) 18f 1 0.0930 (4) 0.1596 (4) 0.2488 (3) 0.0092(9) C (5) 18f 1 0.1879 (4) 0.0867 (4) 0.4149 (3) 0.012(1) C (6) 18f 1 0.1869 (4) 0.0849 (4) 0.0843 (3) 0.011 (1) La 14.00 (6) Sn(MnC 6 ) 3 La(1) 6l 1 0.01158 (5) 0.26378(4) 0.25500 (3) 0.00944 (7) La (2) 3j 1 0.33516 (6) 0.39510 (7) 0 0.00797(9) La (3) 3k 0.89 (3) 0.3974 (5) 0.4059 (4) 1 2 0.0120 (5) La (3a) (2) Mn (1) 2h 1 1 3 2 3 0.2623(2) 0.0078 (3) Mn (2) (2) a U eq is defined as one third of the trace of the orthogonalized U ij tensor.…”

“…This moiety might form in the molten flux during synthesis and act as a template for the final product, adopting slightly different shapes depending on the crystallizing phase. For the cubic phase La 21 T 8 E 7 C 12 [8,16] (T¼Fe, Mn; E¼Ge, Sn, Sb, Te, Bi) the Sn@La 9 polyhedron has a shape of a monocapped square antiprism, while in the structure of La 6 SnNi 3.67 Ru 0.76 Al 3.6 [9b] the tin atom is coordinated by nine La atoms forming a tricapped trigonal prism. Synthesis in La/Ni eutectic flux in the presence of heavy main elements other than Sn (e.g.…”