Influence of the La/M Network on Magnetic Properties of Mn4 Tetrahedra in Intermetallic Compounds La21−δMn8M7C12 (M = Ge, Sn, Sb, Te, Bi)

Crystals of La21−δMn8M7C12 (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, Mn4C6, embedded into a nonmagnetic La/M network. The structures were determined from single-crystal X-ray diffraction (La21Mn8Ge6.2Al0.8C12, Fm3̅m; a = 16.2259(3) Å; Z = 4; R1 = 0.022). While most of the La21Mn8M7C12 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. La21−δMn8M7C12 with M = Sn, Sb, Te, Bi are paramagnets, but the susceptibility of La21Mn8Ge6.2Al0.8C12 exhibits ZFC-FC divergence at T1 ≈ 6 K and an additional feature at T2 ≈ 3 K. Such behavior is indicative of frustration of antiferromagnetically coupled manganese magnetic moments within the Mn4 tetrahedra. AC susceptibility measurements confirm the presence of two cusps at T1 and T2; however, no frequency dependence of cusp maxima was found. This differentiates La21Mn8Ge6.2Al0.8C12 from the Fe analogue, La21Fe8Sn7C12, where frequency dependence indicates spin-glass behavior. Electronic calculations on La21−δMn8M7C12 confirm that the vacancy on one of the La sites in the La20Mn8Te7C12 analogue stabilizes the phase by shifting the Fermi level to a pseudogap in the density of states. 57Fe and 119Sn Mössbauer spectroscopy of spin-glass compound La21Fe8Sn7C12 and paramagnetic La21Mn8Sn7C12 supports the presence of spin glass behavior in the Fe analogue and not in the Mn analogue.