The Crystal Structure and Magnetic Properties of a New Ferrimagnetic Semiconductor:  Ca₂₁Mn₄Sb₁₈

Single crystals of the new transition metal Zintl phase, Ca₂₁Mn₄Sb₁₈, were prepared by high temperature melt synthesis. The crystal structure was determined by single crystal X-ray diffraction to be monoclinic in the space group C2/c. Crystal information was obtained at 90 K, and unit cell parameters were determined (a = 17.100(2) Å, b = 17.073(2) Å, c = 16.857(2) Å, β = 92.999(2)°, Z = 2, R1 = 0.0540, wR2 = 0.1437). The structure can be described as containing 4 discreet units per formula unit:  1 linear [Mn₄Sb₁₀]^22- anion, 2 dumbbell-shaped [Sb₂]^4- anions, 4 individual Sb^3- anions, and 21 Ca²⁺ cations. The [Mn₄Sb₁₀]^22- anion contains four edge-shared MnSb₄ tetrahedra with distances between Mn ions of 3.388(4) Å, 2.782(4) Å, and 2.760(4) Å. Electron counting suggests that the Mn are 2+. Temperature dependent magnetization shows a ferromagnetic-like transition temperature at ∼52 K which is suppressed with increasing magnetic field. The paramagnetic regime is best fit to a ferrimagnetic model, providing a total effective moment of 4.04(2) μ_B, significantly less than that expected for 4 Mn²⁺ ions (11.8 μ_B). Temperature dependent resistivity shows that this compound is a semiconductor with an activation energy of 0.159(2) eV (100−300 K).