Ferrimagnetic Cage Framework in Ca₁₂Fe₁₀Si₄O₃₂Cl₆

The positively charged cage framework of the natural mineral mayenite, which enables various species with negative charge to be stabilized, is one of the key structures to provide the new functionalities exploited in applications. Here we report the structural and magnetic properties of recently found eltyubyuite, Ca₁₂Fe₁₀Si₄O₃₂Cl₆, which is the first compound bearing a transition metal oxide as a main constituent in the mayenite-type structure. From neutron powder diffraction measurements at T = 20 K and the low temperature Mössbauer measurement, we determined the magnetic structure of eltyubyuite to be a ferrimagnet with oppositely aligned magnetic moments of +3.17(3) and -3.05(8) μ_B in two tetrahedral Fe sites with different oxygen ligands, all bridging oxygens or mixed bridging and nonbridging oxygens. As far as is known, this result is likely to be a first example showing ferrimagnetism stemming from only tetrahedral Fe³⁺ ions. The reduced magnetic moment per Fe³⁺ and the resultant small net moment per unit cell of 22 μ_B at μ₀H = 5 T and T = 15 K are attributed to strong covalency in much shorter Fe-O bonds in the FeO₄ tetrahedra.