Swedenborgite-Type Cobaltites and Ferrites: Tetrahedral Frameworks with Exceptional Magnetic Properties

Structural, Magnetic, and Electronic Properties of CaBaCo4- xM xO7 (M = Fe, Zn)

The effect of substituting iron and zinc for cobalt in CaBaCo₄O₇ was investigated using neutron diffraction and X-ray absorption spectroscopy techniques. The orthorhombic distortion present in the parent compound CaBaCo₄O₇ decreases with increasing the content of either Fe or Zn. The samples CaBaCo₃ZnO₇ and CaBaCo_{4- x}Fe _xO₇ with x ≥ 1.5 are metrically hexagonal, but much better refinements in the neutron diffraction patterns are obtained using an orthorhombic unit cell. The two types of substitution have opposite effects on the structural and magnetic properties. Fe atoms preferentially occupy the sites at the triangular layer. Thus, the replacement of Co by Fe suppresses the ferrimagnetic ordering of the parent compound, and CaBaCo_{4- x}Fe _xO₇ (0.5 ≤ x ≤ 2) samples are antiferromagnetically ordered following a new propagation vector k = (1/3,0,0). However, the Zn atoms prefer occupying the Kagome layer, which is very detrimental for the long-range magnetic interactions giving rise to a magnetic glass-like behavior in the CaBaCo₃ZnO₇ sample. The oxidation states of iron and zinc are found to be 3+ and 2+, respectively, independently of the content, as confirmed by X-ray absorption spectroscopy. Therefore, the average Co oxidation state changes accordingly with the Fe³⁺ or Zn²⁺ doping. Also, X-ray absorption spectroscopy data confirm the different preferential occupation for both Fe and Zn cations. The combined information obtained by neutron diffraction and X-ray absorption spectroscopy indicates that cobalt atoms can be either in a fluctuating Co²⁺/Co³⁺ valence state or, alternatively, Co²⁺ and Co³⁺ ions being randomly distributed in the lattice. These results explain the occurrence of local disorder in the CoO₄ tetrahedra obtained by EXAFS. An anomaly in the lattice parameters and an increase in the local disorder are observed only at the ferrimagnetic transition for CaBaCo₄O₇, revealing the occurrence of local magneto-elastic coupling.

Structural invariance upon antiferromagnetic ordering in geometrically frustrated swedenborgite, CaBaCo2Fe2O7

Centimetre-sized single crystals of high-quality CaBaCo2Fe2O7were synthesized by the optical floating zone technique. The metal-to-metal stoichiometry and oxygen content were confirmed by spectroscopy and thermal reduction experiments. The hexagonal symmetryP63mc(No. 186) well describes the powder X-ray and neutron diffraction as well as single-crystal neutron diffraction at all measured temperatures. This symmetry is also consistent with optical second harmonic generation data obtained between 10 and 295 K. However, a satisfactory structure description from single-crystal neutron diffraction data needs an oxygen split position. Specific heat, magnetic susceptibility and powder neutron diffraction data indicate a magnetic phase transition atTN= 159 K to an antiferromagnetic ground state, but with a persisting hexagonal symmetry and intrinsic geometric frustration.