Unique octahedral rotation pattern in the oxygen-deficient Ruddlesden-Popper compound Gd3Ba2Fe4O12.

A novel Ruddlesden-Popper-related compound, Gd3Ba2Fe4O12, was discovered and its crystal structure was determined via single-crystal X-ray diffraction. The structure has an ordered structure of octahedra and pyramids along the c axis. Gd3Ba2Fe4O12 belongs to the tetragonal system P42/ncm, with a = 5.59040 (10) Šand c = 35.1899 (10) Å. The A-site ions in the Ruddlesden-Popper structure, i.e. Gd3+ and Ba2+, exhibit an ordering along the c axis. The perfect oxygen deficiency is accommodated at the GdO layers in the proper Ruddlesden-Popper structure. Using the bond-valence-sum method, the Fe ions in the FeO6 octahedra and FeO5 pyramids represent valence states of +3 and +2.5, respectively, demonstrating a two-dimensional charge disproportionation. The corner-sharing FeO6 octahedra and FeO5 pyramids are tilted in opposite directions, with the neighbours around one axis of the simple perovskite configuration, which, using Glazer's notation, can be represented as a-b0c0/b0a-c0. In the perovskite blocks, the facing FeO5 pyramids across the Gd layer rotate in the same sense, which is a unique rotation feature related to oxygen deficiency.

Spin glass transition of single-crystalline TmFe2O4- δ.

TmFe2O4 is one kind of multiferroic material in which equivalent amounts of Fe2+ and Fe3+ occupy a two-dimensional triangular lattice, leading to charge and spin frustrations. The spin frustration is expected to be increased as the fraction of Fe2+ (Fe3+) becomes larger than that of Fe3+ (Fe2+). We have grown single-crystalline TmFe2O4-δ with oxygen vacancies by using floating zone melting method and examined its magnetic properties. On cooling the compound, a long-range magnetic ordering develops around ∼240 K. With further cooling, a maximum of zero-field-cooled (ZFC) magnetization is observed at 186.2 K. The ac magnetic susceptibility obtained by ZFC process also manifests a maximum in its temperature dependence, and the variation of spin-freezing temperature with frequency of ac magnetic field is explainable in terms of the dynamic scaling law with the critical component of 8.68(8). This value suggests that the spin glass transition occurs at 186.2 K. The effect of external dc magnetic field on the irreversible transition temperature is coincident with the de Almeida-Thouless (AT) line. Aging-memory and rejuvenation effect is also observed below the spin-freezing temperature. These facts support the idea that TmFe2O4-δ undergoes spin glass transition below the ferrimagnetic transition temperature. In other words, TmFe2O4-δ can be regarded as a reentrance spin glass. It is thought that the oxygen vacancies bring about unequal number of Fe2+ and Fe3+ ions and thereby strengthen the magnetic frustration among the iron ions coupled with antiferromagnetic interactions, leading to the spin glass behavior.