Rational design, crystal structure, and frustrated magnetism of the Ge-containing YbFe2O4-type layered oxides In2Zn3-xCoxGeO8 (0 ≤ x ≤ 3).

Abstract

YbFe2O4-type layered oxides have attracted tremendous interest because the unique crystal comprises two distinct geometrically frustrated triangular cation-sublattices. Herein, a series of YbFe2O4-type materials In2Zn3-xCoxGeO8 (0 ≤ x ≤ 3) were rationally designed and experimentally synthesized for the first time. The crystal structures of In2Zn3-xCoxGeO8 were investigated comprehensively by Rietveld refinements against high-resolution monochromatic Cu Kα1 XRD data. Zn2+, Co2+, and Ge4+ cations are distributed randomly on the [MO]2 bilayer and possess a trigonal bipyramid (TBP) coordination geometry. Because Co2+ has an unpaired electron in the dz2 orbital and a larger electronegativity than Zn2+, Co2+-to-Zn2+ equivalent substitution in In2Zn3-xCoxGeO8 results in more compact MO5-TBPs, which is the origin of anisotropic lattice expansion and contraction along the a and c axes, respectively. The Co2+ moments in the [MO]2 bilayer are strongly AFM coupled and geometrically frustrated, therefore resulting in a spin-glass magnetic transition at around Tg = 20 K for In2ZnCo2GeO8, while a long-range AFM ordering is established for In2Co3GeO8 with a Néel temperature of 53 K, attributed to the significantly enhanced AFM interactions and increased In3+/Co2+ anti-site disordering, as compared to those in In2ZnCo2GeO8.