Rational design, crystal structure, and frustrated magnetism of the Ge-containing YbFe 2 O 4 -type layered oxides In 2 Zn 3- x Co x GeO 8 (0 ≤ x ≤ 3).

YbFe 2 O 4 -type layered oxides have attracted tremendous interest because the unique crystal comprises two distinct geometrically frustrated triangular cation-sublattices. Herein, a series of YbFe 2 O 4 -type materials In 2 Zn 3- x Co x GeO 8 (0 ≤ x ≤ 3) were rationally designed and experimentally synthesized for the first time. The crystal structures of In 2 Zn 3- x Co x GeO 8 were investigated comprehensively by Rietveld refinements against high-resolution monochromatic Cu K α1 XRD data. Zn 2+ , Co 2+ , and Ge 4+ cations are distributed randomly on the [MO] 2 bilayer and possess a trigonal bipyramid (TBP) coordination geometry. Because Co 2+ has an unpaired electron in the d z 2 orbital and a larger electronegativity than Zn 2+ , Co 2+ -to-Zn 2+ equivalent substitution in In 2 Zn 3- x Co x GeO 8 results in more compact MO 5 -TBPs, which is the origin of anisotropic lattice expansion and contraction along the a and c axes, respectively. The Co 2+ moments in the [MO] 2 bilayer are strongly AFM coupled and geometrically frustrated, therefore resulting in a spin-glass magnetic transition at around T g = 20 K for In 2 ZnCo 2 GeO 8 , while a long-range AFM ordering is established for In 2 Co 3 GeO 8 with a Néel temperature of 53 K, attributed to the significantly enhanced AFM interactions and increased In 3+ /Co 2+ anti-site disordering, as compared to those in In 2 ZnCo 2 GeO 8 .