Lanthanide-Germanate Adelite, LnCo(GeO4)(OH) (Ln = La-Sm), with Edge-Sharing Octahedral Chains of Co2+ Ions: Spin Frustration Expected to Form Cycloids

A new series of P212121 adelite-type LnCo(GeO4)(OH) (Ln = La-Sm) single crystals were grown by a high-temperature, high-pressure hydrothermal method (650 °C and 100 MPa). Single-crystal diffraction refinements yielded chiral one-dimensional (1D) chains of Co2+ along the a axis with an average 2.98 Å separation between Co2+ centers in the [CoO2(OH)2]∞ ribbon chains. A three-dimensional (3D) superstructure is formed by a bridging lanthanide-germanium framework formed by two unique alternating 5.83 and 6.00 Å distances between interchain Co2+ centers along the a axis. Magnetic studies of the S = 3/2 Co2+ chains in LaCo(GeO4)(OH) revealed a highly anisotropic structure with a common Néel temperature of 32 K. Additionally, a spin-flip transition occurs at 2 K when a 7.3 T field is applied along the chain. Zero-field cooled susceptibility at this critical field resulted in a complex intermediate state consisting of three unique antiferromagnetic transitions at 3, 8, and 16 K. The spin exchanges of LaCo(GeO4)(OH) evaluated by density functional theory calculations show the presence of spin frustration in the 1D chains, which can lead to a cycloidal magnetic structure within the plane of [CoO2(OH)2]∞ chains. The observed magnetic properties are explained by considering the competition between the 1D intrachain and 3D–1D interchain antiferromagnetic interactions.