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.