K₂RENb₅O₁₅ (RE = Ce, Pr, Nd, Sm, Gd–Ho): A Family of Quasi-One-Dimensional Spin-Chain Compounds with Large Interchain Distance

One-dimensional (1D) spin chain systems have received special attention to discover novel magnetic ground states and emergent phenomena, while magnetic studies on rare-earth (RE)-based 1D spin chain materials are still rare. Here, we report the synthesis, structure, and magnetic behaviors of a family of tetragonal tungsten-bronze (TTB) structure K₂RENb₅O₁₅ (RE = Ce, Pr, Nd, Sm, Gd–Ho) compounds, which consist of a 1D linear spin-chain structure built by RE³⁺ ions along the c-axis and well spatially separated by nonmagnetic K/Nb–O polyhedrons with large interchain distances of ∼8.80–8.88 Å in the ab-plane. In this family of K₂RENb₅O₁₅ compounds, the nearest-neighboring interchain exchange interactions through the RE–O–K/Nb–O–RE routes are much smaller than the intrachain ones via the RE–O–RE pathways, and the low-temperature magnetic results reveal the absence of long-range magnetic order down to 1.8 K for all compounds. Among them, K₂GdNb₅O₁₅, with spin-only magnetic moment S = 7/2, exhibits a long-range magnetic order with T_N ∼ 0.31 K and strong spin fluctuations at low temperatures due to its low-dimension characteristics. Moreover, a large magnetocaloric effect under low field change (ΔB) of ΔB = 0–2 T is realized at temperatures below 1 K for K₂GdNb₅O₁₅, making it an ideal candidate for adiabatic magnetic refrigeration applications at sub-Kelvin temperatures. The K₂RENb₅O₁₅ become a rare family of insulating RE-based magnets for exploring the novel 1D spin chain physics beyond the 3d TM-based counterparts in terms of their combination of low dimension, strong spin–orbital coupling, and rich diversity of RE ions.