  One-dimensional spin chain systems have received special attention to
discover the novel magnetic ground states and emergent phenomena, while the
magnetic studies on rare-earth (RE)-based 1D spin chain materials are still
rare. Here, we report the synthesis, structure and magnetic behaviors on a
family of tetragonal tungsten-bronze structure K2RENb5O15 (RE = Ce, Pr, Nd, Sm,
Gd-Ho) compounds, which consist of 1D linear spin-chain structure built by RE3+
ions along the c-axis and well spatially separated by the nonmagnetic K/Nb-O
polyhedrons with large interchain distances of ~ 8.80-8.88 {\AA} in the
ab-plane. The low temperature magnetic measurements reveal the absence of
long-range magnetic order down to 1.8 K for all serial K2RENb5O15 compounds and
the dominant ferromagnetic interactions for RE=Ce,Dy and antiferromagnetic
interactions for other members. Among them, K2GdNb5O15 with spin only magnetic
moment S=7/2, exhibits a long-range magnetic order with TN~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 of 0-2 T is
realized at temperatures below 1 K for K2GdNb5O15, letting it as an ideal
candidate for adiabatic magnetic refrigeration applications at sub-kelvin
temperatures. The K2RENb5O15 become a rare family of insulting RE-based magnets
to explore the novel 1D spin chain physics beyond the 3d TM-based counterparts,
in terms of its combination of low dimension, strong spin-orbital coupling and
the rich diversity of RE ions.
