We have investigated magnetic properties of ${\mathrm{Rb}}_2{\mathrm{Cu}}_2{\mathrm{Mo}}_3{\mathrm{O}}_{12}$ powder. Temperature dependence of magnetic susceptibility and magnetic-field dependence of magnetization have shown that this cuprate can be considered as a compound of a one-dimensional spin-$1∕2$ Heisenberg system with ferromagnetic first-nearest-neighbor (1NN) and antiferromagnetic second-nearest-neighbor (2NN) competing interactions (competing system). Values of the 1NN and 2NN exchange interactions are estimated as $J_1=-138\mathrm{K}$ and $J_2=51\mathrm{K}$ $\left(\alpha \equiv J_2∕J_1=-0.37\right)$. This value of $\alpha$ suggests that the ground state is a spin-singlet incommensurate state. In spite of relatively large $J_1$ and $J_2$, no magnetic phase transition appears down to $2\mathrm{K}$, while an antiferromagnetic transition occurs in other model compounds of the competing system with ferromagnetic 1NN interaction. For that reason, ${\mathrm{Rb}}_2{\mathrm{Cu}}_2{\mathrm{Mo}}_3{\mathrm{O}}_{12}$ is a suitable compound to study properties of the incommensurate ground state that are unconfirmed experimentally.

• Received 19 April 2004

DOI:https://doi.org/10.1103/PhysRevB.70.104426