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We report the observation of a chemical-substitution driven phase transition from a gapped quantum paramagnetic phase to one with long range order in Cs_1-xRb_xFeCl₃. The x = 0 compound in this series of triangular-lattice antiferromagnets has a spin-singlet ground state due to strong easy-plane magnetic anisotropy. In contrast, the x = 1 material orders magnetically in a 120° structure [1]. Calorimetric and magnetic experiments performed on a series of samples with 0 ≤ x ≤ 1 reveal that in the low-temperature limit magnetic order appears at x ~ 0.35. Inelastic neutron scattering experiments show that this coincides with the closure of the gap in the spin excitation spectrum. It appears that disorder effects in this material are more pronounced than those in the only other known phase transition of this type, namely in DTNX [2].

[1] S. Hayashida L. Stoppel et al., Phys. Rev. B 99, 224420 (2019).
[2] K. Yu. Povarov et al., Phys. Rev. B 92, 024429 (2015).