Structural phase transitions in the kagome lattice based materials Cs_2-xRb_xSnCu₃F₁₂ (x = 0, 0.5, 1.0, 1.5)

The solid solution Cs_2-xRb_xSnCu₃F₁₂ (x = 0, 0.5, 1.0, 1.5) has been investigated crystallographically between 100 and 300 K using synchrotron X-ray powder diffraction and, in the case of x = 0, neutron powder diffraction. For Cs₂SnCu₃F₁₂ (x = 0), there is a structural transition from the previously reported room temperature rhombohedral symmetry (R[3 with combining macron]m) to monoclinic (P2₁/n) symmetry at 170 K. This transformation is repeated for the x = 0.5 composition, but with an increased transition temperature of 250 K. For x = 1.0 the monoclinic phase is found at 300 K, suggesting that the transition temperature is increased even further. For x = 1.5 a different behaviour, more akin to that previously reported forCs₂SnCu₃F₁₂, is found: a single phase transition between rhombohedral symmetry (R[3 with combining macron]) and triclinic symmetry (P[1 with combining macron]) is found at 280 K. In agreement with previous single crystal studies, Cs₂SnCu₃F₁₂ powder exhibits strong antiferromagnetic interactions (Θ ~ −268 K) and long-range magnetic order at T_N ~ 19.3 K. The finite magnetic moment observed for T < T_N might be explained by a Dzyaloshinskii–Moriya interaction, due to the lowering of symmetry from rhombohedral to monoclinic, which was not suggested in the earlier single crystal study.