Structural Phase Transition in the S=1/2 Kagome System Cs₂ZrCu₃F₁₂ and a Comparison to the Valence-Bond-Solid Phase in Rb₂SnCu₃F₁₂

The crystal structure of the S = 1/2 kagome system Cs2ZrCu3F12 has been determined at both 295 and 125 K via single-crystal X-ray diffraction. A first-order structural phase transition is seen near 225 K, confirmed by variable-temperature synchrotron powder diffraction studies, in which the structure transforms from rhombohedral (R (3) over barm) to monoclinic (P2(1)/m). A corresponding abrupt change in dielectric constant is observed near the same temperature. The phase transition is driven by a dramatic change in coordination around the Zr site, which changes from regular octahedral in the rhombohedral phase to a seven-coordinate environment in the monoclinic phase. This leads to a severe buckling of the copper fluoride kagome layers and significant changes in the geometry around the Cu sites, which correlates with both the observed dielectric anomaly and a previously observed anomaly in magnetic susceptibility. It is suggested that this structural phase transition ultimately permits long-range antiferromagnetic ordering in Cs2ZrCu3F12, contrasting markedly with the behavior of the analogue Rb2SnCu3F12, which exhibits a "pinwheel" valence-bond-solid ground state.