Low-temperature structural transition in the quasi-one-dimensional spin-12compoundLi2Cu2O(SO4)2

A thorough structural exploration has been made on the quasi-one-dimensional $S=1/2$ compound $\mathrm{L}{\mathrm{i}}₂\mathrm{C}{\mathrm{u}}₂\mathrm{O}{(\mathrm{S}{\mathrm{O}}₄)}₂$ by neutron and synchrotron x-ray diffraction. It reveals the occurrence of a structural transition at 125 K, characterized by a lowering of symmetry from $P{4}₂/m$ to $P\overline{1}$, which is possibly driven by an exchange striction mechanism. This transition involves a dimerization of some Cu in the edge-sharing tetrahedral Cu chains. A symmetry mode analysis indicates that one representation, ${\mathrm{\ensuremath{\Gamma}}}₃{}⁺{\mathrm{\ensuremath{\Gamma}}}₄{}⁺$, dominates the structural transition. Interestingly, no intermediate structure with $P112/m$ symmetry is observed experimentally. Lastly, temperature dependent magnetic susceptibility measurements and neutron diffraction reveal that the magnetic ground state of this compound is a spin-singlet with a spin gap, characterized by the absence of long-range magnetic order down to 1.7 K.