Transformation between spin-Peierls and incommensurate fluctuating phases of Sc-doped TiOCl

Abstract

Single crystals of ${\mathrm{Sc}}_{x}$${\mathrm{Ti}}_{1\ensuremath{-}x}\text{OCl}(x=0.005)$ have been grown by the vapor phase transport technique. Specific heat measurements prove the absence of phase transitions for 4--200 K. Instead, an excess entropy is observed over a range of temperatures that encompasses the incommensurate phase transition at 90 K and the spin-Peierls transition at 67 K of pure TiOCl. Temperature-dependent x-ray diffraction on ${\mathrm{Sc}}_{x}$${\mathrm{Ti}}_{1\ensuremath{-}x}$OCl gives broadened diffraction maxima at incommensurate positions between ${T}_{c1}=61.5\phantom{\rule{0.16em}{0ex}}(3)$ and $\ensuremath{\sim}90$ K, and at commensurate positions below 61.5 K. These results are interpreted as due to the presence of an incommensurate phase without long-range order at intermediate temperatures, and of a highly disturbed commensurate phase without long-range order at low temperatures. The commensurate phase is attributed to a fluctuating spin-Peierls state on an orthorhombic lattice. The monoclinic symmetry and local structure of the fluctuations are equal to the symmetry and structure of the ordered spin-Peierls state of TiOCl. A novel feature of ${\mathrm{Sc}}_{x}$${\mathrm{Ti}}_{1\ensuremath{-}x}\text{OCl}(x=0.005)$ is a transformation from one fluctuating phase (the incommensurate phase at intermediate temperatures) to another fluctuating phase (the spin-Peierls-like phase). This transformation is not a phase transition occurring at a critical temperature, but it proceeds gradually over a temperature range of $\ensuremath{\sim}10$ K wide. The destruction of long-range order requires much lower levels of doping in TiOCl than in other low-dimensional electronic crystals, like the canonical spin-Peierls compound ${\mathrm{CuGeO}}_{3}$. An explanation for the higher sensitivity to doping has not been found, but it is noticed that it may be the result of an increased two-dimensional character of the doped magnetic system. The observed fluctuating states with long correlation lengths are reminiscent of Kosterlitz--Thouless-type phases in two-dimensional systems.