Pressure-induced spin-Peierls to incommensurate charge-density-wave transition in the ground state of TiOCl

The ground state of the spin-Peierls system TiOCl was probed using synchrotron x-ray diffraction on a single-crystal sample at $T=6\text{K}$. We tracked the evolution of the structural superlattice peaks associated with the dimerized ground state as a function of pressure. The dimerization along the $b$ axis is rapidly suppressed in the vicinity of a first-order structural phase transition at $P_C=13.1\left(1\right)\text{GPa}$. The high-pressure phase is characterized by an incommensurate charge-density wave perpendicular to the original spin-chain direction. These results show that the electronic ground state undergoes a fundamental change in symmetry, indicating a significant change in the principal interactions.

Figure 1

(Color online) (a) Crystal structure of TiOCl viewed along the spin-chain direction. The Ti atoms are shown in gray. (b) Ti sublattice showing the orbital arrangement yielding chains along $b$ in the low-pressure phase. (c) Micrograph of the TiOCl sample inside the diamond-anvil cell at room temperature, before (with $P<0.5\text{GPa}$) and after (with $P\sim 3\text{GPa}$) the x-ray measurements. The silver pressure calibrant is visible at the left of the sample.

Figure 2

(Color online) (a) Pressure dependence of the unit-cell parameters of TiOCl at $T=6\text{K}$. (b) Low-temperature unit-cell volume with fit to a Murnaghan equation of state. (c) Measurements of the (200) fundamental Bragg reflection as the pressure is increased in fine steps. Left inset: the sample pressure measured by the Ag lattice constant is linear relative to the membrane load in this pressure range. Right inset: the (200) peak of the high-pressure phase, using pseudo-orthorhombic notation.

Figure 3

(Color online) Pressure dependence of the $\left(0\frac{3}{2}0\right)$ superlattice reflection of TiOCl at $T=6\text{K}$. (a) Representative scans at 11.7 GPa; the intensity is scaled to the closest fundamental Bragg peak (0 2 0). (b) Intensity and (c) wave vector $\mathit{q}$ of the structural superlattice as a function of pressure. The error bars for the intensity is smaller than symbol size while the error bars for $\mathit{q}$ represent the fitted widths of the scans along $K$.

Figure 4

(Color online) (a) Longitudinal scan along the $\left[H00\right]$ direction of the high-pressure phase of TiOCl at $T=6\text{K}$. A new pair of superlattice peaks are observed around the (200) Bragg peak with incommensurate wave vector $q=0.48$. (b) Scans through the incommensurate $(2-q,0,0)$ peak along the $H$, $K$, and $L$ directions, using pseudo-orthorhombic notation.