Enhanced Dimerization of TiOCl under Pressure: Spin-Peierls to Peierls Transition

We report x-ray diffraction and magnetization measurements under pressure combined with ab initio calculations to show that high-pressure TiOCl corresponds to an enhanced ${\mathrm{Ti}}^{3+}\mathrm{\text{−}}{\mathrm{Ti}}^{3+}$ dimerized phase existing already at room temperature. Our results demonstrate the formation of a metal-metal bond between ${\mathrm{Ti}}^{3+}$ ions along the $b$ axis of TiOCl, accompanied by a strong reduction of the electronic gap. The evolution of the dimerization with pressure suggests a crossover from the spin-Peierls to a conventional Peierls situation at high pressures.

Figure 1

Evolution of the x-ray diffraction pattern under pressure. Note the appearance of the new peaks of the monoclinic phase, $P21/m$ above 10.5 GPa, with respect to the low pressure orthorhombic phase, $Pmmn$. The coordinate $z$ of the Wyckoff positions of Ti ($2b\mathrm{\text{:}}0,\frac{1}{2},z$), Cl ($2a\mathrm{\text{:}}0,0,z$), and O ($2a\mathrm{\text{:}}0,0,z$) in the $Pmmn$ phase at 0 GPa (7 GPa) are 0.113 (0.133), 0.320 (0.368), and 0.943 (0.929), respectively. In the $P21/m$ phase there exist two inequivalent positions for each atom ($2e\mathrm{\text{:}}\frac{1}{4},y,z$). The $y$, $z$ at 15 GPa (20 GPa) are 0.415 (0.419) and 0.372 (0.353) for Ti1 and 0.905 (0.864) and 0.373 (0.350) for Ti2, respectively. Inset: Rietveld refinement of the high-pressure monoclinic phase ($+$ are the experimental points and the solid line is the fitting). Asterisks mark the new peaks of the high-pressure monoclinic phase. The left inset shows the structure generated from the refinement, highlighting the short Ti-Ti bonds along the $b$ axis, which are displaced half a period between the chains.

Figure 2

Pressure dependence of the lattice parameters of TiOCl at room temperature as obtained from the Rietveld refinement in both the orthorhombic and monoclinic phase above 10 GPa. The open (closed), $O$, $\Delta$ symbols correspond to $a$, $b$ ($b/2$) and $c/2$, in the orthorhombic (monoclinic) phase, respectively.

Figure 3

Temperature dependence of the magnetic susceptibility at some representative pressures (from left to right: 0, 0.25, 0.43, 0.6, 0.7, and 0.97 GPa). The curves are slightly vertically displaced for clarity. The precise evolution of $T_{\mathrm{ISP}}$ and $T_{\mathrm{SP}}$ is shown in the inset.

Figure 4

Density of states for TiOCl at ambient pressure ($Pmmn$) and at $P=15\mathrm{GPa}$ ($P{2}_1/m$). The vertical line marks the Fermi energy. The insets show the evolution of the band gap (left) and the difference between the short and long distances in the dimerized monoclinic phase (right) with pressure. Note that at low temperatures and ambient pressure TiOCl exhibits monoclinic structure and thereby the dimerized spin-Peirels state.