Two pressure-induced structural phase transitions in TiOCl

We studied the crystal structure of TiOCl up to pressures of $p=25\text{GPa}$ at room temperature by x-ray powder-diffraction measurements. Two pressure-induced structural phase transitions are observed: at $p_{c1}\approx 15\text{GPa}$ emerges an $2a\times 2b\times c$ superstructure with $b$-axis unique monoclinic symmetry (space group $P{2}_1/m$). At $p_{c2}\approx 22\text{GPa}$, all lattice parameters of the monoclinic phase show a pronounced anomaly. A fraction of the sample persists in the ambient orthorhombic phase (space group $Pmmn$) over the whole pressure range.

Figure 1

(Color online) Room-temperature x-ray powder-diffraction diagrams of TiOCl at high pressures. The numbers on the right, vertical axis indicate the applied pressures in gigapascal. The diffraction diagrams at the critical pressures $p_{c1}=15.4\text{GPa}$ and $p_{c2}=22.6\text{GPa}$ are highlighted by bold, blue (black) lines.

Figure 2

(Color online) Room-temperature x-ray powder-diffraction diagrams (full, orange lines) of TiOCl at high pressures together with the Le Bail fits (dotted, blue lines). For the pressures 1.8, 15.4, and 22.6 GPa) the difference curve $\left(I_{obs}-I_{calc}\right)$ between the diffraction diagram and the Le Bail fit is shown. Markers show the calculated peak positions for the various phases. The arrow indicates the reflection from the ruby chip. The asterisks mark the reflections due to the monoclinic phase already present above $\sim 7\text{GPa}$.

Figure 3

(Color online) Results of the Le Bail fits of the room-temperature x-ray diffraction diagrams of TiOCl: lattice parameters and unit-cell volume as a function of pressure [$Pmmn$: black, open symbols; $P{2}_1/m$: red (gray), full symbols]. Lines are guides to the eyes. For the normalization of the lattice parameters and the unit-cell volume, the values of Ref. 13 were used.

Figure 4

(Color online) Angle $\beta$ of the unit cell of the $P{2}_1/m$ monoclinic crystal structure at room temperature as a function of pressure.

Figure 5

Lattice parameter ratio as a function of pressure: Ratio $b/a$ for the orthorhombic phase ($Pmmn$, squares) for the whole studied pressure range; ratio $b/a$ for the monoclinic phase ($P{2}_1/m$, circles) in the pressure range $p_{c1}<p<p_{c2}$; ratio $b/a$ for the monoclinic phase ($P{2}_1/m$, triangles) in the pressure range $p>p_{c2}$.