Layered polymorph of titanium triiodide

A previously unreported layered triangular lattice polymorph of $\mathrm{Ti}{\mathrm{I}}_3$ is described, synthesized under 6 GPa of applied pressure at $900{}^{\circ }\mathrm{C}$, but stable at atmospheric pressure. This air-sensitive material has a $\mathrm{Cd}{\mathrm{I}}_2\text{-type}$ layered structure [ $P-3m1$ (no. 164), $a=4.012\AA$ and $c=6.641\AA$ at 120 K, $Z=1$ of ${\mathrm{Ti}}_{0.667}{\mathrm{I}}_2$ ] with an in-plane triangular lattice, related to that of $\mathrm{Ti}{\mathrm{I}}_4$ (${\mathrm{Ti}}_{0.5}{\mathrm{I}}_2$). Although the $\mathrm{Ti}{\mathrm{I}}_3$ formula is consistent with expectations for a layered honeycomb lattice of spin $1/2$ Ti(III), there is disorder in the crystal structure, and the complex magnetic behavior observed is inconsistent with a simple local moment picture of spin $1/2$ per Ti(III). Magnetic susceptibility and heat capacity measurements suggest that the material undergoes several low-temperature phase transitions.

Figure 1

The crystal structures of $\mathrm{Ti}{\mathrm{I}}_3$. (a) The average $P\text{−}3m1$ unit cell of $\mathrm{HP}\text{−}\mathrm{Ti}{\mathrm{I}}_3$, refined from SCXRD measurements; (b) the $Pmnm$ unit cell of the 1D-chain structure of $\mathrm{AP}\text{−}\mathrm{Ti}{\mathrm{I}}_3$ [22]. Titanium is represented by the light blue spheres and iodine by the purple spheres, while vacancies are shown by white.

Figure 2

The diffraction characterization of $\mathrm{HP}\text{−}\mathrm{Ti}{\mathrm{I}}_3$ and $\mathrm{HP}\text{−}\mathrm{Ti}{\mathrm{I}}_4$. (a) The $hk0$ (left), $hk1$ (middle), and $0kl$ reciprocal-lattice planes of single-crystal $\mathrm{HP}\text{−}\mathrm{Ti}{\mathrm{I}}_3$ at 120 K. (b) The laboratory PXRD patterns of $\mathrm{HP}\text{−}\mathrm{Ti}{\mathrm{I}}_3$ (black pattern with left $y\text{-axis}$) and $\mathrm{HP}\text{−}\mathrm{Ti}{\mathrm{I}}_4$ (green pattern with right $y\text{-axis}$), with the calculated Bragg reflection tics of $\mathrm{HP}\text{−}\mathrm{Ti}{\mathrm{I}}_3$, $\mathrm{HP}\text{−}\mathrm{Ti}{\mathrm{I}}_4$, and $\mathrm{AP}\text{−}\mathrm{Ti}{\mathrm{I}}_3$ as a small amount of impurity.

Figure 3

Three different arrangements of the potential in-plane lattices. (a) A fully occupied triangular lattice ($M{X}_2$) of the $\mathrm{Cd}{\mathrm{I}}_2$ type; (b) a 2/3 occupied honeycomb lattice ($M{X}_3$) as is observed for $\mathrm{Bi}{\mathrm{I}}_3$, and (c) a 1/2 occupied in-plane zigzag chain lattice as is found for $\mathrm{Ti}{\mathrm{I}}_4$ [3, 14, 24].

Figure 4

The magnetic and thermodynamic characterization of $\mathrm{Ti}{\mathrm{I}}_3$. (a) The temperature-dependent magnetic susceptibility measured with zero-field cooling from 1.8 to 300 K under 1 kOe of $\mathrm{HP}\text{−}\mathrm{Ti}{\mathrm{I}}_3$ (black), $\mathrm{AP}\text{−}\mathrm{Ti}{\mathrm{I}}_3$ (green), and $\mathrm{HP}\text{−}\mathrm{Ti}{\mathrm{I}}_4$ (pink). A 1.8–50 K range is presented in the inset to exhibit the magnetic features. (b) Magnetic moments measured vs magnetic field from $-9$ to 9 T under different temperatures, on polycrystalline $\mathrm{HP}\text{−}\mathrm{Ti}{\mathrm{I}}_3$ powders, with ${\mu }_{\mathrm{B}}$ per Ti unit shown on the right axis. Zoomed-in view of $-0.5$ to 0.5 T range is shown in the inset. (c) Heat capacity of $\mathrm{HP}\text{−}\mathrm{Ti}{\mathrm{I}}_3$ measured under zero field from 2 to 120 K. The red curve represents the Debye model fitting of phonon contribution, with Eq. (1). The subtracted $C_{\mathrm{mag}}$ curve is plotted in the inset vs temperature from 2 to 45 K. Corresponding entropy change is exhibited by a pink curve, with a comparison to the value of Heisenberg limit $R\ln \left(2S+1\right)=R\ln \left(2\right)$ shown as the green dashed line.