Characterization of the ${\text{TiSiO}}_4$ structure and its pressure-induced phase transformations: Density functional theory study

Theoretical investigations concerning the possible titanium silicate polymorphs have been performed using density functional theory at B3LYP level. Total-energy calculations and geometry optimizations have been carried out for all phases involved. The following sequence of pressure-driven structural transitions has been found: ${\text{CrVO}}_4$-type, $Cmcm$ (in parenthesis the transition pressure), $\to$ zircon-type, $I{4}_1/amd$ (0.8 GPa), $\to$ scheelite-type, $I{4}_1/a$ (3.8 GPa). At higher pressure the last phase is found to be stable at least up to 25 GPa. The equation of state of the different polymorphs is also reported. We found that the highest bulk modulus corresponds to the zircon and scheelite phases with values of 248 and 238 GPa, respectively. The orthorhombic $Cmcm$ phase is the most compressible of all the studied structures with a bulk modulus of 124 GPa, being also the most stable phase at ambient pressure. Finally, calculations of the electronic structure, vibrational and dielectric properties of ${\text{TiSiO}}_4$ are also reported.

Figure 1

(Color online). Structures of ${\text{TiSiO}}_4$ a) ${\text{CrVO}}_4$-type, (b) zircon and (c) scheelite. ${\text{SiO}}_4$, ${\text{TiO}}_6$, and ${\text{TiO}}_8$ polyhedra are shown.

Figure 2

Internal energy (Hartree) versus volume $\left({\text{Å}}^3\right)$ per formula unit. The entalphy versus pressure curve for the ${\text{TiSiO}}_4$ polymorphs is depicted in the inset (taking the ${\text{CrVO}}_4$-type structure as reference). In the figure we only show the results for those structures which are relevant for $P<25\text{GPa}$.

Figure 3

Evolution with pressure of (a) bond distance and (b) unit-cell parameters. Solid, dashed and dotted lines represents ${\text{CrVO}}_4$-type, zircon, and scheelite phases, respectively.