Unusual compression behavior ofTiO2polymorphs from first principles

Abstract

The physical mechanisms behind the reduction in the bulk modulus of a high-pressure cubic TiO 2 phase are revealed by first-principles calculations. An unusual and abrupt change occurs in the dependence of energy on pressure at 43 GPa, indicating a pressure-induced phase transition from columbite TiO 2 to a modified fluorite TiO 2 with a Pca21 symmetry. Oxygen atom displacement in Pca21 TiO 2 unexpectedly reduces the bulk modulus by 34% relative to fluorite TiO 2 . This discovering provides a direct evidence for understanding the compressive properties of such groups of homologous materials.Titanium dioxide ͑TiO 2 ͒ has rich phase diagrams, namely, the rutile ͑P42/ mnm͒, anatase ͑I41/ amd͒, brookite ͑Pbca͒, columbite ͑Pbcn͒, baddeleyite ͑P21/ c͒, and cotunnite ͑Pnma͒ phases. 1-6 Due to its versatile physical and chemical properties, TiO 2 is extensively used in many industrial applications, such as high efficiency solar cells, photocatalysis, dynamic random access memory modules, and superhard materials. 7-12 The rutile and anatase phases of TiO 2 are abundant in nature. 13,14 Since the phase sequence of TiO 2 is very similar to that of other bulk materials, such as ZrO 2 and HfO 2 , it is highly expected to transform into its cubic polymorphs under pressure. 15 Modified cubic fluorite-structured RuO 2 , SnO 2 , and PbO 2 that possess a Pa3 symmetry, have been successfully synthesized. 16 In particular, RuO 2 is considered to be a potential ultrahard material because of its measured Knoop hardness ͑ϳ20 GPa͒ and bulk modulus ͑399 GPa͒, which is only 10% less than that of sintered diamonds. 17 Moreover, synthesized cotunnite TiO 2 has an extremely high bulk modulus of 431 GPa and is considered as the hardest oxide to date. 1 After the synthesis of cotunnite TiO 2 , scientists expected to synthesize cubic TiO 2 because it showed potential for use as a solar cell or ultrahard material. Ultimately, the highly anticipated cubic TiO 2 was successfully synthesized by heating anatase TiO 2 between 1900 and 2100 K in diamond-anvil cells under a pressure of 48 GPa. 18 Some ambiguities, however, remained both in experiment and in theory. For instance, the theoretical bulk modulus calculated for cubic TiO 2 in the pyrite and fluorite phases was significantly larger than that obtained during the experiments. Kim et al. 13 showed that pyrite TiO 2 is unstable because of the presence of imaginary frequencies in the phonon spectra throughout the entire pressure range, whereas fluorite TiO 2 is stable because of the absence of these imaginary frequencies under pressure. Swamy and Muddle 19 reported that pyrite TiO 2 has theoretical properties closer to the experimental values because it has a relatively lower bulk modulus. In terms of mechanical properties, however, Liang et al. 20 found a minor difference between the fluorite and pyrite phases. At the present stage, there is no theory of the cubic phase of TiO 2 , and that although there is some disagreement between existing calculations on candidate phases fluorite and pyrite, ...

Modeling hardness of polycrystalline materials and bulk metallic glasses

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Superconducting high-pressure phase of platinum hydride from first principles

“…[16][17][18] Moreover, to make sure the reliability of our predicted structure, the additional ab initio evolutionary algorithm is carried out for structural identification. 19 We have performed the detailed evolutionary simulations for four fixed stoichiometries (PtH, PtH 2 , Pt 2 H, and Pt 2 H 3 ) at 120 GPa (6 and 12 atoms per unit cell for PtH, PtH 2 and Pt 2 H, and 10 atoms per unit cell for Pt 2 H 3 , respectively).…”

Prediction of 10-fold coordinated TiO 2 and SiO 2 structures at multimegabar pressures

“…Highpressure studies have shown that the rutile and anatase forms transform to a columbite (α-PbO 2 ) phase (17), then to a baddeleyite structure at around 20 GPa (18,19), and then to an orthorhombic (OI) phase (1), followed by a cotunnite structure (1, 19−21). Recently, a new phase of TiO 2 of Pca2 1 symmetry was predicted in density functional theory (DFT) calculations, which is very close to thermodynamic stability at around 50 GPa (22,23), although it has not so far been observed in experiments. The highest-pressure phase of TiO 2 identified in experiments so far is the Fe 2 P-type ðP62mÞ structure (2).…”