Dynamical stability of the hardest known oxide and the cubic solar material: TiO2

Abstract

The authors have studied dynamical stability of different polymorphs of TiO2 using ab initio phonon calculations based on density functional theory in conjunction with force-constant method. Rutile TiO2 was found stable at ambient pressure, but unstable at high pressure. The calculated Raman frequency and phonon density of states (PDOS) of rutile TiO2 are in a good agreement with experiment. Concerning two cubic phases (solar materials), fluorite stabilized under pressure, whereas pyrite showed instability throughout the whole pressure range. Furthermore, the PDOS of cotunnite (the hardest known oxide) phase confirmed that it exists at high pressure and can be quenched down to a low pressure limit.

Phase composition, crystal structure and microstructure of silver and tungsten doped TiO2 nanopowders with tuneable photochromic behaviour

“…However, the unit cell parameters of the Ag-modified sample are also virtually the same as those of undoped anatase and rutile, suggesting that Ag has not been incorporated into the lattice. Referring to the effective ionic radii of [6] Ti 4+ and [6] Ag + , which are 0.61 and 1.15 Å , respectively [74], the Ag incorporation into the titania lattice is not expected [75] as, during calcination, the Ag atoms have a tendency to aggregate into metallic Ag clusters [69]. This was further confirmed by the TEM observation, where metallic Ag clusters are seen around titania NP (Fig.…”

“…Considering the unit cell parameters of the anatase and rutile present in the tungsten-doped and W/Ag-modified samples fired at 450°C, these are only slightly different from those of the undoped sample. The incorporation of tungsten into the titania lattice is actually predictable, the effective ionic radii of [6] Ti 4+ and [6] W 4+/6+ being 0.61 Å and 0.66/0.60 Å , respectively [74]. However, the unit cell parameters of the Ag-modified sample are also virtually the same as those of undoped anatase and rutile, suggesting that Ag has not been incorporated into the lattice.…”

“…It crystallizes in a large number of polymorphs: the high-pressure columbite-like [2], baddeleyite-like [3], cotunnite-like [4] and fluorite-like structures [5,6]. Besides these, the wellknown titania polymorphs are rutile, anatase and brookite, in order of abundance [7], and a fourth naturally occurring phase known as TiO 2 (B) [8].…”

Compositional Tuning of Structural Stability of Lithiated Cubic Titania via a Vacancy-Filling Mechanism under High Pressure

“…TiO 2 is one of a few metal oxide anodes, which intercalate Li ions at a reasonably low potential for safe operation. Whereas rutile and anatase phases of TiO 2 are abundant in nature and extensively used in many industrial applications, cubic TiO 2 is still a desirable material with superior electronic and mechanical properties [13][14][15]. However, its synthesis is extremely difficult, requiring high temperature (between 1900 and 2100 K) and high pressure (48 GPa) conditions [13].…”

Crystal structure of the pressure-induced metallic phase of SiH 4 from ab initio theory

“…Because hydrogen contributes considerable zero-point energy to the system, it is essential to include zero-point corrections in any enthalpy comparison of different phases (8). Even if we assume that the I4 1 /a structure is indeed the ground state Ͻ100 GPa, there remains the possibility for a dynamically stabilized phase to appear as a metastable one, which is a common phenomenon observed in materials under high pressure (9)(10)(11)(12).…”