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

Abstract

We predict by first-principles methods a phase transition in TiO 2 at 6.5 Mbar from the Fe 2 P-type polymorph to a ten-coordinated structure with space group I4/mmm. This is the first report, to our knowledge, of the pressure-induced phase transition to the I4/mmm structure among all dioxide compounds. The I4/mmm structure was found to be up to 3.3% denser across all pressures investigated. Significant differences were found in the electronic properties of the two structures, and the metallization of TiO 2 was calculated to occur concomitantly with the phase transition to I4/mmm. The implications of our findings were extended to SiO 2 , and an analogous Fe 2 P-type to I4/mmm transition was found to occur at 10 TPa. This is consistent with the lower-pressure phase transitions of TiO 2 , which are well-established models for the phase transitions in other AX 2 compounds, including SiO 2 . As in TiO 2 , the transition to I4/mmm corresponds to the metallization of SiO 2 . This transformation is in the pressure range reached in the interiors of recently discovered extrasolar planets and calls for a reformulation of the equations of state used to model them.ab initio density functional simulation | multimegabar crystalline phase | titanium dioxide | silicon dioxide | super-Earth mantle T he high-pressure behavior of TiO 2 has attracted significant interest in material and Earth sciences. Its pressure-induced phase transitions are not only quenchable to ambient pressure but also serve as lower-pressure analogs to the structures adopted in many other important AX 2 systems (1, 2). In particular, the densities of high-pressure silicas (SiO 2 ) significantly affect viscosity and convection within the Earth's mantle and extrasolar planets. The central pressures of such planets can reach 30 TPa, which is extreme enough to compress electronic shells and involve core electrons in bonding. The phase stability and properties of systems at extreme pressures is mostly the realm of theory (3-11), although recent advances in diamond anvil cell techniques have seen the achievement of pressures as high as 640 GPa (12), and high-energy lasers now permit X-ray diffraction measurements at pressures approaching 1 TPa (13-15). A recent ramped compression experiment on diamond reached about 5 TPa, although diffraction measurements were not attempted (16).TiO 2 also has a rich phase diagram at elevated 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). This phase was predicte...

Ultrahigh‐Pressure Phase Transitions in FeS2 and FeO2: Implications for Super‐Earths' Deep Interior

“…(Liu et al, 2014;Merkel et al, 2002;Thompson et al, 2016). Such a phenomenon was also observed in previous theoretical simulations about SiO 2 and GeO 2 Lyle et al, 2015;. The sum of volume reductions from Pa3 to I4/mmm is comparable to that of NiS 2 (~9%) (Yu & Ross, 2010).…”

“…Since the Kepler space telescope was launched to the space, thousands of exoplanets have been detected and confirmed (Borucki, 2016;Borucki et al, 2010). In addition, according to observations and theoretical models, many super-Earths have terrestrial-type compositions, which has led to extensive investigations into wellknown materials at ultrahigh pressures such as MgSiO 3 , MgO, and SiO 2 (Cebulla & Redmer, 2014;Coppari et al, 2013;Jaffe et al, 2000;Lyle et al, 2015;Niu et al, 2015;Umemoto et al, 2017Umemoto et al, , 2006Wu et al, 2011). In addition, according to observations and theoretical models, many super-Earths have terrestrial-type compositions, which has led to extensive investigations into wellknown materials at ultrahigh pressures such as MgSiO 3 , MgO, and SiO 2 (Cebulla & Redmer, 2014;Coppari et al, 2013;Jaffe et al, 2000;Lyle et al, 2015;Niu et al, 2015;Umemoto et al, 2017Umemoto et al, , 2006Wu et al, 2011).…”

Two successive amorphous‐to‐amorphous phase transformations in TiO2

“…Consequently the metallization of a ‐TiO 2 is expected to occur roughly at 180 GPa if and only if the pressure‐induced structural modifications continue in this manner. We should point out here that this prediction is much less than the metallization pressure, 647‐60 GPa, of the crystalline TiO 2 .…”

“…This estimation includes the self‐interaction error (under estimation of band gap energy) in DFT‐GGA calculations and hence it is a quite rough projection. Therefore, we also try to guess the metallization pressure of a ‐TiO 2 from the variation in the mean CN of Ti atom as the metallization of the crystalline TiO 2 appears when the CN of Ti is 10 (Refs ). The mean CN of Ti from a linear fit beyond 60 GPa is anticipated to be 10 at around 180 GPa, close to the value predicted from the variation in the band gap energy under pressure.…”

Electrical conductivity of SiO 2 at extreme conditions and planetary dynamos

“…The novel high-pressure structure of ref. 28 shows remarkably high 10-fold Si-O coordination. Using the reported atomic coordinates and lattice parameters, we have analyzed the predicted structure and found another remarkable feature: The smallest O-O distance (1.754Å) is less than the smallest Si-O distance (1.790Å).…”