Ge0.57Ti0.43O2: a new high-pressure material with rutile-type crystal structure

Ge0.57Ti0.43O2 adopts the rutile structure type with Ge and Ti sites disordered on one position.


Chemical context
At ambient pressure, the GeO 2 -TiO 2 phase diagram shows the formation of three phases: rutile-type GeO 2 , stable up to 1323 K, -quartz-type GeO 2 , stable above 1323 K and TiO 2 in the form of rutile. A metastable -quartz-type structured GeO 2 has also been reported as the result of the cooling of the -quartz-type structure (Sarver, 1961). Additionally, at ambient pressure GeO 2 and TiO 2 exhibit only limited mutual solubility. The GeO 2 -TiO 2 phase diagram at elevated pressures and temperatures has not been studied in great detail and the mutual solubility of Ge and Ti in the phases stable at these conditions is still largely unknown. GeO 2 is dimorphous at ambient atmospheric conditions, represented by both rutiletype and -quartz-structured phases depending on the temperature, but with increasing pressure the GeO 2 rutile becomes more stable, and is the primary phase above two GPa (Micoulaut et al., 2006). At pressures above 25 GPa, the tetragonal rutile-type phase transforms into an orthorhombic CaCl 2 -type phase (Haines et al., 2000). TiO 2 rutile undergoes two phase transitions under high pressure of up to 12 GPa: rutile-to--PbO 2 -type at around 7 GPa and -PbO 2 -tobaddeleyite at 12 GPa (Gerward & Staun Olsen, 1997). We synthesized the title compound while investigating the GeO 2 -TiO 2 phase diagram at a pressure of 8 GPa at 2028 K by means of the multi-anvil high-pressure technique. Instead of forming Ge-bearing TiO 2 and Ti-bearing GeO 2 , we discovered that the high pressure and temperature conditions led to the formation of a crystalline, single solid-solution material. At temperatures above 1873 K, crystal growth was significant and high-quality ISSN 2056-9890 single crystals of the solid solution with a composition near TiGeO 4 could be obtained.

Structural commentary
The crystal structure of Ge 0.57 Ti 0.43 O 2 corresponds to the TiO 2 rutile type (space group P4 2 /mnm). The shared metal site M is in Wyckoff position 2a and is surrounded by six O atoms, thus forming a sixfold coordination polyhedron. 57% of the 2a positions are occupied by Ge and the remaining 43% are occupied by Ti. Each oxygen atom occupies a 4f position and is surrounded by three M sites, forming triangular MO 3 groups in the (110) lattice plane (Fig. 1). The structure is represented by chains of edge-sharing MO 6 octahedra running parallel to the c-axis direction (Fig. 2) and connected to each other by shared corners. Relevant bond lengths and angles are presented in Table 1 (Howard et al., 1991) and indicates a linear relationship between the unit-cell volume and molar fraction of GeO 2 , adhering to Vegard's Law.
The somewhat large difference in the ionic radii of the sixfold coordinated Ge 4+ and Ti 4+ (0.53 and 0.605 Å , respectively; Shannon, 1976) may be the reason for the limited mutual solubility of Ge and Ti in the rutile structured oxides at ambient pressure. This might explain why the single solidsolution phase is absent in the GeO 2 -TiO 2 system, and why the synthesis of a material with composition near TiGeO 4 requires high-pressure and high-temperature conditions. Disordering at high temperatures (significantly above the ambient-pressure melting point) could assist in the stability of the solid solution even with the two different sized cations.

Figure 1
View of the structure of Ge 0.57 Ti 0.43 O 2 looking down the c axis. The unit cell is outlined in white. The red ellipsoids represent the oxygen atoms and show the orientation of the displacement ellipsoids for 99% probability. The Ti atom is represented by light blue and the Ge atom by purple, with the percentage occupancy of the M site represented as a pie chart on the atom. MO 6 octahedra are represented as transparent polyhedra.
a GeO 2 -TiO 2 glass produced from the corresponding oxide powders with a molar ratio of 60:40 (Sem-Com Company, Toledo, OH). A Pt foil capsule was loaded with the powdered glass and was subjected to high-pressure/high-temperature (HPHT) conditions of 8 GPa and 2028 K for 30 minutes, followed by cooling for 15 minutes to room temperature and releasing pressure non-isobarically to atmospheric pressure to recover the sample. The temperature was monitored with a W3%Re-W26%Re (C-type) thermocouple. The pressure was estimated by recovering and analyzing SiO 2 -GeO 2 glass that was loaded in the Pt foil capsule and pressed in the same highpressure cell. Thus, the pressure standard and the GeO 2 -TiO 2 glass were treated at the same conditions. The details on the pressure calibration technique can be found elsewhere Leinenweber et al., 2015). The applied temperature was sufficient to produce high-quality single crystals with uniform extinction in the optical microscope. A clear colourless tabular-like crystal from the recovered GeO 2 -TiO 2 sample was used for the X-ray crystallographic analysis.

Germanium titanium tetraoxide
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )