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

Stoyanov, E.; Leinenweber, K.; Groy, T.L.; Malik, A.-S.

doi:10.1107/S2056989018008988

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Volume 74| Part 7| July 2018| Pages 1010-1012

https://doi.org/10.1107/S2056989018008988

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Ge_0.57Ti_0.43O₂: a new high-pressure material with rutile-type crystal structure

Emil Stoyanov,^a ^* Kurt Leinenweber,^b Thomas L. Groy ^c and Abds-Sami Malik ^a

^aSandvik Hyperion, 6325 Huntley Road, Worthington, OH 43085, USA, ^bEyring Materials Center, Arizona State University, Tempe, AZ 85287-1604, USA, and ^cSchool of Molecular Sciences, PSD-102 MS-871604, Arizona State University, Tempe, AZ 85287-1604, USA
^*Correspondence e-mail: emil.stoyanov@sandvik.com

Edited by M. Weil, Vienna University of Technology, Austria (Received 12 June 2018; accepted 19 June 2018; online 26 June 2018)

Single crystals of a GeO₂–TiO₂ solid solution with the corresponding composition Ge_0.57Ti_0.43O₂ (germanium titanium tetraoxide) were obtained by devitrification of germania-titania glass at high pressure and temperature. The new compound crystallizes in the rutile structure type (space group P4₂/mnm), where Ge and Ti share the same position M (site symmetry m.mm), with occupancy values of 0.57 (3) and 0.43 (3), respectively, and one O-atom position (m.2m). The M site is in a sixfold O-atom coordination and, as in the original TiO₂ rutile structure, an elongation of the O—M—O bonds along the c-axis direction of the coordination polyhedron and deviation of the angles from 90° lead to a decrease in the coordination symmetry from octahedral to tetragonal. The Ge and Ti atoms are fully disordered in the structure, which indicates that the rutile structure is surprisingly pliant given the differing sizes of the two cations.

Keywords: titania-germania; solid solution; rutile structure; high pressure; high temperature; crystal structure.

CCDC reference: 1850471

1. Chemical context

At ambient pressure, the GeO₂–TiO₂ phase diagram shows the formation of three phases: rutile-type GeO₂, stable up to 1323 K, β-quartz-type GeO₂, stable above 1323 K and TiO₂ in the form of rutile. A metastable α-quartz-type structured GeO₂ has also been reported as the result of the cooling of the β-quartz-type structure (Sarver, 1961 ). Additionally, at ambient pressure GeO₂ and TiO₂ exhibit only limited mutual solubility. The GeO₂–TiO₂ 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. GeO₂ is dimorphous at ambient atmospheric conditions, represented by both rutile-type and α-quartz-structured phases depending on the temperature, but with increasing pressure the GeO₂ 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 CaCl₂-type phase (Haines et al., 2000 ). TiO₂ rutile undergoes two phase transitions under high pressure of up to 12 GPa: rutile-to-α-PbO₂-type at around 7 GPa and α-PbO₂-to-baddeleyite at 12 GPa (Gerward & Staun Olsen, 1997 ). We synthesized the title compound while investigating the GeO₂–TiO₂ 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 TiO₂ and Ti-bearing GeO₂, 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 single crystals of the solid solution with a composition near TiGeO₄ could be obtained.

2. Structural commentary

The crystal structure of Ge_0.57Ti_0.43O₂ corresponds to the TiO₂ rutile type (space group P4₂/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 MO₃ groups in the (110) lattice plane (Fig. 1). The structure is represented by chains of edge-sharing MO₆ 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. In the MO₆ coordination polyhedra, the M—O distances in the xy plane are 1.9080 (12) Å, while the M—O distances along the z axis increase to 1.9441 (19) Å. The M⋯M distances are equal to 2.9121 (13) Å. The elongation of O—M—O bonds along the z direction of the MO₆ coordination polyhedron and the deviation of the angles from 90° lead to a decrease in point group symmetry from octahedral O_h to tetragonal D_4h. The unit-cell volume of Ge_0.57Ti_0.43O₂ [58.79 (6) Å³] falls in between the rutile-type GeO₂ [55.3424 (17) Å³] (Gullikson et al., 2015 ) and TiO₂ rutile [62.435 Å³] (Howard et al., 1991 ) and indicates a linear relationship between the unit-cell volume and molar fraction of GeO₂, adhering to Vegard's Law.

Table 1
Selected geometric parameters (Å, °)

Ge1—O1ⁱ	1.9080 (12)	Ge1—Ti1ⁱⁱ	2.9121 (13)
Ge1—O1	1.9441 (19)

O1ⁱ—Ge1—O1ⁱⁱⁱ	99.48 (8)	Ti1ⁱⁱ—Ge1—Ti1^vi	180
O1ⁱ—Ge1—O1^iv	80.52 (8)	Ti1^vii—O1—Ti1^viii	99.48 (8)
O1^v—Ge1—O1	180	Ge1^vii—O1—Ge1	130.26 (4)
Symmetry codes: (i) $[y-{\script{1\over 2}}, -x+{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (ii) x, y, z+1; (iii) $[y-{\script{1\over 2}}, -x+{\script{1\over 2}}, -z-{\script{1\over 2}}]$ ; (iv) $[-y+{\script{1\over 2}}, x-{\script{1\over 2}}, z+{\script{1\over 2}}]$ ; (v) -x, -y, -z; (vi) x, y, z-1; (vii) $[-y+{\script{1\over 2}}, x+{\script{1\over 2}}, z+{\script{1\over 2}}]$ ; (viii) $[-y+{\script{1\over 2}}, x+{\script{1\over 2}}, z-{\script{1\over 2}}]$ .

Figure 1
View of the structure of Ge_0.57Ti_0.43O₂ 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. MO₆ octahedra are represented as transparent polyhedra.

Figure 2
View of the edge-sharing chain of MO₆ octahedra in Ge_0.57Ti_0.43O₂. The symbols are the same as in Fig. 1

The somewhat large difference in the ionic radii of the sixfold coordinated Ge⁴⁺ and Ti⁴⁺ (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 solid-solution phase is absent in the GeO₂–TiO₂ system, and why the synthesis of a material with composition near TiGeO₄ 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.

3. Synthesis and crystallization

The title compound was synthesized by using an industrial multi-anvil high-pressure apparatus. The starting material was a GeO₂–TiO₂ 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 SiO₂–GeO₂ glass that was loaded in the Pt foil capsule and pressed in the same high-pressure cell. Thus, the pressure standard and the GeO₂–TiO₂ glass were treated at the same conditions. The details on the pressure calibration technique can be found elsewhere (Gullikson et al., 2015; 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 GeO₂–TiO₂ sample was used for the X-ray crystallographic analysis.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. Structure data were standardized according to the STRUCTURE-TIDY program (Gelato & Parthé, 1987 ).

Table 2
Experimental details

Crystal data
Chemical formula	Ge_1.14Ti_0.86O₄
M_r	187.88
Crystal system, space group	Tetragonal, P4₂/mnm
Temperature (K)	298
a, c (Å)	4.493 (2), 2.9121 (13)
V (Å³)	58.79 (6)
Z	1
Radiation type	Mo Kα
μ (mm⁻¹)	17.23
Crystal size (mm)	0.08 × 0.08 × 0.07

Data collection
Diffractometer	Bruker SMART APEX
Absorption correction	Multi-scan (SADABS; Bruker, 2014 )
T_min, T_max	0.31, 0.40
No. of measured, independent and observed [I > 2σ(I)] reflections	722, 77, 76
R_int	0.022
(sin θ/λ)_max (Å⁻¹)	0.772

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.018, 0.050, 1.20
No. of reflections	77
No. of parameters	10
Δρ_max, Δρ_min (e Å⁻³)	0.60, −0.93
Computer programs: APEX2 and SAINT (Bruker, 2014), SHELXT (Sheldrick, 2015a ), SHELXL2014 (Sheldrick, 2015b ), CrystalMaker (Palmer, 2015 ) and publCIF (Westrip, 2010 ).

Supporting information

CCDC reference: 1850471

Crystal structure: contains datablocks global, I. DOI: https://doi.org/10.1107/S2056989018008988/wm5451sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989018008988/wm5451Isup2.hkl

checkCIF report

Computing details top

Data collection: APEX2 (Bruker, 2014); cell refinement: SAINT (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: CrystalMaker (Palmer, 2015); software used to prepare material for publication: publCIF (Westrip, 2010).

Germanium titanium tetraoxide top

Crystal data top

Ge_1.14Ti_0.86O₄	D_x = 5.307 Mg m⁻³
M_r = 187.88	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P4₂/mnm	Cell parameters from 538 reflections
a = 4.493 (2) Å	θ = 4.5–33.3°
c = 2.9121 (13) Å	µ = 17.23 mm⁻¹
V = 58.79 (6) Å³	T = 298 K
Z = 1	Tabular, clear colourless
F(000) = 87	0.08 × 0.08 × 0.07 mm

Data collection top

Bruker SMART APEX diffractometer	77 independent reflections
Radiation source: fine-focus sealed tube, sealed tube	76 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.022
Detector resolution: 8.3330 pixels mm^-1	θ_max = 33.3°, θ_min = 6.4°
ω and φ scans	h = −6→6
Absorption correction: multi-scan (SADABS; Bruker, 2014)	k = −6→6
T_min = 0.31, T_max = 0.40	l = −4→4
722 measured reflections

Refinement top

Refinement on F²	Primary atom site location: iterative
Least-squares matrix: full	Secondary atom site location: notdet
R[F² > 2σ(F²)] = 0.018	w = 1/[σ²(F_o²) + (0.0347P)² + 0.0217P] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.050	(Δ/σ)_max < 0.001
S = 1.20	Δρ_max = 0.60 e Å⁻³
77 reflections	Δρ_min = −0.93 e Å⁻³
10 parameters	Extinction correction: SHELXL2014 (Sheldrick, 2015b)
0 restraints	Extinction coefficient: 0.58 (7)

Special details top

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 (Å²) top

	x	y	z	U_iso*/U_eq	Occ. (<1)
Ge1	0	0	0	0.0049 (3)	0.57 (3)
Ti1	0	0	0	0.0049 (3)	0.43 (3)
O1	0.3059 (3)	0.3059 (3)	0	0.0077 (6)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Ge1	0.0059 (4)	0.0059 (4)	0.0029 (4)	−0.00019 (9)	0	0
Ti1	0.0059 (4)	0.0059 (4)	0.0029 (4)	−0.00019 (9)	0	0
O1	0.0084 (7)	0.0084 (7)	0.0065 (8)	−0.0014 (5)	0	0

Geometric parameters (Å, º) top

Ge1—O1ⁱ	1.9080 (12)	Ge1—Ti1^vii	2.9121 (13)
Ge1—O1ⁱⁱ	1.9080 (12)	Ge1—Ge1^vi	2.9121 (13)
Ge1—O1ⁱⁱⁱ	1.9080 (12)	Ge1—Ge1^vii	2.9121 (13)
Ge1—O1^iv	1.9080 (12)	O1—Ti1^viii	1.9080 (12)
Ge1—O1^v	1.9440 (19)	O1—Ge1^viii	1.9080 (12)
Ge1—O1	1.9441 (19)	O1—Ti1^ix	1.9080 (12)
Ge1—Ti1^vi	2.9121 (13)	O1—Ge1^ix	1.9080 (12)

O1ⁱ—Ge1—O1ⁱⁱ	99.48 (8)	O1ⁱ—Ge1—Ge1^vi	40.26 (4)
O1ⁱ—Ge1—O1ⁱⁱⁱ	80.52 (8)	O1ⁱⁱ—Ge1—Ge1^vi	139.74 (4)
O1ⁱⁱ—Ge1—O1ⁱⁱⁱ	180.0	O1ⁱⁱⁱ—Ge1—Ge1^vi	40.26 (4)
O1ⁱ—Ge1—O1^iv	180.0	O1^iv—Ge1—Ge1^vi	139.74 (4)
O1ⁱⁱ—Ge1—O1^iv	80.52 (8)	O1^v—Ge1—Ge1^vi	90.0
O1ⁱⁱⁱ—Ge1—O1^iv	99.48 (8)	O1—Ge1—Ge1^vi	90.0
O1ⁱ—Ge1—O1^v	90.0	Ti1^vi—Ge1—Ge1^vi	0
O1ⁱⁱ—Ge1—O1^v	90.0	Ti1^vii—Ge1—Ge1^vi	180.0
O1ⁱⁱⁱ—Ge1—O1^v	90.0	O1ⁱ—Ge1—Ge1^vii	139.74 (4)
O1^iv—Ge1—O1^v	90.0	O1ⁱⁱ—Ge1—Ge1^vii	40.26 (4)
O1ⁱ—Ge1—O1	90.0	O1ⁱⁱⁱ—Ge1—Ge1^vii	139.74 (4)
O1ⁱⁱ—Ge1—O1	90.0	O1^iv—Ge1—Ge1^vii	40.26 (4)
O1ⁱⁱⁱ—Ge1—O1	90.0	O1^v—Ge1—Ge1^vii	90.0
O1^iv—Ge1—O1	90.0	O1—Ge1—Ge1^vii	90.0
O1^v—Ge1—O1	180.0	Ti1^vi—Ge1—Ge1^vii	180.0
O1ⁱ—Ge1—Ti1^vi	40.26 (4)	Ti1^vii—Ge1—Ge1^vii	0
O1ⁱⁱ—Ge1—Ti1^vi	139.74 (4)	Ge1^vi—Ge1—Ge1^vii	180.0
O1ⁱⁱⁱ—Ge1—Ti1^vi	40.26 (4)	Ti1^viii—O1—Ge1^viii	0
O1^iv—Ge1—Ti1^vi	139.74 (4)	Ti1^viii—O1—Ti1^ix	99.48 (8)
O1^v—Ge1—Ti1^vi	90.0	Ge1^viii—O1—Ti1^ix	99.48 (8)
O1—Ge1—Ti1^vi	90.0	Ti1^viii—O1—Ge1^ix	99.5
O1ⁱ—Ge1—Ti1^vii	139.74 (4)	Ge1^viii—O1—Ge1^ix	99.48 (8)
O1ⁱⁱ—Ge1—Ti1^vii	40.26 (4)	Ti1^ix—O1—Ge1^ix	0
O1ⁱⁱⁱ—Ge1—Ti1^vii	139.74 (4)	Ti1^viii—O1—Ge1	130.3
O1^iv—Ge1—Ti1^vii	40.26 (4)	Ge1^viii—O1—Ge1	130.26 (4)
O1^v—Ge1—Ti1^vii	90.0	Ti1^ix—O1—Ge1	130.3
O1—Ge1—Ti1^vii	90.0	Ge1^ix—O1—Ge1	130.26 (4)
Ti1^vi—Ge1—Ti1^vii	180.0

Symmetry codes: (i) y−1/2, −x+1/2, −z+1/2; (ii) y−1/2, −x+1/2, −z−1/2; (iii) −y+1/2, x−1/2, z+1/2; (iv) −y+1/2, x−1/2, z−1/2; (v) −x, −y, −z; (vi) x, y, z+1; (vii) x, y, z−1; (viii) −y+1/2, x+1/2, z+1/2; (ix) −y+1/2, x+1/2, z−1/2.

Atomic coordinates and equivalent isotropic atomic displacement parameters (Å²) for TiGeO₄. U(eq) is defined as one third of the trace of the orthogonalized U_ij tensor. top

	x/a	y/b	z/c	U(eq)
Ge(1)	0.0	0.0	0.0	0.0049 (3)
Ti(1)	0.0	0.0	0.0	0.0049 (3)
O(1)	0.3059 (3)	0.3059 (3)	0.0	0.0077 (6)

Anisotropic atomic displacement parameters (Å²) for TiGeO₄. top

	U₁₁	U₂₂	U₃₃	U₂₃	U₁₃	U₁₂
Ge(1)	0.0059 (4)	0.0059 (4)	0.0029 (4)	0	0	-0.00019 (9)
Ti(1)	0.0059 (4)	0.0059 (4)	0.0029 (4)	0	0	-0.00019 (9)
O(1)	0.0084 (7)	0.0084 (7)	0.0065 (8)	0	0	-0.0014 (5)

Selected interatomic distances (Å) and angles for TiGeO₄. top

Ge(1)-O(1)	1.9080 (12)	Ge(1)-O(1)	1.9441 (19)
Ge1-Ti1	2.9121 (13)	O1-Ti1	1.9080 (12)
O1-Ge1	1.9080 (12)
O(1)-Ge(1)-O(1)	99.48 (8)	O(1)-Ge(1)-O(1)	80.52 (8)
O(1)-Ge(1)-O(1)	180.0	O(1)-Ge(1)-O(1)	90.0
O(1)-Ge(1)-Ti(1)	139.74 (4)	O(1)-Ge(1)-Ti(1)	40.26 (4)
O(1)-Ge(1)-Ti(1)	90.0	Ti(1)-Ge(1)-Ti(1)	180.0
Ge1-O1-Ti1	99.48 (8)	Ti1-O1-Ge1	130.26 (4)

Acknowledgements

We acknowledge the use of facilities within the Eyring Materials Center at Arizona State University. The GeO₂–TiO₂ crystals were grown in-house at Sandvik Hyperion.

Funding information

The authors thank Sandvik Hyperion (Worthington, OH, USA) for providing research funding for exploratory experiments and X-ray diffraction related to this project. Exploration of the TiO₂–GeO₂ system at ASU was funded by Sandvik Hyperion. Use of the COMPRES Cell Assembly Project was supported by COMPRES under NSF cooperative agreement EAR 1661511.

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