Structural characterization of a new high-pressure phase of GaAsO₄

As in SiO₂ which, at high pressures, undergoes the α-quartz → stishovite transition, GaAsO₄ transforms into a dirutile structure at 9 GPa and 1173 K. In 2002, a new GaAsO₄ polymorph was found by quenching the compound from 6 GPa and 1273 K to ambient conditions. The powder diagram was indexed on the basis of a hexagonal cell (a = 8.2033, c = 4.3941 Å, V = 256.08 ų), but the structure did not correspond to any known structure of other AXO₄ compounds. We report here the ab initio crystal structure determination of this hexagonal polymorph from powder data. The new phase is isostructural to β-MnSb₂O₆ and it can be described as a lacunary derivative of NiAs with half the octahedral sites being vacant, but it also contains fragments of the rutile-like structure.

Computing details

Data collection: Bruker AXS software' for Fase_GaAsO4. Program(s) used to solve structure: Endeavour for Fase_GaAsO4. For both compounds, program(s) used to refine structure: FULLPROF. Molecular graphics: Diamond for Fase_GaAsO4. Software used to prepare material for publication: FULLPROF for Fase_GaAsO4.

Figures

(Fase_GaAsO4)

Crystal data

GaAsO4	Dx = 6.087 Mg m^−3
Mr = 208.64	Cu Kα radiation, λ = 1.540590 Å
Hexagonal, P3	µ = 33.6 mm^−1
Hall symbol: P 3	T = 273 K
a = 8.203320 (1) Å	Particle morphology: plate like
c = 4.394109 (1) Å	white
V = 256.08 (1) Å^3	cylinder, 30 × 0.5 mm
Z = 4.5	Specimen preparation: Prepared at 1273 K and 6 kPa, cooled at 5 K min^−1

Data collection

Bruker D8 diffractometer	Data collection mode: transmission
Radiation source: X-ray tube'	Scan method: step
Ge monochromator	2θmin = 9.842°, 2θmax = 150.234°, 2θstep = 0.015°
Specimen mounting: 60 rpm rotating glass capillaries of 0.5 mm diameter

Refinement

Refinement on raw data	Profile function: pseudo-Voigt
Rp = 0.089	48 parameters
Rwp = 0.129	0 restraints
Rexp = 0.040	1/sigma
RBragg = 4.722
χ^2 = NOT FOUND	Background function: Linear interpolation'
9590 data points	Preferred orientation correction: none
Excluded region(s): 0-10, 150-180

Special details

Refinement. The profile refinement was carried out with this model in which symmetry- and hkl-dependent anisotropic microstrain parameters were included (Stephens, 1999). This was necessary as the observed diffraction lines with l>h were preferentially broadened. This may be related to static Ga/As disorder.; ?

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
AS1	0.00000	0.00000	0.48368	0.020 (2)*
GE2	0.3610 (6)	0.0267 (6)	0.505 (3)	0.0158 (5)*
GA3	0.3082 (6)	0.3422 (11)	0.998 (4)	0.0395 (10)*
AS4	0.33330	0.66670	0.528 (3)	0.054 (4)*
AS5	0.66670	0.33330	1.010 (4)	0.0101 (11)*
O1	0.451 (4)	0.229 (7)	0.247 (6)	0.0202 (13)*
O2	0.558 (6)	0.112 (4)	0.792 (6)	0.0202 (13)*
O3	0.133 (3)	0.552 (4)	0.299 (5)	0.0202 (13)*
O4	0.066 (3)	0.182 (3)	0.305 (4)	0.0202 (13)*
O5	0.197 (4)	0.438 (3)	0.787 (4)	0.0202 (13)*
O6	0.236 (3)	0.134 (3)	0.742 (5)	0.0202 (13)*

(Fase_As2O3)

Crystal data

As2O3	Dx = 3.867 Mg m^−3
Mr = 197.84	Cu Kα radiation, λ = 1.540590 Å
Cubic, Fd3m	T = 273 K
Hall symbol: -F 4vw 2vw 3	white
a = 11.0771 (5) Å	cylinder, ? × ? × ? mm
V = 1359.20 (10) Å^3	Specimen preparation: Prepared at 1273 K and 6 kPa, cooled at 5 K min^−1
Z = 16

Data collection

Bruker D8 diffractometer	Scan method: step
Ge monochromator	2θmin = 9.842°, 2θmax = 150.234°, 2θstep = 0.015°
Specimen mounting: 60 rpm rotating glass capillaries of 0.5 mm diameter

Refinement

Refinement on raw data	Excluded region(s): 0-10, 150-180
Rp = 0.089	Profile function: pseudo-Voigt
Rwp = 0.129	48 parameters
Rexp = 0.040	0 restraints
RBragg = 0.130	1/sigma
χ^2 = NOT FOUND
0 data points

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
AS1	0.77154	0.77154	0.77154	0.057 (3)*
O1	0.95105	0.12500	0.12500	0.042 (11)*	0.99998