De­hydroxy­lation and structural transition in α-GaOOH investigated by in situ X-ray diffraction

Ga₂O₃ is an ultra-wide-bandgap semiconductor that is receiving considerable attention due to its promising applications in high-frequency, high-power and high-temperature settings. It can be prepared by calcinating the α-GaOOH phase at high temperatures. Understanding the significance of hydroxyl groups in α-GaOOH, de­hydroxy­lation and the structural transition at high temperatures has become a key aspect of preparing high-quality α-Ga₂O₃ crystals, but the underlying mechanism remains unknown. In this research, α-GaOOH nanorods were hydro­thermally synthesized and the structural evolution of α-GaOOH investigated at high temperatures by in situ X-ray diffraction. The hydroxyl group in α-GaOOH squeezes Ga³⁺ from the center of the [GaO₆] octahedron, resulting in deformed [GaO₆] octahedra and significant microstrain in α-GaOOH. The hydroxyl groups are peeled off from α-GaOOH when the temperature exceeds 200°C, resulting in contraction along the c-axis direction and expansion along the a-axis direction of α-GaOOH. When the temperature exceeds 300°C, the Ga—O bond inside the double chains preferentially breaks to generate square-wave-like octahedron chains, and the neighboring chains repack to form hexagonal-like octahedron layers. The octahedron layers are packed up and down by electrostatic interaction to generate the α-Ga₂O₃ structure. This work highlights the role of hydroxyl groups in α-GaOOH, de­hydroxy­lation and the structural transition on the atomic scale, providing valuable guidelines for the fabrication of high-quality α-Ga₂O₃ crystals.

Computing details

(GaOOH_30)

Crystal data

GaHO2	Z = 4
Mr = 102.73	Dx = 5.194 Mg m^−3
Orthorhombic, Pnma	Cu Kα1, 2 radiation, λ = 1.54060, 1.54440 Å
Hall symbol: -P 2ac 2n	T = 303 K
a = 9.78216 (15) Å	Particle morphology: nanorods
b = 2.97127 (4) Å	White
c = 4.51938 (11) Å	flat_sheet, 20 × 20 mm
V = 131.36 (1) Å^3	Specimen preparation: Prepared at 303 K and 101 kPa

Data collection

SmartLabXE diffractometer	Scan method: continuous
Radiation source: rotating-anode X-ray tube, SmartLabXE	2θmin = 10.040°, 2θmax = 120.040°, 2θstep = 0.020°
Data collection mode: reflection

Refinement

Least-squares matrix: full	22 parameters
Rp = 0.060	H-atom parameters not refined
Rwp = 0.079
Rexp = 0.025	(Δ/σ)max = 0.001
R(F^2) = 0.04628	Background function: Background function: "chebyschev-1" function with 6 terms: -18.3(20), 14(3), 29.0(31), -1.5(30), 1.0(25), 13.7(24),
5501 data points	Preferred orientation correction: none
Profile function: Finger-Cox-Jephcoat function parameters U, V, W, X, Y, SH/L: peak variance(Gauss) = Utan(Th)^2+Vtan(Th)+W: peak HW(Lorentz) = X/cos(Th)+Ytan(Th); SH/L = S/L+H/L U, V, W in (centideg)^2, X & Y in centideg 34.645, -15.632, 21.741, 0.583, 0.091, 0.004,

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

	x	y	z	Uiso*/Ueq
Ga1	0.35560 (8)	0.25000	0.55083 (15)	0.0046*
O1	0.1927 (3)	0.25000	0.3001 (7)	0.0043*
O2	0.4437 (3)	0.75000	0.3056 (7)	0.0067*
H1	0.42800	0.75000	0.14000	0.0500*

Geometric parameters (Å, º)

Ga1—O1	1.955 (3)	O1—Ga1^v	1.923 (2)
Ga1—O1^i	1.923 (2)	O1—Ga1^vi	1.923 (2)
Ga1—O1^ii	1.923 (2)	O2—Ga1	2.044 (2)
Ga1—O2^iii	2.044 (2)	O2—Ga1^vii	2.044 (2)
Ga1—O2	2.044 (2)	O2—Ga1^iv	2.068 (3)
Ga1—O2^iv	2.068 (3)	O2—H1	0.764
O1—Ga1	1.955 (3)	H1—O2	0.764
O1^i—Ga1—O1^ii	101.14 (15)	Ga1^v—O1—Ga1^vi	101.14 (15)

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

(Ga2O3_500)

Crystal data

Ga2O3	Dx = 6.390 Mg m^−3
Mr = 187.44	Cu Kα1, 2 radiation, λ = 1.54060, 1.54440 Å
Trigonal, R3c	T = 773 K
Hall symbol: -R 3 2"c	Particle morphology: nanorods
a = 4.9995 (2) Å	white
c = 13.5010 (3) Å	flat_sheet, 20 × 20 mm
V = 292.24 (2) Å^3	Specimen preparation: Prepared at 773 K and 101 kPa
Z = 6

Data collection

SmartLabXE diffractometer	Scan method: continuous
Radiation source: rotating-anode X-ray tube, SmartLabXE	2θmin = 10.040°, 2θmax = 120.040°, 2θstep = 0.020°
Data collection mode: reflection

Refinement

Least-squares matrix: full	Profile function: Finger-Cox-Jephcoat function parameters U, V, W, X, Y, SH/L: peak variance(Gauss) = Utan(Th)^2+Vtan(Th)+W: peak HW(Lorentz) = X/cos(Th)+Ytan(Th); SH/L = S/L+H/L U, V, W in (centideg)^2, X & Y in centideg 34.645, -15.632, 21.741, 0.583, 0.091, 0.004,
Rp = 0.053	17 parameters
Rwp = 0.068
Rexp = 0.026	(Δ/σ)max = 0.005
R(F^2) = 0.06553	Background function: Background function: "chebyschev-1" function with 6 terms: -8.1(15), -7.6(26), 14.6(23), 8.1(23), -3.8(20), 4.1(19), Background peak parameters: pos, int, sig, gam: 34.440, 2.13(6)e5, 28000.000, 0.100,
5501 data points	Preferred orientation correction: none

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

	x	y	z	Uiso*/Ueq
Ga1	0.00000	0.00000	0.35596 (4)	0.0060*
O1	0.3026 (6)	0.00000	0.25000	0.0092*

Geometric parameters (Å, º)

Ga1—O1	2.0821 (19)	Ga1—O1^v	1.9309 (14)
Ga1—O1^i	2.0821 (19)	O1—Ga1	2.0821 (19)
Ga1—O1^ii	2.0821 (19)	O1—Ga1^vi	2.0821 (19)
Ga1—O1^iii	1.9309 (14)	O1—Ga1^vii	1.9309 (14)
Ga1—O1^iv	1.9309 (14)	O1—Ga1^viii	1.9309 (14)
O1^iii—Ga1—O1^iv	103.28 (3)	O1^iv—Ga1—O1^v	103.28 (3)
O1^iii—Ga1—O1^v	103.28 (3)	Ga1^vii—O1—Ga1^viii	118.53 (14)

Symmetry codes: (i) −y, x−y, z; (ii) y−x, −x, z; (iii) y−x+1/3, y−1/3, z+1/6; (iv) −y+1/3, −x+2/3, z+1/6; (v) x−2/3, x−y−1/3, z+1/6; (vi) x−y, −y, −z+1/2; (vii) y−x+2/3, y+1/3, z−1/6; (viii) −x+1/3, −y−1/3, −z+2/3.