Structures and phase transitions of B-Ta₂O₅ and Z-­Ta₂O₅: two high-pressure forms of Ta₂O₅

A sample of Ta₂O₅, ditantalum pentaoxide, heat-treated in a `toroid'-type high-pressure chamber at P = 8 GPa and T = 1470 K, was studied by X-ray powder diffraction and high-resolution transmission electron microscopy (HRTEM). Two high-pressure modifications of Ta₂O₅, isostructural with B-Nb₂O₅ and Z-Nb₂O₅, were identified from the X-ray powder pattern. Both structures were refined by the Rietveld method from the X-ray diffraction data: B-Ta₂O₅, a = 12.7853 (4), b = 4.8537 (1), c = 5.5276 (2) Å, β = 104.264 (2)°, V = 332.45 ų, Z = 4, space group C2/c; Z-Ta₂O₅, a = 5.2252 (1), b = 4.6991 (1), c = 5.8534 (1) Å, β = 108.200 (2)°, V = 136.53 ų, Z = 2, space group C2. The Z-Ta₂O₅ modification is new. The Ta atoms are six-coordinated in B-Ta₂O₅ and seven-coordinated in Z-Ta₂O₅. The two structures are closely related, which makes an intergrowth and a transformation between them possible. An idealized model of the intergrowth structure has been given. The HRTEM study showed defect-rich B-Ta₂O₅ crystals, which could be interpreted as an intergrowth between the B-­Ta₂O₅ and Z-Ta₂O₅ phases.

Computing details

For both compounds, data collection: Stoe Stadi/P, Powdat. Cell refinement: DBW 3.2 S Wiles et al.,1988) for B; DBW 3.2 S, Wiles et al. 1988 for Z. For both compounds, data reduction: Stoe Stadi/P, Rawdat. Program(s) used to refine structure: DBW 3.2 S Wiles et al.,1988) for B; DBW 3.2 S, Wiles et al. 1988 for Z.

(B) ditantalum penta oxide

Crystal data

Ta2O5	Z = 4
Mr = 441.89	Dx = 8.83 Mg m^−3
Monoclinic, C2/c	Cu Kα1 radiation, λ = 1.540562 Å
a = 12.7853 (4) Å	T = 293 K
b = 4.8537 (1) Å	Particle morphology: plate-like
c = 5.5276 (2) Å	white
β = 104.264 (2)°	flat_sheet, 0.1 × 5 mm
V = 332.45 Å^3	Specimen preparation: Prepared at 1470 K and 8000000 kPa, cooled at 6000 K min^−1

Data collection

Stoe PSD diffractometer	Data collection mode: transmission
Radiation source: sealed X-ray tube	Scan method: step
Ge monochromator	2θmin = 12°, 2θmax = 115°, 2θstep = 0.02°
Specimen mounting: packed powder pellet

Refinement

Refinement on Inet	Excluded region(s): 12-23 deg. in 2 theta
Least-squares matrix: fsqd	Profile function: pseudo-Voigt
Rp = 0.054	47 parameters
Rwp = 0.071	1/Yi
Rexp = 0.061	(Δ/σ)max = 0.2
5148 data points	Background function: exponential functions, 6 ranges

Special details

Refinement. Both structures refined from the same data set.

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

	x	y	z	Uiso*/Ueq
Ta	0.1395 (1)	0.2470 (15)	0.2623 (2)	1.10 (3)*
O1	0	0.102 (4)	0.25	1.8 (2)*
O2	0.389 (2)	0.051 (2)	0.990 (4)	1.8 (2)*
O3	0.299 (1)	0.431 (3)	0.355 (3)	1.8 (2)*

Bond lengths (Å)

Ta—Ta^i	3.351 (5)	O1—O3^i	2.94 (5)
Ta—Ta^ii	3.536 (6)	O1—O3^iv	2.89 (5)
Ta—O1	1.90 (2)	O2—O2^vi	2.86 (9)
Ta—O2^iii	2.10 (5)	O2—O2^vii	2.81 (9)
Ta—O2^i	1.67 (6)	O2—O3^viii	2.80 (8)
Ta—O3	2.17 (5)	O2—O3^vii	2.63 (6)
Ta—O3^i	2.24 (5)	O2—O3^i	2.68 (8)
Ta—O3^iv	1.91 (5)	O2—O3^ix	2.79 (6)
O1—O1^v	2.94 (3)	O3—O3^i	2.87 (7)
O1—O2^iii	2.80 (6)	O3—O3^ix	2.84 (7)
O1—O2^i	2.75 (6)	O3—O3^iv	2.84 (6)

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

(Z) ditantalum penta oxide

Crystal data

Ta2O5	Z = 2
Mr = 441.89	Dx = 10.75 Mg m^−3
Monoclinic, C2	Cu Kα1 radiation, λ = 1.54056 Å
a = 5.2252 (1) Å	T = 293 K
b = 4.6991 (1) Å	Particle morphology: plate-like
c = 5.8534 (1) Å	white
β = 108.200 (2)°	flat_sheet, 0.1 × 5 mm
V = 136.53 Å^3	Specimen preparation: Prepared at 1470 K and 8000000 kPa, cooled at 6000 K min^−1

Data collection

Stoe Small PDS diffractometer	Data collection mode: transmission
Radiation source: sealed X-ray tube	Scan method: step
Ge monochromator	2θmin = 12°, 2θmax = 115°, 2θstep = 0.02°
Specimen mounting: packed powder pellet

Refinement

Refinement on Inet	Excluded region(s): 12-23 deg. in 2 theta
Least-squares matrix: fsqd	Profile function: pseudo-Voigt
Rp = 0.054	47 parameters
Rwp = 0.071	1/Yi
Rexp = 0.061	(Δ/σ)max = 0.2
5148 data points	Background function: exponential functions, 6 ranges

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

	x	y	z	Uiso*/Ueq
Ta	0.2172 (3)	0	0.2696 (4)	1.73 (6)*
O1	0.189 (4)	0.164 (5)	0.934 (4)	0.7 (3)*
O2	0.090 (5)	0.681 (5)	0.323 (4)	0.7 (3)*
O3	0	0.201 (7)	0.5	0.7 (3)*

Bond lengths (Å)

Ta—Ta^i	3.262 (9)	O1—O1^ix	2.49 (10)
Ta—Ta^ii	3.321 (9)	O1—O2^x	2.86 (10)
Ta—O1^iii	2.07 (7)	O1—O2^xi	2.57 (10)
Ta—O1^iv	2.22 (8)	O1—O2^v	2.17 (11)
Ta—O1^v	2.13 (7)	O1—O2^iv	2.99 (10)
Ta—O2^vi	1.71 (7)	O1—O3	2.43 (7)
Ta—O2^vii	2.06 (8)	O2—O2^iv	2.53 (10)
Ta—O3	2.22 (4)	O2—O3^xii	2.75 (11)
Ta—O3^vii	2.18 (6)	O2—O3^xiii	2.07 (8)
O1—O1^viii	2.33 (11)	O2—O2^iv	2.53 (11)

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