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

Zibrov, I.P.; Filonenko, V.P.; Sundberg, M.; Werner, P.-E.

doi:10.1107/S0108768100005462

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

I. P. Zibrov, V. P. Filonenko, M. Sundberg and P.-E. Werner

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 Å³, 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 Å³, 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.

Keywords: Structure; Phase transitions; High-pressure forms.

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Supporting information

	Crystallographic Information File (CIF) https://doi.org/10.1107/S0108768100005462/os0050sup1.cif Contains datablocks B, global, Z
	Rietveld powder data file (CIF format) https://doi.org/10.1107/S0108768100005462/os0050sup2.rtv Contains datablock ta2o5_B_Z
	Text file https://doi.org/10.1107/S0108768100005462/os0050sup3.txt Supplementary material
	Text file https://doi.org/10.1107/S0108768100005462/os0050sup4.txt Supplementary material
	Text file https://doi.org/10.1107/S0108768100005462/os0050sup5.txt Supplementary material

Computing details top

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 top

Crystal data top

Ta₂O₅	Z = 4
M_r = 441.89	D_x = 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 Å³	Specimen preparation: Prepared at 1470 K and 8000000 kPa, cooled at 6000 K min⁻¹

Data collection top

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

Refinement top

Refinement on I_net	Excluded region(s): 12-23 deg. in 2 theta
Least-squares matrix: fsqd	Profile function: pseudo-Voigt
R_p = 0.054	47 parameters
R_wp = 0.071	1/Y_i
R_exp = 0.061	(Δ/σ)_max = 0.2
5148 data points	Background function: exponential functions, 6 ranges

Special details top

Refinement. Both structures refined from the same data set.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
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) top

Ta—Taⁱ	3.351 (5)	O1—O3ⁱ	2.94 (5)
Ta—Taⁱⁱ	3.536 (6)	O1—O3^iv	2.89 (5)
Ta—O1	1.90 (2)	O2—O2^vi	2.86 (9)
Ta—O2ⁱⁱⁱ	2.10 (5)	O2—O2^vii	2.81 (9)
Ta—O2ⁱ	1.67 (6)	O2—O3^viii	2.80 (8)
Ta—O3	2.17 (5)	O2—O3^vii	2.63 (6)
Ta—O3ⁱ	2.24 (5)	O2—O3ⁱ	2.68 (8)
Ta—O3^iv	1.91 (5)	O2—O3^ix	2.79 (6)
O1—O1^v	2.94 (3)	O3—O3ⁱ	2.87 (7)
O1—O2ⁱⁱⁱ	2.80 (6)	O3—O3^ix	2.84 (7)
O1—O2ⁱ	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 top