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The crystal structure of the relaxor ferroelectric Pb
2ScTaO
6 has been refined from high-resolution neutron time-of-flight powder diffraction data recorded at various temperatures from 4 to 400 K. Upon warming, Pb
2ScTaO
6 undergoes a first-order transition at 295 K from the rhombohedral ferroelectric state into the cubic paraelectric state. At 4.2 K, in the ferroelectric state, this compound adopts
R3 space-group symmetry, with
a = 8.15231 (7) Å and α = 89.8488 (3)°. At 400 K, in the paraelectric state, this compound adopts
Fmm space-group symmetry, with
a = 8.15345 (3) Å. In the ferroelectric state, the Pb
2+ coordination polyhedra are quite asymmetric, clearly indicating the presence of a stereoactive electron lone pair. The Sc
3+ and Ta
5+ ions are also shifted away from the centers of their respective octahedra, each toward an octahedral face. The large displacement parameters associated with both the Pb and the O ions, in the 400 K structure reveal significant local shifts of these ions from their ideal sites in the paraelectric state. Thus, the paraelectric to ferroelectric transition is driven by long-range cooperative ordering of the cation displacements. Synchrotron X-ray powder diffraction measurements are used to estimate the domain size of the Sc
3+/Ta
5+ ordering and the ferroelectric cation displacements as 88 nm and 10 nm, respectively.
Supporting information
For both compounds, program(s) used to refine structure: GSAS.
Crystal data top
? | V = ? Å3 |
Mr = ? | Z = ? |
Cubic, Fm3m | ? radiation, λ = ? Å |
a = 8.15231 (3) Å | ?, ? × ? × ? mm |
Refinement top
Rp = ? | 4567 data points |
Rwp = ? | 17 parameters |
Rexp = ? | |
χ2 = 6.350 | |
Crystal data top
? | V = ? Å3 |
Mr = ? | Z = ? |
Cubic, Fm3m | ? radiation, λ = ? Å |
a = 8.15231 (3) Å | ?, ? × ? × ? mm |
Refinement top
Rp = ? | χ2 = 6.350 |
Rwp = ? | 4567 data points |
Rexp = ? | 17 parameters |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | Occ. (<1) |
Pb | 0.25 | 0.25 | 0.25 | 0.0440 (3)* | |
Ta(1) | 0.0 | 0.0 | 0.0 | 0.0096 (3)* | .759 (11) |
Sc(1) | 0.5 | 0.5 | 0.5 | 0.0096 (3)* | .759 (11) |
Sc(2) | 0.0 | 0.0 | 0.0 | 0.0096 (3)* | .241 (11) |
Ta(2) | 0.5 | 0.5 | 0.5 | 0.0096 (3)* | .241 (11) |
O | 0.2443 (3) | 0.0 | 0.0 | 0.02621 | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
O | 0.0085 (4) | 0.0 | 0.0 | 0.0351 (4) | 0.0351 (4) | 0.0 |
Crystal data top
? | V = ? Å3 |
Mr = ? | Z = ? |
Trigonal, R3 | ? radiation, λ = ? Å |
a = 8.15231 (7) Å | ?, ? × ? × ? mm |
c = 8.15231 Å | |
Refinement top
Rp = ? | 4589 data points |
Rwp = ? | 43 parameters |
Rexp = ? | 20 restraints |
χ2 = 5.382 | |
Crystal data top
? | V = ? Å3 |
Mr = ? | Z = ? |
Trigonal, R3 | ? radiation, λ = ? Å |
a = 8.15231 (7) Å | ?, ? × ? × ? mm |
c = 8.15231 Å | |
Refinement top
Rp = ? | 4589 data points |
Rwp = ? | 43 parameters |
Rexp = ? | 20 restraints |
χ2 = 5.382 | |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | Occ. (<1) |
Pb(1) | 0.270 (3) | 0.270 (3) | 0.270 (3) | 0.0178 (4)* | |
Pb(2) | 0.764 (3) | 0.764 (3) | 0.764 (3) | 0.0178 (4)* | |
Pb(3) | 0.274 (2) | 0.264 (3) | 0.765 (2) | 0.0178 (4)* | |
Pb(4) | 0.255 (2) | 0.7758 (15) | 0.755 (2) | 0.0178 (4)* | |
Ta(1) | 0.0 | 0.0 | 0.0 | 0.0082 (3)* | .76 |
Ta(1b) | 0.5 | 0.5 | 0.5 | 0.0082 (3)* | .24 |
Ta(2) | 0.5 | 0.5 | 0.0 | 0.0082 (3)* | .76 |
Ta(2b) | 0.5 | 0.0 | 0.0 | 0.0082 (3)* | .24 |
O(1) | 0.240 (2) | −0.019 (3) | 0.002 (3) | 0.0098 (3)* | |
O(2) | −0.249 (2) | −0.026 (3) | −0.012 (3) | 0.0098 (3)* | |
O(3) | 0.2526 (19) | 0.482 (3) | 0.005 (4) | 0.0098 (3)* | |
O(4) | 0.739 (2) | 0.480 (3) | −0.014 (3) | 0.0098 (3)* | |
O(5) | 0.493 (4) | 0.250 (2) | −0.031 (3) | 0.0097 (3)* | |
O(6) | 0.507 (3) | 0.7417 (18) | −0.016 (3) | 0.0097 (3)* | |
O(7) | 0.484 (3) | 0.475 (2) | 0.2403 (18) | 0.0097 (3)* | |
O(8) | 0.475 (3) | 0.499 (4) | −0.249 (2) | 0.0097 (3)* | |
Sc(1b) | 0.0 | 0.0 | 0.0 | 0.0082 (3)* | .24 |
Sc(1) | 0.5 | 0.5 | 0.5 | 0.0082 (3)* | .76 |
Sc(2b) | 0.5 | 0.5 | 0.0 | 0.0082 (3)* | .24 |
Sc(2) | 0.5 | 0.0 | 0.0 | 0.0082 (3)* | .76 |
Experimental details
| (PST_9903_phase_1) | (PST_9824_phase_1) |
Crystal data |
Chemical formula | ? | ? |
Mr | ? | ? |
Crystal system, space group | Cubic, Fm3m | Trigonal, R3 |
Temperature (K) | ? | ? |
a, b, c (Å) | 8.15231 (3), 8.15231, 8.15231 | 8.15231 (7), 8.15231, 8.15231 |
α, β, γ (°) | 90, 90, 90 | 89.8488 (3), 89.8488, 89.8488 |
V (Å3) | ? | ? |
Z | ? | ? |
Radiation type | ?, λ = ? Å | ?, λ = ? Å |
Specimen shape, size (mm) | ?, ? × ? × ? | ?, ? × ? × ? |
|
Data collection |
Diffractometer | ? | ? |
Specimen mounting | ? | ? |
Data collection mode | ? | ? |
Scan method | ? | ? |
2θ values (°) | 2θmin = ? 2θmax = ? 2θstep = ? | 2θmin = ? 2θmax = ? 2θstep = ? |
|
Refinement |
R factors and goodness of fit | Rp = ?, Rwp = ?, Rexp = ?, χ2 = 6.350 | Rp = ?, Rwp = ?, Rexp = ?, χ2 = 5.382 |
No. of data points | 4567 | 4589 |
No. of parameters | 17 | 43 |
No. of restraints | ? | 20 |
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