Crystal structures of the relaxor oxide Pb₂(ScTa)O₆ in the paraelectric and ferroelectric states

The crystal structure of the relaxor ferroelectric Pb₂ScTaO₆ has been refined from high-resolution neutron time-of-flight powder diffraction data recorded at various temperatures from 4 to 400 K. Upon warming, Pb₂ScTaO₆ 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 Pb²⁺ coordination polyhedra are quite asymmetric, clearly indicating the presence of a stereoactive electron lone pair. The Sc³⁺ and Ta⁵⁺ 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 Sc³⁺/Ta⁵⁺ ordering and the ferroelectric cation displacements as 88 nm and 10 nm, respectively.

Computing details

For both compounds, program(s) used to refine structure: GSAS.

Figures

(PST_9903_phase_1)

Crystal data

?	V = ? Å^3
Mr = ?	Z = ?
Cubic, Fm3m	? radiation, λ = ? Å
a = 8.15231 (3) Å	?, ? × ? × ? mm

Data collection

Refinement

Rp = ?	4567 data points
Rwp = ?	17 parameters
Rexp = ?
χ^2 = 6.350

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

	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 (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O	0.0085 (4)	0.0	0.0	0.0351 (4)	0.0351 (4)	0.0

(PST_9824_phase_1)

Crystal data

?	V = ? Å^3
Mr = ?	Z = ?
Trigonal, R3	? radiation, λ = ? Å
a = 8.15231 (7) Å	?, ? × ? × ? mm
c = 8.15231 Å

Data collection

Refinement

Rp = ?	4589 data points
Rwp = ?	43 parameters
Rexp = ?	20 restraints
χ^2 = 5.382

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

	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