The high-temperature phases of the perovskites sodium niobate, NaNbO
3, and sodium tantalate, NaTaO
3, have been re-examined using the high-resolution powder diffractometer HRPD at the ISIS neutron spallation source; the two materials show the same sequence of phases with tilted octahedra. Diffraction patterns were measured every 5 K allowing structural changes with temperature within a single phase to be determined for the first time. Previous structure determinations within one phase had been performed at a single temperature only. The octahedra are tilted about pseudocubic
directions and are also deformed; the magnitude of the deformation is shown to be proportional to the square of the angle of tilt as expected from a phenomenological theory applied to such transitions. The structures of NaNbO
3 between 753 and 793 K and of NaTaO
3 below 758 K are not as reported in the literature.
Supporting information
| Crystallographic Information File (CIF) Contains datablocks global, Nb813, Ta803, Nb888, Ta878 |
| Portable Document Format (PDF) file Supplementary material |
Crystal data top
NaNbO3 | c = 7.8765 (3) Å |
Mr = ? | V = ? Å3 |
Orthorhombic, Cmcm | Z = 8 |
Hall symbol: -C 2c 2 | Neutron radiation, λ = ? Å |
a = 7.85758 (3) Å | T = 813 K |
b = 7.86793 (3) Å | × × mm |
Crystal data top
NaNbO3 | V = ? Å3 |
Mr = ? | Z = 8 |
Orthorhombic, Cmcm | Neutron radiation, λ = ? Å |
a = 7.85758 (3) Å | T = 813 K |
b = 7.86793 (3) Å | × × mm |
c = 7.8765 (3) Å | |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Biso*/Beq | |
Nb | 0.25 | 0.25 | 0.0 | 1.60 (1) | |
Na1 | 0.0 | 0.0022 (14) | 0.25 | 3.56 (12) | |
Na2 | 0.0 | 0.5026 (14) | 0.25 | 4.31 (14) | |
O1 | 0.2230 (4) | 0.0 | 0.0 | ? | |
O2 | 0.0 | 0.2741 (4) | 0.0203 (3) | ? | |
O3 | 0.2710 (3) | 0.2489 (7) | 0.25 | ? | |
Crystal data top
NaO3Ta | c = 7.85728 (4) Å |
Mr = ? | V = ? Å3 |
Orthorhombic, Cmcm | Z = 8 |
Hall symbol: -C 2c 2 | Neutron radiation, λ = ? Å |
a = 7.83729 (4) Å | T = 803 K |
b = 7.84887 (4) Å | × × mm |
Crystal data top
NaO3Ta | V = ? Å3 |
Mr = ? | Z = 8 |
Orthorhombic, Cmcm | Neutron radiation, λ = ? Å |
a = 7.83729 (4) Å | T = 803 K |
b = 7.84887 (4) Å | × × mm |
c = 7.85728 (4) Å | |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Biso*/Beq | |
Ta | 0.25 | 0.25 | 0.0 | 0.98 (2) | |
Na1 | 0.0 | 0.008 (2) | 0.25 | 3.3 (2) | |
Na2 | 0.0 | 0.507 (2) | 0.25 | 3.4 (2) | |
O1 | 0.2253 (5) | 0.0 | 0.0 | ? | |
O2 | 0.0 | 0.2739 (6) | 0.0200 (5) | ? | |
O3 | 0.2737 (5) | 0.2478 (11) | 0.25 | ? | |
Crystal data top
NaNbO3 | V = ? Å3 |
Mr = ? | Z = 2 |
Tetragonal, P4/mbm | Neutron radiation, λ = ? Å |
Hall symbol: -P 4 2ab | T = 888 K |
a = 5.56896 (2) Å | × × mm |
c = 3.94408 (2) Å | |
Crystal data top
NaNbO3 | V = ? Å3 |
Mr = ? | Z = 2 |
Tetragonal, P4/mbm | Neutron radiation, λ = ? Å |
a = 5.56896 (2) Å | T = 888 K |
c = 3.94408 (2) Å | × × mm |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Biso*/Beq | |
Nb | 0.0 | 0.0 | 0.0 | 1.64 (1) | |
Na | 0.0 | 0.5 | 0.5 | 4.26 (3) | |
O1 | 0.0 | 0.0 | 0.5 | ? | |
O2 | 0.2719 (1) | 0.2281 (1) | 0.0 | ? | |
Crystal data top
NaO3Ta | V = ? Å3 |
Mr = ? | Z = 2 |
Tetragonal, P4/mbm | Neutron radiation, λ = ? Å |
Hall symbol: -P 4 2ab | T = 878 K |
a = 5.55465 (2) Å | × × mm |
c = 3.93426 (2) Å | |
Crystal data top
NaO3Ta | V = ? Å3 |
Mr = ? | Z = 2 |
Tetragonal, P4/mbm | Neutron radiation, λ = ? Å |
a = 5.55465 (2) Å | T = 878 K |
c = 3.93426 (2) Å | × × mm |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Biso*/Beq | |
Ta | 0.0 | 0.0 | 0.0 | 1.05 (2) | |
Na | 0.0 | 0.5 | 0.5 | 3.75 (4) | |
O1 | 0.0 | 0.0 | 0.5 | ? | |
O2 | 0.2713 (2) | 0.2287 (2) | 0.0 | ? | |
Experimental details
| (Nb813) | (Ta803) | (Nb888) | (Ta878) |
Crystal data |
Chemical formula | NaNbO3 | NaO3Ta | NaNbO3 | NaO3Ta |
Mr | ? | ? | ? | ? |
Crystal system, space group | Orthorhombic, Cmcm | Orthorhombic, Cmcm | Tetragonal, P4/mbm | Tetragonal, P4/mbm |
Temperature (K) | 813 | 803 | 888 | 878 |
a, b, c (Å) | 7.85758 (3), 7.86793 (3), 7.8765 (3) | 7.83729 (4), 7.84887 (4), 7.85728 (4) | 5.56896 (2), 5.56896 (2), 3.94408 (2) | 5.55465 (2), 5.55465 (2), 3.93426 (2) |
α, β, γ (°) | 90, 90, 90 | 90, 90, 90 | 90, 90, 90 | 90, 90, 90 |
V (Å3) | ? | ? | ? | ? |
Z | 8 | 8 | 2 | 2 |
Radiation type | Neutron, λ = ? Å | Neutron, λ = ? Å | Neutron, λ = ? Å | Neutron, λ = ? Å |
µ (mm−1) | ? | ? | ? | ? |
Crystal size (mm) | × × | × × | × × | × × |
|
Data collection |
Diffractometer | ? | ? | ? | ? |
Absorption correction | ? | ? | ? | ? |
No. of measured, independent and observed (?) reflections | ?, ?, ? | ?, ?, ? | ?, ?, ? | ?, ?, ? |
Rint | ? | ? | ? | ? |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | ?, ?, ? | ?, ?, ? | ?, ?, ? | ?, ?, ? |
No. of reflections | ? | ? | ? | ? |
No. of parameters | ? | ? | ? | ? |
No. of restraints | ? | ? | ? | ? |
Δρmax, Δρmin (e Å−3) | ?, ? | ?, ? | ?, ? | ?, ? |