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The high-temperature phases of the perovskites sodium niobate, NaNbO3, and sodium tantalate, NaTaO3, 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 \langle100\rangle 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 NaNbO3 between 753 and 793 K and of NaTaO3 below 758 K are not as reported in the literature.

Supporting information

cif

Crystallographic Information File (CIF)
Contains datablocks global, Nb813, Ta803, Nb888, Ta878

pdf

Portable Document Format (PDF) file
Supplementary material

Computing details top

Figures top
[Figure 1]
[Figure 2]
[Figure 3]
[Figure 4]
[Figure 5]
[Figure 6]
[Figure 7]
[Figure 8]
[Figure 9]
[Figure 10]
[Figure 11]
[Figure 12]
(Nb813) top
Crystal data top
NaNbO3c = 7.8765 (3) Å
Mr = ?V = ? Å3
Orthorhombic, CmcmZ = 8
Hall symbol: -C 2c 2Neutron radiation, λ = ? Å
a = 7.85758 (3) ÅT = 813 K
b = 7.86793 (3) Å × × mm
Data collection top
h = ??l = ??
k = ??
Refinement top
Crystal data top
NaNbO3V = ? Å3
Mr = ?Z = 8
Orthorhombic, CmcmNeutron radiation, λ = ? Å
a = 7.85758 (3) ÅT = 813 K
b = 7.86793 (3) Å × × mm
c = 7.8765 (3) Å
Data collection top
Refinement top
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzBiso*/Beq
Nb0.250.250.01.60 (1)
Na10.00.0022 (14)0.253.56 (12)
Na20.00.5026 (14)0.254.31 (14)
O10.2230 (4)0.00.0?
O20.00.2741 (4)0.0203 (3)?
O30.2710 (3)0.2489 (7)0.25?
(Ta803) top
Crystal data top
NaO3Tac = 7.85728 (4) Å
Mr = ?V = ? Å3
Orthorhombic, CmcmZ = 8
Hall symbol: -C 2c 2Neutron radiation, λ = ? Å
a = 7.83729 (4) ÅT = 803 K
b = 7.84887 (4) Å × × mm
Data collection top
h = ??l = ??
k = ??
Refinement top
Crystal data top
NaO3TaV = ? Å3
Mr = ?Z = 8
Orthorhombic, CmcmNeutron radiation, λ = ? Å
a = 7.83729 (4) ÅT = 803 K
b = 7.84887 (4) Å × × mm
c = 7.85728 (4) Å
Data collection top
Refinement top
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzBiso*/Beq
Ta0.250.250.00.98 (2)
Na10.00.008 (2)0.253.3 (2)
Na20.00.507 (2)0.253.4 (2)
O10.2253 (5)0.00.0?
O20.00.2739 (6)0.0200 (5)?
O30.2737 (5)0.2478 (11)0.25?
(Nb888) top
Crystal data top
NaNbO3V = ? Å3
Mr = ?Z = 2
Tetragonal, P4/mbmNeutron radiation, λ = ? Å
Hall symbol: -P 4 2abT = 888 K
a = 5.56896 (2) Å × × mm
c = 3.94408 (2) Å
Data collection top
h = ??l = ??
k = ??
Refinement top
Crystal data top
NaNbO3V = ? Å3
Mr = ?Z = 2
Tetragonal, P4/mbmNeutron radiation, λ = ? Å
a = 5.56896 (2) ÅT = 888 K
c = 3.94408 (2) Å × × mm
Data collection top
Refinement top
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzBiso*/Beq
Nb0.00.00.01.64 (1)
Na0.00.50.54.26 (3)
O10.00.00.5?
O20.2719 (1)0.2281 (1)0.0?
(Ta878) top
Crystal data top
NaO3TaV = ? Å3
Mr = ?Z = 2
Tetragonal, P4/mbmNeutron radiation, λ = ? Å
Hall symbol: -P 4 2abT = 878 K
a = 5.55465 (2) Å × × mm
c = 3.93426 (2) Å
Data collection top
h = ??l = ??
k = ??
Refinement top
Crystal data top
NaO3TaV = ? Å3
Mr = ?Z = 2
Tetragonal, P4/mbmNeutron radiation, λ = ? Å
a = 5.55465 (2) ÅT = 878 K
c = 3.93426 (2) Å × × mm
Data collection top
Refinement top
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzBiso*/Beq
Ta0.00.00.01.05 (2)
Na0.00.50.53.75 (4)
O10.00.00.5?
O20.2713 (2)0.2287 (2)0.0?

Experimental details

(Nb813)(Ta803)(Nb888)(Ta878)
Crystal data
Chemical formulaNaNbO3NaO3TaNaNbO3NaO3Ta
Mr????
Crystal system, space groupOrthorhombic, CmcmOrthorhombic, CmcmTetragonal, P4/mbmTetragonal, P4/mbm
Temperature (K)813803888878
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, 9090, 90, 9090, 90, 9090, 90, 90
V3)????
Z8822
Radiation typeNeutron, λ = ? ÅNeutron, λ = ? ÅNeutron, λ = ? ÅNeutron, λ = ? Å
µ (mm1)????
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 Å3)?, ??, ??, ??, ?

 

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