High-temperature phases of NaNbO₃ and NaTaO₃

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 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.

Computing details

Figures

(Nb813)

Crystal data

NaNbO3	c = 7.8765 (3) Å
Mr = ?	V = ? Å^3
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

Data collection

h = ?→?	l = ?→?
k = ?→?

Refinement

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

	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	?

(Ta803)

Crystal data

NaO3Ta	c = 7.85728 (4) Å
Mr = ?	V = ? Å^3
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

Data collection

h = ?→?	l = ?→?
k = ?→?

Refinement

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

	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	?

(Nb888)

Crystal data

NaNbO3	V = ? Å^3
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

Data collection

h = ?→?	l = ?→?
k = ?→?

Refinement

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

	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	?

(Ta878)

Crystal data

NaO3Ta	V = ? Å^3
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

Data collection

h = ?→?	l = ?→?
k = ?→?

Refinement

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

	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	?