Structural study of K_xNa_1 - xNbO₃ (KNN) for compositions in the range x = 0.24-0.36

The structure of the A-site substituted perovskite K_xNa_1 - xNbO₃, x = 0.24-0.36, where a phase boundary was previously reported, has been determined by high-resolution X-ray powder and neutron powder diffraction studies. The structure of the x = 0.3 compound was refined in the monoclinic space group Pm at 293 K and in P4mm at 523 K. The Glazer tilt system of the room-temperature monoclinic phase is a⁰b⁺c⁰, which has implications for the nature of the next symmetry change with composition towards pure potassium niobate. A phase-coexistence region at the transition between monoclinic and tetragonal phases was also identified, consistent with a first-order phase boundary. There is also evidence for an intermediate oxygen-octahedra tilted tetragonal phase.

Computing details

Figures

(293K)

Crystal data

K0.3Na0.7NbO3	V = 124.53 (1) Å^3
Mr = 168.73	Z = 2
Monoclinic, Pm	F(000) = 156.8
Hall symbol: P -2y	Dx = 4.5 Mg m^−3
a = 5.64304 (5) Å	Neutron radiation, λ = ? Å
b = 3.93188 (2) Å	T = 293 K
c = 5.61260 (5) Å	× × mm
β = 89.9136 (6)°

Data collection

HRPD diffractometer	k = ?→?
Radiation source: spallation	l = ?→?
h = ?→?

Refinement

S = 0.65
42 parameters

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Na1	0	0	0	0.005 (3)	0.7
Na2	0.467 (4)	0	0.456 (5)	0.016 (3)	0.7
K1	0	0	0	0.005 (3)	0.3
K2	0.467 (4)	0	0.456 (5)	0.016 (3)	0.3
Nb1	−0.020 (2)	0.5	0.505 (5)	0.0081 (19)
Nb2	0.478 (2)	0.5	0.004 (5)	0.0077 (18)
O1	0.156 (3)	0.5	0.758 (5)	0.030 (4)
O2	0.687 (3)	0.5	0.728 (4)	0.015 (4)
O3	0.718 (3)	0.5	0.231 (5)	0.010 (2)
O4	0.193 (3)	0.5	0.240 (8)	0.017 (5)
O5	0.460 (3)	0	−0.027 (5)	0.012 (3)
O6	−0.071 (3)	0	0.516 (5)	0.007 (2)

Geometric parameters (Å, º)

Nb1—O3^i	2.14 (3)	Nb2—O1^iii	2.29 (3)
Nb1—O1	1.73 (3)	Nb2—O2^iii	1.95 (3)
Nb1—O2^i	2.07 (3)	Nb2—O3	1.86 (3)
Nb1—O6	1.988 (3)	Nb2—O4	2.08 (4)
Nb1—O6^ii	1.988 (3)	Nb2—O5	1.977 (4)
Nb1—O4	1.92 (4)	Nb2—O5^ii	1.977 (4)
O2^i—Nb1—O4	166.0 (16)	O1^iii—Nb2—O3	174.0 (15)
O6—Nb1—O6^ii	163.1 (12)	O2^iii—Nb2—O4	166.6 (15)
O1—Nb1—O3^i	171.2 (14)	O5—Nb2—O5^ii	168.1 (19)

Symmetry codes: (i) x−1, y, z; (ii) x, y+1, z; (iii) x, y, z−1.

(523K)

Crystal data

K0.3Na0.7NbO3	Z = 1
Mr = 168.73	F(000) = 78.4
Tetragonal, P4mm	Dx = 4.478 Mg m^−3
Hall symbol: P 4 -2	Neutron radiation, λ = ? Å
a = 3.95092 (1) Å	T = 523 K
c = 4.00852 (2) Å	× × mm
V = 62.57 (1) Å^3

Data collection

HRPD diffractometer	k = ?→?
Radiation source: spallation	l = ?→?
h = ?→?

Refinement

S = 0.53
24 parameters

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Na1	0	0	0	0.0357	0.7
K1	0	0	0	0.0357	0.3
Nb1	0.5	0.5	0.492 (11)	0.0114
O1	0.5	0.5	0.038 (10)	0.0241
O2	0.5	0	0.539 (10)	0.0221

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Na1	0.0307 (11)	0.0307 (11)	0.046 (3)	0	0	0
K1	0.0307 (11)	0.0307 (11)	0.046 (3)	0	0	0
Nb1	0.0101 (4)	0.0101 (4)	0.014 (3)	0	0	0
O1	0.0248 (8)	0.0248 (8)	0.023 (3)	0	0	0
O2	0.0175 (7)	0.0175 (7)	0.031 (3)	−0.0014 (6)	0	0

Geometric parameters (Å, º)

Nb1—O1	1.82 (6)	Nb1—O2	1.984 (6)
O1^i—Nb1—O1	180	O2—Nb1—O2^iv	169 (3)
O2^ii—Nb1—O2^iii	169 (3)

Symmetry codes: (i) x, y, z+1; (ii) −y, x, z; (iii) −y+1, x, z; (iv) x, y+1, z.