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The paraelectric and the ferroelectric/ferroelastic structures of Pb2KNb5O15 have been refined by the Rietveld method from neutron and X-ray powder diffraction data. The paraelectric phase is tetragonal (space group P4/mbm) with lattice parameters at = 12.646 (1) and ct = 3.9551 (2) Å at 790 K. The Pb- and K-atom distribution has been determined. It was established that the Pb atoms situated on the 15-coordinate sites are not on the special position (2mm) but are distributed over two disordered positions as in the tetragonal phase of Pb2KTa5O15. The ferroelectric/ferroelastic phase is orthorhombic. The space group of the average structure is Cm2m and the lattice parameters are ao = 17.779 (2), bo = 18.015 (2), co = 3.9209 (4) Å at 294 K and ao = 17.756 (1), bo = 18.019 (1), co = 3.9141 (2) Å at 81 K. It is isostructural with the average ferroelectric/ferroelastic structure of PbNb2O6. The orthorhombic distortion is of the same order but the Curie temperature is lowered by the substitution 2K+-Pb2+. The structures of PbNb2O6, PbTa2O6, Pb2KNb5O15 and Pb2KTa5O15 are compared. The ferroelectric polarization of the niobates is due to both the Pb off-site position and the Nb displacements, which induce a distortion of the oxygen lattice. In PbTa2O6, the oxygen-lattice distortion is very weak. The polarization seems to be due only to the Pb atom. In the tantalates the substitution 2K+-Pb2+ should weaken the correlations between the Pb atoms and consequently suppress the ferroelectric transition. Only a local polarization should take place; this might be the origin of the dielectric relaxation observed in this compound.

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cif

Crystallographic Information File (CIF)
Contains datablocks global, 790X, 790N, 294N, 81X

rtv

Rietveld powder data file (CIF format)
Contains datablock {vcif WARNING: Line 0: length exceeds internal buffer size (4096) - rest discarded}

rtv

Rietveld powder data file (CIF format)
Contains datablock 790N

rtv

Rietveld powder data file (CIF format)
Contains datablock {vcif WARNING: Line 0: length exceeds internal buffer size (4096) - rest discarded}

rtv

Rietveld powder data file (CIF format)
Contains datablock 294N

Computing details top

Figures top
[Figure 1]
[Figure 2]
[Figure 3]
[Figure 4]
(790X) top
Crystal data top
KNb5O15Pb2V = ? Å3
Mr = ?Z = 2
Tetragonal, P4/mbmCu Kβ radiation, λ = 1.39217 Å
Hall symbol: -P 4 2abT = 790 K
a = 12.646 (1) Å × × mm
c = 3.9551 (2) Å
Data collection top
h = ??l = ??
k = ??
Refinement top
Crystal data top
KNb5O15Pb2V = ? Å3
Mr = ?Z = 2
Tetragonal, P4/mbmCu Kβ radiation
a = 12.646 (1) ÅT = 790 K
c = 3.9551 (2) Å × × mm
Data collection top
Refinement top
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzBiso*/BeqOcc. (<1)
Pb(p)0.1421 (5)0.6900 (4)0.57.5 (3)0.55
K(p)0.1940 (16)0.6940 (16)0.55.0 (8)0.45
Pb(c)000.52.8 (2)0.90
K(c)000.52.8 (2)0.10
Nb(1)00.501.6 (3)
Nb(2)0.0747 (2)0.2139 (2)01.1 (2)
O(1)00.50.52.5 (2)
O(2)0.0047 (11)0.3457 (10)02.5 (2)
O(3)0.0769 (11)0.2065 (11)0.52.5 (2)
O(4)0.2218 (9)0.2782 (9)02.5 (2)
O(5)0.1385 (10)0.0647 (11)02.5 (2)
(790N) top
Crystal data top
KNb5O15Pb2V = ? Å3
Mr = ?Z = 2
Tetragonal, P4/mbmNeutron radiation, λ = 1.985 Å
Hall symbol: -P 4 2abT = 790 K
a = 12.646 (1) Å × × mm
c = 3.9551 (2) Å
Data collection top
h = ??l = ??
k = ??
Refinement top
Crystal data top
KNb5O15Pb2V = ? Å3
Mr = ?Z = 2
Tetragonal, P4/mbmNeutron radiation, λ = 1.985 Å
a = 12.646 (1) ÅT = 790 K
c = 3.9551 (2) Å × × mm
Data collection top
Refinement top
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzBiso*/BeqOcc. (<1)
Pb(p)0.1428 (7)0.6907 (6)0.55.6 (3)0.55
K(p)0.1886 (17)0.6886 (17)0.58.3 (9)0.45
Pb(c)000.52.7 (2)0.90
K(c)000.52.7 (2)0.10
Nb(1)00.502.1 (2)
Nb(2)0.0746 (2)0.2139 (2)01.5 (1)
O(1)00.50.53.3 (2)
O(2)0.0037 (2)0.3449 (2)02.4 (2)
O(3)0.0758 (2)0.2058 (2)0.52.5 (2)
O(4)0.2150 (2)0.2850 (2)02.1 (2)
O(5)0.1400 (2)0.0684 (2)02.2 (2)
Bond lengths (Å) top
Pb(p)—O(2)i2.685 (6)Pb(c)—O(5)2.792 (2)
Pb(p)—O(4)ii2.927 (7)Nb(1)—O(2)1.962 (3)
Pb(p)—O(1)3.013 (8)Nb(1)—O(1)1.977 (1)
Pb(p)—O(3)iii3.058 (8)Nb(2)—O(2)1.930 (4)
Pb(p)—O(3)i3.059 (9)Nb(2)—O(3)1.980 (1)
Pb(p)—O(5)iii3.063 (6)Nb(2)—O(4)1.990 (4)
Pb(p)—O(2)iv3.087 (6)Nb(2)—O(5)2.017 (4)
Pb(p)—O(3)ii3.564 (9)Nb(2)—O(5)vi2.036 (4)
Pb(p)—O(5)ii3.721 (8)Pb(p)—Nb(1)3.604 (7)
Pb(p)—O(3)v3.876 (8)Pb(p)—Nb(2)i3.595 (8)
K(p)—O(4)ii2.624 (14)Pb(p)—Nb(2)iii3.678 (7)
K(p)—O(3)iii2.99 (2)K(p)—Nb(1)3.910 (19)
K(p)—O(2)i3.091 (17)K(p)—Nb(2)iii3.603 (18)
K(p)—O(5)iii3.304 (17)Pb(c)—Nb(2)3.481 (2)
K(p)—O(1)3.37 (2)Pb(p)—Pb(p)0.86 (2)
K(p)—O(3)i3.60 (2)Pb(p)—K(p)0.58 (3)
Pb(c)—O(3)2.773 (3)
Symmetry codes: (i) x, y+1, z; (ii) x+1/2, y+1/2, z; (iii) y, x+1, z; (iv) y1/2, x1/2, z; (v) y+1/2, x+1/2, z; (vi) y, x, z.
(294N) top
Crystal data top
KNb5O15Pb2c = 3.9209 (4) Å
Mr = ?V = ? Å3
Orthorhombic, Cm2mZ = 4
Hall symbol: C -2 -2Neutron radiation, λ = 1.985 Å
a = 17.779 (2) ÅT = 294 K
b = 18.015 (2) Å × × mm
Data collection top
h = ??l = ??
k = ??
Refinement top
Crystal data top
KNb5O15Pb2V = ? Å3
Mr = ?Z = 4
Orthorhombic, Cm2mNeutron radiation, λ = 1.985 Å
a = 17.779 (2) ÅT = 294 K
b = 18.015 (2) Å × × mm
c = 3.9209 (4) Å
Data collection top
Refinement top
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzBiso*/Beq
K(p1)0.326 (2)0.006 (4)0.51.1 (2)
K(p2)00.145 (4)0.51.1 (2)
K(p3)0.50.321 (4)0.51.1 (2)
Pb(p1)0.3257 (7)0.0435 (8)0.51.1 (2)
Pb(p2)0.0133 (16)0.1965 (13)0.51.1 (2)
Pb(p3)0.4725 (13)0.3481 (15)0.51.1 (2)
Pb/K(c)0.2560 (7)0.2565 (7)0.51.6 (2)
Nb(1)0001.8 (2)
Nb(2)00.4996 (14)01.8 (2)
Nb(3)0.1771 (6)0.1114 (7)01.0 (1)
Nb(4)0.3191 (6)0.3983 (7)01.0 (1)
Nb(5)0.3937 (6)0.1863 (6)01.0 (1)
Nb(6)0.1041 (7)0.3293 (7)01.0 (1)
O(1)00.0138 (10)0.50.8 (1)
O(2)0.50.0088 (13)0.50.8 (1)
O(3)0.1802 (7)0.1013 (8)0.50.8 (1)
O(4)0.3075 (7)0.3857 (9)0.50.8 (1)
O(5)0.3922 (6)0.1753 (8)0.50.8 (1)
O(6)0.1142 (7)0.3047 (8)0.50.8 (1)
O(7)0.0784 (10)0.0736 (8)01.5 (1)
O(8)0.1401 (8)0.2126 (9)01.5 (1)
O(9)0.2154 (5)0.0018 (10)01.5 (1)
O(10)0.2845 (9)0.1419 (7)01.5 (1)
O(11)0.4191 (10)0.0706 (8)01.5 (1)
O(12)0.50.2050 (11)01.5 (1)
O(13)00.2794 (10)01.5 (1)
O(14)0.0802 (8)0.4164 (10)01.5 (1)
O(15)0.2147 (8)0.3517 (9)01.5 (1)
O(16)0.3481 (8)0.2828 (9)01.5 (1)
O(17)0.4225 (8)0.4156 (9)01.5 (1)
Bond lengths (Å) top
K(p1)—O(9)2.78 (3)Nb(1)—O(17)vi2.052 (1)
K(p1)—O(11)2.91 (4)Nb(2)—O(11)vii1.93 (2)
K(p1)—O(14)i2.93 (4)Nb(2)—O(2)iii1.966 (3)
K(p1)—O(4)ii3.07 (6)Nb(2)—O(14)2.07 (2)
K(p2)—O(7)2.73 (4)Nb(3)—O(7)1.882 (18)
K(p2)—O(1)2.86 (8)Nb(3)—O(8)1.94 (2)
K(p3)—O(12)2.87 (6)Nb(3)—O(3)1.970 (2)
K(p3)—O(17)2.94 (8)Nb(3)—O(10)1.99 (2)
K(p3)—O(1)iii2.98 (8)Nb(3)—O(9)2.089 (18)
Pb(p1)—O(11)2.62 (1)Nb(4)—O(17)1.865 (11)
Pb(p1)—O(5)2.65 (2)Nb(4)—O(9)iii1.963 (18)
Pb(p1)—O(10)2.74 (2)Nb(4)—O(4)1.984 (3)
Pb(p1)—O(3)2.79 (2)Nb(4)—O(15)2.037 (18)
Pb(p1)—O(9)2.87 (2)Nb(4)—O(16)2.14 (2)
Pb(p2)—O(13)2.48 (2)Nb(5)—O(16)1.92 (2)
Pb(p2)—O(6)iv2.65 (3)Nb(5)—O(12)1.920 (11)
Pb(p2)—O(6)2.99 (3)Nb(5)—O(5)1.971 (2)
Pb(p2)—O(8)iv3.00 (3)Nb(5)—O(10)2.10 (2)
Pb(p3)—O(17)2.47 (2)Nb(5)—O(11)2.13 (2)
Pb(p3)—O(1)iii2.54 (3)Nb(6)—O(14)1.63 (2)
Pb(p3)—O(17)v2.97 (2)Nb(6)—O(15)2.007 (19)
Pb/K(c)—O(4)2.50 (2)Nb(6)—O(6)2.018 (5)
Pb/K(c)—O(16)2.60 (2)Nb(6)—O(13)2.06 (2)
Pb/K(c)—O(6)2.67 (2)Nb(6)—O(8)2.20 (2)
Pb/K(c)—O(15)2.71 (2)K(p1)—Pb(p1)0.89 (7)
Pb/K(c)—O(5)2.83 (2)K(p2)—Pb(p2)0.96 (7)
Pb/K(c)—O(10)2.89 (2)K(p3)—Pb(p3)0.69 (7)
Pb/K(c)—O(8)2.95 (2)Pb(p2)—Pb(p2)iv0.47 (6)
Nb(1)—O(7)1.924 (15)Pb(p3)—Pb(p3)v0.98 (5)
Nb(1)—O(1)1.976 (2)
Symmetry codes: (i) x+1/2, y1/2, z; (ii) x+1/2, y1/2, z; (iii) x+1/2, y+1/2, z; (iv) x, y, z+1; (v) x+1, y, z; (vi) x1/2, y1/2, z; (vii) x1/2, y+1/2, z.
(81X) top
Crystal data top
KNb5O15Pb2c = 3.9141 (2) Å
Mr = ?V = ? Å3
Orthorhombic, Cm2mZ = 4
Hall symbol: C -2 -2Cu Kβ radiation, λ = 1.39217 Å
a = 17.756 (1) ÅT = 81 K
b = 18.019 (1) Å × × mm
Data collection top
h = ??l = ??
k = ??
Refinement top
Crystal data top
KNb5O15Pb2V = ? Å3
Mr = ?Z = 4
Orthorhombic, Cm2mCu Kβ radiation
a = 17.756 (1) ÅT = 81 K
b = 18.019 (1) Å × × mm
c = 3.9141 (2) Å
Data collection top
Refinement top
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzBiso*/Beq
K(p1)0.329 (3)0.010 (3)0.51.2 (1)
K(p2)00.160 (4)0.51.2 (1)
K(p3)0.50.358 (4)0.51.2 (1)
Pb(p1)0.3263 (4)0.0356 (5)0.51.2 (1)
Pb(p2)0.0116 (9)0.1993 (6)0.51.2 (1)
Pb(p3)0.4725 (6)0.3395 (8)0.51.2 (1)
Pb/K(c)0.2508 (4)0.2526 (5)0.50.8 (1)
Nb(1)0000.3 (1)
Nb(2)00.5009 (9)00.3 (1)
Nb(3)0.1791 (4)0.1087 (6)00.3 (1)
Nb(4)0.3182 (5)0.3979 (6)00.3 (1)
Nb(5)0.3937 (5)0.1815 (6)00.3 (1)
Nb(6)0.1066 (4)0.3234 (6)00.3 (1)
O(1)00.021 (3)0.50.4 (3)
O(2)0.50.014 (3)0.50.4 (3)
O(3)0.194 (3)0.104 (3)0.50.4 (3)
O(4)0.324 (3)0.375 (3)0.50.4 (3)
O(5)0.402 (3)0.182 (3)0.50.4 (3)
O(6)0.121 (3)0.304 (3)0.50.4 (3)
O(7)0.079 (3)0.074 (3)00.4 (3)
O(8)0.181 (3)0.220 (2)00.4 (3)
O(9)0.214 (2)0.005 (4)00.4 (3)
O(10)0.289 (3)0.138 (3)00.4 (3)
O(11)0.416 (3)0.061 (3)00.4 (3)
O(12)0.50.210 (4)00.4 (3)
O(13)00.275 (3)00.4 (3)
O(14)0.071 (3)0.424 (3)00.4 (3)
O(15)0.210 (3)0.336 (3)00.4 (3)
O(16)0.357 (3)0.275 (3)00.4 (3)
O(17)0.425 (3)0.411 (3)00.4 (3)

Experimental details

(790X)(790N)(294N)(81X)
Crystal data
Chemical formulaKNb5O15Pb2KNb5O15Pb2KNb5O15Pb2KNb5O15Pb2
Mr????
Crystal system, space groupTetragonal, P4/mbmTetragonal, P4/mbmOrthorhombic, Cm2mOrthorhombic, Cm2m
Temperature (K)79079029481
a, b, c (Å)12.646 (1), 12.646 (1), 3.9551 (2)12.646 (1), 12.646 (1), 3.9551 (2)17.779 (2), 18.015 (2), 3.9209 (4)17.756 (1), 18.019 (1), 3.9141 (2)
α, β, γ (°)?, ?, ??, ?, ??, ?, ??, ?, ?
V3)????
Z2244
Radiation typeCu KβNeutron, λ = 1.985 ÅNeutron, λ = 1.985 ÅCu Kβ
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)?, ??, ??, ??, ?

Selected bond lengths (Å) for (790N) top
Pb(p)—O(2)i2.685 (6)Pb(c)—O(5)2.792 (2)
Pb(p)—O(4)ii2.927 (7)Nb(1)—O(2)1.962 (3)
Pb(p)—O(1)3.013 (8)Nb(1)—O(1)1.977 (1)
Pb(p)—O(3)iii3.058 (8)Nb(2)—O(2)1.930 (4)
Pb(p)—O(3)i3.059 (9)Nb(2)—O(3)1.980 (1)
Pb(p)—O(5)iii3.063 (6)Nb(2)—O(4)1.990 (4)
Pb(p)—O(2)iv3.087 (6)Nb(2)—O(5)2.017 (4)
Pb(p)—O(3)ii3.564 (9)Nb(2)—O(5)vi2.036 (4)
Pb(p)—O(5)ii3.721 (8)Pb(p)—Nb(1)3.604 (7)
Pb(p)—O(3)v3.876 (8)Pb(p)—Nb(2)i3.595 (8)
K(p)—O(4)ii2.624 (14)Pb(p)—Nb(2)iii3.678 (7)
K(p)—O(3)iii2.99 (2)K(p)—Nb(1)3.910 (19)
K(p)—O(2)i3.091 (17)K(p)—Nb(2)iii3.603 (18)
K(p)—O(5)iii3.304 (17)Pb(c)—Nb(2)3.481 (2)
K(p)—O(1)3.37 (2)Pb(p)—Pb(p)0.86 (2)
K(p)—O(3)i3.60 (2)Pb(p)—K(p)0.58 (3)
Pb(c)—O(3)2.773 (3)
Symmetry codes: (i) x, y+1, z; (ii) x+1/2, y+1/2, z; (iii) y, x+1, z; (iv) y1/2, x1/2, z; (v) y+1/2, x+1/2, z; (vi) y, x, z.
Selected bond lengths (Å) for (294N) top
K(p1)—O(9)2.78 (3)Nb(1)—O(17)vi2.052 (1)
K(p1)—O(11)2.91 (4)Nb(2)—O(11)vii1.93 (2)
K(p1)—O(14)i2.93 (4)Nb(2)—O(2)iii1.966 (3)
K(p1)—O(4)ii3.07 (6)Nb(2)—O(14)2.07 (2)
K(p2)—O(7)2.73 (4)Nb(3)—O(7)1.882 (18)
K(p2)—O(1)2.86 (8)Nb(3)—O(8)1.94 (2)
K(p3)—O(12)2.87 (6)Nb(3)—O(3)1.970 (2)
K(p3)—O(17)2.94 (8)Nb(3)—O(10)1.99 (2)
K(p3)—O(1)iii2.98 (8)Nb(3)—O(9)2.089 (18)
Pb(p1)—O(11)2.62 (1)Nb(4)—O(17)1.865 (11)
Pb(p1)—O(5)2.65 (2)Nb(4)—O(9)iii1.963 (18)
Pb(p1)—O(10)2.74 (2)Nb(4)—O(4)1.984 (3)
Pb(p1)—O(3)2.79 (2)Nb(4)—O(15)2.037 (18)
Pb(p1)—O(9)2.87 (2)Nb(4)—O(16)2.14 (2)
Pb(p2)—O(13)2.48 (2)Nb(5)—O(16)1.92 (2)
Pb(p2)—O(6)iv2.65 (3)Nb(5)—O(12)1.920 (11)
Pb(p2)—O(6)2.99 (3)Nb(5)—O(5)1.971 (2)
Pb(p2)—O(8)iv3.00 (3)Nb(5)—O(10)2.10 (2)
Pb(p3)—O(17)2.47 (2)Nb(5)—O(11)2.13 (2)
Pb(p3)—O(1)iii2.54 (3)Nb(6)—O(14)1.63 (2)
Pb(p3)—O(17)v2.97 (2)Nb(6)—O(15)2.007 (19)
Pb/K(c)—O(4)2.50 (2)Nb(6)—O(6)2.018 (5)
Pb/K(c)—O(16)2.60 (2)Nb(6)—O(13)2.06 (2)
Pb/K(c)—O(6)2.67 (2)Nb(6)—O(8)2.20 (2)
Pb/K(c)—O(15)2.71 (2)K(p1)—Pb(p1)0.89 (7)
Pb/K(c)—O(5)2.83 (2)K(p2)—Pb(p2)0.96 (7)
Pb/K(c)—O(10)2.89 (2)K(p3)—Pb(p3)0.69 (7)
Pb/K(c)—O(8)2.95 (2)Pb(p2)—Pb(p2)iv0.47 (6)
Nb(1)—O(7)1.924 (15)Pb(p3)—Pb(p3)v0.98 (5)
Nb(1)—O(1)1.976 (2)
Symmetry codes: (i) x+1/2, y1/2, z; (ii) x+1/2, y1/2, z; (iii) x+1/2, y+1/2, z; (iv) x, y, z+1; (v) x+1, y, z; (vi) x1/2, y1/2, z; (vii) x1/2, y+1/2, z.
 
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