Structure, phase transitions and ionic conductivity of K₃NdSi₆O₁₅·xH₂O. II. Structure of β-K₃NdSi₆O₁₅

Hydrothermally grown crystals of β-K₃NdSi₆O₁₅, potassium neodymium silicate, have been studied by single-crystal X-ray methods. Under appropriate conditions, the compound crystallizes in space group Bb2₁m and has lattice constants a = 14.370 (2), b = 15.518 (2) and c = 14.265 (2) Å. There are 30 atom sites in the asymmetric unit of the basic structure. With eight formula units per unit cell, the calculated density is 2.798 Mg m⁻³. Refinement was carried out to a residual, wR(F²), of 0.1177 [R(F) = 0.0416] using anisotropic temperature factors for all atoms. The structure is based on (Si₂O₅²⁻)_∞ layers, connected by Nd polyhedra to form a three-dimensional framework. Potassium ion sites, some of which are only partially occupied, are located within channels that run between the silicate layers. The silica–neodymia framework of β-K₃NdSi₆O₁₅, in particular the linkages formed between the silicate layers and Nd polyhedra, bears some similarities to that of the essentially isocompositional phase α-K₃NdSi₆O₁₅·2H₂O. In both, the silicate layers are corrugated so as to accommodate a simple cubic array of NdO₆ octahedra with lattice constant ∼ 7.5 Å. Furthermore, the Si₂O₅ layers in β-K₃NdSi₆O₁₅ are topologically identical to those of the mineral sazhinite, Na₂HCeSi₆O₁₅. Although β-K₃NdSi₆O₁₅ and sazhinite are not isostructural, the structures of each can be described as slight distortions of a high-symmetry parent structure with space group Pbmm.

Computing details

Program(s) used to refine structure: SHELXL (Sheldrick, 1993).

Figures

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Crystal data

K3NdSi6O15	Z = 8
Mr = 670.0	F(000) = 2520
Orthorhombic, Bb21m	Dx = 2.798 Mg m^−3
Hall symbol: B -2 -2b	Mo Kα radiation, λ = 0.71073 Å
a = 14.370 (2) Å	Cell parameters from 22 reflections
b = 15.518 (2) Å	µ = 4.31 mm^−1
c = 14.265 (2) Å	T = 293 K
V = 3181.0 (7) Å^3	0.3 × 0.3 × 0.2 mm

Data collection

Siemens R3m-V diffractometer	θmax = 27.5°, θmin = 2.0°
Absorption correction: ψ scan (empirical (using intensity measurements) ?	h = 0→18
Tmin = 0.057, Tmax = 0.119	k = 0→20
1975 measured reflections	l = 0→18
1975 independent reflections	69 standard reflections
1974 reflections with I > 2σ	intensity decay: min 0.9887, max 1.0353, final 1.0160, mean 1.0137%
Rint = 0

Refinement

Refinement on F^2	1 restraint
Least-squares matrix: Full-matrix least-squares	w = 1/[σ^2Fo^2 + (0.0746P)^2 + 44.0045P] where P = [max(Fo^2,0) + 2Fc^2]/3
R[F^2 > 2σ(F^2)] = 0.042	(Δ/σ)max = 0.177
wR(F^2) = 0.117	Δρmax = 2.59 e Å^−3
S = 1.13	Δρmin = −1.76 e Å^−3
1974 reflections	Absolute structure: Flack, 1983)
244 parameters	Absolute structure parameter: 0.10 (3)

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

	x	y	z	Biso*/Beq
Nd(1)	0.49154 (3)	0.57468 (6)	0.76341 (3)
Si(1)	0.3846 (2)	0.7537 (2)	0.6119 (2)
Si(2)	0.6157 (2)	0.4554 (2)	0.6051 (2)
Si(3)	0.3352 (2)	0.7146 (2)	0.8969 (2)
Si(4)	0.7796 (2)	0.4854 (2)	0.7495 (2)
Si(5)	0.6511 (2)	0.4129 (2)	0.8930 (2)
Si(6)	0.7284 (2)	0.6711 (2)	0.7837 (2)
K(1)	0.4616 (2)	0.3182 (2)	0.7854 (3)
K(2)	0.5754 (3)	0.6302 (4)	0
K(3)	0.8658 (4)	0.4920 (3)	0.5000
K(4)	0.8725 (3)	0.5194 (4)	0
K(5)	0.5896 (3)	0.6691 (3)	0.5000
K(6)	0.8463 (10)	0.3053 (11)	0.5000
O(1)	0.7874 (5)	0.5821 (6)	0.7990 (5)
O(2)	0.4057 (6)	0.8563 (5)	0.6299 (6)
O(3)	0.3788 (6)	0.8116 (5)	0.8869 (5)
O(4)	0.6974 (7)	0.4919 (7)	0.6720 (6)
O(5)	0.6849 (8)	0.4354 (8)	0
O(6)	0.8790 (6)	0.4633 (5)	0.7096 (7)
O(7)	0.7856 (8)	0.7081 (7)	0.5000
O(8)	0.9570 (6)	0.6962 (5)	0.8319 (6)
O(9)	0.1215 (5)	0.6630 (6)	0.6951 (6)
O(10)	0.8906 (9)	0.7351 (9)	0
O(11)	0.7482 (6)	0.4201 (6)	0.8321 (6)
O(12)	0.4142 (5)	0.6453 (5)	0.1137 (6)
O(13)	0.6607 (8)	0.4602 (9)	0.5000
O(14)	0.5710 (6)	0.4750 (5)	0.8597 (7)
O(15)	0.2239 (6)	0.2381 (6)	0.8582 (6)
O(16)	0.2509 (6)	0.2046 (6)	0.6766 (6)
O(17)	0.5265 (7)	0.5127 (6)	0.6165 (6)