Structure, phase transitions and ionic conductivity of K₃NdSi₆O₁₅·xH₂O. I. α-K₃NdSi₆O₁₅·2H₂O and its polymorphs

Hydrothermally grown crystals of α-K₃NdSi₆O₁₅·2H₂O, potassium neodymium silicate, have been studied by single-crystal X-ray methods. The compound crystallizes in space group Pbam, contains four formula units per unit cell and has lattice constants a = 16.008 (2), b = 15.004 (2) and c = 7.2794 (7) Å, giving a calculated density of 2.683 Mg m⁻³. Refinement was carried out with 2161 independent structure factors to a residual, R(F), of 0.0528 [wR(F²) = 0.1562] using anisotropic temperature factors for all atoms other than those associated with water molecules. The structure is based on highly corrugated (Si₂O₅²⁻)_∞ layers which can be generated by the condensation of xonotlite-like ribbons, which can, in turn, be generated by the condensation of wollastonite-like chains. The silicate layers are connected by Nd octahedra to form a three-dimensional framework. Potassium ions and water molecules are located in interstitial sites within this framework, in particular, within channels that extend along [001]. Aging of as-grown crystals at room temperature for periods of six months or more results in an ordering phenomenon that causes the length of the c axis to double. In addition, two phase transitions were found to occur upon heating. The high-temperature transformations, investigated by differential scanning calorimetry, thermal gravimetric analysis and high-temperature X-ray diffraction, are reversible, suggesting displacive transformations in which the layers remain intact. Conductivity measurements along all three crystallographic axes showed the conductivity to be greatest along [001] and further suggest that the channels present in the room-temperature structure are preserved at high temperatures so as to serve as pathways for easy ion transport. Ion-exchange experiments revealed that silver can readily be incorporated into the structure.

Computing details

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

Figures

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Crystal data

H4K3NdSi6O17	Z = 4
Mr = 706.1	F(000) = 1348
Orthorhombic, Pbam	Dx = 2.683 Mg m^−3
Hall symbol: -P 2 2ab	Mo Kα radiation, λ = 0.71073 Å
a = 16.008 (2) Å	Cell parameters from 20 reflections
b = 15.004 (2) Å	µ = 4.18 mm^−1
c = 7.2794 (7) Å	T = 293 K
V = 1748.4 (3) Å^3	0.5 × 0.4 × 0.2 mm

Data collection

Siemens R3m-V diffractometer	Rint = 0.020
Absorption correction: ψ scan (empirical (using intensity measurements)) ?	θmax = 27.5°, θmin = 1.9°
Tmin = 0.061, Tmax = 0.102	h = 0→20
2579 measured reflections	k = 0→19
2173 independent reflections	l = −9→9
1897 reflections with I ≥ 2σ

Refinement

Refinement on F^2	1 restraint
Least-squares matrix: Full-matrix least-squares	H atoms not located
R[F^2 > 2σ(F^2)] = 0.053	w = 1/[σ^2Fo^2 + (0.0816P)^2 + 30.48P] where P = [max(Fo^2,0) + 2Fc^2]/3
wR(F^2) = 0.156
S = 1.02	Δρmax = 2.49 e Å^−3
2161 reflections	Δρmin = −1.77 e Å^−3
144 parameters

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

	x	y	z	Biso*/Beq
Nd(1)	0	0	0
Nd(2)	0.5000	0	0
K(1)	0.2910 (3)	0.3785 (3)	0
K(2)	0.1556 (2)	0.1555 (3)	0.5000
K(3)	0.0699 (2)	0.4105 (2)	0.5000
Si(1)	0.21036 (14)	0.1268 (2)	0
Si(2)	0.35548 (10)	0.13505 (11)	0.2862 (2)
Si(3)	0.0912 (2)	0.2806 (2)	0
Si(4)	0.46288 (11)	0.29877 (11)	0.2821 (3)
O(1)	0.0429 (4)	0.2459 (3)	0.1828 (8)
O(2)	0.1018 (5)	0.3859 (5)	0
O(3)	0.1822 (4)	0.2318 (4)	0
O(4)	0.1346 (4)	0.0599 (4)	0
O(5)	0.2689 (3)	0.1134 (3)	0.1799 (7)
O(6)	0.3264 (4)	0.1335 (5)	0.5000
O(7)	0.4289 (3)	0.0679 (3)	0.2459 (8)
O(8)	0.3803 (3)	0.2390 (3)	0.2397 (8)
O(9)	0.4477 (3)	0.3984 (3)	0.2213 (8)
O(10)	0.4846 (5)	0.2876 (5)	0.5000
Ow(1)1	0.3659 (10)	0.4830 (10)	0.5000
Ow(2)2	0.2281 (13)	0.3284 (14)	0.421 (3)
Ow(3)2	0.2113 (13)	0.4189 (14)	0.292 (3)