Structure of Na₃NdSi₆O₁₅.2H₂O – a Layered Silicate with Paths for Possible Fast-Ion Conduction

Abstract

Hydrothermal investigations of the high silica region of the Na20-Nd203-SiO2 system, carried out in the search for new fast-ion conductors (FIC's), yielded the compound Na3NdSi6Ols.2H20 (sodium neodymium silicate). Single-crystal X-ray methods provided lattice constants of a=7.385(2), b=30.831(7) and c= 7.1168 (13) /~,, space group Cmm2, and 22 atoms in the asymmetric unit. With four formula units per unit cell, the calculated density is 2.68gcm -3. Refinement was carried out with 1113 independent structure factors to a weighted residual wR(F) of 2.63% [8.09% for wR(F2)] using anisotropic temperature factors for all atoms. The structure, based on corrugated Si6015 layers containing four-, five-, six-and eightmembered rings, is related to that of a model previously reported for a compound assigned the composition NaNdSi6013(OH)2.nH20. Our structure differs in the placement of sodium ions and water molecules, and contains no hydroxyl groups. We believe that both studies examined the same phase.

Comparison of the crystal chemistry of selected MSi6O15-based silicates

“…Crystallographic data are provided in Table 1. Complete structure determinations, reported elsewhere (Haile et al 1995(Haile et al , 1997Haile and Wuensch, unpublished manuscript), confirmed the ideal compositions deduced from the microprobe and thermogravimetric analyses.…”

“…† Haile and Wuensch, to be published. ‡ Haile et al (1997). Related to a phase reported by Karpov et al (1977).…”

“…As might be expected, the shared edge is shortened relative to other O-O distances in the structure, whereas the distances between the cations and the bridging O atoms are elongated. The increase in bond lengths results in a bond valence sum of 2.06 at the bridging O atom, which is only slightly greater than the average of 1.96(12) in the Na 3 Nd structure (Haile et al 1997).…”

Mixed Alkali Systems: Structure and 29Si MASNMR of Li2Si2O5 and K2Si2O5

“…Liebau, 1985;Haile et al, 1997) 4.1.2. Alkali coordination and position with respect to the silica sheet.…”

Two Structure Types Based on Si6O15 Rings: Synthesis and Structural and Spectroscopic Characterisation of Cs1.86K1.14DySi6O15 and Cs1.6K1.4SmSi6O15

“…Their various crystal structures and high physical‐chemical stability are advantageous for uses in many fields of application. To the family of the alkali‐ion conductors belong the silicates LiYSiO 4 ,3 K 8 Nd 3 (Si 12 O 32 )(OH),4 Na 3 Nd(Si 6 O 15 )(H 2 O) 2 ,5 K 8 Nd 3 Si 12 O 32 (OH)6 and α‐K 3 Nd(Si 6 O 15 )(H 2 O) 2 7. Among tuneable optical materials a large number of silicates are known: K 7 Eu 3 Si 12 O 32 (H 2 O) 4 , K 7 Sm 3 Si 12 O 32 (H 2 O) 3 ,8 Na 3 LaSi 6 O 15 (H 2 O) 2.5 , Na 4.92 Sm 2.36 Si 12 O 30 (H 2 O) 4.88 ,9 K 3 TbSi 3 O 9 ,10 K 3 TbSi 3 O 8 (OH) 2 , K 3 ErSi 3 O 8 (OH) 2 , K 3 EuSi 3 O 8 (OH) 2 ,11 Na 3 ErSi 3 O 9 ,12 Na 3 TbSi 3 O 9 (H 2 O) 3 ,13 (Eu 8.18 Na 1.08 )(SiO 4 ) 6 (OH) 1.62 ,14 Na 2.4 Ce(Si 2 O 5 ) 3 (H 2 O) 2 15 and Eu‐, Er‐, Nd‐, Tb‐AV‐9‐type Na 4 K 2 Ln 2 Si 16 O 38 · 10 H 2 O 16–18.…”