Mixed alkali systems: Dietzel’s theorem, X-ray structure, hygroscopicity, and 29-Si MAS NMR of NaRbSi2O5 and NaCsSi2O5

Abstract

NaRbSi2O5, Mr 244.63, λ(MoKα)=0.71073 Å, 150K, monoclinic, P21/c, a=4.8570(10)°, b=13.5403(10)°, c=7.7328(10) Å, β=90.908(10)°, V=508.49(13) Å3, μ(MoKα)=10.23 mm-1, Z=4, F(000)=464, Dx=3.1954(8) g/cm3, R1=0.0899 for 743 reflections with I>2.0σ(I), wR2=0.2425 for all 2534 reflections. NaCsSi2O5, Mr 584.13, λ(MoKα)=0.71073 Å, 150K, orthorhombic, Pna21, a=17.0743(10)°, b=4.9011(10)°, c=13.3391(10) Å, V=1116.3(3) Å3, μ(MoKα)=7.08 mm-1, Z=8, F(000)=1072, Dx=3.4757(9)g/cm3, R1=0.0199 for 1103 reflections with I>2.0σ(I), wR2=0.0465 for all 2668 reflections. The crystal structures of NaRbSi2O5 and NaCsSi2O5 have been determined. Though the nominal composition of both phases is that of a phyllosilicate, their anion topology is similar and consists of double chains of [SiO4] tetrahedra forming 4-membered T rings, i.e. rings consisting of 4 tetrahedra in chair conformation and built from Q3 units. The alkali-oxygen distances in the Na2O-Cs2O-SiO2 system tend to support the notion that in mixed alkali systems the alkali-oxygen distance of the smaller cation decreases at the expense of that of the larger one. The observed very high hygroscopicity of both crystalline phases might be rationalized by substantial underbonding of some of the oxygen atoms. 29-Si spin lattice relaxation times, T1, vary largely between 535 and 493 sec for NaRbSi2O5 and 2440 and 2329 sec for NaCs-Si2O5.