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

NaRbSi2O5, M-r 244.63, lambda(Mok(a)) = 0.71073 Angstrom, 150K, monoclinic, P2(1)/c, a = 4.8570(10) Angstrom, b = 13.5403(10) Angstrom, c = 7.7328(10) Angstrom, = 90.908(10)degrees, V = 508.59(13) Angstrom(3), mu(Mok(u)) = 10 .23 mm(-1), Z = 4, F(000) = 463, D-x = 3.1954(8) g/cm(3), R-I = 0.0899 for 743 reflections with I > 2.0 sigma(I), wR(2) = 0.2425 for all 2534 reflecti ons. NaCsSi2O5, M-r 584.13, lambda(MoKu) = 0.71073 Angstrom, 150 K, orthorh ombic, Pna2(1), a = 17.0743(10) Angstrom, b = 4.9011(10) Angstrom, c = 13.3 391(10) Angstrom, V = 1116.3(3) Angstrom(3), mu(MoKa) = 7.08 mm(-1), Z = 8, F(000) = 1072, D-x = 3.4757(9) g/cm(3), R-I = 0.0199 for 1103 reflections with I > 2.0 sigma(I), wR(2) = 0.0465 for all 2668 reflections. The crystal structures of NaRbSi2O5 and NaCsSi2O5 have been determined. Though the nom inal composition of both phases is that of a phyllosilicate, their anion to pology is similar and consists of double chains of [SiO1] tetrahedra formin g 4-membered T rings, i.e. rings consisting of 4 tetrahedra in chair confor mation and built from Q(3) units. The alkali-oxygen distances in the Na2O-C s2O-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 t hat of the larger one. The observed very high hygroscopicity of both crysta lline phases might be rationalized by substantial underbonding of some of t he oxygen atoms. 29-Si spin lattice relaxation times, T-1, vary largely bet ween 535 and 493 sec for NaRbSi2O5 and 2440 and 2329 sec for NaCs-Si2O5.