Location and anisotropic refinement of hydrogen atoms in disodium dihydrogen-silicate tetrahydrate, Na2H2SiO4.4H2O, by neutron diffraction; hydrogen bonding at 173 K

Msub(r)=212.14, triclinic, Panti 1, a=7.916 (2), b=9.558 (2), c=6.643 (1) A, α=70.13 (2), β=104.83 (2), γ=122.56(2)0, V=396.71 A3, Z=2, Dsub(m)=1.74, Dsub(x)=1.776 Mg m-3, lambda=0.7017 A, μ(n)=0.204 mm-1, F(000)=4.09, T=173 K, R=0.028 for 1498 independent reflections. Anisotropic structure refinement including absorption and secondary-extinction correction. The crystal structure of Na2H2SiO4.4H2O consists of linear chains of hydrogen-bonded [H2SiO4]2- tetrahedra running along [110] and columns of edge-linked Na-O polyhedra running parallel to the c axis. The [H2SiO4]2-tetradedron shows the smallest HO-Si-OH angle observed in the series of Na2H2SiO4.nH2O (n=8,7,5,4) hydrates which is due to the arrangement of the hydroxyl H atoms. All Na atoms have coordination number (CN) 5 with short Na-O bonds [means Na-O=2.37 (2) A]. Cn=4 with tetrahedral geometry is observed for the coordination polyhedra of the O atoms of all hydrate water molecules. (orig.)