Hydrothermal Crystallization in the NaOH-GeO_2-H_2O System at 500 deg C and 0.1 GPa: The Mechanism of Structure Assembling of A_4Ge_9O_(20) and A_3HGe_7O_(16) centre dot 4H_2O (A = Li, Na) from Subpolyhedral Clusters

摘要

In the NaOH-GeO_2 (quartz)-H_2O system at 500 deg C and 0.01 GPa, dioxide R-GeO_2 (the rutile structure type) and Na,Ge~[6] germanates Na_4Ge_4~[6]Ge_5~[4] O_(20) and Na_3HGe_4~[6]Ge_3~[4] O_(16) centre dot 4H_2O crystallize. These germanates are open frameworks composed of M octahedra (Ge~[6]) and T tetrahedra (Ge~[4]). The increasing NaOH concentration changes crystallization fields as follows: R-GeO_2 =>R-GeO_2 + Na_4Ge_4~[6]Ge_5~[4] O_(20) =>Na_4Ge_4~[6]Ge_5~[4] O_(20) + Na_3HGe_4~[6]Ge_3~[4] O_(16) centre dot 4H_2O. The results are compared to the LiOH-GeO_2 (quartz)-H_2O) system studied earlier at 500 deg C and 0.1 GPa. Phase-formation features for Na,Ge~[6] and Li,Ge~[6] germanates of compositions A_4Ge_9O_(20) and A_3HGe_7O_(16) (A = Li, Na) are interpreted in terms of the matrix assembling of crystal structures from Ge~[6],Ge~[4] subpolyhedral structural units.