Hydrothermal Crystallization in the LiOH-ZrO_2-SiO_2-H_2O System at 500 deg C: The Structural Mechanism of Self-Assembling of ZrSiO_4 and Li_2ZrSi_6O_(15) from Subpolyhedral Clusters

摘要

In the LiOH-ZrO_2 (nanocr.)-SiO_2-H_2O system at 500 deg C and 0.1 GPa, the following phases have been found to crystallize: SiO_2 (quartz), ZrO_2 (monoclinic), ZrSiO_4 (zircon), Li_2Si_2O_5, and Li_2SiO_3. For the component ratio of ZrO_2: SiO_2 = 1 : 1 mol/mol, ZrSiO_4 is the major phase over a wide range of c_(LiOH) values up to LiOH centre dot H_2O : SiO_2 = 3. When ZrO_2: SiO_2 = 1: 2 to 1:6, phase fields change from ZrSiO_4 + Li_2Si_2O_5 to ZrO_2 +Li_2SiO_3 as the LiOH concentration increases. The features of ZrSiO_2 and Li_2ZrSi_6O_(15) formation have been considered in terms of the model of matrix crystal-structure assembling from subpolyhedral structural units (SSUs), namely, cyclic clusters. The ZrSiO_4 structure is direct packing of four-polyhedron precursor SSUs (composed of two ZrO_8 dodecahedra and two SiO_4 tetrahedra). In topologically different Li_2ZrSi_6O_(15) polymorphs, invariant six-polyhedron precursor SSUs (composed of two ZrO_6 octahedra and four SiO_4 tetrahedra) have been found to exist. The mechanism of their structure assembling involves additional polymerization of both the precursor SSU and higher level SSUs (chains and layers) through linking SiO_4 tetrahedra. The phase constitution of the crystallization products of the ZrO_2 system has been compared against the analogue systems of TiO_2, SnO_2, and GeO_2.