Syntheses And Characterization Of New A₂A'MF₆-type Compounds

Cubic compounds with the highest symmetry structure exhibit promising optical properties in material science, and have been attract widespread attention. They also play an important role in the structural chemistry research.Five new compounds possessing the general formula A₂A'MF₆ were synthesized by hydrothermal reaction in this thesis (A and A' are univalent cations, M is a trivalent cation). Meanwhile, all known compounds with A₂A'MF₆ formula until now were summarized to show the relations between ionic radius, transition metal ions and the compound structure. Then the five new compounds were investigated by some characterization methods, such as, X-ray powder diffraction, single crystal diffraction or Rietveld method, SEM, EDX, FT-IR, UV-visible, optical microscopy measurement. The unit cells of the five compounds were calibrated by X-ray powder to be more exactly.The five compounds possessing the general formula A₂A'MF₆ all crystallize in the cubic system with space group Fm-3m. And all of them belong to the structure of cryolite. The five compounds formula and their crystallographic parameters are as follows: 1, (NH₄)₂NaAlF₆, a=8.3446(3)(?), V=581.14(4)(?)³; 2, (NH₄)₂NaGaF₆, a=8.4401(3)(?), V=601.23(4)(?)³; 3, (NH₄)₂NaFeF₆, a=8.4830(3)(?), V=610.45(4)(?)³; 4, (NH₄)₂NaCrF₆, a=8.45432(6)(?), V=604.277(13)(?)³; 5, [(NH₄)_0.83K_0.17]₂KAlF₆, a=8.7242(3)(?), V=664.02(4)(?)³The crystal structure of those compounds is characterized by an array of alternate [A'F₆] and [MF₆] corner-connected octahedra. The structure can be visualized as [MF₆]^3- octahedra forming a face-centred cubic (fcc) arrangement, with Na+ cations filling all octahedral interstices and A cations filling all tetrahedral interstices. Each A cation lies within four [MF₆]^3- octahedra, and is coordinated by 12 F atoms to form a cuboctahedron(3-6-3). Each [MF₆]^3- octahedron is surrounded by eight A cations in an octahedral shape. The ionic radius of ammonium was estimated by the five compounds, together with all known compounds with A₂A'MF₆ formula until now. At the same time, the growth mechanism are preliminarily discussed.