Fluoridhaltige Gäste in Alumosilicaten: Tetrafluoroborate in dem Sodalithen Na8Al6Si6O24(BF4)2

In the course of investigations on optical properties resulting from the interaction of fluorides with alumosilicate host materials and rare earth guests, a well defined BF₄^– ion wasfound to be incorporated within the sodalite of composition Na₈Al₆Si₆O₂₄(BF₄)₂. The resulting cubic molecular structure, which was determined by Rietveld methods (space group P4 n, a = 906.91 pm, wRp = 0.045, Rp = 0.027), contains one anion in each sodalite cage and is, contrarily to expectations, thermally stable. NMR spectroscopic investigations indicated a fast rotatory motion of the BF₄^– tetrahedra at room temperature and agreed with the tetrahedral BF₄^– ions found in IR and Raman spectra. Preliminary attempts to obtain a luminescent material by incorporation of Eu³⁺ through aqueous ion exchange only yielded low rare earth concentrations, giving rise to characteristic red emission lines at 581 nm (⁵D₀ → ⁷F₁) and 615 nm (⁵D₀ → ⁷F₂) in a 1:2 intensity ratio. The material unexpectedly exhibited a strong broad band emission at 520 nm after calcination under Ar, which is attributed to the formation of an Eu²⁺ species. Further calcination under air partially reestablished the Eu³⁺ emission.     

Crystallization kinetics of Na7.4(AlSiO4)6(CO3)0.7·4H2O, an intermediate phase between cancrinite and sodalite, grown under low temperature hydrothermal conditions

During low temperature hydrothermal alkaline conversion of kaolinite in the presence of sodium carbonate a disordered phase crystallizes showing structural features between the sodalite and cancrinite minerals. A detailed study of its formation field and the kinetics of crystallization is the first step for further investigations of this zeolite-like material. The crystallization kinetics was investigated in the early stage of the reaction for times up to 192 h at a temperature of 353 K. Besides X-ray powder diffraction²⁷Al MAS NMR has been found to be the preferred method to follow the reaction kinetics because a signal assigned to six-coordinated aluminium of the starting kaolinite can be clearly distinguished from a second resonance, being typical for aluminium in four-coordinate position of an aluminosilicate framework structure. Important conclusions can also be drawn from the experiments for the synthesis of ideal ordered cancrinites.     

Application of Vegard's law to mixed cation sodalites: a simple method for determining the stoichiometry

Vegard’s law describes the empirical relationship between the crystal lattice parameter of a mixture and its components. This relationship holds for some sodalites, in particular those containing mixtures of Li, K and Na as the charge balancing cations. By utilizing previously published lattice parameters for Li/Na and K/Na mixed cation chloride sodalites, linear curves were drawn allowing the composition of the mixed cation sodalites to be determined from their lattice parameters. Further, by mathematical addition of the curves for Li/Na and K/Na mixed cation chloride sodalites, a linear curve was developed and tested for the mixed tri-cation Li/Na/K chloride sodalites. This provides a simple way to monitor the composition of mixed cation sodalites and has an application in monitoring the composition of multi-phase materials where the sodalite phase cannot be easily separated for elemental analysis.     

KINETICS AND PHASE TRANSFORMATIONS DURING THE REACTION OF NaCl-SODALITE IN AMMONIUM CHLORIDE SOLUTIONS

The reaction behavior of sodium chloride sodalite Na₈[AlSiO₄]₆Cl₂ in ammonium chloride solution has been investigated under mild hydrothermal conditions (T = 473 K) for reaction times up to 72 h. Reactions under weak acid conditions led to an amorphous aluminosilicate phase comparable to a leaching process. This material forms an amorphous layer around the sodalite grains, preventing the framework from further decomposition. About 52% of sodalite was damaged by acid leaching after 11 hours and this amount remains nearly constant even at longer reaction periods up to 72 hours. Cation exchange was observed in sodalite only on a very low level (< 10%). Beside these reactions under acid conditions (pH » 5) some additional experiments in alkaline solutions were done to improve ion exchange of sodalite. Thus an ammonium/ammonia buffer solution was used (pH » 9) at various temperatures in a range of 353 – 473 K. Neither cation exchange nor decomposition of the sodalite was obtained at 353 and 393 K after 72 hours. Formation of amorphous material started at 433 K. In contrast to the acid conditions a total transformation of sodalite into a crystalline ammonium aluminosilicate phase was observed at 473 K.