Novel Fluoridoaluminates from Ammonothermal Synthesis: Two Modifications of K2AlF5 and the Elpasolite Rb2KAlF6

Abstract

Two new modifications of the pentafluoridoaluminate K2AlF5 were obtained from ammonothermal synthesis at 753 K, 224 MPa and 773 K, 220 MPa, respectively. Both crystallize in the orthorhombic space group type Pbcn, with close metric relations and feature kinked chains of cis-vertex-connected AlF6 octahedra resulting in the Niggli formula ∞1{[AlF2/2eF4/1t]2−}. The differences lie in the number of octahedra necessary for repetition within the chains, which for K2AlF5-2 is realized after four and for K2AlF5-3 after eight octahedra. As a result, the orthorhombic unit cell for K2AlF5-3 is doubled in chain prolongation direction [001] as compared to K2AlF5-2 (1971.18(4) pm versus 988.45(3) pm, respectively), while the unit cell parameters within the other two directions are virtually identical. Moreover, the new elpasolite Rb2KAlF6 is reported, crystallizing in the cubic space group Fm3¯m with a = 868.9(1) pm and obtained under ammonothermal conditions at 723 K and 152 MPa.

Exchange of ammine‐ and fluorido‐ligands in complex salts: The series [Cr(NH3)6][AlF6], [Cr(NH3)5F][SiF6] and K2[Cr(NH3)4F2][Si(NH3)0.5F5.5]2

“…[3] We have observed earlier that both aluminium and silicon are dissolved from the crucible material under the applied conditions in presence of fluoride ions as fluoridoaluminate and -silicate ions and also isolated the products from the hot zone of the autoclave. [5,6] The three title compounds, hexaamminechromium ] 2 crystallizes as rose colored plates in agreement with its tetragonal space group symmetry. All three compounds proofed to be stable in air for at least several days without any apparent change in color or appearance, which we associate with some effective stabilization by hydrogen bonding as discussed below.…”

“…The product crystals were obtained from the hotter zone at the bottom of the autoclave within an intended thermal gradient, indicating so‐called retrograde solubilities, i. e. lower solubility at higher temperatures typically found for ammonobasic growth of GaN crystals, to name only one prominent example [3] . We have observed earlier that both aluminium and silicon are dissolved from the crucible material under the applied conditions in presence of fluoride ions as fluoridoaluminate and ‐silicate ions and also isolated the products from the hot zone of the autoclave [5,6] …”

“…Earlier, we have observed the formation and crystal growth of hexafluoridosilicates and ‐aluminates from supercritical ammonia in presence of fluoride ions. Now we present further information on solid compounds of silicon and aluminium obtained from ammonothermal synthesis, indicating the nature of various dissolved species present under the given conditions [5,6] . Particularly intriguing is the successive interexchange of ammine and fluoride ligands on both the cationic and the anionic complex ions in the title compounds.…”

“…Now we present further information on solid compounds of silicon and aluminium obtained from ammonothermal synthesis, indicating the nature of various dissolved species present under the given conditions. [5,6] Particularly intriguing is the successive interexchange of ammine and fluoride ligands on both the cationic and the anionic complex ions in the title compounds. The observation of a complex ion involving a bond between nitrogen and silicon may lead a way to future crystal growth of nitrodosilicates from supercritical ammonia in presence of fluoride ions.…”

Polymorphism in A3MF6 (A = Rb, Cs; M = Al, Ga) grown using mixed halide fluxes

“…Namely, Li 3 GaF 6 was grown from the melt, 3,10 Na 3 GaF 6 was grown under mild hydrothermal conditions, 12 and Rb 2 KAlF 6 was grown via an ammonothermal method. 17…”