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.
Highlights
Ammonothermal synthesis, which uses supercritical ammonia as a solvent medium, is a pathway to the production and crystal growth of a variety of materials, nitrides, amides, and even halides, depending on the administered mineralizer [1,2]
Two new modifications of the pentafluoridoaluminate K2AlF5 were obtained from ammonothermal synthesis at 753 K, 224 MPa and 773 K, 220 MPa, respectively
Two new modifications of K2AlF5 were obtained from supercritical ammonia under very similar conditions of 753 K, 224 MPa versus 773 K, 220 MPa
Summary
Ammonothermal synthesis, which uses supercritical ammonia as a solvent medium, is a pathway to the production and crystal growth of a variety of materials, nitrides, amides, and even halides, depending on the administered mineralizer [1,2]. Typically supercritical conditions are applied (critical point of pure ammonia at 405.2 K and 11.3 MPa [12]), which are most often realized by applying elevated temperatures to a sealed reaction vessel, reaching a high pressure from the expanding ammonia contained within. In this experimental set-up, pressure and ammonia density fundamentally depend on the temperature and filling degree of the autoclave with ammonia. The mineralizer can provide ammonobasic or ammonoacidic conditions within a wide range of pH and serves for the formation of dissolved complex species
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