1. Introduction
Ammonothermal synthesis, which uses supercritical ammonia as a solvent medium, is a pathway to the production and crystal growth of a variety of materials, particularly nitrides, amides, and even halides, depending on the administered mineralizer [
1,
2]. Especially due to the available nitrides from ammonothermal reactions, such as AlN [
3], GaN [
4,
5], and InN [
6], this technique has gained considerable interest [
7]. Exploratory work in ammonothermal synthesis recently led to solid intermediate amides, which may give valuable information about the dissolved species during the process as well as the condensation process eventually producing nitride materials [
8,
9,
10]. Similar to the hydrothermal technique, we frequently observe the formation of several modifications of the same compound under rather similar pressure–temperature conditions during synthesis [
11].
For ammonothermal synthesis, 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, therefore reaching a high pressure from the expanding ammonia contained within. In this experimental set-up, pressure and thus ammonia density fundamentally depend on the temperature and filling degree of the autoclave with ammonia. The supercritical state of the solvent in combination with suitable mineralizers is intended to provide sufficient solubility and supersaturation for solid product formation and ideally crystal growth. The mineralizer can provide ammonobasic or ammonoacidic conditions within a wide range of pH and serves for the formation of dissolved complex species.
K
2AlF
5 was first reported by de Kozak et al. as a dehydrate of K
2AlF
5·H
2O, which was obtained by hydrothermal synthesis [
13]. During thermogravimetric investigations, the water-free K
2AlF
5 was observed to form upon heating. In the crystal structure of the monohydrate, very slightly kinked infinite trans-vertex-sharing octahedra with Niggli formula
(e = corner-sharing, t = terminal) are prearranged to result in straight chains in the dehydrate. This compound, in the following denoted K
2AlF
5-1, crystallizes in
P4
/mmm and spontaneously transforms back to its monohydrate after days in air. While these fluoridoaluminates are characterized by infinite chains of trans-vertex-sharing octahedra around Al, isolated octahedra [AlF
6]
3– are well known, for example from the mineral elpasolite, K
2NaAlF
6 [
14,
15], which might be described as a double perovskite. Fluorides with the elpasolite structure are numerous [
14,
16,
17]. Here we report two new modifications of K
2AlF
5 featuring infinite chains of cis-vertex-sharing octahedra around Al with different conformations and the novel elpasolite Rb
2KAlF
6, obtained from ammonothermal synthesis using a mineralizer system containing alkali metal amides and fluoride ions in a near-ammononeutral regime.
3. Materials and Methods
The entire handling of all compounds was conducted under argon with
p(O
2) < 0.1 ppm (glovebox: MBRAUN Inertgas-Systeme, Garching, Germany). The synthesis of the reported compounds was performed under similar conditions as discussed for the synthesis of intermediates in ammonothermal InN synthesis [
6,
26]. A custom-made autoclave from nickel base alloy was used, as well as a Si
3N
4 liner (air-pressure sintered silicon nitride, Ingenieurkeramik, a QSIL company, Frankenblick, Germany), which protects the autoclave from corrosion [
27,
28]. The autoclaves were assembled and disassembled in the glovebox. Ammonia (Linde, Pullach, Germany, purity ≥ 99.999) was filled into the autoclave by condensation (cooling in an ethanol/dry ice bath), using a self-made tensi-eudiometer according to Hüttig [
29]. The synthesis was performed in a one-zone tubular furnace (LOBA 1200-60-400-1 OW, HTM Reetz, Berlin, Germany) set up in a vertical position, generating a temperature gradient from the heated lower part of the reaction vessel (warmer temperature zone) to the unheated upper part (colder temperature zone) [
30,
31]. The pressure was monitored with a pressure transmitter and a digital analyzer (HBM P2VA2 and DA 2510, Hottinger Brüel and Kjaer, Darmstadt, Germany).
For the synthesis of K2AlF5-2, equimolar amounts of InF3 (125.4 mg, 0.73 mmol) and AlF3 (61.3 mg, 0.73 mmol) were used as aluminum and fluoride sources together with a sixfold amount of KNH2 (241.3 mg, 4.38 mmol), which served as mineralizer and provided the potassium (equimolar amounts of F– and K+). At the beginning of the experiment, KNH2 was spatially separated from the metal fluorides by placing in a Si3N4 crucible, equipped with a cap with a hole. The crucible prevents the reactants and mineralizer from a premature solid-state reaction, the cap reduces the diffusion rate of dissolved KNH2 into the solution. A total of 19.0 g ammonia were condensed into the autoclave, which corresponds to a filling degree of 100% regarding the free volume in the liner and crucible. The autoclave was heated up to 753 K within five hours and maintained at this temperature for 60 h, reaching a maximum pressure of 224 MPa. It was subsequently cooled within 15 h to room temperature. K2AlF5-3 was obtained from a synthesis using Al (50.0 mg, 1.85 mmol), Cr (96.4 mg, 1.85 mmol), NH4F (205.7, 5.56 mmol), and KNH2 (306.4 mg, 5.56 mmol) in 17.0 g of ammonia (90% filling degree), again providing equimolar amounts of the ammonoacid and the ammonobase. The autoclave was heated to 773 K within five hours and kept at that temperature for 24 h. At maximum, a pressure of 220 bar was reached. The autoclave was subsequently cooled to room temperature over a time of 48 h. Both products were obtained as colorless crystals from the hot zone of the autoclave. Chromium and indium were not found to take part in the reactions.
The synthesis of Rb2KAlF6 included AlF3 (90.7 mg, 1.08 mmol), Ga (75.3 mg, 1.08 mmol), NH4F (119.5 mg, 3.23 mmol), and RbNH2 (355.4 mg, 6.45 mmol), residue KNH2 from earlier reactions, and a total of 17.5 g ammonia (93% filling degree). Loaded with the starting chemicals, the autoclave was heated to 723 K within 4.5 h and then kept at that temperature for 72 h, reaching a maximum pressure of 152 MPa. Subsequently, the autoclave was cooled to room temperature within 72 h, after which the product was found in the hot zone of the autoclave. Elemental gallium was recovered unchanged after the reaction.
InF3 (Alfa Aesar, Thermo Fisher, Kandel, Germany, 99.95% metal basis, anhydrous), AlF3 (abcr, Karlsruhe, Germany, 99.99% metal basis), Al (Alfa Aesar, Thermo Fischer Kandel GmbH, Germany, 99.97% metal basis), Cr (Sigma Aldrich, Taufkirchen, Germany, 99.0%), NH4F (Sigma Aldrich, Taufkirchen, Germany, 99.99%), and self-made KNH2 and RbNH2, synthesized from potassium (Sigma Aldrich, Merck, Darmstadt, Germany, 98%) and rubidium (donation) reacting with ammonia at 373 K for 24 h were used for synthesis.
Single-crystal X-ray diffraction data collection was performed on a
κ-CCD (Bruker Cooperation, Billerica, MA, USA) with Mo-
Kα radiation. Solving and refinement of all crystal structures was done with the SHELX-2013 software package [
32,
33].
Single-crystal Raman spectroscopy was done on an XploRa Raman spectrometer (Horiba Europe, Oberursel, Germany) equipped with a confocal polarization microscope (Olympus BX51, Olympus Europa, Hamburg, Germany). For single-crystal X-ray diffraction and Raman spectroscopy, the crystals were measured in a sealed glass capillary.