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 $\stackrel{1}{\infty }\{{\left[{{\mathrm{AlF}}_{2/2}}^{\mathrm{e}}{{\mathrm{F}}_{4/1}}^{\mathrm{t}}\right]}^{2-}\}$. 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 Fm$\overline{3}$m with a = 868.9(1) pm and obtained under ammonothermal conditions at 723 K and 152 MPa.

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.

K₂AlF₅ was first reported by de Kozak et al. as a dehydrate of K₂AlF₅·H₂O, which was obtained by hydrothermal synthesis [13]. During thermogravimetric investigations, the water-free K₂AlF₅ was observed to form upon heating. In the crystal structure of the monohydrate, very slightly kinked infinite trans-vertex-sharing octahedra with Niggli formula $\stackrel{1}{\infty }\{{\left[{{\mathrm{AlF}}_{2/2}}^{\mathrm{e}}{{\mathrm{F}}_{4/1}}^{\mathrm{t}}\right]}^{2-}\}$ (e = corner-sharing, t = terminal) are prearranged to result in straight chains in the dehydrate. This compound, in the following denoted K₂AlF₅-1, 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 [AlF₆]^3– are well known, for example from the mineral elpasolite, K₂NaAlF₆ [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 K₂AlF₅ featuring infinite chains of cis-vertex-sharing octahedra around Al with different conformations and the novel elpasolite Rb₂KAlF₆, obtained from ammonothermal synthesis using a mineralizer system containing alkali metal amides and fluoride ions in a near-ammononeutral regime.

2. Results

Two new modifications of K₂AlF₅ were obtained from supercritical ammonia under very similar conditions of 753 K, 224 MPa versus 773 K, 220 MPa. While we monitor the pressure in the autoclaves during the process, the temperatures given represent the furnace temperatures outside the autoclave. Average temperatures within the autoclave are about 100 K lower, and an intended temperature gradient of about 50 K naturally develops due to heat loss at the autoclave installations for filling, pressure monitoring, and safety purposes sticking out the furnace [18]. Thus from experimental conditions, we cannot derive any information on the relative stabilities of these different modifications. Upon providing both potassium and rubidium ions in solution at slightly reduced pressures (723 K and 152 MPa) the novel elpasolite Rb₂KAlF₆ was formed.

All three fluoridoaluminates were obtained as colorless crystals in the hot zone of the autoclave in a temperature gradient, thus exhibiting lower solubility at higher temperatures for the given conditions (so-called retrograde solubility). The temperature dependence of the solubility for a given mineralizer system typically depends on the pressure and temperature regime applied [2,19]. The mineralizer system itself has a fundamental impact on solubility and temperature dependence via the formation of dissolved species and intermediates. Here we used a combination of metal fluorides and alkali metal amides in molar ratios such that near-ammononeutral conditions are expected if completely reacted. While the dissolved intermediates are difficult to experimentally study in situ due to the extreme conditions administered, it is likely that they are represented in the present case by complex fluoridoaluminate ions.

2.1. Crystal Structures

Potassium Pentafluoridoaluminates K₂AlF₅

We present two novel modifications of K₂AlF₅, which both crystallize in the orthorhombic space group type Pbcn and are labeled K₂AlF₅-2 and K₂AlF₅-3 in the following. According to structure determination from single-crystal X-ray diffraction, K₂AlF₅-3 (Figure 1, bottom) features a doubled c-axis in comparison to K₂AlF₅-2 (Figure 1, top), while the a and b axes are nearly identical. This fact can readily be understood from the crystal structures discussed in the following. Selected crystallographic data and refinement parameters are gathered in

Table 1. Selected crystallographic parameters and refinement data of K₂AlF₅-2, K₂AlF₅-3, and Rb₂KAlF₆.

Compound	K2AlF5-2	K2AlF5-3	Rb2KAlF6
Crystal system	Orthorhombic	Orthorhombic	Cubic
Space group	Pbcn	Pbcn	Fm $\overline{3}$m
a/pm	718.12(3)	718.370(10)	868.88(11)
b/pm	1265.94(5)	1264.49(2)	-
c/pm	988.45(3)	1971.18(4)	-
Z	8	16	4
Density (calculated)/g∙cm−3	2.959	2.970	3.554
Volume/106 pm3	898.60(6)	1790.56(5)	656.0(2)
Index ranges hkl	–9 ≤ h ≤ 9	–9 ≤ h ≤ 8	–10 ≤ h ≤ 9
	–16 ≤ k ≤ 16	–16 ≤ k ≤ 16	–10 ≤ k ≤ 10
	–12 ≤ l ≤ 11	–25 ≤ l ≤ 25	–11 ≤ l ≤ 11
2θmax/°	54.94	54.96	54.96
F(000)	768	1536	640
T/K	293(2)	293(2)	293(2)
µ(Mo-Kα)/mm−1	2.30	2.31	15.73
Measured reflections/sym. independent	14490/1034	27919/2061	1072/58
Rint/Rσ	0.0857/0.0357	0.0557/0.0201	0.1148/0.0319
R1 with ∣Fo∣ > 4σ(Fo)	0.0308	0.0219	0.0318
R1/wR2/GooF	0.0517/0.0732/1.107	0.0287/0.0554/1.112	0.0392/0.0949/1.189
Largest peak/hole in the difference electron density map/106 pm−3	0.64/–0.43	0.31/–0.32	0.85/–0.41

. Fractional atomic coordinates and isotropic displacement parameters of K₂AlF₅-2 and K₂AlF₅-3 can be found in

Table 2. Fractional atomic coordinates and equivalent isotropic displacement parameters (in 10⁴ pm²) for K₂AlF₅-2.

Atom	Site	x/a	y/b	z/c	Ueq
Al	8d	0.1544(1)	0.09746(6)	0.40752(7)	0.0148(2)
F(1)	8d	0.0028(2)	0.1963(1)	0.4657(2)	0.0237(4)
F(2)	8d	0.2039(2)	0.3988(1)	0.0533(2)	0.0248(4)
F(3)	8d	0.2149(2)	0.4889(1)	0.3435(2)	0.0242(4)
F(4)	8d	0.2904(2)	0.1911(1)	0.3155(2)	0.0234(4)
F(5)	4a	0	0	0	0.0224(5)
F(6)	4c	0	0.0811(2)	¼	0.0215(5)
K(1)	4c	0	0.32802(6)	¼	0.0216(2)
K(2)	8d	0.35685(9)	0.11414(5)	0.07843(6)	0.0288(2)
K(3)	4c	½	0.36182(6)	¼	0.0238(2)

and

Table 3. Fractional atomic coordinates and equivalent isotropic displacement parameters (in 10⁴ pm²) for K₂AlF₅-3.

Atom	Site	x/a	y/b	z/c	Ueq
Al(1)	8d	0.34006(6)	0.00306(4)	0.07804(2)	0.01285(12)
Al(2)	8d	0.15454(6)	0.30892(4)	0.32833(2)	0.01289(12)
F(1)	8d	0.00118(13)	0.21130(8)	0.14313(5)	0.02101(22)
F(2)	8d	0.00529(13)	0.40872(8)	0.37647(5)	0.02002(22)
F(3)	4b	0	½	0	0.01986(30)
F(4)	4c	0	0.32874(11)	¼	0.01899(29)
F(5)	8d	0.28728(14)	0.21407(8)	0.28163(5)	0.02057(22)
F(6)	8d	0.27138(14)	0.38826(7)	0.04254(5)	0.01934(21)
F(7)	8d	0.47905(14)	0.10612(8)	0.10966(5)	0.02259(23)
F(8)	8d	0.30459(14)	0.49862(8)	0.15048(5)	0.02174(23)
F(9)	8d	0.19137(14)	0.08868(8)	0.03231(5)	0.02165(23)
F(10)	8d	0.28759(14)	0.41739(8)	0.29745(5)	0.02110(22)
F(11)	8d	0.29631(14)	0.30022(8)	0.40163(5)	0.02154(22)
K(1)	4c	½	0.04458(4)	¼	0.02156(13)
K(2)	4c	0	0.07646(4)	¼	0.01799(13)
K(3)	8d	0.15370(5)	0.02462(3)	0.41609(2)	0.02395(11)
K(4)	8d	0.36071(5)	0.29171(3)	0.16355(2)	0.02299(11)
K(5)	8d	0.01092(5)	0.26658(3)	0.49444(2)	0.01940(11)

, respectively.

The crystal structure of K₂AlF₅-2 (Figure 1, top) features cis-vertex-sharing AlF₆-octahedra, which form infinite zig-zag chains running parallel [001]. These chains obey the Niggli formula $\stackrel{1}{\infty }\{{\left[{{\mathrm{AlF}}_{2/2}}^{\mathrm{e}}{{\mathrm{F}}_{4/1}}^{\mathrm{t}}\right]}^{2-}\}$ and are aligned in the motif of a hexagonal rod packing (Figure 2, left). Repetition of the zig-zag chain is realized after four octahedra. The terminal fluoride ligands F(1–4) feature Al–F distances in the range of 176 pm to 179 pm, which is noticeably shorter than the distances to the bridging fluorides atoms (F(5) with 189 pm and F(6) with 192 pm). Selected interatomic distances are gathered in

Table 4. Selected interatomic distances (in pm) in K₂AlF₅-2.

	Distance/pm		Distance/pm		Distance/pm		Distance/pm
Al–F(1)	175.5(2)	K(1)–F(2)	259.4(2)	K(2)–F(4)	258.3(2)	K(3)–F(4)	271.2(2)
Al–F(2)	176.5(2)	K(1)–F(2)	259.4(2)	K(2)–F(3)	271.3(2)	K(3)–F(4)	271.2(2)
Al–F(3)	178.1(2)	K(1)–F(1)	270.6(2)	K(2)–F(2)	277.2(2)	K(3)–F(3)	276.3(2)
Al–F(4)	178.5(2)	K(1)–F(1)	270.7(2)	K(2)–F(2)	281.7(2)	K(3)–F(3)	276.3(2)
Al–F(5)	189.37(7)	K(1)–F(3)	271.7(2)	K(2)–F(1)	281.8(2)	K(1)–F(6)	277.6(2)
Al–F(6)	192.24(8)	K(1)–F(3)	271.7(2)	K(2)–F(1)	283.1(2)	K(3)–F(1)	290.5(2)
		K(1)–F(4)	278.8(2)	K(2)–F(4)	291.0(2)	K(3)–F(1)	290.5(2)
		K(1)–F(4)	278.8(2)	K(2)–F(5)	304.23(7)	K(3)–F(2)	291.9(2)
		K(1)–F(6)	312.6(2)	K(2)–F(6)	310.13(7)	K(3)–F(2)	291.9(2)
				K(2)–F(3)	310.6(2)	K(3)–F(5)	302.76(5)
				K(2)–F(3)	311.8(2)	K(3)–F(5)	302.76(5)

. Despite these slightly different distances, the octahedra are close to ideal as indicated by the internal angles near 90° and 180°. The angles at the bridging fluoride ligands ∡(Al–F–Al) vary from linear (180° due to symmetry restrictions) to slightly kinked with 168° at F(5) and F(6), respectively.

The potassium ions interconnect three chains each (Figure 3). As it can be taken from Figure 4, K(1) is coordinated by nine fluoride ions, K(2) and K(3) by eleven fluoride ligands. Coordination by the octahedra of the chains is realized mono-, bi-, and three-dentate. Distances with about 258 to 312 pm are in the expected range, given those found for K₂AlF₅-1, K₂AlF₅-3, and Rb₂KAlF₆ discussed below.

Similar to K₂AlF₅-2, K₂AlF₅-3 crystallizes in the orthorhombic space group type Pbcn, but with a doubled c-axis (Figure 1, bottom). It crystallizes as an isotype of K₂FeF₅, first described by Vlasse et al. in the noncentrosymmetric space group type Pn2₁a, which was later corrected by Le Bail et al. [20,21]. The crystal structure of K₂AlF₅-3 shows close structural similarity to K₂AlF₅-2, while apparently no direct group–subgroup relation exists. The crystal structure of K₂AlF₅-3 also contains infinite zig-zag chains of cis-vertex-sharing AlF₆-octahedra, which are aligned parallel [001] in the motif of a hexagonal rod packing (Figure 2, right) and consistent with the Niggli formula $\stackrel{1}{\infty }\{{\left[{{\mathrm{AlF}}_{2/2}}^{\mathrm{e}}{{\mathrm{F}}_{4/1}}^{\mathrm{t}}\right]}^{2-}\}$. However, in K₂AlF₅-3 the chains exhibit a longer repetition length, completed only after eight octahedra. Figure 1 gives a comparison of the arrangements of the chains in K₂AlF₅-2 and K₂AlF₅-3. The already known modification K₂AlF₅-1 (Cs₂MnF₅ structure type in P4/mmm with a ≈ 597 pm, c ≈ 370 pm), in contrast, is built from straight chains of trans-vertex-sharing octahedra with a 180° angle at the bridging fluoride ligands (Figure 5) [13]. The coordination polyhedron of potassium can be described as a bicapped square prism.

The five different potassium sites in K₂AlF₅-3 are coordinated by nine, ten, or eleven fluoride ions (see Figure 6). In contrast to K₂AlF₅-2, the coordination by the octahedra of the chains is only realized mono- or three-dentate, but no bi-dentate coordination appears. Each site connects three chains of AlF₆-octahedra, similar to the situation in K₂AlF₅-2 (see Figure 7). Still, internal distances in both modifications are rather similar: d(Al–F) in K₂AlF₅-3 to terminal fluoride ligands range from 176 pm to 180 pm, while those to bridging fluoride ions are longer with 191 and 192 pm (

Table 5. Selected interatomic distances (in pm) in K₂AlF₅-3.

	Distance/pm		Distance/pm		Distance/pm		Distance/pm
Al(1)–F(7)	175.60(11)	Al(2)–F(1)	175.82(10)	K(1)–F(5)	2 × 270.50(10)	K(2)–F(8)	2 × 260.54(9)
Al(1)–F(8)	176.68(10)	Al(2)–F(11)	177.12(10)	K(1)–F(4)	1 × 272.93(15)	K(2)–F(10)	2 × 269.24(10)
Al(1)–F(9)	176.79(10)	Al(2)–F(10)	177.92(10)	K(1)–F(10)	2 × 278.02(11)	K(2)–F(1)	2 × 271.02(10)
Al(1)–F(6)	179.95(10)	Al(2)–F(5)	178.75(10)	K(1)–F(7)	2 × 287.76(10)	K(2)–F(5)	2 × 277.05(11)
Al(1)–F(2)	190.68(10)	Al(2)–F(2)	190.85(10)	K(1)–F(8)	2 × 299.57(10)	K(2)–F(4)	1 × 319.00(15)
Al(1)–F(3)	192.04(5)	Al(2)–F(4)	191.82(5)	K(1)–F(2)	2 × 302.77(10)
K(3)–F(9)	271.56(10)	K(4)–F(5)	258.05(10)	K(5)–F(6)	264.78(11)
K(3)–F(10)	273.60(10)	K(4)–F(8)	265.97(10)	K(5)–F(6)	267.91(10)
K(3)–F(6)	277.78(10)	K(4)–F(7)	271.27(11)	K(5)–F(11)	269.94(10)
K(3)–F(9)	279.92(11)	K(4)–F(6)	275.55(10)	K(5)–F(9)	272.95(10)
K(3)–F(8)	284.93(11)	K(4)–F(11)	278.09(11)	K(5)–F(11)	278.04(11)
K(3)–F(1)	285.88(11)	K(4)–F(1)	280.47(11)	K(5)–F(7)	278.49(11)
K(3)–F(11)	287.44(11)	K(4)–F(5)	291.99(11)	K(5)–F(1)	280.16(10)
K(3)–F(7)	287.75(11)	K(4)–F(10)	308.22(11)	K(5)–F(9)	291.23(11)
K(3)–F(2)	295.96(10)	K(4)–F(2)	311.84(10)	K(5)–F(2)	293.93(10)
K(3)–F(3)	300.36(4)	K(4)–F(10)	312.53(11)	K(5)–F(3)	295.47(4)
K(3)–F(6)	334.39(11)	K(4)–F(4)	313.65(4)

). Again the internal angles within the octahedra are close to ideal the of 90° and 180°, while the angles at the bridging fluoride ions ∡(Al–F–Al) diversify between 180° for F(3) (due to symmetry restrictions) and 165° at F(4). Distances between potassium and fluoride ions in both modifications are also rather similar, although the furthest distances in the coordination are increased to 334 pm in K₂AlF₅-3.

2.2. Raman Spectroscopy

The Raman signals obtained from single crystals of K₂AlF₅-2 and K₂AlF₅-3 are assigned in accordance with spectra reported for K₃AlF₆ [22], Na₃AlF₆ [23], and KF:AlF₃ melts [24]. The Raman spectra resulted in three signals between 700 and 300 cm^–1 for both K₂AlF₅-2 and K₂AlF₅-3 (Figure 8). The two strong and sharp signals at approximately 600 and 456 cm^–1 can be assigned to the Al–F stretching modes ν₁ and ν₂. The third signal at around 377 cm^–1 is rather weak and belongs to the F–Al–F bending mode. The remaining signals below 300 cm^–1 relate to lattice vibration modes with signals at 282, 190, 160, and 71 cm^–1 for K₂AlF₅-2 and 240, 178, and 65 cm^–1 for K₂AlF₅-3, respectively.

Compared with the literature, the observed signals are shifted to higher wavenumbers. However, literature data are from compounds containing isolated complex [AlF₆]^3– ions and are mostly concerned with salt melts at higher temperatures, for which a shift of signals towards lower wavenumbers with rising temperature is reported [25].

Potassium Dirubidium Hexafluoridoaluminate, Rb₂KAlF₆

In an experiment, providing rubidium next to potassium ions within the reaction vessel, a new cubic hexafluoridoaluminate with elpasolite structure, Rb₂KAlF₆, was obtained. General crystallographic parameters are gathered in Table 1, the fractional atomic coordinates, as well as isotropic displacement parameters, are collected in

Table 6. Fractional atomic coordinates and equivalent isotropic displacement parameters (in 10⁴ pm²) of Rb₂KAlF₆.

Atom	Site	x/a	y/b	z/c	Ueq
Al	4a	0	0	0	0.0166(17)
K	4b	½	0	0	0.0224(14)
Rb	8c	¼	¼	¼	0.0310(8)
F	24e	0	0.2073(8)	0	0.0386(18)

. The structure features one crystallographic site each for aluminum, fluoride, potassium, and rubidium arranged in an ordered double perovskite motif with an alternating filling of octahedra formed from fluoride ions by aluminum and potassium ions. Consequently, the structure contains mutually isolated [AlF₆]^3– octahedra arranged in the motif of a cubic closed packing. In this hierarchical view, potassium ions occupy octahedral voids, while rubidium ions are located in tetrahedral voids in the packing of [AlF₆]^3– octahedra. This leads to twelvefold coordination of Rb by F in a distorted cuboctahedron (4 × triangular face of [AlF₆]^3– octahedra, Figure 9). A slight antisite disorder by mutual substitution of potassium and rubidium ions appears possible, while refinements resulted in around 5% for both alkali metal sites. However, since for both sites the resulting figures are in the order of the standard deviations and the number of 58 unique reflections is already low in comparison to the number of refined parameters (7 for the fully ordered structure) we have discarded to further consider this minor disorder. Isostructural fluoridoaluminates comprise K₂LiAlF₆ [16], K₂NaAlF₆ [14], and Rb₂NaAlF₆ [17].

Distances within Rb₂KAlF₆ are within the expected ranges, with d(Al–F) = 180.1(7) pm, d(K–F) = 254.3(7) pm, and d(Rb–F) = 309.4(1) pm. Compared to the above-discussed modifications of K₂AlF₅, the slightly longer distance of the terminal fluoride ligands to aluminum reflects the isolated complex ions [AlF₆]^3– including the increased negative charge. The distance between potassium and fluoride ions, on the other hand, presents as shorter, due to the reduced coordination number of six.

3. Materials and Methods

The entire handling of all compounds was conducted under argon with p(O₂) < 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 Si₃N₄ 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-K_α 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.

4. Conclusions

Ammonothermal synthesis in presence of fluoride ions is a promising technique for the production of complex fluorides including new modifications, which possibly are difficult to assess by other methods. We have obtained two new modifications of K₂AlF₅, build up by zig-zag chains of cis-vertex-sharing AlF₆-octahedra with different conformations. Upon addition of rubidium next to potassium ions within the system, formation of the quaternary elpasolite, Rb₂KAlF₆, is preferred, which contains mutually isolated [AlF₆]^3– octahedra.