(Oxo)(Fluoro)-Aluminates in KF-Al2O3 System: Thermal Stability and Structural Correlation

Article abstract of DOI:10.1021/acs.inorgchem.7b02105

Precise investigation of part of the phase diagram of KF-Al₂O₃ system was performed in an experiment combining different techniques. Solidified mixtures of KF-Al₂O₃ were studied by X-ray powder diffraction and high-field solid-state NMR spectroscopy over a wide range of compositions. To help with the interpretation of the NMR spectra of the solidified samples found as complex admixtures, we synthesized the following pure compounds: KAlO₂, K₂Al₂₂O₃₄, α-K₃AlF₆, KAlF₄, and K₂Al₂O₃F₂. These compounds were then characterized using various solid-state NMR techniques, including MQ-MAS and D-HMQC. NMR parameters of the pure compounds were finally determined using first-principles density functional theory calculations. The phase diagram of KF-Al₂O₃ with the alumina content up to 30 mol % was determined by means of thermal analysis. Thermal analysis was also used for the description of the thermal stability of one synthesized compound, K₂Al₂O₃F₂.

Full text of DOI:10.1021/acs.inorgchem.7b02105

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
pubs.acs.org/IC
(
Oxo)(Fluoro)−Aluminates in KF−Al O System: Thermal Stability and
2
3
Structural Correlation
,
†
‡
‡
†
†
̌
̌
k Simko,* Aydar Rakhmatullin, Pierre Florian, Martin Kontrík, Michal Korenko,
Frantise
Zuzana Netriova,
†
Vladimír Danielik,§ and Catherine Bessada
‡
́
†
‡
§
Institute of Inorganic Chemistry, Slovak Academy of Sciences, 845 36 Bratislava, Slovakia
Conditions Extremes et Materiaux: Haute Temperature et Irradiation, UPR 3079−CNRS Univ. Orlea
Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinskeho 9, 812 37 Bratislava, Slovakia
S Supporting Information
̂
́
́
́ ́
ns 450 71, Orleans, France
́
*
ABSTRACT: Precise investigation of part of the phase diagram of
KF−Al O system was performed in an experiment combining
2
3
different techniques. Solidified mixtures of KF−Al O were studied
2
3
by X-ray powder diffraction and high-field solid-state NMR
spectroscopy over a wide range of compositions. To help with the
interpretation of the NMR spectra of the solidified samples found as
complex admixtures, we synthesized the following pure compounds:
KAlO , K Al O , α-K AlF , KAlF , and K Al O F . These
2
2
22 34
3
6
4
2
2
3 2
compounds were then characterized using various solid-state
NMR techniques, including MQ-MAS and D-HMQC. NMR
parameters of the pure compounds were finally determined using
first-principles density functional theory calculations. The phase
diagram of KF−Al O with the alumina content up to 30 mol % was determined by means of thermal analysis. Thermal analysis
2
3
was also used for the description of the thermal stability of one synthesized compound, K Al O F .
2
2
3 2
INTRODUCTION
The presence and the stability of oxo-fluoro-aluminate
species in molten sodium cryolite electrolytes has been widely
discussed in the aluminum industry, where, according to several
studies, oxo-fluoro-aluminates play an essential role as electro-
■
Potassium oxo- and oxo-fluoro-aluminates are new materials
1
2
with the interesting “charge” and optical properties. They are
multicomponent systems known for their high ionic con-
ductivity at relatively low temperatures (<500 °C). These
properties are related to the lattice structure and its changes by
11
active species. Various investigations concluded that the
oxygen ions from dissolved alumina seem to form stable
3
2−
2−
12−15
anionic substitution. Crystalline systems with oxygen partially
Al
2
OF
6
and Al
O
2
F
2
4
species in this molten electrolyte.
replaced by fluorine have the ability to introduce defects into
the structure and thus modify materials properties.
However, their existence has not yet been evidently
confirmed; thus, the presence and the role of these species
3
16
KF-doped Al O systems are also interesting catalyst
in molten cryolite electrolytes are still considered controversial.
The situation becomes different if the sodium cation is
substituted for the next member of the alkali metals
potassium. In this case, the obtained results seem to be more
consistent. Kubel et al. confirmed, more recently, the existence
of potassium oxo-fluoro-aluminate compound in the solid state
2
3
materials for various industrial applications. KF/Al O is an
2
3
effective solid catalyst to promote base-catalyzed reactions. It is
often used in organic synthesis chemistry as a powerful catalyst
4
−6
in various heterogeneous catalytic processes. The system has
been industrially used as catalyst in the preparation of
3
7
−9
biodiesels
and also as green recyclable material in the
and published the crystal structure of K Al O F . The crystal
10
2
2
3 2
preparation of the bactericide agents.
structure consists of individual layers of oxygen-connected
AlO F) tetrahedral groups with the fluorine atoms separating
All potassium (oxo)-(fluoro)-aluminates (fluoro-, oxo-, and
oxo-fluoro-aluminates), and KF-doped Al O systems are part
(
3
2
3
the layers. Such tetrahedrally coordinated structure of
potassium oxo-fluoro-aluminate is unique and is unusual,
since aluminum fluorine compounds almost exclusively
of the KF−Al O system, and that system could be a subsystem
2
3
of a wider reciprocal system, KF−AlF −Al O −K O. The
3
2
3
2
rigorous understanding and characterization of this reciprocal
system refers also to other industrial applications, so besides the
pure scientific reason, since there is no complete data about the
phase diagram of both (all system and subsystems), the
complete analysis of these systems has been attracting even
more attention.
17
crystallize into the octahedral form. Having said that, it is
possible that the size of the cations of alkaline metals has
Received: August 17, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
primary importance in terms of the formation and the stability
of oxo-fluoro-aluminates in the melts.
eutectic crystallization were determined. The cooling rate was set to
1
.4 °C/min. The temperature control and the data processing were
performed using a computerized measuring device (multicomponent
model for thermal analysis data collectionsNational Instruments
where the data collections run online under Labview software)
developed at the Institute of Inorganic Chemistry SAS, Bratislava. The
temperature of the sample was controlled by a Pt-PtRh10
thermocouple, calibrated on the melting points of pure NaCl and
NaF. The accuracy of the temperature difference measurement
between the temperatures of primary crystallizations of the given
The size and the role of the cations is also a key factor for an
understanding of the transport properties of these materials, as
solid conductors. To understand the unusually high transport
ability of potassium ions requires more rigorous character-
ization of the phases and their systems and subsystems, where
these phases and compounds arise and exist. The problem is
that the information about the related phase diagrams is rather
limited, and, accordingly, the thermal stability of potassium
mixtures was (±1 °C). For each run, a new bath of KF and Al
prepared.
O was
3
2
18
oxo-fluoro-aluminates.
A critical evaluation of the chemistry and the thermal stability
Analysis of the Phase Fields. An analysis of the samples from
each phase field was made. At the beginning, we prepared samples
from the phase fields at the same composition, 7 mol % of Al O . The
of (oxo)(fluoro)aluminates in the system KF−Al O will be
2
3
2
3
presented. The obtained data are completely new, and some of
them can solve several discrepancies in literature. In the first
step, the present study will focus on the fundamental
samples were heated in a resistance furnace, under the atmosphere of
dried argon (Messer, 99.996% purity) and kept in the Pt crucible,
covered with a lid, at the temperatures that correspond to particular
field (760, 800, 830, and 860 °C) for 2 h. All mixtures were then
quenched by immersing the platinum crucible with the sample into
liquid nitrogen, cooling rate of ca. 100 °C/min. After each quenching,
the samples were quickly moved to a glovebox, homogenized, and
analyzed by MAS NMR and XRD analyses.
description of the system KF−Al O . We will present the
2
3
results of thermal analysis and subsequent X-ray diffraction
XRD) analysis of solidified samples, where we determined the
(
liquidus temperatures and the phase fields, where the new solid
species were arising. The next step is the synthesis of these new
species as pure compounds, to be used as standards for further
analysis. Those newly synthesized standards are then
characterized by various solid-state nuclear magnetic resonance
In the next step, we prepared the samples with the higher
composition of Al O (12, 20, and 30 mol %). We quenched these
2
3
samples from 860 °C by the same way described above.
Preparation of Pure Compounds. α-K AlF . Potassium cryolite,
3
6
(
NMR) methods along with the first-principles density
19
α-K AlF , was prepared in a glovebox according to Abakumov et al.
3
6
functional theory (DFT) calculations of NMR parameters
using the CASTEP code.
The mechanically homogenized powder mixture of KF and AlF3
(sublimated, dried at 300 °C, min. 99.00%) (molar ratio 3:1) was
placed in a Pt crucible and heated from room temperature to 1010 °C
at a rate of 5 °C/min under an argon atmosphere. The holding time
was 1 h, and then the sample was spontaneously cooled. The sample
was then powdered and analyzed by XRD and solid-state NMR.
The main goal of this work is to address the nature and the
type of the potassium (oxo)(fluoro)aluminate complexes, using
2
7
19
Al and F solid-state NMR in an attempt to gain more insight
into the reaction mechanisms taking place in investigated
systems. The NMR spectroscopy has proved to be a powerful
tool for the investigation of local structure around a selected
nucleus, without limitations due to a disorder. Among the
different experimental parameters that can be extracted from an
experimental spectrum, chemical shifts provide a significant
source of structural information including the nature of first
neighbors and the coordination numbers.
KAlF . Crystals of potassium tetrafluoroaluminate were prepared by
4
the heating of stoichiometric mixtures of KF and AlF . The
3
mechanically homogenized powder mixture was placed in a Pt crucible
and heated from room temperature to 580 °C at a rate of 5 °C/min
under argon atmosphere. The holding time was 1 h, and then the
sample was spontaneously cooled. The obtained crystals of the sample
were powdered and analyzed by XRD and solid-state NMR.
KAlO
. The pure potassium aluminate KAlO (referred to also as
2
2
2
7
K O × 1Al O ) compound was prepared in a glovebox from the
High-field solid-state Al magic angle spinning (MAS) NMR
2
2
3
stoichiometric mixture of KNO (Merck, 99.5%) and Al(NO ) ·9H O
3
3
3
2
is used to monitor the state and exact nature of all species
2
0
1
9
(Lachema, 99.5%) according to Husheer et al. The mixture was
mechanically homogenized, heated to 350 °C and kept for 2 h, and
then spontaneously cooled. The solid sample was then crushed and
heated at 850 °C for 3 h. The spontaneously cooled sample was then
again regrounded and heated at 900 °C for 18 h. The whole treatment
was done under inert argon atmosphere. The sample was powdered
and analyzed by XRD and MAS NMR.
existing in the investigated system, and F MAS NMR is used
to probe the (oxo)(fluoro)aluminate complexes during
reactions. To more accurately describe the chemical environ-
ment of aluminum in investigated multicomponent mixtures,
two-dimensional (2D) multiple quantum (MQ) MAS NMR
experiments were applied. Double-resonance NMR method,
namely, 19F{ Al} D-HMQC-HETCOR (heteronuclear corre-
lation spectroscopy), was employed to help the assignment of
27
K
Al22
O
3
34. Potassium β-alumina K
Al22O34 (referred to also as K O
2
2
2
×
11Al O or KAl O ) is known as an Al O -rich phase, for which
small amounts of K ion are essential to its stability. The crystals of this
compound were randomly obtained after/during the synthesis of
2
11 17
2
3
+
1
9
F NMR peaks and investigate the connectivity of aluminum
with fluorine atoms.
1
KAlO , according to Burmakin et al. from K CO (Lachema, 99.8%)
2
2
3
and alumina. The compound K Al O was found as pure crystals on
2
22 34
EXPERIMENTAL SECTION
■
the crucible holder, and the small amount of this compound was found
in the bulk of the sample with KAlO . Part of the crystals were
homogenized and analyzed by XRD and MAS NMR.
Al . K Al was synthesized by the reaction in solid
state. The mechanically mixed and dried Al /KF (molar ratio: 1/2;
Al O Lachema, 99.9%) powder was placed in a Pt crucible and heated
Thermal Analysis Measurements. For the preparation of
samples, the following chemicals were used: KF (Aldrich, 99.8%),
Al O (Aldrich, 99.8%), NaCl (Aldrich, 99.9%), and NaF (Aldrich,
2
K
O
2
F
3
O F
2 3 2
2
3
2
2
2
3
9
9.9%), all of the reagent quality. Sample preparation consisted of KF
mixtures containing from 0 to 30 mol % Al O . All samples were
O
2 3
2
3
2
3
individually weighted in glovebox under inert atmosphere (Ar, Messer,
9.999% purity), homogenized, placed in a platinum crucible, and
transferred into the preheated furnace at 80 °C under dried argon
atmosphere. The experiments were done in tightly closed vertical
resistance furnace with water cooling. Thereafter the sample was
heated at rates of 7 °C/min up to 50−70 °C above the liquidus
temperature. The temperatures of phase transitions at primary and
from ambient temperature to 800 °C by rate of 5 °C/min under argon
atmosphere. The mixture was kept at this temperature for 18 h and
then cooled to room temperature during 10 h. The sample was at the
end homogenized and analyzed by XRD and solid-state NMR
methods.
NMR Measurements. Solid-state NMR experiments were
performed using Bruker Avance III spectrometer operating at 20.0 T
9
B
DOI: 10.1021/acs.inorgchem.7b02105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
using a double resonance probe using 2.5 mm diameter rotors
RESULTS
■
spinning at a MAS frequency between 30 and 34 kHz. To enhance
resolution in some of the 19F solid-state NMR experiments, we used a
Thermal Analysis. The part of the KF−Al O phase
2
3
diagram was determined by thermal analysis (Figure 1). The
picture also contains the identified species in the phase fields,
which are results of this work and which we will describe in the
next sections.
high-speed probe using 1.3 mm diameter rotors that spin up to 67
kHz. 1 F and Al chemical shifts are referenced to CFCl and 1 M
9
27
3
27
Al(NO ) , respectively. In some cases Al NMR spectra were acquired
3
3
2
1
using a double frequency sweep (DFS) pulse for signal enhance-
ment.
Triple-quantum 27Al MQ-MAS 2D spectra were acquired using a
2
2
three-pulse sequence with a zero-quantum filter, a repetition delay of
1
.0 s, and rotor-synchronized sampling of the indirect dimension ν =
r
3
0 kHz with 256 t increments. Phase cycling involved the states
1
method23 for acquisition of pure absorption line shapes. The
optimized excitation and conversion pulse width values were 2 μs
(
ν = 125 kHz) and 0.9 μs (ν = 278 kHz), respectively.
rf rf
24
2
27
19
The SR41 recoupling pulses in the dipolar Al( F) HMQC
25,26
19
experiments
were applied with nutation frequencies ν( F) = 50
19
kHz (π/2 pulse of 5 μs) on the F channel. The optimum double-
quantum transfer was obtained for τ = 1.8 ms corresponding to 18
2
1
rotor periods (three SR4 supercycles). The radio frequency field on
the 27Al channel was ν( Al) = 23 kHz, the spinning speed was 30 kHz,
and the spectral width in the direct dimension was set to 30 kHz, 592
complex points recorded with 196 transients, and in the indirect
27
dimension, a 30 kHz spectral width was used with 128 t increments.
1
Phase-sensitive detection in the indirect dimension was obtained using
Figure 1. Part of a phase diagram of the system KF−Al O ; (○)
2
3
23
the States method. The recycling delay was set to 2 s. In some cases,
conventional field 27Al NMR measurements were performed on a
Bruker Avance spectrometer operating at 9.4 T. Because most of the
samples were extremely hygroscopic, it was necessary to prepare the
rotors under argon atmosphere in a glovebox.
temperatures of phase transitions obtained in this work by thermal
analysis.
The lines of crystallization ((1) and (4)) are decreasing
(through the inflex point (I1) and invariant point (I2)) with
increasing of amount of alumina in the system. The line (2) is
composed from the data of thermal effects detected only in the
concentration range between 0 and 10 mol % Al O . Beyond
The use of continuous wave fluorine decoupling for MQ-MAS and
HMQC, during evolution and acquisition, only slightly enhances the
resolution in the isotropic dimension without yielding a well-resolved
27
spectrum.
First-Principles Calculations Details. Calculations of NMR
2
3
this concentration range, no thermal effect was detected. The
last line (3) in the phase diagram represents the thermal effects,
below which all compositions are completely solidified. Dashed
lines, emanating from invariant points, are hypothetical lines,
not detected by our thermal analysis because of the significant
difference between the melting points of KF and Al O .
2
8
parameters were performed using the CASTEP DFT code,
employing the gauge including projector augmented wave (GIPAW)
29
NMR calculations. Prior to the calculation of the NMR parameters,
full geometry optimizations were performed (using a cutoff energy of
−1
7
00 eV and k-point spacing of 0.04 Å ), where both the lattice
2
3
parameters and the internal atomic coordinates were allowed to vary
and using the initial atomic positions and unit cell parameters taken
from literature diffraction structures (see part: standard compounds).
Pseudopotentials were calculated “on-the-fly”, and the Perdew, Burke,
The first point (I1) is located at 11 mol % of Al O , and t(I1)
2
3
=
835 °C. The location of the second one (I2) was found to be
at 23 mol % of Al O , and t(I2) = 785 °C.
2
3
30
The line (1) represents the equilibrium between melt,
KAlO , and KF. Further line (4), in the concentration range
along with the Ernzerhof (PBE) functionals in the generalized
31−33
gradient approximation (GGA). The GIPAW
method imple-
2
mented in CASTEP provides accurate computed values of molecular
properties, and its use is well-established in the solid-state NMR
from 10 to 23 mol % of Al
between melt, KAlO , KF, and new species, K
phase field in the highest concentration range of Al O (>23
O
2
3
, represents the equilibrium
Al . The
2
O F
2 3 2
2
31,34
literature.
Cutoff energies were 700 eV. In all cases calculations
2
3
were performed after a geometry optimization of all atom positions,
keeping the unit-cell parameters and other atom positions constrained
at their experimentally determined values. Optimizations of the
geometry were used to ensure that the lowest-energy configuration of
atoms is used. Crystallographic data used in the calculations were
taken from the literature, with the corresponding references given.
mol %, line (5)), contains another new precipitated species,
K Al O . The system is completely solidified below line (3),
2
22 34
where also α-K AlF , as the new last identified phase, is
3
6
solidifying.
On the line (3), in the very low concentration of Al O
2
3
(
below 0.025 mol %), no thermal effects were detected.
36
Linear regressions led to the following relationships:
Thermal effects on this line appeared just beyond this
concentration. The temperatures of crystallization then
increased to the plateau at 785 °C, 10 mol % Al O . This is
19
35
δ (ppm) = −0.795 × σ +3689.444 for F, and δ (ppm) =
iso
iso
iso
−
0.977 × σ + 541.86 for 27Al.
iso
2
3
XRD Measurements. X-ray powder patterns of samples were
collected on PanAnalytical diffractometer equipped with a curved Ge
111) monochromator placed in the primary beam and a linear
the indication of presence of the solid solutions. We proposed,
in this case, the formation of solid solution between α-K AlF
6
(
3
and KAlO , considering the very good solubility of aluminates
position sensitive detector. All samples were placed in sample holders
for air-sensitive materials in Glovebox under an inert atmosphere,
provided with X-ray thin-film polyimide foil from both sides to prevent
access of oxygen and moisture. To achieve a better resolution, Cu Kα
radiation was used. The XRD patterns were taken in the 2θ range of
2
37
in cryolites. The formation of this solid solution is related
with the thermal effects detected on line (2), which is the upper
limit of its formation.
All identified phase fields were analyzed by XRD and NMR
(exact coordinates of the analyzed points are in Supporting
7−70° at room temperature.
C
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Inorganic Chemistry
Article
Information, part 1). At the beginning, we analyzed all fields at
mol %; see Figure 6, line b), but the NMR does. However, note
that developments in X-ray analysis of pair distribution function
2
7
19
the same composition, 7 mol % of Al O . The Al and
F
2
3
(
PDF) have begun to examine disordered oxide fluoride
MAS NMR spectra of these samples are shown in Figure 2.
40
materials.
The presence of two new phases, namely, α-K AlF and
3
6
KAlO , in all samples, suggests a complete reaction between
2
Al O and KF, according to the chemical equation
2
3
6
KF(s) + 2Al O (s) = 3KAlO (s) + α‐K AlF (s)
2
3
2
3
6
This interpretation is in very good agreement with analogous
4
1
NaF−Al O system, reported by Foster. Clearly, the system
2
3
KF−Al O cannot be interpreted as a simple binary system,
since the identification of KAlO and α-K AlF indicates that
2
3
2
3
6
the investigated system is quarternary in nature: KF−
AlF −Al O −K O. α-K AlF comes from the KF−AlF binary
3
2
3
2
3
6
3
system, whereas KAlO belongs to the K O−Al O one. The
2
2
2
3
+
3+
2−
quarternary system with basic ionic constituents (K , Al , O ,
and F ) will much more likely form a high number of complex
−
species.
In the next step, we analyzed the samples with the higher
composition of Al O (12, 20, and 30 mol %), taken at 860 °C.
2
3
The 27Al and F MAS NMR spectra of these samples are
19
shown in Figure 3, and the XRD analysis is shown in Figure 4.
Figure 2. 27Al (a) and 19F (b) MAS NMR spectra of individual
crystallization fields; sample KF + 7 mol % Al O ; line 1, 770 °C; line
2
3
2, 800 °C; line 3, 830 °C; line 4, 860 °C; *, sidebands.
The presence of KF and Al O was detected (Figure 2, lines
) in the sample taken from the first field below line (3), at 770
2
3
1
°
C. One could also have seen in this spectrum the first signs of
2
7
something new. The corresponding Al MAS spectrum shows
2
7
only one strong resonance at δ ( Al) = 15.0 ppm
corresponding to the octahedral Al site in Al O . The
MAS spectra shows as well a single narrow peak at δ ( F) =
−
iso
38
19
F
2
3
19
iso
39
134.4 ppm attributed to KF.
With increasing temperature (Figure 2, lines 2, 3, and 4),
new 27Al and2 F signals arise, while Al signal from Al O is
19
27
2
3
7
fading. The Al MAS spectra shows two distinct resonances
attributed to aluminum signals of KAlO and α-K AlF . The
2
3
6
2
7
overlapping quadrupolar signal at δ ( Al) ≈ −0.5 ppm
corresponds to the Al site in α-K AlF . The second signal at
δ ( Al) ≈ 74 ppm is attributed to the fourfold coordinated
aluminum environment found in KAlO . The F MAS spectra
iso
3
6
27
iso
1
9
2
shows two distinct sets of resonance peaks, a single narrow
peak attributed to KF, and set of peaks in the range between
−
3
156 and 169 ppm attributed to the different F sites in α-
K AlF .
6
Figure 3. 27Al (a) and 19F (b) MAS NMR analysis of the samples with
different amount of alumina content at 860 °C; line 4, 7 mol %; line 5,
The XRD and NMR results correlate to each other (see
Figure 2 and Supporting Information, part 1). It is worth noting
that the NMR method appears to be more sensitive than XRD
for our set of compounds, so one can identify the species in the
concentrations below the detection limit of XRD. In our case,
XRD does not show the presence of the oxofluoroaluminate in
the reaction mixture, containing 10 mol % of alumina (see
Figure 4, line 5). Moreover, the NMR is a tool suitable for the
study of amorphous phases. This can be also seen in our case,
where the XRD does not show the presence of K Al O (30
1
2 mol %; line 6, 20 mol %; line 7, 30 mol %; *, sidebands.
The 27Al and 19F MAS NMR spectra of the sample with 7
mol % of Al O (Figure 3, lines 4) confirmed the presence of
2
3
KF, α-K AlF , and KAlO . The XRD pattern of the same
3
6
2
sample also contains only diffractions of three listed
compounds (Figure 4, pattern 4).
2
22 34
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DOI: 10.1021/acs.inorgchem.7b02105
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Inorganic Chemistry
Article
Al sites of K Al O F , in agreement with the 19F MAS spectra.
2
2
3 2
A set of peaks in the range between −156 and −169 ppm is
attributed to the different F sites in α-K AlF , and the single,
3
6
19
strong narrow peak, located at δ ( F) = −170.5 ppm, is
iso
attributed to K Al O F .
2
2
3 2
The presence a lot of a complex fluoro- and oxo-aluminates
existing side by side suggests that they could be taken as
product of partial decomposition of K Al O F . They can be
2
2
3 2
predicted as metastable precursors of thermodynamically more
stable structure, and K Al O F is probably more stable under
2
2
3 2
high-temperature conditions.
To understand the synthesis mechanism, the preparation of
the K Al O F was performed at different temperatures to
2
2
3 2
determine its stability. It was first observed at concentration of
0 mol % of alumina and reaches a maximum at a
3
concentration of 33 mol % of alumina according to the
following reaction:
2
KF(s) + Al O (s) = K Al O F (s)
2
3
2
2
3 2
Figure 4. XRD analysis of the samples with different amount of
alumina content at 850 °C: 4, 7 mol %; 5, 12 mol %; 6, 20 mol %; 7,
The thermal analysis of dried and mechanically homogenized
2KF/Al O sample was realized (Figure 5).
42
19
20
1
3
0 mol %; *, KF; (X) α-K AlF ; (Y) α-KAlO2; (●) K Al O ,
2
3
3
6
2
22 34
3
(
○) K Al O F .
2 2 3 2
The situation seems to be quite different for samples
2
7
prepared with 12 and 20 mol % of Al O . The Al MAS spectra
2
3
showed the known distinct resonances attributed to aluminum
signals of α-K AlF and KAlO (Figure 3, lines 5 and 6).
3
6
2
Besides that, a new weak wide signal is observed in both
spectra, and it is growing with increasing amount of Al O . To
2
3
2
7
19
attribute this peak, we realized the Al− F D-HMQC
19
HETCOR MAS NMR experiment that correlates F and
2
7
Al nuclei only if they belong to the same chemical phase (see
Supporting Information, part 2). In the 2D map we observe
only two spots between Al and F: first narrow and very intense
2
7
at δ ( Al) ≈ 0 ppm, and second wide signal with quadrupolar
iso
27
line shape at δ ( Al) ≈ 70 ppm. These signals were attributed
to the octahedrally coordinated Al atoms in α-K AlF and
iso
3
6
tetrahedrally coordinated Al atoms in K Al O F , respectively
2
2
3 2
Figure 5. Notable part of thermal analysis curve of the mixture 2KF/
(
see part: standard compounds).
The one-dimensional (1D) 19F MAS spectra confirmed this
statement. Besides two known fluorine signals of KF and α-
Al O ; (black line) measured thermocouple, (red line) reference
2
3
−1
thermocouple, the heating rate: 2 °C × min .
1
9
K AlF , a new signal at δ ( F) ≈ −170.5 ppm is growing,
which is also associated with the fluorine site(s) of K Al O F .
3
6
iso
The heating curve contains a two-endothermic effect at 792
and 840 °C, respectively. The endothermic effect indicates
some phase changes. To better understand this thermal region,
2
2
3 2
XRD patterns of the same samples contain the diffractions
attributed to KAlO and α-K AlF , intensity of KF diffractions
2
3
6
are fading and a new ones are detected, which were attributed
the synthesis reaction process of KF/Al O3 mixture (see
2
13
to K Al O F (Figure 4, pattern 6). It means that K Al O F
experimental section: preparation of K Al O F ) was per-
2
2
3
2
2
2
3
2
2
2
3 2
as a new compound precipitates from the melted system with
increasing concentrations of alumina.
formed at three different temperatures; below the region of
thermal effects (at 700 °C), inside the region (at 800 °C) and
above (at 900 °C), respectively.
The X-ray pattern of 30 mol % alumina sample is notably
difficult to evaluate (Figure 4, pattern 7). KF lines are lost. The
XRD pattern contains the known lines of α-K AlF and KAlO
The XRD pattern of the first sample, synthesized at 700 °C
(Figure 6, pattern a), contains only observable diffractions of α-
K AlF and KAlO , respectively.
3
6
2
1
4
3
and lot of new reflections that can be attributed to K Al O F
and K Al O as a new phase (see part: pure compounds).
2
2
3
2
3
6
2
3
The opposite situation is found for the sample, synthesized
above the region of thermal effect, at 900 °C (Figure 6, pattern
c). The XRD pattern of this sample contains intense
diffractions of all three precursors: α-K AlF , KAlO , and
2
22 34
27
1
9
Both F and Al NMR spectra of the last sample with 30 mol
of alumina confirm the presence of these compounds (Figure
%
3
, lines 7). The 2 Al MAS spectrum contains well-known
7
3
6
2
signals of α-K AlF and KAlO and two distinct signals with the
K Al O . The diffractions of K Al O F were spotted with
3
6
2
2
22 34
2
2
3 2
maximum at ∼15 and 70 ppm. They are attributed to the
different octahedral and tetrahedral Al sites in the structure of
K Al O . The last intense signal with a typical quadrupolar
smaller intensities. The conversion of K Al O F to precursors
2 2 3 2
with heating demonstrates that it is a metastable phase. The
best situation occurs between thermal effects (sample prepared
at 800 °C). The XRD pattern of the corresponding sample
2
22 34
27
line shape at δ ( Al) = 69.0 ppm is attributed to tetrahedral
iso
E
DOI: 10.1021/acs.inorgchem.7b02105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
aluminum fluorine compounds, which generally crystallize with
an octahedral environment. To the best of our knowledge only
two examples of oxo-fluoro-aluminates in the solid state have
17
been reported: AlOF and K Al O F .
2
2
3 2
Our best synthesized sample is contaminated with the some
oxide phases and fluorides. By comparing the NMR data with
27
the ones of the pure described compounds, Al signals at
2
7
δ ( Al) = 75.0 ppm are attributed to tetrahedral coordinated
iso
27
Al nuclei in structure of KAlO , signals at δ ( Al) = 17.5 and
2
iso
6
9.0 ppm are attributed to the two different Al sites in the
27
K Al O , and the small peak at δ ( Al) = 0 ppm belongs to
2
22 34
iso
sixfold coordinated Al nuclei in α-K AlF . The remaining signal
3
6
27
with a typical quadrupolar line shape at δ ( Al) = 69.0 ppm is
iso
then attributed to the tetrahedral Al sites of K Al O F . The
2
2
3 2
relative 27Al intensities of K Al O F /oxo-aluminates/α-K AlF
2
2
3
2
3
6
=
73/22/5.
On the one hand, the F spectra also shows a set of peaks in
the range between −156 and −169 ppm attributed to the
different F sites in α-K AlF . On the other hand, the signal at
19
Figure 6. XRD analysis of the KF/Al O mixtures, prepared at 700 °C
2
3
19
20
(
a), 800 °C (b), and 900 °C (c); (X) α-K AlF , (Y) KAlO2, (●)
3
6
3
6
19
43
3
δ ( F) = −170.5 ppm is dominant and therefore associated
K Al O , (○) K Al O F .
iso
2
22 34
2
2
3 2
with the fluorine site of K Al O F (Figure 7b, line a).
2
2
3 2
2
7
19
Additionally we realized 2D Al− F D-HMQC HETCOR
contains the dominating diffractions of K Al O F : the pattern
2
2
3
2
27
19
experiment. The 2D Al− F D-HMQC HETCOR MAS
spectrum contains only two correlation peaks between signals
from the K Al O F and α-K AlF , and it confirms the presence
also contains small diffractions of α-K AlF and KAlO . One
3
6
2
can also notice a characteristic, low-intensity diffraction pattern
2
7
2
2
3
2
3
6
attributed to K Al O (confirmed by Al MAS NMR analysis,
2
22 34
only two fluorinated compounds in the synthesis and makes it
possible to separate the signals from oxo-aluminates and fluoro-
aluminates (Figure 7c).
see Figure 7a, line a). The last observation is in good agreement
with the findings of Kubel et al.
3
Another important measurement for the structure elucida-
tion is the 2D MQ-MAS technique, which enhances resolution
of the quadrupole-broadened aluminum spectra and enables to
constrain the NMR parameters of the simulations performed
on the (quantitative) 1D experiment. Fit of the 27Al MAS and
2
D MQ-MAS spectra shown in Figure 7d gives a quadrupolar
coupling constant of 9.33 MHz, an asymmetry parameter η =
Q
0
.15, and an 27Al isotropic chemical shift of 69.0 ppm. This
large quadrupolar interaction can be related to the distortion of
the Al coordination polyhedron inherent to the mixed chemical
nature of the [AlO F] polyhedron.
3
All experimental F and 27Al NMR parameters for KAlO2,
19
KAlF , and K Al O F are given in Table 1 along with the
4
2
2
3 2
results of their first-principles calculation. One can notice, for
K Al O F , that the calculated NMR parameters are in excellent
2
2
3 2
agreement with the experimental values.
The presence of α-K AlF , KAlO , K Al O , and K Al O F
3
6
2
2
22 34
2
2
3 2
occur side by side at all performed synthesis experiments. It
seems that this coexistence is not casual and that the first three
of them could be taken as the product of partial decomposition
of K Al O F . We therefore discuss in the following the stability
Figure 7. NMR spectra of mixture 2KF/Al O ; (a) 27Al MAS NMR
2
3
2
2
3 2
spectrum: a, mixture of 2KF/Al O (black line, spectrum; color lines,
2
3
simulated spectra); (b) 1 F MAS NMR spectrum: a, mixture of 2KF/
9
of these structures.
27
β-alumina, M Al O (also referred to as M O × 11Al O or
Al O (black line, spectrum; color lines, simulated spectra); (c) Al
2
22 34
2
2
3
2
3
2
7
19
MAl O ; M = Na, K, Rb), was first reported as sodium form
MQ-MAS NMR spectrum; (d) Al/ F D-HMQC-HETCOR NMR
11 17
44
spectrum.
by Rankin and Merwin in 1916. The name “beta-alumina”
derives from the assumption that this substance is one of the
allotropic forms of aluminum oxide. This kind of “unfortunate”
designation has been continuing for a long time in the
46
K Al O F NMR Structure Investigations. The crystal
2
2
3 2
structure of K Al O F consists of individual layers of oxygen
2
2
3 2
45
connected to AlO F tetrahedral groups, with the fluorine atoms
literature. Bragg et al. in 1931 and Beevers and Ross in 1937
3
separating the layers. Consequently, we have one four-
determined the main structural features of β-alumina. Beevers
and Ross confirmed the structure proposed on the basis of the
“ideal” formula Na Al O , as well as its isomorphous form
+
coordinated Al site and one nonbridging (Al−F···K ) fluorine
3
site (ICSD-421736). These [AlO F] tetrahedra are joined by a
3
2
22 34
common oxygen corner, constituting the basic building unit
K Al O . The unit cell is composed of two spinel-like blocks
separated at a mirror plane by a loosely packed layer of oxygen
and alkali metal ions. This layer is linked to the blocks by
2 22 34
+
[
Al O F ] of [Al O F ] layers, where K cations are mobile
2
5
2
2
3 2
between the layers. Such [AlO F] polyhedron is unusual in
3
F
DOI: 10.1021/acs.inorgchem.7b02105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Experimental and Computed Using CASTEP of 27Al and 19F NMR Parameters
δ(ppm) ± 0.1 ppm
27Al51 CQ(MHz) ± 0.1 MHz
ηQ ± 0.05
19F35 δ(ppm) ± 0.1 ppm
KAlO 2Al sites; 1/1
computed
74.5
0.95
1.25
2.00
9.52
0.15
0.65
0.61
0
2
7
2.8
experimental
computed
76.4
KAlF 1Al site; 2F sites; 1/1
−5.4
−166.1
4
−
169.6
−166.0
166.7
experimental
−5.7
10.8
0
−
K Al O F Al site; 1F site
computed
68.8
69.0
9.45
9.33
0.18
0.15
−171.4
−170.5
2
2
3 21
experimental
bridging oxygen ions, located at the top of [AlO ] tetrahedrons.
conditions; however, no sodium oxo-fluoro-aluminate species
were found. From these observations, a mullite-type oxo-
aluminate could have been tentatively considered as a skeletal
arrangement, sharing some characteristics with the oxo-fluoro-
4
Vacant sites in these planes give rise to a mobility of the alkali
metal cations resulting in high values of ionic conduction. This
type of structure is also referred to as mullite-type alkali
aluminate structure family with composition M O × xAl O (x
+
+
+
aluminate species. The substitution of Na by K or Rb cations
2
2
3
ranging between 5 and 12; M = Na, K, or Rb), where β-alumina
falls also with its metastable form, referred to as β″-alumina
with alkali metal contents between M O × 5Al O and M O ×
in channels perpendicular to the [AlO F] layers in the structure
3
of oxo-fluoro-aluminates can lead to a better geometrical fit of
+
+
oxo-aluminates and oxo-fluoro-aluminates, since K or Rb have
2
2
3
2
4
7
9
Al O . β″-Alumina, as metastable form, mostly occurs
together with β, regardless of the method of preparation.
an ionic radius that is nearly identical to that of oxygen, while
2
3
+
50
Na is somewhat smaller. As a consequence, the stability of
oxo-fluoro-aluminate species probably depends mainly on
monovalent cation size.
A similar structural arrangement was observed at alkali metal
3
oxo-fluoro-aluminates. Kubel et al. confirmed, first in 2011, the
existence of potassium oxo-fluoro-aluminate compound in the
Standard Compounds. The XRD patterns of all prepared
standard compounds are presented in Supporting Information,
solid state. Its structure is built with [AlO F] tetrahedrons to
3
the blocks layers, which are connected with bridging oxygen
ions, located at the top of these tetrahedrons. The potassium
ions might have moved in to the channels perpendicular to the
layers.
part 3.
−
KAlF
octahedra sharing four corners that form [AlF
. The structure of KAlF is built from (AlF
)3
4 6
4
−
] layers. The
4
crystalline structure exhibits two kinds of fluorine atoms (in a
1/1 ratio), one shared, one unshared, and one kind of
The instability of oxo-fluoro-aluminates was first predicted by
5
2,53
Foster, for sodium form of oxo-fluoro-aluminate, in Na AlF −
aluminum atom (ICSD-60524).
The attribution of NMR
3
6
4
1
54
Al O melts. The realized experiments ruled out the
lines to fluorine sites has been done by Bureau et al., but they
2
3
19
possibility of a stable solid oxo-fluoride compound. XRD
did not resolve two lines. F NMR chemical shift computations
55
powder patterns of the quenched liquids showed only Na AlF6
were performed by Body et al. using GIAO method. Appling
high field (20.0 T) and very fast spinning rate (60 kHz)
enabled us to obtain the 19F NMR spectrum with better
resolution of the peaks and to distinguish the two lines with
integral intensities 1:1 corresponding to the two crystallo-
3
and Al O or mullite-type form of Al O (listed as m-Al O );
2
3
2
3
2
3
depending on the cooling rate of the melts. That means that the
oxo-fluorides are less stable and do not retain their
configuration upon quenching.
The behavior of “oxygen” (in the form FeO) in Na AlF
graphic fluorine sites of the KAlF crystal structure (Figure 8).
4
3
6
48
̌
melts was studied by Simko et al. All the reaction products
escaped from the melts at the used experimental conditions.
Two volatile (high-temperature and low-temperature) fractions
were separated. Both fractions contain different products.
Oxygen from FeO was present in the high-temperature fraction
and contained solid Na AlF and α-Al O , respectively. The
3
6
2
3
presence of Na AlF and α-Al O in condensed high-temper-
3
6
2
3
ature vapors was surprising, and it was suggested that oxygen
was bounded to aluminum forming some oxo-fluoro-aluminates
that had evaporated and decomposed, resulting in the
solidification of Na AlF and α-form of Al O .
Figure 8. 27Al (a) and 19F (b) and MAS NMR experimental (black
line) spectra and their simulation (color lines).
3
6
2
3
4
9
́
Kucharik et al. have prepared a deeply undercooled
Na AlF −Al O melt by rapid solidification processing with a
3
6
2
3
5
6
cooling rate in the interval of 1 ×10 −1 × 10 K/s. Besides
27
Na AlF , the next mullite-type of Al O (listed as i-Al O ) was
The Al spectrum of KAlF is characterized by a very strong
3
6
2
3
2
3
4
16
detected. In his second paper, a mixture of Na AlF and
quadrupolar broadening giving a nuclear quadrupolar coupling
3
6
Al O were heated under different conditions of temperature,
constant (C = 10.7 MHz), which is associated with a strong
2
3
Q
time, and atmosphere. The existence/nonexistence of oxo-
fluoro-species in solidified samples at room temperature was
investigated by XRD analysis, IR, and MAS NMR spectroscopy.
The formation of different mullite-type of Al O (listed as
distortion of the octahedron. Its value of chemical shift remains
nevertheless within the range of those observed for the
3−
[AlF ] . A comparison between the previous reports and our
6
measurements shows a good agreement for the NMR
2
3
51,54
sodium β-Al O ) was detected, as a function of heating
parameters.
2
3
G
DOI: 10.1021/acs.inorgchem.7b02105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Our 19F and 27Al NMR results show also good agreement
with calculated data (Table 1). The calculated CQ is found
slightly lower than the experimental one, but the distortion of
the quadupolar line shape (stemming probably from some
structural disorder) prevents a very precise measurement.
α-K AlF . α-K AlF adopts the elpasolite structure. Al cations
(Figure 9c). As it has been mentioned above, in α-K AlF , the
3
6
[AlF ] octahedra are only slightly distorted. Perhaps, the Al5
6
site has a more distorted octahedral environment: distances
between aluminum and fluorine atoms are 1.78 ± 0.1 Å, and
the octahedral angles are 172° ± 6°. In this respect, the
observations of a relatively small CQ and close values of
chemical shift is expected. The MQ-MAS technique does not
allow us to distinguish and assign all five sites.
3
6
3
6
are situated in an octahedral fluorine environment, and the
structure can be considered to be composed of symmetrically
equivalent layers of the [AlF ] octahedra, containing the
octahedra rotated by π/4 around the c and a or b axes. Five
Calculated values of the chemical shifts are shifted by ∼10
ppm from the measured values (Table 3). We explain this
6
[
AlF ] sites are in a slightly distorted octahedral environment.
6
Table 3. Computed Using CASTEP 27Al NMR Parameters
The structure contains 30 crystallographically nonequivalent
fluorine sites (ICSD-260574).
F NMR spectrum (see Figure 9b) consists of a broad
envelope of signals in the −156 to −160 ppm range and four
19
27Al site
δ (ppm)
C
Q
(MHz)
η
Q
1
9
1
2
3
4
5
10.5
11.0
8.0
−3.6
−4.4
1.4
0.8
0.7
0.2
0.6
0.4
13.5
8.7
1.0
3.6
discrepancy using the linear correlation between experimental
δiso and calculated δ based on the data for oxides. As it was
iso
19
shown for F chemical shifts, both the slope and intercept of
the linear regression are different for various families of
35,51,56−58
fluorides.
With this in mind the agreement is overall
reasonable considering that we do not resolve all individual
components.
Three correlation peaks appear in the 27Al− F D-HMQC
MAS NMR spectrum (Figure 9d): first intense and broad at the
maximum intensity 19F −159 ppm, second at −162 ppm, and
third with low intensity at −168 ppm. Both the first and second
peaks correlate with the full aluminum peak range. The last
signal is connected only with the aluminum site at 0.5 ppm.
This clearly shows that all observed F sites connect with at least
19
Figure 9. 27Al (a) and 19F (b) MAS NMR experimental (black line)
27
19
27
one Al atom and that the F signals at −162, −164.1, −167.3,
spectra and its simulation (color lines); Al MQ-MAS NMR (c) and
2
7
19
and −168.5 ppm are not contaminations as Ahrens et al.
Al/ F D-HMQC-HETCOR NMR (d) at 20 T.
59
suggested in their paper. Unfortunately we are not able to
assign the signals in these spectra to the 30 fluorine sites.
KAlO (K O × 1Al O ). The room-temperature form of
small signals at −162, −164.1, −167.3, and −168.5 ppm. The
complete simulation of the spectrum using Lorentzian lines
revealed a minimum of seven signals.
2
2
2
3
KAlO has a cristobalite-related orthorhombic structure with
2
1
,60
symmetry lower than cubic.
The three-dimensional frame-
The 27Al MAS NMR spectrum is shown in Figure 9a. This
spectrum evidences the presence of aluminum only in sixfold
coordination, based on the chemical shift range observed close
to 0 ppm. Despite the use of high magnetic field and very fast
spinning rate, the overlapping signals are not resolved. To
improve the resolution, we used the two-dimensional MQ-MAS
technique (Figure 9c) at the two magnetic fields of 9.4 and 20
T. The whole set of 1D and 2D spectra at different fields were
modeled using a single consistent set of parameters given in
Table 2. Because of the strong partial overlap, only three of five
expected aluminum sites could be unambiguously separated
work is composed of corner-shared [AlO ] tetrahedra that are
4
almost regular. There are rather large voids of two types in the
+
framework, where two nonequivalent K cations surrounded by
2−
eight O anions are located. The structure is made of six
[
AlO ] tetrahedra arranged in a hexagonal pattern, five of which
4
are oriented in one direction and one of which is inverted
through a 180° flip. Thus, it consists of two domains of a
structure of lower symmetry in which the tetrahedra are rotated
to give favorable Al−O−Al bond angles.
Two tetrahedrally coordinated aluminum sites could be
expected corresponding to their crystal structures (ICSD-
2
0
8
8774). We note that KAlO is very hygroscopic.
2
Table 2. 27Al Isotropic Chemical Shift (δ ), Mean
27
iso
Al MAS and MQ-MAS NMR spectra (Figure 10) contain
Quadrupolar Constants C , and Relative Intensities
two lines of relative intensity 70:30. The first “narrow”
Q
2
7
Obtained from the Simulation of the Al MAS and MQ-
MAS at 9.4 and 20 T
resonance (C < 2.0 MHz) at an isotropic chemical shift δ
Q
iso
of 75.4 ppm is assigned to KAlO , and the broad line caused by
2
a distribution of second-order quadrupolar interaction to the
27Al
site
δ (ppm) ±
CQ (MHz) ±
intensity (%)
hydrated form KAlO × H O (C ≈ 7.5 MHz, η ≈ 0.6, δ
≈
2
2
Q
Q
iso
0.1 ppm
0.1 MHz
ηQ ± 0.1
± 1
7
5 ppm). The previous studies have shown the similar results:
#
#
#
1
2
3
−1.1
0.5
2.2
2.8
1.1
0.6
0.0
0.1
42
17
39
narrow line with maximum at 75−76 ppm (C = 1.1 MHz, η
Q
Q
=
0.7) for KAlO and broad line for potassium aluminate
2
0.2
hydrate KAlO × H O (C = 6.5 MHz, η = 0.6, δ = (83 ±
2
2
Q
Q
iso
H
DOI: 10.1021/acs.inorgchem.7b02105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 4. 27Al Isotropic Chemical Shift (δ ), Mean
iso
Quadrupolar Constants (C ), and Relative Intensities
Q
2
7
Obtained from the Simulation of the Al MAS and MQ-
MAS at 9.4 and 20 T
27Al
site
δ (ppm) ±
0.2 ppm
CQ (MHz) ±
0.03 MHz
intensity (%)
ηQ ± 0.1
± 1
1
2
3
4
5
6
14.8
16.7
18.7
70.0
71.3
74.3
4.5
7.5
5.3
3.4
6.9
5.6
0.0
0.5
0.6
0.1
0.5
0.1
13
15
37
10
14
11
Figure 10. 27Al MAS (a) and MQ-MAS (b) NMR experimental (black
line) spectra at 20 T and its simulation (color lines) with the
parameters reported in the text of the pure KAlO2.
6
1,62
It is also noted that we observe only one 27Al
3) ppm).
NMR line for KAlO under the experimental conditions applied
2
here (20.0 T and 60 kHz), while the structure contains two Al
2
7
sites. Calculations of Al NMR parameters are in acceptable
agreement with the experimental data. The calculated C values
Q
of 0.95 and 1.25 MHz and calculated chemical shifts of 74.5 and
7
2.8 ppm for KAlO are very similar and in agreement with our
2
measurement.
Figure 11. 27Al MAS (a) and MQ-MAS (b) NMR experimental (black
line) spectra and its simulation (color lines) with the parameters
reported in Table 4 of the pure K Al O .
K Al O (K O × 11Al O ). Potassium β-alumina is known
2
22 34
2
2 3
+
as Al O -rich phase containing small amounts of K ions that
2
3
2
22 34
are essential to its stability. Its hexagonal structure contains two
3
+
2−
formula units and is composed of spinel blocks of Al and O
ions (first parentheses from formula ([Al O ]−[O−K ]−
1
1
16
(1+x)
O ) separated from one another by loosely packed layers of
x/2
+
2− 42
CONCLUSIONS
cations (K and O ), which are mirror planes in the
structure. Every formula unit of the spinel block contains
aluminum ions in two different types of tetrahedral and
octahedral coordination, respectively. The crystalline structure
exhibits four kinds of aluminum atoms 18[AlO ]/6[AlO ]/
■
We have shown a strong advantage to combine different
techniques, such as NMR spectroscopy, X-ray diffraction, and
thermal analysis, to better define the different phases in the
6
4
KF−Al O3 system and thus to help the precise phase
2
43
6
[AlO ]/3[AlO ] (ICSD-201177 K Al O ). Moreover,
4 6 1.58 11 17
description. The part of the KF−Al O phase diagram system
2
3
two crystallographically inequivalent potassium sites have
partial occupancies: 0.1633, 0.3122.
has been established in temperature range of 750−900 °C. This
system cannot be interpreted as a simple binary system, but it
represents the stable diagonal of the quarternary system KF−
Al O −AlF −K O. K AlF and KAlF falling to the simple
Generating an initial model for first-principle DFT
calculations is more complicated when partially occupied sites
are involved. The average structures that result from diffraction
cannot be used directly, and obtaining a statistically relevant
ensemble of structure or large superstructure is computationally
challenging. Therefore, for these compounds different
approaches were applied, but the results obtained were not
satisfactory. To the extent of our knowledge no NMR analysis
has been performed for this compound. The crystals of
K Al O sample were analyzed by X-ray diffraction, and it
2
3
3
2
3
6
4
binary system KF−AlF and K O × 1Al O , K O × 11Al O are
3
2
2
3
2
2
3
oxo-aluminate complexes from simple binary system K O−
2
Al O , respectively. A new, complex compound K Al O F
2
3
2
2
3 2
represents, as the only one, the full member of overall
quarternary system. A similar structural coherence between
oxo-aluminates of mullite type (K O × 3−12Al O ) was
2
2
3
predicted, which was reached particularly by use of new NMR
techniques. The structure characterization using various solid-
state NMR methods including MQ-MAS and D-HMQC has
provided new and clearer insights into links between the
structures of individual existing complexes.
2
22 34
contains only patterns corresponding to the ICSD-201177.
27
Solid-state Al MAS NMR showed a sharp strong peak at 75.2
ppm indicating tetrahedral coordination sites and a broader
peak at 15 ppm indicating octahedral coordination sites. The
intensity ratio of the two contribution I[AlO ]/I[AlO ] = 1.94
6
4
ASSOCIATED CONTENT
is in rather good agreement with the value of 1.87 expected
from the sites multiplicities. As in the case of α-K AlF we
■
3
6
*
S
Supporting Information
employed MQ-MAS method to improve resolution. The whole
set of 1D MAS and 2D MQ-MAS spectra can be modeled using
the parameters reported in Table 4 (Figure 11). As shown in
Figure 9 we can separate three Al , three Al sites. The
structure contains four nonequivalent aluminum sites−but a
reasonable fit of the spectra requires at least six peaks. This is
probably due to Al sites with different surroundings and
symmetry as well as possible distributions of those due to
partial occupations of potassium sites.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02105.
IV
VI
XRD analysis of the samples with 7 mol % of Al O from
2
3
different phase fields, 27Al− F D-HMQC HETCOR
19
MAS NMR experiments of the samples with 12 and 20
mol % of Al
O , the X-ray patterns of the prepared
2
3
reference compounds (PDF)
I
DOI: 10.1021/acs.inorgchem.7b02105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
̌
́
12) Grjotheim, K.; Holm, J. L.; Krohn, C.; Matiasovsky, K.
AUTHOR INFORMATION
■
Discussion of the constitution and properties of the electrolyte in
relation to aluminium production. Svensk Kem. Tidskr. 1966, 78 (10),
Corresponding Author
*
E-mail: uachsim@savba.sk. Phone: 00421-2-59410495. Fax:
0421-2-59410444.
ORCID
5
(
47−567.
0
13) Sterten, Å Structural Entities in NaF−AlF Melts Containing
3
Alumina. Electrochim. Acta 1980, 25, 1673−1677.
̌
(14) Robert, E.; Olsen, J. E.; Danek, V.; Tixhon, E.; Østvold, T.;
Gilbert, B. Structure and Thermodynamics of Alkali Fluoride−
Aluminum Fluoride−Alumina Melts. Vapor Pressure, Solubility, and
Raman Spectroscopic Studies. J. Phys. Chem. B 1997, 101, 9447−9457.
(15) Lacassagne, V.; Bessada, C.; Florian, P.; Bouvet, S.; Ollivier, B.;
Coutures, J.-P.; Massiot, D. Structure of High-Temperature NaF−
Frantisek Simko: 0000-0003-2390-1349
̌
Vladimír Danielik: 0000-0002-3517-820X
Author Contributions
F.S. and A.R. conceived the ideas, designed the experiments,
and performed the thermal analysis experiments. A.R.
conducted of the all NMR experiments and calculations with
assistance from P.F. and C.B., while Z.N. and M.K. performed
the synthesis experiments. M.K. and V.D. cowrote the
manuscript. All authors have given approval to the final version
of the manuscript.
AlF
106, 1862−1868.
16) Kucharík, M.; Boca
Oxofluoroaluminates Exist at Room Temperature? Eur. J. Inorg. Chem.
005, 2005, 1781−1787.
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−Al O Melts: A multinuclear NMR study. J. Phys. Chem. B 2002,
3
2 3
(
̌
, M.; Bessada, C.; Fuess, H. Do Sodium
2
(
Notes
and Selected Physical Properties of Inorganic Fluorides and Oxide-
The authors declare no competing financial interest.
Fluorides. Chem. Rev. 2015, 115, 1191−1254.
(18) Schaefer, G. W.; De Kroon, A. P.; Aldinger, F. Effect of
ACKNOWLEDGMENTS
aluminum raw materials on the formation of potassium-beta-aluminas.
■
Solid State Ionics 1995, 81, 43−51.
This study was financially supported by the CAMPUS
FRANCE (PHC STEFANIK Project No. 31799NM), the
Slovak bilateral project (No. SK-FR-2013-0039), and Slovak
Grant Agencies (APVV-15-0738, APVV-15-0479, VEGA-2/
116/14, and VEGA-2/0114/16. For DFT calculations, we
thank the “Centre de Calcul Scientifique en region Centre”
(19) Abakumov, A. M.; King, G.; Laurinavichute, V. K.; Rozova, M.
G.; Woodward, P. M.; Antipov, E. V. The Crystal Structure of α-
K AlF : Elpasolites and Double Perovskites with Broken Corner-
3
6
Sharing Connectivity of the Octahedral Framework. Inorg. Chem.
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Cristobalite-Related Phases in the KAlO −KAlSiO System. J. Solid
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