Phase transitions in K3AlF6

Article abstract of DOI:10.1016/j.jssc.2005.10.044

Phase transitions in the elpasolite-type K₃AlF₆ complex fluoride were investigated using differential scanning calorimetry, electron diffraction and X-ray powder diffraction. Three phase transitions were identified with critical temperatures T1=132°C, T2=153°C and T3=306°C. The α-K₃AlF₆ phase is stable below T₁ and crystallizes in a monoclinic unit cell with a=18.8588(2)A, b=34.0278(2)A, c=18.9231(1)A, β=90.453(1)° (a=2a_c-c_c, b=4b_c, c=a_c+2c_c; a_c, b_c, c_c - the basic lattice vectors of the face-centered cubic elpasolite structure) and space group I2/a or Ia. The intermediate β phase exists only in very narrow temperature interval between T₁ and T₂. The γ polymorph is stable in the T2a=36.1229(6)A, b=17.1114(3)A, c=12.0502(3)A (a=3a_c-3c_c, b=2b_c, c=a _c+c_c) at 250 °C and space group Fddd. Above T ₃ the cubic δ polymorph forms with ac=8.5786(4)A at 400 °C and space group Fm3m. The similarity between the K ₃AlF₆ and K₃MoO₃F₃ compounds is discussed.

Full text of DOI:10.1016/j.jssc.2005.10.044

ARTICLE IN PRESS
Journal of Solid State Chemistry 179 (2006) 421–428
www.elsevier.com/locate/jssc
Phase transitions in K AlF6
3
a,
Artem M. Abakumov , Marta D. Rossell , Anastasiya M. Alekseeva , Sergey Yu. Vassiliev ,
Ã
b
a
a
a
Svetlana N. Mudrezova , Gustaaf Van Tendeloo , Evgeny V. Antipov
b
a
a
Department of Chemistry, Moscow State University, Moscow 119992, Russia
b
EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
Received 26 September 2005; received in revised form 26 October 2005; accepted 30 October 2005
Available online 5 December 2005
Abstract
Phase transitions in the elpasolite-type K
3 6
AlF complex fluoride were investigated using differential scanning calorimetry, electron
diffraction and X-ray powder diffraction. Three phase transitions were identified with critical temperatures T1 ¼ 132 1C, T2 ¼ 153 1C
˚
and T3 ¼ 306 1C. The a-K
3
AlF
6
phase is stable below T
b ¼ 34:0278ð2ÞA, c ¼ 18:9231ð1ÞA, b ¼ 90:453ð1Þ1 (a ¼ 2a ꢀc
centered cubic elpasolite structure) and space group I2/a or Ia. The intermediate b phase exists only in very narrow temperature interval
between T and T
. The g polymorph is stable in the T2oToT3 temperature range and has an orthorhombic unit cell with
a ¼ 36:1229ð6ÞA, b ¼ 17:1114ð3ÞA, c ¼ 12:0502ð3ÞA (a ¼ 3a
1
and crystallizes in a monoclinic unit cell with a ¼ 18:8588ð2ÞA,
˚
˚
c
c
, b ¼ 4b , c ¼ a +2c ; a , b , c —the basic lattice vectors of the face-
c
c
c
c
c
c
1
2
˚
˚
˚
c
ꢀ3c
c
, b ¼ 2b
c
, c ¼ a
c c 3
+c ) at 250 1C and space group Fddd. Above T the
˚
¯
cubic d polymorph forms with ac ¼ 8:5786ð4ÞA at 400 1C and space group Fm3m. The similarity between the K AlF and K MoO F
3
6
3
3 3
compounds is discussed.
r 2005 Elsevier Inc. All rights reserved.
3 6
Keywords: K AlF ; Elpasolite; Phase transition; Electron diffraction
1
. Introduction
that the room temperature form of K AlF has a
3 6
tetragonally distorted elpasolite-type structure, which
transforms into the high symmetry cubic phase above
300–310 1C [4,5]. Jenssen reported two phase transitions at
131 1C from the tetragonal to an orthorhombic phase and
at 318 1C from the orthorhombic to the cubic phase [6]. No
structural characterization is available, however. Never-
theless, this compound deserves particular attention, not
only as an important representative of a wide structural
family, but also as a compound of high technological
importance. Due to increasing ecological demands to the
aluminum production using high-temperature electrolysis
of cryolite-alumina melt, alternative technologies based on
modified melts with lower melting point are widely
discussed. K AlF is considered as one of the basic
Many fluorides, oxyfluorides and oxides with the general
0
A BB X composition are known to crystallize in the
2
6
elpasolite (K NaAlF ) (or ordered double perovskite)
6
2
structure. The parent structure of these compounds can
be easily derived from the perovskite structure type by
0
ordering the octahedrally coordinated B and B cations
over the B-sublattice in a chess-board manner reminiscent
of the atomic arrangement in the rock salt structure
(
Fig. 1). According to Anderson et al. [1], such ordering is
0
driven by charge and size differences between the B and B
cations. The pecularities of the preparation techniques,
crystal chemistry and phase transitions in these compounds
are described in several comprehensive reviews [1–3].
However, only limited information is available on
K AlF being compared with its closest analogues cryolite
3
6
components of the melts for low-temperature electrolysis
in aluminum smelting [7]. A precise knowledge of the cell
dimensions of all melt constituents is necessary to analyze
the frozen melt using X-ray powder diffraction. We
discovered that the room temperature X-ray powder
diffraction pattern of K AlF cannot be fully interpreted
3
6
Na AlF and elpasolite K NaAlF . It has been reported
3
6
2
6
Ã
Corresponding author. Fax: 95 939 47 88.
E-mail address: abakumov@icr.chem.msu.ru (A.M. Abakumov).
3
6
0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2005.10.044

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422
Thermogravimetric (TG) and differential scanning ca-
lorimetric (DSC) measurements were performed on a
NETZSCH STA 409 thermoanalyzer in the temperature
range from 20 to 800 1C with heating/cooling rates of
10 1C/min.
Electron diffraction (ED) investigation was made on a
crushed K AlF sample deposited on holey carbon grids.
3
6
ED patterns in the temperature range from room
temperature to 350 1C were obtained in a Philips CM20
electron microscope equipped with a double-tilt heating
holder.
3. Results
The TG curve in the temperature range from 20 to
800 1C (Fig. 2, top) does not show any feature, confirming
the constant chemical composition of the sample over the
entire temperature range. The heating DSC curve exhibits
three well pronounced peaks (Fig. 2, top) at T1 ¼ 132 1C,
T2 ¼ 153 1C and T3 ¼ 306 1C. The sharpness of the peaks
allows to attribute them to first-order phase transitions.
0
Fig. 1. The crystal structure of the A
2
BB X
6
elpasolite. Larger B cations
0
are situated at the centers of the grey octahedra, the smaller B X
octahedra are darker shaded. The A cations are shown as spheres.
6
The transition temperature T ¼ 306 1C coincides well with
3
with the tetragonal unit cell proposed by Steward et al. [5].
This inspired us to undertake a more detailed investigation
of this compound at different temperatures. In this
contribution we report a sequence of phase transitions in
K AlF along with the data on unit cell dimensions and
the temperature of the polymorphic transformation
(310 1C) in K AlF measured by K. Grojtheim et al. [4].
3
6
Steward et al. [5] reported that the cubic K AlF phase
3
6
exists above 300 1C and transforms into a tetragonal phase
on cooling, but Jenssen [6] observed a transition between
an orthorhombic and the cubic polymorph at 318 1C.
According to Jenssen [6], the critical temperature T1
corresponds to the tetragonal 2 orthorhombic phase
3
6
space symmetry of three major polymorphs.
2
. Experimental
transition. The critical temperature T coincides on heating
3
Samples of K AlF were prepared from anhydrous AlF
3
‘‘Reakhim’’, ‘‘pure’’ purity grade) and KF. The raw AlF3
was additionally annealed at 300 1C for 12 h to remove
possible traces of absorbed water. According to Xu et al.
[8] such thermal treatment allows to avoid pyrohydrolisis.
Anhydrous KF was prepared by a dehydration of
and cooling curves (Fig. 2, bottom), whereas the two
transitions at 132 and 153 1C produce a single peak at
129 1C on the cooling DCS curve due to a noticeable
hysteresis. This thermal behavior was reproduced in several
heating and cooling cycles, indicating the reversibility of
the phase transitions. One can therefore expect the
existence of four K AlF polymorphs in the studied
3
6
(
KF ꢁ 2H O (‘‘Reakhim’’, ‘‘pure for analysis’’ purity grade)
2
3
6
at 500 1C. The purity of the initial materials was confirmed
using X-ray powder diffraction. A sample with an overall
temperature interval: the a polymorph is stable at room
temperature, the intermediate b polymorph only exists in a
very narrow temperature interval between 132 and 153 1C,
the g polymorph between 153 1C and 306 1C and the high-
temperature d polymorph, that is stable above 306 1C.
In order to determine the crystallographic characteristics
of K AlF at different temperatures an electron diffraction
mass of 60 g and a KF:AlF ¼ 3.1:1 ratio was prepared in a
3
glove box, placed in a closed graphite crucible, melted at
1
000 1C for 1.5 h and cooled down to room temperature
with 100 1C/h cooling rate. The crystallized melt was
crushed and the excess of KF was removed by washing
with distilled water.
Phase analysis and cell parameter determination at room
temperature were performed using X-ray powder diffrac-
tion (XRPD) with a Huber G670 Guinier diffractometer
3
6
investigation was undertaken. The most prominent ED
patterns of the room temperature a phase are shown in
Fig. 3. The brighter spots can be attributed to the elpasolite
sublattice; they can be indexed on a face centered cubic
˚
(
CuKa -radiation, image plate detector) and a STADI-P
lattice with a ꢂ 8:5 A. Weaker spots are the superlattice
1
diffractometer (CuKa -radiation, curved Ge monochroma-
reflections. Breaking the crystal symmetry at a phase
transition introduces numerous orientational variants due
to the loss of rotation axes and mirror planes [10], and
many investigated crystals were found to be highly
twinned. Nevertheless, single domain areas were also
observed, and the ED patterns on Fig. 3 were taken from
crystallites free from twinning. The superlattice reflections
1
tor, transmission mode, linear PSD). High-temperature
XRD patterns were obtained with a SIEMENS D500
powder diffractometer (CuKa radiation, reflection mode)
equipped with a BRAUN position-sensitive detector. The
JANA2000 program package was used for the refinement
of the lattice parameters by Le Bail fitting [9].

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423
3 6
Fig. 2. TG and DSC curves for K AlF .
can be indexed on a body-centered tetragonal lattice with
microscope is difficult due to the additional heating by the
electron beam, and only the direct a ! g transformation
was observed. The ED patterns of g-K AlF taken in the
˚
˚
unit cell parameters a ꢂ 18:9 A, c ꢂ 34:0 A. However, the
precision of the electron diffraction technique allowed to
detect deviations from the tetragonal symmetry: the a=b
3
6
temperature range 200–250 1C are shown in Fig. 4. As for
ꢄ
ratio was found to be ꢃ0.996 and b ꢂ 89:551. Further
a-K AlF , the elpasolite sublattice can easily be identified
3
6
interpretation of the XRPD pattern of the room tempera-
ture phase (see Fig. 8, bottom) supports decreasing
symmetry down to monoclinic. According to that, the
indexation on Fig. 3 is consistently done with a monoclinic
by the brighter basic spots. A complete indexation of the
ED patterns was performed on the base of a face-centered
˚
orthorhombic lattice with unit cell parameters a ꢂ 36:1 A,
˚
˚
b ꢂ 17:1 A, c ꢂ 12:04 A. The reflection conditions 0kl,
k þ l ¼ 4n, h0l, h þ l ¼ 4n, hk0, h þ k ¼ 4n allow to chose
the space group Fddd. Very weak reflections which do not
obey the reflection conditions imposed by d glide planes
can be observed on the [203]* and [101]* ED patterns. They
appear due to multiple diffraction since these forbidden
reflections do not show up in the [010]* and [001]* ED
patterns. The transformation matrix from the cubic
˚
˚
˚
unit cell with a ꢂ 18:9 A, b ꢂ 34:0 A, c ꢂ 18:9 A, b ꢂ 89:61.
The reflection conditions for a body-centered lattice
(
hkl; h þ k þ l ¼ 2n) and a glide plane perpendicular to
the b axis (h0l; h; l ¼ 2n) were clearly obeyed, resulting in
the possible space groups I2/a and Ia (non-standard
settings of the space groups C2/c (No. 15) and Cc (No.
9
), respectively). The transformation matrix from the cubic
˚
˚
elpasolite subcell with a ꢂ 8:5 A to the monoclinic supercell
elpasolite subcell with a ꢂ 8:5 A to the orthorhombic
is given as
supercell is given as
0
1
0
1
2
0
1
0
4
0
ꢀ1
3
0
2
0
3
B
@
C
B
C
0 A.
@ 0
0 A.
2
1
ꢀ1
Various slow heating rates (5 and 10 1C/min) were
Heating the sample above T3 makes the compound
extremely unstable under the incident electron beam.
Nevertheless, the observation of the behavior of the
superlattice reflections on going across the third phase
applied in order to observe the b polymorph, which exists
only in a very narrow temperature range. However, a
precise temperature control while heating in the electron

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424
Fig. 3. Electron diffraction patterns of a-K
3
AlF
6
. The hkl
c
indexes belong to the parent cubic face-centered elpasolite structure.
transition was performed. Fig. 5 shows the ED pattern
taken from a crystallite of the g-K AlF at 220 1C along the
unavoidably results in strongly twinned crystallites. This
is depicted in Fig. 6. An ED pattern from a crystallite of
the a-K AlF polymorph at room temperature along the
3
6
[203]* zone axis. Upon heating up to 340 1C the superlattice
spots disappear completely resulting in the [001]* ED
3
6
[010]* zone axis is shown at the top of the figure. Upon
heating up to 175 1C the a ! g transformation takes place
resulting in the [010]* ED pattern of the g polymorph. On
cooling down again to 25 1C the single domain is not
recovered and a twinned structure with two [010]*
orientational variants of the a-K AlF phase is formed.
pattern of the d polymorph, which has a face-centered
˚
cubic elpasolite-type structure with a ꢂ 8:55 A. Being
cooled down to 275 1C the g polymorph is completely
restored, but now in the [010]* orientation. Also the
reversibility of the a2g phase transition was observed by
heating and cooling the sample inside the electron
microscope, but the g ! a transformation on cooling
3
6
Twinned structures are also observed for the g-K AlF
3
6
polymorph from a direct heating of the a-K AlF phase. In
3
6

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A.M. Abakumov et al. / Journal of Solid State Chemistry 179 (2006) 421–428
425
3 6 c
Fig. 4. Electron diffraction patterns of g-K AlF . The hkl indexes belong to the parent cubic face-centered elpasolite structure.
Fig. 7 an ED pattern is shown with four orientational
variants of the g-K AlF polymorph.
the pattern of the a phase (the indices of the sublattice
reflections can be revealed when comparing with the
XRPD pattern of the d phase (top in Fig. 8)). On the
XRPD pattern of the g phase the superlattice reflections
change their position and intensity. Both the superlattice
reflections and splitting of the peaks of the basic structure
disappear on the XRPD pattern of the d polymorph. The
sublattice reflections on the XRPD pattern of the a phase
were indexed on a tetragonal F-centered lattice with
3
6
Due to the instability of the sample under the focused
electron beam, required for recording high resolution
electron microscopy images, no investigation of the atomic
and defect structure in real space has been successful.
The unit cells derived from the ED data were further
used for indexation of the XRPD patterns. The XRPD
patterns of the a, g and d phases are shown in Fig. 8. The
superlattice reflections and splitting of the basic reflections
due to the pseudotetragonal unit cell are clearly visible on
˚
˚
a ¼ 8:427ð1ÞA, c ¼ 8:497ð1ÞA, in agreement with the unit
cell reported by Steward et al. [5]. Due to the very large unit

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426
Fig. 5. Transformation of the [203]* ED pattern of the g phase into the
[
3
010]* ED pattern of the d phase upon heating above T and back to the
[
010]* pattern of the g phase on cooling.
3
˚
cell volume (412,000 A ) of the superlattice for a-K AlF
3
6
and the large amount of overlapping reflections, the final
refinement of the unit cell parameters was performed using
Le Bail fitting procedure. The Le Bail fit was performed
using a monoclinic unit cell with I2/a space symmetry,
according to the results of the electron diffraction
observations and obvious analogy with the superlattice
observed recently in K MoO F [11] (see also Section 4
Fig. 6. a 2 g transformations by heating and cooling a K
inside the electron microscope. From the top to the bottom: [010]* ED
pattern of a-K AlF at room temperature, [010]* ED pattern of the g
polymorph at 175 1C, twinned [010]* type zone axis ED pattern of the a-
AlF at 25 1C and schematic representation of the twinning structure.
Black circles indicate the elpasoite sublattice reflections and black and
open squares mark the two different [010]* orientations of the superlattice
reflections, respectively.
3 6
AlF crystallite
3
6
K
3
6
3
3 3
below). The refinement resulted in the unit cell parameters
˚
˚
˚
a ¼ 18:8588ð2ÞA,
b ¼ 34:0278ð2ÞA,
c ¼ 18:9231ð1ÞA,
b ¼ 90:453ð1Þ1. For the g polymorph the Le Bail fitting

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427
the mechanisms of the structural distortions behind these
phase transitions could possibly be obtained using a
synchrotron X-ray source, providing XRPD patterns with
an ultimate resolution, which can play key role in the
structure solution.
4. Discussion
The a-K AlF superstructure is identical to that found in
3
6
the room temperature polymorph of K MoO F [11].
3
3 3
However, in addition to the sharp superlattice spots, a
highly structured three-dimensional continuous diffuse
intensity is present on the ED patterns of K MoO F .
3
3 3
Withers et al. suggested that the diffuse intensity origi-
nates from the O/F ordering in K MoO F , whereas the
3
3 3
sharp superlattice spots have a displacive, rather than a
diffusion controlled origin [12]. In our experiments on
K AlF we never observed structured diffuse intensity,
3
6
which is in agreement with the presence of only one type
of anion in the structure. The obvious similarity between
the superstructures in a-K AlF and K MoO F leads to
3
6
3
3 3
the assumption about an identical mechanism of struc-
tural distortions resulting in breaking down the cubic
symmetry of the parent elpasolite-type structure. It is
hardly possible to expect that the phase transitions in
K AlF occur due to diffusion process, for example due to
3
6
re-distribution of the K and Al atoms over the octahedral
sites, because fast atomic long range motion at the
temperatures of 132–306 1C seems to be highly unlikely.
One must assume that the phase transitions in K AlF
3
6
have a displacive character. The most common origin of
displacive phase transitions in the elpasolite-like fluo-
rides and oxyfluorides, as well as in the double perovskites
0
A BB (O,F) , are cooperative tilts and rotations of the
2
6
0
B(O,F)6 and B (O,F) octahedra in order to optimize
6
the coordination environment for the A-cations, if they
are too small to be placed into 12-coordinated cavities
¯
of the parent cubic Fm3m structure. Group theory ana-
Fig. 7. ED pattern with four orientational twinned variants of the g-
AlF polymorph. Black circles indicate the sublattice reflections, while
K
3
6
black and open squares and black and open triangles mark the superlattice
reflections of the two different [010]* and [203]* orientations, respectively.
lysis performed for such distortions by Howard et al. [2]
and Flerov et al. [3] revealed the unit cell expansions
and space symmetries for all possible tilt systems, origi-
nally derived by Glazer [13]. The superstructures observed
in K AlF and K MoO F are not among these possibi-
was performed with the orthorhombic Fddd unit cell. The
refined values of the lattice parameters are a ¼
3
6
3
3 3
lities. However the tilt systems and corresponding symme-
tries were derived assuming that both the B(O,F)
˚
˚
˚
3
6:1229ð6ÞA, b ¼ 17:1114ð3ÞA, c ¼ 12:0502ð3ÞA at 250 1C.
6
0
The XRPD pattern of the d phase was indexed on a face-
and B (O,F) octahedra behave as rigid units. As pro-
6
˚
centered cubic lattice with a ¼ 8:5786ð4ÞA at 400 1C and
posed by Withers et al, this is not necessarily true for
the KF6 octahedra [12]. In this case a rotation of the
AlF6 octahedra acquires additional degrees of freedom
that can result in much more complicated structural
distortions, which can be combined with the off-center
displacements of the B-cations creating puzzling super-
structures. A structure solution for the low temperature
K AlF and K MoO F phases is needed to make an
¯
space group Fm3m.
Unfortunately, the extremely large unit cell volumes for
3
3
˚
˚
a-K AlF (12144 A ) and g-K AlF (7448 A ) together with
3
6
3
6
the low symmetry make it impossible to determine and
refine these structures from X-ray powder diffraction data.
It is hardly possible to expect that single crystals of these
phases with a quality suitable for structure determination
will be prepared because of the unavoidable twinning due
to the phase transitions. Further progress in understanding
3
6
3
3 3
unambiguous conclusion about the mechanism of the
phase transitions.

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428
3 6 3 6 3 6
Fig. 8. Parts of the X-ray powder diffraction patterns of a-K AlF (RT), g-K AlF (250 1C) and d-K AlF (400 1C).
Acknowledgement
[5] E.G. Steward, H.P. Rooksby, Acta Crystallogr. 6 (1953) 49.
[
6] B. Jenssen, Thesis, Institute of Inorganic Chemistry, Norwegian
Technical University, Trondheim, 1969 (cited from R. Chen,
Q. Zhang, Thermochim. Acta 335 (1999) 135).
This work was supported by the RusAl project ‘‘Devel-
opment of the composition of new electrolytes’’. AMA is
grateful to the Russian Science Support Foundation for the
financial support.
[
7] J. Yang, J.N. Hryn, B.R. Davis, A. Roy, G.K. Krumdick,
J.A. Pomykala Jr., in: T. Alton Tabereaux, (Ed.), Light Metals
2004, TMS (The Minerals, Metals & Materials Society), 2004, p. 321.
8] X. Delong, L. Yongqin, J. Ying, Z. Longbao, G. Wenkui,
Thermochim. Acta 352–353 (2000) 47.
[
[
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