Pseudo-binary systems CsAlF4-KAlF4 and Cs3AlF6-K3AlF6

Article abstract of DOI:10.1016/s0040-6031(99)00196-3

Phase relations in the system CsAlF₄-KAlF₄ and Cs₃AlF₆-K₃AlF₆ were investigated by DTA and XRD methods. In the system CsAlF₄-KAlF₄, two intermediate compounds 3CsAlF₄·KAlF₄ and CsAlF₄·KAlF₄ were confirmed. The first one was indexed as orthorhombic with a=9.079±0.003, b=7.958±0.002, c=7.648±0.004A?. The second one α-CsAlF₄·KAlF₄ is also orthorhombic, a=10.982±0.005, b=10.093±0.006, c=6.160±0.002A?. Among the two compounds and KAlF₄, two eutectic temperatures e₁ and e₂ are formed. The first eutectic temperature e₁ is located in 42mol % KAlF₄ at 525°C, and e₂ in 70mol % KAlF₄ at 510°C. The system Cs₃AlF₆-K₃AlF₆ is a successive solid solution system with the lowest melting point, the composition of which is 75mol % Cs₃AlF₆ at 775°C. In the solid solution, compound 2Cs₃AlF₄·K₃AlF₆ was formed at 656°C, and was indexed as a cubic cell with a=8.891A?.

Full text of DOI:10.1016/s0040-6031(99)00196-3

Thermochimica Acta 335 (1999) 135±139
Pseudo-binary systems CsAlF4±KAlF4 and Cs3AlF6±K3AlF6
Rong Chen*, Qiyun Zhang
Department of Chemistry, Peking University, Beijing 100871, P.R. China
Received 24 March 1999; accepted 22 June 1999
Abstract
Phase relations in the system CsAlF4±KAlF4 and Cs3AlF6±K3AlF6 were investigated by DTA and XRD methods. In the
system CsAlF4±KAlF4, two intermediate compounds 3CsAlF4ÁKAlF4 and CsAlF4ÁKAlF4 were con®rmed. The ®rst one was
Ê
indexed as orthorhombic with a = 9.079 Æ 0.003, b = 7.958 Æ 0.002, c = 7.648 Æ 0.004 A. The second one a-CsAlF4ÁKAlF4
Ê
is also orthorhombic, a = 10.982 Æ 0.005, b = 10.093 Æ 0.006, c = 6.160 Æ 0.002 A. Among the two compounds and KAlF4,
two eutectic temperatures e1 and e2 are formed. The ®rst eutectic temperature e1 is located in 42 mol % KAlF4 at 5258C, and
e2 in 70 mol % KAlF4 at 5108C. The system Cs3AlF6±K3AlF6 is a successive solid solution system with the lowest melting
point, the composition of which is 75 mol % Cs3AlF6 at 7758C. In the solid solution, compound 2Cs3AlF4ÁK3AlF6 was formed
Ê
at 6568C, and was indexed as a cubic cell with a = 8.891 A. # 1999 Elsevier Science B.V. All rights reserved.
Keywords: CsAlF4; KAlF4; Cs3AlF6; K3AlF6; Phase diagram
1. Introduction
Tianjin Chem. Works) was heated in N2 plus HF
atmospheres at 6008C for 2 h, the product was identi-
®ed as anhydrous AlF3 by XRD. All ¯uorides were
kept in a desiccator.
The systems CsAlF4±KAlF4 and Cs3AlF6±K3AlF6
are two polythermal sections in the ternary system
AlF3±CsF±KF, in which the three-side binary systems
AlF3±KF [1], AlF3±RbF [2] and AlF3±CsF [3] have
been well investigated. However, no one has published
about the inner sections. Given that the chemical
formula of the components is similar, we were ®rstly
interested in the research of the mentioned systems.
2.2. Preparation of samples
Samples were prepared by reacting mixtures of
anhydrous AlF3 with a solution of CsF in HF. Products
were placed in Pt crucibles and heated until dry at
2008C, then annealed for 48 h at a higher temperature
at which no melting of any phase would occur. During
the annealing process, grinding and mixing of the
samples were repeatedly carried out in order to obtain
homogeneous and equilibrium samples.
2. Experimental
2.1. Preparation of fluorides
CsF (purity > 99.5%, Beijing Chem. Works) was
dehydrated at 4008C for 3 h; AlF3Á3.5H2O (A.R.;
2.3. Method for determination
*Corresponding author. Fax: +86-10-627-51496
E-mail address: qyzh@chemms.chem.pku.edu.cn (R. Chen)
A CR-G type high-temperature DTA equipment
(Beijing Optical Instrument) was employed and
0040-6031/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ( 9 9 ) 0 0 1 9 6 - 3

136
R. Chen, Q. Zhang / Thermochimica Acta 335 (1999) 135±139
Table 1
DTA data of the section CsAlF4±KAlF4
KAlF4 (mol %)
Temperatures (8C)
Polymorphic transition
Eutectic 1
Eutectic 2
Liquidus
0
6
507
487
452
480
523
530
585
590
589
575
545
535
540
560
553
536
523
520
521
550
556
563
575
10
13
16
20
25
30
33.3
35
40
42
45
50
55
60
65
70
75
80
85
90
100
480
527
484
485
524
517
485
483
479
522
526
510
513
514
512
495
478
475
490
511
511
507
513
calibrated by standard substances with known
melting point (calibrating both, the heating and
cooling curves). Al2O3 was used as a reference sub-
stance. The heating rate was 158/min. Liquidus
temperature was determined from the cooling
curve, and the other temperatures were determined
by extended extrapolating initial temperature of the
peaks in the heating curve. Experiments were carried
out in dry air (relative humidity <30%) in the static
state. The error in measuring the temperature was
Æ38C.
3. Results and discussion
Phase diagrams of CsAlF4±KAlF4 and Cs3AlF6±
K3AlF6 based on the results of DTA data listed in
Tables 1 and 2 are given in Figs. 1 and 2.
Fig.
1 reveals two intermediate compounds
3CsAlF4ÁKAlF4 and CsAlF4ÁKAlF4 congruently melt-
ing at 5858C and 5608C, respectively.
CsAlF4 formed in the solid state just below the
eutectic temperature between Cs3AlF6 and CsFÁ2AlF3
at 4438C [4], so we can hardly consider that the
vertical axis of CsAlF4 in Fig. 1 is a dependent
component. The phase relations between CsAlF4
and 3CsAlF4ÁKAlF4 were too complicated to draw
up the exact picture. We could only leave the liquidus
on the diagram. The lowest point (m) on liquidus is
located in 90 mol % CsAlF4 at 4508C, and also the two
eutectic temperatures e1 and e2 were found in 58 mol
% CsAlF4 at 5258C and 30 mol % CsAlF4 at 5108C,
respectively.
In addition, visual polythermal method was also
used to determine some liquidus temperatures.
2.4. X-ray powder diffraction analysis
The intermediate compounds in the sections were
determined by Rigaku Dmax 2400 X-ray diffract-
Ê
ometer (Radiation Cu Ka, ꢀ = 1.5409 A, ®lter Ni).
Si powder was added as a cross reference for ®ne-
tuning the results during determination.
The compounds 3CsAlF4ÁKAlF4 and CsAlF4Á
KAlF4 have not been reported so far. Their XRD

R. Chen, Q. Zhang / Thermochimica Acta 335 (1999) 135±139
137
Table 2
DTA data of the system Cs3AlF6±K3AlF6
K3AlF6 (mol %)
Temperatures (8C)
Solid reaction
Solidus
Liquidus
0
3.0
6.0
10
790
775
780
775
775
778
775
784
795
807
810
832
850
877
902
920
960
995
668
663
645
660
610
650
645
660
665
665
654
658
658
670
669
770
770
15
20
25
770
30
779
790
790
790
793
795
850
33.3
40
45
50
60
70
Fig. 2. Diagram of the section CsAlF4±KAlF4.
75
80
890
943
90
100
Fig. 2 gives a result that the pseudo-binary system
Cs3AlF6±K3AlF6 is a successive solid solution system
with a lowest melting point `m', at which the compo-
sition is 75 mol % Cs3AlF6 at 7758C. The intermediate
compound 2Cs3AlF4ÁK3AlF6 formed at 6568C when
the solid solution was cooled. XRD and indexed data
for 2Cs3AlF4ÁK3AlF6 is presented in Table 5. The
and indexed data are presented in Tables 3 and 4.
3CsAlF4ÁKAlF4 is orthorhombic with a = 9.079 Æ
Ê
0.003, b = 7.958 Æ 0.002, c = 7.648 Æ 0.004 A. The
transformation of CsAlF4ÁKAlF4 takes place at 4858C.
a-CsAlF4ÁKAlF4 is also orthorhombic, a = 10.982 Æ
Ê
compound is in cubic, a = 8.891 A.Each system in
CsAlF4±KAlF4 and Cs3AlF6±K3AlF6 has the same
component formula; however, different types of phase
diagram were observed. In the system CsAlF4±KAlF4,
the two components are not isostructural. KAlF4 has
only one type of structure Ð tetragonal, however,
CsAlF4 has two types, and the polymorphic transition
is as follows [3]:
Ê
0.005, b = 10.093 Æ 0.006, c = 6.160 Æ 0.002 A.
ꢁ-CsAlF4 ꢀhexagonal†
ꢁ
422 C ꢂ-CsAlF4 ꢀorthorhombic†
!
The ionic structures of Cs+ and K+ are quite
similar, and the difference between their ionic radii
is not so large. The formation of two intermediate
compounds 3CsAlF4ÁKAlF4 and CsAlF4ÁKAlF4 could
only be explained as due to their being non-iso-
structural.
In the system Cs3AlF6±K3AlF6 continous solid
solution with lower melting temperature was
observed. Cs3AlF6 has only cubic structure (Amorasit
[4] reported tetragonal formation below 2908C), and
Fig. 1. Diagram of the pseudo-binary system Cs3AlF6±K3AlF6.

138
R. Chen, Q. Zhang / Thermochimica Acta 335 (1999) 135±139
Table 3
Indexed dataa of 3CsAlF4ÁKAlF4
Table 4
Indexed dataa of CsAlF4ÁKAlF4
Ê
d (A) obs.
Ê
d (A) calc.
Ê
d (A) obs.
Ê
d (A) calc.
I/I0
h
k
l
I/I0
h
k
l
5.832
3.651
3.545
3.211
3.027
2.917
2.377
2.301
2.198
2.095
5.840
3.645
3.531
3.217
3.025
2.920
2.373
2.300
2.194
2.097
2.090
2.039
1.971
1.943
1.940
1.895
1.894
1.803
1.773
1.770
1.769
1.767
1.765
1.652
1.539
1.524
1.438
1.350
1.335
1.307
1.285
1.283
1.243
1.204
1.203
23
77
100
51
70
25
80
28
23
33
1
1
0
1
3
2
3
3
2
4
1
3
4
1
2
4
3
1
2
5
3
5
0
5
1
3
4
0
1
0
4
6
6
6
1
0
2
2
1
0
0
0
2
3
1
2
2
2
4
2
1
1
3
4
1
3
0
4
2
3
4
1
5
5
6
5
1
3
4
2
1
0
1
2
0
2
2
1
1
1
3
2
0
0
3
2
3
3
1
0
2
1
2
0
4
2
4
3
3
1
1
3
2
0
6
6.146
5.467
3.907
3.726
3.455
3.166
3.134
3.078
3.011
2.564
2.457
2.373
6.145
5.496
3.910
3.720
3.450
3.184
3.151
3.076
3.005
2.561
2.460
2.372
2.303
2.296
2.294
2.197
1.779
1.658
1.657
1.540
15
12
24
31
23
28
42
100
20
23
20
37
0
2
0
2
3
2
3
0
3
1
1
2
3
3
2
5
4
6
5
0
0
0
2
2
1
2
0
0
1
2
4
2
3
1
4
0
4
2
4
0
1
0
1
0
0
1
1
2
1
2
0
2
1
2
0
0
1
1
0
4
2.040
1.971
23
22
2.295
14
1.942
62
2.194
1.778
1.659
13
15
15
1.892
1.804
19
17
1.540
15
1.769
1.650
75
20
a Orthorhombic cell: a = 10.982 Æ 0.005, b = 10.093 Æ 0.006,
Ê
c = 6.160 Æ 0.002 A.
1.526
1.438
1.350
1.335
1.307
22
22
25
19
23
hombic and tetragonal, and the intermediate com-
pound 2Cs3AlF4ÁK3AlF6 is thus formed.
From the above-mentioned discussion, it appears
that the different crystal structure is a necessary factor
1.283
1.243
1.205
14
13
12
Table 5
Indexed dataa of 2Cs3AlF6ÁK3AlF6
Ê
d (A) obs.
Ê
d (A) calc.
I/I0
h
k
l
a Orthorhombic cell: a = 9.079 Æ 0.003, b = 7.958 Æ 0.002,
Ê
c = 7.648 Æ 0.004 A.
4.462
3.151
2.570
2.224
2.041
1.989
1.816
1.572
1.482
1.406
1.340
1.283
1.233
1.188
1.111
4.458
3.140
2.569
2.225
2.038
1.988
1.816
1.572
1.482
1.405
1.340
1.283
1.233
1.188
1.111
19
100
45
43
4
2
2
2
4
3
4
4
4
6
6
6
4
6
6
8
0
2
2
0
3
2
2
4
0
2
2
4
4
4
0
0
0
2
0
1
0
2
0
0
0
2
4
0
2
0
for K3AlF6, three crystal structures were reported [5].
The polymorphic transition is as follows:
21
40
21
9
ꢁ-K3AlF6 ꢀtetragonal†
131ꢁC
!
ꢂ-K3AlF6 ꢀorthorhombic†
ꢁ
19
8
318 Cꢃ-K3AlF6 ꢀcubic†
!
6
At higher temperatures, the two components have
isostructural cubic cell structures. They form a con-
tinuous solid solution, and as the temperature is
decreased, K3AlF6 changes its structure into orthor-
3
16
2
a
Ê
Cubic cell: a = 8.891 Æ 0.001 A.

R. Chen, Q. Zhang / Thermochimica Acta 335 (1999) 135±139
139
for the formation of the intermediate compounds in a
References
molten salt system, even though the components have
the same chemical formula, same anion, similar ionic
structure and almost identical chemical properties.
[1] B. Phillips, C.M. Warshaw, I. Mockrin, J. Am. Ceram. Soc.
49(12) (1966) 631.
[2] R. Chen, Q.Y. Zhang, Thermochim. Acta 297 (1997) 125.
[3] R. Chen, J. Cao, Q.Y. Zhang, Thermochim. Acta 303 (1997)
145.
Acknowledgements
[4] M. Amorasit, B.J. Holm, J.L. Holm, Acta Chem. Scand. 27(5)
(1973) 1831.
[5] B. Jenssen, Thesis, Institute of Inorgnic Chemistry, Norwe-
gian Technical University, Trondheim, 1969.
The authors wish to acknowledge the support from
Beijing Science Fund.