Ternary chlorides in the systems ACl/YbCl3 (A=Cs,Rb,K)

Article abstract of DOI:10.1016/s0040-6031(98)00326-8

The phase diagrams of the ACl/YbCl₃ (A=Cs,Rb,K) systems were investigated by DTA and XRD. Compounds A₃YbCl₆, A₂YbCl₅ and AYb₂Cl₇ exist in all systems, enneachlorides A₃Yb₂Cl₉ in the systems with A=Cs,K. In the group AYb₂Cl₇, the Yb ions have coordination number 7, in all other compounds the coordination is octahedral. Thermodynamic functions determined by solution calorimetry and emf vs T measurements in galvanic cells for solid electrolytes reveal that the ternary chlorides A_nYbCl_3+n are stable compared with mixtures (nACl+YbCl₃); the compounds Cs₂YbCl₅ and Rb₂YbCl₅ are stable only at temperatures >0 K.

Full text of DOI:10.1016/s0040-6031(98)00326-8

Thermochimica Acta 318 (1998) 29±37
Ternary chlorides in the systems ACl/YbCl3 (AˆCs,Rb,K)
È
Jorg Sebastian, Hans-Joachim Seifert
*
FB19 Inorganic Chemistry, University Gh Kassel, Heinrich-Plett-Str. 40, D-34109 Kassel, Germany
Received 9 June 1997; accepted 15 October 1997
Abstract
The phase diagrams of the ACl/YbCl3 (AˆCs,Rb,K) systems were investigated by DTA and XRD. Compounds A3YbCl6,
A2YbCl5 and AYb2Cl7 exist in all systems, enneachlorides A3Yb2Cl9 in the systems with AˆCs,K. In the group AYb2Cl7, the
Yb ions have coordination number 7, in all other compounds the coordination is octahedral. Thermodynamic functions
determined by solution calorimetry and emf vs T measurements in galvanic cells for solid electrolytes reveal that the ternary
chlorides AnYbCl3‡n are stable compared with mixtures (nACl‡YbCl3); the compounds Cs2YbCl5 and Rb2YbCl5 are stable
only at temperatures >0 K. # 1998 Elsevier Science B.V.
Keywords: Phase diagrams; Ternary ytterbium chlorides; Thermodynamic properties
1. Introduction
methods which have proved very successful for yield-
ing predictions about the mutual stabilities of all
ternary chlorides in the respective systems.
In connection with our research work on ternary
lanthanide chlorides, we have investigated the pseudo-
binary systems of ytterbium(III) chloride with the
alkali metal chlorides CsCl, RbCl and KCl. Hitherto,
DTA measurements on the KCl/YbCl3 [1] and CsCl/
YbCl3 systems [2] were performed and the unit cells
of those compounds, which could be prepared directly
from melts, were determined by Meyer [3] with X-ray
powder patterns.
Our special interests were directed to the question
of whether systematic rules found recently from inves-
tigations of the analogous systems with HoCl3 [4],
ErCl3 [5] and TmCl3 [6] are also valid for ytterbium
compounds. In addition to DTA and XRD, we have
measured solution enthalpies and (free) Gibbs'enthal-
pies with galvanic cells for solid electrolytes ±
2. Experimental
2.1. Chemicals
The starting compound was YbCl3Á6H2O, prepared
from a solution of Yb2O3 (99.9%, Heraeus/Hanau) in
hot hydrochloric acid. Alkali metal chlorides were
dried at 5008C.
2.2. Differential thermal analysis (DTA)
The DTA measurements were performed in a home-
built device for samples (ꢀ0.5 g) in vacuum-sealed
silica ampoules. Samples with <50 mol% YbCl3 were
homogenized by melting in a gas ¯ame, shaking and
*Corresponding author. Fax: +49 561 804 4010.
0040-6031/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved
P I I S 0 0 4 0 - 6 0 3 1 ( 9 8 ) 0 0 3 2 6 - 8

30
J. Sebastian, H.-J. Seifert / Thermochimica Acta 318 (1998) 29±37
annealing. Because YbCl3 melts react with silica,
YbCl3-rich mixtures were melted in the DTA furnace
ꢁ20 K above the alleged liquidus temperatures. In
general, heating curves were recorded (heating rate-
ˆ2 K min 1).
kind of glass. Therefore, measurements could be
performed only for Rb- and K-compounds, AYb2Cl7
excluded.
3. Results
2.2.1. X-ray powder patterns
Powder patterns at ambient temperature were taken
with a Philips PW 1050/25 goniometer equipped with
a proportional counter and a vacuum attachment.
During exposure (CuKꢀ radiation) the samples were
kept under He atmosphere. For high-temperature
photographs, a Simon±Guinier camera (Enraf-Non-
ius) was applied. Corundum powder was used as the
internal standard; at 208C: aˆ475.92 pm and cˆ
1299.00 pm; at 5008C, aˆ479.32 and cˆ1308.92 pm.
3.1. Ytterbium(III) chloride
The starting compound YbCl3Á6H2O with a correct
content of crystal water was prepared by precipitation
from a saturated solution at 08C with HCl-gas. After
®ltration and washing with ether it was dried at 408C.
Surprisingly, its crystal structure was still unknown
[9]. As for the other hexahydrates, it is isotypic to
PrCl3Á6H2O [10], space group P2/n, Zˆ2:
a ˆ 777:3ꢂ4† pm; b ˆ 643:0ꢂ5† pm;
2.2.2. Solution calorimetry
c ˆ 947:4ꢂ9† pm
The heats of solution were measured using an
isoperibolic underwater calorimeter ®rst described
in 1978 [7]. In a silver vessel of 1.3 l volume, samples
of 2 to 4 g in thin-walled glass ampoules are cracked
under water at the beginning of the measurement. The
change of temperature, T, against the surroundings,
in a thermostat with a temperature constancy of
2±10 4 K, generated by the solution enthalpy is
measured with a thermopile. The calibration is done
by Joule heating.
In this structure type, the lanthanide ion is surrounded
by two chloride ions and six water molecules,
‡
[YbCl2(H2O)6] . These cationic groups are held
together by the third Cl -ions.
When heated in a drying oven at 1008C, a trihy-
drate, YbCl3Á3H2O, is formed. According to X-ray
powder patterns, it is isotypic to ErCl3Á3H2O and
TmCl3Á3H2O; the structure is still unknown.
Further dehydration must occur in a stream of HCl
to avoid hydrolysis. At 1208C, a monohydrate
YbCl3ÁH2O with unknown structure is formed. Above
1808C, the last water is removed. For the preparation
of ca. 10 g YbCl3, the temperature must be raised
slowly up to 3508C. (If the product looks grey because
of impurities of carbon, it can be puri®ed by heating
for 1 h in a stream of Cl2 at ca. 5008C.)
At least two samples of each substance were mea-
sured. From the enthalpies of solution, ÁsolH0298, the
enthalpies of formation of the ternary chlorides from
(nACl‡YbCl3), ÁfHo298 were calculated according to
the following:
ÁfH0 ˆ fÁsolH0ꢂYbCl3† ‡ nÁsolH0ꢂACl†g
ÁsolH0ꢂAnYbCl3‡n
†
Laptev [11] has prepared YbCl3 from the oxide and
CCl4 gas at 7008C. His product crystallizes with the
layer structure of AlCl3 (space group C2/m, Zˆ2). If
the compound is prepared at a lower temperature, e.g.
by dehydration of the hydrate, the structure is more or
less disturbed. We have already described this phe-
nomenon previously for DyCl3 [12].
2.2.3. Measurement of emf
A detailed description of the galvanic cell has been
given earlier [8]. The measured potentials were gen-
erated by the solid state reactions:
nACl ‡ AxYbCl3‡x ˆ Aꢂn‡x†YbClꢂ3‡n‡x†
in the 300±4008C range. The solid electrolytes (com-
pressed discs) were separated by A -conducting dia-
3.2. Ternary ytterbium(III) chlorides from solutions
‡
phragms of sintered glass powder. It was not possible
to prepare Cs-glasses, stable against YbCl3; further-
more, YbCl3 itself gave no stable potentials with each
Some ternary chlorides could be prepared conve-
niently from aqueous solutions or from acetic acid, as
far as they are congruently soluble in these solvents. In

J. Sebastian, H.-J. Seifert / Thermochimica Acta 318 (1998) 29±37
31
the case of aqueous solutions, ACl and Yb2O3 were
dissolved with the correct molar ratio in hydrochloric
acid. These solutions were evaporated to dryness at
temperatures of 100±1208C. The following com-
pounds could be prepared.
other incongruently melting ternary chlorides:
K2YbCl5 and K3Yb2Cl9, which are stable only in
the temperature range from 3948 to 4258C.
A typical dif®culty for the investigations of phase
diagrams with incongruently melting compounds
arose. Because of the large temperature difference
of the melting point of K3YbCl6 (8208C) and the
eutectic (4098C at 49.3 mol% YbCl3) a large amount
of K3YbCl6 crystallizes from the melt above the
peritectic for K2YbCl5 at 4558C. The crystals separate
from the melt and, therefore, the peritectic reaction at
4558C is not complete. As a consequence, all other
effects at lower temperatures, namely the peritectic for
K3Yb2Cl9 at 4258C and the eutectic, will appear also
in the range from K3YbCl6 to K2YbCl5. Thus, the
quenched samples have to be annealed to get equili-
brium conditions. The proof for the existence of the
two compounds was eventually found by a combina-
tion of DTA measurements for these annealed samples
and X-ray patterns:
Cs4YbCl7 ± The crystal structure could be solved
by single-crystal measurements, as previously
described [13];
Cs3YbCl6 ± In the orthorhombic modification,
space group Pbcm [14];
Cs2YbCl5ÁH2O ± Erythro-siderite structure (space
group Pnma, Zˆ4).
Theothorhombiccellparametersare:aˆ1455.3(9) pm;
bˆ1046.6(9) pm; cˆ751.9(2) pm ± Cs2YbCl5ÁH2O
can be dehydrated by heating in a stream of HCl at
temperatures rising slowly from 200 to 3008C.
From acetic acid the compounds Cs3YbCl6 and
Rb3YbCl6, both with the Cs3BiCl6 structure (space
group C2/c) could be precipitated with HCl gas,
K3YbCl6 (space group P21/c) was formed with KCl.
The solutions were prepared from YbCl3Á6H2O and
alkali metal carbonate in the correct molar ratio.
K2YbCl5: A sample with 33.3 mol% YbCl3,
annealed at ca. 4308C, only gave one effect at
4558C (peritectic temperature). The X-ray photo
was different from that of a mixture between
K3YbCl6 and KYb2Cl7.
3.3. Phase diagrams and crystal structures
K3Yb2Cl9: A 40% sample, annealed at ca. 3808C,
gave two thermal effects in a heating curve at 3948
and 4258C in addition to the peritectic effect of
K2YbCl5 at 4558C. The X-ray pattern was a
superposition of the peaks of K2YbCl5 and
KYb2Cl7. In a dynamic high temperature photo
new lines appeared at ca. 4008C.
Fig. 1 illustrates the results of the DTA measure-
ments on the systems ACl/YbCl3 (AˆCs,Rb,K). As
already found for ErCl3, YbCl3 also melts and reacts
slowly with quartz. Therefore, the melting point was
measured in a corundum crucible; the value is 1133 K
(8608C). (In the literature: 8648C [11] and 854±
8808C)
In the CsCl/YbCl3 system, four compounds were
found: one congruently melting (Cs3YbCl6) and the
others, incongruently melting Cs2YbCl5, Cs3Yb2Cl9
and CsYb2Cl7. This result is in good agreement with
the ®ndings of Blachnik and Selle [2].
In the RbCl/YbCl3 system, one congruently melting
compound, Rb3YbCl6, and the incongruently melting
compounds Rb2YbCl5 and RbYb2Cl7 exist.
In the KCl/YbCl3 system, according to Novikov [1],
one congruently melting compound, K3YbCl6, exists
with a phase transition at 3858C. We found this
transition at 3928C and a second, which is not shown
in Fig. 1, at 478C. Another compound ± KYb2Cl7 ± for
which the structure was determined by Meyer [17],
melts incongruently at 4538C. Still unknown were two
3.3.1. Thermodynamic measurements
Solution enthalpies could be measured for all com-
pounds, K3Yb2Cl9 excepted, since is not stable at
ambient temperature. The results are given in Table 1,
together with two values determined by Blachnik and
Selle [15]. For the calculation of the ÁfH0 values, the
solution enthalpies of the alkali metal chlorides were
taken from published papers: CsClˆ18.1; RbClˆ17.6;
KClˆ17.9 kJ mol 1. The solution enthalpy of YbCl3
was found to be 212.8(7) kJ mol 1 (Ref. [16]: 212.9±
216.1 kJ mol 1). In the last column of Table 1 `syn-
reaction enthalpies' are listed. These are enthalpies of
formation from the two direct neighbours of a com-
pound in the phase diagram at ambient temperature.

32
J. Sebastian, H.-J. Seifert / Thermochimica Acta 318 (1998) 29±37

J. Sebastian, H.-J. Seifert / Thermochimica Acta 318 (1998) 29±37
33
Table 1
Solution enthalpies (ÁsolH0), enthalpies of formation from the binary compounds (ÁfH0) and synreaction enthalpies (ÁsynH0289) (all in
kJ mol 1) (Range of error for the last digit within parentheses)
Compound
Á
solH0
Ref. [15]
65.3
ÁfH0
Á
synH0
289
289
298
Cs4YbCl7
54.(2)
86.1
94.9
93.9
75.2
74.8
29.5
‡8.8
14.3
Ð
Cs3YbCl6 (Pbcm)
Cs3YbCl6 (C2/c)
Cs2YbCl5
63.6(3)
64.6(1)
101.4(1)
110.8(6)
174.2(3)
‡6.3
14.8
4.6
1/2 Cs3Yb2Cl9
1/2 CsYb2Cl7
Rb3YbCl6
74.6(1)
114.5(1)
166.7(1)
85.4
63.1
35.4
22.3
‡2.3
21.5
Rb2YbCl5
1/2 RbYb2Cl7
K3YbCl6
105.5(1)
133.7(1)
188.8(4)
105.0
53.8
43.3
15.1
10.5
5.0
K2YbCl5
1/2 KYb2Cl7
4.3
These reactions are:
Cs0:5YbCl3:5
Cs1:5YbCl4:5
Cs compounds : Cs2YbCl5
ˆ
ˆ
ˆ
ˆ
ˆ
1=3Cs1:5YbCl4:5 ‡ 2=3YbCl3
1=3Cs0:5YbCl3:5 ‡ 2=3Cs2YbCl5
2=3Cs1:5YbCl4:5 ‡ 1=3L Cs3YbCl6
1=2Cs4YbCl7 ‡ 1=2Cs2YbCl5
CsCl ‡ L Cs3YbCl6
1=4A2YbCl5 ‡ 3=4YbCl3
0:4A0:5YbCl3:5 ‡ 0:6L A3YbCl6
ACl ‡ A2YbCl5
L Cs3YbCl6
Cs4YbCl7
A
Rb; K compounds : A2YbCl5
A3YbCl6
0:5YbCl3:5
ˆ
ˆ
ˆ
Measurements on emf could be performed only for the
compounds A2YbCl5 and A3YbCl6 (AˆRb,K) as
pointed out in Section 2.2.3. The dependence of
emf (") on Twas linear. Thus, the measured regression
lines "ˆa‡b T would be transformed to Gibbs±
Helmholtz equation, ÁrG0ˆÁrH0 ÁrS0T, by means
of the relation Gˆ nF".
3.3.3. Potassium compounds
Reaction: 1.5 KCl‡K0.5YbCl3.5ˆK2YbCl5
(Samples of 50 and 37.5 mol% YbCl3 in the range
of 580±635 K)
emf/mVˆ179.4‡0.1152 T/K
1
ÁrG0/kJ mol ˆ 26.0 0.0167 T/K
Measurements above the formation temperature of
K3Yb2Cl9 yield two reactions:
50 mol%: KCl‡K0.5YbCl3.5ˆK1.5YbCl4.5 (Tˆ
630±670 K)
3.3.2. Rubidium compounds
Reaction: 1.5 RbCl‡Rb0.5YbCl3.5ˆRb2YbCl5 (Tˆ
620±690 K)
emf/mVˆ252.3‡0.0014 T/K
emf/ mVˆ211.3‡0.1346 T/K
1
ÁrG0/kJ mol ˆ 24.3
1
ÁrG0/kJ mol ˆ 30.6-0.0195 T/K
with emf(K2YbCl5)ˆemf(K1.5YbCl4.5), the forma-
tion temperature Tf for K3YbCl6 is 641 K (3688C)
37.5 mol%: 0.5KCl‡K1.5YbCl4.5ˆK2YbCl5 (Tˆ
630±670 K)
Reaction: RbCl‡Rb2YbCl5ˆRb3YbCl6 (Tˆ620±
690 K)
emf/mVˆ259.8
1
ÁrG0/kJ mol ˆ 25.1
emf/mVˆ121.3‡0.2045 T/K

34
J. Sebastian, H.-J. Seifert / Thermochimica Acta 318 (1998) 29±37
Fig. 2. Plot of emf vs. T for K3YbCl6.
1
ÁrG0/kJ mol ˆ 5.8 0.0099 T/K.
(AˆRb, K) are negative, and the compounds are stable
at all temperatures.
From emf(HT)ˆemf(LT), it follows that Tfˆ650 K
(3778C)
The synreactions of the 2 : 1 compounds are 0.4A0.5
YbCl3.5‡0.6 A3YbCl6ˆA2YbCl5, the free enthalpies
Reaction: KCl‡K2YbCl5ˆH K3YbCl6 (Tˆ650±
680 K)
Á
synG0ˆÁfG0(A2YbCl5) {0.4ÁfG0(A0.5YbCl3.5
)
‡0.6ÁfG0(A3YbCl6)}. With A0.5YbCl3.5 as standard
compound, one can formulate) ÁfG0 values:
ÁfG0(A0.5YbCl3.5)ˆ0
emf/mVˆ9.4‡0.3065 T/K
1
ÁrG0/kJ mol ˆ 0.9 0.0296 T/K
ÁfG0(A2YbCl5)ˆÁrG0
Reaction: KCl ‡ K2YbCl5 ˆ M K3YbCl6 (Tˆ
ÁfG0(A3YbCl6)ˆÁrG0(A2YbCl5)‡ÁrG0(A3YbCl6).
Then: ÁsynG0(A2YbCl5)ˆ0.4ÁrG0(A2YbCl5) 0.6
ÁrG0(A3YbCl6).
590 to 650 K)
emf/mVˆ90.4‡0.1818 T/K
1
ÁrG0/kJ mol ˆ 8.7 0.0175T/K
1
ForRb2YbCl5:ÁsynG0/kJ mol ˆ‡2.8 0.0078T/K
The results of the measurements are depicted in
Fig. 2.
Á
synG0ˆ0 at 359 K (868C)
1
For K2YbCl5: ÁsynG0/kJ mol ˆ 5.2‡0.0038T/K
From emf(H K3YbCl6)ˆemf(M K3YbCl6), it
follows that Tˆ650 K (3778C). The transformation
enthalpy is ÁrH0ˆ7.8 kJ mol
,
the entropy
1
3.3.4. Crystal structures
For several compounds, the unit cells have already
been determined by Meyer et al. from powder patterns.
ÁrS0ˆ12.1 J K 1 mol 1. It should be pointed out that
Tt from the emf measurements is lower than the value
from DTA heating curves (3928C) because of kinetic
effects. In DTA cooling curves, the transformation
could not be detected at all.
[17] CsYb2Cl7 and RbDy2Cl7 type; space group
RbYb2Cl7:
Pnma
KDy2Cl7 type; space group P21/c
Cs3BiCl6 type; space group C2/c
The ÁrG0 values for the reactions ACl‡A2YbCl5ˆ
[17] KYb2Cl7:
[18] Rb3YbCl6:
A3YbCl6 are identical with ÁsynG0. For Tˆ0, both

J. Sebastian, H.-J. Seifert / Thermochimica Acta 318 (1998) 29±37
35
[19] Cs2YbCl5:
[20] Cs3Yb2Cl9:
Cs2DyCl5 type; space group
Pbnm (id. Pnma)
ꢀ
Cs3Tl2Cl9 type; space group R3c
[21]. The results of the single-crystal work are shown
in Tables 2 and 3.
H-Cs3YbCl6 and H-modi®cations of A3YbCl6
(AˆRb,K): Guinier patterns at 5008C could be indexed
ꢀ
Our own results about hitherto unknown crystal
structures are compiled in Table 3:
in the cubic space group Fm3m.
Rb2YbCl5: isotypic to Cs2YbCl5, space group
Pnma.
Cs4YbCl7: The structure for this compound was
solved recently by single-crystal work [13].
Cs3YbCl6: The structure of the modification in
Pbcm is discussed elsewhere [14], the structure of
the modification in C2/c was determined by single-
crystal work.
L-and M-K3YbCl6: recently we found, by structure
determination with single crystals of the analogous
erbium compound, again obtained from supercritical
acetic acid, that L-K3ErCl6 has the monoclinic
K3MoCl6-structure [22] in space group P21/c. L-
K3YbCl6 proved to be isotypic. From an X-ray photo
at 1008C, we found that the monoclinic a- and c-axes
had become equal, the angle ꢁ was 109.48, which is
We succeeded in growing suitable crystals from super-
critical acetic acid, as recently described for Cs3CrCl6
Table 2
Single crystal data for Cs3YbCl6 and structure refinement
Crystal data
Cs3YbCl6; Mrˆ784.47; monoclinic;
S.G. C2/c; Zˆ8
single-crystal data
Ê
powder diffraction data
Ê
aˆ26.748(7) A
aˆ26.837(8) A
Ê
Ê
bˆ8.1010(5) A
bˆ8.174(4) A
Ê
Ê
cˆ13.028(2) A
cˆ13.035(3) A
ꢁˆ100.00(2)8
ꢁˆ100.2(3)8
Ê 3
Vˆ2780.1(9) A
Data collection
Ê
MoKꢀ radiation
ꢂˆ0.71073 A
Enraf Nonius CAD-4 diffr.
1
ꢃˆ2±248
ꢄˆ15.570 mm
Tˆ293(2) K
! scans
h ˆ
3 0 !0 ; k ˆ 9 !0 ;
lˆ 14!14
Refinement
Full-matrix least sqares on F2
R[I>2sigma (I)]
R(all data)
Goodness-of-fit on F2 ˆ1.077
R1ˆ0.0365, wR2ˆ0.1008
R1ˆ0.0459, wR2ˆ0.1073
Rintˆ0.0321
Ê 2
Fractional atomic coordinates and equivalent isotropic displacement parameters (A )
x
y
z
U(eq)
22(1)
21(1)
33(1)
35(1)
53(1)
32(1)
40(1)
45(1)
42(1)
52(1)
45(1)
Yb1
Yb2
Cs3
Cs4
Cs5
Cl1
Cl2
Cl3
Cl4
Cl5
Cl6
2500
2500
0
0
2193(1)
3135(1)
3095(1)
2530(1)
1154(3)
139(4)
2240(4)
4444(4)
405(4)
637(4)
2500
3545(1)
3016(1)
4330(1)
1807(2)
3076(2)
4306(2)
1847(2)
367(2)
1525(1)
3367(1)
502(1)
2502(1)
561(1)
568(1)
557(1)
1825(2)
3260(1)
786(2)

36
J. Sebastian, H.-J. Seifert / Thermochimica Acta 318 (1998) 29±37
Table 3
Hitherto unknown cell parameters of ternary ytterbium chlorides
Compound
Space group
Z
a/pm
b/pm
c/pm
ꢁ/8
Cs4YbCl7
R3 m
Pbcm
C2/c
3
8
8
4
764.6(4)
808.9(3)
2628.9(10)
2636.6(7)
1303.5(3)
120.00
Cs3YbCl6(aq.sol.)
L-Cs3YbCl6
H-Cs3YbCl6
1307.7(3)
817.4(4)
2683.7(8)
1152.2(3)
100.2(3)
Fm3m
H-Rb3YbCl6
Rb2YbCl5
Fm3m
Pnma
4
4
1114.5(2)
952.7(3)
726.1(3)
1494.2(4)
L-K3YbCl6
M-K3YbCl6
H-K3YbCl6
K2YbCl5
P21/c
4
8
4
4
1299.8(5)
1504.4(3)
1086.4(2)
1141
762.4(3)
1248.5(4)
752.7(2)
109.85(2)
104.1
Cmmm
Fm3m
(P21/c)
2128.1(5)
1008
824
the tetrahedral angle. Thus, the cell could be trans-
formed into an orthorhombic C-centred cell, space
group Cmmm. We tried to ®nd the point positions by a
Rietveld analysis, by constructing a suitable sur-
veyor's cell.
K2YbCl5: The structure of this compound neither
belongs to the K2PrCl5 nor to the Cs2DyCl5 type.
Because of a certain similarity to the X-ray pattern of
L-K3YbCl6, we could perform a monoclinic trial
indexing with the extinctions of space group P21/c.
K3Yb2Cl9: A dynamic Guinier photo in a small
temperature range above ca. 3808C displayed lines in
addition to those of the mixture (K2YbCl5‡KYb2Cl7).
The structure is probably that of the K3W2Cl9 type,
space group P63/m.
Cs2DyCl5 type in which the compounds Cs2YbCl5 and
Rb2YbCl5 crystallize.
The compounds Rb2LnCl5 have an increasing sta-
bility with decreasing radius of the Ln3‡ ion.
Rb2HoCl5 has only a range of existence between
687 and 785 K. Rb2ErCl5 is stable above 593 K, its
synreaction enthalpy is ‡6.0 kJ mol 1. The values for
Rb2TmCl5 are 353 K and ‡2.4 kJ mol 1, for the
1
ytterbium compound ± 359 K and ‡2.8 kJ mol
,
respectively. The 10/11 interstices, generated by the
‡
[LnCl6]-octahedra reach the optimal size for the Rb
ions.
The synreaction enthalpies for the compounds
1
Cs2LnCl5 are: 6.1 kJ mol for LnˆHo, 1.7 for
Er, 1.7 for Tm and ‡6.3 for Yb. The explanation
‡
must be, that the Cs ion increase for the 10/11
coordination.
4. Discussion
Comparing the systems of ytterbium chloride with
the foregoing systems ACl/TmCl3, only one difference
exists; with ytterbium a high-temperature compound
K3Yb2Cl9 is formed. It does not have the structure of
Cs3Yb2Cl9, but probably belongs to the K3W2Cl9
type. In the group ALn2Cl7, the lanthanide ion has
coordination number 7, in other groups ± A3LnCl6,
A2LnCl5 and A3Ln2Cl9 ± the Ln3‡ ions are octahed-
rally surrounded by chloride ions. That will be true
also for the still unknown structures of K2YbCl5 and
K2TmCl5. It is to be expected that there the coordina-
Acknowledgements
This work was supported by the Deutsche For-
schungsgemeinschaft and the Fonds der Chemischen
Industrie. Their help is gratefully acknowledged.
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