1
28
ZHENG AND SEIFERT
(
graphite#Cl )/ACl/A>-conducting diaphragm/A TmCl
CsTm Cl two phase transitions were found: one at 172°C
ꢂ
ꢂ
ꢃ
ꢂ ꢆ
(#A TmCl )/(graphite#Cl )/temperature range 300— and a second at 432°C.
ꢁ
ꢄ
ꢂ
4
00°C.
In the system RbCl/TmCl two congruently melting com-
3
The collected emf vs ¹ values were subjected to a linear pounds—Rb TmCl and RbTm Cl , both dimorphic—and
ꢁ
ꢄ
ꢂ ꢆ
regression analysis.
the incongruently melting Rb TmCl with the Cs DyCl
ꢂ
ꢃ
ꢂ
ꢃ
Measurements of solution enthalpies, which need at least structure exist.
5
g of each compound, were not performed as for the former
The system KCl/TmCl contains three compounds as in
3
systems because of the restricted amount of thulium oxide. the RbCl system. However, KTm Cl is now incongruently
ꢂ
ꢆ
melting. K TmCl is the first member of a new structure
ꢂ
ꢃ
3
. RESULTS
family, which is actually investigated with the isotypic com-
pound K YbCl ; probably the space group is P2 /c. The
Ternary Chlorides from Solutions
ꢂ
ꢃ
ꢀ
third compound, K TmCl , has in addition to the cubic
ꢁ
ꢄ
Some ternary chlorides could be prepared conveniently high-temperature and the monoclinic low-temperature
from aqueous solutions or from acetic acid, provided they modifications a middle-temperature phase above &120°C.
are congruently soluble in these solvents.
The correct structure (Cmmm?) is not yet proved; however, it
In the case of aqueous solutions, ACl and Tm O were can be described with a pseudomonoclinic cell with a"c.
ꢂ ꢁ
dissolved with the correct molar ratio in hydrochloric acid.
The unit cells of the majority of the found compounds
These solutions were evaporated to dryness at temperatures already have been determined by Meyer and co-workers
of 100—120°C. The following compounds could be prepared: from powder patterns: CsTm Cl and RbTm Cl : RbDy
ꢂ
ꢆ
ꢂ
ꢆ
ꢂ ꢆ
ꢂ
Cs TmCl [the crystal structure could be solved by single- Cl type, space group Pnma (9); KTm Cl : KDy Cl type,
ꢅ
ꢆ
ꢆ
ꢂ
ꢆ
ꢁ ꢂ ꢇ
crystal measurement, as previously described (6)]; space group P2 /c (9); Cs Tm Cl : Cs Tl Cl type, space
ꢀ
ꢁ
ꢂ
ꢇ
Cs TmCl [orthorhombic modification, space group Pbcm group R 3ꢀ c (10); Cs TmCl and Rb TmCl : Cs DyCl type,
ꢁ
ꢄ
ꢂ
ꢃ
ꢂ
ꢃ
ꢂ
ꢃ
(
7)]; and Cs TmCl ) H O [erythro-siderite structure (space space group Pbnm (id. Pnma) (10); ¸-Rb TmCl : Cs BiCl
ꢂ ꢃ ꢂ ꢁ ꢄ ꢁ ꢄ
group Pnma, Z"4); the orthorhombic cell parameters type, space group C2/c (11). The unit cells of the compounds
are a"1460.4(3), b"1049.4(4), and c"753.0(2) pm]. listed in Table 1, were still unknown; the lattice parameters
Cs TmCl ) H O can be dehydrated by heating in an HCl for Cs Tm Cl are refined.
ꢂ
ꢃ
ꢂ
ꢁ
ꢂ ꢇ
stream at slowly rising temperature from 200 to 300°C.
A synopsis of the structures of all ternary lanthanide
From acetic acid, the compounds Cs TmCl and halides, including the fluorides, bromides, and iodides,
ꢁ
ꢄ
Rb TmCl , both crystallizing with the Cs BiCl structure which were known in 1982 was given by Meyer (1). Accord-
ꢁ
ꢄ
ꢁ
ꢄ
(
space group C2/c), could be precipitated with HCl gas. ing to this summary, the Cs DyCl type and the structure
ꢂ
ꢂ ꢆ
ꢃ
K TmCl (space group P2 /c) could be obtained only to- for the compounds A¸n Cl are originally determined by
gether with KCl. The solutions were prepared from Meyer himself. The elpasolite type, which is the aristotype
ꢁ
ꢄ
ꢀ
TmCl ) 6H O and alkali metal carbonate in the correct for all 3 : 1 compounds, and the structure of enneachlorides
ꢁ
ꢂ
molar ratio.
are well known. The K PrCl type is identical with the
ꢂ
ꢃ
Y HfS structure, solved by Jeitschko (13).
ꢂ
ꢃ
Phase Diagrams and Crystal Structures
Results of emf Measurements
Figure 1 illustrates the results of the DTA measurements
on the systems ACl/TmCl (A"Cs, Rb, K). Phase transi-
Emf values were measured for the formation of each
ꢁ
tions below 350°C are not included in the diagrams.
compound from ACl and the adjacent TmCl -richer com-
ꢁ
As already found for ErCl , melts of TmCl also react pound in the temperature range &300—400°C. In this
ꢁ
ꢁ
slowly with quartz. Therefore, the melting point was meas- range, the dependence of emf on ¹ was linear. Thus, the
ured in a corundum crucible. The measured value of 1094 K equations for the regression lines could be transformed by
(
821°C) agrees well with that recently found by Gaune- multiplication with !nF to the Gibbs—Helmholtz equation
Escard (8).
* G°"* H°—¹* S°.
ꢃ ꢃ ꢃ
In the system CsCl/TmCl four compounds were found:
Emf measurements could not be performed for the ErCl -
3
ꢁ
two congruently melting ones (Cs TmCl and CsTm Cl ) richest compounds ATm Cl . According to our present
and two incongruently melting ones (Cs TmCl and experience, the emf cells break down for emf values higher
ꢁ
ꢄ
ꢂ
ꢆ
ꢂ ꢆ
ꢂ
ꢃ
Cs Tm Cl ). Cs TmCl is the first tetramorphic tricesium than &500 mV.
ꢁ
ꢂ
ꢇ
ꢁ
ꢄ
hexachloro compound of the lanthanides: besides ¸-
In the following, the Gibbs—Helmholtz equations for the
Cs TmCl with the Cs BiCl structure, the H-modification reaction in the cell (* G°) are listed together with the tem-
ꢁ
ꢄ
ꢁ
ꢄ
ꢃ
in the cubic elpasolite type, and orthorhombic Cs TmCl
perature ranges of the measurements. The range of error
from aqueous solution, a fourth modification exists between was smaller than 1 kJ mol\ꢀ for the energy values and
76 and 409°C with a still unknown space group. For 0.8J K\ꢀ mol\ꢀ for the entropies.
ꢁ
ꢄ
3
Zheng, Chagoui
