3
About Trihalides with TiI Chain Structure
As the super structure was known this model was trans-
solubility in the gas phase. Temperatures of 875 K (source) and
50 K (sink) were chosen. After 14 days small needles with a silver-
metallic lustre were found in the cooler zone (β-RuCl ) and thin
black platelets in the hot zone (α-RuCl ).
Needle-like crystals of RuBr were grown without additional trans-
port medium in a temperature gradient 925 K Ǟ 870 K. By using
mixtures of powdered β-RuCl and RuBr under these conditions
mixed crystals RuCl3ϪxBr were yielded. X-ray investigations and
electron micro probe analyses (EDX) showed the formation of
needles with chain type structure for higher bromide contents
(x>0.6) and thin black platelets with layer type structure for
x<0.6 [29].
6
fered to further representatives of the TiI type. The results
3
3
are listed in the Table 2.
3
The usual way to yield single crystals is high temperature
deposition from the gas phase. This leads to the assumption
that at first a structure with equidistant metal atoms is
formed. On cooling to room temperature a phase transition
takes place leading to paired metal atoms and the formation
of threefold twins [12, 13, 16]. In those cases where no super
structure reflections could be detected (see Table 1:
β-RuCl , ZrBr ) an undistorted TiI type must be assumed
3
3
3
x
3
3
3
at room temperature. If TiI3 type compounds can form
stuctures with and without metal pairs phase transitions in-
duced by pressure and temperature may occur. The high
temperature phases should have the ideal and undistorted
hexagonal structure while on cooling the formation of me-
tal pairs is expected. The conditions for a higher order tran-
sition according to the Landau theory are fulfilled [27, 28].
3 Structure Determinations
It was known from previous investigations [7, 13, 19] the
orthorhombic super structure can only be detected with
very small single crystals, which were isolated grown from
the gas phase at the tube wall, but slightest mechanical
stress destroys the very thin needles immidiately.
Single crystals were fixed in quartz glass capillaries with
small amounts of vacuum grease or for HT-measurements
with a thin glass capillary.
Intensity measurements of the weak super structure
reflections are difficult because of the small diffraction vol-
ume. In order to get higher intensities especially for meas-
urements at different temperatures larger crystals were used,
which were usually twin intergrowths in which three twin
domains with nearly equal amounts are intimately ar-
For MoBr we have found hints for this expected tran-
3
sition at 580 K by conductivity measurements using the res-
onance frequency perturbation method [19]. For compair-
ing studies the couple β-RuCl / RuBr seemed to be more
3
3
appropriate because at room temperature the chloride has
an ideal hexagonal structure but the bromide forms a dis-
torted orthorhombic structure with Ru -pairs. Indeed we
2
could show by means of temperature dependent structure
analyses on single crystals the reversible transitions between
both forms as it was expected [29]. The temperature depen-
dence was monitored qualitatively by Rietveld refinements
of powder data and measuring electric conductivities via
impedance spectroscopy and magnetic susceptibilities
ranged. For β-RuCl no super structure reflections were
3
found at room temperature by film methods or with a single
crystal diffractometer. This result agrees to the observations
by Hönle using electron diffraction [8]. In the case of RuBr3
the superstructure reflections were clearly detected leading
to the well known hexagonal super cell with aЈ ϭ 2a. No
deviations from the hexagonal metric were observed even
for high angle reflections. The cell constants we recieved
correspond within the experimental errors to the more rely-
able values resulting from Rietveld refinements of powder
data. Because of the crystal shape the “needle-modus” was
used for the measurement of intensity data.
While the intensities of reflections of the hexagonal sub-
cell were determined with good accuracy, the refinements
gave R-values of 0.01-0.02, the super structure reflections
showed larger experimental errors because of their lower
intensities and broader profiles. Especially measurements
near the transition temperature are difficult because the
transition covers a larger temperature range and the inten-
sities vanish to the background. But a sufficient number of
super structure reflections is necessary for reliable structure
refinements because they contain the information about the
super structure. The crystallographic data are collected in
Table 3 and 4. Further details may be obtained from the
Fachinformationszentrum Karlsruhe, Gesellschaft für wis-
senschaftlich-technische Zusammenarbeit, D-76344 Eggen-
stein-Leopoldshafen, on quoting the dispository numbers,
for β-RuCl3 (RT): CSD 414040, β-RuCl3 (TT): CSD
[
29, 30]. It turned out the metal metal distances change con-
tinuously in a temperature range of about 100 K around
the transition temperature. From the turning points we have
derived transitions temperatures of 206 K for β-RuCl and
3
3
84 K für RuBr in good agreements with the temperature
3
dependencies of lattice constants and electric and magnetic
properties as well. In the following we report on single crys-
tal syntheses and the proof of phase transitions by means
of single crystals X-ray structure analyses. In a later contri-
bution to this journal we will give further details on the
course of the transitions.
2
Synthesis and Experimental Details
3 3
Single phase powder samples of β-RuCl and RuBr were yielded
by high pressure halogenation of finely powedered Ru metal.
Quartz ampoules (lϭca. 5 cm, dϭ10 mm, Vϭ2-3 ml) were filled
with 100-200 mg Ru (Degussa 99.9 %) and 1 ml halide (Br
9.5 %; Cl , Preussag, dried with conc. H SO /CaCl ). The sealed
ampoules were heated in a steel autoclav (back pressure generated
by CO ) for 6-8 days at temperatures of 600 K (β-RuCl ) resp.
25 K (RuBr ). Starting from powder samples single crystals were
2
, Merck,
9
2
2
4
2
2
3
7
3
grown by chemical transport methods.
It must be taken into consideration that β- RuCl transforms
3
irreversible above 670 K [6, 7] to α-RuCl
with a layer structure of the monoclinic AlCl
high excess of AlCl was added as a transport agent to enlarge the
3
, a high temperature form
-type. Therefore a
414041, RuBr (RT): CSD 414042, β-RuBr (HT): CSD
3
3
3
3
414043, the name of the authors and this journal.
Z. Anorg. Allg. Chem. 2004, 630, 2199Ϫ2204
zaac.wiley-vch.de
2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
2201
Hillebrecht
