Real-time in-situ monitoring of the topotactic transformation of TlCu3Se2 into TlCu2Se2

Article abstract of DOI:10.1016/j.jallcom.2015.02.180

The solid state transformation of monoclinic TlCu₃Se₂ into tetragonal TlCu₂Se₂ by oxidative copper leaching in concentrated ammonia solution has been studied in situ by the use of synchrotron radiation. The diffraction patterns of parent and daughter phase are both sharp, indicating a strong topotactic relationship between them that effectuates a rapid change by chimie douce performed at +19 °C. The transformation rate is strongly connected to the access of oxygen from the surrounding air. The transformation was followed in a real-time mode, being almost complete after 2.5 h with 1 h incubation due to low oxygen content, as shown from refining the diffraction patterns by Rietveld profile technique.

Full text of DOI:10.1016/j.jallcom.2015.02.180

Journal of Alloys and Compounds 637 (2015) 321–325
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Real-time in-situ monitoring of the topotactic transformation of TlCu Se2
3
into TlCu Se2
2
⇑
J. Ångström , M. Sahlberg, R. Berger
Department of Chemistry, Ångström Laboratory, Uppsala University, P.O. Box 538, SE-75121 Uppsala, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
The solid state transformation of monoclinic TlCu Se2 into tetragonal TlCu Se by oxidative copper
3
2
2
Received 5 December 2014
Received in revised form 16 January 2015
Accepted 25 February 2015
Available online 3 March 2015
leaching in concentrated ammonia solution has been studied in situ by the use of synchrotron radiation.
The diffraction patterns of parent and daughter phase are both sharp, indicating a strong topotactic
relationship between them that effectuates a rapid change by ‘‘chimie douce’’ performed at +19 °C.
The transformation rate is strongly connected to the access of oxygen from the surrounding air. The
transformation was followed in a real-time mode, being almost complete after 2.5 h with 1 h incubation
due to low oxygen content, as shown from refining the diffraction patterns by Rietveld profile technique.
Ó 2015 Elsevier B.V. All rights reserved.
Keywords:
Inorganic materials
Chemical synthesis
Crystal structure
Phase transitions
Synchrotron radiation
X-ray diffraction
1
. Introduction
Syntheses of phases containing heavy elements are usually
synthesis). The phase occurrence in the systems Tl–Cu–Se and
Tl–Cu S is very similar [4], probably due to the fact that the elec-
tronegativity of S and Se is almost the same. Thus, tetragonal
TlCu and TlCu have pairwise (X = S or Se) the same structure.
This opened the door to preparing TlCu by soft chemistry [5]
where ordinary high-temperature methods had failed [3,6]. The
explanation for that is that TlCu is not stable above 120 °C
made by high-temperature methods. However, in some very spe-
cial cases a route based on phase transformations within the solid
state may be possible, a route that instead avoids high tempera-
tures. The basis for such a synthesis is a topotactic relationship
between a ‘‘parent’’ and ‘‘daughter’’ phase that takes advantage
of the fact that most of the parent structure is left intact while a
lowered temperature still permits the movement of some of the
atoms within the structure to produce the change. This also means
that the nucleation step is extremely easy and that the growth of
the new phase may occur through shear of whole structure frag-
ments. The phenomenon falls under the concept ‘‘chimie douce’’
4
X
3
2 2
X
2 2
S
2 2
S
and, once made at room temperature, it decomposes into a
three-phase mixture on heating.
A topotactic relation was suggested [4] between the structures
3 2 2 2
of TlCu X and TlCu X (X = S, Se) which was tested both experi-
mentally and theoretically for the sulphide phases [7]. The oxida-
tive copper extraction does not proceed smoothly, and the
transformation could therefore be followed to some extent by per-
forming a gradual extraction and intermittent powder neutron
diffraction recordings. In the selenide case, however, the trans-
formation occurs much easier, even by using air as oxidizing agent,
and the extent of transformation is not tunable in the same way.
The use of synchrotron radiation now offered a possibility to follow
the transformation more closely in a series of real-time in-situ
diffraction experiments while the oxidation proceeded. From such
an experiment information might be gained as to the mechanism
or at least the time scale for the transformation. Thus, no crystal
structure gain was expected since both structures are already
known.
(
soft chemistry).
In earlier work, some examples were given where the soft
chemistry method was applied to the Tl–Cu–Se system. A new
form of TlCu Se (orthorhombic) was obtained from room-tem-
perature copper extraction from TlCu
TlCu Se was obtained from TlCu Se
extraction.
The normal form of TlCu
perature synthesis [3] and that is also the usual way to synthesize
TlCu Se . The soft-chemistry route is in that case an alternative
method, once TlCu Se is available (from high-temperature
4
3
Se
5 3
[1] and, correspondingly,
2
2
3
2
[1,2], both by oxidative
4 3
Se (tetragonal) is made by high-tem-
2
2
3
2
The essential structural parameters of the parent and daughter
structures are collected in Table 1. The two structures are depicted
⇑
Corresponding author.
http://dx.doi.org/10.1016/j.jallcom.2015.02.180
925-8388/Ó 2015 Elsevier B.V. All rights reserved.
0

3
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J. Ångström et al. / Journal of Alloys and Compounds 637 (2015) 321–325
in Fig. 1 as projections along a short axis. There, the background
model of transformation [4] is also presented.
3. Results and discussion
All atoms occur in two parallel planes shifted apart by half the
axis along the direction of the projection, yielding two fixed coor-
dinates of these mirror planes. This fact is illustrated by filled or
unfilled symbols. The circles with the smallest radius denote cop-
per atoms and those with the largest radius denote selenium. The
thallium atoms are represented by intermediate sized circles. In
Fig. 2 gives an overview of the process, starting with the diffrac-
tion pattern of single-phase TlCu Se at the top (t = 0). The powder
3
2
diffraction lines appear as white streaks towards a dark back-
ground. The brighter the streaks are the stronger is the scattering.
Thus, the growing and waning as a function of time (i.e. down-
wards along the vertical axis) show how the parent phase gradu-
ally disappears while new lines appear, the main change
occurring after slightly more than 1 h. These new lines belong to
TlCu Se . During the experiment the liquid evaporates slowly, so
2 2
TlCu Se , all atoms are situated in special positions except on the
selenium site which carries a free z-parameter. No single crystal
study has been made, and its determination from X-ray powder
diffraction (zSe = 0.36) is not especially precise. However, the
experiments performed aimed at yielding a fair quantitative phase
analysis where small structural details do not matter for the out-
come. In particular, a refinement of copper occupancies would be
futile in the light of the strong scatting of the other atoms. On
the other hand, since the dimensions of the short axes are very sus-
ceptible to copper content, significant changes in those cell
parameters are important indicators of copper deficiency.
2
2
that the interface facing air at the end of the capillary moves
inwards. The growth of the new phase accelerates at a point of
time at which the meniscus approaches the position of the X-ray
beam. At the same time there is a radical change of background
when the interface air/liquid has passed so that the main liquid
phase is no longer in the beam. Still, the powder is in contact with
the ammonia solution that is trapped by capillary forces, and air is
henceforth highly available. The transformation process is also
depicted in Fig. 3 which in a qualitative way shows the increasing
amount of the new tetragonal phase by time. The data are the same
as in Fig. 2, only being cut – both in time and in angle. Fig. 4 is a
diagram based on the refinements of selected data sets to show
in more detail what relative amount the mother phase takes during
the process. The degradation of the mother phase is slow in the
beginning until oxygen is easily available. No proper analysis can
be made from the oxidation process since parallel mechanisms
are at play simultaneously. Only a few representative refinements
in the form of X-ray powder patterns are shown in Fig. 5. Towards
the end, very small amounts of copper(II) hydroxide are seen
2
. Experimental
In-situ powder diffraction experiments were performed at the I 711 beamline
[
8] at the Max II synchrotron of the Max IV laboratory (Lund, Sweden) using a large
area Titan CCD detector.
3 2
The sample preparation was rather easy: Previously synthesized TlCu Se [9],
checked by X-ray diffraction to be single-phase material, was finely ground in an
agate mortar. A small amount of the powder was placed at the tip of a single-crystal
sapphire tube with an inner diameter of 1 mm. Concentrated aqueous ammonia
was sucked into it through the capillary force to wet the powder, just before the
experiment. The capillary was mounted horizontally and aligned normal to the syn-
chrotron X-ray beam with the temperature held at 19 °C. The sample distance and
(
Figs. 2 and 5) which was included in the refinements, now in a
the radiation wavelength (k = 0.9866 Å) were determined using a NIST LaB
6
stan-
dard (a = 4.15689 Å) [10].
three-phase mixture.
The X-ray beam entered through the sample close to the tip of the capillary and
data were taken using an exposure time of 10 s for each recording, with a total
duration of nearly 3 h (640 exposures). During the experiment the solution slowly
evaporated at the end, and the liquid/gas interface eventually passed the position
that was probed by the X-ray beam. The 2d images were reduced to 1d diffrac-
tograms using the fit2d software [11].
Refinements of the diffraction pattern parameters were performed using the
Rietveld method [12] as implemented in the Fullprof program [13]. As an improve-
ment of the statistics and at the same time reducing the number of refinements, the
number of patterns was reduced from 640 to 160 by adding every 4 of them. Out of
these patterns, a still more limited number of data sets were selected so as to yield a
fair picture of the copper extraction process from start of the transformation to fin-
The transformation is too fast to be analysed in detail as to the
local changes of the parent structure, and the much distorted back-
ground in the beginning precludes an adequate analysis because
the refinement would be too susceptible to the manual setting of
the background in an angular range covered by diffraction peaks.
The structural change is very likely to proceed in the same way
as that of the sulphide analogue, i.e. 50% of those copper atoms that
carry a trigonal chalcogen environment are vulnerable for chemical
attack and leave the solid. The model applied to the isostructural
sulphides [4] was supported by neutron powder diffraction and
electronic calculation [7] and is illustrated in Fig. 1. The parent
ish. The data sets were cut to cover the 2
H range of 6.0–31.5°. The high-angle cut
from 45° was motivated by the fact that the diffraction rings of the 2d diffractograms
were so incomplete that the reduction to 1d introduced erroneous intensities.
Because of a very complicated background for the first sets of data (vide infra,
Fig. 5), that was determined largely manually with additional refinements of points
within the program. The cell parameters of the phases were in excellent agreement
with the published values and hardly needed any refinement, but the position of the
0
0
0
0
structure is expressed by a slab sequence ABA B ABA B . The primes
denote a difference in projection height only. The B-type slabs have
a composition TlCu Se and lose half of the copper atoms on leach-
4
2
2 2
ing from which their average composition changes into TlCu Se . A
diffusion train of copper atoms along a slab is started, and in the
next mechanistic step of the transformation a whole slab is moved
by crystal shear and connects again to an adjacent slab. Only minor
rearrangements transform them all into A-type slabs, i.e. into a
sequence AAA. . . Hence, the new composition and structure is
2 2
selenium atom in TlCu Se was found to give the best refinement for z = 0.357, and
that value was further fixed in all refinements. The scale factor and the peak profile
width w were refined for each phase. The profile form chosen was that of Thomson–
Cox–Hastings pseudo-Voigt. At each selected data set of the refinements, the rela-
tive mass amounts of the phases were calculated in the program, using the proper
cell contents and volumes. These values were quite insensitive to profile parame-
ters. In the beginning, only the two phases of the transformation occurred, but at
the end of the experiment small extra peaks appeared, interpreted as belonging
2 2
identical to what is found for TlCu Se .
From the diffraction result it is seen that there is no apparent
change of the parent compound until a certain time has passed.
However, even if no occupancy parameters may be refined, the
removal of copper should be revealed by a subtle change in metrics
of the cell. For the corresponding sulphide there was a 1% decrease
of the b-axis during a slow leaching process, illustrating a very high
2
to Cu(OH) [14].
Table 1
3 2 2 2
Selected structural data for monoclinic TlCu Se and tetragonal TlCu Se .
3 2
sensitivity to copper content. For TlCu Se , the change is hardly
Phase
Space
group
Cell parameters
Refs.
significant, only a decrease from 4.0115 Å to 4.0089 Å at the begin-
ning of the copper leaching according to refinement.
The rate of the transformation is probably closely connected to
the access of oxidant, the macroscopic reaction occurring accord-
ing to the formula:
TlCu
3
Se
Se
2
2
C 2/m
a = 15.2128 Å; b = 4.0115 Å; c = 8.3944 Å;
b = 111.70°
[9]
TlCu
2
I 4/mmm
a = b = 3.8572 Å; c = 14.038 Å
[3,16]

J. Ångström et al. / Journal of Alloys and Compounds 637 (2015) 321–325
323
3 2 2 2
Fig. 1. Comparison between the structures of TlCu Se (to the left, monoclinic) and TlCu Se (to the right, tetragonal), both in projection along the short axis of about 4 Å.
Accentuated are structure slabs (A, B) of different composition. In the lower left picture, copper atoms have been removed from the B slabs in the same way as found for the
sulphur analogue, which has brought these to the same composition as that of the A slabs. The figure was adapted from Ref. [7].
Fig. 3. Part of the whole transformation illustrated as a 3-d plot of the diffraction
Fig. 2. Densitometric view of the powder X-ray diffraction patterns as a function of
time where intensity is shown as a greyscale. Upper part shows the powder
pattern. The selection covers the time just before and just after the meniscus of the
liquid/air interface has passed the location of the beam. The formation of the
diffraction pattern of single-phase TlCu
that of TlCu Se (tetragonal). The very small rising amount of accompanying
Cu(OH) can be seen as a trace at 10.5 degrees at the bottom after 2 h.
3
Se
2
(monoclinic) and lower part mainly
TlCu
the most intense peak of TlCu
in Fig. 5. The low-angle part has been omitted (cf. Figs. 2 and 5).
2
Se
2
phase can easiest be seen in the growth of the peak at 19 degrees (which is
2
2
2
Se ). The peak positions of the phases are easiest seen
2
2

3
24
J. Ångström et al. / Journal of Alloys and Compounds 637 (2015) 321–325
by is paramount is directly obvious by the naked eye, since the blue
copper ammine complex is not seen until the magnetic stirrer has
been set in action for whipping in oxygen from the surrounding air.
While the capillary resides fixed, mounted in its holder, water
evaporates through its mouth and the corresponding meniscus
travels inward. That interface carries a large oxygen activity com-
pared to the liquid deep inside, an air-borne reagent necessary for
the change to occur. When the meniscus passes the position where
the X-ray goes through, it is shown in the diffraction pattern in two
ways: The high background based on the contributions from the
capillary filled with liquid starts to change and the new phase
appears to form at a higher rate. The breakthrough of the trans-
formation is thus directly coupled to the access to air. Once the
process has started (the nucleation step) it can proceed by further
growth of the new phase whose content is increasing while the
background becomes more linear.
The connection to oxygen access is easily noted from a compar-
ison between Figs. 2 and 4. The latter shows how the trans-
formation rate is suddenly greatly enhanced just before 1.25 h
after starting the exposures. That time corresponds to a sudden
change in background of Fig. 2 (seen as a white fuzzy horizontal
streak), the appearance of which is illustrated in more detail in
the series of diffractograms of Fig. 5. The effects of difficult back-
ground description in the refinements are reflected in the profile
R-values of Table 2.
Fig. 4. The relative abundance of TlCu
time. The condensed data sets of this sequence started with nr 63 and ended with nr
40 (cf. Fig. 5).
3 2
Se as found from refinement, plotted vs.
1
2
TlCu
3
Se
2
ðsÞ þ O
2
ðaqÞ þ 2H
2
OðlÞ ! 2TlCu
2
Se
ꢀ
2
ðsÞ þ 2Cu2þðammÞ
þ 4OH ðaqÞ
Previous experiments of this reaction were performed by simply
swirling powder of the parent phase in a beaker containing aqueous
ammonia. The transformation to the daughter phase was finished
within minutes at room temperature [1]. That the access to oxygen
The role of the ammonia is to change the oxidation potential
favourably through ammine complex formation. The ammonia
then keeps copper in solution until saturation is eventually
reached:
Fig. 5. Examples of Rietveld powder profile refinements along the transformation time path, starting with 99.8% (Run 63) and ending with 4.8% (Run 140) of the parent phase
cf. Fig. 3). The tick-marks below each pattern denote the angular positions of the three crystalline phases considered: respectively TlCu Se , TlCu Se and, eventually,
. The irregular horizontal pattern at the bottom illustrates the difference between observed and calculated profiles, deviations being positive and negative around
(
3
2
2
2
Cu(OH)
zero.
2

J. Ångström et al. / Journal of Alloys and Compounds 637 (2015) 321–325
325
Table 2
shown by electrochemistry that the reverse process is also possi-
ble, that TlCu Se can incorporate copper to create TlCu Se [2].
It may be noted that the cell parameters of TlCu Se are kept,
Figures of merit of the refinements shown in Fig. 5.
2
2
3
2
2
Data set
v
Rp (%)
Rwp (%)
2
2
even with this phase resting in the aerated ammonia, a strong indi-
cation that the composition does not change. One electron hole per
formula unit seems to be the limit. In fact, one method of making
Run 63
Run 83
Run 95
Run 140
0.45
0.68
0.88
1.02
17.4
15.4
13.6
12.7
13.8
13.8
12.0
11.0
2 2
single phase TlCu Se is to deliberately add a small excess of cop-
per when synthesizing it at high temperature from the elements,
and subsequently leach the two-phase product. Such a procedure
ascertains that not instead a three-phase mixture is obtained if
by accident too little copper has been weighed in. This inertness
2
þ
ꢀ
Cu ðammÞ þ 2OH ðaqÞ ! CuðOHÞ ðsÞ
2
Thus, at a certain degree of transformation when almost all
TlCu Se has been leached out, crystalline Cu(OH) precipitates
from the oversaturated ammonia solution. This is quite in line with
methods of growing copper(II) hydroxide crystals from ammonia
solution [14]. In the end, the parent compound is almost entirely
consumed.
to leaching of TlCu
published by Ohtani et al. [17] who claimed the existence of
TlCu2ꢀdSe (d < 0.10), however only from nominally changing the
ratios between the elements at synthesis. It is a pity that no cell
parameters were published, since in all compounds TlCu
X = S, Se) that experience copper leaching, particularly a short axis
2 2
Se appears to be in contradiction to the results
3
2
2
2
n m
X
(
is always sensitive to copper deficiency. For instance, the sulphur
analogue parent compound quoted above shows a huge 1%
4
. Conclusions
2
decrease of the b-axis at the composition TlCu2.92S [7]. Likewise,
The use of the synchrotron technique has proven very useful for
TlCu7 S4 forms a good example of a strong decrease of the short
ꢀd
a real-time study of an example of solid state transformations, not
achievable before.
axis on copper leaching, which moreover effectuates a phase trans-
formation on cooling [18].
It would be tempting to try to apply empirical mathematical
models (as expressed by an Avrami equation) to the degree of
transformation, but they are likely to fail when compared to alloy
transformations where grains are in fixed contact and diffusion
through the solid material is the only mechanism. The ease of
the nucleation has precluded an analysis of the actual mechanism
In view of the success of this experiment, other couples of com-
pounds may be worthwhile examining in the same manner.
Acknowledgments
The authors thank the head of the research group, Professor U.
Jansson, for giving free hands to perform these investigations.
Access to beam time at the MAX-II synchrotron at the MAX IV
Laboratory as well as financial support from the Swedish
Research Council is gratefully acknowledged. The professional aid
at the synchrotron facility by Dr. Olivier Balmes was required for
the success and is highly appreciated.
by investigating the copper depletion of TlCu
of the transformation.
3 2
Se in the beginning
From Figs. 3 and 4 it is seen that the transformation proceeds
rather fast after the nucleation step. It is known from investiga-
tions of the diffusivity of copper in its sulphides that there is a con-
siderable enhancement factor through the thermodynamical factor
[
15]. The same most certainly applies to copper selenides, also with
mixed conduction. Diffusion most probably plays a dominant role
in the nucleation step. However, the growth may in this case
include a shear mechanism of whole slabs, a mechanism that is
not thermally activated – the (nominal) temperature of +19 °C is
rather low for solid state reactions based on diffusion only.
In this case, where the beam introduces energy on a small vol-
ume, the actual temperature may be higher than nominal but not
too high, since the evaporation of the liquid (in the order of
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2 2
lic properties. TlCu Se is namely an alloy with p-type conduction,
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