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especially for A ˛ {K, Rb, Cs} as it involves high temperature Bruker apparatus used in taking the XRD spectra is regularly
heating of monoalkali acetylides mixed with alkali metals in used to produce good quality powder XRD spectra. Instead, the
3,12
high vacuum. To avoid the direct use of bialkali acetylides we broadness off the peaks is an indication of disordered crystal-
have developed an alternative synthesis based on monoalkali line materials where the heavy tellurium atoms form a crystal-
acetylides only. It consists of rst reacting the Te powder with line structure that is lled with very disordered acetylide and
two molar equivalent of monoalkali acetylides according to
alkali ions, as discussed in the following. DFT calculations
strongly support the tted structure for Na TeC , while for
Li TeC there is a larger deviation, the origins of which will be
discussed below.
The main geometric parameters of Li TeC and Na TeC , as
2
2
2
AC H + Te / ATeC H + AC (3)
2
2
2
H
2
2
that is followed by the disproportionation of the resulting
mixture:
2
2
2
2
obtained from Rietveld tting of XRD data, are listed in Table 1.
24
Full details of the tting are available as ESI† For comparison,
ATeC
2
H + AC
2
H / A
2
TeC
2
2
+ H C
2
[.
(4)
also the DFT-predicted structural parameters and those of the
2 2 2 2
related ternary acetylides Na PdC and Na PtC are listed. Note
that XRD data for 2q values smaller than those shown in Fig. 1
The rst step of this reaction is familiar from analogous
acetylenic organo-telluride reactions when the H of ATeC H is
replaced by an organic functional group, typically an
2
and 2 are all due to the sample holder, including a sharper peak
o
14,15,20
at about 13 and a broader one at about 10 ( ) 2q. Fig. 4 and 5
arene.
solvent NH
The second step may largely complete when the
is evaporated along with the acetylene gas, with the
display the tted structures of Li TeC and Na TeC , respec-
2
2
2
2
3
ꢀ
tively. The space group of Li TeC is P3m1, identical with the
2
2
possibility that the rest of the acetylene gas is removed when the
product is ground in a mortar. A repeated condensation of NH3
and sonication of the mixture, followed by repeated evaporation
space group of all known A MC (M ¼ Pt, Pd; A ¼ Na, K, Rb, Cs)
2
2
compounds. The space group of Na
2 2
TeC , however, is I4/mmm,
representing a new structure in ternary acetylides. Also note
of NH helps to better mix the reaction components and remove
3
that the tting of Na
one of the actual Na
2
TeC
TeC
2
data identied two phases, a major
2
material with I4/mmm space group
as much residual acetylene gas as possible, before grinding the
2
product in a mortar. We have not characterized ATeC
above two reactions are part of an equilibrium and the isolation
of ATeC H appeared difficult, if at all possible. The identity of
the products for both Na TeC syntheses are indicated by
2
H, as the
2 2
and a minor one of the unreacted excess Na C used in the
synthesis.
2
˚
The C^C distance in Li TeC is 1.044 A, interpreted as a
2
2
2
2
projection of a wobbling C^C dumbbell onto the c axis,
therefore it is shorter than an acetylenic bond (C^C distance is
overlapping powder X-ray diffraction (XRD) spectra. The mon-
oalkali acetylide based synthesis of ternary acetylides may be
applicable to the synthesis of a wide variety of ternary acetylides
avoiding the cumbersome production of heavier bialkali acety-
lides and the use of high-temperature solid state reactions.
˚
1
.203 A in acetylene gas). The C^C distance in Na
2
TeC
2
, as
˚
projected on the a and b axes, is 1.208 A very close to that in
acetylene. The wobbling motion of the C^C dumbbell and its
effect on the projected C^C distance has also been observed in
Powder XRD spectra of Li
comparison of XRD patterns of Na
different synthesis routes described above are shown in Fig. 1–
, respectively. The XRD spectra consist of a few broad peaks in
2
TeC
2
and Na
2
2
TeC as well as the
other ternary acetylides, such as K PdC . The short, 1.727 A˚ ,
1
0
2
2
2
TeC
2
made by the two
Te–C distance in Li TeC should also be interpreted as a pro-
2
2
˚
jected distance, while the long, 2.333 A, Te–C distance in
3
13
2 2
Na TeC is closer to expectations based on DFT predictions.
the case of both compounds. The broadness of the peaks is not
a consequence of the resolution of the measurements, as the
The Te–C distance in bis[(4-methylphenyl) ethynyl] telluride is
˚
2
.045 A˚ ,20 while it is expected to be about 2.4 A in ternary ace-
tylides with Te on the basis of DFT calculations, assuming the
13
ꢀ
P3m1 space group. The observed and DFT-predicted lattice
˚
parameters of Li TeC differ a lot: the observed a ¼ b ¼ 6.2981 A
2
2
˚
and the c ¼ 4.4987 A values appear to be approximately reversed
as compared to the predictions. As a consequence of this
change in the lattice parameters, the Li–Te and Li–C distances
will also become much longer than predicted: 3.851 and 3.665
˚
˚
A, instead of 2.982 and 2.636 A, respectively, while the Li–Li
distances remain relatively close to the DFT-predicted values:
˚
˚
4
.131 A (observed) vs. 4.356 A (predicted).
The long experimental a and b lattice parameters in Li TeC2
2
suggest that the Te–C^C–Te units actually run along the a and
b axes instead of the c, while the short c parameter (twice about
˚
2
.25 A) suggests Li ions complexed by acetylide ions along the c
direction, with the acetylide ions lying in the a or b directions.
We have explored such structural models with 1/6 probability
for Li and carbon positions using the P6/mmm space group or
closely related ones, however the resulting ts were signicantly
Fig. 3 Powder X-ray diffraction spectra of Na
two different synthesis methods. Synthesis (A): Na
2 2
Na TeC . Synthesis (B): 2NaC H + Te / Na TeC + C H .
2
TeC
2
as obtained by
2 2
C
+ Te /
2
2
2
2
2
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 55986–55993 | 55989