Synthesis of the titanium phosphide telluride Ti2PTe2: A thermochemical approach

Article abstract of DOI:10.1016/j.jssc.2008.01.003

The phosphide telluride Ti₂PTe₂ can be synthesised from the elements or from oxides in a thermite type reaction. Both ways have been optimised by consideration of the thermodynamic behaviour of the compound. Hence, the investigation of phase equilibria in the ternary system Ti/P/Te and of the thermal decomposition of Ti₂PTe₂ was necessary. This investigation was performed by using different experimental approaches as total pressure measurements, thermal analysis and mass spectrometry. The results were supported and further analysed by thermodynamic modelling of the ternary system. It was shown that Ti₂PTe_2(s) decomposes to Ti₂P_(s) and Te_2(g) in six consecutive steps. The growth of single crystals of Ti₂PTe₂ is thermodynamically described as a chemical vapour transport with TiCl_4(g) acting as the transport agent.

Full text of DOI:10.1016/j.jssc.2008.01.003

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Journal of Solid State Chemistry 181 (2008) 758–767
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Synthesis of the titanium phosphide telluride Ti PTe :
2
2
A thermochemical approach
a
a,
Ã
b
b
Frauke Philipp , Peer Schmidt , Edgar Milke , Michael Binnewies , Stefan Hoffmann
c
a
Inorganic Chemistry, Dresden University of Technology, HelmholtzstraXe 10, 01069 Dresden, Germany
Institute for Inorganic Chemistry, Leibniz University Hannover, Callinstr. 9, 30167 Hannover, Germany
b
c
Max Planck Institute for Chemical Physics of Solids Dresden, N o¨ thnitzer Str. 40, 01187 Dresden, Germany
Received 22 October 2007; received in revised form 21 December 2007; accepted 6 January 2008
Available online 10 January 2008
Abstract
The phosphide telluride Ti
2 2
PTe can be synthesised from the elements or from oxides in a thermite type reaction. Both ways have been
optimised by consideration of the thermodynamic behaviour of the compound. Hence, the investigation of phase equilibria in the ternary
system Ti/P/Te and of the thermal decomposition of Ti PTe was necessary. This investigation was performed by using different
2
2
experimental approaches as total pressure measurements, thermal analysis and mass spectrometry. The results were supported and
further analysed by thermodynamic modelling of the ternary system. It was shown that Ti PTe2(s) decomposes to Ti (s) and Te2(g) in six
consecutive steps. The growth of single crystals of Ti PTe is thermodynamically described as a chemical vapour transport with TiCl
2
2
P
2
2
4(g)
acting as the transport agent.
r 2008 Elsevier Inc. All rights reserved.
Keywords: Phosphide telluride; Thermal decomposition; Total pressure; Chemical vapour transport; Thermodynamic modelling
1
. Introduction
the thermodynamic properties of the ternary system Ti/P/
Te have been investigated in detail as well by experimental
methods as by thermodynamic modelling. The crystal
structure and the physical properties of Ti PTe will be
Although many metal pnictide chalcogenides are known
as antimonides selenides [1–5], antimonide tellurides [4–19],
arsenides selenides [4,18–31], arsenides tellurides [18,30–37]
and even phosphide sulphides [6,18,29,38–42] and phos-
phide selenides [4,18,23,40–42], by now there are only few
phosphide tellurides: UPTe [43,44], IrPTe [45], OsPTe,
2
2
presented soon [48].
. Experimental part
Syntheses have been performed in evacuated closed silica
2
RuPTe [46] and BaP Te [47]. This must be due to the fact,
2
4
that either no one ever tried to synthesise phosphide
tellurides, or that the synthesis cannot be performed
straightforward from the elements. Even the non-existence
of a binary compound made up from phosphorus and
tellurium shows that both elements cannot be combined
easily. Anyhow, we succeeded in synthesising the com-
tubes. Adherent moisture has been removed by heating
under dynamic vacuum prior to use. Starting materials
used were titanium powder (Alfa Aesar, 99.9%), tellurium
(Acros, 99.8%), red amorphous phosphorus (Fluka, 99%,
washed according to Ref. [49]), tellurium dioxide (Alfa
Aesar, ultrapure), phosphorus pentoxide (Laborchemie
Apolda, 99%, sublimed), tellurium oxide phosphate
Te O (PO ) (synthesised according to Ref. [50]) and
pound Ti PTe , but the optimisation of this synthesis was
2
2
possible only by consideration of the thermal behaviour of
the compound. To understand the processes taking place,
8
10
4 4
tellurium tetrachloride (synthesised from Te(s) and Cl2(g)
sublimed). All starting materials have been handled in an
,
Ã
argon-filled glove box (c(O , H O)o0.1 ppm). Ti PTe is
2
2
2
2
Corresponding author. Fax: +49 351 463 37287;
URL: http://www.peer-schmidt.de.
E-mail address: peer.schmidt@chemie.tu-dresden.de (P. Schmidt).
not sensitive to oxygen and moisture and therefore was
handled under air.
0022-4596/$ - see front matter r 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2008.01.003

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F. Philipp et al. / Journal of Solid State Chemistry 181 (2008) 758–767
759
Total pressure measurements were carried out in a silica
zero-point membrane manometer as described in Ref. [51].
A point was recorded every 20 K while each point was
recorded over 1 day, so that thermal equilibrium between
solid phase and gas phase can be assured.
the heating temperature is at a point where the decom-
position of the compound to the gas phase is already
running and the rebuilding of Ti PTe requires some time.
Although no explicit kinetic studies were carried out no
single-phase product was observed at higher cooling rates.
In these cases, gaseous tellurium condenses out of
thermodynamic equilibrium conditions. Anyway, at lower
temperatures no formation of the compound could be
observed.
2
2
Thermogravimetric measurements have been performed
under argon atmosphere on a Netzsch STA 449C with a
ꢀ
1
heating rate of 10 K min in the temperature range from
room temperature up to 1500 1C.
Investigations of the composition of the gas phase at the
decomposition of Ti PTe were performed using the mass
The reduction of the oxides with titanium to form the
goal compound Ti PTe besides Ti O can be considered as
2
2
2
2
2
3
spectrometer Varian MAT 212 coupled to a Knudsen-
effusion cell. The sample was heated isothermally to 400,
an example of a thermite reaction. The formation of the
titaniumsesquioxide from elemental titanium lowers sig-
nificantly the free Gibbs energy of the total reaction
(2), respectively, (3) and therefore is the driving force
for building Ti PTe as a side product. The resulting
6
00 and 800 1C. The evaporated species were ionised
by electron impact ionisation with an energy of 70 eV
and detected by a quadrupole mass spectrometer with
m/zp350.
2
2
o
free Gibbs energy for reaction (2) is DRG
¼
1000 K
ꢀ1
o
ꢀ1
Thermodynamic modelling of phase equilibria have been
performed by using Calphad methods based on the
Eriksson Gibss energy minimiser implemented in the
programs Chemsage [52] and Tragmin [53]. The phase
diagrams of the binary systems Ti/P and Ti/Te have been
calculated with the two dimensional phase mapping mode
ꢀ2295 kJ mol
compared to DRG1
for reaction (1); (used thermodynamic data are listed in
¼ ꢀ405 kJ mol
000 K
Tables 1 and 2).
This kind of redox reaction is enabled by the difference in
oxygen partial pressure between the titanium oxides and the
tellurium, respectively, phosphorus oxides. As only one
distinct partial pressure can exist above the reaction
mixture, an equalisation is reached through the reaction.
(
Chemsage [52]) in the temperature range 300–3000 K. The
solid state–gas phase equilibria in the ternary and
quaternary systems Ti/P/Te/(Cl) have been calculated
If also the gaseous species P O
4
is concerned, it can be
seen, that the level of this equalisation lies at Ti O beside
2 3
6(g)
(
Tragmin [53]) at different given compositions isothermally
in the temperature range 800–1300 K with DT ¼ 100 K.
elemental tellurium and phosphorus (Fig. 1). In the sense of
an electromotive series of solids there is a potential gradient
between titanium and the tellurium and phosphorus oxides,
which will be compensated by reaction (2); [54].
3
. Results and discussion
The advantage in using Te O (PO ) instead of TeO
2
8
10
4 4
3
.1. Methods of synthesis
and P O is the retention of the wanted composition of the
2
5
reaction mixture up to temperatures of 650 1C, while TeO2
The compound Ti PTe can be synthesised in two
2
different ways. Besides a common solid state reaction
from the elements under controlled thermal conditions
2
and even more P O would be lost by sublimation at much
lower temperatures.
2
5
The formed phosphide telluride can be separated from
the oxide by chemical vapour transport from T to T
(
tellurium oxide phosphate Te O (PO ) [50] with elemen-
Eq. (1)), the reduction of either P O10 and TeO or of the
4 2
2
1
8
10
4 4
(
T 4T ), e.g. from 1073 to 973 K by adding small amounts
2
1
tary titanium to Ti O and Ti PTe (Eq. (2), respectively,
2
3
2
2
of TeCl4 The observed transport rate with approximately
0.02 mg h
(s).
ꢀ
(
3)) leads to the aimed compound. Both ways show special
advances towards the other one, so they can be used
complementary.
1
is that small, that this method is not
convenient for synthesising large amounts of sample. On
the other hand, it is very usable for growing single crystals
for crystal structure determination and physical properties
measurements [48].
2
Ti þ ð1=4Þ P4ðs;gÞ þ 2 Teðl;gÞ ! Ti PTe
(1)
(2)
(3)
ðsÞ
2
2ðsÞ
ð19=3Þ TiðsÞ þ ð1=4Þ P O
þ 2 TeO2ðs;gÞ
4
10ðs;gÞ
3.2. Measurements of total pressure
!
Ti2PTe2ðsÞ þ ð13=6Þ Ti2O3ðsÞ
The measured total pressure above solid Ti PTe is
2
ð19=3Þ TiðsÞ þ ð1=4Þ Te8O10ðPO4Þ4ðsÞ
2
shown logarithmically versus the inverse temperature in
Fig. 2. At low temperatures, a slow increase of pressure can
be seen. This is probably due to the evaporation of
remaining elemental phosphorus (and eventually also
tellurium) from the synthesis and thermal expansion of
the gas phase. Around 600 1C, a sudden decrease of
pressure appears. At this point, the reaction of the gas
phase with the remaining starting material in completion of
!
Ti PTe þ ð13=6Þ Ti O
2
2ðsÞ
2
3ðsÞ
The reaction from the elements was carried out by
heating the grinded educts in a silica tube of small volume
ꢀ
1
(
left at this temperature for 1 day and cooled down to
approximately 1 ml) up to 800 1C with a rate of 10 K h
,
ꢀ
1
4
00 1C with a cooling rate of 1 K h . This very slow
cooling results in a long reaction time, but is necessary, as

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F. Philipp et al. / Journal of Solid State Chemistry 181 (2008) 758–767
760
Table 1
Thermodynamic data from the optimisation of solid compounds in the system Ti/P/Te/Cl
DH2
ðkJ molꢀ1
Þ
DS
o
298K
ðJ molꢀ1
K
ꢀ1Þ
C
p
(J mol
ꢀ
1
K
ꢀ1 a
)
Reference
o
98K
Compound
a
b
c
Ti(s)
Te(s)
0
0
30.761
49.706
22.803
22.238
19.121
16.736
138.490
65.187
53.070
73.346
86.280
64.140
234.400
170.280
148.140
42.000
187.700
142.900
101.600
60.400
99.474
10.205
22.092
14.895
–0.008
[61]
[61]
P
(red,s)
ꢀ17.489
ꢀ323.841
ꢀ323.423
ꢀ1520.884
ꢀ944.747
ꢀ331.800
ꢀ321.600
ꢀ1265.000
ꢀ941.700
ꢀ852.300
ꢀ248.000
ꢀ621.240
ꢀ582.610
ꢀ397.090
ꢀ196.620
ꢀ447.988
[61]
[61]
TeCl4(s)
TeO2(s)
203.012
74.308
14.560
163.444
3.054
ꢀ0.502
ꢀ1.703
[61]
[61]
Ti
2
O
3(s)
77.253
TiO2(rutile,s)
50.626
[61]
Ti
3
Ti
2
Ti
7
Ti
5
Ti
4
P
P
P
P
P
(s)
114.400
83.700
32.600
22.300
78.900
56.600
46.300
12.000
139.400
119.300
87.000
54.300
79.491
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
(s)
4(s)
3(s)
3(s)
303.312
220.400
189.700
51.650
TiP(s)
Ti
5
Ti
3
Ti
2
Te4(s)
Te4(s)
Te3(s)
351.000
287.490
208.210
136.380
183.800
0.040
TiTe2(s)
Ti PTe2(s)
2
ꢀ0.016
a
¼ a+b ꢁ 10ꢀ3T+c ꢁ 106T
ꢀ2
C
p
.
Table 2
Thermodynamic data of gaseous compounds in the system Ti/P/Te/Cl
DH2
ðkJ molꢀ1
Þ
DS
o
298K
ðJ molꢀ1
K
ꢀ1Þ
C
p
(J mol
ꢀ1
ꢀ1 a
K )
Reference
o
98K
Compound
a
b
c
Ti(g)
473.628
ꢀ237.232
ꢀ539.317
ꢀ763.161
211.710
160.372
ꢀ111.716
ꢀ205.852
333.883
144.301
59.119
180.297
278.345
316.838
354.803
182.699
262.153
305.662
401.773
163.201
218.129
279.981
345.745
406.697
311.984
364.878
233.078
165.184
191.610
23.304
60.124
87.257
107.169
19.414
34.644
58.028
96.734
20.669
36.296
81.839
216.355
292.830
82.366
131.587
36.610
23.736
30.418
1.001
[61] C
[61]
[61]
[61]
[61]
[61]
[61]
[61]
[61]
[61]
[61]
[61]
[61]
[61]
[61]
[61]
[61]
[61]
p
estimated
TiCl2(g)
TiCl3(g)
TiCl4(g)
Te(g)
2.218
ꢀ0.711
0.490
1.841
6.615
0.092
0.155
0.172
0.799
0.678
8.665
19.192
0.406
0.837
1.079
ꢀ1.284
2.544
ꢀ0.276
ꢀ1.293
ꢀ1.050
0.075
Te2(g)
ꢀ0.025
ꢀ0.331
ꢀ0.548
TeCl2(g)
TeCl4(g)
P
P
P
P
P
(g)
2(g)
4(g)
ꢀ0.414
ꢀ1.343
ꢀ6.795
4
O
O
6(g)
ꢀ2214.290
ꢀ2902.076
ꢀ286.922
ꢀ374.769
0
4
10(g)
ꢀ10.715
ꢀ1.067
ꢀ1.778
ꢀ0.272
ꢀ0.126
ꢀ0.238
PCl3(g)
PCl5(g)
Cl2(g)
Cl(g)
121.294
0
N
2(g)
a
¼ a+b ꢁ 10ꢀ3T+c ꢁ 106T
ꢀ2
C
p
.
the chemical equilibrium takes place, so that the total
pressure is lowered. This observation reconfirms the fact
found from experiments for synthesis optimisation, that
high temperatures W4700 1C are needed for the formation
of Ti PTe .
finished at 1000 1C, as it has not reached atmospheric
pressure up to this temperature. Hence, you can find
Ti PTe beside TiP and small amounts of Ti Te in the
2
2
2
3
remaining solid phase after the measurement. This is a first
hint, that Ti PTe decomposes by release of Te or other
2
2
2
2
2(g)
If one continues the measurement with increasing
temperature, another rise of total pressure is observed
starting from approximately 700 1C. This effect is finally
due to the thermal decomposition of Ti PTe . It is not
tellurium-containing gaseous species. Thereby, the decom-
position pressure of Ti PTe is lower than the one of pure
Te(s) due to the stabilisation of the ternary phase relative to
pure tellurium.
2
2(s)
2
2

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F. Philipp et al. / Journal of Solid State Chemistry 181 (2008) 758–767
761
Fig. 1. Oxygen partial pressure and electrochemical potential above the
oxides of titanium, tellurium and phosphorus calculated at 1000 K,
marked: level of equalisation of oxygen partial pressure.
Fig. 3. Relative frequency of gaseous species detected by mass spectro-
metry after decomposition of Ti PTe in a Knudsen-cell at 400 1C (
2
2
),
600 1C (
) and 800 1C (’).
Fig. 4. Thermogravimetric decomposition of Ti
shows the differential thermogravimetric curve.
2
PTe2(s); the dashed line
Fig. 2. Measured and modelled total pressure above solid Ti
Ti Te
2 2
PTe and
2
3
.
As TiP(s) cannot be formed directly from Ti PTe2(s), only
2
distinct species is defined by the temperature dependent
equilibrium between both of them.
by delivery of Te2(g) to the gas phase, the decomposition
must follow a more complex mechanism. This assumption
is supported by the slow kinetics of the decomposition of
Ti PTe , as equilibrium between solid phase and gas phase
The detection of as well phosphorus gas species as
tellurium gas species at lower temperatures again is due
to the evaporation of small amounts of remaining starting
material from synthesis as it also was found in the total
pressure measurements at low temperatures.
2
2
was only reached after 1 day.
.3. Knudsen-cell mass spectrometry
To confirm that Ti PTe decomposes under release of
3
3.4. Thermogravimetric measurement
2
2(s)
tellurium-containing species and to investigate, if phos-
phorus-containing species are released simultaneously, the
substance was pyrolised in a Knudsen-cell coupled to a
mass spectrometer and the resulting gaseous molecules
were examined. As the decomposition of Ti PTe starts at
The thermogravimetric measurement shows in the first
instance a very small mass loss of 2(1)% of the initial mass
at temperatures up to 600 1C and a big decomposition step
starting from approximately 650 1C, which is not finished
at 1500 1C (Fig. 4). Up to this point, the mass loss is nearly
50(1)%. In agreement with the results of the measurement
of total pressure and the mass spectrometry, the first mass
loss can be explained by the evaporation of elemental
phosphorus and tellurium. Hence, the molar mass of
Ti PTe is not treated as 100% but as 98% of the initial
2
2
7
00 1C, only the gaseous species detected at 800 1C can be
considered as decomposition products. Here, the most
+
frequently found species is Te . Furthermore, high
+
amounts of monoatomic Te can be found, but only very
2
few amounts of phosphorus-containing species (Fig. 3).
This confirms, that the decomposition process of Ti PTe
leads to the release of Te2(g) and Te , the amount of each
2
2(s)
2
2
mass.
(g)

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762
By regarding the differential thermogravimetric curve,
performed using the program Chemsage [52]. Using this
program, thermodynamic data of all included solid phases
and of the liquid phase have been varied that way that
known thermic effects of the compounds as melting points,
eutectica and peritectica could be represented correctly in
comparison with literature data [55,56]. All optimised data
have been introduced into the modelling of the ternary
system Ti/P/Te, adjusted to experimental observations for
this system and reoptimised in modelling of the binary
systems, until the whole set of data was consistent with all
experimental results.
the total decomposition step can be separated into several
distinct steps. The first one ends close to 1300 1C and is
coupled with a mass loss of 21(1)%. It can be explained by
the formation of 1/3 Ti Te +TiP per formula unit Ti PTe
3
4
2
2
and the release of 1/3 Te2(g) (Dm/mtheoretical ¼ ꢀ22%),
Eq. (4)). The next step gives another mass loss of 4(1)%. It
(
can be explained by the phase width of TiP which is
passed from the upper phase border (TiP1.0) to the
lower phase border (TiP0.92) [55] by the release of 4/69
Te2(g) (Dm/mtheoretical ¼ ꢀ4%) as described in Eq. (5).
Afterwards, another mass loss of 12(1)% is observed
due to the formation of Ti4P3(s) and 34/207 Te2(g)
In the system Ti/Te, standard enthalpies and entropies of
formation were known for the compounds TiTe2(s) and
(
is made up of 2/9 Ti Te +1/3 Ti P (Eq. (6)).
Dm/mtheoretical ¼ ꢀ11%), while the remaining solid phase
Ti Te
2
and for a compound having the nominal
3(s)
composition TiTe , but being noted for showing a wide
(s)
3
4
4
3
All described steps are consistent with the decomposition
process obtained by the thermodynamic modelling of the
ternary system Ti/P/Te, which will be regarded later on in
this work. According to this modelling, the next step of
decomposition has to be the formation of Ti P besides
homogeneity range [57]. As there is not such a phase in the
published phase diagram, which was used for comparison
[55], the standard entropy published for TiTe(s) multiplied
by five, have been used to describe the compound Ti Te4(s).
5
The enthalpy of formation for this compound has been
determined by heat flow calorimetry [58]. For the last
binary titanium telluride Ti Te4(s), no data at all have been
5
3
Ti Te (Eq. (6)), but this step is not completed at 1500 1C,
3
4
when the gravimetric measurement was stopped. Anyway,
the formation of Ti5P3(s) was verified by means of X-ray
powder diffraction in the residual from the thermogravi-
metric measurement.
3
accessible, so they are newly estimated here. The calculated
phase diagram is in good agreement with the experimental
one [55] concerning the thermal effects of Ti , Ti Te4(s),
(s)
5
TiTe2(s) and Te(s) (Fig. 5); the melting point of Ti Te
4
unknown. The obtained thermodynamic data for the
system Ti/Te are listed in Table 3.
is
3(s)
Ti2PTe2ðsÞ"ð1=3Þ Ti3Te4ðsÞ þ TiPðsÞ þ ð1=3Þ Te2ðgÞ
ð1=3Þ Ti Te þ TiP "ð7=23Þ Ti Te
(4)
3
4ðsÞ
ðsÞ
3
4ðsÞ
þ ð25=23Þ TiP0:92ðsÞ
þ ð4=69Þ Te2ðgÞ
(5)
ð7=23Þ Ti Te þ ð25=23Þ TiP0;92ðsÞ"ð2=9Þ Ti Te
3
4ðsÞ
3
4ðsÞ
þ ð1=3Þ Ti4P3ðsÞ
þ ð34=207Þ Te2ðgÞ
(6)
ð2=9Þ Ti3Te4ðsÞ þ ð1=3Þ Ti4P3ðsÞ"ð1=9Þ Ti3Te4ðsÞ
þ ð1=3Þ Ti P
5
3ðsÞ
þ ð2=9Þ Te2ðgÞ
(7)
3.5. Thermodynamic modelling of the binary systems Ti/P
and Ti/Te
To understand the complex mechanism of formation and
decomposition of Ti PTe and to optimise the synthesis of
2
2
this and other phosphide tellurides, a detailed thermo-
dynamic modelling and description of the ternary system
Ti/P/Te was aspired. In this modelling, at least some of the
numerous binary titanium phosphides and -tellurides had
to be included, to reach a realistic picture of the phase
relations in the ternary system. Since thermodynamic data
for the binary compounds have only fragmentary been
reported afore, an estimation and adjustment of these data
by modelling of the binary systems Ti/P and Ti/Te was
Fig. 5. Phase diagram of the binary system Ti/Te as published [55] (top)
and as resulted from thermodynamic modelling using the program
Chemsage [52] (bottom).

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763
Table 3
Thermodynamic data for the system Ti/Te yield from the modelling in
comparison to literature [57,58]
o
98K
ðkJ molꢀ1
o
ðJ Kꢀ1 molꢀ1Þ
DH2
Þ
DS
298 K
Optimisation Literature
Optimisation Literature
TiTe2(s) ꢀ196.62
ꢀ213.1 [57]
136.38
208.21
287.49
117.2 [57]
207725 [57]
–
Ti
2
Ti
3
Ti
5
Te3(s) ꢀ397.09
Te4(s) ꢀ582.61
Te4(s) ꢀ621.24
ꢀ355740 [57]
–
a
a
ꢀ6057100 [57] 351.00
420740 [57]
ꢀ
630710 [58]
a
5
ꢂ TiTe(s): ‘‘wide homogeneity range’’ [57].
In the system Ti/P, far less thermodynamic data have
been known in advance to this work. The existence of the
compounds Ti P, Ti P, Ti P , Ti P , Ti P , TiP and TiP is
3
2
7
4
5
3
4
3
2
shown in the phase diagram [56], but only the enthalpy of
formation for TiP(s) has been determined on two different
ꢀ
1
ꢀ1
ways to ꢀ265 kJ mol , respectively, to ꢀ324 kJ mol [59].
As there is not even an existence range determined for TiP ,
2
this compound was excluded from modelling. For all other
compounds, the entropies of formation have been esti-
mated according to the Neumann–Kopp rule [60] as the
sum of the entropies of elemental titanium and red
phosphorus. For the enthalpies of formation normalised
to one phosphorus atom per formula unit a linear
devolution from Ti(s) to TiP(s) with increasing phosphorus
content was assumed and used as a starting point.
Neither for any titanium telluride nor for the titanium
phosphides the functions of heat capacity were known, so
these were estimated for all compounds again according to
the Neumann-Kopp rule.
Fig. 6. Phase diagram of the binary system Ti/P as published [56] (top)
and as resulted from thermodynamic modelling using the program
Chemsage [52] (bottom).
Table 4
Thermodynamic data for the system Ti/P yield from the modelling in
comparison to literature [57,59]
o
98K
ðkJ molꢀ1
o
ðJ Kꢀ1 molꢀ1Þ
DH2
Þ
DS
298 K
Optimisation
Literature
Optimisation
51.65
Literature
–
The experimental phase diagram [56] for the system Ti/P
is covered by the calculated very well (Fig. 6), the gained
data are presented in Table 4.
TiP(s)
ꢀ248.0
ꢀ265.3
ꢀ
–
–
–
–
–
324
Ti4P3(s)
ꢀ852.3
ꢀ941.7
ꢀ1265.0
ꢀ321.6
ꢀ331.8
189.7
220.4
303.32
83.7
–
–
–
–
–
Ti
5
Ti
7
Ti
2
Ti
3
P
P
P
P
3(s)
4(s)
(s)
3.6. Thermodynamic modelling of the ternary system
Ti/P/Te
(s)
114.4
Using the optimised thermodynamic data for the binary
titanium phosphides and tellurides, solid state–gas phase
equilibria in the ternary system Ti/P/Te have been
modelled using the program Tragmin [53]. In Tragmin,
there is a limit of the number of solid phases; hence, only
the phosphides TiP , Ti P and Ti2P(s) and the tellurides
Fig. 7 shows the resulting ternary phase diagram with all
appearing coexistence ranges. The ternary areas Ti PTe2(s)/
2
Ti Te3(s)/TiP(s) and Ti PTe /Te /TiP have also been
(s)
2
2
2(s)
(s)
proved by solid-state reactions. If the decomposition of
Ti PTe runs by the release of Te2(g), as shown by mass
(s)
5 3(s)
TiTe2(s), Ti Te
2
and Ti Te
3
were included besides
4(s)
3(s)
2
2(s)
Ti PTe
2
and the elements Ti , Te and P(red,s)
(s)
.
spectrometry, the composition of the solid phase has to
follow the dashed line until finally Ti2P(s) remains. Each
time it crosses a solid line and therewith enters a new
coexistence range, another decomposition step takes place.
As the phase width of TiP(s) was not incorporated in the
modelling, the passing through its phase region as found in
the thermogravimetric measurement cannot be seen here.
So, the presented phase diagram shows the decomposition
of Ti PTe in six steps, if also the existence of Ti4P3(s) and
2(s)
(s)
As already described, the modelling was done in
alternation with the modelling of the binary systems, so
that experimental results could be reproduced for both
binary systems and the ternary one. Not until this
agreement was reached, new results were considered. First
of all, the agreement between model and experiment was
proved on the measurement of total pressure. Finally the
model was able to reflect the total pressure above solid
Ti PTe and Ti Te very well (Fig. 2).
2
2(s)
Ti7P4(s) between TiP(s) and Ti2P(s) is considered. These steps
2
2
2
3

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764
Fig. 7. Calculated phase diagram (T ¼ 1000 K) of the system Ti/P/Te-
calculation using the program Tragmin [53], (ꢂ ꢂ ꢂ) decomposition of
2 2
Ti PTe under release of Te2(g).
Fig. 8. Calculated composition of the gas phase above solid Ti
calculation using the program Tragmin [53].
2 2
PTe -
can be described by the Eqs. (8)–(13), which give in sum the
total decomposition to Ti2P(s) and Te2(g) (14). It is
remarkable for this process, that most of the steps are
not typical decomposition reactions in that one compound
decays into another one plus a gaseous species. These steps
detail reactions in that a new compound is built from two
starting compounds, while a gaseous side product is
released. This explains the very slow progress of the whole
decomposition process.
Ti PTe "ð1=2Þ Ti Te þ TiPðsÞ þ ð1=4Þ Te2ðgÞ
(8)
(9)
2
2ðsÞ
2
3ðsÞ
ð1=2Þ Ti2Te3ðsÞ"ð1=3Þ Ti3Te4ðsÞ þ ð1=12Þ Te2ðgÞ
ð1=9Þ Ti3Te4ðsÞ þ TiP "ð1=3Þ Ti4P þ ð2=9Þ Te2ðgÞ (10)
ðsÞ
3ðsÞ
ð1=9Þ Ti Te þ ð1=3Þ Ti P "ð1=3Þ Ti P
3
4ðsÞ
4
3ðsÞ
5
3ðsÞ
Fig. 9. Tellurium partial pressures p(i), (i ¼ Te2(g)) above all coexistence
lines subtended during the decomposition of Ti PTe2(s) to Ti (s) and
4(s)-calculation using the program
þ ð2=9Þ Te2ðgÞ
(11)
(12)
2
2
P
Te2(g), unmarked area: Ti
3 7
Te4(s)+Ti P
ð1=36Þ Ti Te þ ð1=3Þ Ti P "ð1=4Þ Ti P
3
4ðsÞ
5
3ðsÞ
7
4ðsÞ
Tragmin [53].
þ ð1=18Þ Te2ðgÞ
ð1=12Þ Ti Te þ ð1=4Þ Ti P "Ti P þ ð1=6Þ Te2ðgÞ
of the gas phase above solid Ti PTe as obtained from
2 2
3
4ðsÞ
7
4ðsÞ
2
ðsÞ
complex modelling of the phase equilibria is shown in
Fig. 8. Here the dominance of the species Te2(g) and Te(g)
versus the phosphorus species can be seen, as it was found
by mass spectrometry. Nevertheless, the dissociation of
Te2(g) in the regarded temperature range creates a partial
pressure p(Te ) three orders of magnitude lower than
(
13)
X
Ti PTe "Ti P þ Te2ðgÞ
(14)
2
2ðsÞ
2
ðsÞ
In comparison to the decomposition steps found from
thermogravimetric measurements, the first measured step
covers equilibria (8) and (9). The second step originates
from passing through the phase width of TiP , which is
(g)
p(Te2(g)). Therefore the contribution of the dissociation can
be neglected in calculations of the free Gibbs energy of
reactions (8)–(14).
In Fig. 9, the tellurium pressure for equilibria (8)–(13)
according to equation (15) above the distinct coexistence
ranges is shown. Here, the same sequence of phases can be
seen which confirms the decomposition process (8)–(13).
(
s)
not included in the modelling. So the second and third
thermogravimetrically measured step can be assigned to
equation (10). The thermogravimetric measurement was
stopped at 1500 1C. At this temperature, the fourth
decomposition step (equation (11)) is taking place; hence,
Ti5P3(s) can be found in the residual. In contrast, in the
total pressure measurement up to 1000 1C, only reaction (8)
takes place, so TiP(s) is found afterwards. The composition
o
o
ꢀ
DRH
DRS
lg pðTe Þ ¼
þ
.
(15)
2
2:303 R T 2:303 R

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765
3.7. Thermodynamic modelling of the chemical vapour
transport of Ti PTe
pressure for each component, it is necessary, that the
solubility of each component in the gaseous phase is
temperature dependent at such a rate that the direction of
the transport equilibrium can be changed by a moderate
temperature gradient. To check this possibility, the
transport efficiency was examined. It is defined as the
difference of partial pressure between source and sink
normalised to the solvent. According to this definition, a
transport agent should show negative values for the
transport efficiency, as it gets consumed in the source and
released in the sink, and transport-relevant species should
show positive values of transport efficiency.
2
2
Subsequent to the ternary system Ti/P/Te modelling of
the quaternary system Ti/P/Te/Cl has been performed to
investigate the mechanism of the chemical vapour trans-
port. Therefore, chlorine-containing gaseous species and
solid TeCl were included in addition to the same species as
4
regarded in the ternary system. Fig. 10 shows the gas phase
above the area Ti PTe /Ti Te /TiP /TeCl4(s). It can be
2
2(s)
2
3(s)
(s)
seen easily, that TiCl4(g) is built from the starting
compounds and exhibits a very high partial pressure. If
ꢀ
5
the region down to a pressure of 10 bar is regarded, the
tellurium-containing species Te2(g), Te(g) and TeCl2(g) and
the phosphorus-containing species P2(g) can be found. This
region is said to be relevant for chemical transport
reactions, so the basic condition for a chemical transport
In a first result, the calculation of transport efficiency
shows only TiCl4(g) as transport agent and TiCl3(g) and
Te2(g) as transport relevant species (Fig. 11, left). That way
the chemical transport of Ti PTe would not be possible,
2
2(s)
as there would be no transport of phosphorus. Only by
regarding very low transport efficiencies, a transport
relevance of P2(g) can be seen (Fig. 11, right). This very
low transport efficiency of P2(g) fits to the very low
transport rate which was found experimentally (Dm/t ¼
of Ti PTe
2
is fulfilled. But beside a sufficient partial
2(s)
ꢀ
1
0
.02 mg h ). However, the chemical transport of
Ti PTe
2
is possible and can be described by Eq. (16).
2(s)
Moreover, the modelling shows that TiTe2(s) is transported
simultaneously to Ti PTe according to Eq. (17).
2
2(s)
2
Ti PTe þ 12 TiCl4ðgÞ"16 TiCl þ 2 Te2ðgÞ þ P
2
2ðsÞ
3ðgÞ
2ðgÞ
(
16)
TiTe2 þ 3 TiCl4ðgÞ"4 TiCl þ Te2ðgÞ
(17)
ðsÞ
3ðgÞ
In both transport reactions, TiCl4(g) acts as the transport
agent. In the experiment, it is built from the reaction
mixture immediately, although solid TeCl was added. This
4
reaction proceeds to reach an equalisation in chlorine
partial pressure. The tellurium chlorides TeCl4(g) and
TeCl2(g)—and as well the phosphorus chlorides PCl5(g)
and PCl3(g)—have no common existence range with
elemental titanium, so they cannot coexist (Fig. 12). By
Fig. 10. Calculated composition of the gas phase p(i) ¼ f(T) above a solid
mixture Ti PTe2(s)/Ti Te3(s)/TiP(s)/TeCl4(s)-calculation using the program
Tragmin [53], grey marked area: transport relevant region.
2
2
*
Fig. 11. Transport efficiency D(p(i)/p (L)) above a solid mixture Ti
side shows the limiting efficiency of P2(g) for the chemical transport of Ti
2
PTe2(s)/Ti
2
Te3(s)/TiP(s)/TeCl4(s)-calculation using the program Tragmin [53], the right
2
PTe2(s).

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766
vapour transport of Ti PTe with TiCl4(g), which shows
2
2
very low transport rates due to the low partial pressure of
P2(g), the only transport relevant species for phosphorus.
Finally, the results cannot be passed on directly to the
synthesis of other ternary phosphide tellurides M PTe , as
x
y
the distinct conditions have to be regarded individually for
each system. Especially the basic binary systems M/P and
M/Te have to be taken into account. The synthesis from
the oxides should be applicable for every metal that can
form an oxide, whose oxygen partial pressure is below the
one of P O6(g), that means in the same range as the one of
4
Ti O3(s). But, still a suitable way of separating the aimed
2
Fig. 12. Chlorine partial pressure lg(p(Cl )/bar) and electrochemical
potential E at T ¼ 1000 K above the chlorides of titanium, tellurium
2
compound from the oxide, e.g. by chemical transport has
to be found.
and phosphorus, marked: level of equalisation of chlorine partial pressure.
Acknowledgments
¨
The authors thank Mrs. S. Muller, Max Planck Institute
the oxidation of titanium to TiCl4(g) and the reduction of
TeCl4(g), a shared range of existence can be reached,
analogical to the description of the redox processes in the
oxidic system. Likewise, the use of phosphorus chlorides or
even elemental chlorine would lead to the formation of
TiCl4(g), so it is possible to use the most practicable
chlorine supplying agent, which here is solid TeCl4.
for Chemical Physics of Solids, Dresden, for performing
the thermogravimetric measurements and Mrs. J. Krug,
Dresden University of Technology, for help with the
synthesis. Financial support by the Deutsche Forschungs-
gemeinschaft is gratefully acknowledged.
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