Study of the formation of bi2te4o11

Article abstract of DOI:10.1006/jssc.1996.0036

The solid state reaction in a 1:4 mole ratio mixture of Bi₂O₃ and TeO₂ and the polymorphic phase transition of Bi₂Te₄O₁₁ have been investigated using differential scanning calorimetry (DSC), electron microprobe, X-ray powder diffraction (XPD), and selected area electron diffraction (SAED) analysis in the 25-730°C temperature range. Upon heating first a 8Bi₂Te₄O₁₁ + 23TeO₂ eutectic is formed, which melts at 598.9°C. In this melt the excess of Bi₂O₃ reacts further and the Bi₂O₃ + 4TeO₂ = Bi₂Te₄O₁₁ reaction takes place. The cubic modification is formed by fast crystallization of the Bi₂Te₄O₁₁ melt. The structure of the cubic Bi₂Te₄O₁₁ can be characterized by the lattice constant of a = 5.6397(5) A and space group Fm3m. The main product of a slow cooling is the same cubic polymorph although a subordinate formation of the monoclinic phase is also observed. The β-Bi₂Te₄O₁₁ cubic phase undergoes a monotropic transformation into the α-Bi₂Te₄O₁₁ monoclinic modification at temperatures higher than 400°C. The cubic → monoclinic transition is the result of an ordering in one set of {111} planes and the orthogonality of the cubic phase in the [110] projection changes to monoclinic symmetry. The melting enthalpies of the cubic β-phase and the monoclinic α-phase are 35.9 ± 3.3 J/g and 84.3 ± 4.3 J/g respectively.

Full text of DOI:10.1006/jssc.1996.0036

JOURNAL OF SOLID STATE CHEMISTRY 121, 251–261 (1996)
ARTICLE NO. 0036
Study of the Formation of Bi2Te4O11
Zs. Szaller
Research Laboratory for Crystal Physics, Hungarian Academy of Sciences, 1502 Budapest,Pf.132, Hungary
L. Poppl
¨
Institute of Inorganic and Analytical Chemistry of L.Eo¨tvo¨s University, 1518 Budapest,112 Pf.32, Hungary
Gy. Lovas and I. Dodony
´
Mineralogical Department of L.Eo¨tvo¨s University 1088 Budapest, Mu´zeum krt. 4/a, Hungary
Received March 6, 1995; in revised form June 27, 1995; accepted July 7, 1995
For single-crystal growth of bismuth tellurites it is im-
The solid state reaction in a 1 : 4 mole ratio mixture of Bi2O3 portant to know the details of the solid state reaction be-
and TeO2 and the polymorphic phase transition of Bi2Te4O11
have been investigated using differential scanning calorimetry
(DSC), electron microprobe, X-ray powder diffraction (XPD),
and selected area electron diffraction (SAED) analysis in the
25–730؇C temperature range. Upon heating first a 8Bi2Te4O11 ؉
23TeO2 eutectic is formed, which melts at 598.9؇C. In this melt
the excess of Bi2O3 reacts further and the Bi2O3 ؉ 4TeO2 ؍
Bi2Te4O11 reaction takes place. The cubic modification is formed
by fast crystallization of the Bi2Te4O11 melt. The structure of
the cubic Bi2Te4O11 can be characterized by the lattice constant
tween Bi2O3 and TeO2 . There are some contradictions in
the published phase diagrams of this reaction (12, 13). In
spite of the fact that bismuth tellurites can easily be oxi-
dized in air there are still some open questions concerning
the oxidation process (12–14). Bismuth tellurium oxide
compounds have anion-deficient fluorite structures. It is
not yet clear whether complete oxidation leading to the
filling of all vacant oxygen sites is possible.
The chemical reaction between Bi2O3 and TeO2 can be
described by the following equation:
˚
of a ؍ 5.6397(5) A and space group Fm3m. The main product
of a slow cooling is the same cubic polymorph although a
subordinate formation of the monoclinic phase is also observed.
The ␤-Bi2Te4O11 cubic phase undergoes a monotropic transfor-
mation into the ␣-Bi2Te4O11 monoclinic modification at temper-
atures higher than 400؇C. The cubic Ǟ monoclinic transition
is the result of an ordering in one set of ͕111͖ planes and the
orthogonality of the cubic phase in the [110] projection changes
to monoclinic symmetry. The melting enthalpies of the cubic
␤-phase and the monoclinic ␣-phase are 35.9 ؎ 3.3 J/g and
(1 Ϫ x)/2Bi2O3 ϩ xTeO2 ϭ Bi1 xTexO(3 x)/2.
Ϫ
ϩ
Bi2Te4O11 (x ϭ 0.667) is the first compound formed in the
solid state reaction of Bi2O3 and TeO2 . The structure of
Bi2Te4O11 was first studied by Frit et al. (17) and the com-
plete structure analysis was carried out by Rossel et al.
(18). These authors described only the monoclinic Ͱ-modi-
fication. Demina et al. (15) reported on X-ray investigation
of bismuth tellurites but they did not find polymorphic
modifications either. The existence of a cubic ͱ-polymorph
of Bi2Te4O11 above 609ЊC was first reported by Astaf’ev
et al. (16), but no structural details of this phase were
given. In the present paper the synthesis and properties
of Bi2Te4O11 , in particular for the almost unknown
ͱ-modification, are discussed in detail.
84.3 ؎ 4.3 J/g respectively.
1996 Academic Press, Inc.
INTRODUCTION
Bismuth-oxide-based compounds have a number of im-
portant applications such as in high-Tc superconductors
(1), oxygen ion conductors (2, 3), and nonlinear optical
materials (4). Among the nonlinear optical materials there
are sillenites related to Ͳ-Bi2O3 , such as bismuth ger-
manate, and its silicate and titanate counterparts, and
ͳ-Bi2O3-type compounds such as bismuth tellurites
EXPERIMENTAL
Starting materials were Bi2O3 (Johnson Matthey
(Bi2Te4O11 and Bi2TeO5). The latter are also known as Grade 1) and TeO2 prepared from tellurium metal
photorefractive (5–10) and acousto-optical materials (11) (99.999%) by oxidation with HNO3 (Carlo Erba for analy-
and might also have applications as oxygen ion conductors. sis). After a short anneal in air at 450ЊC the oxides were
251
0022-4596/96 $18.00
Copyright 1996 by Academic Press, Inc.
All rights of reproduction in any form reserved.

252
SZALLER ET AL.
FIG. 1. DSC curve of a Bi2O3 ϩ 4TeO2 mixture.
ground and sieved. The fraction with a grain size smaller A was made of a Bi2O3 ϩ 4TeO2 mixture. The mixture
than 63 Ȑm was used for the synthesis and for the DSC was put into a platinum crucible, heated in a furnace up
measurements.
to 720ЊC under argon atmosphere, and kept 2 hr at this
For X-ray powder diffraction (XPD) and selected area temperature with subsequent cooling to room temperature
electron diffraction (SAED) analysis three samples of at 10ЊC/min. Sample B was prepared under the same condi-
Bi2Te4O11 were prepared (Samples A, B, and C). Sample tions except for a different cooling rate (0.5ЊC/min). Sam-
FIG. 2. Intensities of d-lines of oxides heated to different temperatures.

STUDY OF THE FORMATION OF Bi2Te4O11
253
FIG. 3. Back-scattered electron pictures of the cross section of a pellet heated at 650ЊC for 60 min. (a) Unreacted Bi2O3 (bottom of the picture),
solidified melting phase (top of the picture), and the microcrystalline Bi2Te4O11 between them. Scale, 100 Ȑm. (b) Magnified details of solidified
melting phase. White prismatic and needle-like crystals have Bi2Te2O7 composition, the gray dendrite-like crystals have Bi2Te4O11 composition.
Scale, 100 Ȑm. (c) Magnified details of (b). Scale 10 Ȑm.
ple C was prepared like Sample A but in a second heating in argon atmosphere. This sample was broken into two
cycle it was annealed at 500ЊC.
pieces. The broken area was polished. The cross section
For electron microprobe analysis a pellet was prepared was analyzed with a Jeol Microprobe 733 instrument using
from Bi2O3 powder and annealed at 800ЊC in air. The pellet an accelerating voltage of 20 kV and a beam current of
was polished, pressed with polished TeO2 single crystal, 20 mA. Bi2O3 and TeO2 were used as standards and ZAF
and annealed for 1 hr at 650ЊC with subsequent quenching correction was applied.
FIG. 4. DSC curves of different samples demonstrating the formation of Bi2Te4O11
.

254
SZALLER ET AL.
FIG. 5. DSC curve of the phase transition of cubic Bi2Te4O11 modification showing the irreversibility of the monotropic transformation.
The DSC measurements were carried out by a PL Ther- 1ЊC/min in flowing argon atmosphere. For the temperature
mal Sciences 1500 Differential Scanning Calorimeter and the enthalpy calibration of the instrument, ICTA stan-
(DSC) in flowing argon atmosphere (10 ml/min), using dards were used. All enthalpy results shown are the mean
10ЊC/min heating and cooling rates. In the course of the of from three to five runs and errors are calculated from
determination of the melting enthalpy and melting point range estimates of the results.
of Ͱ-Bi2Te4O11 , the heating was performed at a rate of
X-ray powder diffraction experiments were carried out
FIG. 6. DSC curve of the melting characteristics of Ͱ- and ͱ-Bi2Te4O11
.

STUDY OF THE FORMATION OF Bi2Te4O11
255
TABLE 1
Thermoanalytical Results of DSC Measurements
Endothermic
Exothermic
Endothermic
Tonset
(ЊC)
Tmax
(ЊC)
⌬H
(J/g)
Tonset
(ЊC)
Tmax
(ЊC)
⌬H
(J/g)
Tonset
(ЊC)
Tmax
(ЊC)
⌬H
(J/g)
4 : 27.5 mixture
4 : 27.5 eutectic
Eutectic ϩ Bi2O3
1 : 4 mixture
Ͱ-Bi2Te4O11
ͱ-Bi2Te4O11
601.5
575.3
574.1
598.9
604.0
580.5
579.6
607.7
90.5 Ϯ 2.1
89.9 Ϯ 2.2
10.6 Ϯ 1.1
8.9 Ϯ 1.1
584.4
612.5
588.8
615.8
Ϫ6.4 Ϯ 1.2
Ϫ4.2 Ϯ 1.1
650.9
646.7
650.8
626.7
653.7
654.9
655.3
658.7
37.8 Ϯ 1.7
44.0 Ϯ 2.1
84.3 Ϯ 4.3
35.9 Ϯ 3.3
Solidification of ͱ-Bi2Te4O11
655.2
651.6
Ϫ32.5 Ϯ 1.1
from these samples using Cu-radiation (␭
ϭ 0.154178
SAED measurements of the above-mentioned samples
CuK
Ͱ
nm) on a Siemens D5000 powder diffractometer system were performed on a JEOL JEM 100CX transmission elec-
using theta–theta geometry. The system was equipped with tron microscope applying a 100 kV accelerating voltage.
a pyrolytic graphite curved crystal secondary monochro- Selected grains were oriented with their main crystallo-
mator. Data collection and all subsequent calculations were graphic axis parallel to the beam using a side-entry tilt
performed on the integrated computing facilities of the (Ϯ60Њ)-rotating (Ϯ360Њ) goniometer stage.
D5000 system. The intensity data were collected with the
step-scan technique in the 5–65Њ and 22–110Њ 2⌰ ranges, mined by dissolution in HCl and bismuth and tellurium
using 0.02Њ 2⌰ stepwidth. contents were measured by Zeiss AAS3 Atomic Absorp-
The chemical composition of Bi2Te4O11 was deter-
FIG. 7. X-ray powder pattern of different modifications of Bi2Te4O11. (a) X-ray powder pattern of cubic Bi2Te4O11. (b) X-ray powder pattern
of monoclinic Bi2Te4O11. (c) X-ray powder pattern of the coexistence of both modifications.

256
SZALLER ET AL.
˚
˚
tion and PSX 75 Sequential ICP AES spectrometers, d-line intensities (d ϭ 3.25 A for Ͱ-Bi2O3 and d ϭ 3.40 A
respectively. for TeO2) of the oxides. The results can be seen in Fig. 2.
The density measurement was carried out in a pic- The amount of Bi2O3 decreases significantly above 500ЊC,
nometer with carbon tetrachloride at 21ЊC.
while the diffraction line intensity of TeO2 starts to de-
crease from 450ЊC and less than 50% of the oxide is present
at 550ЊC. Our measurements confirm that the reaction
among the two oxides starts near 450ЊC and the product
should be a TeO2-rich compound. The thermal motion of
the atoms is considered as constant in this temperature
range. The first endothermic peak on the DSC curve can
be considered as a melting process. This was confirmed by
optical microscope using a heated sample holder. Localized
melting was observed at 600ЊC, then the sample solidified
and melted again. The substance was a metallic gray color
when cooled to room temperature.
RESULTS
Preliminary Examinations and Observations
The most characteristic features of the solid state reac-
tion of the Bi2O3 ϩ 4TeO2 mixture investigated by DSC
can be seen in Fig. 1. Heating up the sample the base line
remains stable up to 450ЊC, but above this temperature an
exothermic shift can be observed. Then an endothermic
process takes place at 607.7ЊC peak maximum, upon which
an exothermic one at 615.8ЊC is superimposed. A second
endothermic process can be seen at 654.9ЊC peak maxi-
mum. Only one sharp exothermic peak with a long tail on
the lower temperature side can be observed during cooling
of the sample.
Phase Identification by Electron Microprobe Analyses
Electron microprobe analyses were performed to iden-
tify the different phases existing in the course of Bi2O3 ϩ
To elucidate the origin of these DSC traces further ex- 4TeO2 reaction. Results of these measurements cannot be
periments were carried out. From the exothermic base- strictly correlated with Bi2Te4O11 formation, because the
line shift it could be expected that the reaction commences reaction could only take place within the contact surface
below 600ЊC. In order to prove this the Bi2O3 ϩ 4TeO2 layer and the mole ratio varies locally.
mixtures were heated to 300, 350, 400, 450, 500, and 550ЊC,
The back-scattered electron pictures in Figs. 3a–3c were
respectively, in a Mettler TA-1 thermobalance under argon taken from the cross section of the sample. Surprisingly,
atmosphere, and kept for 30 min at these temperatures. unreacted TeO2 could not be identified, it was fully con-
After preparation the samples were examined by X-ray sumed. The analysis confirmed that the composition of the
powder diffractometry supposing that the onset tempera- homogeneous phase with light color at the bottom of Fig.
ture of the reaction can be estimated from the decreasing 3a is unmodified Bi2O3 reactant. At the top of the picture
TABLE 2
Powder Data and Unit Cell Refinement Results for ␤-Bi2Te4O11 (Cubic)
Unit cell dimensions
a
Volume
Reciprocal cell
Direct cell
0.177314D ϩ 00
0.563978D ϩ 01
0.557488D Ϫ 02
0.179375D ϩ 03
a
Volume
Reciprocal cell standard errors
Direct cell standard errors
0.177063D Ϫ 04
0.563165D Ϫ 03
0.537352D Ϫ 01
n
h
k
l
dobs
I/Io
2Th(calc)
2Th(obs)
2Th(diff)
Figure of merit
1
2
3
4
5
6
7
8
9
1
2
2
3
2
4
3
4
4
5
4
5
1
0
2
1
2
0
3
2
2
1
4
3
1
0
0
1
2
0
1
0
2
1
0
1
3.260
2.823
1.997
1.702
1.6293
1.4098
1.2937
1.2607
1.1510
1.0852
0.9968
0.9531
100
37
28
22
6
2
4
4
2
27.390
31.731
45.488
53.917
56.524
66.231
73.074
75.296
83.997
90.421
101.180
107.807
27.354
31.689
45.446
53.871
56.475
66.235
73.077
75.319
84.010
90.437
101.191
107.828
0.0361
0.0421
0.0425
0.0467
0.0496
Ϫ0.0040
Ϫ0.0021
Ϫ0.0222
Ϫ0.0127
Ϫ0.0157
Ϫ0.0105
Ϫ0.0200
F 1 ϭ 28(0.036, 1)
F 2 ϭ 26(0.039, 2)
F 3 ϭ 25(0.040, 3)
F 4 ϭ 24(0.042, 4)
F 5 ϭ 23(0.043, 5)
F 6 ϭ 27(0.037, 6)
F 7 ϭ 31(0.032, 7)
F 8 ϭ 33(0.031, 8)
F 9 ϭ 35(0.029, 9)
F10 ϭ 37(0.027, 10)
F11 ϭ 39(0.026, 11)
F12 ϭ 39(0.025, 12)
10
11
12
2
1
1

STUDY OF THE FORMATION OF Bi2Te4O11
257
TABLE 3
Powder Data and Unit Cell Refinement Results for ␣-Bi2Te4O11 (Monoclinic)a
Unit cell dimensions
a
b
c
Alpha
90.0
90.0
Beta
8458.5
951.5
Gamma
90.000
90.000
Volume
0.954357D Ϫ 03
0.104782D ϩ 04
Reciprocal cell
Direct cell
0.53117D Ϫ 01
0.18898D ϩ 02
0.12549D ϩ 00
0.79683D ϩ 01
0.14371D ϩ 00
0.69848D ϩ 01
a
b
c
Alpha
0.0
0.0
Beta
0.9
0.9
Gamma
0.0
0.0
Volume
Reciprocal cell standard errors
Direct cell standard errors
0.756065D Ϫ 05
0.279406D Ϫ 02
0.29160D Ϫ 04
0.18515D Ϫ 02
0.25985D Ϫ 04
0.12223D Ϫ 02
0.242303D ϩ 00
n
h
k
l
dobs
2Th(calc)
2Th(obs)
2Th(diff)
Figure of merit
2
8
11
13
16
19
20
29
31
32
34
41
46
49
54
60
62
68
2
Ϫ2
0
1
Ϫ2
2
Ϫ3
Ϫ4
Ϫ1
0
Ϫ1
Ϫ2
2
6
3
Ϫ4
Ϫ4
4
6
4
5
Ϫ3
8
0
1
1
1
1
1
1
1
0
2
2
2
0
0
2
0
2
2
1
1
2
3
0
3
2
0
1
3
2
1
0
2
2
2
0
0
4
3
0
3
1
2
1
2
4
0
0
1
1
1
1
1
1
2
1
1
1
2
0
1
2
1
1
1
2
1
0
0
0
1
2
3
2
3
1
3
3
3
3
4
2
0
0
1
2
1
2
2
4
3
9.434
6.086
5.247
4.950
4.718
4.460
4.155
3.626
3.474
3.466
3.426
3.292
3.173
3.138
2.971
2.927
2.846
2.727
2.613
2.545
2.488
2.442
2.352
2.314
2.275
2.223
2.127
2.084
2.004
1.987
1.948
1.871
1.794
1.751
1.739
1.720
1.682
1.644
1.630
1.621
1.596
1.585
1.566
1.548
1.463
9.395
14.564
16.916
17.864
18.827
19.925
21.358
24.573
25.627
25.766
25.998
27.084
28.112
28.445
30.046
30.586
31.430
32.800
34.262
35.243
36.022
36.741
38.244
38.933
39.618
40.559
42.491
43.425
45.237
45.620
46.611
48.724
50.800
52.216
52.618
53.210
54.518
55.903
56.433
56.806
57.709
58.168
58.888
59.676
63.522
9.374
14.555
16.896
17.919
18.809
19.906
21.386
24.550
25.640
25.700
26.005
27.085
28.123
28.443
30.080
30.536
31.428
32.836
34.319
35.261
36.096
36.797
38.258
38.920
39.607
40.576
42.502
43.412
45.233
45.647
46.625
48.664
50.845
52.209
52.597
53.207
54.524
55.875
56.406
56.736
57.711
58.150
58.907
59.661
63.540
0.021
0.009
0.020
Ϫ0.055
0.018
0.019
Ϫ0.027
0.023
Ϫ0.013
0.066
Ϫ0.007
Ϫ0.001
Ϫ0.011
0.002
Ϫ0.034
0.050
F 1 ϭ 24(0.021, 2)
F 2 ϭ 17(0.015, 8)
F 3 ϭ 16(0.017, 11)
F 4 ϭ 12(0.026, 13)
F 5 ϭ 13(0.025, 16)
F 6 ϭ 13(0.024, 19)
F 7 ϭ 14(0.024, 20)
F 8 ϭ 11(0.024, 29)
F 9 ϭ 13(0.023, 31)
F10 ϭ 12(0.027, 32)
F11 ϭ 13(0.025, 34)
F12 ϭ 13(0.023, 41)
F13 ϭ 13(0.022, 46)
F14 ϭ 14(0.021, 49)
F15 ϭ 13(0.022, 54)
F16 ϭ 11(0.024, 60)
F17 ϭ 12(0.022, 62)
F18 ϭ 11(0.023, 68)
F19 ϭ 10(0.025, 75)
F20 ϭ 10(0.024, 82), M20 ϭ 6
F21 ϭ 9(0.027, 84)
F22 ϭ 8(0.028, 93)
F23 ϭ 8(0.028, 101)
F24 ϭ 8(0.027, 108)
F25 ϭ 8(0.026, 113)
F26 ϭ 8(0.026, 122)
F27 ϭ 8(0.025, 136)
F28 ϭ 8(0.025, 145)
F29 ϭ 7(0.024, 162)
F30 ϭ 7(0.024, 166)
F31 ϭ 7(0.024, 180)
F32 ϭ 6(0.025, 199)
F33 ϭ 6(0.026, 221)
F34 ϭ 6(0.025, 230)
F35 ϭ 6(0.025, 236)
F36 ϭ 6(0.024, 246)
F37 ϭ 6(0.024, 273)
F38 ϭ 5(0.024, 288)
F39 ϭ 6(0.024, 293)
F40 ϭ 5(0.025, 298)
F41 ϭ 5(0.025, 309)
F42 ϭ 5(0.025, 317)
F43 ϭ 5(0.025, 329)
F44 ϭ 5(0.024, 340)
F45 ϭ 5(0.024, 397)
0.002
Ϫ0.035
Ϫ0.057
Ϫ0.018
Ϫ0.074
Ϫ0.056
Ϫ0.014
0.013
75
82
84
93
101
108
113
121
136
145
162
166
180
199
221
230
236
246
272
288
293
298
309
317
329
340
397
Ϫ4
6
Ϫ7
2
Ϫ2
0
Ϫ9
Ϫ6
3
0.011
Ϫ0.017
Ϫ0.011
0.013
0.004
Ϫ0.027
Ϫ0.013
0.060
Ϫ0.045
0.007
0.021
0.003
Ϫ0.006
0.028
0.027
0.070
Ϫ0.002
0.018
Ϫ0.019
0.015
Ϫ0.018
4
Ϫ6
Ϫ2
Ϫ10
Ϫ6
Ϫ9
11
7
11
9
10
Ϫ4
Ϫ4
a Space group P21 after E. Spiridonov et al, Moscow Univ. Geology Bull. 42(6), 71 (1987).

258
SZALLER ET AL.
there is a heterogeneous phase which shows a very complex
morphological structure. Between these regions a light-
gray layer can be observed. Investigating the heteroge-
neous phase with higher magnifications (Figs. 3b and 3c)
it is revealed to consist of three well-distinguishable com-
pounds. The dark background is a solidified melt phase
consisting of 4Bi2O3 ϩ 29TeO2 . The gray dendrite-like
crystals with rounded outline boundary have a composition
of Bi2O3 ϩ 4TeO2 (Bi2Te4O11). The white prismatic and
needle-like crystals are identified as Bi2O3 ϩ 2TeO2
(Bi2Te2O7). The light-gray layer consists of microcrystal-
line Bi2Te2O7 .
Study of Bi2Te4O11 Formation by DSC
According to the phase diagram of the Bi2O3 Ϫ TeO2
system (12) the 4Bi2O3 ϩ 27.6TeO2 eutectic has the lowest
melting point. This eutectic is a mixture of Bi2Te4O11 and
TeO2 as confirmed by X-ray diffraction analysis. The accu-
rate value of the eutectic composition was determined by
DSC and it was found to be 8Bi2Te4O11 ϩ 23TeO2 or
4Bi2O3 ϩ 27.5TeO2 . To prove that the first endothermic
peak at 607.7ЊC peak maximum in Fig. 1 arises from the
melting of eutectic, an analysis of the 4Bi2O3 ϩ 27.5TeO2
mixture was carried out. In the course of heating the eutec-
tic was formed and melted at 601.5ЊC (Fig. 4, Curve A). If
already present the melting point of the eutectic is 575.3ЊC
(Curve B). It was consistent during the experimental work
that the melting temperature of the eutectic depends
strongly on the nature of the reactants. If the starting
materials were pure oxides the melting temperature of the
eutectic was always higher (Curves A and D in Fig. 4.). The
explanation is that the chemical (i.e., oxide) composition of
the eutectic is 4Bi2O3 ϩ 27.5TeO2 , but Bi2Te4O11 and TeO2
are the compounds which form the eutectic. Namely if
Bi2Te4O11 is not present as reactant it has to be formed and
the melting temperature will be higher. But if Bi2Te4O11 is
already present the real melting temperature of the eutectic
can be measured (curve B and C in Fig. 4.).
To show that the origin of the exothermic peak in Fig. 1
is due to the reaction of unreacted Bi2O3 with the TeO2
content of the eutectic we added the stoichiometric quan-
tity of Bi2O3 needed for the formation of Bi2Te4O11 to the
eutectic. The obtained DSC curve (Curve C) is similar to
that of the Bi2O3 ϩ 4TeO2 mixture (Curve D), but the
endothermic and exothermic peaks are obtained at lower
temperatures.
FIG. 8. SAED patterns of Bi2Te4O11 modifications. (a) [001] pro-
jected SAED patterns of the cubic Bi2Te4O11 modification. (b) [1 1ළ 0]
projected SAED patterns of the cubic Bi2Te4O11 modification. (c) Super-
structure of fcc sublattice.
In both cases the chemical reaction proceeds faster in
the molten phase so the formation of Bi2Te4O11 is com-
pleted with accompanying heat release. The crystalline
Step 1. Bi2Te4O11 forms above 450ЊC, which is probably
Bi2Te4O11 melts at 650.8ЊC. We summarize the thermoana- a diffusion-controlled reaction.
lytical results in Table 1.
Step 2. When the amount of Bi2Te4O11 is sufficient the
The chemical sequence of Bi2Te4O11 formation as de- (8Bi2Te4O11 ϩ 23TeO2) eutectic is formed.
duced from the results given above:
Step 3. Melting of the eutectic.

STUDY OF THE FORMATION OF Bi2Te4O11
259
TABLE 4
Unit Cell Parameters of ␣-Bi2TeeO11 (Monoclinic)
Density (calc.)
(g/cm3)
Њ
Њ
Њ
Reference
a [A]
b [A]
c [A]
ͱ
Frit (17)
Demina (15)
Chekhovichite natural
mineral, Spiridonov (23)
Astaf’ev (16)
19.107(5)
19.11(1)
19.00
18.89
18.894
7.971(5)
7.995(6)
7.982
7.968
7.960
6.904(5)
6.910(6)
6.938
9.919
6.974
95.74(5)
95.78(5)
95.67
95.89
95.30
7.01
7.01
6.88
7.04
7.00
7.00
Rossel (18)
Present work
18.8963(8)
18.898(3)
7.9593(3)
7.968(2)
6.9909(3)
6.985(1)
95.176(3)
95.025(15)
Step 4. The formation of Bi2Te4O11 is completed in the mined melting point and melting enthalpy of Ͱ- and
molten phase.
ͱ-Bi2Te4O11 using 1ЊC/min and 10ЊC/min heating rates are
Step 5. Melting of Bi2Te4O11 .
summarized in Table 1.
Study of the Polymorphic Transition of Bi2Te4O11
by DSC
Results of the Chemical Analysis and
Density Measurement
In our preparation process when the sample was cooled
The results of the chemical analysis of Bi2Te4O11 are the
down from the melt by 10ЊC/min (Sample A) only cubic following: bismuth content is 11.66 Ϯ 0.12 mole% (theo-
modification could be obtained. The detailed X-ray diffrac- retical concentration 11.765 mole%), tellurium content
tion analysis is described in the next section. The most is 22.7 Ϯ 0.7 mole% (theoretical concentration 23.530
obvious feature identified for the cubic phase was the exo- mole%).
thermic peak on the heating curve at about 450ЊC (Fig. 4,
The density of ͱ-Bi2Te4O11 was found to be 6.81 Ϯ
Curve E). Figure 5 shows the results of a heating–cooling– 0.02 g/cm3.
heating cycle below the melting point. There is no DSC
Study of the Polymorphic Modifications using XPD and
SAED Analysis
peak for the second heating. We checked the structure of
the sample after the DSC measurement by X-ray diffrac-
tion and the monoclinic form was identified. Once the
monoclinic modification is formed it cannot be transformed
to cubic phase below the melting point in either the heating
or the cooling process. Thus it is obvious that this exother-
mic process is an irreversible monotropic transformation
from cubic to monoclinic phase. Accordingly the mono-
clinic modification is the stable form of Bi2Te4O11 and the
cubic is the metastable one. In accordance with Oswald’s
rule, the least stable modification often forms from the
melts of polymorphic substances on cooling.
The crystallographic study of the Bi2Te4O11 (Sample A)
was carried out by X-ray diffraction. The powder pattern
of this material is shown in Fig. 7a. An attempt of a phase
analysis using the current ICDD-JCPDS database (19, 20)
failed, indicating that there is not powder data for this
phase. Following a data reduction (smoothing, background
treatment, and peak search) performed by the integrated
DIFFRAC-AT package of D5000, an indexing of the pat-
tern was attempted using the ‘‘Trial and Error’’ technique
as described by Werner (21). The solution with the highest
The melting point and the melting enthalpy normally
measured belongs to the monoclinic phase. However, cool-
ing the melt to just below the solidification temperature
(ȁ500–530ЊC) and heating it again, the second melting
process is due to the cubic phase. Figure 6 shows the results.
It can be established that the measured heat of crystalliza-
tion is equal to the heat of fusion for the cubic form and the
solidification temperature and enthalpy of the monoclinic
form could not be detected. It also explains the formation
of cubic modification in every cooling process. The accu-
rate value of the melting point and the melting enthalpy
of the ͱ-Bi2Te4O11 modifications could not be determined
because the cubic phase transformed into monoclinic phase
when the heating rate was lower than 10ЊC/min. The deter-
consistency mark was a cubic unit cell with a dimension
3
˚
˚
of 5.6 A and volume of 179 A . This unit cell was used in
a subsequent cell refinement (22) and indexing using a
locally modified version of the UNITCELL program of
TABLE 5
Unit Cell Parameters of ␤-Bi2Te4O11 (Cubic)
Density (exp.)
(g/cm3)
Density (calc.)
(g/cm3)
Њ
Reference
a [A]
Astaf’ev (16)
Present work
5.640
5.6397(5)
6.88 Ϯ 0.05
6.81 Ϯ 0.02
6.82
6.83

260
SZALLER ET AL.
Appelman et al. (23). The final results of the refinement the lowest melting point (575.3ЊC) in the bismuth oxide-
are given in Table 2. The results show that this material tellur dioxide pseudo-binary system.
is a cubic modification of Bi2Te4O11 . The given unit cell
In the melt of the eutectic the reaction of still unreacted
suitably indexed all reflections and the indexing is consis- starting materials takes place (Bi2O3 ϩ 4TeO2
ϭ
tent with the Fm3m space group. Calculations using the Bi2Te4O11). Increasing the temperature, Bi2Te4O11 melts
measured density, the formula confirmed by chemical anal- at 646.7ЊC. when the sample is cooled there is a tail exo-
ysis, and the unit cell volume showed that a fractional site thermic peak on the DSC curve which shows a slow solidi-
occupancy can be expected in this structure. The powder fication process.
pattern of Sample C can be seen in Fig. 7b. A phase analysis
When the Bi2Te4O11 melt is cooled, the cubic phase can
of this material using the ICDD-JCPDF database (19, 20) form. Except for a partial conversion at an extremely slow
immediately led to the identification of the pure monoclinic (0.5ЊC/min) cooling rate this cubic phase cannot be trans-
modification of Bi2Te4O11 described by Frit et al. (14). formed into a thermodynamically stable monoclinic modi-
Subsequent to a data reduction, a unit cell refinement was fication because of kinetic hindrance. The cubic phase un-
launched using the P21 monoclinic structural model sug- dergoes a monotropic transformation into the monoclinic
gested by Spiridonov et al. (24). The final results shown in above 400ЊC. This observation is in contrast with those of
Table 3 indicated that this material—including the unit Astaf’ev et al. (16).
cell dimensions—is similar to the one described by Frit
The two modifications have different melting points and
et al. (17), but shows differences from the data of Spirido- melting enthalpies. The heat of fusion of the metastable
˚
nov et al. (24) as no traces of 100 reflection (18.9 A) were cubic form is smaller than that of the stable monoclinic
found. Accordingly a more detailed structural investigation one and the melting point is also lower for the monoclinic
is indicated.
form. The solidification temperature and enthalpy of the
The powder pattern of Sample B is shown in Fig. 7c. monoclinic form could not be detected.
The phase analysis of this sample showed that it was not
The monoclinic Bi2Te4O11 crystals are characterized by
a single-phase material but evidently a mixture of both a superstructure of the fcc sublattice. The cubic monoclinic
modifications. The majority of the material consists of the transition is the result of an ordering in one set of ͕111͖
cubic phase but distinct marks of the presence of the mono- planes with periodicity of 3xd(111)fcc. As a consequence of
clinic structure can also be recognized. This kind of coexis- these ordering processes, the orthogonality of the [110]
tence of both modifications indicates that even a fairly projection vanishes and the structure changes to mono-
slow (0.5 C/min) cooling rate is insufficient to ensure a clinic symmetry.
considerable transition from the cubic modification into
the monoclinic one.
Finally, we summarize the unit cell parameters of both
modifications published up to now (Tables 4 and 5).
The SAED patterns proved the fcc symmetry of the
Bi2Te4O11 (Sample A). The [001] and [1 1ළ 0] projected
SAED patterns of this phase are shown in Figs. 8a and
8b. The Ͱ-Bi2Te4O11 crystals (Sample C) can be character-
ized by a superstructure of the fcc sublattice. The cubic Ǟ
monoclinic transition is the result of an ordering in one
set of ͕111͖ planes with periodicity of 3xd(111)fcc (Fig. 8c).
As a consequence of this ordering processes the orthogo-
nality of the [110] projection vanishes and the structure
changes to the monoclinic symmetry. The detailed crystal-
lographic description of the phase transformation between
the cubic and monoclinic phases is the aim of a subse-
quent paper.
ACKNOWLEDGMENTS
One of the authors (Zs. Szaller) was supported by a young scientist
grant of the National Research Foundation for the (OTKA F-4357) and by
a financial support of the Hungarian Science Foundation of the Hungarian
Credit Bank. The analytical work of Dr. O. Szakacs is kindly acknowl-
edged.
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