First evidence of a phase transition in a high-pressure metal iodate: Structural and thermal studies of AgIO3 polymorphs

Article abstract of DOI:10.1002/ejic.201300191

A new phase of silver iodate, β-AgIO₃, was obtained at 2.7 GPa by heating the α-AgIO₃ precursor above 240 °C. β-AgIO₃ is the first iodate obtained at high pressure that stabilizes under normal conditions. The crystal structure was determined by X-ray powder diffraction and Rietveld refinement. In comparison to α-AgIO₃, which crystallizes in the Pbc2₁ acentric orthorhombic space group with the lattice parameters a = 7.265(2) A, b = 15.17(4) A, c = 5.786(2) A, and V = 638(2) A³, β-AgIO₃ crystallizes in the Pbca centric orthorhombic space group, with the following parameters: a = 6.137(2) A, b = 16.980(4) A, c = 5.827(1) A, and V = 607.3(3) A³. Differential thermal analysis under high pressure (2.7 GPa) and structural studies showed an endothermic transformation at 260°C corresponding to a first-order reconstructive phase transition from α-AgIO₃ to β-AgIO₃. The thermal behaviors of α-AgIO₃ and β-AgIO₃ were studied by differential scanning calorimetry at ambient pressure and in situ temperature-dependent X-ray powder diffraction. Copyright

Full text of DOI:10.1002/ejic.201300191

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Silver Iodate Polymorphs
Y. Suffren, I. Gautier-Luneau,* C. Darie,
C. Goujon, M. Legendre,
O. Leynaud ....................................... 1–8
First Evidence of a Phase Transition in a
High-Pressure Metal Iodate: Structural
and Thermal Studies of AgIO Polymorphs
3
Keywords: High-pressure chemistry / Phase
transitions / Structure elucidation / Iodates
β-AgIO
3
is the first iodate phase obtained
3
phase to the dense β-AgIO centric phase
under high pressure. The reconstructive
phase transition from the α-AgIO acentric
3
is an endothermic transformation that oc-
curs at 260 °C at 2.7 GPa.
0000, 0–0
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DOI:10.1002/ejic.201300191
First Evidence of a Phase Transition in a High-Pressure
Metal Iodate: Structural and Thermal Studies of AgIO
Polymorphs
3
[
a]
[a]
[a]
Yan Suffren, Isabelle Gautier-Luneau,* Céline Darie,
[
a]
[a]
[a]
Céline Goujon, Murielle Legendre, and Olivier Leynaud
Keywords: High-pressure chemistry / Phase transitions / Structure elucidation / Iodates
A new phase of silver iodate, β-AgIO
.7 GPa by heating the α-AgIO precursor above 240 °C. β-
AgIO is the first iodate obtained at high pressure that stabi-
lizes under normal conditions. The crystal structure was de-
termined by X-ray powder diffraction and Rietveld refine-
3
, was obtained at
rhombic space group, with the following parameters: a =
6.137(2) Å, 16.980(4) Å, 5.827(1) Å, and
607.3(3) Å . Differential thermal analysis under high pressure
(2.7 GPa) and structural studies showed an endothermic
transformation at 260 °C corresponding to a first-order recon-
2
3
b
=
c
=
V =
3
3
ment. In comparison to α-AgIO
Pbc2 acentric orthorhombic space group with the lattice pa- mal behaviors of α-AgIO
rameters a = 7.265(2) Å, b = 15.17(4) Å, c = 5.786(2) Å, and V differential scanning calorimetry at ambient pressure and
3
, which crystallizes in the
structive phase transition from α-AgIO
3
to β-AgIO
3
. The ther-
1
3
and β-AgIO
3
were studied by
3
=
638(2) Å , β-AgIO
3
crystallizes in the Pbca centric ortho- in situ temperature-dependent X-ray powder diffraction.
Introduction
analysis (DTA) at high pressure. Structural studies of these
two phases were carried out to understand the formation
In the 1970s, a large number of metal iodates compounds
were extensively investigated for their nonlinear optics
of β-AgIO . Furthermore, the thermal behaviors of α-
3
AgIO and β-AgIO were studied by differential scanning
3
3
(NLO) properties as well as for their ferroelectric, piezo-
calorimetry (DSC) at ambient pressure and in situ tempera-
ture-dependent X-ray powder diffraction.
[1–9]
electric, and pyroelectric properties.
Interest in these
compounds resumed in the early 2000s.[
10–23]
We have
shown that metal iodates are particularly interesting for IR
applications, as they possess a large domain of transparency
from the visible region to the beginning of the far IR region
Results and Discussion
(
12.5 μm), thus covering the three atmospheric trans-
[
24,25]
Crystal Structure of α-AgIO3
parency windows.
The synthesis of metal iodates has
so far been mainly investigated by solution chemistry, under
hydrothermal conditions or by the flux method. The solid-
state synthesis of these compounds at high pressure has
never been explored. To date, only the structural evolution
The crystal structure of α-AgIO has already been pub-
3
[27]
lished
and is briefly described in this section to better
understand the transformation of α-AgIO into β-AgIO .
3
3
α-AgIO3 crystallizes in the Pbc21 acentric orthorhombic
space group with the following lattice parameters: a =
of α-LiIO with pressure has been studied by X-ray powder
3
[
26]
diffraction.
It was shown that, at room temperature, α-
7
.265(2) Å, b = 15.17(4) Å, c = 5.786(2) Å, and V =
LiIO is stable up to 75 GPa; only compression of the lat-
3
3
638(2) Å . The asymmetric unit consists of two AgIO enti-
3
tice parameters with pressure was observed.
ties at general positions. The crystal structure of α-AgIO3
is constituted of sheets parallel to the (010) plane, as shown
in Figures 1 and 2. The cohesion of crystal packing is pro-
vided by I···O interactions [I1···O4 2.99(1) Å, I2···O2
2.71(1) Å] between the sheets. The I···I intersheet distances
are in the range 4.01(1)–4.64(1) Å. The sheet thickness is
equal to 0.5b (7.585 Å).
In this work, we present a new phase of silver iodate
obtained at high pressure from α-AgIO3[
27]
and charac-
terized by X-ray powder diffraction. The α-AgIO to β-
3
AgIO transition was characterized by differential thermal
3
[
a] Institut Néel, CNRS et Université Joseph Fourier,
BP 166, 38042 Grenoble Cedex 9, France
Fax: +33-4-76881038
The coordination spheres of the silver atoms, previously
[
27]
described as octahedral,
prisms.
are in fact closer to trigonal
E-mail: Isabelle.Gautier-Luneau@grenoble.cnrs.fr
Homepage: http://neel.cnrs.fr
[28]
The Ag–O bond lengths and O–Ag–O bond
angles are in the range 2.37(1)–2.78(1) Å and 61.3(4)–
148.4(4)°, respectively. The bond valence sum (BVS) values
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300191.
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The edge-sharing polyhedra lead to the shortest Ag···Ag
distances [3.18(1)–3.79(1) Å], whereas the vertex-sharing
polyhedra lead to Ag···Ag distances in the range 4.70(1)–
4.98(1) Å. As shown in Figure 1, the c mirror and the a and
c translations build a sheet. The 2 screw axis and the b
1
mirror duplicate this sheet along the [010] direction.
3 3
Figure 3. Coordination scheme of the (a) I(1)O and (b) I(2)O
iodate groups in α-AgIO
3
.
3
Figure 1. Representation of the α-AgIO crystal packing and sym-
metry elements of the Pbc2
direction.
1
space group projected along the [001] Characterization of the α-AgIO to β-AgIO Transition by
3 3
High-Pressure DTA
Figure 4 shows the high-pressure (HP)-DTA curve ob-
tained at a heating and cooling rate of approximately
–1
2
0 °Cmin . An increasing (and a decreasing) baseline is
observed during heating (cooling) as a result of the thermal
mass difference between the sample and the reference. The
signal is also noisy with fluctuations up to 2.5 μV, which
are inherent to the setup itself and can be eliminated by
performing a polynomial fitting. Despite these effects, an
endothermic deviation of the HP-DTA signal is clearly ob-
served with an onset temperature of 254 °C. After correc-
tion for pressure effects on the S-thermocouple electro-
[
32]
motive force (emf), the onset temperature is 260 °C. The
endothermic event is large, as it occurs over a range greater
than 100 °C, and it exhibits a small intensity. During the
cooling, no exothermic deviation was detected, which indi-
cated that the reaction is not reversible. Moreover, the X-
ray diffraction pattern of the HP-DTA product shows a
Figure 2. Representation of one sheet in the α-AgIO
jected (a) along the [100] direction and (b) along the [010] direction,
showing the connection of {AgO } polyhedra.
3
structure pro-
6
for the Ag atoms are 0.99 and 0.95, respectively.[29] The
iodine atoms of the iodate anions have the usual tetrahedral
arrangement completed by three I···O interactions arranged
around the direction of the lone pair of electrons, and this
pure phase of β-AgIO . Therefore, the high-pressure phase
3
transition of α-AgIO to β-AgIO occurs at approximately
3
3
[
30]
leads to an octahedral geometry.
The BVS values for I1
and I2 are 5.52 and 5.17, respectively.[ The difference be-
tween the calculated BVS value and the theoretical valence
value is due to the presence of the lone pair of electrons that
distorts the coordination polyhedron.[ The coordination
scheme of the iodate groups is presented in Figure 3: the
29]
31]
I(1)O iodate group links four silver atoms (2 Ag1 and 2
3
Ag2) by chelation, involving the μ -O5 bridging oxygen
4
atom, whereas the I(2)O iodate group coordinates six silver
3
atoms (3 Ag1 and 3 Ag2) through the monodentate O1 and
O3 oxygen atoms and the μ -O6 bridging oxygen atom.
4
Therefore, in a sheet, a silver atom has eight silver atom
neighbors: four of them share one edge of one square face
and the four others share only a vertex of this same face
Figure 4. DTA (grey curve) and polynomial fit (black curve) of α-
AgIO at 2.7 GPa showing an endothermic deviation with an onset
at 254 °C (insertion of the dotted lines). After correction for pres-
(Figure 2, b). These connections through the O5 and O6
3
oxygen atoms lead to a compact wavy plane (Figure 2, a).
The iodine atoms are placed above the square faces in the
[32]
sure effects on the S-thermocouple emf,
the onset temperature
void left by the two polyhedra having the same orientation. is 260 °C.
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260 °C at 2.7 GPa. This in situ value, measured in our HP-
DTA setup in Conac anvils, is consistent with temperatures
of the ex situ syntheses performed in the Belt apparatus (see
the Experimental Section).
Crystal Structure of β-AgIO3
Crystal structure refinement of β-AgIO revealed a cen-
3
tric structure that was confirmed by the negative second
harmonic generation (SHG) activity (Kurtz and Perry pow-
[
33]
der test),
whereas α-AgIO exhibits intense SHG effi-
3
Figure 6. Representation of one sheet in the β-AgIO
jected (a) along the [100] direction and (b) along the [010] direction
showing the connection of {AgO } polyhedra.
3
structure pro-
ciency. β-AgIO crystallizes in the Pbca orthorhombic space
3
group (a nonisomorphic supergroup of Pbc2 in which α-
1
6
AgIO crystallizes), with the following lattice parameters: a
3
=
6.137(2) Å, b = 16.980(4) Å, c = 5.827(1) Å, and V =
3
607.3(3) Å . Consequently, the asymmetric unit consists of other (with a difference in the z atomic coordinates equal
only one AgIO at a general position. The unit-cell volume to 0.5), which is a consequence of the pressure leading to
3
is reduced by approximately 4.2% with a contraction of the the slipping of the sheets relative to each other along the
a parameter (15.4%) and an elongation of the b (12.1%) [100] direction. The cohesion of crystal packing is provided
and c (0.9%) parameters. The density increased from by I1···O12 interactions [2.87(1) Å] between the sheets. The
–
3
–3
5
.92 gcm for α-AgIO to 6.19 gcm for β-AgIO3.
sheet thickness of β-AgIO [8.49 Å (0.5b)] is larger than that
3
3
Like α-AgIO , the crystal structure of β-AgIO is consti- of α-AgIO (7.585 Å), but the upper limit of the I···I in-
3
3
3
tuted of sheets parallel to the (010) plane, as illustrated in tersheet distances is smaller [4.128(3) to 4.242(2) Å, see
Figures 5 and 6. However, the position of the sheets is Table 1].
shifted by 0.25b relative to the symmetry elements shown
The coordination sphere of silver atoms is a trigonal
in Figure 1. Thus, the c mirror, which built a sheet in α- prism; however, the dihedral angle between the two triangu-
AgIO , now generates an infinite stacking of sheets in β- lar faces is larger (19.35°) in β-AgIO than in α-AgIO (2.42
3
3
3
AgIO . Inversion centers are located in the sheets. Further- and 3.64°). The deformation of the trigonal prism could be
3
more, the iodate groups of two successive layers face each explained by the lengthening of the Ag–O13 bond
Figure 5. Representation of the β-AgIO
3
crystal packing and symmetry elements of the Pbca space group projected along the [001]
direction.
Table 1. Comparison of selected interatomic distances in the α-AgIO [
27]
and β-AgIO
structures.
3
3
Distance (Å)
Ag···Ag
Sheet
thickness (Å)
Ag–O
I–O
I···O
I···Iintrasheet
I···Iintersheet
α-AgIO
β-AgIO
3
2.37(1)–2.78(1) 1.78(1)–1.85(1)
2.35(1)–2.81(1) 1.80(1)–1.82(1)
2.71(1)–2.99(1)
2.61(1)–2.97(1)
3.18(1)–4.98(1)
2.973(2)–4.372(3)
3.63(1)–3.70(1)
4.121(3)–4.345(3)
4.01(1)–4.64(1)
4.128(3)–4.242(2)
7.585
8.49
3
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[
2.81(1) Å]. The five other Ag–O bond lengths range from
2.35(1) to 2.57(1) Å. The O–Ag–O bond angles are in a
range (57.6 to 141.6°) that is similar to that in α-AgIO .
3
The BVS calculation for Ag is 1.06,[ which confirms the
structural model obtained by X-ray powder diffraction. The
iodine atom of the iodate anions is still in the usual tetrahe-
dral arrangement completed by three I···O interactions [be-
tween 2.61(1) and 2.97(1) Å], and this leads to an octahe-
dral geometry. The iodine BVS value is 5.47, which is
between the values calculated for the α-phase.[ The coor-
dination scheme of the iodate group, linking five silver
atoms, is presented in Figure 7. Its coordination number is
the average of those observed for the two iodate groups of
29]
29]
α-AgIO , which requires reorganization of the Ag–O bond.
3
This structural reorganization corresponds to a recon-
structive phase transition (first order). The μ -O11 and μ -
3
3
O13 oxygen atoms each coordinate three silver atoms, but Figure 8. DSC curve of α-AgIO3 versus temperature between 50
only one silver atom is chelated. The O12 oxygen atom is and 400 °C showing reversible thermal behavior.
uncoordinated.
In addition, in situ temperature-dependent X-ray powder
diffraction of α-AgIO shows a shift in the Bragg peak to
3
lower angles during heating because of the thermal expan-
sion of the lattice parameters, as shown in Figure 9. At ap-
proximately 330 °C, the intensity of the Bragg peak de-
creases and fluctuates, which suggests a loss of long-range
order and texturing effects, and this takes place before the
melting of the compound. This behavior can be related to
Figure 7. Coordination scheme of the I(1)O
AgIO
3
iodate group in β- the weak endothermic peak detected by DSC at 326 °C.
Then, during cooling, the intensity of the Bragg peak in-
3
.
creases at approximately 320 °C as a result of the recrystalli-
A sheet is constituted of two facing planes deducted by zation of α-AgIO (weak exothermic peak in DSC analysis).
3
the a glide mirror, the 2 screw axis parallel to [001] direc- Finally, thermal contraction of the lattice parameters shifts
1
tion, and the inversion center. In one plane, a silver atom is the Bragg peak to higher angles.
bonded to four other atoms of silver and shares four verti-
ces of a square face, which leads to empty spaces (Figure 6,
b) in which the upper plane is inserted. Therefore, a silver
atom has four other silver neighbors, and each of them
shares an edge of the triangular faces. In this dense stacking
of polyhedra, the iodate groups are placed outside the
sheet, on both surfaces of the double planes, and this results
in increases in the sheet thickness and the b parameter. The
edge-sharing polyhedra lead to the shortest Ag···Ag dis-
tances [2.973(2)–3.796(3) Å], whereas the vertex-sharing
polyhedra lead to Ag···Ag distances in the range of
4.096(3)–4.372(3) Å.
Figure 9. Temperature-dependent X-ray powder diffraction of α-
AgIO performed between room temperature and 350 °C. The
3
black line shows the temperature cycle. Two temperature steps were
applied at 300 and 350 °C during heating and two other ones at
Thermal Behavior of α-AgIO3
330 and 300 °C during cooling.
DSC analysis (Figure 8) shows reversible thermal behav-
–1
ior of α-AgIO : a weak endothermic peak (E = 0.139 Jg )
3
at 326 °C is observed during heating, whereas a weak exo-
thermic peak (E = 0.097 Jg ) at 323 °C is present during
Thermal Behavior of β-AgIO3
–
1
cooling. Another DSC analysis (Supporting Information)
Figure 10 presents two consecutive DSC analyses of β-
shows a melting peak at 418 °C followed by a decomposi- AgIO . The first analysis shows three exothermic peaks at
3
–1
–1
tion peak at 440 °C. The residue of the DSC experiments 220 (E = 2.141 Jg ), 299, and 323 °C (E
= 0.6447 Jg )
total
was identified by powder XRD as AgI (JSPDS file no. 01- as the temperature increases. Conversely, a single exother-
–1
078-1614).
mic peak at 320 °C (E = 0.2011 Jg ) is observed during
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cooling. The X-ray powder diffraction pattern obtained at (at 299 and 323 °C) are explained by the superposition of
room temperature after the first DSC analysis indicates that two opposite effects: a broad exothermic phenomenon (due
β-AgIO was transformed into α-AgIO . Furthermore, the to residual stresses relaxation) and a weak endothermic
3
3
XRD pattern performed at 250 °C shows that the transition event (due to the loss of long-order range). Upon cooling,
from β-AgIO to α-AgIO has already occurred. Conse- the recrystallization of α-AgIO is marked by sharp peaks.
3
3
3
quently, the first exothermic peak at 220 °C corresponds to The centric β-AgIO phase is therefore a dense metastable
3
this transformation, and the two successive exothermic phase that transforms into α-AgIO in an exothermic way.
3
peaks observed above 250 °C can be attributed to thermal The acentric α-AgIO3 phase is the thermodynamically
phenomena of the α-AgIO phase.
stable phase.
3
Figure 11. In situ temperature-dependent X-ray powder diffraction
starting from β-AgIO
temperature is 380 °C. Three temperature steps were applied at 250,
40, and 380 °C during heating and one at 300 °C during cooling.
3
at room temperature. The maximum heating
Figure 10. Two successive DSC analyses of β-AgIO
3
. The first DSC
to
at 220 °C. The second DSC analysis (dotted curve) shows
a phase transition similar to that observed for α-AgIO in Figure 8.
analysis (black curve) shows a phase transition from β-AgIO
α-AgIO
3
3
3
3
The second DSC analysis illustrates the behavior of the
α-AgIO3 phase, as described previously in Figure 8, but
with a shift in the temperatures. Indeed, Figure 10 is charac-
Conclusions
A new phase of silver iodate, β-AgIO , was obtained by
3
terized by a broad endothermic peak starting at 307 °C and heating the α-AgIO precursor under high pressure. The
3
–
1
reaching a maximum at 317 °C (E = 0.4393 Jg ) as the crystal structure was determined by X-ray powder diffrac-
temperature is increased and an exothermic peak at 320 °C tion and Rietveld refinement. The phase transition from α-
–
1
(
E = 0.2345 Jg ) as the temperature is decreased.
To explain the two exothermic peaks (299 and 323 °C) setup developed in our laboratory, is an endothermic trans-
only observed after the transition of β-AgIO to α-AgIO3 formation that occurs at 260 °C and 2.7 GPa. Structural
AgIO to β-AgIO , characterized by using the HP-DTA
3 3
3
(
first DSC analysis), in situ temperature-dependent X-ray comparison of both phases showed a first-order recon-
powder diffraction experiments were carried out for the β- structive phase transition at high pressure from the α-
AgIO phase. AgIO acentric phase to the dense β-AgIO centric phase.
3
3
3
As shown in Figure 11, an irreversible phase transition This work is the first characterization of a high-pressure
from β-AgIO to α-AgIO occurs at 220 °C, which is consis- transition of a metal iodate.
3
3
tent with the DSC analysis. Beyond 220 °C, the X-ray pow-
In situ temperature-dependent X-ray powder diffraction
der diffraction patterns of the α-AgIO3 phase show an and thermal analyses also demonstrated that β-AgIO is a
3
abrupt shift in the Bragg peak at 300 °C as a result of a metastable phase that, upon heating, transforms into the
significant expansion of the lattice parameters. A quantita- thermodynamically stable α-AgIO phase over two exother-
3
tive estimation of the amplitude of that lattice parameters mic steps. The first step occurs at 220 °C and corresponds
shift is obtained with a sequential Le Bail refinement of all to the transformation of β-AgIO into α-AgIO . The sec-
3
3
3
patterns. The cell volume increases from 642 to 658 Å , ond step at 300 °C is explained by the relaxation of the
which is too large to be only due to thermal expansion. pressure-induced stresses and is characterized by a signifi-
Refinement results extracted while ramping down reinforce cant expansion of the lattice parameters.
3
that statement, as at 250 °C, the volume is 653 Å (signifi-
3
cantly higher than 642 Å while ramping up). This sudden
expansion of the α-phase is due to relaxation of the pres- Experimental Section
sure-induced stresses and occurs with energy release (exo-
thermic peak at 299 °C on DSC). Then, the intensity of the
Bragg peak decreases, which was previously described as a
, previously obtained by the flux method,[27]
was synthesized under hydrothermal conditions as the precursor of
Syntheses: α-AgIO
3
[34]
β-AgIO
3
.
3
A white milky mixture of AgNO (0.510 g, 3 mmol),
loss of long-range order (endothermic phenomenon). Con- HIO (0.528 g, 3 mmol), and water (10 mL) was prepared. The mix-
3
sequently, the weak double peaks observed by DSC analysis ture was then placed into a Teflon-lined steel autoclave and heated
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to 220 °C for 48 h. Afterwards, the temperature was decreased at a
rate of 10 °Ch–1 to room temperature. The colorless small blocks
obtained in solution were filtered, washed with water, and dried at
6
0 °C. Polycrystalline samples of β-AgIO
3
were obtained by solid-
was
state reaction of α-AgIO under high pressure. α-AgIO
3
3
ground, packed into a gold capsule (3 mm diameter), and placed
in the pressure cell. High-pressure syntheses were performed in a
Belt press at 2.7 GPa and at temperatures between 120 and 300 °C
for 30 min. After thermal treatment, the samples were quenched to
room temperature, and the pressure was slowly released. At
2
2
1
.7 GPa, it was observed that the temperature has to be higher than
40 °C to obtain β-AgIO . A larger volume synthesis (using
0 mm-diameter capsules in a Conac-type press) was also per-
3
Figure 13. Experimental (dots), calculated (line), and difference
formed to provide enough powder (1.5 g) for characterizing the new
material.
(
bottom) X-ray powder diffraction pattern (λCuKα1 = 1.54056 Å) of
β-AgIO 37r] efined in the Pbca space group by the Rietveld
3
[
method.
3 3
X-ray Powder Diffraction: α-AgIO and β-AgIO powder patterns
were collected under ambient conditions by using a Siemens D5000
diffractometer with copper radiation (λCuKα1 = 1.54056 Å, 40 mA,
performed with the Fullprof suite (Figure 13 and Table 2).[37] All
atom positions were refined with an overall Debye-Waller factor.
4
1
0 kV) in the 2θ scan range 10–100° for α-AgIO
0–90° for β-AgIO with a step size of 0.016° and an acquisition
X-ray powder diffraction pattern (Fig-
ure 12) was indexed in the Pbc2 orthorhombic space group by
using the single-crystal unit-cell parameters as initial data [a =
3
and in the range
3
time of 10 s. The α-AgIO
3
In situ temperature-dependent X-ray powder diffraction experi-
ments were performed (under a helium atmosphere) to study the
structural evolution of the α-AgIO and β-AgIO phases. A CoKα1
3 3
anode X-ray generator (Inel XRG 3000, λ = 1.78897 Å) equipped
with an Inel curved detector and a high-temperature cell was used
to collect the diffraction patterns in the 2θ range 0–120° every
1
3
[27]
7
.265(2) Å, b = 15.17(4) Å, c = 5.786(2) Å, and V = 638(2) Å ].
The results of the refinement by whole-pattern decomposition
[
35]
(
Le Bail method) are listed in Table 2. The structure of β-AgIO
3
[36]
was determined by using Fox software,
and its refinement was
2
min between room temperature and 380 °C.
Thermal Analyses: Differential thermal analyses under high pres-
sure (HP-DTA) were performed with a 1500 t press equipped with
Conac” toroid anvils.[
38,39]
The HP-DTA setup was specially de-
“
[
40]
signed for Conac anvils and is described in detail in ref.
The
pressure was first increased up to 2.7 GPa. Next, the temperature
–
1
was ramped at a heating rate of approximately 20 °Cmin (the
–
1
average value was 18.4 °Cmin ). Then, the cooling was applied at
the same rate, and finally, the pressure was slowly decreased. The
temperature and the DTA signal, given by the differential electro-
motive force (Δemf) between the sample (ca. 0.15 g of AgIO
3
pow-
der) and the reference (ca. 0.12 g of Al powder), were measured
2
O
3
by S-type thermocouples and collected every second by an Euro-
therm Chessel recorder with precisions of 0.1 °C and 1 μV, respec-
tively. The onset temperatures were taken at the intersection of the
baseline with the linear extrapolation of the HP-DTA curve. A
NETZSCH DSC 404S was used to analyze the thermal stability of
Figure 12. Experimental (dots), calculated (line), and difference
bottom) X-ray powder diffraction pattern (λCuKα1 = 1.54056 Å) of
α-AgIO 35]refined in the Pbc2 space group by the Le Bail
(
α-AgIO
ing and cooling rates of 5 °Cmin were applied from room tem-
perature to 500 °C for α-AgIO and from room temperature to
80 °C for β-AgIO
3 3
and β-AgIO at ambient pressure in an argon flux. Heat-
3
[
1
–
1
method.
3
3
3
.
Supporting Information (see footnote on the first page of this arti-
cle): Scanning electron microscopy micrographs, additional struc-
tural data for α-AgIO and β-AgIO , DSC curve showing melting
Table 2. Crystallographic data for α-AgIO
3
(Le Bail refinement)
and β-AgIO (Rietveld refinement) under ambient conditions.
3
3
3
and decomposition of α-AgIO
of α-AgIO obtained from the patterns of the temperature-depend-
3
ent X-ray powder diffraction.
3
, and variation of the volume cell
α-AgIO β-AgIO
3
3
Crystal system
Space group
a (Å)
orthorhombic
orthorhombic
Pbca (61)
6.137(2)
16.980(4)
5.827(1)
607.3(3)
8/8
Pbc2 (29)
1
7.2506(3)
15.1510(6)
5.7732(3)
634.21(5)
b (Å)
Acknowledgments
c (Å)3
V (Å )
The authors thank Dr. Philippe Plaindoux for his technical assist-
ance and discussions concerning the DSC analyses.
Z/multiplicity of SG 8/4
–
3
D
R(F )
x
(gcm )
5.92
–
6.19
0.0369
2
Rwp
χ
0.0453
0.0222
0.0926
0.0498
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Received: February 6, 2013
Published Online: ᭿
Eur. J. Inorg. Chem. 0000, 0–0
8
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim