On the Use of Dynamical Diffraction Theory to Refine Crystal Structure from Electron Diffraction Data: Application to KLa5O5(VO4)2, a Material with Promising Luminescent Properties

Article abstract of DOI:10.1021/acs.inorgchem.5b02663

A new lanthanum oxide, KLa₅O₅(VO₄)₂, was synthesized using a flux growth technique that involved solid-state reaction under an air atmosphere at 900 °C. The crystal structure was solved and refined using an innovative approach recently established and based on three-dimensional (3D) electron diffraction data, using precession of the electron beam and then validated against Rietveld refinement and denisty functional theory (DFT) calculations. It crystallizes in a monoclinic unit cell with space group C2/m and has unit cell parameters of a = 20.2282(14) ?, b = 5.8639(4) ?, c = 12.6060(9) ?, and β = 117.64(1)°. Its structure is built on Cresnel-like two-dimensional (2D) units (La₅O₅) of 4?3 (OLa₄) tetrahedra, which run parallel to (001) plane, being surrounded by isolated VO₄ tetrahedra. Four isolated vanadate groups create channels that host K⁺ ions. Substitution of K⁺ cations by another alkali metal is possible, going from lithium to rubidium. Li substitution led to a similar phase with a primitive monoclinic unit cell. A complementary selected area electron diffraction (SAED) study highlighted diffuse streaks associated with stacking faults observed on high-resolution electron microscopy (HREM) images of the lithium compound. Finally, preliminary catalytic tests for ethanol oxidation are reported, as well as luminescence evidence. This paper also describes how solid-state chemists can take advantages of recent progresses in electron crystallography, assisted by DFT calculations and powder X-ray diffraction (PXRD) refinements, to propose new structural types with potential applications to the physicist community.

Full text of DOI:10.1021/acs.inorgchem.5b02663

Article
pubs.acs.org/IC
On the Use of Dynamical Diffraction Theory To Refine Crystal
Structure from Electron Diffraction Data: Application to
KLa O (VO ) , a Material with Promising Luminescent Properties
5
5
4 2
,
†
‡
†
†
†
†
́ ́
Marie Colmont,* Lukas Palatinus, Marielle Huve, Houria Kabbour, Sebastien Saitzek, Nora Djelal,
and Pascal Roussel†
†
Univ. Lille, CNRS, ENSCL, Centrale Lille, Univ. Artois, UMR 8181UCCS−Unite
́
de Catalyse et de Chimie du Solide, F-59000
Lille, France
‡
Department of Structure Analysis, Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 182 21 Praha 8, Czech
Republic
*
S Supporting Information
ABSTRACT: A new lanthanum oxide, KLa O (VO ) , was synthesized using a flux growth
5
5
4 2
technique that involved solid-state reaction under an air atmosphere at 900 °C. The crystal
structure was solved and refined using an innovative approach recently established and based on
three-dimensional (3D) electron diffraction data, using precession of the electron beam and then
validated against Rietveld refinement and denisty functional theory (DFT) calculations. It
crystallizes in a monoclinic unit cell with space group C2/m and has unit cell parameters of a =
2
0.2282(14) Å, b = 5.8639(4) Å, c = 12.6060(9) Å, and β = 117.64(1)°. Its structure is built on
Cresnel-like two-dimensional (2D) units (La O ) of 4*3 (OLa ) tetrahedra, which run parallel
to (001) plane, being surrounded by isolated VO tetrahedra. Four isolated vanadate groups
create channels that host K ions. Substitution of K cations by another alkali metal is possible,
going from lithium to rubidium. Li substitution led to a similar phase with a primitive monoclinic
unit cell. A complementary selected area electron diffraction (SAED) study highlighted diffuse
streaks associated with stacking faults observed on high-resolution electron microscopy
5
5
4
4
+
+
(
HREM) images of the lithium compound. Finally, preliminary catalytic tests for ethanol oxidation are reported, as well as
luminescence evidence. This paper also describes how solid-state chemists can take advantages of recent progresses in electron
crystallography, assisted by DFT calculations and powder X-ray diffraction (PXRD) refinements, to propose new structural types
with potential applications to the physicist community.
1
. INTRODUCTION
polyanions or halogens in most cases. Some of these phases
present many potentially interesting physical properties, such as
One of the main motives in contemporary inorganic chemistry
research is tailoring existing compounds to reach better and
optimized properties, which is appears to provide promising
results faster, with more certainty than looking for entirely new
materials. However, the danger of this approach is a lack of
interesting new structural architectures to be studied by
chemists, physicists, or even industrial partners. A systematic
research for new structural architectures is the only way to
increase the number of compounds available in the databases
and potentially investigated for their properties, and, as such,
should receive special attention in fundamental research.
In this context, materials based on oxo-centered units have
2
3,4
oxygen ion conductivity, ferroelectricity, catalytic proper-
5
−7
8
ties,
or superconductivity. In order to modify architectures
and vary properties, bismuth(III) may be substituted by other
cations. Particularly, the La3+ cation is well-known to be able to1
adopt the same configuration of oxo-centered tetrahedra OLa4,
as often encountered in many bismuth compounds. Indeed,
3
+
3+
Bi and La have almost identical ionic radii (1.03 Å), but the
3
+
La cation does not exhibit the pronounced stereoactivity of
2
bismuth, because of the absence of the p-hybridized 6s lone
electron pair. To date, only a few architectures have been
obtained that could adopt alternatively bismuth or lanthanum
in the cationic backbone. This paper mainly focuses on our
attempts to synthesize the La homologue of BiCu O (VO ),
1
been thoroughly studied over the last decades, leading to a
wide panel of interesting architectures and topologies. Plethora
of bismuth-based compounds with various dimensionalities
were intensively isolated and characterized. Their cationic
backbone is built on an association of O(Bi,M)n (M =
transition metal or alkali metal and n = 1, ..., ∞) tetrahedra
sharing edges or corners to form one-dimensional (1D) ribbons
or columns, two-dimensional (2D) layers, or three-dimensional
2
2
4
9
,10
i.e., LaCu O (VO ).
For that, because of the well-known
2
2
4
refractory behavior of lanthanum oxides, in a first step,
La(OH) , CuO, and V O were mixed together and heated
3
2
5
to 1200 °C. Unfortunately, this “brutal” approach systematically
Received: November 17, 2015
(
3D) networks associated with XO (X = P, As, V, Mo, Cr, ...)
4
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b02663
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Inorganic Chemistry
Article
11
led to lanthanum orthovanadate (LaVO4 ) and copper(II)
oxide. To overcome this problem, an alternative flux growth
technique was attempted, to modify the result, and KCl was
tested as flux, as proposed in ref 12, leading, by serendipity, to
the title compound and also a broad series of new phases. The
new structural type was solved by a combination of electron
diffraction, density functional theory (DFT) calculations, and
powder X-ray Rietveld refinement. In addition, to improve the
accuracy of the electron data refinement, instead of using the
kinematical approximation (only valid for X-ray, but not for
electron diffraction), we applied the newly developed method
of dynamical refinement from electron diffraction tomography
data that takes into account multiple scattering. To our
knowledge, this is the first application of this innovative
approach in the analysis of an unknown compound. Finally, to
reveal potential functions to this new compound, first physical
characterizations were undertaken, on the basis of expected
properties, i.e., oxidation catalytic and luminescence properties,
of 550 eV and 30 k-points in the Irreducible Brillouin Zone (IBZ). The
convergence was reached with residual Hellman−Feynman forces on
−1
the atoms smaller than 0.03 eV Å .
2
.5. Catalytic Test. Reaction of ethanol oxidation was performed
using a fixed-bed reactor. Before the experiment, 200 mg of sample
was mixed with 200 mg of SiC in order to homogenize the
temperature inside the catalytic bed. Catalytic test was carried out in a
glass reactor consisting of a 10-mm-diameter tube connected online
with a gas chromatography−mass spectroscopy (GC-MS) apparatus,
which performed an analysis every 3 min. The reactor was located in
an oven that was controlled by two thermocouples, allowing a precise
temperature of the catalytic bed. Reaction was performed in a range of
temperature comprise between 250 °C and 375 °C at constant
atmospheric pressure. The total gas flow rate was not changed (gas
hourly space velocity (GHSV) of 2256 h ) during reaction, and the
composition was maintained at 13.2% ethanol, 6% oxygen, and 40.8%
helium (as a carrier gas). The conversion of ethanol for each product is
defined as
−1
0
(n − n) × 100
ethanol conversion of X (%) =
because of distorted VO groups. The luminescence is induced
0
4
n
by these vanadate groups, because it has no electron in the 4f
orbitals for the La3+ cation.
0
where n is the initial number of moles of ethanol and n is the number
of moles of ethanol after the analysis has been performed, and the
selectivity for each product (S) is presented and defined as
2
. EXPERIMENTAL SECTION
nCX × n
2
.1. Synthesis. For the synthesis of polycrystalline ALa O (VO ) ,
x
5
5
4
2
S (%) = nCEth
0
a stoichiometric amount of La(OH) , V O , and A CO (where A =
(n − n)
3
2
5
2
3
Li, Na, and Rb) was ground in an agate mortar, placed in an alumina
crucible, preliminary heated at 600 °C for 12 h, and then heated at
where nCX is the number of carbons in the product X, n the number
of moles of product after the analysis has been performed, and nCEth
the number of carbons in ethanol (i.e., nCEth = 2).
2.6. Luminescence. The excitation and emission spectra were
performed on a SAFAS-Monaco Xenius spectrofluorometer that was
equipped with a xenon lamp as an excitation source. All measurements
were conducted at room temperature.
X
1
000 °C for 48 h. The furnace was then switched off. To date, our
attempts to obtain single crystals of the corresponding powder
systematically failed, probably because of the refractory character of
lanthanum.
2
.2. Electron Microscopy and Structure Refinement. Trans-
mission electron microscopy (TEM) studies and energy-dispersive X-
ray spectroscopy (EDX) analysis were performed on a FEI Tecnai G2
2
0 microscope. Electron diffraction (ED) patterns were obtained with
3
. RESULTS AND DISCUSSION
a precession system (Nanomegas Spinningstar). The material was
crushed and dispersed on a holey carbon film that was deposited on a
copper grid. The 3D electron diffraction tomography data were
collected in steps of 1°, 111 nonoriented PED patterns were collected
3.1. Preliminary Study. As already stated, the precursors
mentioned above were ground in an agate mortar, then placed
in an alumina crucible in order to undergo a heat treatment at
(
at tilt angles from −60° to +50°), with a precession angle of 1.2°,
6
00 °C for 12 h and then 1200 °C for 48 h and systematically
which is not too high to minimize the superposition of spots, but is
led to a mixture of LaVO and CuO. This first attempt was
4
large enough to reduce dynamical effects. The series of diffraction
1
8
unsuccessful, but KCl was then added to decrease the
temperature of the reaction and increase the reactivity. After
washing with hot water, the resulting powder of a sample of
nominal composition (La O )Cu (VO ) , melted in KCl, was
investigated using PXRD. The pattern obtained evidenced two
images was processed using the computer program PETS. The
indexing and integration procedure is analogous to commonly used
procedures in X-ray diffraction (XRD). Its particular implementation
in PETS is described in ref 41. The structure was solved using the
program Superflip and refined in Jana2006, first in the kinematical
approximation and then also using the dynamical refinement
method.
using the JEMS program.
.3. Powder X-ray Diffraction. Powder X-ray diffraction (PXRD)
data were collected at room temperature (RT) using a Bruker AXS D8
Avance diffractometer (Bragg−Brentano geometry) that was equipped
with a 1D PSD detector (Lynx-Eye); Cu Kα1 radiations were used in
the range of 2θ = 5°−120°, with a 0.02° step and a counting time
adjusted to obtain a good statistical count. The JANA 2006 program
was used for Rietveld refinement, using a Pseudo-Voigt function for
the peak profiles; the atomic positions, as well as the isotropic atomic
displacements of all atoms, were refined.
4 4 2 4 2
20
19
crystallized phases: the first one, a minor phase, is consistent
21−23
11
The computer-simulated HREM images were calculated
with the presence of LaVO4, while the second one, which is
predominant, was not found in the ICDD database (within a
tolerance of 1.5 Å for unit-cell parameters). It was indexed
afterward with DICVOL in a C-centered monoclinic unit cell
with parameters a = 20.29 Å, b = 5.96 Å, c = 12.59 Å, and β =
42
2
13
,2
1
17.89°. EDXstudy confirmed the presence of LaVO and a
4
1
9
second phase, showing, besides lanthanum and vanadium, the
unexpected presence of potassium instead of the expected
copper, in an average composition of K:La:V = 7:40:9 (average
of 10 different crystals). Attempts to solve the structure ab initio
with powder X-ray data were not successful, and, because our
attempts to get single crystals systematically failed, we decided
to find the structure model using precession-assisted electron
diffraction tomography.
2.4. Computational Methods. Electronic structure calculations
were carried out within the DFT formalism, using the Vienna Ab Initio
Simulation Package (VASP). The calculations were carried out
within the generalized gradient approximation (GGA) for the electron
43
4
4
exchange and correlation corrections, using the Perdew−Wang
45
3
.2. Crystal Structure Determination by Transmission
functional. Projector augmented-wave (PAW) pseudo-potentials
were employed. Structural relaxation of both atomic positions and
unit-cell parameters was carried out using a plane wave energy cutoff
Electron Microscopy. The procedure detailed in ref 14 has
been applied in this study.
B
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Inorganic Chemistry
.2.1. Preliminary Study from Oriented Patterns. In the
first step, a “classical” SAED study was conducted on a
crystallite containing lanthanum, potassium, and vanadium. The
reconstitution of the reciprocal space led to a monoclinic cell
with approximate lattice parameters of a = 20.2 Å, b = 5.9 Å, c =
Article
3
intensities were imported in Jana 2006. Analysis of intensities
suggested the following space groups: C2/m, C2, or Cm. The
structure was solved by the program Superflip, which
confirmed the space group C2/m. The structure was then
refined by Jana2006.
20
1
2.6 Å, and β = 118°. The comparison between the Zero Order
Despite the use of PED (which is known to reduce the
dynamical effects due to multiple diffraction) and the use of
“nonoriented” patterns (which are also less affected by multiple
diffraction and, thus, are more kinematical), the data are still
affected by dynamical diffraction effects, and the deviation from
the kinematical approximation results in relatively large
refinement (R) values and limited accuracy of the refined
model. Nevertheless, the derived structural model is coherent
from chemical point of view and it provided also the chemical
composition of the compound. The formula deduced from the
kinematical PEDT model was KLa V O , in good agreement
Laue Zone (ZOLZ) and the First Order Laue Zone (FOLZ)
allows one to determine the extinction symbol, according to the
tables given in ref 15. The examination of the difference of
periodicity and shift between the ZOLZ and FOLZ of the
[
010] zone axis pattern (ZAP) leads to the extinction symbol
C- (see Figure 1).
5
2
13
with the previous EDXanalysis. With this composition, a pure
powder was obtained without any trace of LaVO impurity. All
4
data were recollected on the pure sample.
3.2.3. Accurate Structure Model by Dynamical Refinement
of PEDT Data. To improve the accuracy of the structure model,
we employed a recently developed method of dynamical
refinement. This method, which was described in a series of
21−23
publications,
uses (P)EDT data for the least-squares
refinement in a similar way as described in the previous section,
but the theory used to calculate the model intensities is not the
kinematical diffraction theory but the full dynamical diffraction
theory. As a result, the figures of merit are much lower and the
structure models are more accurate, compared to the
kinematical refinement. In ref 23, it was shown that the typical
average error in atomic positions obtained with this method is
<
0.02 Å, and occupancies of partially occupied sites can also be
reliably refined.
The refinement proceeded along the guidelines presented in
ref 23. Recommended settings were used for all parameters.
The refinement of anisotropic displacement parameters led to a
few nonpositive definite displacement tensors, and the
displacement parameters were therefore refined as isotropic.
No other constraints or restraints were employed in the
refinement. The refinement resulted and details of the
refinement are summarized in Table 1.
Figure 1. KLa O (VO ) : [010]-oriented zone axis pattern (ZAP).
5
5
4 2
The comparison between the Zero Order Laue Zone (ZOLZ) and
Higher Order Laue Zone (HOLZ) leads to a C- extinction symbol,
because no difference of periodicity is observed but a shift is detected.
The labels −1, 1, and 0 refer to the order of the Laue Zone.
3
.3. Rietveld Analysis and DFT Optimizations. Powder
X-ray data for the title compound were collected under
experimental conditions given in Table 1. For all atoms, the
initial atomic positions have been taken from the structural
model obtained from kinematic PEDT data. The refinement
rapidly converged without any particular problem, with
acceptable values of isotropic atomic displacement parameters.
The experimental and calculated patterns are shown in Figure
2.
To validate the different structure models, we decided to
optimize the structures using DFT calculations starting from
both the kinematical and dynamical models (see Table 2, as
well as Table S3 in the Supporting Information). DFT serves to
pinpoint the clearly wrong aspects of the structure, while
certain small differences are possible and expected. For the
kinematical model, the full optimization provided unit-cell
parameters in very good agreement with the experiment, with
discrepancies of <2%. However, some noticeable changes
occurred within the structure, in particular, a significant shift of
the O9 position with local relaxation around this atom. The
comparison between the dynamically refined model with the
DFT optimization shows only one important discrepancy: the
3
.2.2. Structure Model by Precession Electron Diffraction
Tomography−Kinematical Refinement. A precession electron
diffraction tomography (PEDT) dataset was collected on the
16,17
nanocrystal based on the approach proposed by Kolb et al.
11 nonoriented PED patterns were collected (every 1° from a
tilt range of −60° to +50°) on a TEM FEI Tecnai, operating at
00 kV (λ = 0.0251 Å) with a precession angle of 1.2°. This
series of diffraction images was processed using the PETS
1
2
1
8
software. After peak hunting and microscope parameters
refinement (i.e., the adjustment of the angle between the
projection of the tilt axis on the pattern and the horizontal axis,
angle depending on the camera length and on the exact
focusing conditions), the 3D positions of diffraction spots were
1
9
indexed in Jana 2006, leading to the following cell
parameters: a = 20.293 Å, b = 5.958 Å, c = 12.680 Å, α =
90.12°, β = 118.11°, and γ = 90.15°. After the indexing, the
reflection intensities were integrated by PETS, resulting in a list
of 3782 pseudo-kinematical reflection intensities. These
C
DOI: 10.1021/acs.inorgchem.5b02663
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Inorganic Chemistry
Article
Table 1. Refinement Details for KLa O (VO )
4 2
Table 2. Unit-Cell Parameters Comparison between
Kinematical, Optimized Kinematical, Dynamical, Optimized
Dynamical, and Rietveld Refinements
5
5
parameter
value
General Data
ref
a (Å)
b (Å)
c (Å)
β (deg)
space group
C2/m (No. 12)
cell parameters
kin
20.2282
20.3099
20.2962
20.3051
20.2093
5.8639
5.9589
5.9249
5.9605
5.8710
12.6060
12.6649
12.5966
12.6623
12.5869
117.64
a
b
c
20.2093(11) Å
5.8710(1) Å
12.5869(1) Å
117.71(1)°
OPT kin
dyn
118.212
117.5825
118.243
117.71
OPT dyn
Riet
β
V
1322.11(1) Å3
Statistical Parameters for Rietveld Refinement
with a soft restraint on the V−O6 distance set to 1.70(1) Å. We
consider this model to be the best structure model of
KLa O (VO ) .
No. of reflections
1099
57
No. of refined parameters
5
5
4 2
N−P+C
RP
5561
3.47
The results show that the dynamical model compares much
better to the DFT-optimized one than the kinematical
refinement, with lower divergence for both unit-cell parameters
and atomic positions, and with only one slightly anomal
distance (see Table 3, as well as Table S1 in the Supporting
Information). As a further validation procedure, HREM image
simulations were calculated on the basis of this model, and
compared to the experimental ones. For a defocus of −35 nm
and a thickness of 35 Å, the simulated [010] image matches
perfectly with the experimental one (Figure 3). For these
conditions, the cations appear as black dots and the tunnel is
highlighted as white squares. The crystal-structure data for
KLa O (VO ) phase was deposited at the Cambridge
ωRP
4.96
goodness of fit, GOF
2.96
Rall
4.64
Robs
ωRobs
4.58
5.91
G (texture)
1
0.0651(26)
0.070(1)
G (texture)
2
Statistical Parameters for Dynamical Refinement from EDT Data
No. of reflections
8355
171
No. of refined parameters
No. of diffraction patterns
data completeness
114
5
5
4 2
72.5%
10.06
14.33
2.85
Structure Depository (No. CSD-430461).
Robs
3.4. Crystal Structure Description. The structure may be
described using the antiphase approach, meaning an association
of polyhedra centered on anions and surrounded by cations.
Here, the aforementioned polyhedra are OLa4 tetrahedra,
sharing edges to build 2D Cresnel-like layers condensed in a
stepwise manner with weight and steps equal to 3 and 4
tetrahedra, respectively (Figure 4). These layers are surrounded
Rall
goodness of fit, GOF
V2−O6 distance, which refines to 1.58 Å and is estimated by
DFT as 1.73 Å (see Table S1 in the Supporting Information).
The distance of 1.58 Å appears to be too short for a V−O
24
distance in a VO tetrahedron. To obtain the final refined
by isolated VO groups organized together, in groups of four, to
4
4
model, we have therefore repeated the dynamical refinement
build the so-called “tunnels” already observed in related
Figure 2. Final observed (red), calculated (black), and difference plots (blue) obtained for the powder X-ray diffraction (PXRD) Rietveld refinement
of KLa O (VO ) .
5
5
4 2
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Inorganic Chemistry
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Table 3. Atomic Positions Deviations between Kinematical/
Dynamical Refinement and Respective DFT Optimizations
and between the Two Optimizations
kinematical
opt./
dynamical opt.
deviation
dynamical/
dynamical
opt.
dynamical +
restraint/
dynamical
opt.
kinematical/
kinematical
opt. deviation
atom−
atom
deviation
K1−K1
0.00119
0.00000
0.00127
0.00609
0.00363
0.00238
0.00404
0.00758
0.00140
0.00494
0.01478
0.00808
0.01077
0.01136
0.00874
0.00000
0.00238
0.00591
0.01478
0.02287
0.04221
0.07360
0.07131
0.05312
0.18872
0.19617
0.00878
0.11631
0.12391
0.16954
0.12019
0.15440
0.15085
0.33877
0.08969
0.34128
0.13304
0.34128
0.00473
0.01714
0.01296
0.05472
0.03779
0.07809
0.02925
0.01965
0.05265
0.02425
0.10999
0.02839
0.01691
0.18484
0.05518
0.05682
0.02610
0.04762
0.18484
0.00309
0.01754
0.01432
0.05504
0.03776
0.07834
0.02840
0.02110
0.05223
0.02505
0.10323
0.03274
0.01937
0.10040
0.05275
0.06269
0.02764
0.04304
0.10323
La1−La1
La2−La2
La3−La3
La4−La4
La5−La5
V1−V1
V2−V2
O1−O1
O2−O2
O3−O3
O4−O4
O5−O5
O6−O6
O7−O7
O8−O8
O9−O9
average
maximum
Figure 4. KLa O (VO ) crystal structure showing the 4*3 Td
5
5
4 2
Cresnel-type 2D units of (OLa ) tetrahedra (the “⊗” symbol denotes
4
the infinite character of the ribbons perpendicular to the plane of the
figure).
The stabilization of oxo-centered inorganic structure with
Cresnel-like geometry is rarely observed in the literature, and
the final dimensionality of the blocks is closely related to the
nature of the main supporting cation. In the case of lead-based
materials, polycationic blocks are only chains built on edge-
shared OPb tetrahedra, leading to chains of variable width:
4
2
7
2
Td*2Td in Pb SiO
(Figure 5a)), 3Td*2Td in
2
4
2
8
Pb O (MoO ) (Figure 5b)), or 7Td*4Td in [Pb O ]-
5
4
4
10
7
2
9
(
OH) F (SO ) (Figure 5c)). Concerning lanthanide- and
2 2 4
bismuth-based materials, Cresnel-like geometry is usually
associated with the presence of 2D layers. For example, in
3
0
Nd O (GeO ) (Figure 5d)), [Nd O ] layers exhibit a
4
4
4
4
4
ladderlike topology with the width and height of steps being
equal to 3Td*3Td. Landa-Canovas et al. recently reported a
family of bismuth oxides with the general formula
Figure 3. (a) High-resolution image corresponding to the diffraction
pattern of Figure 1. The comparison with a simulated image, shown as
panel (b), obtained for a defocus of 35 nm and a thickness of 3.5 nm
allows the superimposition of the projected structure. For these
conditions, the cations appear as black dots and the white squares
highlight the vanadate groups.
Bi Mon−2O
(n = 5−8) (the case of n = 5 is shown in
2n
6n−6
31
Figure 5e)). In this series, ribbons of variable width equal to n
overlap by terminal edge sharing to create a zigzag layer.
32
Another compound, Bi18.71Cr0.27P6O43.22 (Figure 5f)) is built
on n = 8 ribbons sharing edges with perpendicular n = 3 ribbon
connections. It is noteworthy that, in case of zigzag layers, the
general stability of the edifice is ensured by additional O atoms
filling empty space at the angles of the zigzag layers through
bismuth-based materials. These tunnels are formed by four XO4
OBi triangular edifices. In the case of our compound, the
3
groups, running parallel to the b-axis (∼5.5 Å, corresponding to
global stability of the structure is ensured by similar additional
OLa units (Figure 4). Finally, the condensation of OM (M =
the height of two O(La,Bi) tetrahedra). These cationic
4
3
4
+
channels are hosting here K cations. The residual electronic
Pb, Bi, Ln) leads to units of variable sizes, related to the
dimensionality of the parent structure: PbO being 2D leads to
1D chains, whereas La O and Bi O are 3D and give 2D
1
1
density remains quite high around / , y, / , which is quite
2
2
25
usual in such tunnel structures. Indeed, it is noteworthy that
several cations (two in maximum) may occur in the tunnels.
2
3
2
3
26
Cresnel-like layers.
Here, the occupancy rate of K was arbitrary fixed to 1, resulting
in one cation per one tunnel and one unit cell.
3.5. Alkali-Metal Substitution. With the objective of
measuring the impact of the ionic radius of the alkali metal on
E
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Figure 5. Various width and height of Cresnel-like layers of various compounds: (a) 2Td*2Td in Pb SiO , (b) 3Td*2Td in Pb O (MoO ), (c)
2
4
5
4
4
7
Td*4Td in Pb O (OH) F (SO ), (d) 3Td*3Td in Nd O (GeO ), (e) 2Td*5Td in Bi Mo O , and (f) 3Td*8Td in Bi18.71Cr0.27P6O43.22.
10 7 2 2 4 4 4 4 2 8 24
Figure 6. Powder X-ray patterns of Li, Na, K, and Rb compounds. As shown in the insert, the unit-cell volume is a function of the ionic radii of
+
alkaline apart for Li cation. In this case, the Bravais lattice is different, P-centered instead of C-centered, and shown by (hkl) index enhancement.
Table 4. Unit-Cell Parameters of ALa O (VO ) Powders Calculated Using Jana 2006 Software
5
5
4 2
A
a (Å)
b (Å)
c (Å)
β (deg)
V (Å3)
Li
Na
K
22.691(2)
19.949(2)
20.228(2)
20.326(2)
5.905(1)
5.912(1)
5.864(4)
5.845(1)
11.828(1)
12.460(1)
12.606(1)
12.660(1)
120.068(5)
117.463(3)
117.636(3)
117.690(3)
1371.48(19)
1303.87(14)
1324.68(22)
1331.72(16)
Rb
F
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Inorganic Chemistry
Article
3
3
the crystal structure, the substitution of K by Li, Na, Rb, and Cs
was tested. Powder syntheses were performed using the
procedure detailed in Experimental Section. Na and Rb
substitution resulted in compounds giving similar XRD patterns
to the title compound, with only small displacements of peak
positions related to the modification of unit-cell parameters,
probably due to variations of the atomic size of the alkaline
component (see Figure 6 and Table 4). However, the
substitution of lithium induced a rather strong modification
of the powder X-ray pattern. Therefore, the lithium-based
compound was studied in detail by electron diffraction, both in
diffraction and high-resolution image modes. The former study
evidenced the same kind of monoclinic unit cell, slightly
distorted (Table 4), but with a primitive Bravais lattice, instead
of being C-centered. This violation of C centering is due to the
presence of additional spots evidenced by white arrows on
families of compounds such as tungsten bronze WO
or in
3−x
34
the (Pb,Bi)1−xFe1+xO3−y series. These defects are probably
related to a partial or inhomogeneous mixing with severe
lithium loss/separation of reactants during the synthesis
process. Thus, even if the starting powder has the expected
Li:La:V ratio, any inhomogeneity would create regions with
either rich or poor lithium content. In addition, Li loss may
happen, because of sublimation or evaporation at high synthesis
temperature (lithium evaporation occurs at 950 °C and
accelerates at 1050 °C ). As long as the cationic tunnels are
not filled in the structure, the cationic backbone is probably
modified and, as a consequence, the general periodicity is
potentially locally changed.
35
3.6. Physical Characterization. 3.6.1. Catalytic Tests.
KLa O (VO ) is a potentially interesting compound for
5
5
4 2
catalysis, because of both its redox character (given by the
vanadium element that is well-known to be adaptive, in terms
of the degree of oxidation) and its acid behavior (linked to the
presence of lanthanum). Since redox and acid properties are
key points for ethanol oxidation reaction, we used this reaction
to test the catalytic properties. Pure powder sample was tested
at different temperatures (250, 275, 300, 325, 350, and 375 °C).
Figure 8 presents the evolution of the ethanol conversion as a
[
010] ZAP (Figure 7). The observation of this ZAP shows
Figure 8. (a) Presentation of ethanol conversion and carbon balance
for KLa O (VO ) registered at different temperatures, and (b)
5
5
4 2
selectivity to acetaldehyde and CO2.
Figure 7. [010] zone axis: HREM images showing shear planes (green
arrows) in case of the lithium sample and the corresponding electron
diffraction pattern, showing diffuse streaks. The white arrows indicate
the weak spots responsible of the P Bravais lattice. The size of
intergrowth is variable, with several examples evidenced here.
function of temperature for the title sample. For oxidation
reactions, the ethanol conversion increases with the temper-
ature but still remains very low, under 30%, even for the highest
temperatures tested. During ethanol conversion, the catalyst
acts in favor of acetaldehyde production, at least at 250, 275,
and 300 °C. Above this temperature, the selectivity in
acetaldehyde suddenly decreases, probably because of the co-
diffuse streaks, in accordance with the HREM study, which
proved the presence of defects observed on Figure 7. Many
crystallographic shear planes with a small shift along the c-axis
are observed with variable spacing (in terms of numbers of
groups of tunnels between the faults). Of course, this defect is
interesting and, based on a predictive approach, may help to
synthesize new related materials, as already observed in several
production of CO , preventing the use of such a compound as
2
an efficient catalyst. Anyway, as many new materials, this
preliminary test was necessary to evaluate its capacity for
further applications.
3.6.2. Luminescence. The photoluminescence emission
(PL) and photoluminescence excitation (PLE) spectra of
G
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Inorganic Chemistry
Article
KLa O (VO ) are shown in Figure 9. The PLE spectrum
exhibits two broad peaks, centered at 295 and 338 nm,
calculations. The structure analysis leads us to evidence a new
series of compounds with potentially interesting optical
properties. The title compound is a new oxo-centered material,
5
5
4 2
based on OLa building units connected together to build two-
4
dimensional Cresnel-like layers surrounded by vanadate groups
+
that form tunnels hosting the K cation. In the literature, other
materials have been described with related Cresnel-like layers
but with various geometries observed for different unifying
3+
2+
cations (Bi , Pb , etc.). From a methodological point of view,
it encouraged us to diversify the nature of cations in order to
obtain new architectures and attain other types of properties.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02663.
Additional tables, showing a comparison between
interatomic distances using different refinements (PDF)
Crystallographic data for KLa O (VO ) (CIF)
5
5
4 2
Figure 9. Photoluminescence emission and photoluminescence
excitation spectra of KLa O (VO ) (inset shows a photograph of
the sample under same excitation).
AUTHOR INFORMATION
5
5
4 2
■
Corresponding Author
E-mail: marie.colmont@ensc-lille.fr.
Notes
*
respectively. These bands are due to an absorption process
within VO43 groups starting at 370 nm (3.8 eV). In fact, the
origin is the charge transfer (CT) of an electron between the 2p
−
The authors declare no competing financial interest.
5
+
3−
orbitals of oxygen and vacant 3d orbitals of V in the VO4
ACKNOWLEDGMENTS
■
36,37
groups with tetrahedral symmetry.
In this case, the ground
state is A and excited states are T , T , T , T . The two
The Fonds Europee
́
en de Dev
ion Nord Pas-de-Calais, and Minister
ieur et de la
́
eloppement Reg
́
ional (FEDER),
1
1
1
3
3
38
1
1
2
1
1
CNRS, Reg
́
̀
e de
bands observed are explained for allowed transitions, i.e., A →
1
l’Education Nationale de l’Enseignement Super
́
1
1
1
1
T and A → T . The PL spectrum, monitored at 338 nm,
2
1
1
Recherche are acknowledged for funding X-ray and TEM
facilities. This work was carried out under the framework of the
Multi-InMaDe project supported by the ANR (Grant No. ANR
2011-JS-08 003 01). The authors thank Dr. Mickael Capron,
Georgiana Bucataru, and Anita Borowiec for catalytic measure-
ments.
presents two intense peaks, centered at 466 and 495 nm, that
3
3
1
can be attributed to ( T , T → A ) transitions, respectively.
2
1
1
However, these transitions are forbidden in the ideal Td
symmetry by the spin selection rule. Nevertheless, the
structural study of the compound has highlighted a distortion
of VO43 tetrahedron (V−O bond lengths between 1.582 Å
and 1.729 Å). It is coherent with a lowering of the symmetry of
−
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