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Vibrational spectroscopy study and ab initio calculation on ZnMoO4 system
Journal Pre-proof Vibrational spectroscopy study and ab initio calculation on ZnMoO4 system J.G. da Silva Filho, G.D. Saraiva, A.J. Ramiro de Castro, V.O. Sousa Neto, A. Saraiva-Souza, C.B. Silva, J.A. Lima, A.M.R. Teixeira, P.T.C. Freire, W. Paraguassu, F.F. de Sousa PII:
S0022-2860(20)30100-9
DOI:
https://doi.org/10.1016/j.molstruc.2020.127776
Reference:
MOLSTR 127776
To appear in:
Journal of Molecular Structure
Received Date: 3 October 2019 Revised Date:
2 January 2020
Accepted Date: 20 January 2020
Please cite this article as: J.G. da Silva Filho, G.D. Saraiva, A.J.R. de Castro, V.O.S. Neto, A. SaraivaSouza, C.B. Silva, J.A. Lima, A.M.R. Teixeira, P.T.C. Freire, W. Paraguassu, F.F. de Sousa, Vibrational spectroscopy study and ab initio calculation on ZnMoO4 system, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2020.127776. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
AUTHOR CONTRIBUTIONS:
G. D. Saraiva and P. T. C. Freire wrote the manuscript and performed the assignments of the Raman and infrared modes. J. G. da Silva Filho, A. Saraiva-Souza and A. M. R. Teixeira performed and reported the theoretical study of the electronic properties. Paraguassu and F. F. de Sousa predicted the assignments of the Raman and infrared modes using first-principle calculation through density functional perturbation theory (DFPT). A. J. Ramiro de Castro, V. O. Sousa Neto, C. B. Silva and J. A. Lima Jr performed all experiments (synthesis and spectral acquire) and provided reagents.
Vibrational spectroscopy study and ab initio calculation on ZnMoO4 system G. D. Saraiva, J. G. da Silva Filho, A. J. Ramiro de Castro, V. O. Sousa Neto, A. Saraiva-Souza, C. B. Silva, J. A. Lima Jr, A. M. R. Teixeira, P. T. C. Freire, W. Paraguassu and F. F. de Sousa.
1
Vibrational spectroscopy study and ab initio
2
calculation on ZnMoO4 system
3 4
J. G. da Silva Filhoa*, G. D. Saraivab*, A. J. Ramiro de Castroc, V. O. Sousa Netoa,
5
A. Saraiva-Souzad, C. B. Silvae, J. A. Lima Jre, A. M. R. Teixeiraf, P. T. C. Freiree,
6
W. Paraguassug and F. F. de Sousag. a
7 8 b
9 10
Centro de Ciências Sociais, Saúde e Tecnologia, Universidade Federal do Maranhão, Imperatriz, MA 65900-410, Brazil
Faculdade de Educação Ciências e Letras do Sertão Central, Universidade Estadual do Ceará, CEP 63902-098, Quixadá, CE, Brazil c
11 12 13
d
Universidade Federal do Ceará, 63902-58, Quixadá, CE, Brazil
Departamento de Física, Universidade Federal do Maranhão (UFMA), Campus Universitário do Bacanga, São Luís, Maranhão 65080-805, Brazil e
14 15
Departamento de Física, Universidade Federal do Ceará, P. O. Box 6030, CEP 60455-970, Fortaleza, CE, Brazil f
16 17
Departamento de Física, Universidade Regional do Cariri, Juazeiro do Norte, CE, 63040-000, Brazil
g
18 19
Instituto de Ciências Exatas e Naturais, Universidade Federal do Pará, CEP 66075-110, Belém, PA, Brazil
20 21
Abstract
22
This report presents the results of experimental and theoretical Raman and infrared
23
vibrational spectroscopic studies on the triclinic phase of zinc molybdate α-ZnMoO4
24
polycrystals. The assignments of the Raman and infrared modes were predicted using
25
first-principle calculation through density functional perturbation theory (DFPT).
26
Additionally, a theoretical study of the electronic properties of ZnMoO4 in relation to
27
both the band structure and the associated projected density of states (PDOS) was
28
performed by using first-principle calculations under the local density approximation
29
(LDA). ZnMoO4 presents an indirect band-gap from the gamma point on the valence
30
band to the Y point on the conduction band. The calculated electronic gap is in line with 1
1
the well-known trend of DFT-LDA underestimation of band gap, confirming that the
2
ZnMoO4 crystal is a semiconductor compound with an indirect band-gap of about 3.35
3
eV.
4
Keywords: A. semiconductors, B. phase transitions, C. Raman spectroscopy
5
*Corresponding author- Fax: +55 8834451036
6
E-mail address:[email protected], [email protected]
7
8
9
1. Introduction
10
The molybdate compounds are well documented in literature due to their important
11
physical and chemical properties such as [1-3]: thermal expansion, thermal annealing
12
effects,
13
characteristics [9, 10]. In addition, they have a large range of technological applications
14
to electrochemical devices [11, 12], sensors [13, 14], electronic properties [15-17], and
15
catalysts [18], beyond presenting interesting vibrational and structural properties and
16
phase transformations [2, 19-23]. Recently, Saravanakumar et al. [24] synthesized
17
nanoparticles of MnMoO4 and used them as an electrode for energy storage
18
applications.
19
Regarding zinc molybdate, there are research being performed on structural and
20
vibrational properties [25-29], supercapacitor applications [30], near-zero thermal
21
expansion and polymorphism [31], synthesis of micro- and nanostructures [32] and a
22
flower-like zinc molybdate based high-performance symmetric supercapacitor [33]. The
23
Co-doped
luminescence,
scintillation
ZnMoO4:Eu3+/Tb3+
has
properties
been 2
[4-8],
synthesized
and
to
photoluminescence
study
enhanced
1
photoluminescence [27], as well as its physical and chemical properties for developing
2
new technologies [19]. Finally, the preparation and investigation of Eu3+-activated
3
ZnMoO4 phosphors for white LED has been reported on [34], beyond the study of phase
4
equilibria, crystal structures [35, 36] and phase transformations [21].
5
In this investigation α-ZnMoO4 polycrystals were studied under ambient-temperature
6
and pressure conditions. Raman and infrared spectroscopy were combined with first-
7
principle calculations through the DFPT to predict all the vibrational normal modes,
8
which show good agreement with the experimental data. Additionally, first-principle
9
calculations were performed to obtain the band structure and PDOS, which led to the
10
conclusion that the ZnMoO4 crystal is a semiconductor with a reasonable indirect band-
11
gap of about 3.35 eV.
12
2. Experimental
13
The sample of the ZnMoO4 was synthesized by solid state chemical reaction method,
14
according to equation 1. In a typical procedure, was used a molar ration of 1:1 of the
15
ZnO and MoO3 of the powder oxides (with purity of 99.8%, as furnished by Sigma-
16
Aldrich) used as starting material. The stated oxides were mixed and macerated in the
17
Gral with Pistil for 30 minutes and then placed in the reactor at 900ºC during 720
18
minutes and then cooled naturally until room temperature.
19
ZnO + MoO3→ ZnMoO4
.
(1)
20
The X-ray diffraction (XRD) measurements confirm that the room temperature phases
21
of ZnMoO4 were in agreement with those previously reported in the literature [36]. The
22
formation of the product (crystalline phase) and its structural characterization were
23
confirmed by XRD measurements, using a Bruker D-8 Advance XRD diffractometer 3
1
with the CuKα radiation of λ = 1.54 Å in the 2θ range from 5° to 60°. The Fig. 1S
2
shows the structural refinement, which was done using the Rietveld method along with
3
the GSAS program, which confirmed a triclinic structure (ICSD Nº 17030) without any
4
secondary phase with the following structural refinement details: χ2 = 2.29 is the square
5
of goodness-of-fit indicator and the RWP = 0.20 is the refinement quality parameter.
6
The Raman spectra were collected with a Jobin Yvon T64000 triple-grating
7
spectrometer in the subtractive mode. In this study the slits were set for a resolution of
8
about 2 cm-1. The 514.5 nm line of an Argon ion laser was used as the excitation source.
9
An Olympus microscope lens, with a focal distance of f=20.5 mm and a numeric
10
aperture of NA=0.35, was used to focus the laser on the sample surface. The Fourier-
11
transform infrared (FT-IR) measurements were performed using a Bruker spectrometer,
12
model Vertex 70, equipped with an accessory-setting A225/Q Platinum attenuated total
13
reflectance (ATR) technique, and with a detector-setting RT-Dla TGS wide-range MIR-
14
FTIR. The width aperture of 6 mm was used, allowing measurements of down to 100
15
cm-1 with a spectral resolution of about 4 cm-1 for 120 scans. The spectral region
16
analyzed spanned from 100 to 1020 cm-1.
17
3. Computational details
18
Calculations were performed using the plane wave code QUANTUM-ESPRESSO [37]
19
under the local density approximation (LDA) [38] considering the Perdew-Zunger [39]
20
functional with 4 × 4 × 4 MonkHorst-Pack [40] K-points and a plane waves cut-off of
21
100 Ry. The structure of the triclinic phase of α-ZnMoO4 [36] was fully relaxed,
22
including the cell parameters, until the forces became smaller than 1 × 10-4 Ry/Bohr and
23
the stress lower than 0.01 kbar, with the energy threshold set to 1 × 10-12 Ry. The
24
relaxed lattice parameters were found to be a = 9.627 Å, b = 6.938 Å, c = 8.126 Å, α = 4
1
102.358º, β = 95.334º, and γ = 108.114º, which are in good agreement with
2
experimental values obtained from the literature [36]. The electronic band structure and
3
the density of states (DOS) were computed for a relaxed unit cell of α-ZnMoO4 using
4
the same level theory.
5
The vibrational frequencies were evaluated via dynamical matrix, calculated using
6
density functional perturbation theory (DFPT), also implemented in QUANTUM-
7
ESPRESSO [38]. The non-resonant Raman coefficients were computed using the DFPT
8
linear response method at the gamma (Г) point [41, 42]. The computed Raman activities
9
were based on Placzek's theory of the Raman effect [43]. The theoretical Raman
10
intensity , which simulates the measured Raman spectrum, can be calculated with
11
equation (2) below [44, 45].
( − ) . = , [1 − exp(−ℎ /)]
12
where is the laser exciting wavenumber in cm-1, is the vibrational wavenumber of
13
the ith normal mode, and is the Raman scattering activity of the normal mode . is
14
a suitable normalization factor for all peak intensities (10-13). h, k, c and T are Planck
15
and Boltzmann constants, the speed of light and temperature in Kelvin, respectively.
16
The simulated spectra were plotted using a Lorentzian line shape with a full width at
17
half maximum of 10 cm-1.
18
4. Results and discussion
19
4.1. Raman spectra of ZnMoO4 at room temperature
20
Zinc molybdate (ZnMoO4) has a triclinic structure belonging to the space group P1
21
( ), with six formulae per unit cell (Z = 6) (Fig. 1 and Fig. 2S). The diamagnetic α-
22
ZnMoO4 is a stable phase at standard conditions for temperature and pressure with 5
(2)
1
lattice constants a = 9.625±0.015 Å, b = 6.965±0.010 Å, c = 8.373±0.015 Å, α =
2
103.28°±0.15°, ß = 96.30°±0.15°, and γ = 106.72±0.15° at 298°K. Two of the
3
independent zinc atoms occupy distorted octahedron of oxygen atoms, while the three
4
crystallographically independent Mo atoms are surrounded by distorted tetrahedron of
5
oxygen atoms. The octahedral Zn–O distances range from 1.977±0.011 to 2.243±0.010
6
Å, with an average value of 2.078 Å, and the Mo–O distances vary between
7
1.707±0.009 and 1.842±0.009 Å, with the average being 1.764 Å. The remaining zinc
8
atom is in a distorted ”square'' pyramidal arrangement of oxygen atoms with Zn–O
9
distances that vary from 1.954±0.012 to 2.106±0.007 Å, with the average being 2.025 Å
10
[36].
11
Fig. 2 shows the experimental and calculated Raman spectra of the ZnMoO4 crystal at
12
room temperature in the 20 to 1100 cm-1 spectral range. The experimental and
13
calculated IR spectra of the ZnMoO4 in the 120 to 1020 cm-1 region are shown in Fig. 3.
14
The optical modes are distributed among the irreducible representations of the Ci factor
15
group as 54Ag + 51Au. The selection rules require that only Ag modes are Raman-active
16
modes, while the Au modes are IR-active, and 39 Raman modes were observed from a
17
total of 54 Ag Raman modes predicted by group theory. Table 1 shows the observed and
18
calculated Raman and infrared modes for the ZnMoO4 together with their assignment
19
based on lattice dynamic calculations for the triclinic phase.
20
According to the analyses presented on table 1, the Raman modes observed in the 40 to
21
125 cm-1 range, correspond to combinations of vibrational modes, which are described
22
as: translations of the MoO4 (tetrahedron) plus translations of the ZnO6 (octahedron)
23
units, translations of the MoO4 plus librations of the ZnO6, and translations of the MoO4
24
plus scissoring of the ZnO6. The librational modes of the MoO4 are present in the 6
1
spectral range of 130 to 272 cm-1, which correspond to librational modes of the MoO4
2
units plus a bending, scissoring or translation of the ZnO6 units. From 273 to 315 cm-1
3
scissoring modes of the MoO4 are present. Also, from 320 to 460 cm-1 all Raman modes
4
are described as a combination of the bending modes of the MoO4 plus a bending of the
5
ZnO6. The vibrational spectroscopy analyses of the internal Raman modes based on
6
first-principle calculations allow the assignment of the modes and the particular
7
vibrations of the tetrahedron and octahedron present in the crystal structure. The Raman
8
modes located at about 981.05, 962.21, 956.27, 939.69, 902.80 and 893.77 cm-1,
9
according to our first principle calculations, present a projection of the motions along
10
the O–Zn bond of the octahedron, as well as the O–Mo bond of the tetrahedron.
11
However, calculated modes near 865.25, 835.53, 826.79, 796.00, 775.07 and 725.80,
12
cm-1 present a projection of the motions predominantly along the Mo–O bond.
13
Regarding the IR modes, the calculated vibrational modes located between 846.82 and
14
1001.19 cm-1 are associated with a combination of the stretching modes of the MoO4
15
tetrahedron plus stretching modes of the ZnO6 octahedron, while the four modes located
16
at about 734.73, 762.29, 800.55 and 802.29 cm-1 are assigned as a combination of the
17
anti-symmetric stretching modes of the tetrahedron plus a bending mode of the
18
octahedron. The spectral region from 460.50 to 311.84 cm-1 is rich in the bending and
19
scissoring vibrational modes. The remaining modes ranging from 302.91 to 65.68 cm-1
20
are associated with different combination of translational, librational, and bending
21
modes of the tetrahedral plus octahedral units. A better description of the IR and Raman
22
modes appears on Table 1.
23
24 7
1
4.2. Electronic band structure
2 3
Table 2 shows a comparative between the calculated bond lengths and bond
4
angles computed at LDA with those reported by X-ray data measurement [36]. As one
5
can see, we found a reasonable correspondence between the calculated and observed
6
intramolecular geometry parameters. The larger variations were found for the Zn1–O8
7
(-3.05 %), Zn1–O8 (-3.60 %) and Zn1–O8 (-3.83 %) bond lengths. The average
8
discrepancy between experimental measurements and simulation results for the Mo–O
9
bond angles were found to be lower than 1 %, which constitutes an excellent result. In
10
addition, the relaxed bond angles deviates from the experimental values for the varied
11
structure presents. Morevoer, the avarage deviation of the relaxed bond angles was
12
found to be 2.46 %. The Zn1-O6-Mo1 bond angle presents the maximum variation of
13
5.71 %. It is important to note that distorted character of the octahedral ZnO6 clusters
14
are maintained after the structural optimization.
15
To further understand the electronic properties of ZnMoO4 structure we
16
calculate the electronic band structure, projected density of states (PDOS) and the
17
corresponding squared wave function of the valence band maximum (VBM) and
18
conduction band minimum (CBM), as shown in Fig. 4. The LDA electronic band
19
structure of the zinc molybdate crystal is shown in Fig. 4 (a), the corresponding high-
20
symmetry points in the first Brillouin zone (BZ) are Γ(0,0,0), Y(0,1/2,0), T(0,1/2,1/2),
21
Z(0,0,1/2) and X(1/2,0,0). This structure presents an indirect band gap of 3.35 eV whose
22
the VBM and CBM are located at the Γ- and X- point in the BZ zone, respectively. The
23
calculated electronic gap is in line with the well-known trend of DFT-LDA
24
underestimation of band gaps [46]. However, the results qualitatively agree with the 8
1
existing experiments [8]. Both the valence and conduction bands present a low
2
dispersive character. In fact, the energy difference between the maximum and minimum
3
of the valence band is less than 0.1 eV. The same analysis for the conduction band leads
4
to an energy difference of ~ 0.2 eV. The PDOS in the Fig. 4(b) indicates that the top of
5
the valence band is dominated by O states, while the Mo states give rise to the
6
conduction band with substantial contributions of O states. Note that the Mo PDOS is
7
very small in the valence energy range, which is characteristic of molybdate crystals
8
[47-49].
9
It is relevant to investigate the electronic structure in detail to get basic
10
information about the semiconducting state. This can be directly probed by the spatial
11
orbital distribution of highest occupied molecular orbital, HOMO and lowest
12
unoccupied molecular orbital (LUMO), as shown in Fig. 4(c) and (d), respectively. The
13
HOMO (VBM) is composed mainly of localized px and pz orbitals of the O atoms that
14
connect with both Zn and Mo atoms. In addition, there are small contribution from dx2-y2
15
and dyz orbitals of Zn atoms. In that case, the molybdenum atoms present an absence of
16
relevant states. Meanwhile the LUMO (CBM) orbitals are distributed on the O and Mo
17
atoms, with a distinct contribution in each one. As expected, the O atoms are composed
18
by slight contribution of px orbitals, whereas the Mo atoms present a strong contribution
19
of dz2 and dyz type orbitals. In order to understand the properties and the composition of
20
the energy bands, the orbital per-atom PDOS were plotted for each atomic species Zn,
21
Mo, and O, describing the relative orbital contributions to the total DOS, as shown in
22
Fig. 5. As can be seen, O 2p and Zn 3d give the most significant contribution to the
23
valence bands in the energy range from -0.9 to 0 eV. The other orbital states have
9
1
negligible contributions in this energy interval. On the other hand, the valence bands are
2
composed mainly of Mo 4d and O 2p levels.
3
In the following, we will investigate the Electron Localization Function (ELF)
4
mapping to further understand the chemical bond formation, as shown in Fig 6. From
5
the physical point of view, the ELF can be understood from the contours of electron-
6
pair density at the structure [50, 51] . The red zones (strong localization) denotes that
7
the occurrence probability of valence electrons is 100%, the blue zones (zero
8
localization) means the absence of electrons in the area, and the green zones
9
corresponding to a free electron gas behavior that indicates the border of covalent bonds
10
(50% localization). The Fig. 6 (a)-(c) illustrates the ELF contours map for the Mo–O
11
bonding's, in these cases the Mo atom and the neighboring O atom share a green area
12
(value of 50%) showing a significant electron-pair density, which means the formation
13
of strong bond formation. Whereas, the Zn–O bonding's present a lower electron-pair
14
density (value of 25%). It is important to note that those data are in accordance with the
15
bond lengths ascribed on Table 2.
16
In this point it should be interesting to compare our findings with very recent
17
results published in the literature [52-54]. In the Ref. [52], a bimetallic coordination
18
cluster,
19
Co4MoO8 (where H4TC4A means p-ter-butylthiacalix[4]arene), shows the particular
20
importance of Zn (and Ni) ions for eletrocalalysis processes involving molybdates.
21
Co4MoO8 was used as working electrodes, showing improved electrocatalytic activities
22
for the oxygen evolution reaction. However, when Co is replaced in Co4MoO8 clusters
23
with Ni and Zn, an extraordinary oxygen evolution reaction performance is verified
24
because of the Co – Mo synergistic effect [52], indicating the importance of Zn in the
(NH4){CO4II(TC4A)Cl[(MoVO2)2S(CH3O)]4}(+solvent),
10
abbreviated
as
1
catalytic process. Also, a theoretical study has recently investigated the evolutional rule
2
of the Cu-Zn nanoalloy clusters with sizes [53]. Using density functional theory, authors
3
obtained the global minimum of several nanoalloy clusters, discovering the phase
4
diagrams and realizing that the possible geometric structures of Cu – Zn are dependent
5
from the total number of valence electrons, like what happen with the bulk material. The
6
study showed that clusters with even valence electron numbers are more stable and
7
depending on these numbers the compounds can appear with planar motifs, as spherical
8
structure or with prolate profiles [53]. Another thorough investigation of the role played
9
by valence electrons and bonding in a particular system, Mo8Ga41, was also given
10
recently in the work presented in Ref. [54]. Authors show that the substitution of the Zn
11
for Ga leads to a shortening of the interatomic distances in the Mo8Ga41 compound and,
12
at the same time, decrease of the electron valence count for the mixed system. An
13
additional interesting feature noted by the calculation is the possibility of formation of
14
Zn clusters in the crystal structure [54]. Such aspect is not glimpsed in our investigation
15
on ZnMoO4, where no evidence of cluster formation is achieved.
16
5. Conclusions
17
In summary, this paper reported the detailed vibrational and electronic properties of the
18
P1 ( ) triclinic phase of zinc molybdate α-ZnMoO4 polycrystals. The experimental and
19
theoretical Raman and infrared vibrational properties were discussed considering first-
20
principle calculations through density functional perturbation theory (DFPT). The
21
assignments of the Raman and infrared modes were found to be in good agreement with
22
experiments. Additionally, first-principle calculations under the local density
23
approximation were used to study the electronic properties of the ZnMoO4 system in
24
relation to the band structure and its associated PDOS. The coordinates corresponding 11
1
to the high-symmetry points in the first BZ are Γ(0,0,0), Y(0,1/2,0), T(0,1/2,1/2),
2
Z(0,0,1/2) and X(1/2,0,0). ZnMoO4 presents an indirect band gap from the Γ point on
3
the valence band to the X point on the conduction band. The calculated electronic gap of
4
3.35 eV, in litne with the well-known trend of DFT-LDA underestimation of band-gaps,
5
confirms that the ZnMoO4 crystal is a semiconductor compound.
6
7
Acknowledgments
8
G.D. Saraiva acknowledges support from MCT/CNPq Edital 14/2010 (process
9
476569/2010-9), FUNCAP/Edital 02/2010 (process BP10031001350100/10) and
10
FUNCAP/Edital 05/2009 (process 186.01.00/09). PTCF thanks FUNCAP/CNPq for
11
grant PRONEX PR2-011-00006.01-00/15. We acknowledge the support from
12
FAPEPI/MCT/CNPq/CT – INFRA Nº 010/2009. The other authors acknowledge CNPq
13
and FUNCAP for partial financial support.
14
15
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16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
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13 14 15
16
17 18
19 20 21 22 23 24 25 26 27 28 29 30
16
1
List of Figures
2
3
Fig. 1: Unit cell of the ZnMoO4 crystal in the monoclinic phase, P1 ( ), space group.
4
Fig. 2: Experimental and calculated (scaled) Raman spectra of ZnMoO4 in the (a) 40 to
5
700 cm−1 region and in the (b) 700–1100 cm−1 region.
6
Fig. 3: Experimental and calculated (scaled) IR spectra of ZnMoO4 in the 120 to 1020
7
cm−1 region.
8
Fig. 4: (a) The LDA calculated band structure of ZnMoO crystal around the region of
9
the main energy band gap. (b) The projected density of states displaying the
10
contributions of the Zn (yellow), O (red) and Mo (green) atoms is shown at the right
11
side panel, as well as the total contribution (black). (c) and (d) the spatial orbital
12
distribution of highest occupied molecular orbital, HOMO and lowest unoccupied
13
molecular orbital (LUMO), respectively.
14
Fig. 5: Zinc molybdate projected density of states (PDOS) for O, Zn and Mo: s (dotted,
15
red), p (solid, black) and d (solid, blue) orbitals.
16
Fig. 6: The ELF mapping for ZnMoO4 crystal structure. (a-c) Contours map for the Mo–
17
O bonding's. (d-f) Contours map for the Zn–O bonding's.
18 19
Fig.1S. Rietveld refinement of X-ray diffraction pattern ZnMoO4 crystals measured at
20
ambient conditions.
21 22
Fig. 2S: Unit cell of the ZnMoO4 crystal in the monoclinic phase, P1 ( ), space group
23
along of different axis.
24 25 26 27 17
1
List of Tables
2 3 4
Table 1. Observed and calculated Raman and infrared modes for ZnMoO4, together with
5
their assignments based on the basis of lattice dynamic calculations for the triclinic
6
phase.
7
Table 2. The geometrical parameters of ZnMoO4: bond lengths (Å) and bond angles (°)
8
computed at DFT-LDA and determined by X-ray diffraction data [36].
9
18
Raman ωobs 51 60 70 78 85 92
102 109 116 121 129 144 163 172 189 206 225 239
289 298 321
Raman ωcal 44.90 54.97 65.68 76.75 77.94 86.20 91.21 92.70 105.93 111.99 115.32 122.60 132.66 146.13 149.18 173.32 178.03 183.94 200.04 205.14 214.42 225.23 238.73 247.63 251.72 270.85 274.79 277.58 290.43 295.08 312.11 321.16 325.95 335.60
sym Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag
IR ωobs T(tetra)+ T(oct) T (tetra)+T(oct) Lib(tetra)+T(oct) T(tetra)+T(oct) T(tetra)+T(oct) T(tetra)+T(oct) T (tetra)+T(oct) T (tetra)+Lib(oct) T(tetra)+Sci(oct) Lib(tetra)+Sci(oct) T(tetra)+T(oct) Lib (tetra)+T(oct) Lib (tetra)+Bend(oct) Lib (tetra)+Lib(oct) Lib (tetra)+Bend(oct) Lib (tetra)+Sci(oct) Lib(tetra)+Sci(oct) Lib(tetra)+T(oct)+Bend(oct) Lib(tetra)+Sci(oct) Lib(tetra)+Sci(oct) Lib(tetra)+T(oct) Sci(tetra)+T(oct) Lib (tetra)+Sci(oct) Lib(tetra)+Sci(oct)+T(oct) Lib(tetra)+Sci(oct)+T(oct) Lib(tetra)+Lib(oct) Sci (tetra)+Bend(oct) Sci (tetra)+Lib(oct) Sci (tetra)+Bend(oct) Sci (tetra)+Bend(oct) Sci (tetra)+Bend(oct) Bend (tetra)+Bend(oct) Bend (tetra)+Bend(oct) Bend (tetra)+Bend(oct)
131 137 144 154 159 163 171 175 182 203 212 228 233 245 255 261
289 301 308 318 337 357
IR ωcal 65.68 71.99 84.93 87.43 94.80 97.36 105.31 130.17 132.66 140.86 150.38 159.10 164.19 165.80 181.16 183.79 191.78 208.22 217.34 224.66 236.33 244.16 254.10 262.44 270.98 276.26 288.21 291.65 302.91 311.84 323.60 330.86 335.52 352.13
sym Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au
Lib (tetra)+Lib(oct) T(tetra)+Lib(oct) T(tetra)+Bend(oct) Lib(tetra)+Bend(oct) Lib(tetra)+Lib(oct) Lib(tetra)+Lib(oct) T(tetra)+T(oct) T(tetra)+Bend(oct) T(tetra)+T(oct) Lib(tetra)+ Bend(oct) T(tetra)+T(oct)+Bend(oct) T(tetra)+Bend(tetra)+T(oct)+Bend(oct) T(tetra)+Bend (tetra)+T(oct)+Bend(oct) T(tetra)+Bend (tetra)+T(oct)+Bend(oct) T(tetra)+Lib(tetra)+ T(oct)+Bend(oct) T(tetra)+Bend(tetra)+T(oct)+Bend(oct) T(tetra)+Bend(tetra)+ T(oct)+Bend(oct) T(tetra)+Lib (tetra)+ T(oct)+Sci(oct) T(tetra)+Lib (tetra)+T(oct)+Bend(oct) T(tetra)+Lib (tetra)+T(oct)+Bend(oct) T(tetra)+Sci (tetra)+T(oct)+Bend(oct) T(tetra)+Lib(tetra)+T(oct)+Bend(oct) T(tetra)+Lib(tetra)+T(oct)+Bend (oct) T(tetra)+Bend (tetra)+T(oct)+Lib(oct) Sci(tetra)+Bend(oct) Lib(tetra)+Lib(oct) Bend(tetra)+Bend(oct) Bend(tetra)+T(oct) Bend(tetra)+Bend(oct) Sci(tetra)+Sci(oct)+Bend(oct) Sci(tetra)+Sci(oct)+Bend(oct) Sci(tet)+Bend (tetra)+Sci(oct)+Bend(oct) Sci(tetra)+Sci(oct) Sci(tetra)+Sci(oct)
331 338 343 369 404 417 727 755 788 817 842 862 883 896 932 949 957 969
338.62 343.45 373.92 384.58 396.53 417.81 422.21 458.44 725.80 775.07 796.00 826.79 835.53 865.25 893.77 902.80 939.69 956.27 962.21 981.05
Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag
Bend(tetra)+Bend(oct) Bend(tetra)+Bend(oct) Bend(tetra)+Bend(oct) Bend(tetra)+Bend(oct) Bend(tetra)+Bend(oct) Bend(tetra)+Bend(oct) Bend(tetra)+Bend(oct) Ben (tetra)+Bend(oct) υ1 (tetra)+Bend(oct) υ1 (tetra)+Bend(oct) υ1 (tetra)+Bend (oct) υ3 (tetra)+Bend (oct) υ3 (tetra)+Bend (oct) υ3 (tetra)+Bend (oct) υ3 (tetra)+Bend (oct) υ1 (tetra)+Bend (oct) υ1 (tetra)+Bend (oct) υ1 (tetra)+υ3 (oct) υ1 (tetra)+υ3T(oct) υ1 (tetra)+υ1T(oct)
387 405 432 455 739 753 798 806 843 867 892 911 940 969 987
370.29 378.21 392.11 423.45 429.29 460.50 734.73 762.29 800.55 802.52 846.82 859.85 900.11 904.67 923.84 949.82 960.26 1001.19
Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au
Sci(tetra)+Sci(oct) Sci(tetra)+Sci(oct) Sci(tetra)+Sci(oct) Bend (tetra)+Bend(oct) Bend (tetra)+Bend(oct) Sci (tetra)+Bend(oct) υ3(tetra)+Bend(oct) υ3(tetra)+Bend(oct) υ3(tetra)+Bend(oct) υ3(tetra)+Bend(oct) υ3(tetra)+υ3(oct) υ3(tetra)+υ3(oct) υ1(tetra)+υ3(oct) υ3 (tetra)+υ3(oct) υ1(tetra)+υ1(oct) υ1(tetra)+υ3(oct) υ1(tetra)+υ3(oct) υ3(tetra)+υ1(oct)
Tetra: Tetrahedron, Oct: Octahedron, Bend: Bending, Lib: Libration, T: Translation, Sc: Scissor, υ1: Symmetric Stretching, υ3: Antisymmetric Stretching.
Table 2. The geometrical parameters of ZnMoO4: bond lengths (Å) and bond angles (°) computed at DFT-LDA and determined by X-ray diffraction data [36].
Bond Length (Å) Calc.
Exp.
∆%
Bond Angle (º)
Calc.
Exp.
∆%
Zn1–O2
2.10
2.13
-1.03
Mo3-O1-Zn2
167.24
164.80
1.48
Zn1–O6
2.06
2.11
-2.61
Zn1-O2-Zn3
98.84
99.20
-0.36
Zn1–O7
2.01
2.04
-1.39
Zn1-O2-Mo3
116.36
121.80
-4.47
Zn1–O8
2.00
2.07
-3.05
Zn2-O3-Zn3
98.27
97.00
1.31
Zn1–O11
1.98
2.03
-2.44
Zn2-O3-Mo3
123.37
125.70
-1.85
Zn1–O12
2.05
2.08
-1.50
Zn2-O4-Zn2
99.99
99.30
0.69
Zn2–O1
1.98
2.06
-3.60
Zn2-O4-Mo1
141.57
138.8
1.99
Zn2–O3
2.03
2.02
0.86
Zn3-O5-Mo2
148.43
150.60
-1.44
(Zn2–O4)a
2.24
2.22
0.84
Zn1-O6-Zn3
98.49
100.30
-1.81
(Zn2–O4)a
1.94
1.98
-1.58
Zn1-O6-Mo1
141.02
133.40
5.71
Zn2–O9
2.00
2.05
-2.50
Zn1-O7-Mo2
167.17
169.80
-1.55
Zn2–O10
2.12
2.12
0.17
Zn1-O8-Mo3
153.01
148.60
2.97
Zn3–O2
1.95
2.03
-3.83
Mo2-O9-Zn2
155.86
160.10
-2.65
Zn3–O3
2.05
2.11
-2.64
Zn2-O10-Zn3
96.90
96.80
0.11
Zn3–O5
1.90
1.95
-2.64
Zn2-O10-Mo2 129.49
129.20
0.23
Zn3–O6
2.01
2.01
0.11
Zn1-O11-Mo1 160.63
152.40
5.40
Zn3–O10
1.98
2.03
-2.14
Zn1-O12-Mo1 132.98
138.30
-3.85
Mo1–O4
1.78
1.78
0.07
Mo1–O6
1.79
1.78
0.69
Mo1–O11
1.73
1.74
-0.58
Mo1–O12
1.77
1.77
-0.17
Mo2–O5
1.76
1.75
0.59
Mo2–O7
1.74
1.75
-0.38
Mo2–O9
1.74
1.74
0.29
Mo2–O10
1.80
1.81
-0.40
Mo3–O1
1.73
1.72
0.98
Mo3–O2
1.79
1.78
0.71
Mo3–O3
1.83
1.84
-0.45
Mo3–O8
1.72
1.71
0.89
Universidade Estadual do Ceará Faculdade de Educação Ciências e Letras do Sertão Central CEP 63.900-000 Quixadá, Ceará Phone: 55-88 – 3445 1036
Quixadá, October 2nd, 2019.
“Journal Molecular Structure”
Highlights Raman and infrared vibrational spectroscopic studies of the ZnMoO4. First principle calculation through the density functional perturbation theory (DFPT). A theoretical study on the electronic properties of the ZnMoO4. The band structure and the associated projected density of states (PDOS).
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0022-2860(20)30100-9
DOI:
https://doi.org/10.1016/j.molstruc.2020.127776
Reference:
MOLSTR 127776
To appear in:
Journal of Molecular Structure
Received Date: 3 October 2019 Revised Date:
2 January 2020
Accepted Date: 20 January 2020
Please cite this article as: J.G. da Silva Filho, G.D. Saraiva, A.J.R. de Castro, V.O.S. Neto, A. SaraivaSouza, C.B. Silva, J.A. Lima, A.M.R. Teixeira, P.T.C. Freire, W. Paraguassu, F.F. de Sousa, Vibrational spectroscopy study and ab initio calculation on ZnMoO4 system, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2020.127776. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
AUTHOR CONTRIBUTIONS:
G. D. Saraiva and P. T. C. Freire wrote the manuscript and performed the assignments of the Raman and infrared modes. J. G. da Silva Filho, A. Saraiva-Souza and A. M. R. Teixeira performed and reported the theoretical study of the electronic properties. Paraguassu and F. F. de Sousa predicted the assignments of the Raman and infrared modes using first-principle calculation through density functional perturbation theory (DFPT). A. J. Ramiro de Castro, V. O. Sousa Neto, C. B. Silva and J. A. Lima Jr performed all experiments (synthesis and spectral acquire) and provided reagents.
Vibrational spectroscopy study and ab initio calculation on ZnMoO4 system G. D. Saraiva, J. G. da Silva Filho, A. J. Ramiro de Castro, V. O. Sousa Neto, A. Saraiva-Souza, C. B. Silva, J. A. Lima Jr, A. M. R. Teixeira, P. T. C. Freire, W. Paraguassu and F. F. de Sousa.
1
Vibrational spectroscopy study and ab initio
2
calculation on ZnMoO4 system
3 4
J. G. da Silva Filhoa*, G. D. Saraivab*, A. J. Ramiro de Castroc, V. O. Sousa Netoa,
5
A. Saraiva-Souzad, C. B. Silvae, J. A. Lima Jre, A. M. R. Teixeiraf, P. T. C. Freiree,
6
W. Paraguassug and F. F. de Sousag. a
7 8 b
9 10
Centro de Ciências Sociais, Saúde e Tecnologia, Universidade Federal do Maranhão, Imperatriz, MA 65900-410, Brazil
Faculdade de Educação Ciências e Letras do Sertão Central, Universidade Estadual do Ceará, CEP 63902-098, Quixadá, CE, Brazil c
11 12 13
d
Universidade Federal do Ceará, 63902-58, Quixadá, CE, Brazil
Departamento de Física, Universidade Federal do Maranhão (UFMA), Campus Universitário do Bacanga, São Luís, Maranhão 65080-805, Brazil e
14 15
Departamento de Física, Universidade Federal do Ceará, P. O. Box 6030, CEP 60455-970, Fortaleza, CE, Brazil f
16 17
Departamento de Física, Universidade Regional do Cariri, Juazeiro do Norte, CE, 63040-000, Brazil
g
18 19
Instituto de Ciências Exatas e Naturais, Universidade Federal do Pará, CEP 66075-110, Belém, PA, Brazil
20 21
Abstract
22
This report presents the results of experimental and theoretical Raman and infrared
23
vibrational spectroscopic studies on the triclinic phase of zinc molybdate α-ZnMoO4
24
polycrystals. The assignments of the Raman and infrared modes were predicted using
25
first-principle calculation through density functional perturbation theory (DFPT).
26
Additionally, a theoretical study of the electronic properties of ZnMoO4 in relation to
27
both the band structure and the associated projected density of states (PDOS) was
28
performed by using first-principle calculations under the local density approximation
29
(LDA). ZnMoO4 presents an indirect band-gap from the gamma point on the valence
30
band to the Y point on the conduction band. The calculated electronic gap is in line with 1
1
the well-known trend of DFT-LDA underestimation of band gap, confirming that the
2
ZnMoO4 crystal is a semiconductor compound with an indirect band-gap of about 3.35
3
eV.
4
Keywords: A. semiconductors, B. phase transitions, C. Raman spectroscopy
5
*Corresponding author- Fax: +55 8834451036
6
E-mail address:[email protected], [email protected]
7
8
9
1. Introduction
10
The molybdate compounds are well documented in literature due to their important
11
physical and chemical properties such as [1-3]: thermal expansion, thermal annealing
12
effects,
13
characteristics [9, 10]. In addition, they have a large range of technological applications
14
to electrochemical devices [11, 12], sensors [13, 14], electronic properties [15-17], and
15
catalysts [18], beyond presenting interesting vibrational and structural properties and
16
phase transformations [2, 19-23]. Recently, Saravanakumar et al. [24] synthesized
17
nanoparticles of MnMoO4 and used them as an electrode for energy storage
18
applications.
19
Regarding zinc molybdate, there are research being performed on structural and
20
vibrational properties [25-29], supercapacitor applications [30], near-zero thermal
21
expansion and polymorphism [31], synthesis of micro- and nanostructures [32] and a
22
flower-like zinc molybdate based high-performance symmetric supercapacitor [33]. The
23
Co-doped
luminescence,
scintillation
ZnMoO4:Eu3+/Tb3+
has
properties
been 2
[4-8],
synthesized
and
to
photoluminescence
study
enhanced
1
photoluminescence [27], as well as its physical and chemical properties for developing
2
new technologies [19]. Finally, the preparation and investigation of Eu3+-activated
3
ZnMoO4 phosphors for white LED has been reported on [34], beyond the study of phase
4
equilibria, crystal structures [35, 36] and phase transformations [21].
5
In this investigation α-ZnMoO4 polycrystals were studied under ambient-temperature
6
and pressure conditions. Raman and infrared spectroscopy were combined with first-
7
principle calculations through the DFPT to predict all the vibrational normal modes,
8
which show good agreement with the experimental data. Additionally, first-principle
9
calculations were performed to obtain the band structure and PDOS, which led to the
10
conclusion that the ZnMoO4 crystal is a semiconductor with a reasonable indirect band-
11
gap of about 3.35 eV.
12
2. Experimental
13
The sample of the ZnMoO4 was synthesized by solid state chemical reaction method,
14
according to equation 1. In a typical procedure, was used a molar ration of 1:1 of the
15
ZnO and MoO3 of the powder oxides (with purity of 99.8%, as furnished by Sigma-
16
Aldrich) used as starting material. The stated oxides were mixed and macerated in the
17
Gral with Pistil for 30 minutes and then placed in the reactor at 900ºC during 720
18
minutes and then cooled naturally until room temperature.
19
ZnO + MoO3→ ZnMoO4
.
(1)
20
The X-ray diffraction (XRD) measurements confirm that the room temperature phases
21
of ZnMoO4 were in agreement with those previously reported in the literature [36]. The
22
formation of the product (crystalline phase) and its structural characterization were
23
confirmed by XRD measurements, using a Bruker D-8 Advance XRD diffractometer 3
1
with the CuKα radiation of λ = 1.54 Å in the 2θ range from 5° to 60°. The Fig. 1S
2
shows the structural refinement, which was done using the Rietveld method along with
3
the GSAS program, which confirmed a triclinic structure (ICSD Nº 17030) without any
4
secondary phase with the following structural refinement details: χ2 = 2.29 is the square
5
of goodness-of-fit indicator and the RWP = 0.20 is the refinement quality parameter.
6
The Raman spectra were collected with a Jobin Yvon T64000 triple-grating
7
spectrometer in the subtractive mode. In this study the slits were set for a resolution of
8
about 2 cm-1. The 514.5 nm line of an Argon ion laser was used as the excitation source.
9
An Olympus microscope lens, with a focal distance of f=20.5 mm and a numeric
10
aperture of NA=0.35, was used to focus the laser on the sample surface. The Fourier-
11
transform infrared (FT-IR) measurements were performed using a Bruker spectrometer,
12
model Vertex 70, equipped with an accessory-setting A225/Q Platinum attenuated total
13
reflectance (ATR) technique, and with a detector-setting RT-Dla TGS wide-range MIR-
14
FTIR. The width aperture of 6 mm was used, allowing measurements of down to 100
15
cm-1 with a spectral resolution of about 4 cm-1 for 120 scans. The spectral region
16
analyzed spanned from 100 to 1020 cm-1.
17
3. Computational details
18
Calculations were performed using the plane wave code QUANTUM-ESPRESSO [37]
19
under the local density approximation (LDA) [38] considering the Perdew-Zunger [39]
20
functional with 4 × 4 × 4 MonkHorst-Pack [40] K-points and a plane waves cut-off of
21
100 Ry. The structure of the triclinic phase of α-ZnMoO4 [36] was fully relaxed,
22
including the cell parameters, until the forces became smaller than 1 × 10-4 Ry/Bohr and
23
the stress lower than 0.01 kbar, with the energy threshold set to 1 × 10-12 Ry. The
24
relaxed lattice parameters were found to be a = 9.627 Å, b = 6.938 Å, c = 8.126 Å, α = 4
1
102.358º, β = 95.334º, and γ = 108.114º, which are in good agreement with
2
experimental values obtained from the literature [36]. The electronic band structure and
3
the density of states (DOS) were computed for a relaxed unit cell of α-ZnMoO4 using
4
the same level theory.
5
The vibrational frequencies were evaluated via dynamical matrix, calculated using
6
density functional perturbation theory (DFPT), also implemented in QUANTUM-
7
ESPRESSO [38]. The non-resonant Raman coefficients were computed using the DFPT
8
linear response method at the gamma (Г) point [41, 42]. The computed Raman activities
9
were based on Placzek's theory of the Raman effect [43]. The theoretical Raman
10
intensity , which simulates the measured Raman spectrum, can be calculated with
11
equation (2) below [44, 45].
( − ) . = , [1 − exp(−ℎ /)]
12
where is the laser exciting wavenumber in cm-1, is the vibrational wavenumber of
13
the ith normal mode, and is the Raman scattering activity of the normal mode . is
14
a suitable normalization factor for all peak intensities (10-13). h, k, c and T are Planck
15
and Boltzmann constants, the speed of light and temperature in Kelvin, respectively.
16
The simulated spectra were plotted using a Lorentzian line shape with a full width at
17
half maximum of 10 cm-1.
18
4. Results and discussion
19
4.1. Raman spectra of ZnMoO4 at room temperature
20
Zinc molybdate (ZnMoO4) has a triclinic structure belonging to the space group P1
21
( ), with six formulae per unit cell (Z = 6) (Fig. 1 and Fig. 2S). The diamagnetic α-
22
ZnMoO4 is a stable phase at standard conditions for temperature and pressure with 5
(2)
1
lattice constants a = 9.625±0.015 Å, b = 6.965±0.010 Å, c = 8.373±0.015 Å, α =
2
103.28°±0.15°, ß = 96.30°±0.15°, and γ = 106.72±0.15° at 298°K. Two of the
3
independent zinc atoms occupy distorted octahedron of oxygen atoms, while the three
4
crystallographically independent Mo atoms are surrounded by distorted tetrahedron of
5
oxygen atoms. The octahedral Zn–O distances range from 1.977±0.011 to 2.243±0.010
6
Å, with an average value of 2.078 Å, and the Mo–O distances vary between
7
1.707±0.009 and 1.842±0.009 Å, with the average being 1.764 Å. The remaining zinc
8
atom is in a distorted ”square'' pyramidal arrangement of oxygen atoms with Zn–O
9
distances that vary from 1.954±0.012 to 2.106±0.007 Å, with the average being 2.025 Å
10
[36].
11
Fig. 2 shows the experimental and calculated Raman spectra of the ZnMoO4 crystal at
12
room temperature in the 20 to 1100 cm-1 spectral range. The experimental and
13
calculated IR spectra of the ZnMoO4 in the 120 to 1020 cm-1 region are shown in Fig. 3.
14
The optical modes are distributed among the irreducible representations of the Ci factor
15
group as 54Ag + 51Au. The selection rules require that only Ag modes are Raman-active
16
modes, while the Au modes are IR-active, and 39 Raman modes were observed from a
17
total of 54 Ag Raman modes predicted by group theory. Table 1 shows the observed and
18
calculated Raman and infrared modes for the ZnMoO4 together with their assignment
19
based on lattice dynamic calculations for the triclinic phase.
20
According to the analyses presented on table 1, the Raman modes observed in the 40 to
21
125 cm-1 range, correspond to combinations of vibrational modes, which are described
22
as: translations of the MoO4 (tetrahedron) plus translations of the ZnO6 (octahedron)
23
units, translations of the MoO4 plus librations of the ZnO6, and translations of the MoO4
24
plus scissoring of the ZnO6. The librational modes of the MoO4 are present in the 6
1
spectral range of 130 to 272 cm-1, which correspond to librational modes of the MoO4
2
units plus a bending, scissoring or translation of the ZnO6 units. From 273 to 315 cm-1
3
scissoring modes of the MoO4 are present. Also, from 320 to 460 cm-1 all Raman modes
4
are described as a combination of the bending modes of the MoO4 plus a bending of the
5
ZnO6. The vibrational spectroscopy analyses of the internal Raman modes based on
6
first-principle calculations allow the assignment of the modes and the particular
7
vibrations of the tetrahedron and octahedron present in the crystal structure. The Raman
8
modes located at about 981.05, 962.21, 956.27, 939.69, 902.80 and 893.77 cm-1,
9
according to our first principle calculations, present a projection of the motions along
10
the O–Zn bond of the octahedron, as well as the O–Mo bond of the tetrahedron.
11
However, calculated modes near 865.25, 835.53, 826.79, 796.00, 775.07 and 725.80,
12
cm-1 present a projection of the motions predominantly along the Mo–O bond.
13
Regarding the IR modes, the calculated vibrational modes located between 846.82 and
14
1001.19 cm-1 are associated with a combination of the stretching modes of the MoO4
15
tetrahedron plus stretching modes of the ZnO6 octahedron, while the four modes located
16
at about 734.73, 762.29, 800.55 and 802.29 cm-1 are assigned as a combination of the
17
anti-symmetric stretching modes of the tetrahedron plus a bending mode of the
18
octahedron. The spectral region from 460.50 to 311.84 cm-1 is rich in the bending and
19
scissoring vibrational modes. The remaining modes ranging from 302.91 to 65.68 cm-1
20
are associated with different combination of translational, librational, and bending
21
modes of the tetrahedral plus octahedral units. A better description of the IR and Raman
22
modes appears on Table 1.
23
24 7
1
4.2. Electronic band structure
2 3
Table 2 shows a comparative between the calculated bond lengths and bond
4
angles computed at LDA with those reported by X-ray data measurement [36]. As one
5
can see, we found a reasonable correspondence between the calculated and observed
6
intramolecular geometry parameters. The larger variations were found for the Zn1–O8
7
(-3.05 %), Zn1–O8 (-3.60 %) and Zn1–O8 (-3.83 %) bond lengths. The average
8
discrepancy between experimental measurements and simulation results for the Mo–O
9
bond angles were found to be lower than 1 %, which constitutes an excellent result. In
10
addition, the relaxed bond angles deviates from the experimental values for the varied
11
structure presents. Morevoer, the avarage deviation of the relaxed bond angles was
12
found to be 2.46 %. The Zn1-O6-Mo1 bond angle presents the maximum variation of
13
5.71 %. It is important to note that distorted character of the octahedral ZnO6 clusters
14
are maintained after the structural optimization.
15
To further understand the electronic properties of ZnMoO4 structure we
16
calculate the electronic band structure, projected density of states (PDOS) and the
17
corresponding squared wave function of the valence band maximum (VBM) and
18
conduction band minimum (CBM), as shown in Fig. 4. The LDA electronic band
19
structure of the zinc molybdate crystal is shown in Fig. 4 (a), the corresponding high-
20
symmetry points in the first Brillouin zone (BZ) are Γ(0,0,0), Y(0,1/2,0), T(0,1/2,1/2),
21
Z(0,0,1/2) and X(1/2,0,0). This structure presents an indirect band gap of 3.35 eV whose
22
the VBM and CBM are located at the Γ- and X- point in the BZ zone, respectively. The
23
calculated electronic gap is in line with the well-known trend of DFT-LDA
24
underestimation of band gaps [46]. However, the results qualitatively agree with the 8
1
existing experiments [8]. Both the valence and conduction bands present a low
2
dispersive character. In fact, the energy difference between the maximum and minimum
3
of the valence band is less than 0.1 eV. The same analysis for the conduction band leads
4
to an energy difference of ~ 0.2 eV. The PDOS in the Fig. 4(b) indicates that the top of
5
the valence band is dominated by O states, while the Mo states give rise to the
6
conduction band with substantial contributions of O states. Note that the Mo PDOS is
7
very small in the valence energy range, which is characteristic of molybdate crystals
8
[47-49].
9
It is relevant to investigate the electronic structure in detail to get basic
10
information about the semiconducting state. This can be directly probed by the spatial
11
orbital distribution of highest occupied molecular orbital, HOMO and lowest
12
unoccupied molecular orbital (LUMO), as shown in Fig. 4(c) and (d), respectively. The
13
HOMO (VBM) is composed mainly of localized px and pz orbitals of the O atoms that
14
connect with both Zn and Mo atoms. In addition, there are small contribution from dx2-y2
15
and dyz orbitals of Zn atoms. In that case, the molybdenum atoms present an absence of
16
relevant states. Meanwhile the LUMO (CBM) orbitals are distributed on the O and Mo
17
atoms, with a distinct contribution in each one. As expected, the O atoms are composed
18
by slight contribution of px orbitals, whereas the Mo atoms present a strong contribution
19
of dz2 and dyz type orbitals. In order to understand the properties and the composition of
20
the energy bands, the orbital per-atom PDOS were plotted for each atomic species Zn,
21
Mo, and O, describing the relative orbital contributions to the total DOS, as shown in
22
Fig. 5. As can be seen, O 2p and Zn 3d give the most significant contribution to the
23
valence bands in the energy range from -0.9 to 0 eV. The other orbital states have
9
1
negligible contributions in this energy interval. On the other hand, the valence bands are
2
composed mainly of Mo 4d and O 2p levels.
3
In the following, we will investigate the Electron Localization Function (ELF)
4
mapping to further understand the chemical bond formation, as shown in Fig 6. From
5
the physical point of view, the ELF can be understood from the contours of electron-
6
pair density at the structure [50, 51] . The red zones (strong localization) denotes that
7
the occurrence probability of valence electrons is 100%, the blue zones (zero
8
localization) means the absence of electrons in the area, and the green zones
9
corresponding to a free electron gas behavior that indicates the border of covalent bonds
10
(50% localization). The Fig. 6 (a)-(c) illustrates the ELF contours map for the Mo–O
11
bonding's, in these cases the Mo atom and the neighboring O atom share a green area
12
(value of 50%) showing a significant electron-pair density, which means the formation
13
of strong bond formation. Whereas, the Zn–O bonding's present a lower electron-pair
14
density (value of 25%). It is important to note that those data are in accordance with the
15
bond lengths ascribed on Table 2.
16
In this point it should be interesting to compare our findings with very recent
17
results published in the literature [52-54]. In the Ref. [52], a bimetallic coordination
18
cluster,
19
Co4MoO8 (where H4TC4A means p-ter-butylthiacalix[4]arene), shows the particular
20
importance of Zn (and Ni) ions for eletrocalalysis processes involving molybdates.
21
Co4MoO8 was used as working electrodes, showing improved electrocatalytic activities
22
for the oxygen evolution reaction. However, when Co is replaced in Co4MoO8 clusters
23
with Ni and Zn, an extraordinary oxygen evolution reaction performance is verified
24
because of the Co – Mo synergistic effect [52], indicating the importance of Zn in the
(NH4){CO4II(TC4A)Cl[(MoVO2)2S(CH3O)]4}(+solvent),
10
abbreviated
as
1
catalytic process. Also, a theoretical study has recently investigated the evolutional rule
2
of the Cu-Zn nanoalloy clusters with sizes [53]. Using density functional theory, authors
3
obtained the global minimum of several nanoalloy clusters, discovering the phase
4
diagrams and realizing that the possible geometric structures of Cu – Zn are dependent
5
from the total number of valence electrons, like what happen with the bulk material. The
6
study showed that clusters with even valence electron numbers are more stable and
7
depending on these numbers the compounds can appear with planar motifs, as spherical
8
structure or with prolate profiles [53]. Another thorough investigation of the role played
9
by valence electrons and bonding in a particular system, Mo8Ga41, was also given
10
recently in the work presented in Ref. [54]. Authors show that the substitution of the Zn
11
for Ga leads to a shortening of the interatomic distances in the Mo8Ga41 compound and,
12
at the same time, decrease of the electron valence count for the mixed system. An
13
additional interesting feature noted by the calculation is the possibility of formation of
14
Zn clusters in the crystal structure [54]. Such aspect is not glimpsed in our investigation
15
on ZnMoO4, where no evidence of cluster formation is achieved.
16
5. Conclusions
17
In summary, this paper reported the detailed vibrational and electronic properties of the
18
P1 ( ) triclinic phase of zinc molybdate α-ZnMoO4 polycrystals. The experimental and
19
theoretical Raman and infrared vibrational properties were discussed considering first-
20
principle calculations through density functional perturbation theory (DFPT). The
21
assignments of the Raman and infrared modes were found to be in good agreement with
22
experiments. Additionally, first-principle calculations under the local density
23
approximation were used to study the electronic properties of the ZnMoO4 system in
24
relation to the band structure and its associated PDOS. The coordinates corresponding 11
1
to the high-symmetry points in the first BZ are Γ(0,0,0), Y(0,1/2,0), T(0,1/2,1/2),
2
Z(0,0,1/2) and X(1/2,0,0). ZnMoO4 presents an indirect band gap from the Γ point on
3
the valence band to the X point on the conduction band. The calculated electronic gap of
4
3.35 eV, in litne with the well-known trend of DFT-LDA underestimation of band-gaps,
5
confirms that the ZnMoO4 crystal is a semiconductor compound.
6
7
Acknowledgments
8
G.D. Saraiva acknowledges support from MCT/CNPq Edital 14/2010 (process
9
476569/2010-9), FUNCAP/Edital 02/2010 (process BP10031001350100/10) and
10
FUNCAP/Edital 05/2009 (process 186.01.00/09). PTCF thanks FUNCAP/CNPq for
11
grant PRONEX PR2-011-00006.01-00/15. We acknowledge the support from
12
FAPEPI/MCT/CNPq/CT – INFRA Nº 010/2009. The other authors acknowledge CNPq
13
and FUNCAP for partial financial support.
14
15
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13 14 15
16
17 18
19 20 21 22 23 24 25 26 27 28 29 30
16
1
List of Figures
2
3
Fig. 1: Unit cell of the ZnMoO4 crystal in the monoclinic phase, P1 ( ), space group.
4
Fig. 2: Experimental and calculated (scaled) Raman spectra of ZnMoO4 in the (a) 40 to
5
700 cm−1 region and in the (b) 700–1100 cm−1 region.
6
Fig. 3: Experimental and calculated (scaled) IR spectra of ZnMoO4 in the 120 to 1020
7
cm−1 region.
8
Fig. 4: (a) The LDA calculated band structure of ZnMoO crystal around the region of
9
the main energy band gap. (b) The projected density of states displaying the
10
contributions of the Zn (yellow), O (red) and Mo (green) atoms is shown at the right
11
side panel, as well as the total contribution (black). (c) and (d) the spatial orbital
12
distribution of highest occupied molecular orbital, HOMO and lowest unoccupied
13
molecular orbital (LUMO), respectively.
14
Fig. 5: Zinc molybdate projected density of states (PDOS) for O, Zn and Mo: s (dotted,
15
red), p (solid, black) and d (solid, blue) orbitals.
16
Fig. 6: The ELF mapping for ZnMoO4 crystal structure. (a-c) Contours map for the Mo–
17
O bonding's. (d-f) Contours map for the Zn–O bonding's.
18 19
Fig.1S. Rietveld refinement of X-ray diffraction pattern ZnMoO4 crystals measured at
20
ambient conditions.
21 22
Fig. 2S: Unit cell of the ZnMoO4 crystal in the monoclinic phase, P1 ( ), space group
23
along of different axis.
24 25 26 27 17
1
List of Tables
2 3 4
Table 1. Observed and calculated Raman and infrared modes for ZnMoO4, together with
5
their assignments based on the basis of lattice dynamic calculations for the triclinic
6
phase.
7
Table 2. The geometrical parameters of ZnMoO4: bond lengths (Å) and bond angles (°)
8
computed at DFT-LDA and determined by X-ray diffraction data [36].
9
18
Raman ωobs 51 60 70 78 85 92
102 109 116 121 129 144 163 172 189 206 225 239
289 298 321
Raman ωcal 44.90 54.97 65.68 76.75 77.94 86.20 91.21 92.70 105.93 111.99 115.32 122.60 132.66 146.13 149.18 173.32 178.03 183.94 200.04 205.14 214.42 225.23 238.73 247.63 251.72 270.85 274.79 277.58 290.43 295.08 312.11 321.16 325.95 335.60
sym Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag
IR ωobs T(tetra)+ T(oct) T (tetra)+T(oct) Lib(tetra)+T(oct) T(tetra)+T(oct) T(tetra)+T(oct) T(tetra)+T(oct) T (tetra)+T(oct) T (tetra)+Lib(oct) T(tetra)+Sci(oct) Lib(tetra)+Sci(oct) T(tetra)+T(oct) Lib (tetra)+T(oct) Lib (tetra)+Bend(oct) Lib (tetra)+Lib(oct) Lib (tetra)+Bend(oct) Lib (tetra)+Sci(oct) Lib(tetra)+Sci(oct) Lib(tetra)+T(oct)+Bend(oct) Lib(tetra)+Sci(oct) Lib(tetra)+Sci(oct) Lib(tetra)+T(oct) Sci(tetra)+T(oct) Lib (tetra)+Sci(oct) Lib(tetra)+Sci(oct)+T(oct) Lib(tetra)+Sci(oct)+T(oct) Lib(tetra)+Lib(oct) Sci (tetra)+Bend(oct) Sci (tetra)+Lib(oct) Sci (tetra)+Bend(oct) Sci (tetra)+Bend(oct) Sci (tetra)+Bend(oct) Bend (tetra)+Bend(oct) Bend (tetra)+Bend(oct) Bend (tetra)+Bend(oct)
131 137 144 154 159 163 171 175 182 203 212 228 233 245 255 261
289 301 308 318 337 357
IR ωcal 65.68 71.99 84.93 87.43 94.80 97.36 105.31 130.17 132.66 140.86 150.38 159.10 164.19 165.80 181.16 183.79 191.78 208.22 217.34 224.66 236.33 244.16 254.10 262.44 270.98 276.26 288.21 291.65 302.91 311.84 323.60 330.86 335.52 352.13
sym Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au
Lib (tetra)+Lib(oct) T(tetra)+Lib(oct) T(tetra)+Bend(oct) Lib(tetra)+Bend(oct) Lib(tetra)+Lib(oct) Lib(tetra)+Lib(oct) T(tetra)+T(oct) T(tetra)+Bend(oct) T(tetra)+T(oct) Lib(tetra)+ Bend(oct) T(tetra)+T(oct)+Bend(oct) T(tetra)+Bend(tetra)+T(oct)+Bend(oct) T(tetra)+Bend (tetra)+T(oct)+Bend(oct) T(tetra)+Bend (tetra)+T(oct)+Bend(oct) T(tetra)+Lib(tetra)+ T(oct)+Bend(oct) T(tetra)+Bend(tetra)+T(oct)+Bend(oct) T(tetra)+Bend(tetra)+ T(oct)+Bend(oct) T(tetra)+Lib (tetra)+ T(oct)+Sci(oct) T(tetra)+Lib (tetra)+T(oct)+Bend(oct) T(tetra)+Lib (tetra)+T(oct)+Bend(oct) T(tetra)+Sci (tetra)+T(oct)+Bend(oct) T(tetra)+Lib(tetra)+T(oct)+Bend(oct) T(tetra)+Lib(tetra)+T(oct)+Bend (oct) T(tetra)+Bend (tetra)+T(oct)+Lib(oct) Sci(tetra)+Bend(oct) Lib(tetra)+Lib(oct) Bend(tetra)+Bend(oct) Bend(tetra)+T(oct) Bend(tetra)+Bend(oct) Sci(tetra)+Sci(oct)+Bend(oct) Sci(tetra)+Sci(oct)+Bend(oct) Sci(tet)+Bend (tetra)+Sci(oct)+Bend(oct) Sci(tetra)+Sci(oct) Sci(tetra)+Sci(oct)
331 338 343 369 404 417 727 755 788 817 842 862 883 896 932 949 957 969
338.62 343.45 373.92 384.58 396.53 417.81 422.21 458.44 725.80 775.07 796.00 826.79 835.53 865.25 893.77 902.80 939.69 956.27 962.21 981.05
Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag Ag
Bend(tetra)+Bend(oct) Bend(tetra)+Bend(oct) Bend(tetra)+Bend(oct) Bend(tetra)+Bend(oct) Bend(tetra)+Bend(oct) Bend(tetra)+Bend(oct) Bend(tetra)+Bend(oct) Ben (tetra)+Bend(oct) υ1 (tetra)+Bend(oct) υ1 (tetra)+Bend(oct) υ1 (tetra)+Bend (oct) υ3 (tetra)+Bend (oct) υ3 (tetra)+Bend (oct) υ3 (tetra)+Bend (oct) υ3 (tetra)+Bend (oct) υ1 (tetra)+Bend (oct) υ1 (tetra)+Bend (oct) υ1 (tetra)+υ3 (oct) υ1 (tetra)+υ3T(oct) υ1 (tetra)+υ1T(oct)
387 405 432 455 739 753 798 806 843 867 892 911 940 969 987
370.29 378.21 392.11 423.45 429.29 460.50 734.73 762.29 800.55 802.52 846.82 859.85 900.11 904.67 923.84 949.82 960.26 1001.19
Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au
Sci(tetra)+Sci(oct) Sci(tetra)+Sci(oct) Sci(tetra)+Sci(oct) Bend (tetra)+Bend(oct) Bend (tetra)+Bend(oct) Sci (tetra)+Bend(oct) υ3(tetra)+Bend(oct) υ3(tetra)+Bend(oct) υ3(tetra)+Bend(oct) υ3(tetra)+Bend(oct) υ3(tetra)+υ3(oct) υ3(tetra)+υ3(oct) υ1(tetra)+υ3(oct) υ3 (tetra)+υ3(oct) υ1(tetra)+υ1(oct) υ1(tetra)+υ3(oct) υ1(tetra)+υ3(oct) υ3(tetra)+υ1(oct)
Tetra: Tetrahedron, Oct: Octahedron, Bend: Bending, Lib: Libration, T: Translation, Sc: Scissor, υ1: Symmetric Stretching, υ3: Antisymmetric Stretching.
Table 2. The geometrical parameters of ZnMoO4: bond lengths (Å) and bond angles (°) computed at DFT-LDA and determined by X-ray diffraction data [36].
Bond Length (Å) Calc.
Exp.
∆%
Bond Angle (º)
Calc.
Exp.
∆%
Zn1–O2
2.10
2.13
-1.03
Mo3-O1-Zn2
167.24
164.80
1.48
Zn1–O6
2.06
2.11
-2.61
Zn1-O2-Zn3
98.84
99.20
-0.36
Zn1–O7
2.01
2.04
-1.39
Zn1-O2-Mo3
116.36
121.80
-4.47
Zn1–O8
2.00
2.07
-3.05
Zn2-O3-Zn3
98.27
97.00
1.31
Zn1–O11
1.98
2.03
-2.44
Zn2-O3-Mo3
123.37
125.70
-1.85
Zn1–O12
2.05
2.08
-1.50
Zn2-O4-Zn2
99.99
99.30
0.69
Zn2–O1
1.98
2.06
-3.60
Zn2-O4-Mo1
141.57
138.8
1.99
Zn2–O3
2.03
2.02
0.86
Zn3-O5-Mo2
148.43
150.60
-1.44
(Zn2–O4)a
2.24
2.22
0.84
Zn1-O6-Zn3
98.49
100.30
-1.81
(Zn2–O4)a
1.94
1.98
-1.58
Zn1-O6-Mo1
141.02
133.40
5.71
Zn2–O9
2.00
2.05
-2.50
Zn1-O7-Mo2
167.17
169.80
-1.55
Zn2–O10
2.12
2.12
0.17
Zn1-O8-Mo3
153.01
148.60
2.97
Zn3–O2
1.95
2.03
-3.83
Mo2-O9-Zn2
155.86
160.10
-2.65
Zn3–O3
2.05
2.11
-2.64
Zn2-O10-Zn3
96.90
96.80
0.11
Zn3–O5
1.90
1.95
-2.64
Zn2-O10-Mo2 129.49
129.20
0.23
Zn3–O6
2.01
2.01
0.11
Zn1-O11-Mo1 160.63
152.40
5.40
Zn3–O10
1.98
2.03
-2.14
Zn1-O12-Mo1 132.98
138.30
-3.85
Mo1–O4
1.78
1.78
0.07
Mo1–O6
1.79
1.78
0.69
Mo1–O11
1.73
1.74
-0.58
Mo1–O12
1.77
1.77
-0.17
Mo2–O5
1.76
1.75
0.59
Mo2–O7
1.74
1.75
-0.38
Mo2–O9
1.74
1.74
0.29
Mo2–O10
1.80
1.81
-0.40
Mo3–O1
1.73
1.72
0.98
Mo3–O2
1.79
1.78
0.71
Mo3–O3
1.83
1.84
-0.45
Mo3–O8
1.72
1.71
0.89
Universidade Estadual do Ceará Faculdade de Educação Ciências e Letras do Sertão Central CEP 63.900-000 Quixadá, Ceará Phone: 55-88 – 3445 1036
Quixadá, October 2nd, 2019.
“Journal Molecular Structure”
Highlights Raman and infrared vibrational spectroscopic studies of the ZnMoO4. First principle calculation through the density functional perturbation theory (DFPT). A theoretical study on the electronic properties of the ZnMoO4. The band structure and the associated projected density of states (PDOS).
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: