X-ray photoelectron spectroscopic study and electronic structure of double-perovskites A2SmTaO6 (A= Ba, Sr, Ca)

Accepted Manuscript X-ray photoelectron spectroscopic study and electronic structure of doubleperovskites A2SmTaO6 (A= Ba, Sr, Ca) Binita Ghosh, Saswata Halder, Santiranjan Shannigrahi, T.P. Sinha PII:

S1293-2558(16)30900-1

DOI:

10.1016/j.solidstatesciences.2017.02.002

Reference:

SSSCIE 5461

To appear in:

Solid State Sciences

Received Date: 11 November 2016 Revised Date:

3 February 2017

Accepted Date: 9 February 2017

Please cite this article as: B. Ghosh, S. Halder, S. Shannigrahi, T.P. Sinha, X-ray photoelectron spectroscopic study and electronic structure of double-perovskites A2SmTaO6 (A= Ba, Sr, Ca), Solid State Sciences (2017), doi: 10.1016/j.solidstatesciences.2017.02.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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Intensity ( arbitary unit)

Intensity ( arbitary unit)

800 600

Ta 4p3/2

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400

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O1s

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Ba 3d5/2

C1s

200 0

Ta 4d3/2 Ta 4d5/2 Ba 4p Sm 4d Ba 4d Ta 4f

Sm 3d

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1000

Binding Energy (eV)

Ba 3d3/2

CST

SST

BST

1200

Sm 3d

Sm 3d Ba 3p

Intensity ( arbitary unit)

O1s

Ta 4p3/2 C1s Sr 3p Ta 4d3/2

Ta 4d5/2 Sm 4d Ta 4f

O1s Ca 2s Ta 4p3/2 Ca 2p C1s Ta 4d3/2 Ta 4d5/2 Sm 4d Ta 4f

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X-ray photoelectron spectroscopic study and electronic structure of doubleperovskites A2SmTaO6 (A= Ba, Sr, Ca) Binita Ghosh1*, Saswata Halder2, Santiranjan Shannigrahi3 and T.P. Sinha2

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1

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St. Paul’s Cathedral Mission College, Department of Physics, 33/1Raja Rammohan Roy Sarani, Kolkata 700009, India. 2 Department of Physics, Bose Institute, 93/1 Acharya Prafulla Chandra Road, Kolkata 700009, India. 3 Institute of Materials Research and Engineering (IMRE), 3 Research Link, Singapore 117602, Singapore Abstract

X-ray photoelectron spectroscopy (XPS) measurements of double perovskite oxides, A2SmTaO6

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(A= Ba, Sr, Ca) are performed in the energy window of 0-1300 eV. The electronic structure investigations of AST have been performed using density functional theory. The calculated DOS is compared with the experimental DOS obtained by XPS. It has been observed that the Sm-f and O-2p states are hybridized in the valence band near the Fermi level. The chemical shifts of

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the constituent elements determined from the core-level XPS spectra deliver information on charge transfer and nature of chemical bonds. These results have been used to explain the

Keywords

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conduction mechanism in these materials.

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Double perovskite X-ray photoemission spectroscopy Electronic structure

*Corresponding author. Tel.: +91 33 23031179; fax: +91 33 23506790 E-mail address: [email protected] (Binita Ghosh).

Introduction:

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Double perovskite oxides (DPOs) of general formula A2B'B''O6 with Ba-, Sr- and Ca- at the A-site and lanthanides at the B-site have been studied from the point of view of their microwave dielectric applications, such as resonators and filters [1-9]. For the microwave applications of

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these DPO's the knowledge of their electronic structure is important. A prime object for understanding the electronic properties of solids is the determination of their band structure or its one-dimensional projection, namely the density of states (DOS). In recent years considerable

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progress has been achieved in determining these densities of states by X-ray photoelectron spectroscopy (XPS) and photoelectric methods [10]. Recently, we have studied the crystal

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structures, dielectric and vibrational properties of A2GdTaO6 (A= Ba, Sr and Ca) [11-13] and A2SmTaO6 (AST) [14]. But no data are available which directly and quantitatively compare experimental measurements of XPS of these DPOs with those derived from band structure calculations. To our knowledge, there exists no report of plane wave based basic electronic

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structure calculations of AST. With these findings in mind we have systematically measured Xray photoelectron spectra of these DPOs and tried to interpret them in terms of calculated theoretical DOS. We have attempted to clarify the electronic structure of the system as it

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emerges from a combined spectroscopic study and first-principles calculations. The aim of this work is a XPS characterization of the electronic structure of Ba2SmTaO6

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(BST) ceramic and comparison of the results with those obtained for the Sr2SmTaO6 (SST) and Ca2SmTaO6 (CST). The chemical shifts determined from the core-level spectra deliver information on charge transfer and nature of chemical bonds. The results are correlated with the structures and electrical properties of these materials.

Experimental:

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In analogy to our previous work [11-13], the conventional solid state reaction technique was used for the synthesis of AST. The starting materials were reagent grades of BaCO3 (Loba Chemie, 99% pure), SrCO3 (Loba Chemie, 99% pure), CaCO3 (Loba Chemie, 99% pure), Sm2O3

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and Ta2O5 (Alfa Aesar, 99% pure) which were taken in stoichiometric ratios for the synthesis of AST. A detail of the sample preparation has already been discussed in our previous communication [14]. The X-ray photoemission spectra of the samples were taken by X-ray

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photoelectron spectroscopy (XPS) (VG ESCALAB 220i-XL Imaging System, England). XPS profiles of the samples were acquired using monochromatic Al-Kα source with energy hν=1486.6

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eV and operated at 15 kV and 15 mA. The binding energy was determined by reference to the C 1s line at 284.8 eV. The energy spectra were analyzed with a hemispherical mirror analyzer with an energy resolution of 0.7 eV. All the measurements were done in vacuum below the pressure

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of 6 × 10−10 Torr.

Computational methods:

The electronic structure calculations for AST were initiated using the Vienna ab initio

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Simulation Package (VASP 5.2) which is a code for planewave pseudopotential density functional theory (DFT) calculations [15-17]. It uses the projected augmented wave (PAW) to

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all-electron potentials and the generalized gradient approximation (GGA) and GGA+U to the exchange-correlation potential. It is to be mentioned that due to self interaction errors f electrons are not handled well by GGA in the density functional theory. They are typically treated using GGA+U or hybrid functionals. For electronic structure calculations, we included ten valence electrons for Ba (5s2 5p6 6s2), Sr (4s2 4p6 5s2) and Ca (3s2 3p6 4s2), eight for Sm (4f 6 6s2), eleven for Ta (5p6 5d3 6s2) and six for O (2s2 2p4) ions. Effective U value of 7 eV was used for the Sm

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4f states. Spin polarized calculations were performed using the experimentally determined lattice parameters. The cubic structure with lattice parameter a = 8.475 Å having Fm3m symmetry [14] was used for BST and monoclinic structure with a = 5.826 Å, b = 5.900 Å, c = 8.281 Å, β =

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90.1620 and a = 5.570 Å, b = 5.831 Å, c = 8.081 Å, β = 89.7370 having P21/n symmetry [14] was used for SST and CST respectively. The complete structural data is presented in Table I. We relaxed the ions until a total energy convergence of better than 10-8 eV per formula unit was

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obtained using 4 × 4 × 4 mesh of k-points. The plane-wave energy cutoff and the number of ionic steps for iterations were so chosen to obtain good convergence for total energy and forces

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acting on the atoms. Binding energies were referenced to the Fermi level (EF = 0). Spin-orbit interactions were also taken into account. All magnetic moments were taken w.r.t (0,0,1) axis. This implies that the spin quantization axis is parallel to Z direction.

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Results & discussions:

The spin polarised density of states (DOS) along with the partial DOS of Sm f, Ta d and O p states of AST are depicted in figure 1. The Fermi level is indicated at 0 eV. In figure 1(a)

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for BST, in the up spin channel (shown by ↑ in the figure 1(a)) the DOS shows an insulating behaviour with a bandgap of 3.02 eV between the valence band and the conduction band. The d

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states of Ta, f states of Sm and p states of O have major contributions to the total DOS near Fermi level. The band extending from -3.93 eV to the Fermi level is composed mainly of the O 2p, Ta 5d and Sm 4f states. It is to be mentioned that Ta 5d contribution is zero at the valence band maximum but rises with increasing binding energy. Conversely, the O 2p contribution rises from zero at the conduction band minimum with increasing energy. This reflects the Ta 5d to O

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2p covalency. It means the Ta

orbitals are hybridized with the O 2p to form O–Ta–O

covalent bond. In the down spin channel (shown by ↓ in the figure 1(a)) the DOS also shows an insulating

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behaviour with a bandgap of 3.54 eV. The band from -3.93 to 0 eV is mainly contributed by the O 2p states together with bonding states of Ta 5d. The band extending from 3.60 to 5.78 eV is mainly occupied by the antibonding states of Ta 5d and Sm 4f states. It is evident from the partial

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DOS that the Ta 5d and the O 2p states are strongly hybridized with each other. Comparing the spin-up band with the spin-down, it is found that the spin-up band is almost symmetrical with the

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spin-down band except in the region from 4.69 to 5.78 eV, which is mainly due to the contribution of Sm 4f states. The valency of the Sm ion in the BST is +3 ([Xe]4f5). Since the 4f orbital is less than half filled for Sm, its spin up channel is not completely filled up. So the major part of it lies in the valence band. On the other hand the spin down part of Sm 4f is completely

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empty and totally lies in the conduction band. The DOS of SST and CST along with partial DOS of Smf, Tad and Op are shown in figure 1(b) and 1(c) respectively. SST also shows an insulating behavior with a gap of 3.38 eV in the spin up channel and a bandgap of 3.77 eV in the spin down

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channel (figure 1(b)). In CST the bandgap in the up spin channel is 3.45 eV and in the down spin channel it is about 3.98 eV (figure 1(c)). It is evident from the partial DOS features, that the

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band edges of valence and conduction bands are mainly contributed from the 2p-orbital of O and the empty d-orbital of the transition-metal Ta cation, respectively. In order to verify the electronic structure calculations experimentally, we have performed

the XPS study of AST in a wide energy window of 0-1300eV. The XPS spectra of AST are shown in figure 2, where the profiles of the spectra are identified and indexed. Except carbon, no other contamination is found in the experimental spectra. Long time exposure to atmosphere

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and/or trace of the starting materials may be responsible for carbon contamination in the studied samples. We see a distinct shift in the peak positions of Ta, Sm, Ba, Sr and Ca when compared

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with the pure element. The spectra of the Ta 4p, Sm 3d, Ba 3d, Ba 4p, Sr 3p and Ca 2p states have been investigated and the values of the binding energies of the experimental emission lines are tabulated in Table II. The peak position of Sm 4d at 135.42 eV is shifted in comparison with

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the pure Sm metal where the main line is at 141.73 eV. The splitting of the Sm 3d doublet with the most prominent peaks at 1083 eV and 1111 eV for 3d5/2 and 3d3/2 respectively is found to be

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28 eV which is comparable with the values characteristic of Sm metal. The location of Ta 4f is also modified with respect to pure Ta at 43.52 eV. Ta 4p states have a natural linewidth. The valence band photoemission spectra of the samples are shown in figure 3. It is clear from the calculated total DOS and partial DOS in figure 1 that the valence band spectrum

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exhibits the domination of the O 2p states which are strongly hybridized with Sm 4f and Ta 5d states which results in the broad peaks in the XPS spectra. The valence band spectra are generated from the calculated DOS where the partial DOS of Sm 4f, Ta 5d and O 2p are

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multiplied by their respective atomic photo-emission cross-sections for 1486.6 eV [18] and are added. The added DOS spectra are convoluted with a Lorentzian of 0.4 eV full width at half

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maximum (which is close to the experimental resolution). The calculated spectra are compared with the respective experimental valence band XPS spectra, as shown in figure 3. It has been observed that the calculated DOS of AST are qualitatively similar to that of the XPS spectra in terms of spectral features, energy positions and relative intensities. The calculated DOS spectra exhibit sharper peaks than the experimental spectra since we have not included the lifetime

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broadening in the DOS curve. We observed similar reduction of bandwidth in other DPOs [19, 20] also. The core level line of O1s in SST is broadened in comparison to that of BST (figure 4).

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This is because BST is a compound with higher symmetry and structural order (cubic, Fm3m) than SST (monoclinic, P21/n) [14]. For the calcium compound, the broadening is even more (figure 4), in agreement with its stronger distortion from cubic crystal symmetry. The presence of

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a small satellite peak at around 529.51 eV in SST and CST signifies the presence of absorbed oxygen in addition to the lattice oxygen.

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The high resolution core level spectra of Ta 4d for AST are shown in figure 5. The splitting of the Ta 4d into 4d3/2 and 4d5/2 is due to the spin-orbit interaction. The spin orbit splitting of Ta 4d level is about 12.43 eV for BST, 12.39 eV for SST and 11.16 eV for CST. We have also compared the XPS spectra of Ta 4d with the corresponding convoluted

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partial DOS spectra as shown by dotted lines in figure 5. All the calculated spectra appear to be very similar with the peak positions and relative intensities of XPS spectra, thus implying a good agreement between the experimental results and our theoretical calculations.

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The high resolution core level experimental spectra of Ta 4f, splitted into 4f5/2 and 4f7/2 are also shown in figure 6. It is evident from figure 6 that there is a shift in the two peak

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positions of Ta 4f as we move from BST to SST and CST. Table III gives a comparison of the experimental binding energies (BEE) of the main lines of the elements present in BST, SST and CST with their literature values. BEE is the minimum energy needed to remove an electron from a particular orbital of the atom. In BST, the BEE of Ta 4f7/2 is 25.27 eV, whereas that in SST and CST it increases progressively (Table III). This indicates that the electron density around Ta atom decreases as we move from BST to SST and then to CST. The electronegativity of O atom

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is very high (3.44). Decrease in electron density around Ta means that the bonding energy between Ta and O also decreases. In other words delocalization of electrons around O ion decreases with decrease in electron density around Ta. The extent of the decrease is correlated to

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the degree of the delocalization, which can be described as the sequence of CST
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narrower, and it is easier for the electron to transfer from valence band to conduction band. We have calculated the band gap as stated earlier and found out that band gap of BST
(∆) for O 1s, Ta 4d5/2 and Ta 4f

7/2.

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To explain the conductivity mechanism in AST we have calculated the chemical shifts The obtained chemical shifts suggest mixed ionic-covalent

character of the chemical bonds. The chemical shift (∆) in O1s is -0.39 eV and in Ta 4d5/2 is +4.47 eV in BST. The negative and positive values of ∆ for O and Ta respectively suggest a

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strong coupling between these elements in BST. For SST the chemical shift is -0.14 eV for O1s and +4.49 eV for Ta 4d5/2. For CST the chemical shift is +0.1 eV for O1s and +4.66eV for Ta 4d5/2. Due to positive values of chemical shifts for Ta and O there exists a weaker coupling

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between these elements in CST. Hence one can conclude that the bond between O and Ta has a strong covalency in BST than that in SST. CST has got the least of all. This suggests a smaller

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value of conductivity for CST than SST than BST which has been observed experimentally in our earlier communication [14]. Table I gives the values of the bond lengths and bond angles [14] for all the three samples. It is observed that the bond angles for all the three oxygen atoms in SST and CST are less than that of BST. Hence, the hybridization between O-2p states and Ta-5d states arising from electron transfer interaction is weakened in CST and SST and may be responsible for the

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decrease in conductivity of CST than SST and BST (ConductivityCST < ConductivitySST < ConductivityBST). Conclusion:

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X-ray photoelectron spectroscopy (XPS) measurements of double perovskite oxides, Ba2SmTaO6 (BST), Sr2SmTaO6 (SST) and Ca2SmTaO6 (CST) are performed in the energy window of 0-1300 eV. Density functional theory calculations reveal a direct energy gap of 3.02 eV, 3.38 eV and

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3.45 eV for BST, SST and CST respectively. The calculated DOS has been compared with the valence band XPS spectra. It has been observed that the Sm-f and O-2p states are hybridized in

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the valence band near the Fermi level. In addition, it also reveals that the electrical properties of the samples are dominated by the interaction between transition-metal (Ta) and oxygen ions. The chemical shifts of these compounds suggest a mixed ionic and covalent character of the bonds,

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which has been used to explain the electrical properties of the systems.

References:

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[2]

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[8]

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H. Haeuseler,

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Blachnik, P. Balan, N. Santa and M. T. Sebastian, J. Appl. Phys., 88, 2813 (2000).

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[10]

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B. Ghosh, S. Halder and T. P. Sinha, J. Am. Ceram. Soc., 97, 2564 (2014).

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J. J. Yeh and I. Lindau, Atomic Data & Nuclear tables, 32, 1 (1985).

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[21]

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[11]

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Table I: XRD fitting and structural parameters for BST, SST and CST [14] x

y

z

Ba 0.2500

0.2500

Lattice Bond length (Å) Bond angle parameters 0.2500 a = b = c = 8.475 Å = 2.4482 = 1800

BST Sm 0.0000

0.0000

0.0000

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Atoms

= 2.1287

0.5000

0.5000

0.5000

O

0.264(2) 0.0000

0.0000

Sr

0.0045

0.0346

0.2583 a = 5.826 Å, b = 5.900 Å = 2.4420 = 1610

Sm 0.5000

0.0000

0.0000 c = 8.281 Å, β = 90.1620 = 2.1173

0.5000

0.0000

0.5000

O1 0.2580

0.2869

0.0335

O2

0.2808 -0.2114

O3 -0.1083

Ca

0.4872

Sm 0.5000

0.4882

0.2295

0.5571 0.2437 a = 5.570 Å, b = 5.831 Å = 2.4041 = 1480 0.0000 0.5000 c = 8.081 Å, β = 89.7370 = 2.1240 0.0000 0.0000

O1 0.3145

0.2863 0.0512

O2 0.2153

0.8060 0.0504

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CST Ta 0.5000

O3 0.5967

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0.1477

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SST Ta

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Ta

0.0717 0.2704

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3d

Ba

4p

189.31

179.45

Sr

3p

270.28

268.58

Ca

2p

356.16

346.59

Sm

3d

1109.33

1082.16 [BST] 1082.32 [SST] 1082.70 [CST]

389.02

402.74 [BST] 400.85 [SST] 402.74 [CST]

4p

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Ta

BEE (eV) 799.03

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Ba

BEC (eV) 809.21

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Table II: Comparison of binding energies of different elements

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BEE: experimental binding energy; BEC: binding energy literature value [21].

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Table III. XPS line positions and chemical shifts of O 1s, Ta 4d5/2, Ta4f 7/2, obtained for BST, SST and CST.

BEC (eV)

531.6eV

225.34eV

BST BEE (eV) ∆ (= BEE- BEC)

531.21eV -0.39

229.32eV +4.47

SST BEE (eV) ∆ (= BEE- BEC)

531.46eV -0.14

229.83eV +4.49

CST BEE (eV) ∆ (= BEE- BEC)

531.71eV +0.1

Ta4f 7/2

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Ta 4d5/2

21.70eV

25.27eV +3.57

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O1s

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Binding Energy

230.13eV +4.79

25.33eV +3.63

25.68eV +3.98

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BEE: experimental binding energy; BEC: binding energy literature value [21].

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Figure captions: Fig. 1. Total DOS and partial DOS of Sm f, Ta d and O p of BST (a), SST (b) and CST (c). Zero of the energy is set at the Fermi energy. Fig. 2. XPS spectra of BST, SST and CST.

Fig. 4. The XPS O 1s lines of BST, SST and CST.

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Fig. 3. Comparison of the XPS spectra in the valence band region with the calculated spectra (dotted lines) for BST, SST and CST.

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Fig. 5. The XPS and convoluted DOS spectra (dotted lines) of Ta 4d for BST, SST and CST.

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Fig. 6. The XPS Ta 4f lines of BST, SST and CST.

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Figure 1

0.0

-5 3 0

-3 12

-2 18 Smf

0

Smf

0

-15 35

0 -28

-5 15

Smf

0

-15 24 0

-45

-45

-10 -5 0 5 10 Energy (eV)

Ta d

0

0

Ca2SmTaO6

-10 -5 0 5 10 Energy (eV)

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-10 -5 0 5 10 Energy (eV)

Sr2SmTaO6

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Ba2SmTaO6

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Ta d

DOS (states/eV)

0

-2 5

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DOS (states/eV)

Tad

Op

0

0

-1.5 2

DOS (states/eV)

2

Op

Op

-22 28

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5

1.5

Fig. 1(a)

Fig. 1(b)

Fig. 1(c)

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Figure 2

800

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Ba 3d5/2

600

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Ta 4p3/2

C1s

200

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400

Binding Energy (eV)

Ba 3d3/2

Ta 4p3/2

0

Ta 4d3/2 Ta 4d5/2 Ba 4p Sm 4d Ba 4d Ta 4f

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Sm 3d

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1000

Ba 3p

O1s

CST

SST

BST

1200

Sm 3d

Sm 3d

Intensity ( arbitary unit)

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Intensity ( arbitary unit)

Intensity ( arbitary unit)

C1s Sr 3p Ta 4d3/2

Ta 4d5/2 Sm 4d Ta 4f

O1s

O1s Ca 2s Ta 4p3/2 Ca 2p C1s Ta 4d3/2 Ta 4d5/2 Sm 4d Ta 4f

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Figure 3

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Experimental

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CST

Intensity (arbitary unit)

Experimental

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SST

BST

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Experimental

10

8

6 4 2 Binding Energy (eV)

0

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Figure 4

O1s

CST

O1s

SST

BST

538

O1s

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FWHM 2.59

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FWHM 3.76

536

534

532

530

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Binding Energy (eV)

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Intensity ( arbitary unit)

FWHM 4.32

528

526

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Figure 5

4d5/2

4d3/2

CST

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Ta 4d

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Experimental

Experimental SST

4d5/2

4d3/2

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Ta 4d

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Experimental

4d5/2

4d3/2

Ta 4d

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BST

250

245

240

235

230

Binding Energy (eV)

225

220

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Figure 6

Ta 4f5/2

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Ta 4f7/2

Ta 4f7/2

Ta 4f5/2

Ta 4f5/2

32

Ta 4f7/2

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BST

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SST

30

28

26

24

EP

Binding Energy (eV)

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Intensity ( arbitary unit)

CST

22

20

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Research highlights: DFT calculations of Ba2SmTaO6, Sr2SmTaO6 and Ca2SmTaO6 have been performed with VASP.

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XPS measurements are performed in the energy window of 0 - 1300 eV.

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The calculated DOS has been compared with the valence band XPS spectra.

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Chemical shifts from XPS spectra have been used to explain the conduction mechanism.

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