Impact of A cation size of double perovskite A2AlTaO6 (A = Ca, Sr, Ba) on dielectric and catalytic properties

Accepted Manuscript Impact of A cation size of double perovskite A2AlTaO6 (A = Ca, Sr, Ba) on dielectric and catalytic properties I. Gorodea, M. Goanta, M. Toma PII: DOI: Reference:

S0925-8388(15)00409-0 http://dx.doi.org/10.1016/j.jallcom.2015.01.310 JALCOM 33375

To appear in:

Journal of Alloys and Compounds

Received Date: Revised Date: Accepted Date:

12 June 2014 14 November 2014 31 January 2015

Please cite this article as: I. Gorodea, M. Goanta, M. Toma, Impact of A cation size of double perovskite A2AlTaO6 (A = Ca, Sr, Ba) on dielectric and catalytic properties, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/j.jallcom.2015.01.310

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Impact of A cation size of double perovskite A2AlTaO6 ( A = Ca, Sr, Ba) on dielectric and catalytic properties I. Gorodeaa), M. Goantaa), M. Tomaa) a) Faculty of Chemistry, Alexandru Ioan Cuza” University of Iasi, Bulevardul Carol I no. 11, 700506, Iasi, Romania

Abstract Double perovskite-type oxide A2AlTaO6 materials, where A = Ca, Sr and Ba, were prepared by using conventional solid state reaction. The role of different A-site cations on their synthesis, structures, dielectric and catalytic properties was investigated. Double perovskite oxide structures were evaluated using X-ray diffraction (XRD). As the average cation size decreases, the crystallographic structure at room temperature evolves from cubic to monoclinic. The influence of the nature of the divalent A-site cations on the dielectric properties was evaluated by resistivity measurements in the frequency range of 10 Hz–106 Hz. It can be found that relative permittivity and dielectric loss regularly changed with A cation size. Catalytic properties of the obtained compounds were evaluated in water splitting process, under gamma-rays irradiation emitted by a 60Co source for the first time. From experimental data it was noticed that the double perovskite Ca2AlTaO6 had a higher catalytic effect.

Keywords: Double - perovskite, Solid state reaction; X-ray diffraction; Dielectric properties; Catalytic properties

1. Introduction Complex metal oxides with the general formula A2B’B’’O6, where B’ and B’’ sites are occupied alternately by different cations, depending on their valences and relative ionic radii, are known as double perovskites or epsolites [1]. The perovskite type oxides have some flexibility in the chemical composition and the crystal structure; the combination of many kinds of ions and the control of their crystal structure are possible. The modification of structural and magnetic properties by changing the A, B’ and/or B’’ site cations has gained interest in recent years in order to better understand the mechanism of colossal magnetoresistance [2]. If in the unit cell of a perovskite structure exist two cations ordered on the B site this is becoming a unit cell of a double perovskite structure as in AB0.5xB’0.5-0.5xO3, which corresponds to the well known formula A2B’xB’’1-xO6. Due to the technological importance substantial research efforts have also been devoted to understand the electronic and magnetic properties of the doubleperovskite compounds containing magnetic ions in B positions [3, 4]. In general, the size of A ion influences the crystal symmetry significantly while that of the B ion does not change the symmetry, but changes the lattice volume proportionally [5, 6]. Complex tantalum oxides have been of interest for their photocatalytic activity [7, 8] ferroelectric behavior [9] and their dielectric properties [10, 11]. This mixed oxides with perovskite structure have many applications such as materials, dielectric resonators, voltage controlled oscillators and filters duplex in mobile phones and satellite communications that operating in the microwave frequency range (300 MHz to 30GHz). Dielectric materials used in these applications must have a high dielectric constant (or a higher relative permittivity) and a small dielectric loss (tan δ <0.001). The dielectric constant of a compound is dependent on

2

the conditions choice of synthesis. Thus porosity compound crystallites and ionic conductivity are factors that can decrease the dielectric constant and dielectric loss may increase compound. For this reason, compounds with a density greater than 90% are required to have very good dielectric measurements [12, 13]. A variety of compounds have been studied as catalysts active in the radiolysis of water, for example compounds of lamellar structure are very interesting because of the possibility to change the chemical composition and microstructure as well and by changing the ions and the interaction. It was also observed that the double perovskites may favor water radiolysis due to their particular structure and large contact surface [14]. The objective of this investigation was to prepare, using conventional solid state method, and investigate the structural, dielectric and catalytic properties of the double perovskite-type oxides with the general formula A2AlTaO6 (A = Ca, Sr, Ba).

2. Experimental Details 2.1 Sample preparation The polycrystalline sample of A2AlTaO6 (A = Ca, Sr, Ba) was prepared by a standard ceramic method from a stoichiometric mixture of ACO3, (SrCO3, BaCO3), CaO, Al2O3 and Ta2O5 with a purity ranging between 99.99 and 99.999%. These carbonates and oxides were dried at 120oC for 2 h before weighing. Afterwards, stoichiometric amounts of these powders were ground and subjected to thermal treatment at 1100oC/24h. The calcined materials were reground and pressed in disks and were sintered in air at 1200oC and 1300oC for several days with intermediate regrinding and repelletizing. This procedure (grinding, pelletizing and firing) was repeat until single-phase perovskite could be obtained. Analysis of crystalline

3

structure and identification of perovskites phases were performed by means of X-ray diffraction technique. 2.2 X-ray Diffraction Analysis X-ray diffraction (XRD) patterns of the sample were recorded with a SHIMADZU LabX6000 diffractometer equipped with a graphite monochromator and CuKα radiation (λ = 1.5406 Å). The samples mounted in reflection mode were analyzed in ambient atmosphere with scanning rate of 0.02◦ s-1 over the 2θ = 20–80◦ range. 2.3 IR- spectroscopic characterization IR spectra were obtained using a JASCO 660 PLUS spectrophotometer with wave number range 4000 – 400 cm-1 used to complete the structure studies. The samples were mixed with KBr in the mass ratio 0.04:1, and then compacted into pellets with a thickness of 0.5 - 0.75 mm and a diameter of 13 mm under a pressure of 0.3 GPa in atmospheric air. 2.4 Dielectric measurements The frequency dependence of dielectric permittivity and dielectric losses were studied using a Agilent 4292-1 device in the range 10 Hz – 106 Hz . The samples obtained after sintering at 1300 °C were mixed very well with a polyvinyl alcohol type densification and was pressed again in the form of pellets. The pellets were pressed into a cilindrical disk at 400 Kpa/cm2 and were inserted between two electrodes. 2.5 Catalytic measurements The catalytic properties of double perovkites A2AlTaO6 with different interlayer cations (A = Ca, Ba, Sr) was studied for pure water splitting under γ-rays irradiation, for the first time. The irradiation is performed using 60Co as a source with

4

3·104 Ci activity and 8.3kGy/h dose rate, which simulated the radioactive wastes, resulted from reprocessing of spent nuclear fuel elements much more active: 108-109 Ci. The stable products of radiolysis, as well as the other chemical species are measured by mass spectrometry. Sample preparation for the analysis followed several steps. In the 30 ml glass vials different quantities of each catalyst were introduced and 5 ml of double distilled water were poured. Each vial was tightly closed with rubber cork and outside paraffined to hinder the release of gaseous radiolysis product. These vials were γirradiated in different experimental conditions, under different dose rates, at IFINBucharest. The radiolysis products were analyzed by mass spectrometry (Hiden Analytical Mass Spectrometer) using an original connection device between the irradiated sample and the spectrometer, based on a metallic capillary.

3. Results and discussion 3.1 Crystal structure Perovskites with general composition A2M13+M25+O6 have a tendency to the atomic ordering of M1 and M2 ions, and in some cases, the perovskite lattice parameters become double, indicating the setting of a long-range crystallographic ordering of metallic ions. The X-ray powder diffraction profiles collected for A2AlTaO6 (A = Ca, Sr, Ba) are shown in Fig. 1.. The recorded patterns present sharp and well-defined peaks, indicating that the as prepared materials have a highly crystalline nature. Insert figure 1

The strongest reflection peak at 2θ of around 30°, assigned to the (112), (110) and (220) the XRD diffraction planes, was identified in the pattern of each sample, confirming the formation of the double perovskites phase [15]. In the case of

5

Sr2AlTaO6 and Ba2AlTaO6 materials all the peaks in the XRD pattern fit well to a cubic unit cell with Fm3m space group. The Ca2AlTaO6 system have monoclinic structure with the space group P/21/n. Structural parameters : tolerance factor (t), latice parameters, cell volume (V), cell angle (β) evaluated using Structure Prediction Diagnostic Software (SPuDs) [16] are summarized in Table 1. Insert table 1 From Table 1 a monotonic increase of lattice parameters and, consequently, increase in cell volume with increase of the A site cationic effective ionic radii (rCa2+ < rSr2+ < rBa2+ ) are observed. The increase of the effective ionic radii of the A cation leads to a increase of the tolerance factor wich determines the transition from the distortion monoclinic structure to the ideal cubic perovskite structure. It must be mentioned that the tilt angle and the tolerance factor give contribution to the ideal cubic structure distortion of the double perovskite [17]. When the tolerance factor value is smaller than the unity, the compound presents a structure with a lower symmetry, different from the cubic one. [18] In our case the tolerance factor increases with increasing of the ionic radius in the serie rCa2+ < rSr2+ < rBa2+, and we can say therefore that when A = Sr we obtain perovskite-type cubic structure with a high degree of order and symmetry, which will be seen and properties of this compound. The FTIR spectra of the perovskite structure have three characteristic absorption bands between 850 - 400 cm-1, respective to composition and these are usually used to identify the perovskite phase formation [19]. All spectra present the typical band pattern caracteristic of the perovskite structure: the strong high energy band centered at about 660 cm-1 can surely be

6

assigned to the antysimmetric stretching mode of TaO6 octahedra due to higher charge of this cation, a band at 840 cm-1 which can be assigned to the symmetric stretching vibration of these octahedra and the strong IR-band at around 450 cm-1 which can be assigned to the Ta(Al)O6 deformation (figure 2). Insert figure 2 The size of the A cation seems to be relevant to the structural distorsion; in our case the tolerance factor increase with the size of cation A and all the IR-bands are displacement to the lower frequencies For A=Sr the presence of the shoulder at the 840 cm-1 can by assigned to symmetric streching vibration of TaO6 octahedra and for A = Ba the band spectrum are the lower energy because the greater Ba(II) cations determined the increase of unit cell and a slight diminution of the bond strenght. For all the compounds with A = Ca, Sr and Ba, the presence of these bands confirm the formation of perovskite phase.

3.2 Dielectric properties Variation of real (ε’), imaginary (ε”) parts of dielectric constant and dielectric loss tangent (tan δ) in the frequency range 102 Hz – 105 Hz for the A2AlTaO6 system is shown in figure 3 (a)-(c). Variation of dielectric constant with frequency indicates a normal dielectric dispersion due to Maxwell-Wagner [20, 21] type interfacial polarization in accordance with Koop’s phenomenological theory [22]. Among the compounds examined, only the formula Sr2AlTaO6 reported in the literature with significant dielectric properties [23]. Figs.3 a, b show that the real and imaginary part of dielectric constants decrease with increasing frequency and then reach a constant value at high frequency.

7

The

Maxwell–Wagner

relaxation

mechanism

is

associated

with

uncompensated surface charges at interfaces inside the composite ceramics. Such phenomenon is determined by the local inhomogeneity of sample caused by a high number of charged defects located inside the ceramic grains and at the grain boundaries, which produce local charge unbalance in the sample volume. These charges are created by external (contacts) or internal (grain boundaries, domainwalls, inhomogeneities) boundary layers and induce local conductivity variation. Fig.3 (a) show that the dielectric constant dispersion is at a maximum for Sr2AlTaO6 because degree of symmetry is the highest. The reduction of the dielectric constant value can be attributed to structural changes caused by the size of the A cation . Insert figure 3 a), b), c) Figure 3 (c) shows the variation of dielectric loss tangent (tan δ) with frequency , measured at room temperature. It can be observed that all materials investigated show normal dielectric behavior. Dielectric loss tangent decreases as the frequency of the alternating field increases. Maximum dispersion in the loss tangent for the all the sample is obtained at low frequencies and it decreases linearly with increase in frequency. Loss values are very small, ranging between 0.04 and 0.7 for the Sr2AlTaO6 a maximum value of the tan δ is observed. As stated above and taking into account the experimental observations according to which the dielectric constant and the dielectric loss tangent decreases with increasing frequency, all synthesized materials show a normal dielectric behavior. Dielectric constant and dielectric loss tangent are influenced by size of A site cation, structure, porosity and chemical homogeneity of the compounds. 3.3 Catalytic properties

8

Photocatalytic water splitting in the presence of perovskites has received an extensive attention, as it can provide a clean and renewable source for hydrogen fuel. Li et al. investigated the photocatalytic properties of water splitting over a series of perovskite-type compounds A2MM’O6 (A=Ca, Sr, Ba; M=Ni, Co, Zn, Mg; M’= Mo, W). Although H2 and O2 evolution was observed under ultraviolet or visible light irradiation in the presence of CH3OH or AgNO3 as sacrificial reagents, it has been found that most ofthese compounds suffer instability in aqueous solution under light irradiation and only Ca2NiWO6 remained chemically stable [24]. An ideal catalyst should not react with the solvent used must be resistant to the action of light or nuclear radiation. The compounds of formula A2AlTaO6 have been used, for the first time as catalysts in the radiolysis of water. It’s generally known that the radiolysis of water leads to the formation of different chemical species, such as: H2, O2, H2O2, HO•, O, HO2•, etc. via a number of reactions outlined below, as: H2O → H2O* → H• + HO

(1)

+

(2)

H2O → H2O + eH2O+ e- → H2O+

(3)

H2O + H2O- → 2H2O* → 2H. + 2HO

(4)

H• + H• → H2

(5)

HO• + HO• → HO2• + H•

(6)

H• + O → HO

(7)

HO2• + HO2• → H2O2 + O2

(8)

H2O2 +HO. → HO2• + H2O

(9)

H• +O2 → HO2•

(10)

Radiolytic yield of hydrogen, GH2 (number of transformed or appeared molecules for 100eV absorbed energy, by γ-ray irradiation) was calculated using a formula deduced from Henglein expression [25]:

9

G H2 =

b⋅ Ix ·9.66.106 D ⋅ t ⋅ ρ ⋅ I et

(11)

where: D·t = Da means absorbed dose (1J/kg or 6.24·1015 eV/g) ρ - the density of irradiated material (g/cm3);

NA – the Avogadro number; b - hydrogen amount determined from the calibration of mass spectrometer (mole H2/1kg H2O);

Iet - is peak intensity value of molecular hydrogen resulted from the mass spectrometer calibration reaction;

Ix - is peak intensity value of molecular hydrogen resulted from the catalyzed water radiolysis; Several species (H•, HO•, HO2•, and H2O2) were found in the mass spectra, but the radiolytic yield was determined only for molecular hydrogen. The experimental results and also the catalysts efficiency in water radiolysis process were shown by the radiolytic yield values of the molecular hydrogen in several working conditions (Figs 4 a and b). Insert figure 4 a) and b) Experimental data pointed out that radiolytic yields of molecular hydrogen

increased when the absorbed dose (for a given quantity of catalyst) or perovskites mass (for the same dose) are higher and higher. The subsequent action of the gamma rays produces a radiolytical split of water, releasing a radiolytic yield GH2 higher than that produced from pure water splitting in the absence of the catalyst (0.43), irradiated in the same conditions as the samples with catalyst.

Explaining catalytic effect: Perovskite oxides contain large concentrations of oxygen vacancies, this being a consequence of the cation stoichiometries and valence. It can be considered that the neutral water molecule will fill these oxide vacancies. Anionic vacancies (VO-) are acting as Lewis acid and the water molecules as Lewis base.

10

VO- + OOx + H2O↔ 2 OHO-

(12)

From experimental data it was noticed that the double perovskite Ca2AlTaO6 had a higher catalytic effect comparing with Ba2AlTaO6 and Sr2AlTaO6. This might be explained on the base of ionic radii variation (rCa2+ < rSr2+ < rBa2+). A smaller ionic radius will determine a higher number of anionic vacancy (VO-). Also, Ca2+ has a higher polarizing action on the water molecules, due of his smaller ionic radius. The changes in stoichiometry that occur on annealing water in the perovskite structure can also be summarized: A2AlTaO6 + xH2O → A2AlTaO6-x (OH)2x

(13)

Conclusions We have performed a detailed analysis of the influence of cation nature of the position A on the the structural, electrical and catalytic characterization of the double perovskite A2AlTaO6 (A = Ca, Sr, Ba). The synthesis method selected proved to be quite good, although in order to achieve the formation of perovskite phase without impurities is required fairly high temperature sintering. From the point of view of the ideal perovskite, Sr cation proved to be the best position for cavity type A. Thus Sr is the best size to get a cubic space group Fm3m structure with close tolerance factor 1. Other cations proved to be too small (Ca) and too large (Ba) for this cavity so that AlO6 octahedral of TaO6 will have to bend leading to changes in symmetry and unit cell volume. The dielectric constant and dielectric loss decrease with increase in frequency and reach a constant value at high frequency. Good dielectric properties of the sintered samples suggest their possible application as high frequency transformers and solid dielectric capacitors.

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On the other hand, the perovskite with the cation A = Ca due to the distortions that occur consist of several anionic vacancies so that it may be better to have the catalytic properties in the radiolysis of water.

References

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comparison of techniques, IEEE Trans. Dielectrics and Electrical Insulation 5(4), (1998), 571-577. 24. D. Li, J. Zheng, Z. Zou, „Band structure and photocatalytic properties of perovskite-type compound Ca2NiWO6 for water splitting”J. Phys. Chem. Solids, (2006), 67, 801-806. 25. A. Henglein, W. Schnabel and J. Wendenburg, Einfuehrung in die

Strahlenchemie, Verlag Chemie, Weinheim, (1969), 19-23.

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Fig. 1 X-ray diffraction patterns of Ca2AlTaO6, Sr2AlTaO6, Ba2AlTaO6 sintered at o

1300 C

3000

intensity (a.u)

Ba2AlTaO6

2000

Sr2AlTaO6 1000

Ca2AlTaO6 0 20

40

60

2Θ (degree)

16

80

o

Fig.2 FTIR spectra of A2AlTaO6, A = (Ca, Sr, Ba) sintered at 1300 C

4

A=Ba absorbance (a.u)

3

2

A=Ca

1

A=Sr

0 1000

800

600 -1

w a ven u m b er (c m )

17

400

Fig. 3 (a) The real part variation of the dielectric constant with frequency, (b) the imaginary part variation of the dielectric constant with frequency and (c) the dielectric o loss function on frequency for A2AlTaO6 sintered at 1300 C

Real part of permitivity ε'

400

300

Sr2AlTaO6

200

Ca2AlTaO6 Ba2AlTaO6

1000

1000000

100000

10000

Frequency (Hz)

Fig. 3(a) Imaginary part of permitivity ε"

100

80

Sr2AlTaO6

60

40

20

Ba2AlTaO6 Ca2AlTaO6

0 10000

100000

1000000

Fig. 3 (b)

Frequency (Hz) 0.9

0.8

Dielectric loss tan δ

0.7 0.6

Sr2AlTaO6

0.5 0.4

Ba2AlTaO6

0.3 0.2 0.1

Ca2AlTaO6

0.0 1000

10000

100000

Frequency (Hz)

Fig. 3 (c)

18

1000000

Fig. 4a) Radiolytic yield vs. absorbed dose 4b) Radiolytic yield vs catalysts mass;

Radiolytic yield, GH2

0.64

Sr2AlTaO6 Ba2AlTaO6 Ca2AlTaO6

0.60

0.56

0.52

0.48

0.44 0

5

10

15

20

25

30

Absorbed dose (kGy)

Fig. 4 a)

Radiolytic yield, GH2

0.9

0.8

Sr2AlTaO6 Ba2AlTaO6 Ca2AlTaO6

0.7

0.6

0.5

0.00

0.03

0.06

Mass (g)

Fig. 4 b)

19

0.09

0.12

Table 1. Crystallographic data calculated from SPuDS for powders sintered at o

1300 C

Compoun

rA2

d

+

(Å) Ca2AlTaO

1.0

6

0

t

Cell-

V(Å3)

Averag

Averag

Averag

parameter

e Bond

e Bond

e Bond

(Å)

A-O(Å)

Al-O

Ta-

(Å)

O(Å)

0.9626 a = 5.5245 b = 5.5245

235.85

ß(˚)

89.99

2.7226

1.8765

1.8219

90.00

2.7538

1.8987

1.9953

90.00

2.8316

1.9565

2.074

7

c = 7.7278 Sr2AlTaO6 1.1

1.0178 a = 7.7878

8 Ba2AlTaO

1.3

6

5

20

461.50 4

1.0790 a = 8.078

513.50 8

Graphical abstract

21

Highlights

•

Synthesis by solid state reaction of the double perovskite - type oxide A2AlTaO6 materials, where A = Ca, Sr and Ba.

•

The role of different A-site cations on their synthesis and structures was investigated.

•

The influence of the nature of the divalent A-site cations on the dielectric properties was evaluated by resistivity measurements

•

Catalytic properties of the obtained compounds were evaluated in water splitting process, under gamma-rays irradiation emitted by a 60Co source, for the first time.

22