Influence of Na doping on the magnetic properties of LaFeO3 powders and dielectric properties of LaFeO3 ceramics prepared by citric sol-gel method

Author’s Accepted Manuscript Influence of Na doping on the magnetic properties of LaFeO3 powders and dielectric properties of LaFeO3 ceramics prepared by citric sol-gel method Ensi Cao, Yanrong Qin, Tingting Cui, Li Sun, Wentao Hao, Yongjia Zhang www.elsevier.com/locate/ceri

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S0272-8842(17)30488-1 http://dx.doi.org/10.1016/j.ceramint.2017.03.119 CERI14888

To appear in: Ceramics International Received date: 28 December 2016 Revised date: 16 March 2017 Accepted date: 18 March 2017 Cite this article as: Ensi Cao, Yanrong Qin, Tingting Cui, Li Sun, Wentao Hao and Yongjia Zhang, Influence of Na doping on the magnetic properties of LaFeO3 powders and dielectric properties of LaFeO3 ceramics prepared by citric sol-gel method, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.03.119 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 galley proof before it is published in its final citable 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.

Influence of Na doping on the magnetic properties of LaFeO3 powders and dielectric properties of LaFeO3 ceramics prepared by citric sol-gel method Ensi Cao*,1, Yanrong Qin1, Tingting Cui, Li Sun, Wentao Hao , Yongjia Zhang* Key Lab. of Advanced Transducers & Intelligent Control System, College of Physics & Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, People’s Republic of China

[email protected] (E. Cao), [email protected] (Y. Zhang) *Corresponding author

Abstract

The emergence of ferromagnetism in perovskite oxide LaFeO3 nanoparticles and colossal dielectric response in ceramics has inspired researchers to study the effect of various dopants on the magnetic and dielectric properties of LaFeO3 powders and ceramics. However, the influence of alkali element Na doping has not been studied yet, and the origin of such ferromagnetic behavior is still ambiguous. The primary objective of the present work is to elucidate the effect of Na doping on the magnetic properties of La1-xNaxFeO3 (x=0, 0.1 and 0.2) powders and dielectric properties of corresponding ceramics prepared by citric sol-gel method. FE-SEM results showed that the introduction of Na dopant actually resulted in the formation of nonstoichiometric La1-xFeO3 and x/2 Na2O. Compared to the canted AFM behavior for the pure powder, ferromagnetic behavior with enhanced magnetization of 2.11emu/g at 10kOe could be obviously observed at room temperature for the powder with x=0.2. XPS measurement suggested nonstoichiometric Fe/La ratio which leads to the distortion of lattice structure and enlarged canting angle between the two AFM coupled Fe sublattice should be responsible for the enhancement of magnetization in the Na-doped samples. Meanwhile, the introduction of Na dopant lowered the growth temperature of grains of the parent LFO and resulted in larger average grain size, which in turn leaded to great enhancement of ε′ into the order of 105 at 100Hz at the cost of high tanδ for the Na-doped ceramics. Key words: LaFeO3; sol-gel; XPS; magnetism; Na doping; dielectric

1.Introduction 1

1 These authors contributed equally to this paper

Nowadays, perovskite oxide LaFeO3 (LFO) has become one of the most important materials for the interesting physical properties and potential applications in the fields of electrode, catalyst and sensors [1-3]. Extensive researches have been done on the synthesis, characterization, structural and physical properties of LFO [4-8]. With regard to the magnetic properties, bulk LFO is antiferromagnetic (AFM) due to the superexchange (SE) interaction of Fe3+-O2--Fe3+ between neighboring Fe3+ ions. Recently, ferromagnetism (FM) has been reported for LFO nanoparticles, and the magnetization increased with decreasing particle size [5-8]. Several possible reasons have been proposed to be responsible for such weak FM. Uncompensated surface spin of nanocrystals and canted internal spin have be commonly accepted as the origins of such FM behavior [5-8]. The uncompensated surface spin creates FM shell surrounding AFM core, resulting in a net magnetization. The decrease of particle size would increase the number of uncompensated spins of Fe moments, leading to the enhancement of M. In addition, tilting of Fe3+-O2--Fe3+ SE interaction [9, 10], double exchange (DE) interaction of Fe3+-O2--Fe4+ between neighboring Fe3+ and Fe4+ ions [7], defect induced magnetic interactions between Fe3+/Fe3+ and Fe3+/Fe2+ through oxygen vacancy [10, 11] and antisymmetric Dzialoshinsky–Moriya interaction [12] have also been put forward to contribute to such FM behavior. Therefore, the origin of such ferromagnetic behavior is still ambiguous. In addition, the substitutions at A and/or B site of LFO carried out by many groups are observed to have great influence on the particle size, magnetic properties of LFO powders and dielectric properties of LFO ceramics. So far, monovalent element K [13], Ag [14], divalent mental element Zn [9], Pb [10], Sr/Ni [15], trivalent element Co[12], Al [16], Cr [17], Ce [18], and tetravalent element Ti [19] have already been doped into LFO powders or ceramics, and the modulation on the magnetic or dielectric properties have been investigated. However, no studies report on the magnetic or dielectric properties of alkali element Na-doped LFO. Therefore, it is of great interest to investigate the effect of Na doping on the structural, magnetic

properties of LFO powders and dielectric properties of LFO ceramics. In the present work, La1-xNaxFeO3 (x=0, 0.1 and 0.2) powders and ceramics were prepared by citric sol-gel method, and the origins of observed FM behavior in the Na-doped powders and enhanced dielectric constant in the Na-doped ceramics were discussed, respectively. 2.Experimental For the preparation of La1-xNaxFeO3 (x=0, 0.1 and 0.2) powders, citric sol-gel method was employed, where analytical grade La(NO3)3·6H2O, Fe(NO3)3·9H2O, Na2CO3, citric acid (CA), and polyethylene glycol (PEG6000) were employed as raw materials. Firstly, appropriate amount of La(NO3)3·6H2O, Fe(NO3)3·9H2O and Na2CO3 were dissolved in ion-free water under continuous stirring to get a homogeneous solution. Then CA in 2:1 molar ratio with respect to the sum of metallic cations was added to the solution as a chelator, and PH value of 2 was obtained. After that, PEG in 2:1 molar ratio with respect to the sum of metallic cations was added to the mixed solution under stirring at 80oC for 6h to form wet gel. Thereafter, gel pieces were formed through combustion process, and were ground to form fine powders, followed by calcination at 600oC for 2h in an oven to get the Na-doped LaFeO3 powder samples. Then the resultant powders were pressed into disks and calcined at 1200 oC for 3h to obtain the corresponding ceramic samples. X-ray diffraction patterns of the obtained powder samples were measured by X-ray diffractometer (D/max 2500, Rigaku Corporation, Japan) using Cu Kα radiation. Surface morphology was checked by Field Emission Scanning Electron Microscope (FE-SEM) (JSM-7100, JEOL, Japan). X-ray photoelectron spectroscopy (XPS) measurements were performed with monochromated Al Kα radiation using X-ray photoelectron spectrometer (ESCALAB 250, Thermo Electron Corporation, USA). Magnetic properties were measured by Superconducting Quantum Interference Device (SQUID, Quantum Design, USA). The dielectric properties of ceramic samples coated with Ag on both surfaces were measured by an LCR meter (E4980A, Agilent, USA) with an oscillation voltage of 0.5 V over the frequency range from

20Hz to 2MHz. 3.Results and discussion Fig. 1 shows the X-ray diffraction patterns of La1-xNaxFeO3 (x=0, 0.1 and 0.2) powders. The diffraction peaks of all samples are in accordance with orthorhombic LaFeO3 (PDF#37-1493) with perovskite structure, showing no trace of impurity phase. Since the main peaks of (200) (220) (222) (400) for cubic Na2O (PDF#23-0528) are adjacent to the main peaks of (121) (202) (240) (242) for orthorhombic LFO (PDF#37-1493) respectively, besides, the lattice parameter 5.56Å of cubic Na2O is close to the 5.56Å and 5.57Å of orthorhombic LFO, the possibility of the presence of Na2O cannot be excluded merely by XRD. We speculate that Na2O in the cubic structure might be present and act as bridge to connect adjacent LFO particles. The average crystallite size is estimated using the Scherrer formula D=0.89λ/βcosθ, where D is the mean crystallite size, λ is the wavelength of the X-ray radiation (λ=0.154 nm for Cu Kα1 radiation), and β is the full width at half-maximum of diffraction peak at 2θ [20]. As listed in Table 1, it decreases gradually upon Na doping concentration, suggesting that Na dopant could lead to the suppression of crystallite growth. XRD patterns of the ceramic samples are similar to those of the powders with higher crystallinity due to higher calcination temperature, which are not shown here. Fig. 2 exhibits the FE-SEM images of La1-xNaxFeO3 (x=0, 0.1 and 0.2) powders and corresponding fractured surface of ceramics, where uniformly nanosized particles and porous structure could be observed for the powders, while bigger grains and dense structure for the corresponding ceramics. For powders, the average particle size is 35nm for x=0, 50nm for x=0.1 and x=0.2. For ceramic samples, the average grain size is 0.74μm for x=0, 4.9μm for x=0.1 and 7.5μm for x=0.2. In both cases, the introduction of Na dopant results in the increase of particle/grain size, and the effect is more obvious for the ceramics. Although Na2O cannot be observed directly from the FE-SEM images of the powders due to the small particle size and detecting limit, the presence could be observed at the grain boundaries of LFO in the FE-SEM images of the corresponding ceramics in Fig. 2(b) and (c) (indicated by the green ellipses), which indirectly verifies the presence of Na2O in the powders. Therefore, the

introduction of Na dopant actually results in the formation of nonstoichiometric La1-xFeO3 and x/2 Na2O. The

magnetization

curves

obtained

from

SQUID

measurements

for

La1-xNaxFeO3 (x=0, 0.1 and 0.2) powders measured at room temperature (RT) with magnetic field (H) in the range of ±10kOe are shown in Fig. 3. The pure sample exhibits canted AFM due to uncompensated spins of Fe ions at the surface. The values of magnetization (M) measured at H and RT for our LaFeO3 powders and others in the Ref. are summarized in Table 1. In comparison to other results in the references, the M value of 0.04emu/g for our pure sample is smaller than the values reported for other pure LFO particles. On the other side, saturation-like hysteresis loop could be observed for x=0.1 and 0.2, indicative of the existence of ferromagnetic behavior at RT. Surprisingly, the M value at 10kOe for x=0.2 can reach 2.11emu/g, which is so far the highest value reported for doped LFO powders synthesized by various methods, as listed in Table.2. The value is only lower than the Pb-doped LFO pellets prepared by solid state reaction, which can reach 3.47emu/g at 10kOe and 6.9emu/g at 50kOe for La0.75Pb0.25FeO3 [10]. So far, most of the possible origins for the FM behavior of LFO nanoparticles are associated with large surface area of nanoparticles and the state of Fe ions at the surface region, such as uncompensated Fe spins and DE interaction between neighboring Fe3+ and Fe4+ ions. Therefore, it is of great importance to examine the surface composition and valence state of each element, especially Fe, in these samples, thus surface sensitive XPS measurement was carried out. Fig. 4 displays the typical whole survey and elemental XPS of La3d, Fe2p, O1s, C1s and Na1s with peak deconvolutions for La0.8Na0.2FeO3 powder. The whole survey spectrum shows that no impurity element is present in our samples. Inspection of La3d and Fe2p XPS shows that La ions are in the valence state of +3, and Fe ions are present in the mixed state of +3 and +4 [21]. Three types of oxygen are present, namely lattice oxygen, adsorbed oxygen (Oads) and oxygen in surface La-carbonate as analyzed in our previous work [21,22]. The existence of La-carbonate is verified by the examination of C1s XPS, where peaks of C-C and C-O bond come from

adventitious carbon, and the third peak is ascribed to La-carbonate due to the reaction of CO2 produced by the combustion process with the lanthanum oxide surface by citric sol-gel method [23, 24], but does not contribute to the FM behavior due to all paired electrons. Perfect fitting of acquired Na1s XPS requires only one peak located at the BE of 1071.27eV, indicating that Na element exists in the form of Na+ ion. Surface atomic composition and corresponding atomic ratios for La1-xNaxFeO3 powders are displayed in Table 2, features of which include: (a) the relative concentration of Na is much higher than designed, and it is comparable to that of La and Fe, indicating that the Na dopant is mainly distributed at the surface region and it does not exist in the form of substitution, otherwise, it would be homogeneously distributed in the whole particle and the concentration could not be so high. As Na2O makes no contribution to the observed FM behavior due to all paired electrons, the magnetic properties of the resultant particles should be determined by La1-xFeO3; (b) the pure LFO shows a La-rich surface due to the formation of La-carbonate, while with increasing Na doping concentration, the atomic ratio of Fe/La increases and finally becomes a Fe-rich surface for x=0.2, suggesting that the Na dopant, or more exactly Na2O, could suppress the formation of La carbonate to great extent; (c) relatively high content of the adsorbed oxygen in the total surface oxygen (Oads/Ototal) concentration could be observed for these samples, which is attributed to the small particle size and porous structure that could provide more adsorption sites for oxygen as revealed by FE-SEM; (d) the atomic ratio of Fe4+/Fe3 for the pure LFO is rather high, which originates from the high content of adsorbed oxygen that could capture electron from the Fe3+ ions and result in the formation of Fe4+ ions. While the sample with x=0.1 showed lower ratio of Fe4+/Fe3+ with respect to the pure one due to the presence of Na, as Na ions could also provide adsorption sites for oxygen, and the amount of adsorbed oxygen on the Fe ions would be reduced. As for the sample with x=0.2, it showed higher ratio of Fe4+/Fe3+ with respect to the sample with x=0.1, which is ascribed to its Fe-rich surface and the presence of La vacancies could induce the transformation from Fe3+ ions to Fe4+ ions in order to maintain the material to be charge neutrality.

Given the small particle size and high atomic ratio of Fe4+/Fe3 at the surface region for the pure LFO, the magnetic properties of LFO particles should not be mainly determined by the surface uncompensated spins or double exchange interaction between Fe4+ and Fe3+ ions, otherwise, such a small M value of 0.04emu/g for the pure LFO and such a large M value of 2.11emu/g for the sample with x=0.2 would not be obtained. Instead, for pure LFO, the intrasublattice FM coupling within each Fe sublattice is enhanced by the DB exchange between Fe4+ and Fe3+ ions, but the intersublattice interaction is still AFM coupled. However, when more La vacancies are introduced into the LFO lattice, both Fe sublattice would be distorted, and the canting angle between the two sublatice would be increased, leading to a net M and weak FM behavior at RT. The temperature dependence of magnetization from 50K to 300K at 1kOe for La1-xNaxFeO3 (x=0, 0.1 and 0.2) powders is displayed in Fig. 4. The M decreased gradually with increasing temperature for all samples due to the thermal fluctuations which caused the randomization of polarization direction. The Na-doped sample showed higher M value than the pure one over the whole measuring range of temperature, and the weak temperature dependence of saturation magnetization suggests a temperature-independent canting angle [25]. Therefore, it is the nonstoichiometric Fe/La ratio which leads to the distortion of lattice structure and enlarged canting angle between the two AFM coupled Fe sublattice that should be responsible for the enhanced magnetization in the Na-doped samples. Fig. 6(a) and (b) illustrates the frequency dependence of ε′ and tanδ for La1-xNaxFeO3 ceramics measured at room temperature, respectively. Both ε′ and tanδ decreases with increasing frequency from 20Hz to 2MHz. In comparison with previous results on pure LFO ceramics at 100Hz [26-28], our value of ε’ is relatively small. However, the introduction of Na dopant results in great enhancement of ε′ into the order of 105 at the cost of high tanδ. According to the work of M. Idrees [26], the colossal dielectric response in LaFeO3 is an extrinsic effect and is related with high capacitance of the grain boundaries. Therefore, the IBLC model by Sinclair et al. [29, 30] should be applicable to study the dielectric properties of LFO, and the ε′ can be approximated by the equation as follows:

𝜀𝑟 ≈ 𝜀𝑔𝑏

𝐴 𝑡

(1)

where εr and εgb represent the ε′ of the samples and grain boundary, A and t represent the average grain size of semiconducting grains and the average thickness of grain boundaries, respectively. As discussed above, the introduction of Na dopant actually results in the formation of nonstoichiometric La1-xFeO3 and x/2 Na2O. In combination with the FE-SEM results in Fig.2, the great enhancement of ε’ for the Na-doped ceramics compared to the pure one should ascribed to the increase of average grain size and and decreased thickness of grain boundaries. In addition, the existence of Na2O at the grain boundaries would influence the εgb to some extent. On the other side, the tanδ can be estimated according to this equation [31]: tanδ ≈

𝜎𝑑𝑐

𝜔𝜀0 𝜀𝑠′

(2)

where 𝜀0 and 𝜀𝑠′ represent the dielectric constant of free space and the static dielectric constant, respectively, and 𝜎𝑑𝑐 represents the DC conductivity which can be expressed as σdc = [C0 (R g + R gb )]−1 , where C0, Rg and Rgb are empty cell capacitance, grain resistances and grain boundary resistances, respectively [30]. Hence, at a fixed measuring frequency, tanδ ∝ [(R g + R gb )𝜀𝑠′ ]−1

(3)

For the pure sample, both Rg and Rgb are very large, as shown by the complex impedance spectroscopy in Fig. 7(a). Thus relatively low tanδ was obtained for the pure sample. However, for the Na-doped samples, the presence of La vacancy inside the LFO lattice would induce the transformation of partial Fe3+ ions into Fe4+ ions in order to maintain the material to be charge neutrality, thus result in the increase of hole carriers and semiconducting behavior as shown in Fig. 7(b). Therefore, high tanδ was obtained for the Na-doped samples due to rather small Rgb. 4.Conclusions La1-xNaxFeO3 (x=0, 0.1 and 0.2) powders and ceramics with orthorhombic perovskite structure were synthesized by citric sol-gel route to study the effect of Na doping on the structural, magnetic properties of LaFeO3 powders and dielectric

properties of corresponding ceramics. Ferromagnetic behavior with enhanced magnetization and great enhancement of ε′ into the order of 105 at 100Hz at the cost of high tanδ could be obviously observed at room temperature for the Na-doped powders and ceramics. FE-SEM images showed larger average grain size, which contributed to the colossal dielectric response of the Na-doped ceramics, and the presence of Na2O at the grain boundaries, which means that the introduction of Na dopant could lower the growth temperature of grains of the parent LFO and actually results in the formation of nonstoichiometric La1-xFeO3 and x/2 Na2O. XPS measurement indicated that the enhancement of magnetization for the powders should be attributed to the nonstoichiometric Fe/La ratio which leads to the distortion of lattice structure and temperature-independent canting angle between the two AFM coupled Fe sublattice. Our work demonstrates that appropriate Na doping is an effective way to achieve enhanced ferromagnetism and dielectric response at room temperature in LFO powder and ceramics, respectively. Acknowledgments This work was supported by National Natural Science Foundation of China (11404236, 11604234 and 51602214), Natural Science Foundation of Shanxi Province (2014021018-2, 2015021026, and 201601D202010), and China Scholarship Council (2016081409162).

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Fig. 1: X-ray diffraction patterns of La1-xNaxFeO3 powders with (a) x=0, (b) x=0.1 and (c) x=0.2. Fig. 2: FE-SEM images of La1-xNaxFeO3 powders and corresponding fractured surface of ceramics with (a) x=0, (b) x=0.1 and (c) x=0.2. Fig. 3: Field dependence of magnetization at room temperature for La1-xNaxFeO3 (x=0, 0.1 and 0.2) powders. Fig. 4: XPS of La0.8Na0.2FeO3 powder with (a) whole survey, (b) La3d, (c) Fe2p, (d) O1s, (e) C1s and (f) Na1s. Fig. 5: Temperature dependence of magnetization at 1kOe for La1-xNaxFeO3 (x=0, 0.1 and 0.2) powders. Fig. 6: Frequency dependence of ε′ (a) and tanδ (b) for La1-xNaxFeO3 (x=0, 0.1 and 0.2) ceramics at RT. Fig. 7: Complex impedance spectroscopy for La1-xNaxFeO3 ceramics at RT with (a) x=0 and (b) x=0.1 and 0.2.

Table 1 Magnetization (M) measured at magnetic field (H) at room temperature for our La1-xNaxFeO3 powders and others in the Ref. Composition

Preparation Method

LaFeO3 La0.9Na0.1FeO3 La0.8Na0.2FeO3

sol-gel

LaFeO3

polymer pyrolysis

Crystallie size(nm) 23.8 22.1 21.5 34.8 47.2 65.7

H (kOe) 10

10

M(emu/g) 0.04 0.28 2.11 0.32 0.11 0.08

Ref. ours

[8]

LaFeO3 LaFe0.9Ti0.1O3 LaFe0.8Ti0.2O3

74.0 47.0 32.0 25.0 30.0 50.0 27.0 31.9 25.4 28.7 24.1 25.2 21.9 15.9 12.7 18.2 20.5 24.0 57 49 39 31

polymer pyrolysis sol-gel milling solution combustion co-precipitation solution combustion co-precipitation solution combustion co-precipitation

LaFeO3 LaFeO3 La0.95Ce0.05FeO3 La0.9Ce0.1FeO3 LaFeO3 La0.9Sr0.1Fe0.9Ni0.1O3 La0.8Sr0.2Fe0.8Ni0.2O3 LaFeO3 La0.9Zn0.1FeO3 La0.7Zn0.3FeO3 LaFeO3 LaFe0.9Cr0.1O3 LaFe0.7Cr0.3O3 LaFe0.5Cr0.5O3

sol-gel

co-precipitation

sol-gel

10

13.5

18

20

50

70

0.07 0.1 0.1 0.32 1.4 0.4 0.22 0.17 0.32 0.27 0.82 0.56 0.37 0.16 0.12 0.5 0.65 0.7 0.6 0.65 0.9 1.0

[19]

[5]

[18]

[15]

[9]

[17]

Table 2 Surface atomic composition and corresponding atomic ratios for La1-xNaxFeO3 powders. x

La

Fe

O

C

Na

Fe/La

Na/La

Oads/Ototal

Fe4+/Fe3+

0 0.1 0.2

14.34% 10.79% 7.62%

6.68% 7.62% 8.64%

45.83% 45.00% 45.02%

33.15% 27.62% 28.67%

0 8.97% 10.05%

0.47 0.70 1.14

0 0.83 1.32

0.42 0.45 0.48

0.46 0.33 0.42