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Научни трудове на Съюза на учените в България-Пловдив. Серия В. Техника и технологии, естествен ии хуманитарни науки, том XVI., Съюз на учените сесия "Международна конференция на младите учени" 13-15 юни 2013. Scientific research of the Union of Scientists in Bulgaria-Plovdiv, series C. Natural Sciences and Humanities, Vol. XVI, ISSN 1311-9192, Union of Scientists, International Conference of Young Scientists, 13 - 15 June 2013, Plovdiv.
Magnetic properties and XPS studies of NdAlO3 nanopowder
D. Petrov*, I. Slavova, V. Ivanova, M. Stoyanova, St. Christoskova Faculty of Chemistry, Dept. of Physical Chemistry, Plovdiv University "Paisii Hilendarski" 24, Tsar Asen Str., 4000 Plovdiv Bulgaria
Abstract
Nanocrystals of single-phase neodymium monoaluminate (NdAlO3) were prepared by modified sol-gel method. Tartaric acid was employed as a new complexing agent that has facilitated the low-temperature synthesis at 900OC, without adding 1,2-ethanediol. The nanopowder was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and specific surface area. The binding energies of core-level electrons in the nanocrystalline samples were obtained by X-ray photoelectron spectroscopy (XPS) and found slightly shifted in respect to the corresponding values for the same elements. The temperature dependence of the AC magnetic susceptibility was measured between 2 and 250 K. Effective Bohr magneton number (peff), Curie constant (C), Weiss temperature (0), were also determined.
Keywords: neodymium monoaluminate; sol-gel method; magnetic susceptibility; effective magnetic moment; XPS spectroscopy
1. Introduction
Lanthanide monoaluminates (LnAlO3) form in pseudo-binary system Ln2O3-Al2O3.These materials have been widely used for buffer layers of various perovskite-type films, high-temperature superconductors or ferroelectrics, as well as activated solids for luminescence and dielectric laser hosts and colossal magnetoresistance oxides. It has been noted that the lanthanide monoaluminates possess very good microwave dielectric properties, especially SmAlO3 [1]
Complex-oxide nanoparticles of size 50 nm or less may also exhibit other valuable properties such as superparamagnetism, UV shielding, energy upconversion, antifungal, etc. [2, 3].
The sol-gel methods have been used to prepare nanoparticles of neodymium monoaluminate dispersed in Al2O3 matrix and the optical and mechanical properties of Nd3+ ions have been studied [4-6]. Nanopowders of SmAlO3, DyAlO3, and GdAlO3 were synthesized by modified sol-gel reaction and their magnetic properties were also studied [7-9]. It has been noted that these compounds possess unusually low effective magnetic moments of the Ln3+ ions, obtained from the
temperature dependence of molar magnetic susceptibility in the range 2-300 K.
The sol-gel syntheses, variations of the Pechini's method [10] based on reactions of hydrolysis and condensation, provide good quality products, simple pH control and chemical homogeneity during the process.
The aim of this study is to report the results of a modified sol-gel procedure, the magnetic susceptibility measurements and surface properties of neodymium monoaluminate (NdAlO3) nanocrystals.
2. Experimental
2.1 Preparation
The samples were synthesized by aqueous sol-gel method. The gel was prepared using stoichiometric amounts of analytical-grade, high-purity Nd2O3 (Rare Earth Products, 99.99%), Al(NO3)3.9H2O and tartaric acid (2,3-dihydroxybutanedioic acid, HO2CCH(OH)CH(OH)CO2H). In the sol-gel process, 4.0376 g Nd2O3 was first dissolved in 50 cm3 HNO3 ("Merck", aq., 2 mol/ dm3) at room temperature. To this solution, 15.0052 g Al(NO3)3 dissolved in 400 cm3 bidistilled water (0.1 mol/dm3), and tartaric acid 0.2 mol/dm3 were added. Ammonia was used to lower the acidity to pH = 5. The mixture was then stirred at 80OC for 6 h. The gel obtained by concentrating was transferred to a Pt crucible and preheated in air at 600OC for 6 h. After an intermediate grinding in an agate mortar, the black powder obtained was additionally sintered at 800OC for 4 h and at 900OC for 4 h in air to a pale, pinkish powder.
2.2 Experimental techniques
Powder X-ray diffraction spectra were collected within the 20- range from 10O to 80O with a constant step 0.04O and counting time ls/step on Bruker D8 Advance diffractometer with CuKa radiation and SolX detector. The spectra were evaluated within the Diffracplus EVA package.
Scanning electron microscope (SEM) JEOL JSM-3790 was used to study the surface morphology, whereas transmission micrographs of the samples were obtained by means of transmission electron microscope (TEM) JEOL TEM 200 kV.
X-ray photoelectron spectra were performed on ESCALAB Mkll (VG Scientific) spectrometer. C ls, O ls, Al 2p, Nd 3d, OKLL, and NdMNN electron core levels were excited with Al K a and Mg K a radiation. The pressure was 6.7x10-7 Pa in the UHV chamber; all spectra were calibrated with C ls line reference at 285.0 eV. The total instrumental resolution was ~ 1 eV
The AC magnetic susceptibility was measured in the temperature range 2.5 K - 250 K with PPMS - 9 (Physical Properties Measurement System), Quantum Design. The parameters of the AC magnetic field were 1 kHz and 2.3 87x103 Am-1 (=30 Oe).
3. Results and discussion
3.1 Powder X-ray diffraction
The XRD pattern of NdAlO3 prepared using tartaric acid as a complexing agent is presented in Fig.1. The diffraction peaks belong to the rhombohedral neodymium monoaluminate; the peaks are sharp and well resolved.
The vertical lines and asterisks correspond to the positions and the intensities, respectively, of the reference sample, file 01.071.1596(C); temperature T=298.15 K. Phase analysis has confirmed the existence of a single-phase neodymium aluminate. Mean crystallite sizes deduced by Debye-Scherrer equation are about 20-40 nm.
500
400
■"i? 300 CO 300
200
100
10 20 30 40 50 60
70 80
2 © (deg)
0
Fig.1. X-ray diffraction pattern of NdAlO3 sample
The pyknometric density of NdAlO3 was determined at T=293.15 K, p =4.03 g/cm3; this value is lower than that one for the bulk crystal, p =7.03 g/cm3 [11]. Specific surface area of the powder was 6.3 m2/g. This value is compatible with the specific surface area obtained for nanopowders of Nd3+:Y3Al5O12, varying from 25 to 4 m2/ g, for samples annealed between 900 and 1300OC, respectively; it has been found that the decay times of certain radiative transitions of Nd3+ are affected by the specific surface area of the nanocrystalline grains [12].
3.2. SEM and TEM studies
A typical scanning electron micrograph of the same sample is shown in Fig.2. The morphology of the heat-tr^^^^^er rcvcalsU^mdm^ia^i^lalHn^a
Fig. 2. Scanning electron micrograph of NdAlO3 NdAlO3 sample
Fig. 3. HRTEM micrograph of
High resolution transmission electron photograph is presented in Fig.3, in which a cluster of
individual particles is evident. The smallest particles are of size 20 nm each and are relatively more transparent for the beam compared with the larger agglomerates since the depth of penetration is about 5 nm; in addition, the individual particles are anisometric, e.g. platelets.
3.3 X-ray photoelectron spectra
Electron binding energies (BE) for Nd, Al and O atoms are observed as peaks in the survey spectrum (Fig. 4) registered in the range 0, -1200 eV
Binding energies / eV
Fig.4. X-ray photoelectron spectra of NdAlO3
For comparison, the corresponding values from two reference sources with close data were used [13, 14]. The depth achieved in the XPS experiments depends on the probed core level and is supposed to reach ca. 5 nm [15].
Here, we report the BE-values (in increasing order) of Al 2p, Al 2s, Nd 4d, O 1s, Nd 3d. Our measured value of the Al 2p signal was found 74.2 eV; this value is close to that reported for NdAlO3/Al2O3, about 75 eV [7]. The asymmetrical peak at ca.120 eV contains Nd 4d and Al 2s electron core levels. The core-level peak of O 1s is located at 530.2 eV as a single, narrow peak.
Similar feature is interpreted as an evidence for the existence of one type of oxygen [15, 16]. The experimental peaks of Nd 3d are distinctly found at 982 eV and 1004 eV.
Table 1. Positions of core-level binding energies in nanocrystalline NdAlO3 Reference BE / eV
Electron core level Experimental BE / eV Chemical shift / eV _[13 14]_1_
Al 2p 74, 72.9
^1/2 74.2 +1.1
Al 2p 73, 72.5
Al 2s Nd 4d O 1s
Nd 3d5/2
Nd 3d
117.8, 118 118, 120.5 532, 543.1 978, 980.4 1000, 1003.3
119.2 121.7 530.2 982.2 1004.2
+1.3 +2.4 -7.3 +3.0 +2.6
The chemical shifts presented in the last column of Table 1 are small, as expected for the ionic Nd - O and Al - O bonds. All shifts are positive in sign, except that one for O 1s. This fact could be explained with a slightly higher kinetic energy (negative shift) of these core electrons in respect to the reference value. Unlike oxygen, all metal (Nd, Al) core-level electrons are in states of lower kinetic energy, thus producing positive shifts of the BE. This difference reflects certain degree of electron mobility of surface oxygen in the nanocrystalline NdAlO3. Our unpublished results on the BE of other lanthanide monoaluminates nanocrystals also exhibit negative shifts for O 1s.
3.4 Magnetic susceptibility
The electronic configuration of Nd3+ ions is [Xe] 4f i.e. contains an odd number of electrons. The ground level 4I9/2 is magnetic and has basic contribution to the experimental magnetic susceptibility that follows the Curie-Weiss law, presented as Eq. 1. The variation of the reciprocal AC molar magnetic susceptibility 1/xm between 2 K and 250 K is presented in Fig. 5.
C
(1)
The experimental effective magnetic moment in Bohr magnetons has been determined by the following equation:
Aeff
/ \1/2 ' 3 kB C |
Mo NA tB j
(2)
where: x - molar magnetic susceptibility
C - Curie constant, m3 mol-1 K
0 - Weiss constant, K
kB - Boltzmann constant
Na - Avogadro's number
^ = 9.27401 x 10-24 J T-1 (=A m2)
^ = 4n 10-7 T2 J-1 m3 (=J A2 m-1 = 5.58494 J mol-1)
8
7-
6-
"o 5-
4-
o
3
2-
1-
0-
I
\
leal part
X '' m - imaginary part
~l-r-
0
—I—
50
100 150 Temperature / K
200
250
Fig. 5. Temperature dependence of molar magnetic susceptibility of nanocrystalline NdAlO3
The linear plot 1/xm - T from 2 to 250 K (not presented here) yields Weiss constant 0 = -(9.530 ± 0.297) K and Hie molar Curie constant C = (1.27 ± 0.1) x 10-8 m3 mol-1K. The coefficient of regression of the linear equation 1/^-T from 2 K to 250 K, R2 = 0.99991.
From the experimental Curie constant, we have determined the experimental effective Bohr magneton number of Nd3+ in nanocrystalline NdAlO3, ^eff = 0.09 ± 0.06x10-3. The theoretical value of the effective magnetic moment of Nd3+ free ion, ^eff (theor.) = 3.62 [19].
4. Conclusions
Tartaric acid facilitates a modified sol-gel preparation of nanocrystalline neodymium aluminate (NdAlO3) as the calcination at 900OC is well below the phase-transition temperature, 1700OC. The XRD confirms single phase and the surface SEM and HRTEM micrographs reveal nanocrystallinity with certain degree of agglomeration. The binding energies of core electrons are chemically shifted in NdAlO3 compared with the reference elements and correspond to ionic metal - oxygen bonds. The value of the effective magnetic moment (in Bohr magnetons, deduced for Nd3+, 0.09, in the interval 2 - 250 K is very low compared to the range of values (3.60 - 3.90) usually found in bulk compounds or single crystals of Nd3+. This fact is due to the nano-size of the particles leading to the superparamagnetic behaviour of this material.
Acknowledgements
The authors gratefully acknowledge the financial support by the University of Plovdiv Research Fund (Project NI HF-2013). The authors also would like to thank Prof. B. Angelov for the valuable suggestions and Assoc. Prof. V. Lovchinov for making the magnetic measurements.
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