Temperature dependent electron paramagnetic resonance (EPR) of SrZrO3

Journal of Magnetism and Magnetic Materials 391 (2015) 101–107

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Temperature dependent electron paramagnetic resonance (EPR) of SrZrO3 Santosh K. Gupta a,n, Nimai Pathak a, P.S. Ghosh b, B. Rajeshwari a,b, V. Natarajan b, R.M. Kadam a a b

Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India Materials Science Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

art ic l e i nf o

a b s t r a c t

Article history: Received 27 March 2015 Received in revised form 22 April 2015 Accepted 27 April 2015 Available online 28 April 2015

SrZrO3 (SZO), a distorted perovskite was synthesized using gel-combustion route employing citric acid as a fuel and ammonium nitrate as oxidizer followed by characterization using X-ray diffraction (XRD) and electron paramagnetic resonance (EPR). Purity of the sample is confirmed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis. Broadening and shift of the resonance field position in EPR spectrum to the lower field was observed as the temperature is lowered; which is the characteristic of ferromagnetic resonance spectra. The value of Curie–Weiss temperature obtained for SZO particles is 8.7 K. The positive sign of the Curie–Weiss temperature indicates that some of the spins are ferromagnetically coupled in this sample. Theoretical investigation using density functional theory (DFT) calculation revealed that Vacancy at zirconium site contribute maximum to the magnetic moment. & 2015 Elsevier B.V. All rights reserved.

Keywords: SrZrO3 Defect EPR Ferromagnetism

1. Introduction As quoted by Stolen et al. [1] perovskite structures have been termed an inorganic chameleon due to their large flexibility since the cubic mother structure easily distorts and adapts to the relative size of the ions forming the compound. The source of fascination is the diversity of the properties and their high sensitivity to crystal chemical tuning; i.e., a tiny change in chemical composition or/and crystal structure may induce huge changes in chemical and physical properties. Strontium zirconate (SZO), with chemical formula SrZrO3, is a complex oxide with a number of useful properties for device applications. Its features include high temperature proton conductivity [2], a large dielectric constant [3], resistance switching [4,5], and ferroelectricity in artificial superlattices [6,7]. SZO crystallizes in the perovskite (ABO3) structure, a class of compounds that is currently of significant research interest due to the emergence of novel interface phenomena [8,9]. Native points defects are known to play an important role in oxides and in many cases dominate the electronic and optical properties. For example, native defects commonly act as carrier-compensation centers, introduce optically active states in the band gap [10,11], and are sometimes invoked as sources of free carriers [12]. Recently, there has been a great deal of interest in the study of n

Corresponding author. Fax: þ 91-22-25505151. E-mail address: [email protected] (S.K. Gupta).

http://dx.doi.org/10.1016/j.jmmm.2015.04.108 0304-8853/& 2015 Elsevier B.V. All rights reserved.

magnetism in nonmagnetic semiconductors/insulator diluted with magnetic impurities due to possible applications in spin-based electronic systems [13]. However, there are controversies over the existence or absence of ferromagnetism in many of these materials [14]. Even if there is magnetism, the origin seems not to be intrinsic to the main phase and possibly associated with magnetic impurity phases [15–17]. Room temperature d0 ferromagnetism (FM) in pure semiconductors or insulators without any ferromagnetism elements has attracted much attention in recent years. The experimental results have indicated that the observed room temperature FM originates from the oxygen vacancies for HfO2, CeO2, Al2O3, ZnO, In2O3, SnO2, TiO2 and CuO films/nanograins [18–23], but from neutral cation vacancies for MgO films/nanograins [24–27]. Most of Ab initio calculations have demonstrated that neutral cation vacancies are responsible for the magnetic moment in oxides such as CaO, HfO2, TiO2, ZnO, SnO2, ZrO2and MgO [28–39]. The origin of magnetic moment in the nanoparticles of nonmagnetic materials was suggested to be due to cation or anion defects present at the surface of the particles. The nature of defect responsible for magnetism seems to depend on specific material. Conversely, the bulk samples obtained by sintering the nanoparticles at high temperatures became diamagnetic. The magnetism in these nanoparticles has been suggested to be intrinsic and originates from cation or anion vacancies at the surfaces of nanoparticles depending on the nature of cations. In fact, the surface ferromagnetism has been envisaged to be a universal feature of

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nanoparticles of inorganic materials [40]. Recently the concept of defect mediated ferromagnetism (FM) has been extended to ABO3 type perovskite oxide mostly to titanates. Yang et al. [41] has observed weak FM in BaTiO3 (BTO) nanoparticles; which they have attributed to the oxygen vacancies at the surface. Mangalam et al. [42] has also attributed oxygen vacancies at the surface for existence of ferromagnetism in nanocrystalline BTO which they have confirmed using positron annihilation spectroscopy. The ab initio calculation by Mangalam et al. [42] with local density approximation (LDA) method indicated that FM in BTO originates from the oxygen vacancy. However, Ab initio calculations by Cao et al. [43] with generalized gradient approximations (GGA′s) method showed that both cation (Ti) vacancies and anion (O) vacancies could induce magnetism in BTO. In case of SrTiO3 (STO) defect induced FM is attributed Ti and O vacancies. The maximum contribution to magnetic moment is from Ti vacancy whereas Sr vacancy does not contribute at all [44]. Potzger et al. [45] and Bannikov et al. [46] have concluded that the oxygen vacancies are responsible for the ferromagnetic properties in STO. Zhang et al. [47] extensively employed Ab initio calculations to get information about the defect responsible for FM in STO. The GGA calculated results indicate that Ti or O vacancy could induce magnetism rather than Sr vacancy. The LDA and LDA þU calculations show that the Ti vacancy could induce magnetism, while Sr and O vacancies could not. Based on their results FM in STO nanoparticles is attributed to Ti vacancies. Some other perovskites are also reported to show defect induced FM [48,49]. Among several, electron paramagnetic resonance (EPR) is proven to be very sensitive to the paramagnetic ions in either magnetic or nonmagnetic substance. Beside the configuration of magnetic ions, EPR also provides the information of the local environment, including site-occupation, structural symmetry, and the oxygen vacancies neighboring to paramagnetic ions. The EPR technique is a very sensitive method for detecting ferromagnetic ordering as well as the existence of other magnetic species, as it is capable of detecting 1013 spins [50]. Further, it is reported that the temperature dependence of the line position, line width and integral intensity of EPR spectra were effectively used to obtain information about magnetic ordering in a number of molecular ferromagnetic nanomaterials [51–53]. SZO is a distorted perovskite and an insulator having band gap of 5.5 eV. As far as SrZrO3 is concerned possible defect centers are strontium vacancies (VSr), zirconium vacancies (VZr), Oxygen vacancies (VO), strontium interstitial (Sri), zirconium interstitial (Zri) and oxygen interstitial (Oi). Since Zri4 þ and Sri2 þ (Electron Configuration (EN)¼ Kr) are diamagnetic, they cannot be detected by EPR spectrometer. There are three possibilities for oxygen on interstitial sites (Oi) which are neutral Oi, singly negative charged Oi, and doubly negative charged Oi2  with their electron configurations ending with 2p4, 2p5, and 2p6 respectively. However Oi2  is diamagnetic and hence will not be detect by EPR while Oi is a triplet one and a non Cramer system. The paramagnetic center Oi  state with the 2p5 electron configuration can easily be detected by EPR. Again in case of oxygen vacancies (VO), only Vȯ is paramagnetic and can be detected by EPR while the diamagnetic Vo€and Vo2 þ cannot be detected. The paramagnetic oxygen vacancy (Vȯ ) which may be attributed to F-center in alkali halides, originates due reduction by electron from the conduction band [54]. Earlier reports including ours have shown by photoluminescence study that in SZO, the EPR signal at g ¼1.976 is due to paramagnetic oxygen vacancies [55,56]. Liu et al. has reported in ZrO2, the EPR signal at g┴ ¼ 1.974, gll¼1.961 could be due to Zr3 þ originated due to reduction of bulk Zr4 þ ions which are adjacent to

the bulk oxygen vacancies and could capture electrons [57]. Both strontium vacancies (VSr2  ), zirconium vacancies (VZr4  ) are diamagnetic and hence unable to detect by EPR. However the adjacent oxygen vacancies i.e. Vo2 þ can capture a electron and thereby producing paramagnetic oxygen vacancy (Vȯ ) which is EPR active and can be detected. But whatever may be the origin; the long range ordering of these paramagnetic species can be responsible for inducing ferromagnetism (FM) in the system. To the best of our knowledge there is no report on defect induced FM in SZO perovskite. Therefore, the origin of ferromagnetism in SZO and the exact correlation with defects, remain at best, a poorly understood phenomenon. In the present work, an attempt has been made to establish experimentally as well as theoretically the correlation between the defects (both oxygen vacancy and cation defect) and the magnetic property of SZO in its nanocrystalline phase. The use of the EPR technique offers some interesting advantages over the generally used superconducting quantum interference device (SQUID) magnetization measurements: it has a higher sensitivity which is required for studies of thin films of lightly doped diluted magnetic semiconductors (DMS) and its resonant character allows one to easily eliminate substrate contributions and effects of second phase inclusions.

2. Experimental 2.1. Synthesis: citric acid assisted gel-combustion All the chemicals used in the sample preparation were of ‘Analytical Reagent’ grade and procured from Sigma Aldrich. Zirconyl oxychloride (ZrOCl2), strontium nitrate Sr(NO3)2, ammonium nitrate (NH4NO3) and citric acid (C6H8O7  H2O) were used as starting materials for the synthesis in the molar ratio 1:1:10:1.25. The synthesis procedure adopted is already reported in our earlier work on doped SrZrO3 [58,59]. A flow chart for the synthesis procedure is presented in Fig. S1 (Supplementary file). 2.2. Computational methodology Ab initio calculations were performed using the plane wave based Vienna Ab initio Simulation Package (VASP) [60,61]. VASP is based on the density functional theory (DFT) and we used generalized gradient approximation (GGA) for the exchange and correlation potentials as parameterized by Perdew, Burke and Ernzerhof (PBE) [62]. The projector augmented wave (PAW) potentials [63] were used for the ion–electron interactions including the valence states of Sr (4s, 4p, 5s – 10 valence electrons), Zr (4s, 4p, 5s, 4d – 12 valence electrons) and O (2s, 2p – 6 valence electrons). For orthorhombic unit-cell of SrZrO3 (SZ), optimization was carried out with respect to Ecut and k-point meshes to ensure convergence of total energy to within a precision of 0.1 meV/atom. The expansion of electronic wave functions in plane waves was set to optimized kinetic energy cut-off (Ecut) of 500 eV throughout this study. The Brillouin-zone (BZ) integrations were performed on an optimized Monkhorst–Pack [64] k-point grid of 12  12  8 for SZ and 6  12  8 for SZ supercells, respectively. The total energy of SZ was optimized with respect to volume (or lattice parameter), b/a, c/a ratio and atomic positions as permitted by the space group symmetry of the crystal structure. The structural relaxations (b/a, c/a ratio and atomic positions) were performed for each structure using the conjugate gradient algorithm until the residual forces and stress in the equilibrium geometry were of the order of 0.005 eV/Å and 0.01 GPa, respectively. The final calculation of total electronic energy and density of states (DOS) were performed using the tetrahedron method with Blöchl corrections [65].

S.K. Gupta et al. / Journal of Magnetism and Magnetic Materials 391 (2015) 101–107

Table 1 Instrumental parameters used for recording the EPR spectra. Receiver

Microwave

Modulation amplitude – 2.00 G

Microwave frequency – 9.4186 GHz Modulation freMicrowave power quency – 100 kHz – 7.838 mW Receiver gain – 1*103

Field

Signal channel

Center field – 3500 G

Conversion – 80.00 ms

Sweep width – 3000 G Resolution – 1024 points

Time constant – 40.960 ms Sweep time – 81.920 ms

2.3. Instrumentation Powders XRD of the samples were carried out using RIGAKU Miniflex-600 diffractometer operating in the Bragg–Brentano focusing geometry. Cu-Kα radiation (λ ¼ 1.5406 Å) has been used as X-ray source. The instrument was operated at 40 kV and 30 mA current. The XRD pattern has been taken from 15 ° to 115 ° 2θ range with scan rate of 1°/min. The EPR spectra were recorded on a Bruker ESP-300 spectrometer operating at X-band microwave frequency (9.4186 GHz). The detail of the measurement parameters are listed in Table 1 Diphenyl picrylhydrazyl (DPPH) was used for the calibration of gvalues of paramagnetic species. The analysis of trace metallic impurities in the sample was determined by ICP-AES method using a computer controlled, high resolution, simultaneous atomic emission spectrometer (Spectro Arcos, Germany) equipped with ICP and D.C. arc as the excitation sources and a Charged Coupled Device (CCD) based detector. This technology offers the best advantage of choosing additional, interference free analytical lines for the elements under study.

3. Results and discussion 3.1. Phase purity: X-ray diffraction (XRD) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) The absence of extrinsic phases was checked by X-ray diffraction (XRD). No additional phase has been detected. The XRD patterns of the SrZrO3 sample are shown in Fig. 1, from which it can be seen that all the diffraction peaks could be indexed to the orthorhombic phase of SrZrO3 (JCPDS card no. 44-0161). The XRD data was indexed on an orthorhombic system with space group

Fig. 1. XRD pattern of SrZrO3 perovskite and the standard JCPDS pattern for file no. 44-0161.

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Pnma. On Rietveld refinement using Pnma space group, XRD pattern for SrZrO3 was fitted perfectly. EDXRF pattern shown in Fig. S2 (Supplementary file) further confirms the purity of SrZrO3 at 600 °C. It shows the presence of Sr and Zr peak without any impurity. ABO3 type perovskite structure is exhibited by various oxide based inorganic compounds. In an ideal perovskite which is having cubic structure, ‘A’ atom occupies corner of the cube (0, 0, 0), ‘B’ atom occupies body center position BCC (1/2, 1/2, 1/2) and oxygen atoms occupies face centered positions FCC (1/2, 1/2, 0). Size requirement for A and B atom to be in ideal perovskite is quite any stringent and deviation from the required size; leads to distortion in the perovskite structure wherein coordination of either A, B or both are reduced compared to idealized geometry. Such distortion can leads to various low symmetry structures. Slight tilting of the BO6 octahedra decreases the coordination number of A cation from 12 to as low as 8. The ideal cubic perovskite ABO3 have space group pm3m. This particular structure can be visualized as threedimensional (3D) network of regular corner-linked BO6 octahedra where B-ion is at the center of octahedra and A ion is localized in the spaces between them. In an ideal cubic perovskite the coordination number of A ion is 12 e.g. BaZrO3. The most commonly occurring distortion in perovskites is octahedral tilting. This means the tilting of the BO6 octahedra about one or more of their symmetry axes, maintaining both regularity of the octahedral (approximately) and their corner connectivity (strictly). Such tilting allows greater flexibility in the coordination of the A-cation, while leaving the environment of the B-cation essentially unchanged. A schematic of the crystal structure for the SrZrO3 is presented in Fig. S3 (Supplementary file) which is drawn using VESTA visualization program [66]. The size of the hole for the A-ion is determined by the radius of the B-ion. If the B-ion is Zr4 þ , the radius of the hole is 0.160 nm, which is exactly the radius of Ba2 þ coordinated by 12 oxygen ions. BaZrO3 is an example of a cubic perovskite with almost no distortions. In SrZrO3, size of Sr ion is smaller than the hole of the undistorted structure. The incorporation of a smaller Sr-ion in the perovskite-type structure is accompanied by rotations of the octahedra. This leads to a lowering of the symmetry with respect to the ideal perovskite structure. SrZrO3 can be either pseudo-cubic or orthorhombic at room temperature [67]. To rule the possibility for the presence of paramagnetic transition metal ion (EPR active ion); inductively coupled plasma atomic emission spectroscopy (ICP-AES) was carried out. About 200 mg of SrZrO3 sample was dissolved in a mixture of HF (0.05 M)–conc. HNO3 and heated under I.R lamp. The dissolved aliquot of supernatant solution from this mixture was leached out into a separate beaker. The residual sample was repeatedly treated with HF–HNO3 acid mixture till the time there was no residual sample left out. Simultaneously, the leachant solution containing the dissolved sample was repeatedly heated under I.R lamp after addition of conc. HNO3 to obtain a clear nitrate solution free from any traces of HF which may cause interference during the analysis stage. The nitrate mixture thus obtained was finally made into a known volume by addition of dilute HNO3 (0.5 M) and analyzed by ICP-AES method. The standard solutions for the impurity analysis was prepared from Certi PUR-ICP multi-element standard solutions (E-Merck, Darmstadt, Germany) by appropriate dilutions with 0.5 M Suprapure HNO3 (E-Merck, Darmstadt, Germany) and quartz double distilled water. A two-point calibration for each of the analyte element was carried out using 0.5 M HNO3 and 10 ppm of multielement standard solution which served as the lower and the higher standards respectively. Prior to the calibration of standards, the detector calibration was also carried out using 100 ppm of the multi-element standard solution so as to have a peak search and

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identify the most suitable peak for each analyte. Using this method, the transition metal impurities such as Co, Cr, Cu, Fe, Mn and Ni were determined. The estimates for almost all the elements was found to be in the range 0.002–0.005 ppm (BDL) with a precision of 72% RSD which confirmed a near total purity of the sample with respect to the above impurities. 3.2. EPR studies Fig. 2 shows the temperature dependence of EPR signal at g ¼1.9925 in the range 77–560 K for the SrZrO3 sample annealed at 873 K, showing a marked broadening and shift of the resonance field position to the lower field as the temperature is lowered; which is the characteristic of ferromagnetic resonance spectra. Fig. S4 (Supplementary file) are plots of the temperature dependence of the g factor and line width of the ferromagnetic resonance (FMR) signal. The broad signal having a peak-to-peak line width of 340 mT was attributed to ferromagnetic resonance (FMR), which arises from transition within the ground state of the ferromagnetic domain. With the lowering of T, a broadening of the signal and a shift of the center of resonance to lower H values were observed. It may be mentioned that such a T-dependent change in the line width and line position of the resonance signal would not occur in the paramagnetic state. The observed broadening of the signal and a shift of the center of resonance to the lower H values is attributed to the presence of a nonhomogeneous local magnetic field, which modifies both the resonance H and the line shape of the signal in the ferromagnetic state [52]. Low-field EPR spectra of the as prepared SrZrO3 possess a large broad peak, conclusively showing that the spins in the system are strongly coupled and form spin clusters. The presence of the FMR signal even at 560 K indicated that the ferromagnetic TC for this sample is much above 500 K. These data clearly show that ferromagnetism persists well above room temperature. The strong temperature dependent effects occur in the ferromagnetic state because the field position and the line width depend on the magnetization and anisotropy constant, which are temperature dependent in a ferromagnet [68]. Direct information about the magnetic state can be obtained from the variation of the integral intensity of the EPR spectrum, IEPR, with temperature [69]. Integral intensity, IEPR(T) is proportional to the spin susceptibility of the paramagnetic species taking

Fig. 3. Temperature dependence of the inverse of EPR integral intensity, 1/IEPR , corresponding to the FMR signal in SZO particles.

part in resonance. Fig. 3 shows the temperature dependence of 1/IEPR of the FMR signal in SZO sample annealed at 873 K. Here, IEPR was obtained by double integration of the corresponding EPR resonance. In the high temperature limit, the temperature variation of IEPR(T) can be described by

IEPR (T )~ C T −θ

(1)

C is the Curie constant and θ the Curie–Weiss temperature. The numerical value of θ is obtained from the linear extrapolation of the high temperature part of 1/IEPR as indicated by the straight line in Fig. 3. For SZO particles annealed at 873 K a value of θ  8.7 K is obtained. But in order to confirm the exact value of Tc one has to perform EPR experiment at Liq He temperature. The positive sign of the Curie–Weiss temperature indicates that some of the spins are ferromagnetically coupled in this sample. A possible cause of the observed ferromagnetism in our SZO sample could be the defects which appear in the synthesis process. It is important to note here that in Fig. 2 below 120 K one extra signal S2 at g¼ 2.0015 starts to appear. For further clarification simulation spectra of the experimental curve at 77 K is shown in

Fig. 2. Temperature dependence of the EPR spectra of SrZrO3 in the range 77–560 K.

S.K. Gupta et al. / Journal of Magnetism and Magnetic Materials 391 (2015) 101–107

Fig. 4. Simulation spectra of the EPR spectra at 120K.

Fig. 4. This extra signal S2 is attributed to the uncoupled paramagnetic species. From Fig. 2 it is clear that as the temperature is decreased from 120 K to 77 K only the intensity of the signal S2 has increased and no change in line shape or resonance field is observed. Hence this signal S2 is purely non ferromagnetic [70]. 3.3. Theoretical calculations The calculated lattice parameters for SZ are a ¼5.845 Å, b¼ 5.909 Å and c ¼8.295 Å using GGA-PBE, which agree well with our XRD data of a ¼5.797 Å, b¼5.817 Å and c ¼8.204 Å [55] and other experimentally determined values [71] within less than 1.5% deviations. These calculated values matches very well with other theoretically calculated values of a ¼5.84 Å, b ¼5.902 Å and c ¼8.289 Å [72]. Therefore, the GGA approximation is able to

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Fig. 6. Position of Sr (a) and Zr (b) vacancy in the 2  1  1 SZ supercell. It also shows location of the oxygen atoms (nearest to the vacancy) giving dominant contribution to the magnetic moment in the supercell with Sr and Zr vacancy.

provide reliable results for the equilibrium lattice constants of the present system. Fig. 5a shows the spin resolved total and partial DOSs for stoichiometric SrZrO3. No spin-polarization emerges around the Fermi energy level. So, the stoichiometric SrZrO3 is nonmagnetic, due to the absence of unpaired electrons. The vacancy formation energies in SrZrO3 are calculated to testify the stability of vacancy defects. Each atom of Sr, Zr or O is removed from the 2  1  1 SZ supercell with Pbnm structure, which corresponds to 12.5%, 12.5% and 4.167% for Sr, Zr and O vacancies in supercell, respectively. Positions of the Sr and Zr vacancy in the supercell are shown in Fig. 6. After creating vacancy supercells are allowed to relax (in terms of supercell volume, shape and atomic positions) to attain a

Fig. 5. Spin-resolved total (solid red line) and O 2p partial DOS (dotted blue line) for (a) stoichiometric SrZrO3 (Mtot ¼ 0 μB) (b) SZ with one Sr vacancy (Mtot ¼ 1.89 μB) (c) SZ with one Zr vacancy (Mtot ¼3.89 μB) and (d) SZ with one O vacancy (Mtot ¼ 0 μB). A positive DOS indicates spin-up states, and a negative DOS indicates spin-down states. The vertical dotted line indicates the Fermi level.

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Table 2 Calculated values of the neutral vacancy formation energies (Ef), total magnetic moments per 40 atom unit-cell (Mtot) and the energy difference (ΔEN  M) between nonmagnetic and magnetic states for the SrZrO3. SrZrO3

V0Sr

V0Zr

V0O

Ef (eV) Mtot (μB) ΔEN  M (meV)

8.59 1.57 77.87

15.83 3.42 700.06

1.55 0.00 0.00

ground state structure with defects. The vacancy formation energy is defined as: Ef(j) ¼{Etot(SrZrO3,j) þ Etot(j)} Etot(SrZrO3) [43], where Etot(SrZrO3) and Etot(SrZrO3,j) are the total energies of the stoichiometric supercell and the supercell with a species j vacancy, respectively. Etot(j) is the total energy of per atom in elemental solid except for oxygen. Etot(O) is defined the total energy per atom in O2 molecule. The calculated ground state cohesive energies of bulk FCC Sr, HCP Zr and O2 molecule (in triplet state) are 1.685, 6.582 and 4.929 eV/atom, respectively. Using these values, the calculated vacancy formation energies for defective SZ supercell are listed in Table 2. It shows that the vacancy formation energy for O vacancy in SZ is the lowest among three types of vacancy. In order to check the stability of the magnetic state we calculate the total energies of the supercell with various neutral vacancies in SZ for spin-polarized and non-spin-polarized states. The corresponding energy difference ΔEN  M ¼EN  EM between nonmagnetic and magnetic states is also listed in Table 2, where EN and EM are the total energies of nonmagnetic and magnetic states. The results show that the magnetic state is more stable than the nonmagnetic one in the presence of one Sr or Zr vacancy. But magnetic and nonmagnetic states are equally stable in the presence of O vacancy. The Sr defect in strontium-deficient Sr0.875ZrO3 behaves as a p-type dopant, as a result the Fermi level is moved into the top of the valence band, Fig. 5(b), hence the non-stoichiometric Sr0.875ZrO3 is metallic. Our calculations show that spin splitting between majority and minority spin states occurs mostly for the O 2p states and this non-stoichiometric perovskite is magnetic. The magnetic moments on the 4O atoms nearest to the Sr vacancy (shown in Fig. 6(a)) are about 0.2 μB, whereas no noticeable magnetic moments are found for the other atoms located on the larger distances from vacancy. The Zr defect in zirconium-deficient SrZr0.875O3 also behave as a p-type dopants, as result the Fermi level is moved into the top of the valence band, Fig. 5(c), hence the non-stoichiometric SrZr0.875O3 is metallic. Additionally, the partially occupied O 2p band adopts the spin splitting. The magnetic moments on the 5O atoms nearest to the Zr vacancy (shown in Fig. 6(b)) are about 0.5 μB, whereas no noticeable magnetic moments are found for the other atoms located on the larger distances from vacancy. Note that this situation resembles the reported cases for Ca vacancy in CaO [28,29], Hf vacancy in HfO2 [19] and Mg vacancy in MgO [73], where it has been shown that cationic vacancies in ordinary oxides create a non-zero local magnetic moments. Finally, for the oxygen-deficient SrZrO2.875, the oxygen vacancies behave as n-type dopants, as result the Fermi level is in the bottom of the conduction band and the DOS at the Fermi level is almost zero. Some additional state appears just below the conduction band but no spin splitting is observed between majority and minority spin components, as shown in Fig. 5(c).

and EPR spectroscopy. XRD shows the formation of pure phase at 873 K. Temperature dependence of EPR spectra in the range 77– 560 K for the SrZrO3 sample annealed at 873 K showed a marked broadening and shift of the resonance field position to the lower field as the temperature is lowered; which is the characteristic of ferromagnetic resonance spectra. The observed broadening of the signal and a shift of the center of resonance to the lower H values is attributed to the presence of a nonhomogeneous local magnetic field, which modifies both the resonance H and the line shape of the signal in the ferromagnetic state. For SZO particles annealed at 873 K a value of Curie–Weiss temperature θ  8.7 K is obtained. This speculative Tc value should be confirmed only by performing EPR measurements below to Liq He temperature. The positive sign of the Curie–Weiss temperature indicates that some of the spins are ferromagnetically coupled in this sample. DFT calculations using Ab initio formulation shows that zirconium vacancy is the root cause of ferromagnetic behavior in case of SZO particles.

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jmmm.2015.04.108.

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4. Conclusion SrZrO3 (SZO) with distorted perovskite was synthesized using gel-combustion route and characterized systematically using XRD

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