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The structural, magnetic and vibrational properties of Ti-doped BaMnO3
Journal of Alloys and Compounds xxx (2016) 1e10
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The structural, magnetic and vibrational properties of Ti-doped BaMnO3 D.P. Kozlenko a, N.T. Dang b, *, T.L. Phan c, S.E. Kichanov a, L.H. Khiem d, S.G. Jabarov e, T.A. Tran f, T.V. Manh g, A.T. Le b, T.K. Nguyen h, B.N. Savenko a a
Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, 141980 Dubna, Russia Institute of Research and Development, Duy Tan University, 550000 Da Nang, Viet Nam Department of Physics and Oxide Research Center, Hankuk University of Foreign Studies, Yongin 449-791, South Korea d Advanced Center for Physics, Institute of Physics, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, 100000 Hanoi, Viet Nam e Institute of Physics, ANAS, АZ-1143 Baku, Azerbaijan f Ho Chi Minh City University of Technology and Education, 700000 Ho Chi Minh, Viet Nam g Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea h Dong Hoi Center for Education and Vocational Training, 510000 Quang Binh, Viet Nam b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 April 2016 Received in revised form 9 November 2016 Accepted 10 November 2016 Available online xxx
The structural, magnetic and vibrational properties of the BaMn1-xTixO3 system (0 x 0.25) have been studied by X-ray and neutron powder diffraction, electron spin resonance (ESR) and Raman spectroscopy methods. A sequence of the structural phase transitions 15R / 8H / 9R / 10H / 12R was observed upon Ti doping. The structural parameters of the observed phases were determined. Antiferromagnetic ordering in the 15R, 9R and 8H structural phases was observed at low temperatures. The magnetic propagation vector is q ¼ (0 0 1/2) for the 15R and 9R phases and q ¼ (0 0 0) for the 8H phase. The magnetic ordering temperatures and the ordered Mn magnetic moments are suppressed rapidly with increase of the Ti concentration from TN ¼ 230 (x ¼ 0) to 100 K (x ¼ 0.25) and m ¼ 2.0 to 1.2 mB, respectively. The relationship between the Raman spectra of different structural phases of BaMn1-xTixO3 and concentration dependencies of the vibrational modes were analyzed. © 2016 Elsevier B.V. All rights reserved.
Keywords: Magnetoelectrics Crystal structure Magnetic structure Vibrational property Phase transformation Diffraction methods
1. Introduction Multiferroic materials, exhibiting simultaneously ferroelectric and magnetic orders, are at the forefront of extensive fundamental and applied research. The coupling between ferroelectricity and magnetism giving rise to magnetoelectric effects, opens the opportunity to produce multifunctional devices with the possibility of the ferroelectric polarization control by a magnetic field and, conversely, the manipulation of magnetization by an electric field [1e9]. Mn-based oxides are promising systems for a search of novel magnetoelectric functional materials. BiMnO3 is an unusual example of a multiferroic with ferromagnetism and ferroelectricity realized within the centrosymmetric monoclinic C2/c structure
* Corresponding author. E-mail address: [email protected] (N.T. Dang).
[10]. In perovskite Sr1-xBaxMnO3 compounds, the displacementtype ferroelectricity coexisting with the long range antiferromagnetic order has been recently discovered [11]. A ferroelectric ground state with a large polarization was also predicted for the hypothetical ideal cubic perovskite BaMnO3 [12]. However, the pffiffiffi structural tolerance factor of BaMnO3, t (t ¼ hA Oi= 2hMn Oi) [13], is greater than unity, and it crystallizes naturally in the 2H hexagonal structure with all face-sharing oxygen octahedra instead of a perovskite structure with all corner-sharing MnO6 octahedra [14e17]. The emergence of ferroelectricity in the antiferromagnetic 2H BaMnO3 was also theoretically predicted at low temperature [18]. Despite this, the 2H-BaMnO3 phase was found to be rather unstable with a tendency to remove oxygen upon variation of temperature and/or pressure, and different external actions [17,19e21]. Numerous intermediate structures between the ideal hexagonal and cubic perovskite structures with different ratios of cubic layers (corner-sharing octahedra) to hexagonal layers (face-sharing
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octahedra) were observed in the oxygen deficient BaMnO3-d system [14,17,19,21e26]. It has been established that the ratio of corner to face-sharing octahedra smoothly expands with increasing the oxygen vacancy level [21,22], which is accompanied by the following sequence of structural transformations: 2H / 15R / 8H / 9R / 6H / 10H / 4H [14,17,19,21e26]. The pronounced magnetoelectric properties of the 15R BaMnO2.99 were recently evidenced [27]. The ferroelectric state in Sr1-xBaxMnO3 compounds is of the same nature as found in classical ferroelectric BaTiO3 [11]. Therefore, a doping of BaMnO3 with titanium is expected to provide a promising route for a synthesis of novel functional multiferroic and magnetoelectric materials [28]. Previous studies of the BaMn1xTixO3 system were concentrated mostly on the Ti-rich region 1/ 3 < x < 1. The consecutive structural transformations from initial tetragonal BaTiO3 structure to 6H hexagonal at x ¼ 0.99, then to 12R rhombohedral at x ¼ 0.12 accompanied by significant changes of the dielectric properties [29e36] were detected. However, no evidence for long range magnetic order was found for x > 1/3 [34,35], although weak ferromagnetism of a diluted nature was observed for x ~ 0.99 e 0.9 [36e41]. In contrast, the Mn-rich concentration range of the BaMn1xTixO3 system remains poorly studied. In this paper, we present a systematic study of the structural, magnetic and vibrational properties of BaMn1-xTixO3 in the concentrations range 0 < x < 0.25. 2. Experimental details Polycrystalline BaMn1-xTixO3 (0 x 0.25) samples were prepared by conventional solid-state reaction. High-purity powdered precursors (4 N) of BaCO3, TiO2, and MnO were combined with stoichiometric quantities and mixed well by using an agate mortar and pestle. These mixtures were then calcined at 1050 C for 24 h. After the calcination, the obtained mixtures were pressed into pellets and sintered at 1300 C for 5 h in air. X-ray diffraction patterns of the sintered powders were collected at room temperature using a Siemens D5000 diffractometer equipped with a Cu-Ka1 (l ¼ 1.5406 Å) X-ray radiation source. Neutron powder diffraction measurements were performed in the wide temperature range 10 e 300 K with the DN-12 diffractometer at the IBR-2 high-flux pulsed reactor (FLNP JINR, Dubna, Russia) [42]. Diffraction patterns were collected at a scattering angle of 90 with resolution Dd/d ¼ 0.015. The typical data collection time at one temperature was 2 h. Experimental data of the X-ray and neutron powder diffraction experiments were analyzed by the Rietveld method using the Fullprof program [43]. Electron spin resonance (ESR) measurements were carried out on a JEOL-TE300 spectrometer operating at 9.4 GHz (corresponding to the X-band). For this experiment, an amount of 20-mg samples in powder was loaded in a quartz tube and then put in the resonant cavity, where the magnetic field can be changed from 0 to 21 kOe. Raman spectra were collected at room temperature using a XPLORA-Plus Horiba micro-Raman spectrometer (Ar ion laser) with wavelength of 532 nm, 1800 grating, confocal hole of 300 mm, slit of 100 mm, and a 50 objective. 3. Results and discussion 3.1. Crystal structures The ambient temperature X-ray powder diffraction patterns of studied BaMn1-xTixO3 compounds are shown in Fig. 1. For undoped BaMnO3 the observed diffraction peaks correspond to the 15R rhombohedral unit cell of R3 m symmetry with lattice parameters a z 5.7 Å, c z 35.3 Å. Such a structure was previously reported by
Adkin et al. [22] for BaMnO2.99(1) material and it has the ratio of cubic stacking layers with corner sharing MnO6 octahedra to hexagonal stacking layers with face sharing MnO6 octahedra 1:5 (Fig. 2). The crystal structure visualization as displayed in Fig. 2 was performed by using the Diamond software [44]. The Rietveld refinement of the neutron diffraction data (Fig. 3) within the 15R structural model provided a good fitting quality with Rp ¼ 6.38% and Rwp ¼ 8.34%. The obtained values of the cell parameters and atomic positions for the 15R structure of BaMnO3 (Table 1) are consistent with those previously reported [22]. In the X-ray diffraction patterns of the BaMn0.95Ti0.05O3 compound, an appearance of the extra peaks located at 2q positions of 26.19 and 29.86 , in addition to characteristic reflections of the 15R phase of the pure BaMnO3, was detected (Fig. 1). Data analysis has shown that these peaks can be indexed in the 8H hexagonal crystal structure of P63/mmc symmetry with lattice parameters a z 5.7 Å, c z 18.8 Å. In the 8H structure (Fig. 2), reported previously for the oxygen-deficient BaMnO2.95 [22], the ratio of cubic and hexagonal stacking layers with corner- and face-sharing MnO6 octahedra is reduced to 1:4. The Rietveld refinement of the neutron powder diffraction data (Fig. 3) involving a combination of 15R and 8H phases provided a satisfactory fitting quality (Rp ¼ 7.17 and Rwp ¼ 9.69%). The volume ratio of the 15R and 8H phases of 47.6 : 52.4% was determined (Table 2). The obtained values of the cell parameters and atomic positions for the 8H phase of BaMn0.95Ti0.05O3 are listed in Table 1. The X-ray diffraction patterns of BaMn0.9Ti0.1O3 demonstrate a presence of peaks corresponding to the 8H phase and full suppression of those relevant to the 15R phase (Fig. 1). In addition, extra peaks located at 2q positions of 24.84, 27.95, 35.07 and 47.82 were observed. These peaks correspond to the formation of the rhombohedral 9R phase with R3 m symmetry (Fig. 2) and lattice parameters a z 5.7 Å, c z 20.9 Å, characterized by the ratio of cubic and hexagonal stacking layers with corner- and face-sharing MnO6 octahedra of 1:3. A combination of the 8H and 9R phases provides a satisfactory fitting of the neutron diffraction data. The determined volume ratio of these phases is 43.8 : 56.2% (Table 2). The X-ray and neutron powder diffraction data of the BaMn0.85Ti0.15O3 compound evidence the single 9R structural phase (Figs. 1 and 3), similar to those reported previously for Ba0.875Sr0.125MnO3 [22] and oxygen-deficient Ba0.85Ti0.15MnO2.93 [31]. The obtained values of the cell parameters and atomic positions are listed in Table 1. The X-ray diffraction patterns of BaMn0.8Ti0.2O3 show a presence of the majority 9R phase, while extra peaks, appearing at 2q z 26.3, 28.8 and 29.3 , point to formation of additional structural phases (Fig. 1). The data analysis has shown that all three peaks cannot be assigned to a single phase. Considering the possible polymorphs observed in oxygen-deficient BaMnO3-d and Ti-rich compounds BaMn1-xTixO3 (0.5 < x < 1) [14,17,19,21e26,29e36], it was found that a combination of the 10H hexagonal structure of P63/mmc symmetry and lattice parameters a z 5.7 Å, c z 23.3 Å and the 12R rhombohedral structure of R3 m symmetry and lattice parameters a z 5.7 Å, c z 27.9 Å successfully matches all the observed extra peaks. For the 12R phase a structural model based on the data reported for BaTi0.5Mn0.5O3 [32e35] was used (Fig. 2). For the 10H phase two structural models have been tested. The first one reported for BaMnO2.91 [22] contains face-sharing octahedral trimers alternated by face-sharing octahedral dimers along the crystallographic c-axis (see Fig. 2g). The second one reported for Ce-doped BaMnO3 [45] is composed of face-sharing octahedral tetramers along the crystallographic c-axis isolated by single octahedral layers (see Fig. 2f). Both these models have the same number of independent atomic parameters to be refined. The Rietveld refinements have shown that a combination of the 9R þ 10H þ 12R
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Fig. 1. X-ray diffraction patterns of BaMn1-xTixO3, x ¼ 0, 0.05, 0.1, 0.15, 0.2 and 0.25 measured at ambient temperature. The experimental points and profiles calculated in matching mode are shown. The ticks below represent calculated positions of the nuclear peaks from different structural phases. The inset illustrates an enlarged part of the patterns in the 2q range of 21 e 32.5 .
phase allows satisfactory description of the neutron powder diffraction data (Fig. 3). The best fitting quality with Rp ¼ 6.92 and Rwp ¼ 9.70% for the Ce-doped BaMnO3 model for the 10H phase was obtained and it was finally chosen for this phase description. The alternative structural model for the 10H phase resulted in larger Rfactor values Rp ¼ 7.73 and Rwp ¼ 10.52%. The evaluated volume ratio of the structural phases observed in BaMn0.8Ti0.2O3 is presented in Table 2. The obtained cell parameters and atomic positions for the hexagonal 10H structure of BaMn0.8Ti0.2O3 are listed in Table 1. In the X-ray diffraction patterns of BaMn0.75Ti0.25O3 only reflections corresponding to the 9R and 12R structures were observed. The volume ratio of the 12R phase grows upon increase of the Ti concentration to 38.4% (Table 2). The values of the cell parameters and atomic positions of the 12R structure of BaMn0.75Ti0.25O3 obtained from the Rietveld refinement of the neutron powder diffraction data are shown in Table 1. The values of the characteristic bond distances and angles in different phases of BaMn1-xTixO3 system are summarized in Table 3. It should be mentioned that in order to examine the oxygen stoichiometry of the BaMn1-xTixO3 compounds, trial refinements, where the oxygen site occupancies in the crystal structures of the phases of BaMn1-xTixO3 were allowed to refine, were performed on neutron powder diffraction data collected at room temperature. The refined values showed that all oxygen sites were fully occupied. This indicates that the investigated BaMn1-xTixO3 samples were nearly oxygen stoichiometric.
The structural phase diagram of the BaMn1-xTixO3 system in the Ti concentration range up to 0.25, constructed on the basis of the present experimental data, is shown in Fig. 4. Upon increase of the titanium concentration, the following sequence of the polymorphic phase transitions occurs: 15R / 8H / 9R / 10H / 12R. One should note that the 12R structural phase was observed in BaMn0.5Ti0.5O3 [32e35]. Therefore, a large domain of this phase is expected for Ti concentrations 0.25 < x < 0.5.
3.2. Magnetic structures and properties The electron spin resonance spectra of BaMn1-xTixMnO3 compounds recorded under the excitation of a microwave frequency n ¼ 9.4 GHz at ambient temperature are shown in Fig. 5. They correspond to a single symmetrical broad line of Lorentzian shape for all the studied samples, typical for a paramagnetic state with collective behavior of magnetic ions, and the resonance positions were found to be weakly dependent on the Ti content, Hr ~ 3409 e 3411 Oe. From the resonance condition hn ¼ gmBHr (h e Planck factor constant, mB e Bohr magneton) one can calculate the Lande g z 1.97, which is quite close to the one for free electrons with g z 2.00. The peak-to-peak linewidth DH, calculated as the difference between the minimal and maximal positions in the ESR dP/ dH spectra (P e microwave power absorption) exhibits a strong reduction in the x ¼ 0 e 0.15 Ti concentration range by about 50%, and it becomes nearly constant for x > 0.15 (Fig. 5). Based on the theory of ESR [46e49], in the case of strong
Please cite this article in press as: D.P. Kozlenko, et al., The structural, magnetic and vibrational properties of Ti-doped BaMnO3, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.11.159
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Fig. 2. Structural models of possible polytypes of BaMn1-xTixO3: a) 2H b) 9R c) 15R d) 4H e) 8H f) 10H g) 10H- BaMnO2.91 and h) 12R.
exchange coupled systems such as manganites, the ESR peak-topeak first-derivative linewidth is proportional to Ed/J, where Ed is the dipolar energy and J is the exchange energy. The increase of the concentration of the nonmagnetic ions Ti4þ leads to the decrease of the exchange integral J, resulting in spectral broadening. On the other hand, as the content x increases, the distance between the Mn4þ ions increases, resulting in the decrease of the dipolar interaction between them and narrowing of the spectra. Thus, such a crossover from nearly linear dependence of DH on x to a concentrationeindependent behavior indicates the competition between magnetic superexchange and dipolar interactions in the BaMn1-xTixMnO3 compounds, mediating by the Mn magnetic ions
content. In order to investigate the long range magnetic order features in BaMn1-xTixMnO3 compounds, neutron powder diffraction patterns were collected at temperatures down to 10 K. In all studied samples, the appearance of extra peaks of magnetic nature was observed at low temperatures (Fig. 3). The data analysis has shown that in all compounds the antiferromagnetic (AFM) order is established. The magnetic unit cells corresponding to the 15R and 9R structural phases are doubled along the c-axis with respect to the crystallographic unit cell of the dominating structural phases. This corresponds to the propagation vector of the magnetic structure q ¼ (0 0 1/2). In the case of the 8H phase, the magnetic unit cell
Please cite this article in press as: D.P. Kozlenko, et al., The structural, magnetic and vibrational properties of Ti-doped BaMnO3, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.11.159
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Fig. 3. Neutron diffraction patterns of BaMn1-xTixO3, x ¼ 0, 0.05, 0.1, 0.15, 0.2 and 0.25, measured at 290 and 10 K and refined by the Rietveld method. The experimental points and calculated profiles are shown. The ticks below represent calculated positions of the nuclear peaks form different structural phases. The most intense magnetic peaks are labeled above by adding “M” symbol to the structural phase notation.
coincides with the crystallographic one, resulting in the propagation vector of the magnetic structure q ¼ (0 0 0). The relevant magnetic structures are illustrated in Fig. 6. Using the temperature dependencies of the ordered Mn magnetic moments obtained from neutron diffraction data, the magnetic ordering temperature TN values were evaluated (Fig. 7). TN demonstrates high sensitivity to the concentration of magnetic ions (Mn) and it is suppressed rapidly upon Ti substitution from 230 K for BaMnO3 to 100 K for BaMn0.75Ti0.25O3. Simultaneously, the ordered Mn magnetic moment value m decreases nearly linearly from about 2.0 to 1.2 mB upon increase of the titanium concentration x from 0 to 0.25 (Fig. 7). The TN and m values obtained for the 15R BaMnO3 are close to those reported in Ref. [22]. The upper inset of Fig. 7 shows the positive linear correlation between m and TN, which can be attributed to the magnetic disorder in the Mn sublattice, caused by the Ti doping.
3.3. Raman spectroscopy The Raman spectra of BaMn1-xTixO3 compounds are shown in Fig. 8. The spectra of the compositions with 0.1 x 0.25 containing the dominant structural fraction of the 9R phase look similar. The factor group analysis predicts 9 Raman active modes for the 9R structure, GRam ¼ 4Ag1 þ 5Eg. In accordance to this, nine well resolved modes located at about 635, 567, 528, 458, 422, 405, 350, 260 and 175 cm1 were observed. Taking into account previous studies of the relevant 9R BaRuO3 [50e52], 2H BaMnO3 [53e55]
and hexagonal SrMnO3 [56,57] compounds, the higher frequency modes can be assigned to A1g (635 cm1), Eg (567, 528 cm1) stretching and A1g (458 cm1), Eg (422, 405 cm1) bending vibrations of oxygen atoms. The lower frequency modes can be assigned to A1g (350, 175 cm1) and Eg (260 cm1) vibrations of Mn/Ti atoms. The mass ratio of these atoms of 1.15 only and the detectable splitting of the modes associated with their particular vibrations is not expected. The Raman spectrum of the 15R phase of BaMnO3 is more complex and demonstrates the appearance of extra peaks at 195, 230 cm1, as well as a splitting of the peaks initially located around 635, 567 and 422 cm1 for the 9R phase into groups of peaks with frequencies (632, 651), (592, 575, 560, 548), (425, 410, 390) cm1 (Fig. 8). The irreducible representation of this structure predicts 18 Raman active phonons, GRam ¼ 8A1g þ 10Eg. The 15R structure contains more nonequivalent positions of Mn/Ti and O atoms in comparison with the 9R one (Table 1). From the close frequency values it is reasonable to suggest that the groups of modes (632, 651 cm1) and (592, 575, 560, 550 cm1) are of stretching character and those (425, 410, 390 cm1) of bending character, respectively. The additional low frequency peaks at 230, 195 cm1 can be associated with vibrations of Mn/Ti atoms. According to the structural data, the BaMn0.95Ti0.05O3 compound contains comparable volume fractions of 15R and 8H phases. The Raman spectrum of BaMn0.95Ti0.05O3 looks intermediate between those of 15R and 9R phases. In comparison to BaMnO3, no pronounced splitting of the peak located around 567 cm1 and no
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Table 1 Structural parameters of the observed polymorphic phases of BaMn1-xTixO3. For the rhombohedral unit cells, the lattice parameters are given in the hexagonal setting. x (Ti)
0
0.05
0.15
0.2
0.25
Phase (space group)
15R (R3 m)
8H (P63/mmc)
9R (R3 m)
10H (P63/mmc)
12R (R3 m)
5.6786(3) 35.3076(5)
5.6819(4) 18.7784(5)
5.6800(4) 20.9944(6)
5.6525(4) 23.2622(6)
5.6606(4) 27.9628(5)
0 0 0 0 0 0.1337(1) 0 0 0.2653(1) 0 0 0.3618(1) 0 0 0.4316(2) 0 0 0.5 0.5 0 0 0.1860(2) 0.1860(2) 0.0642(2) 0.4833(3) 0.4833(3) 0.1328(2)
0 0 0 0 0 0.25 1/3 2/3 0.1343(3) 1/3 2/3 0.5542(3) 1/3 2/3 0.6857(2) e e e 0.5 0 0 0.5165(4) 0.5165(4) 0.25 0.1852(4) 0.1852(4) 0.8795(4)
0 0 0 0 0 0.2160(2) e e e 0 0 0.3817(3) 0 0 0.5 e e e 0.5 0 0 0.1473(4) 0.1473(4) 0.5580(3) e e e
2/3 1/3 1/4 2/3 1/3 0.4682(5) 0 0 0.3392(4) 0 0 0.5 1/3 2/3 0.4045(3) 1/3 2/3 0.2989(3) 0.1670(2) 0.3352(3) 0.4482(4) 0.1780(3) 0.3560(4) 1/4 0.4800(2) 0.9600(3) 0.3500(4)
0 0 0.2798(2) 0 0 0.1264(2) e e e 0 0 0 0 0 0.4081(3) 0 0 0.5 0.1678(4) 0.8251(4) 0.6235(4) 0.1470(4) 0.8351(4) 0.4587(4) e e e
Lattice parameters a (Å) c (Å) Atomic coordinates Ba1:
Ba2:
Ba3:
M1 (Mn/Ti):
M2 (Mn/Ti):
M3 (Mn/Ti):
O1:
O2:
O3:
x y z x y z x y z x y z x y z x y z x y z x y z x y z
Table 2 The phase fractions in BaMn1-xTixO3 with 0 x 0.25 determined from neutron powder diffraction data refinement. Sample
Phase
Phase fraction (%)
x¼0 x ¼ 0.05
15R 15R 8H 8H 9R 9R 9R 10H 12R 9R 12R
100 47.6 52.4 43.8 56.2 100 71.5 19.7 8.8 61.6 38.4
x ¼ 0.1 x ¼ 0.15 x ¼ 0.2
x ¼ 0.25
extra peaks at 190 and 230 cm1 were detected. Moreover, there is no peak at 350 cm1, observed in the Raman spectra of the 9R phase (Fig. 8).
3.4. Discussion In the BaMn1-xTixO3 system moderate Ti doping x < 0.2 leads to formation of structural polytypes composed by chain units containing face-shared MnO6 octahedra with a direct corner-shared connection between them (15R, 8H, 9R phases), Fig. 2. The Ti and Mn atoms have similar values of coherent neutron scattering lengths and it is difficult to determine preferential occupancies of the relevant crystallographic sites. Nevertheless, it is wellestablished that Ti4þ ions in octahedral O2 coordination tend to form longer bond lengths of about 2.00 Å in comparison with those of about 1.92 Å for Mn4þ ions [34]. The MieOie bonds (M ¼ Mn/Ti,
i ¼ 1 e 3) range from 1.883 to 1.930 Å (Table 3), pointing to chemical disorder of Ti and Mn ions within the M1-M3 crystallographic sites of the 15R phase and the M1 e M2 sites of the 8H and 9R phases. The magnetic ordering temperature in the Ti concentration range x ¼ 0 e 0.15 weakly decreases from TN ¼ 230 to 200 K. Simultaneously, the average MneOeMn bonding angle between facesharing MnO6 octahedra, controlling dominant magnetic superexchange interactions, increases from 79.6 to 81.2 (Table 3). The increase of the MneOeMn bonding angle itself is expected to strengthen the magnetic interactions and enlarge the magnetic ordering temperature [22,58,59]. However, a doping by the diamagnetic Ti4þ ions simultaneously leads to the disorder effects on the magnetic Mn sublattice, which should result in a gradual suppression of the long range magnetic order. Therefore, the observed reduction of the TN value is mainly controlled by enhanced magnetic disorder due to the increasing concentration of the diamagnetic Ti4þ ions. In the BaMn1-xTixO3 compounds with larger Ti doping x 0.2, additional 10H and 12R phases are formed. These phases incorporate chain units containing face-sharing MnO6 octahedra connected via intermediate corner-sharing octahedra, Fig. 2. It is important to note that upon increase of the Ti content the MneOeMn bonding angle between corner-sharing MnO6 octahedra gradually decreases from 180 for the 15R, 8H and 9R phases to 175.8 for the 10H phase and then to 170.4 for the 12R phase (Table 3). This leads to significantly weaken the magnetic interaction between facesharing octahedral blocks in the 10H and 12R phases [22,58,59]. In other hand, the M1 site, located in the center of corner-sharing octahedra, has long M1eO1 bond lengths of 2.036 (x ¼ 0.2) and 1.995 Å (x ¼ 0.25), evidencing preferential occupation by Ti4þ ions. The relevant oxygen bonds for the M2 and M3 sites range from 1.864 to 1.917 Å (Table 3), implying chemical disorder of Ti4þ and
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Table 3 Bond distances and angles in the polymorphic phases of BaMn1-xTixO3. x (Ti)
0
0.05
0.15
0.2
0.25
Phase
15R (R3 m)
8H (P63/mmc)
9R (R3 m)
10H (P63/mmc)
12R (R3 m)
1.929(3) 3 1.924(6) 3 e e 1.893(4) 6 e e e
2.036(6) 6
1.995(7) 6
3
1.898(7) 3 e e 1.917(7) 3 e
Bond distances MeO (Å), M ¼Mn/Ti M1-O1 1.923(2) 3 M1-O2 1.921(3) 3 M1-O3 e M2-O1 e M2-O2 1.883(4) 3 M2-O3 1.914(5) 3 M3-O2 e M3-O3 1.899(5) 6 Ba-O (Å) Ba1-O1 2.839(3) 6
2.841(3) 6
2.840(2) 6
Ba1-O2 Ba1-O3 Ba2-O1
2.913(5) 6 e e
e 2.906(5) 6 e
2.925(5) 6 e 2.959(4) 3
Ba2-O2
3.061(6) 3
2.846(2) 6
2.852(3) 6, 2.903(6) 3
Ba2-O3
2.844(2) 2.970(5) 2.908(3) 2.849(3) 2.912(5)
3.039(6) 6
e
3.300(7) 3
3.008(5) 3 2.823(5) 3 2.859(2) 6
e e e
3.020(7) 3 2.710(7) 3 2.844(2) 6
180.0 e e 80.5(3) 78.5(2) e e
180.0 e 81.2
Ba3-O1 Ba3-O2 Ba3-O3 Bond angles MeOeM (deg.) M1-O1-M1 180.0 M1-O1-M2 e M1-O2-M2 80.7(2) M1-O3-M2 e M2-O2-M2 e M2-O2-M3 e M2-O3-M3 78.6(3) M3-O2-M3
6, 3 3 6 3
1.930(3) e 1.917(6) e 1.908(3) 1.904(6) e e
3 3 3 3
Fig. 4. The structural phase diagram of the BaMn1-xTixO3 system in the Ti concentration range x 0.25. Left Y-axis: Concentration dependence of volume fraction of the 15R and 9R phases of BaMn1-xTixO3. Right Y-axis: Concentration dependence of volume fraction of the 8H, 10H and 12R phases of BaMn1-xTixO3. The solid circles represent the experimental data.
Mn4þ ions over these positions. The preferential occupation of diamagnetic Ti4þ ions in corner-sharing octahedra leads to formation of isolated magnetic tetramers and trimers in 10H and 12R structures with a weak magnetic coupling between them. For the BaTi0.5Mn0.5O3 compound also possessing the 12R structure, it was
e
1.915(5) e 1.916(5) 1.899(5) 1.864(5)
2.828(2) 2.958(7) 2.530(7) 2.861(3) e
175.8(3) e e e e 81.1(4) 73.6(2)
3 3 3
6 6 3, 6
2.826(2) 3, 2.866(3) 3, 3.18(1) 3 2.66(1) 3 2.82(1) 3 2.782(3) 3, 2.870(7) 3, 2.883(3) 3
170.4(2)
79.8(4)
Fig. 5. The concentration dependence of peak-to-peak ESR linewidth and measured ESR spectra dP/dH (inset, P - microwave power absorption) for BaMn1-xTixO3.
pointed out that the preferential occupation of diamagnetic Ti4þ ions for corner-sharing octahedral sites prevents magnetic interaction between face-sharing octahedral blocks, consequently, leads to the lack of long range magnetic order [32e35]. We have not found any evidence for the appearance of long range magnetic order in the 10H and 12R phases of BaMn0.8Ti0.2O3 and BaMn0.75Ti0.25O3 compounds, which likely remain magnetically disordered
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Fig. 6. The magnetic structures corresponding to the 15R, 8H and 9R phases of BaMn1-xTixO3.
down to the lowest temperature of our study of 10 K, similarly to BaTi0.5Mn0.5O3 [32e35]. A coexistence of the regions with intrinsic magnetic disorder in the 10H and 12R phases and magnetically ordered regions in the 9R phase is a possible reason for a rapid TN suppression to 100 K and significantly suppressed ordered Mn magnetic moments in BaMn1-xTixO3 compounds with x ¼ 0.2 e 0.25. The formation of isolated face-sharing octahedral blocks in 10H and 12R phases of these compounds may also be responsible for the peculiar behavior of the ESR peak-to-peak linewidth. One should note that while Ti doping leads to an increase of the averaged bonds from about 1.906 to 1.967 Å, the averaged bonds are weakly modified and exhibit a weak tendency to decrease from about 2.895 to 2.861 Å in the observed structural polytypes. In the Raman spectra the modes located at the stretching region with frequencies of 567 and 528 cm1 (at x ¼ 0) have a qualitatively similar concentration behavior with a frequency increase for x < 0.15 followed by a decrease at higher Ti concentrations (Fig. 8). This trend correlated with an opposite concentration variation of the average MneO bond for M2,3 sites (Table 3) and may be associated with the stretching of face-sharing MnO6 octahedra. The mode with initial frequency of 635 cm1 exhibits a tendency towards decreasing upon the growing of the x value and reflects the opposite behavior of the average MneO bond over all the types of sites Mn1 e 3. Therefore this could be a breathing mode involving both face- and corner-sharing octahedra. The modes located at the bending region with frequencies of about 455, 425 and 400 cm1 (at x > 0.05) exhibit a tendency towards decreasing upon increase of the Ti content. Such a behavior correlates with a lengthening of the average Ba1eO bonds. Similar correlations between variation of the stretching and bending mode frequencies and bond distances were also observed in perovskitelike manganites containing corner-sharing MnO6 octahedra only [60]. It is more difficult to establish correlations between particular bond distances and angles for the lower frequency modes due to the complexity of the observed structures. 4. Conclusions The results of our study show that the BaMn1-xTixO3 system
Fig. 7. Concentration dependencies of the magnetic ordering temperature TN (top) and ordered Mn magnetic moment (below) of the structural phases with dominant concentration in BaMn1-xTixO3. The inset above: correlation between the ordered Mn magnetic moment and the magnetic ordering temperature TN. The inset below: selected temperature dependencies of the ordered Mn magnetic moment and their interpolation by the function m(T) ¼ m0,(1 (T/TN)a)2b, with a ¼ 1.65(6), b ¼ 0.14(2) for x ¼ 0; a ¼ 1.29(7), b ¼ 0.26(4) for x ¼ 0.1 and a ¼ 1.10(9), b ¼ 0.21(6) for x ¼ 0.2.
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9
Fig. 8. Left: Raman spectra collected at room temperature for BaMn1-xTixO3. Right: Concentration dependencies of the selected Raman mode frequencies of BaMn1-xTixO3.
exhibits a complex structural phase diagram in a rather narrow range of Ti concentrations 0 x 0.25, with a consequence of polymorphic phase transitions 15R / 8H / 9R / 10H / 12R upon increase of the x value. The 15R, 8H and 9R phases are ordered antiferromagnetically at low temperatures. The magnetic propagation vector is q ¼ (0 0 1/2) for the 15R and 9R phases, whereas q ¼ (0 0 0) for the 8H phase. The magnetic ordering temperatures and ordered Mn magnetic moments are suppressed rapidly upon growth of the Ti content. In contrast, the 10H and 12R phases for x ¼ 0.2 and x ¼ 0.25 remain magnetically disordered in the studied temperature range. It is most likely due to the formation of isolated magnetic tetramers and trimers in these phases, which may also be responsible for the peculiar behavior of the ESR peak-to-peak linewidth at x > 015 due to the enhanced role of magnetic dipolar interactions over superexchange ones. The Raman spectra of BaMn1-xTixO3 reflect the observed structural modifications. A relationship between the concentration variation of the stretching and bending modes and particular MeO (M ¼ Mn/Ti) and BaeO distances was revealed. Acknowledgments The work has been supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02e2014.11 and the RFBR grant N 15-5254008_viet_а. This work has been jointly supported by the Vietnam Academy of Science and Technology and the Russian Academy of Sciences under project VAST.HTQT.NGA.01/15e16. References [1] S.-W. Cheong, M. Mostovoy, Multiferroics: a magnetic twist for ferroelectricity, Nat. Mater 6 (2007) 13e20, http://dx.doi.org/10.1038/nmat1804. [2] R.E. Cohen, Origin of ferroelectricity in perovskite oxides, Nature 358 (1992) 136e138, http://dx.doi.org/10.1038/358136a0. [3] W. Eerenstein, N.D. Mathur, J.F. Scott, J.F. Scott, Multiferroic and magnetoelectric materials, Nature 442759 (2006) 759e765, http://dx.doi.org/10.1038/ nature05023. [4] M. Fiebig, Revival of the magnetoelectric effect, J. Phys. D. Appl. Phys. 38 (2005) R123eR152, http://dx.doi.org/10.1088/0022-3727/38/8/R01.
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The structural, magnetic and vibrational properties of Ti-doped BaMnO3 D.P. Kozlenko a, N.T. Dang b, *, T.L. Phan c, S.E. Kichanov a, L.H. Khiem d, S.G. Jabarov e, T.A. Tran f, T.V. Manh g, A.T. Le b, T.K. Nguyen h, B.N. Savenko a a
Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, 141980 Dubna, Russia Institute of Research and Development, Duy Tan University, 550000 Da Nang, Viet Nam Department of Physics and Oxide Research Center, Hankuk University of Foreign Studies, Yongin 449-791, South Korea d Advanced Center for Physics, Institute of Physics, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, 100000 Hanoi, Viet Nam e Institute of Physics, ANAS, АZ-1143 Baku, Azerbaijan f Ho Chi Minh City University of Technology and Education, 700000 Ho Chi Minh, Viet Nam g Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea h Dong Hoi Center for Education and Vocational Training, 510000 Quang Binh, Viet Nam b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 April 2016 Received in revised form 9 November 2016 Accepted 10 November 2016 Available online xxx
The structural, magnetic and vibrational properties of the BaMn1-xTixO3 system (0 x 0.25) have been studied by X-ray and neutron powder diffraction, electron spin resonance (ESR) and Raman spectroscopy methods. A sequence of the structural phase transitions 15R / 8H / 9R / 10H / 12R was observed upon Ti doping. The structural parameters of the observed phases were determined. Antiferromagnetic ordering in the 15R, 9R and 8H structural phases was observed at low temperatures. The magnetic propagation vector is q ¼ (0 0 1/2) for the 15R and 9R phases and q ¼ (0 0 0) for the 8H phase. The magnetic ordering temperatures and the ordered Mn magnetic moments are suppressed rapidly with increase of the Ti concentration from TN ¼ 230 (x ¼ 0) to 100 K (x ¼ 0.25) and m ¼ 2.0 to 1.2 mB, respectively. The relationship between the Raman spectra of different structural phases of BaMn1-xTixO3 and concentration dependencies of the vibrational modes were analyzed. © 2016 Elsevier B.V. All rights reserved.
Keywords: Magnetoelectrics Crystal structure Magnetic structure Vibrational property Phase transformation Diffraction methods
1. Introduction Multiferroic materials, exhibiting simultaneously ferroelectric and magnetic orders, are at the forefront of extensive fundamental and applied research. The coupling between ferroelectricity and magnetism giving rise to magnetoelectric effects, opens the opportunity to produce multifunctional devices with the possibility of the ferroelectric polarization control by a magnetic field and, conversely, the manipulation of magnetization by an electric field [1e9]. Mn-based oxides are promising systems for a search of novel magnetoelectric functional materials. BiMnO3 is an unusual example of a multiferroic with ferromagnetism and ferroelectricity realized within the centrosymmetric monoclinic C2/c structure
* Corresponding author. E-mail address: [email protected] (N.T. Dang).
[10]. In perovskite Sr1-xBaxMnO3 compounds, the displacementtype ferroelectricity coexisting with the long range antiferromagnetic order has been recently discovered [11]. A ferroelectric ground state with a large polarization was also predicted for the hypothetical ideal cubic perovskite BaMnO3 [12]. However, the pffiffiffi structural tolerance factor of BaMnO3, t (t ¼ hA Oi= 2hMn Oi) [13], is greater than unity, and it crystallizes naturally in the 2H hexagonal structure with all face-sharing oxygen octahedra instead of a perovskite structure with all corner-sharing MnO6 octahedra [14e17]. The emergence of ferroelectricity in the antiferromagnetic 2H BaMnO3 was also theoretically predicted at low temperature [18]. Despite this, the 2H-BaMnO3 phase was found to be rather unstable with a tendency to remove oxygen upon variation of temperature and/or pressure, and different external actions [17,19e21]. Numerous intermediate structures between the ideal hexagonal and cubic perovskite structures with different ratios of cubic layers (corner-sharing octahedra) to hexagonal layers (face-sharing
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octahedra) were observed in the oxygen deficient BaMnO3-d system [14,17,19,21e26]. It has been established that the ratio of corner to face-sharing octahedra smoothly expands with increasing the oxygen vacancy level [21,22], which is accompanied by the following sequence of structural transformations: 2H / 15R / 8H / 9R / 6H / 10H / 4H [14,17,19,21e26]. The pronounced magnetoelectric properties of the 15R BaMnO2.99 were recently evidenced [27]. The ferroelectric state in Sr1-xBaxMnO3 compounds is of the same nature as found in classical ferroelectric BaTiO3 [11]. Therefore, a doping of BaMnO3 with titanium is expected to provide a promising route for a synthesis of novel functional multiferroic and magnetoelectric materials [28]. Previous studies of the BaMn1xTixO3 system were concentrated mostly on the Ti-rich region 1/ 3 < x < 1. The consecutive structural transformations from initial tetragonal BaTiO3 structure to 6H hexagonal at x ¼ 0.99, then to 12R rhombohedral at x ¼ 0.12 accompanied by significant changes of the dielectric properties [29e36] were detected. However, no evidence for long range magnetic order was found for x > 1/3 [34,35], although weak ferromagnetism of a diluted nature was observed for x ~ 0.99 e 0.9 [36e41]. In contrast, the Mn-rich concentration range of the BaMn1xTixO3 system remains poorly studied. In this paper, we present a systematic study of the structural, magnetic and vibrational properties of BaMn1-xTixO3 in the concentrations range 0 < x < 0.25. 2. Experimental details Polycrystalline BaMn1-xTixO3 (0 x 0.25) samples were prepared by conventional solid-state reaction. High-purity powdered precursors (4 N) of BaCO3, TiO2, and MnO were combined with stoichiometric quantities and mixed well by using an agate mortar and pestle. These mixtures were then calcined at 1050 C for 24 h. After the calcination, the obtained mixtures were pressed into pellets and sintered at 1300 C for 5 h in air. X-ray diffraction patterns of the sintered powders were collected at room temperature using a Siemens D5000 diffractometer equipped with a Cu-Ka1 (l ¼ 1.5406 Å) X-ray radiation source. Neutron powder diffraction measurements were performed in the wide temperature range 10 e 300 K with the DN-12 diffractometer at the IBR-2 high-flux pulsed reactor (FLNP JINR, Dubna, Russia) [42]. Diffraction patterns were collected at a scattering angle of 90 with resolution Dd/d ¼ 0.015. The typical data collection time at one temperature was 2 h. Experimental data of the X-ray and neutron powder diffraction experiments were analyzed by the Rietveld method using the Fullprof program [43]. Electron spin resonance (ESR) measurements were carried out on a JEOL-TE300 spectrometer operating at 9.4 GHz (corresponding to the X-band). For this experiment, an amount of 20-mg samples in powder was loaded in a quartz tube and then put in the resonant cavity, where the magnetic field can be changed from 0 to 21 kOe. Raman spectra were collected at room temperature using a XPLORA-Plus Horiba micro-Raman spectrometer (Ar ion laser) with wavelength of 532 nm, 1800 grating, confocal hole of 300 mm, slit of 100 mm, and a 50 objective. 3. Results and discussion 3.1. Crystal structures The ambient temperature X-ray powder diffraction patterns of studied BaMn1-xTixO3 compounds are shown in Fig. 1. For undoped BaMnO3 the observed diffraction peaks correspond to the 15R rhombohedral unit cell of R3 m symmetry with lattice parameters a z 5.7 Å, c z 35.3 Å. Such a structure was previously reported by
Adkin et al. [22] for BaMnO2.99(1) material and it has the ratio of cubic stacking layers with corner sharing MnO6 octahedra to hexagonal stacking layers with face sharing MnO6 octahedra 1:5 (Fig. 2). The crystal structure visualization as displayed in Fig. 2 was performed by using the Diamond software [44]. The Rietveld refinement of the neutron diffraction data (Fig. 3) within the 15R structural model provided a good fitting quality with Rp ¼ 6.38% and Rwp ¼ 8.34%. The obtained values of the cell parameters and atomic positions for the 15R structure of BaMnO3 (Table 1) are consistent with those previously reported [22]. In the X-ray diffraction patterns of the BaMn0.95Ti0.05O3 compound, an appearance of the extra peaks located at 2q positions of 26.19 and 29.86 , in addition to characteristic reflections of the 15R phase of the pure BaMnO3, was detected (Fig. 1). Data analysis has shown that these peaks can be indexed in the 8H hexagonal crystal structure of P63/mmc symmetry with lattice parameters a z 5.7 Å, c z 18.8 Å. In the 8H structure (Fig. 2), reported previously for the oxygen-deficient BaMnO2.95 [22], the ratio of cubic and hexagonal stacking layers with corner- and face-sharing MnO6 octahedra is reduced to 1:4. The Rietveld refinement of the neutron powder diffraction data (Fig. 3) involving a combination of 15R and 8H phases provided a satisfactory fitting quality (Rp ¼ 7.17 and Rwp ¼ 9.69%). The volume ratio of the 15R and 8H phases of 47.6 : 52.4% was determined (Table 2). The obtained values of the cell parameters and atomic positions for the 8H phase of BaMn0.95Ti0.05O3 are listed in Table 1. The X-ray diffraction patterns of BaMn0.9Ti0.1O3 demonstrate a presence of peaks corresponding to the 8H phase and full suppression of those relevant to the 15R phase (Fig. 1). In addition, extra peaks located at 2q positions of 24.84, 27.95, 35.07 and 47.82 were observed. These peaks correspond to the formation of the rhombohedral 9R phase with R3 m symmetry (Fig. 2) and lattice parameters a z 5.7 Å, c z 20.9 Å, characterized by the ratio of cubic and hexagonal stacking layers with corner- and face-sharing MnO6 octahedra of 1:3. A combination of the 8H and 9R phases provides a satisfactory fitting of the neutron diffraction data. The determined volume ratio of these phases is 43.8 : 56.2% (Table 2). The X-ray and neutron powder diffraction data of the BaMn0.85Ti0.15O3 compound evidence the single 9R structural phase (Figs. 1 and 3), similar to those reported previously for Ba0.875Sr0.125MnO3 [22] and oxygen-deficient Ba0.85Ti0.15MnO2.93 [31]. The obtained values of the cell parameters and atomic positions are listed in Table 1. The X-ray diffraction patterns of BaMn0.8Ti0.2O3 show a presence of the majority 9R phase, while extra peaks, appearing at 2q z 26.3, 28.8 and 29.3 , point to formation of additional structural phases (Fig. 1). The data analysis has shown that all three peaks cannot be assigned to a single phase. Considering the possible polymorphs observed in oxygen-deficient BaMnO3-d and Ti-rich compounds BaMn1-xTixO3 (0.5 < x < 1) [14,17,19,21e26,29e36], it was found that a combination of the 10H hexagonal structure of P63/mmc symmetry and lattice parameters a z 5.7 Å, c z 23.3 Å and the 12R rhombohedral structure of R3 m symmetry and lattice parameters a z 5.7 Å, c z 27.9 Å successfully matches all the observed extra peaks. For the 12R phase a structural model based on the data reported for BaTi0.5Mn0.5O3 [32e35] was used (Fig. 2). For the 10H phase two structural models have been tested. The first one reported for BaMnO2.91 [22] contains face-sharing octahedral trimers alternated by face-sharing octahedral dimers along the crystallographic c-axis (see Fig. 2g). The second one reported for Ce-doped BaMnO3 [45] is composed of face-sharing octahedral tetramers along the crystallographic c-axis isolated by single octahedral layers (see Fig. 2f). Both these models have the same number of independent atomic parameters to be refined. The Rietveld refinements have shown that a combination of the 9R þ 10H þ 12R
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Fig. 1. X-ray diffraction patterns of BaMn1-xTixO3, x ¼ 0, 0.05, 0.1, 0.15, 0.2 and 0.25 measured at ambient temperature. The experimental points and profiles calculated in matching mode are shown. The ticks below represent calculated positions of the nuclear peaks from different structural phases. The inset illustrates an enlarged part of the patterns in the 2q range of 21 e 32.5 .
phase allows satisfactory description of the neutron powder diffraction data (Fig. 3). The best fitting quality with Rp ¼ 6.92 and Rwp ¼ 9.70% for the Ce-doped BaMnO3 model for the 10H phase was obtained and it was finally chosen for this phase description. The alternative structural model for the 10H phase resulted in larger Rfactor values Rp ¼ 7.73 and Rwp ¼ 10.52%. The evaluated volume ratio of the structural phases observed in BaMn0.8Ti0.2O3 is presented in Table 2. The obtained cell parameters and atomic positions for the hexagonal 10H structure of BaMn0.8Ti0.2O3 are listed in Table 1. In the X-ray diffraction patterns of BaMn0.75Ti0.25O3 only reflections corresponding to the 9R and 12R structures were observed. The volume ratio of the 12R phase grows upon increase of the Ti concentration to 38.4% (Table 2). The values of the cell parameters and atomic positions of the 12R structure of BaMn0.75Ti0.25O3 obtained from the Rietveld refinement of the neutron powder diffraction data are shown in Table 1. The values of the characteristic bond distances and angles in different phases of BaMn1-xTixO3 system are summarized in Table 3. It should be mentioned that in order to examine the oxygen stoichiometry of the BaMn1-xTixO3 compounds, trial refinements, where the oxygen site occupancies in the crystal structures of the phases of BaMn1-xTixO3 were allowed to refine, were performed on neutron powder diffraction data collected at room temperature. The refined values showed that all oxygen sites were fully occupied. This indicates that the investigated BaMn1-xTixO3 samples were nearly oxygen stoichiometric.
The structural phase diagram of the BaMn1-xTixO3 system in the Ti concentration range up to 0.25, constructed on the basis of the present experimental data, is shown in Fig. 4. Upon increase of the titanium concentration, the following sequence of the polymorphic phase transitions occurs: 15R / 8H / 9R / 10H / 12R. One should note that the 12R structural phase was observed in BaMn0.5Ti0.5O3 [32e35]. Therefore, a large domain of this phase is expected for Ti concentrations 0.25 < x < 0.5.
3.2. Magnetic structures and properties The electron spin resonance spectra of BaMn1-xTixMnO3 compounds recorded under the excitation of a microwave frequency n ¼ 9.4 GHz at ambient temperature are shown in Fig. 5. They correspond to a single symmetrical broad line of Lorentzian shape for all the studied samples, typical for a paramagnetic state with collective behavior of magnetic ions, and the resonance positions were found to be weakly dependent on the Ti content, Hr ~ 3409 e 3411 Oe. From the resonance condition hn ¼ gmBHr (h e Planck factor constant, mB e Bohr magneton) one can calculate the Lande g z 1.97, which is quite close to the one for free electrons with g z 2.00. The peak-to-peak linewidth DH, calculated as the difference between the minimal and maximal positions in the ESR dP/ dH spectra (P e microwave power absorption) exhibits a strong reduction in the x ¼ 0 e 0.15 Ti concentration range by about 50%, and it becomes nearly constant for x > 0.15 (Fig. 5). Based on the theory of ESR [46e49], in the case of strong
Please cite this article in press as: D.P. Kozlenko, et al., The structural, magnetic and vibrational properties of Ti-doped BaMnO3, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.11.159
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Fig. 2. Structural models of possible polytypes of BaMn1-xTixO3: a) 2H b) 9R c) 15R d) 4H e) 8H f) 10H g) 10H- BaMnO2.91 and h) 12R.
exchange coupled systems such as manganites, the ESR peak-topeak first-derivative linewidth is proportional to Ed/J, where Ed is the dipolar energy and J is the exchange energy. The increase of the concentration of the nonmagnetic ions Ti4þ leads to the decrease of the exchange integral J, resulting in spectral broadening. On the other hand, as the content x increases, the distance between the Mn4þ ions increases, resulting in the decrease of the dipolar interaction between them and narrowing of the spectra. Thus, such a crossover from nearly linear dependence of DH on x to a concentrationeindependent behavior indicates the competition between magnetic superexchange and dipolar interactions in the BaMn1-xTixMnO3 compounds, mediating by the Mn magnetic ions
content. In order to investigate the long range magnetic order features in BaMn1-xTixMnO3 compounds, neutron powder diffraction patterns were collected at temperatures down to 10 K. In all studied samples, the appearance of extra peaks of magnetic nature was observed at low temperatures (Fig. 3). The data analysis has shown that in all compounds the antiferromagnetic (AFM) order is established. The magnetic unit cells corresponding to the 15R and 9R structural phases are doubled along the c-axis with respect to the crystallographic unit cell of the dominating structural phases. This corresponds to the propagation vector of the magnetic structure q ¼ (0 0 1/2). In the case of the 8H phase, the magnetic unit cell
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Fig. 3. Neutron diffraction patterns of BaMn1-xTixO3, x ¼ 0, 0.05, 0.1, 0.15, 0.2 and 0.25, measured at 290 and 10 K and refined by the Rietveld method. The experimental points and calculated profiles are shown. The ticks below represent calculated positions of the nuclear peaks form different structural phases. The most intense magnetic peaks are labeled above by adding “M” symbol to the structural phase notation.
coincides with the crystallographic one, resulting in the propagation vector of the magnetic structure q ¼ (0 0 0). The relevant magnetic structures are illustrated in Fig. 6. Using the temperature dependencies of the ordered Mn magnetic moments obtained from neutron diffraction data, the magnetic ordering temperature TN values were evaluated (Fig. 7). TN demonstrates high sensitivity to the concentration of magnetic ions (Mn) and it is suppressed rapidly upon Ti substitution from 230 K for BaMnO3 to 100 K for BaMn0.75Ti0.25O3. Simultaneously, the ordered Mn magnetic moment value m decreases nearly linearly from about 2.0 to 1.2 mB upon increase of the titanium concentration x from 0 to 0.25 (Fig. 7). The TN and m values obtained for the 15R BaMnO3 are close to those reported in Ref. [22]. The upper inset of Fig. 7 shows the positive linear correlation between m and TN, which can be attributed to the magnetic disorder in the Mn sublattice, caused by the Ti doping.
3.3. Raman spectroscopy The Raman spectra of BaMn1-xTixO3 compounds are shown in Fig. 8. The spectra of the compositions with 0.1 x 0.25 containing the dominant structural fraction of the 9R phase look similar. The factor group analysis predicts 9 Raman active modes for the 9R structure, GRam ¼ 4Ag1 þ 5Eg. In accordance to this, nine well resolved modes located at about 635, 567, 528, 458, 422, 405, 350, 260 and 175 cm1 were observed. Taking into account previous studies of the relevant 9R BaRuO3 [50e52], 2H BaMnO3 [53e55]
and hexagonal SrMnO3 [56,57] compounds, the higher frequency modes can be assigned to A1g (635 cm1), Eg (567, 528 cm1) stretching and A1g (458 cm1), Eg (422, 405 cm1) bending vibrations of oxygen atoms. The lower frequency modes can be assigned to A1g (350, 175 cm1) and Eg (260 cm1) vibrations of Mn/Ti atoms. The mass ratio of these atoms of 1.15 only and the detectable splitting of the modes associated with their particular vibrations is not expected. The Raman spectrum of the 15R phase of BaMnO3 is more complex and demonstrates the appearance of extra peaks at 195, 230 cm1, as well as a splitting of the peaks initially located around 635, 567 and 422 cm1 for the 9R phase into groups of peaks with frequencies (632, 651), (592, 575, 560, 548), (425, 410, 390) cm1 (Fig. 8). The irreducible representation of this structure predicts 18 Raman active phonons, GRam ¼ 8A1g þ 10Eg. The 15R structure contains more nonequivalent positions of Mn/Ti and O atoms in comparison with the 9R one (Table 1). From the close frequency values it is reasonable to suggest that the groups of modes (632, 651 cm1) and (592, 575, 560, 550 cm1) are of stretching character and those (425, 410, 390 cm1) of bending character, respectively. The additional low frequency peaks at 230, 195 cm1 can be associated with vibrations of Mn/Ti atoms. According to the structural data, the BaMn0.95Ti0.05O3 compound contains comparable volume fractions of 15R and 8H phases. The Raman spectrum of BaMn0.95Ti0.05O3 looks intermediate between those of 15R and 9R phases. In comparison to BaMnO3, no pronounced splitting of the peak located around 567 cm1 and no
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Table 1 Structural parameters of the observed polymorphic phases of BaMn1-xTixO3. For the rhombohedral unit cells, the lattice parameters are given in the hexagonal setting. x (Ti)
0
0.05
0.15
0.2
0.25
Phase (space group)
15R (R3 m)
8H (P63/mmc)
9R (R3 m)
10H (P63/mmc)
12R (R3 m)
5.6786(3) 35.3076(5)
5.6819(4) 18.7784(5)
5.6800(4) 20.9944(6)
5.6525(4) 23.2622(6)
5.6606(4) 27.9628(5)
0 0 0 0 0 0.1337(1) 0 0 0.2653(1) 0 0 0.3618(1) 0 0 0.4316(2) 0 0 0.5 0.5 0 0 0.1860(2) 0.1860(2) 0.0642(2) 0.4833(3) 0.4833(3) 0.1328(2)
0 0 0 0 0 0.25 1/3 2/3 0.1343(3) 1/3 2/3 0.5542(3) 1/3 2/3 0.6857(2) e e e 0.5 0 0 0.5165(4) 0.5165(4) 0.25 0.1852(4) 0.1852(4) 0.8795(4)
0 0 0 0 0 0.2160(2) e e e 0 0 0.3817(3) 0 0 0.5 e e e 0.5 0 0 0.1473(4) 0.1473(4) 0.5580(3) e e e
2/3 1/3 1/4 2/3 1/3 0.4682(5) 0 0 0.3392(4) 0 0 0.5 1/3 2/3 0.4045(3) 1/3 2/3 0.2989(3) 0.1670(2) 0.3352(3) 0.4482(4) 0.1780(3) 0.3560(4) 1/4 0.4800(2) 0.9600(3) 0.3500(4)
0 0 0.2798(2) 0 0 0.1264(2) e e e 0 0 0 0 0 0.4081(3) 0 0 0.5 0.1678(4) 0.8251(4) 0.6235(4) 0.1470(4) 0.8351(4) 0.4587(4) e e e
Lattice parameters a (Å) c (Å) Atomic coordinates Ba1:
Ba2:
Ba3:
M1 (Mn/Ti):
M2 (Mn/Ti):
M3 (Mn/Ti):
O1:
O2:
O3:
x y z x y z x y z x y z x y z x y z x y z x y z x y z
Table 2 The phase fractions in BaMn1-xTixO3 with 0 x 0.25 determined from neutron powder diffraction data refinement. Sample
Phase
Phase fraction (%)
x¼0 x ¼ 0.05
15R 15R 8H 8H 9R 9R 9R 10H 12R 9R 12R
100 47.6 52.4 43.8 56.2 100 71.5 19.7 8.8 61.6 38.4
x ¼ 0.1 x ¼ 0.15 x ¼ 0.2
x ¼ 0.25
extra peaks at 190 and 230 cm1 were detected. Moreover, there is no peak at 350 cm1, observed in the Raman spectra of the 9R phase (Fig. 8).
3.4. Discussion In the BaMn1-xTixO3 system moderate Ti doping x < 0.2 leads to formation of structural polytypes composed by chain units containing face-shared MnO6 octahedra with a direct corner-shared connection between them (15R, 8H, 9R phases), Fig. 2. The Ti and Mn atoms have similar values of coherent neutron scattering lengths and it is difficult to determine preferential occupancies of the relevant crystallographic sites. Nevertheless, it is wellestablished that Ti4þ ions in octahedral O2 coordination tend to form longer bond lengths of about 2.00 Å in comparison with those of about 1.92 Å for Mn4þ ions [34]. The MieOie bonds (M ¼ Mn/Ti,
i ¼ 1 e 3) range from 1.883 to 1.930 Å (Table 3), pointing to chemical disorder of Ti and Mn ions within the M1-M3 crystallographic sites of the 15R phase and the M1 e M2 sites of the 8H and 9R phases. The magnetic ordering temperature in the Ti concentration range x ¼ 0 e 0.15 weakly decreases from TN ¼ 230 to 200 K. Simultaneously, the average MneOeMn bonding angle between facesharing MnO6 octahedra, controlling dominant magnetic superexchange interactions, increases from 79.6 to 81.2 (Table 3). The increase of the MneOeMn bonding angle itself is expected to strengthen the magnetic interactions and enlarge the magnetic ordering temperature [22,58,59]. However, a doping by the diamagnetic Ti4þ ions simultaneously leads to the disorder effects on the magnetic Mn sublattice, which should result in a gradual suppression of the long range magnetic order. Therefore, the observed reduction of the TN value is mainly controlled by enhanced magnetic disorder due to the increasing concentration of the diamagnetic Ti4þ ions. In the BaMn1-xTixO3 compounds with larger Ti doping x 0.2, additional 10H and 12R phases are formed. These phases incorporate chain units containing face-sharing MnO6 octahedra connected via intermediate corner-sharing octahedra, Fig. 2. It is important to note that upon increase of the Ti content the MneOeMn bonding angle between corner-sharing MnO6 octahedra gradually decreases from 180 for the 15R, 8H and 9R phases to 175.8 for the 10H phase and then to 170.4 for the 12R phase (Table 3). This leads to significantly weaken the magnetic interaction between facesharing octahedral blocks in the 10H and 12R phases [22,58,59]. In other hand, the M1 site, located in the center of corner-sharing octahedra, has long M1eO1 bond lengths of 2.036 (x ¼ 0.2) and 1.995 Å (x ¼ 0.25), evidencing preferential occupation by Ti4þ ions. The relevant oxygen bonds for the M2 and M3 sites range from 1.864 to 1.917 Å (Table 3), implying chemical disorder of Ti4þ and
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Table 3 Bond distances and angles in the polymorphic phases of BaMn1-xTixO3. x (Ti)
0
0.05
0.15
0.2
0.25
Phase
15R (R3 m)
8H (P63/mmc)
9R (R3 m)
10H (P63/mmc)
12R (R3 m)
1.929(3) 3 1.924(6) 3 e e 1.893(4) 6 e e e
2.036(6) 6
1.995(7) 6
3
1.898(7) 3 e e 1.917(7) 3 e
Bond distances MeO (Å), M ¼Mn/Ti M1-O1 1.923(2) 3 M1-O2 1.921(3) 3 M1-O3 e M2-O1 e M2-O2 1.883(4) 3 M2-O3 1.914(5) 3 M3-O2 e M3-O3 1.899(5) 6 Ba-O (Å) Ba1-O1 2.839(3) 6
2.841(3) 6
2.840(2) 6
Ba1-O2 Ba1-O3 Ba2-O1
2.913(5) 6 e e
e 2.906(5) 6 e
2.925(5) 6 e 2.959(4) 3
Ba2-O2
3.061(6) 3
2.846(2) 6
2.852(3) 6, 2.903(6) 3
Ba2-O3
2.844(2) 2.970(5) 2.908(3) 2.849(3) 2.912(5)
3.039(6) 6
e
3.300(7) 3
3.008(5) 3 2.823(5) 3 2.859(2) 6
e e e
3.020(7) 3 2.710(7) 3 2.844(2) 6
180.0 e e 80.5(3) 78.5(2) e e
180.0 e 81.2
Ba3-O1 Ba3-O2 Ba3-O3 Bond angles MeOeM (deg.) M1-O1-M1 180.0 M1-O1-M2 e M1-O2-M2 80.7(2) M1-O3-M2 e M2-O2-M2 e M2-O2-M3 e M2-O3-M3 78.6(3) M3-O2-M3
6, 3 3 6 3
1.930(3) e 1.917(6) e 1.908(3) 1.904(6) e e
3 3 3 3
Fig. 4. The structural phase diagram of the BaMn1-xTixO3 system in the Ti concentration range x 0.25. Left Y-axis: Concentration dependence of volume fraction of the 15R and 9R phases of BaMn1-xTixO3. Right Y-axis: Concentration dependence of volume fraction of the 8H, 10H and 12R phases of BaMn1-xTixO3. The solid circles represent the experimental data.
Mn4þ ions over these positions. The preferential occupation of diamagnetic Ti4þ ions in corner-sharing octahedra leads to formation of isolated magnetic tetramers and trimers in 10H and 12R structures with a weak magnetic coupling between them. For the BaTi0.5Mn0.5O3 compound also possessing the 12R structure, it was
e
1.915(5) e 1.916(5) 1.899(5) 1.864(5)
2.828(2) 2.958(7) 2.530(7) 2.861(3) e
175.8(3) e e e e 81.1(4) 73.6(2)
3 3 3
6 6 3, 6
2.826(2) 3, 2.866(3) 3, 3.18(1) 3 2.66(1) 3 2.82(1) 3 2.782(3) 3, 2.870(7) 3, 2.883(3) 3
170.4(2)
79.8(4)
Fig. 5. The concentration dependence of peak-to-peak ESR linewidth and measured ESR spectra dP/dH (inset, P - microwave power absorption) for BaMn1-xTixO3.
pointed out that the preferential occupation of diamagnetic Ti4þ ions for corner-sharing octahedral sites prevents magnetic interaction between face-sharing octahedral blocks, consequently, leads to the lack of long range magnetic order [32e35]. We have not found any evidence for the appearance of long range magnetic order in the 10H and 12R phases of BaMn0.8Ti0.2O3 and BaMn0.75Ti0.25O3 compounds, which likely remain magnetically disordered
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Fig. 6. The magnetic structures corresponding to the 15R, 8H and 9R phases of BaMn1-xTixO3.
down to the lowest temperature of our study of 10 K, similarly to BaTi0.5Mn0.5O3 [32e35]. A coexistence of the regions with intrinsic magnetic disorder in the 10H and 12R phases and magnetically ordered regions in the 9R phase is a possible reason for a rapid TN suppression to 100 K and significantly suppressed ordered Mn magnetic moments in BaMn1-xTixO3 compounds with x ¼ 0.2 e 0.25. The formation of isolated face-sharing octahedral blocks in 10H and 12R phases of these compounds may also be responsible for the peculiar behavior of the ESR peak-to-peak linewidth. One should note that while Ti doping leads to an increase of the averaged
Fig. 7. Concentration dependencies of the magnetic ordering temperature TN (top) and ordered Mn magnetic moment (below) of the structural phases with dominant concentration in BaMn1-xTixO3. The inset above: correlation between the ordered Mn magnetic moment and the magnetic ordering temperature TN. The inset below: selected temperature dependencies of the ordered Mn magnetic moment and their interpolation by the function m(T) ¼ m0,(1 (T/TN)a)2b, with a ¼ 1.65(6), b ¼ 0.14(2) for x ¼ 0; a ¼ 1.29(7), b ¼ 0.26(4) for x ¼ 0.1 and a ¼ 1.10(9), b ¼ 0.21(6) for x ¼ 0.2.
Please cite this article in press as: D.P. Kozlenko, et al., The structural, magnetic and vibrational properties of Ti-doped BaMnO3, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.11.159
D.P. Kozlenko et al. / Journal of Alloys and Compounds xxx (2016) 1e10
9
Fig. 8. Left: Raman spectra collected at room temperature for BaMn1-xTixO3. Right: Concentration dependencies of the selected Raman mode frequencies of BaMn1-xTixO3.
exhibits a complex structural phase diagram in a rather narrow range of Ti concentrations 0 x 0.25, with a consequence of polymorphic phase transitions 15R / 8H / 9R / 10H / 12R upon increase of the x value. The 15R, 8H and 9R phases are ordered antiferromagnetically at low temperatures. The magnetic propagation vector is q ¼ (0 0 1/2) for the 15R and 9R phases, whereas q ¼ (0 0 0) for the 8H phase. The magnetic ordering temperatures and ordered Mn magnetic moments are suppressed rapidly upon growth of the Ti content. In contrast, the 10H and 12R phases for x ¼ 0.2 and x ¼ 0.25 remain magnetically disordered in the studied temperature range. It is most likely due to the formation of isolated magnetic tetramers and trimers in these phases, which may also be responsible for the peculiar behavior of the ESR peak-to-peak linewidth at x > 015 due to the enhanced role of magnetic dipolar interactions over superexchange ones. The Raman spectra of BaMn1-xTixO3 reflect the observed structural modifications. A relationship between the concentration variation of the stretching and bending modes and particular MeO (M ¼ Mn/Ti) and BaeO distances was revealed. Acknowledgments The work has been supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02e2014.11 and the RFBR grant N 15-5254008_viet_а. This work has been jointly supported by the Vietnam Academy of Science and Technology and the Russian Academy of Sciences under project VAST.HTQT.NGA.01/15e16. References [1] S.-W. Cheong, M. Mostovoy, Multiferroics: a magnetic twist for ferroelectricity, Nat. Mater 6 (2007) 13e20, http://dx.doi.org/10.1038/nmat1804. [2] R.E. Cohen, Origin of ferroelectricity in perovskite oxides, Nature 358 (1992) 136e138, http://dx.doi.org/10.1038/358136a0. [3] W. Eerenstein, N.D. Mathur, J.F. Scott, J.F. Scott, Multiferroic and magnetoelectric materials, Nature 442759 (2006) 759e765, http://dx.doi.org/10.1038/ nature05023. [4] M. Fiebig, Revival of the magnetoelectric effect, J. Phys. D. Appl. Phys. 38 (2005) R123eR152, http://dx.doi.org/10.1088/0022-3727/38/8/R01.
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Please cite this article in press as: D.P. Kozlenko, et al., The structural, magnetic and vibrational properties of Ti-doped BaMnO3, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.11.159