Ti-Substituted BaFe12O19 Single Crystal Growth and Characterization

Article pubs.acs.org/crystal

Ti-Substituted BaFe12O19 Single Crystal Growth and Characterization Denis A. Vinnik,*,† Dmitry A. Zherebtsov,† Lubov S. Mashkovtseva,† Sandra Nemrava,‡ Nikolai S. Perov,§,⊥ Anna S. Semisalova,§ Igor V. Krivtsov,† Ludmila I. Isaenko,∥,# Gennady G. Mikhailov,† and Rainer Niewa‡ †

South Ural State University, 76 Lenin Ave, 454080 Chelyabinsk, Russia Institute of Inorganic Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70550 Stuttgart, Germany § Faculty of Physics, Lomonosov Moscow State University, Leninskie Gory 1-2, 119991 Moscow, Russia ∥ Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Ac. Koptyuga ave. 3, 630090 Novosibirsk, Russia ⊥ Baltic Federal University, Nevskogo street 14, 236041 Kaliningrad, Russia # Novosibirsk State University, Pirogova street 2, 630090 Novosibirsk, Russia ‡

S Supporting Information *

ABSTRACT: Ti-substituted barium hexaferrite BaFe12O19 single crystals BaFe12−xTixO19 with x up to 1.3 and sizes 2−8 mm were grown by spontaneous crystallization from molten sodium carbonate flux. The distribution of Ti on different crystallographic sites was determined from single crystal X-ray diffraction data. For low Ti contents up to x = 0.8 the unit cell expands; on further increase of the Ti amount the unit cell starts to shrink. This behavior for low Ti contents is most likely due to a reduction of Fe3+ to Fe2+ for charge balance. At higher Ti concentrations, supposedly vacancies in the transition metal substructure are formed. An increasing Ti concentration results in a monotonous reduction of the Curie temperature from 452 to 251 °C and the saturation magnetization at room temperature from 64.8 to 24.8 emu/g for powder samples and from 70.0 to 60.1 emu/g for single crystals (for x up to 0.78).

1. INTRODUCTION Barium hexaferrites for long time have attracted considerable attention due to large axial magnetic anisotropy, high resistivity, and high permeability at high frequencies, making applications, e.g., for components of magnetically tunable devices at frequencies up to 100 GHz as well as permanent magnets or spintronic devices, possible.1 Traditionally, hexagonal ferrites are applied in microwave and millimeter-wave engineering, for example, in gyromagnetic devices for the EHF range (30−300 GHz).2 Recently, the interest in barium hexaferrites has been renewed due to novel applications such as electronic components for mobile and wireless communications.3 Barium hexaferrite can be used for detecting radiation, for frequencyselective measurements of signal parameters, and to ensure proper insulation in the channels generating, transmitting, and receiving over a wide frequency range up to several hundred GHz.2 The interest in scientific work directed on structural and magnetic properties modification by partial substitution of iron with different diamagnetic and paramagnetic cations in BaFe12O19 has increased considerably within the latest 20 years.1 Early investigations were already concerned with multiion substitution;4−7 however, recently the interest in understanding the magnetic properties of hexaferrites substituted by a single cation rose again.8−11 For the case of simultaneous iron © 2014 American Chemical Society

substitution by Co and Ti, materials with modified properties for magnetic recording applications were obtained.12 Substitution of Fe by Ti has a strong influence on the magnetic properties (saturation magnetization and coercive field) and improves dielectric properties at microwave frequencies.9 This effect makes it possible to use this ferrite as a microwave absorber. Large barium hexaferrite crystals of good quality can be grown from molten salt fluxes. In the past most studies were concerned with either sodium borate or sodium carbonate fluxes.13,14 For some time we are investigating the possibilities of sodium carbonate fluxes for partially substituted barium hexaferrite crystals.15,16 We have grown Ti-substituted single crystals of barium hexaferrite BaFe12−xTixO19 (x ≤ 1.3) to study the location of Ti within the crystal structure and to relate this information with the magnetic properties.

2. EXPERIMENTAL SECTION Barium hexaferrite crystals with size up to 8 mm were grown from iron oxide (γ-Fe2O3) and titanium dioxide (60 wt % rutile and 40 wt % anataze) in a flux composed of barium carbonate and sodium Received: July 17, 2014 Revised: September 18, 2014 Published: September 23, 2014 5834

dx.doi.org/10.1021/cg501075c | Cryst. Growth Des. 2014, 14, 5834−5839

Crystal Growth & Design

Article

Table 1. Unit Cell Parameters, Curie Temperatures and Saturation Magnetizations of Ti-Substituted Barium Hexaferrites BaFe12−xTixO19 Msat [emu/g] no.

x

Ti (wt %)

a (Å)

V (Å3)

c (Å)

0

0

0

5.893

23.194

697.519

1 2 3 4 5 6

0 0.7 2.0 3.4 3.9 5.5

0 0.16 0.46 0.78 0.9 1.3

5.8929(4) 5.8952(4) 5.8977(4) 5.900(1) 5.8990(3) 5.8972(4)

23.1943(1) 23.205(2) 23.213(1) 23.237(3) 23.2334(2) 23.2238(2)

697.54(7) 698.41(7) 699.24(6) 700.6(2) 700.17(5) 699.45(5)

TC (°C) 45720 45022 452 423 376 322 287 251

powder

crystal 7222

64.8 63.7 61.3 58.9 51.7 23.95

69.98 63.71 60.07 N/Aa N/Aa

a N/A: single crystals of sufficient size were not available due to decreasing crystal sizes with increasing Ti concentration in the same growth conditions.

carbonate.13 The initial mixture was ground in an agate mortar and filled into a 30 mL platinum crucible. The crucible was placed in a resistance furnace equipped with a thermocouple type B and a precision thermoregulator RIF-101. To homogenize the starting materials, the furnace was maintained at 1260 °C for 3 h followed by cooling at a rate of 4.5 K/h to 900 °C. The system was then allowed to naturally cool to room temperature. The spontaneously obtained crystals with a size up to 8 mm were separated from the flux by leaching in hot nitric acid. The additional phases such as NaFeO2 and Fe2O3 were found to be present in small amounts. The batch composition for growth of undoped BaFe12O19 crystals consisted of 67.465 wt % Fe2O3, 18.64 wt % Na2CO3, and 13.895 wt % BaCO3. To grow Ti-substituted crystals, 2, 4, 8, 12, and 20 mass % TiO2 (or 1.20, 2.40, 4.80, 7.19, and 11.99 wt % Ti) were added (samples 2−6 in Table 1). The total mass of one batch for crystal growth accounted to up to 50 g. Typical crystal yield was about 35−40 wt %. Because of the spontaneous nature of crystallization, the compositions of the individual crystals can vary slightly even within one batch. For investigations, 23 hexagonal single crystal plates were selected from all experiments. Several crystals have been studied with regard to their thermal and magnetic properties as well as composition to account for this fact. For average values, several crystals were ground in an agate mortar and examined by EDX, powder X-ray diffraction, thermal analysis, and magnetometry. The compositions of the samples were determined using an electron microscope Jeol JSM7001F with an energy dispersive spectrometer Oxford INCA X-max 80. The correlation of titanium concentrations in the melt and in the crystals is presented in Figure 1.

Powder X-ray diffraction analysis was performed on a Rigaku Ultima IV diffractometer in the angular range from 10° to 90° with filtered (Ni foil) CuKα1,2 radiation. Single crystal X-ray diffraction was carried out using a four-circle diffractometer NONIUS κ-CCD, Bruker AXS, at ambient temperature with monochromatic MoKα radiation (λ = 0.7107 Å). Structural refinements were performed with isotropic displacement factors for the oxygen atoms to reduce the number of variables and resulted in BaFe11.3(1)Ti0.7O19 as final composition, in nice agreement to the Ti content from chemical analysis. The Curie temperatures were determined using a simultaneous thermal analyzer Netzsch 449C Jupiter. The samples were placed into a corundum crucible and heated in air at 2 K/min from 25 to 800 °C. The Curie temperatures were determined from the peak maximum on cooling and compared to literature values for pure BaFe12O19.17−19 Magnetic measurements of powder and single crystal samples were performed on a vibrating sample magnetometer VSM LakeShore 7407. For the experiments the ground samples with a maximum weight of 2 mg were sealed in plastic capsules. For a more detailed study of the magnetic properties, the single crystal samples 1−6 were investigated for different crystal orientations.

3. RESULTS AND DISCUSSION 3.1. Crystal Growth. Using carbonate flux technique we were able to grow high-quality Ti-substituted Ba-ferrite crystals of composition BaFe12−xTixO19 with x up to 1.3 and control the Ti content via the initial Ti concentration in the flux (compare Table 1). By application of the flux technique we were able to reduce the crystal growth temperature to below 1300 °C, compared with temperatures well above 2000 °C necessary in other crystal growth methods. For example crystal growth applying radiation heating to temperatures up to 2500 °C under gas flow conditions and to 1650 °C under pressure was earlier presented.20,21 In this process oxygen or an alternative gas atmosphere was used to minimize evaporation of the melt and crystal components. Figure 2 shows selected crystals with edge length up to 8 mm and the typical shape of hexagonal plates obtained in our study. 3.2. Crystal Structure Details. On aliovalent substitution of Fe3+ in BaFe12O19 by Ti4+ either some iron must be converted to Fe2+ or vacancies in the transition metal substructure have to be produced to maintain electroneutrality. Although this issue appears not finally settled,8 cerimetric titrations and Mössbauer studies on Ti-substituted samples give strong indications on the presence of Fe2+.22 PXRD patterns of samples (from bottom to top) 1 to 6 as denoted in Table 1 are presented in Figure 3. Figure 4 shows the development of the unit cell parameters with increasing Fe substitution by Ti. As was observed earlier in a qualitatively similar fashion9 that the

Figure 1. Correlation of titanium concentration in the melt and in the crystals. 5835



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r(Fe2+) = 0.77 Å; CN = 6, r(Fe2+) = 0.92 Å). However, above x = 0.8 the unit cell parameters start to significantly decrease indicating a different charge balance mechanism starting to dominate: Supposedly the formation of vacancies in the transition metal substructure now compensates the increased charge of Ti4+ compared to Fe3+ (1/3rd per Ti substitution).24 Our single crystal diffraction data on a crystal from sample 4 (see Table 1) with the Ti content of x = 0.78 according to EDX analysis clearly support the first hypothesis on the dominating mechanism at low substitution levels: The refinement in the structural model with purely substitution of Fe by Ti results in a composition with x = 0.7(1), while a deficiency in the transition metal substructure would considerably reduce the electron density on the respective sites and thus mimic a much higher Ti concentration (compare Tables 2 and 3).

Figure 2. BaFe12−xTixO19 single crystals obtained from flux growth. Left: Photograph with mm-scale at the bottom. Right: SEM pictures.

Table 2. Selected Data Collection and Refinement Parameters for BaFe12−xTixO19, x = 0.7(1) crystal system space group a (Å) c (Å) Z density (calcd) (g·cm−3) volume (Å3) wavelength (Å) index ranges h, k, l Θmax (deg) F(000) μ(MoKα) (mm−1) reflections collected/independent no. of parameters data averaging: Rint/Rσ R1|F0| ≥ 4σ(F0) R1/wR2/GooF largest e− diff. peak/hole (Å−3)

Figure 3. PXRD patterns of samples (from bottom to top) 1 to 6.

hexagonal P63/mmc 5.8981(1) 23.2029(7) 2 5.252 699.03(3) MoKα: 0.7104 ±8, −8−7, ±32 30.50 1034 14.56 7699/469 38 0.056/0.021 0.023 0.025/0.060/1.25 0.90/−0.92

The crystal structure of the M-type ferrite BaFe12O19 (magnetoplumbite type) may be described as a closed packed motif with cubic and hexagonal stacking in the sequence ...BAB′ABCAC′AC... along the [001] direction. In this sense, the layers A, B, and C constitute exclusively from oxide ions, while the layers B′ and C′ are represented by the composition of BaO3.24,25 The iron atoms occupy voids exclusively formed by oxide ions in five different crystallographic sites of space group P63/mmc. The structure can be partitioned into two alternating sections: spinel-like S-blocks with cubic stacking sequence of close packed layers and R-blocks with hexagonal stacking sequence. In the S-blocks three layers of edge-sharing octahedra FeO6 (Fe(1) in site 2a and Fe(5) in 12k) share vertices with additional tetrahedra FeO4 (Fe(3) in 4f1). In the R-blocks with hexagonal sequence pairs of face-sharing octahedra, Fe2O9 occur (Fe(4) in 4f 2). Within the R-blocks, additionally trigonal bipyramids are occupied (ideal site 2b). As it is well-known for magnetoplumbites and related ferrites, this site might be described with a 50% occupied split position on both sides of the mirror plane located in (001), resulting in tetrahedral environment of Fe(2) in 4e. A section of the crystal structure including site assignments is depicted in Figure 5. The Ti distribution on the different crystallographic sites in BaFe12−xTixO19 was several times investigated using various techniques but is still under debate. While some data support a

Figure 4. Dependence of cell parameters a (Å), c (Å), V (Å3), and Curie temperature (°C) of BaFe12−xTixO19 on the Ti content x.

unit cell parameters monotonously increase up to 3.4 wt % (corresponding to BaFe12−xTixO19 with x ≈ 0.8), despite the smaller ionic radius of the Ti4+ ions compared to the Fe3+ ions with the same coordination number (e.g., CN = 4, r(Ti4+) = 0.42 Å, r(Fe3+) = 0.63 Å; CN = 6, r(Ti4+) = 0.745 Å, r(Fe3+) = 0.785 Å).23 Supposedly this is due to a dominating charge balance mechanism of Fe-reduction to Fe2+ since the increase in ionic radii from Fe3+ to Fe2+ would overcompensate the volume reduction effect by the smaller Ti4+ ions (CN = 4, 5836



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Table 3. Atomic Coordinates, Occupation Factors, and Isotropic Displacement Parameters (pm2) for BaFe12−xTixO19, x = 0.7(1); U values in 104 Å2 atom

site

x

y

z

occ.c

Ueq/Uiso

Ba Fe(1) Fe(2)/Ti(2) Fe(3)/Ti(3) Fe(4)/Ti(4) Fe(5) O(1)b O(2)b O(3)b O(4)b O(5)b

2d 2a 4ea 4f 4f 12k 4e 4f 6h 12k 12k

1/3 0 0 1/3 1/3 0.16836(5) 0 1/3 0.1818(3) 0.1552(2) 0.5020(2)

2/3 0 0 2/3 2/3 2x 0 2/3 2x 2x 2x

3/4 0 0.2427(3) 0.02696(4) 0.19000(4) 0.60812(3) 0.1507(2) 0.5554(2) 1/4 0.0522(1) 0.1493(1)

1 1 0.44(1)/0.06 0.88(2)/0.12 0.86(2)/0.14 1 1 1 1 1 1

61(2) 43(3) 47(14) 49(3) 39(3) 43(2) 38(8) 60(9) 59(8) 59(8) 33(5)

a

Split position 50% occupied. bRefined using isotropic displacement parameters to reduce the number of variables. cAll sites except the mixed occupied M(2), M(3), and M(4) were refined applying full occupancy with the respective atom.

respectively).8 We have analyzed the X-ray diffraction data of several crystals BaFe12−xTixO19 with x = 0.7(1) obtained from sample 4 (Table 1), all leading to the following result: The sites Fe(1) (2a) and Fe(5) (12k) do not show any significant reduction of electron density, thus no relevant substitution by Ti. However, the substitution occurs to similar degrees on sites Fe(2) (4e), Fe(3) (4f1), and Fe(4) (4f 2). Tables 2 and 3 summarize selected information on the structure determination and positional and occupation parameters. In comparison to the previous studies, different synthesis and crystal growth conditions, particularly different temperatures and perhaps cooling history of the samples, have to be taken into account. Even some cosubstitution effect depending on the synthesis procedure may play a role: For our crystals obtained from Na2O flux, we have detected a minor Na content via EDX analysis, linearly increasing from 0.1 to 0.6 wt % for samples 1− 6, which should lead to a minor increase in Fe3+ content. 3.3. Magnetic Characterization. Pure BaFe12O19 orders ferrimagnetically with Fe on sites 4f1 (Fe(3)) and 4f 2 (Fe(4)) exhibiting antiparallel spin direction relative to those of Fe(1) (2a), Fe(2) (2b = 4e), and Fe(5) (12k).26 Ti4+ can modify the magnetic properties due to preferred substitution of these different sites and by disturbance of the magnetic order in its nearest surrounding. Additionally, Fe2+ produced via reduction of Fe3+ on introduction of Ti4+ is expected to be located in the near neighborhood of Ti4+. For very small substitution rates of BaFe12−xTixO19 of x < 0.2, according to some experimental data, Ti preferentially occupies the 4f 2 site (Fe(4)).22,24,26 Because of the substitution of minority spin Fe by Ti carrying no magnetic moment, the saturation magnetization is expected to rise. At higher substitution rates the saturation magnetization clearly decreases (compare Table 1 and Figures 4 and 6). This can be taken as an indication for an additional occupation of further sites next to 4f 2 as found in our structure analysis. However, in literature the analyses of occupation of the different sites in Ti-substituted BaFe12O19 are contradictive.7,22,24,27 It appears that the occupation factors on the different crystallographic sites critically depend on the substitution level and the synthesis conditions of the studied samples. Table 1 and Figure 6 present the values of the specific saturation magnetizations at room temperature in an external magnetic field of 12.0 kOe for powder and single crystal samples. The external field is parallel to the easy axis and perpendicular to the main extension directions of the crystal

Figure 5. Crystal structure of magnetoplumbite BaFe12−xTixO19. Top: General atomic stacking indicating R- and S-blocks. Bottom: Transition metal site assignments; Fe(1) in 2a (octahedral coordination), M(2) in 4b (tetrahedral, 50% occupied split position), M(3) in 4f1 (tetrahedral), M(4) in 4f 2 (octahedral), and Fe(5) in 12k (octahedral). Sites denoted with M are found to be mixed occupied by Fe and Ti.

substitution of Fe3+ by Ti4+ mainly affecting the 12k site,22 other authors suppose the Ti4+ ions occupying preferentially the 4f sites at low concentrations.24,26 However, a combined theoretical and Mössbauer spectroscopic study indicated Ti to be preferentially located on all octahedrally coordinated sites (Fe(1) 2a, Fe(4) 4f, and Fe(5) 12k), while even some substitution should occur at the 2b site (Fe(2), 4e, 5837



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and no magnetization saturation can be observed (90° at Figure 7).

4. CONCLUSIONS Ti-substituted barium hexaferrite single crystals BaFe12−xTixO19 with sizes up to 8 mm were grown by spontaneous crystallization from molten sodium carbonate flux. The dependence of the Ti concentration in the flux during crystal growth on its content in the crystal was investigated. Substitution by Ti has a significant impact on the magnetic properties. The distribution of Ti between different sites in the unit cell was determined from single crystal X-ray diffraction data. An increasing amount of Ti changes the cell parameters in a nontrivial way and influences the magnetic properties resulting in a reduction of both the Curie temperature and the saturation magnetization.

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Figure 6. Specific saturation magnetizations at room temperature and an external magnetic field of 12.0 kOe for different Ti concentrations. Values for pure single crystal BaFe12O19 are taken from ref 19.

ASSOCIATED CONTENT

S Supporting Information *

CIF file, CSD-428062. This material is available free of charge via the Internet at http://pubs.acs.org.

with the shape of a hexagonal platelet. The saturation magnetization dependence in general is similar for single crystal and powder samples: on increasing substitution level, the saturation magnetization decreases. However, particularly for small Ti contents the quantitative differences are significant. Apparently, substitution by Ti reduces the magnetic anisotropy. For a more detailed study of the magnetic properties, single crystal samples 1−4 were investigated in different relative orientations with respect to the magnetic field. Magnetic hysteresis curves are typical for a barium ferrite with M-type structure (magnetoplumbite). The above dependence is the same for these samples. Figure 7 presents the hysteresis loops

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AUTHOR INFORMATION

Corresponding Author

*Tel: +7-951-457-2286. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Falk Lissner (University of Stuttgart) for single crystal X-ray diffraction intensity collection. REFERENCES

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Figure 7. Dependence of the magnetization on the magnetic field for a BaFe11.84Ti0.16O19 crystal for different orientations of the crystal relative to the magnetic field direction. The 0° orientation belongs to the crystallographic ⟨001⟩ orientation (parallel to the c-axis).

M vs H of a BaFe11.84Ti0.16O19 single crystal at different orientation of external magnetic field and direction of c-axis of single crystal. The direction parallel to the crystallographic caxis corresponds to the easy axis of magnetization, and saturation is observed in the hysteresis curve M vs H (0° at Figure 7). When the magnetic field is applied parallel to the plane of the BaFe11.84Ti0.16O19 single crystal (i.e., perpendicular to the c-axis), the hysteresis curve reveals the hard axis behavior, 5838



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(17) Townes, W. D.; Fang, J. H.; Perrotta, A. J. Z. Kristallogr. 1967, 125, 437−449. (18) Mou, F.-Z.; Guan, J.-G.; Sun, Z.-G.; Fan, X.-A.; Tong, G.-X. J. Solid State Chem. 2010, 183, 736−743. (19) Mohsen, Q. J. Alloys Compd. 2010, 500, 125−128. (20) Balbashov, A. M.; Egorov, S. K. J. Cryst. Growth 1981, 52, 498− 504. (21) Bugaris, D. E.; Loye, H.-C. Angew. Chem. 2012, 51, 3780−3811. (22) Quoiroz, P.; Halbedel, B.; Bustamante, A.; Gonzalez, J. Hyperfine Interact. 2001, 202, 97−106. (23) Shannon, R. D. Acta Crystallogr. 1976, A32, 751−767. (24) Marino-Castellanos, P. A.; Anglada-Rivera, J.; Cruz-Fuentes, A.; Lora-Serrano, R. J. Magn. Magn. Mater. 2004, 280, 214−220. (25) Siegrist, T.; Vanderah, T. A. Eur. J. Inorg. Chem. 2003, 2003 (8), 1483−1501. (26) Brabers, V. A. M.; Stevens, A. A. E.; Dalderop, J. H. J.; Šimša, Z. J. Magn. Magn. Mater. 1999, 196/197, 312−314. (27) Cabañas, M. V.; Gonzlez-Calbet, J. M.; Rodríguez-Carvajal, J.; Vallet-Regi, M. J. Solid State Chem. 1994, 111, 229−237.

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Ti-Substituted BaFe12O19 Single Crystal Growth and Characterization

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