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Sr- and Ti-site substitution, lattice dynamics, and octahedral tilt transition relationship in SrTiO3:Mn ceramics
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Acta Materialia 58 (2010) 577–582 www.elsevier.com/locate/actamat
Sr- and Ti-site substitution, lattice dynamics, and octahedral tilt transition relationship in SrTiO3:Mn ceramics Alexander Tkach a,1, Paula M. Vilarinho a,*, Dmitry Nuzhnyy b, Jan Petzelt b a
Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal b Department of Dielectrics, Institute of Physics ASCR, Na Slovance 2, 18040 Prague 8, Czech Republic Received 18 April 2009; received in revised form 21 September 2009; accepted 21 September 2009 Available online 23 October 2009
Abstract SrTiO3:Mn ceramics, prepared according to the chemical formulae Sr1–xMnxTiO3 and SrTi1–yMnyO3, are studied by Fourier transform infrared and time-domain terahertz spectroscopy in the temperature range of 10–300 K to support the incorporation of Mn ions into the perovskite lattice of SrTiO3, and to ascertain their different lattice site locations. The polar soft mode of the incipient ferroelectric SrTiO3 is found to be hardened in the whole temperature range by the substitution of Mn ions on Ti sites, and only in the low-temperature range by the Sr site substitution. Activation of the mode, associated with the R point condensation of the Brillouin zone due to the doubling of the unit cell by antiphase rotations of the O-octahedra below the structural transition temperature Ta, shows that the substitution of Mn ions on the Sr sites increases Ta, whereas the Ti-site substitution suppresses Ta with respect to the undoped SrTiO3. Ó 2009 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. Keywords: SrTiO3; Dopants; Structural phase transition; Phonon dynamics; Infrared and terahertz spectroscopy
1. Introduction SrTiO3-based compounds have been attracting considerable interest both from a fundamental point of view and for a wide range of applications, particularly in tunable electronic devices [1]. On the other hand, multiferroic materials, combining at least two of three properties – ferromagnetism, ferroelectricity and ferroelasticity – in the same phase [2], are widely studied nowadays and have tremendous potential for multifunctional applications, although magnetoelectric multiferroics are difficult to obtain [3]. Currently, to the best of the authors’ knowledge, the Sr1–xMnxTiO3 compound is a unique material, revealing antiferrodistortive elastic [4], polar dielectric [5], and spin glass magnetic [6] behavior simultaneously. Moreover, the dielectric and magnetic anomalies were found to be *
Corresponding author. Tel.: +351 234 370 354; fax: +351 234 370 204. E-mail address: [email protected] (P.M. Vilarinho). 1 Present address: Department of Physics of Science Faculty, IFIMUP, University of Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal.
coupled according to the so-called “multiglass” scenario, where freezing of electric dipoles, created by off-central Mn2þ Sr ions in highly polarizable SrTiO3 lattice, initiates the transition of the magnetic Mn2+ spin moments into a spin glass state at the dipolar glass temperature Tg = 38 K [6]. However, in spite of a number of indirect confirmations [4–11], direct evidence of Mn ion location on Sr sites, and its key role in the induced multiglass behavior, is required. In the meantime, lattice dynamic studies can partially provide such information. Undoped strontium titanate (SrTiO3–ST) is a quantum paraelectric material, where zero-point fluctuations preclude the condensation of the polar lattice soft mode, saturating the dielectric permittivity at low temperatures T < 4 K [12]. Hence, no ferroelectric phase transition can be observed in ST. Only an improper ferroelastic phase transition from cubic (Pm3m) to a centrosymmetric tetragonal (I4/mcm) phase with a doubled primitive unit cell, associated with tilts of the O-octahedra in antiphase around [0 0 1] direction, occurs at cooling below the transition temperature, Ta 110 K [13–15]. The mode at the
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Brillouin zone boundary (R point), softening as the temperature approaches Ta, is responsible for this structural transition in SrTiO3 [16]. The doubling of the lattice unit cell at the phase transition leads to a folding of the Brillouin zone, so that the R point is in the zone center, and hence six new zone-center phonons appear, whose progenitors were the zone-boundary phonons, one of them being infrared (IR) active [17]. Both Ta and the dielectric response of strontium titanate can be altered by dopant cations, occupying either Sr2+ (A) or Ti4+ (B) sites of the ABO3 perovskite lattice. When Sr2+ is substituted by larger Ba2+ or Pb2+ ions, the structural transition is suppressed due to the increase of the Goldschmidt tolerance factor t [4], whereas a ferroelectric phase transition, due to the cooperative displacements of the Ti4+ ions in the loosely packed oxygen octahedral, can be observed [18]. When Sr2+ is substituted by smaller ions like Ca2+, t decreases, Ta increases and elastic domains can be observed [4], and concomitantly a dielectric relaxation of the polar clusters [19] formed by the off-center dopant ions [20] can be induced. Moreover, while Sr-site doping can induce a ferroelectric phase transition or dielectric relaxation, the dielectric response of Ti-site doped ST is driven away from ferroelectric instability, just by decreasing the dielectric permittivity [18]. Manganese is an element of special interest, because its oxidation states include Mn2+ and Mn4+ and might isovalently substitute Sr2+ and Ti4+. In ST single crystals Mn has been shown to be incorporated into the perovskite lattice preferably as Mn4+ (i.e., on the Ti site) [14]. However, the estimated ratios of ionic radii of Mn2+ to Sr2+ and of Mn4+ to Ti4+, for 12-fold and 6-fold coordination respec2þ 4þ 4þ tively, are found to be equal: r2þ Mn =rSr ¼ rMn =rTi ¼ 0:88 [21]. Hence from the ionic size considerations, ST structure allows for the substitution with Mn cations on both Sr and Ti sites of the lattice. Indeed, the authors of the present work have previously reported that SrTiO3:Mn ceramics, prepared according to the chemical formula Sr1–xMnxTiO3 (SMnT) possess remarkably different dielectric and structural properties with respect to SrTi1–yMnyO3 (STMn) [4– 11,22]. Ti-site substitution was found to reduce the dielectric permittivity, reinforcing the quantum paraelectric behavior [22]. On the contrary, Sr-site substitution induces a notable low-frequency dielectric relaxation in the lowtemperature region, dependent on the frequency and Mn content [5]. This dielectric relaxation was attributed to the off-center displacements of small Mn2+ ions on large Sr sites of the highly polarizable ST lattice [5]. Moreover, the freezing of the electric dipoles formed by off-center Mn2þ Sr into the polar glass was found to trigger a spin freezing of Mn in SMnT [6], while no magnetic anomaly was observed in STMn. The microstructural characterization of SMnT has detected a MnTiO3 secondary phase above the solid solubility limit of x 0.03 in contrast to the monophasic STMn ceramics, characterized by a strong linear decrease of the lattice parameter with increasing Mn content up to y = 0.15 [7]. Qualitative studies of SrTiO3:Mn
ceramics by micro-Raman spectroscopy and electron diffraction within transmission electron microscopy have shown that Ta increases with the decreasing tolerance factor for SMnT and decreases with the increasing t for STMn ceramics [4]. However, these techniques give a rather local response from the material on a few grains, or even one single ceramic grain, and therefore a more macroscopic response is required. Moreover, to the best of the authors’ knowledge, no quantitative lattice vibrational studies with the temperature variation have been reported so far for SrTiO3:Mn ceramics, although vibrational modes and their temperature dependence are expected to be sensitive to the lattice site occupancy of dopant ions. Finally, while the lattice dynamics of undoped SrTiO3 are well known, there is currently a very limited number of works on the lattice dynamics of doped ST. On this background, in this work, SMnT and STMn ceramics are studied by low-temperature Fourier transform infrared (FTIR) and time-domain THz (TDT) spectroscopy techniques. The macroscopic effect of the lattice site substitution on the lattice dynamics and the structural transition is investigated and discussed based on ionic size considerations. 2. Experimental section Sr1–xMnxTiO3 and SrTi1–yMnyO3 ceramics were prepared by the conventional mixed-oxide method, as described in detail elsewhere [7]. Samples with x = 0.025 (about the solid solubility limit concentration) and y = 0.05 were chosen for a detailed analysis. Polished disk-shaped ceramic samples with a diameter of 8 mm and a thickness of 1 mm were used for the IR reflectivity studies. The spectra were obtained using a FTIR Bruker IFS 113 v spectrometer in the frequency range of 20– 650 cm1 (0.6–20 THz). A continuous flow He cryostat Optistat CF with polyethylene windows was used for measurements down to 10 K, and liquid-He cooled (1.5 K) Si bolometer was used as high-sensitive detector. To enhance the accuracy of the data in the far IR range, TDT transmission measurements were performed with the same samples, polished to a thickness of 49 lm. A custom-made TDT spectrometer, based on a femtosecond Ti:sapphire laser and using interdigited photoconducting switch for generation of THz pulses and electro-optic sampling scheme with [1 1 0] ZnTe crystal as THz detector, was used to obtain directly the complex dielectric response of the samples in the range from 7 to 53 cm1 (0.2–1.6 THz). A cryostat Optistat CF with mylar windows was used for measurements down to 10 K. Joint FTIR and THz data were fitted with the standard factorized damped harmonic oscillator model [23] in order to calculate the complex dielectric response function of the polar lattice modes. The dielectric permittivity measurements in the frequency range of 102– 106 Hz and temperature range of 10–300 K were performed in a He closed-cycle cryogenic system Displex ADP-Cryostat HC-2, using precision LCR meter HP 4284A.
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3. Results and discussion IR reflectivity spectra, obtained from 300 to 10 K and normalized to corresponding data in the THz range, are presented in Fig. 1a and b for SMnT and STMn ceramics, respectively. The spectra are similar to those of undoped ST [17], revealing three transverse optical modes: TO1, TO2, and TO4. TO1 is a soft mode, mostly related to Ti– O–Ti bending, whereas TO2 and TO4 modes are almost temperature-independent and related to Sr against TiO6 octahedra translations and Ti–O stretching, respectively [24]. The mode from the R point of the Brillouin zone (characteristic for undoped ST below Ta 110 K and related to the oxygen octahedra tilting) is also observed in the low-temperature FTIR spectra of SMnT and STMn ceramics, as shown in Fig. 1a and b and enlarged in Fig. 2a and b. However, for SMnT ceramics the R mode remains visible up to Ta 150 K and is more intense, compared to that of STMn ceramics, vanishing on heating above Ta 80 K. These results are in good agreement with previous studies of SrTiO3:Mn ceramics by electron diffraction within transmission electron microscopy and by Raman spectroscopy [4]. Therefore, the strong relationship between the structural transition and the lattice site, on which the Mn substitution occurs, is confirmed by three different techniques. The alteration of Ta in doped ST is attributed to modification of the Goldschmidt tolerance
factor, t, which is often used to express the stability of the perovskite lattice and is given by [4]. r A þ rO ð1Þ t ¼ pffiffiffi 2ðrB þ rO Þ where rA and rB stands for the average ionic radii of the Aand B-site respectively, and rO for the ionic radius of the ˚ [21]). For the coordination number oxygen (1.40 A 2þ ˚ Nc = 12, characteristic of the A-site, r2þ Sr is 1.44 A and r Mn ˚ , while for Nc = 6, characteristic of is estimated as 1.27 A 4þ ˚ ˚ the B-site, r4þ Ti is 0.605 A, and r Mn is 0.53 A [21]. For SMnT, t decreases with increasing Mn content x due to the smaller size of Mn2+ ions isovalently substituting Sr2+ ions. In such a way the average A-cation size becomes smaller, favoring tilting of the oxygen octahedra. Hence Ta increases, as observed. For STMn, the size of Mn4+ ions is smaller than that of isovalently substituted Ti4+ ions. Therefore, the average B-cation size decreases and tolerance factor increases with increasing Mn content y. Consequently, the octahedra tend to shrink rather than to tilt, which is reflected in the decrease of Ta, as observed. Thus, the difference in the appearance of the R mode in the IR spectra as a function of temperature, indicating the increase of Ta for SMnT and its decrease for STMn ceramics, strongly support the assumed occupancy of Mn ions onto two different sites in the perovskite lattice. Furthermore, it excludes the dominance of Mn accumulated at the grain boundaries, because if Mn segregates at the grain boundaries, R mode would appear from the quasi-undoped ST grains right below 110 K and no changes in Ta would be expected. There are also evident differences between THz reflectivity of SMnT and STMn ceramics, as shown enlarged in Fig. 2c and d for selected temperatures, implying a dissimilarity in phonon contribution to the dielectric response. In order to quantify the phonon parameters of SMnT and STMn ceramics, their THz and FTIR reflectivity spectra shown in Fig. 1 were fitted to the generalized-oscillator model with the factorized form of the complex dielectric permittivity [23]: pffiffiffiffiffiffiffiffiffiffiffi e ðxÞ 12 Y x2LOj x2 þ ixcLOj RðxÞ ¼ pffiffiffiffiffiffiffiffiffiffiffi with e ðxÞ ¼ e1 e ðxÞ þ 1 x2TOj x2 þ ixcLOj j
Fig. 1. IR reflectivity spectra (solid symbols) together with the data obtained by TDT transmission spectroscopy (open symbols) of Sr0.975Mn0.025TiO3 (a) and Sr0.95Mn0.05TiO3 (b) ceramics and their fits (lines) at selected temperatures.
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ð2Þ
where xTOj and xLOj denote the transverse and longitudinal frequencies of the jth polar phonon, respectively, and cTOj and cLOj are their corresponding damping constants. The high-frequency permittivity e1, resulting from the electronic absorption processes, was obtained from the room temperature frequency-independent reflectivity tails, above the phonon frequencies, and was assumed temperatureindependent. As also depicted in Fig. 1, the fit lines follow well the measurement results for both SMnT and STMn. Spectra of real e0 and imaginary e00 parts of the complex dielectric permittivity e*, calculated from the IR reflectivity fits and as directly obtained from the TDT transmission spectroscopy, are present in Figs. 3a and b and 4a and b for SMnT and STMn ceramics, respectively. Notice that
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Fig. 2. IR reflectivity spectra around the R mode (a and b) and THz reflectivity spectra (c and d) of Sr0.975Mn0.025TiO3 (a and c) and Sr0.95Mn0.05TiO3 (b and d) ceramics.
Fig. 3. Spectra of real e0 (a) and imaginary e0 0 (b) parts of the dielectric permittivity, calculated from the IR reflectivity fits of Sr0.975Mn0.025TiO3 ceramics together with data obtained from TDT transmission spectroscopy, at selected temperatures.
Fig. 4. Spectra of real e0 (a) and imaginary e00 (b) parts of the dielectric permittivity, calculated from the IR reflectivity fits of SrTi0.95Mn0.05O3 ceramics together with data obtained from TDT transmission spectroscopy, at selected temperatures.
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TDT data of e00 on the low-frequency soft mode wing (below 1 THz) are much higher for SMnT sample (Fig. 3b) than for the STMn one (Fig. 4b) and cannot be well fitted with the model of Eq. (2), just by assuming higher damping for the soft mode in SMnT, particularly at high temperatures. It appears that it is connected with the low-frequency dielectric relaxation which naturally shifts up in frequency on heating being thermally activated. Similar features in the THz e00 were also observed in KTaO3:Li crystals [25], in which the A-site substitution is widely accepted. Along with the effect on the appearance of low-frequency dielectric relaxation (as well as of R mode), the effect of Mn on the transverse optical modes of ST is also seen in Figs. 3 and 4. Whereas frequencies of TO2 and TO4 modes keep constant values of 176–177 and 547 cm1, respectively, polar soft TO1 mode is strongly affected by Mn, as well seen from Fig. 5a, where the soft mode frequency is plotted as a function of temperature. For both SrTiO3:Mn ceramics, TO1 modes continuously soften under cooling, as in the paraelectric phase of ferroelectric materials, but no soft mode condensation occurs. However, the soft mode frequency of STMn ceramics is higher than that of SMnT in the whole temperature range. Data points for the polar soft mode of undoped ST ceramics taken from Ref. [18] are also plotted in Fig. 5a for comparison. With respect to the undoped ST, the TO1 mode of STMn ceramics is stiffened in all the temperature range 10–
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300 K as well. On the other hand, the soft mode of SMnT ceramics behaves similarly to that of undoped ST above 200 K but is harder below this temperature. So, the effect of Mn on the polar soft TO1 mode of ST in the case of Sr-site substitution clearly differs from that of Ti-site substitution. Since TO1 mode in ST is known to be mostly related to Ti–O–Ti bending [24], breaking of Ti–O–Ti chains by substitution of highly polarizable Ti4+ ions with less polarizable Mn4+ ions is expected to harden the soft polar mode, as clearly seen from Fig. 5a for STMn ceramics. On the other hand, the TO1 mode in ST is known to be stiffened at low temperatures by the bias electric field [26]. In the case of SMnT ceramics, small Mn2+ ions are supposed to jump between symmetrical off-center positions, resulting in low-frequency polar dielectric relaxation at low temperatures [5]. The quasistatic (compared to the soft mode frequency) polarization, related to the off-center Mn2þ Sr ions and enhancing for decreasing temperature [5,10], perform a bias field action on the soft mode. Corresponding hardening of TO1 mode at low temperatures is clearly seen from Fig. 5a for SMnT ceramics, supporting once more the different incorporation schemes for SMnT and STMn. As can be seen from the far IR part of spectra presented in Figs. 3 and 4 and separately presented in Fig. 5, the stiffening of the soft mode corresponds to the lowering of THz permittivity values according to the Lyddane–Sachs–Teller relation D0j x ¼ const [27]. It should be noted also from Fig. 5b that the dielectric permittivity of STMn ceramics in the THz range matches well to that in the low-frequency range, indicating an absence of any dielectric relaxation from 102 to 1012 Hz. In the case of SMnT ceramics, as shown in Fig. 5b, the dielectric permittivity in the THz range does not exceed its low-frequency value as well, but shows the phonon contribution to the dielectric permittivity as a background for the low-frequency dielectric relaxation. Thus, the dielectric response of SMnT and STMn ceramics in a wide frequency and temperature range differs very much, and the main difference is the dielectric relaxation, where off-center Mn2þ Sr ions indeed play a key role. 4. Conclusions In conclusion, Sr1–xMnxTiO3 and SrTi1–yMnyO3 ceramics prepared by the conventional mixed-oxide method were studied as a function of temperature by FTIR and TDT spectroscopy. Lattice vibrational studies have shown that:
Fig. 5. Temperature dependence of the soft mode frequency xTO1 of SrTi0.95Mn0.05O3, Sr0.975Mn0.025TiO3, and SrTiO3 ceramics (a) and of the real part of the dielectric permittivity e0 of Sr0.975Mn0.025TiO3 and Sr0.95Mn0.05TiO3 ceramics at selected frequencies (b).
(i) Substitution of Sr by Mn increases Ta from 110 K up to 150 K in agreement with decreasing tolerance factor t, whereas substitution of Ti by 5% of Mn raises t and, therefore, decreases Ta to 80 K. (ii) Mn4+ incorporation on the Ti site breaks the highly polarizable Ti–O–Ti chains and increases the frequency of their lattice vibrations, i.e. stiffens TO1 mode in the whole temperature range.
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(iii) Incorporation of Mn2+ ions on the Sr sites and quasistatic polarization due to their off-central “freezing” at low temperatures causes stiffening of the polar soft mode below 200 K. Thus, the relationship between the soft lattice dynamics, octahedral tilt transition, and site occupancy for Mn ions in the perovskite lattice of studied SrTiO3:Mn ceramics is established, excluding the dominance of Mn segregations at the grain boundaries.
Acknowledgments The authors are thankful to Dr. Tatiana Ostapchuk for the help with IR characterizations. This work was funded by FCT, FEDER and European Network of Excellence FAME under the contract FP6-500159-1. References [1] Tagantsev AK, Sherman VO, Astafiev KF, Venkatesh J, Setter N. J Electroceram 2003;11:5. [2] Spaldin NA, Fiebig M. Science 2005;309:391. [3] Hill NA. J Phys Chem B 2000;104:6694. [4] Tkach A, Vilarinho PM, Kholkin AL, Reaney IM, Pokorny´ J, Petzelt J. Chem Mater 2007;19:6471 [and references therein]. [5] Tkach A, Vilarinho PM, Kholkin AL. Appl Phys Lett 2005;86: 172902.
[6] Shvartsman V, Bedanta S, Borisov P, Kleemann W, Tkach A, Vilarinho PM. Phys Rev Lett 2008;101:165704. [7] Tkach A, Vilarinho PM, Kholkin AL. Acta Mater 2005;53:5061. [8] Tkach A, Vilarinho PM, Kholkin AL, Pashkin A, Veljko S, Petzelt J. Phys Rev B 2006;73:104113. [9] Tkach A, Vilarinho PM, Kholkin AL. Acta Mater 2006;54:538. [10] Tkach A, Vilarinho PM, Kholkin AL. J Appl Phys 2007;101:084110. [11] Laguta VV, Kondakova IV, Bykov IP, Glinchuk MD, Tkach A, Vilarinho PM, et al. Phys Rev B 2007;76:054104. [12] Mu¨ller KA, Burkard H. Phys Rev B 1979;19:3593. [13] Mu¨ller KA. Phys Rev Lett 1959;2:341. [14] Unoki H, Sakudo T. J Phys Soc Jpn 1967;23:546. [15] Shirane G, Yamada Y. Phys Rev 1969;177:858. [16] Fleury PA, Scott JF, Worlock JM. Phys Rev Lett 1968;21:16. [17] Petzelt J, Ostapchuk T, Gregora I, Rychetsky´ I, Hoffmann-Eifert S, Pronin AV, et al. Phys Rev B 2001;64:184111. [18] Lemanov VV. Ferroelectrics 1999;226:133. [19] Bianchi U, Dec J, Kleemann W. Phys Rev B 1995;51:8737 (and references therein). [20] Bednorz JG, Mu¨ller KA. Phys Rev Lett 1984;52:2289. [21] Shannon RD. Acta Crystallogr A 1976;32:751. [22] Tkach A, Vilarinho PM, Kholkin AL. Ferroelectrics 2004;304:917. [23] Hlinka J, Petzelt J, Kamba S, Noujni D, Ostapchuk T. Phase Transitions 2006;79:41. [24] Perry CH, Khanna BN, Rupprecht G. Phys Rev 1964;135:A408. [25] Zelezny V, Pashkin A, Petzelt J, Savinov M, Trepakov V, Kapphan S. Ferroelectrics 2004;302:195. [26] Worlock JM, Fleury PA. Phys Rev Lett 1967;19:1176. [27] Lyddane RH, Sachs RG, Teller E. Phys Rev 1941;59:673.
Acta Materialia 58 (2010) 577–582 www.elsevier.com/locate/actamat
Sr- and Ti-site substitution, lattice dynamics, and octahedral tilt transition relationship in SrTiO3:Mn ceramics Alexander Tkach a,1, Paula M. Vilarinho a,*, Dmitry Nuzhnyy b, Jan Petzelt b a
Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal b Department of Dielectrics, Institute of Physics ASCR, Na Slovance 2, 18040 Prague 8, Czech Republic Received 18 April 2009; received in revised form 21 September 2009; accepted 21 September 2009 Available online 23 October 2009
Abstract SrTiO3:Mn ceramics, prepared according to the chemical formulae Sr1–xMnxTiO3 and SrTi1–yMnyO3, are studied by Fourier transform infrared and time-domain terahertz spectroscopy in the temperature range of 10–300 K to support the incorporation of Mn ions into the perovskite lattice of SrTiO3, and to ascertain their different lattice site locations. The polar soft mode of the incipient ferroelectric SrTiO3 is found to be hardened in the whole temperature range by the substitution of Mn ions on Ti sites, and only in the low-temperature range by the Sr site substitution. Activation of the mode, associated with the R point condensation of the Brillouin zone due to the doubling of the unit cell by antiphase rotations of the O-octahedra below the structural transition temperature Ta, shows that the substitution of Mn ions on the Sr sites increases Ta, whereas the Ti-site substitution suppresses Ta with respect to the undoped SrTiO3. Ó 2009 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. Keywords: SrTiO3; Dopants; Structural phase transition; Phonon dynamics; Infrared and terahertz spectroscopy
1. Introduction SrTiO3-based compounds have been attracting considerable interest both from a fundamental point of view and for a wide range of applications, particularly in tunable electronic devices [1]. On the other hand, multiferroic materials, combining at least two of three properties – ferromagnetism, ferroelectricity and ferroelasticity – in the same phase [2], are widely studied nowadays and have tremendous potential for multifunctional applications, although magnetoelectric multiferroics are difficult to obtain [3]. Currently, to the best of the authors’ knowledge, the Sr1–xMnxTiO3 compound is a unique material, revealing antiferrodistortive elastic [4], polar dielectric [5], and spin glass magnetic [6] behavior simultaneously. Moreover, the dielectric and magnetic anomalies were found to be *
Corresponding author. Tel.: +351 234 370 354; fax: +351 234 370 204. E-mail address: [email protected] (P.M. Vilarinho). 1 Present address: Department of Physics of Science Faculty, IFIMUP, University of Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal.
coupled according to the so-called “multiglass” scenario, where freezing of electric dipoles, created by off-central Mn2þ Sr ions in highly polarizable SrTiO3 lattice, initiates the transition of the magnetic Mn2+ spin moments into a spin glass state at the dipolar glass temperature Tg = 38 K [6]. However, in spite of a number of indirect confirmations [4–11], direct evidence of Mn ion location on Sr sites, and its key role in the induced multiglass behavior, is required. In the meantime, lattice dynamic studies can partially provide such information. Undoped strontium titanate (SrTiO3–ST) is a quantum paraelectric material, where zero-point fluctuations preclude the condensation of the polar lattice soft mode, saturating the dielectric permittivity at low temperatures T < 4 K [12]. Hence, no ferroelectric phase transition can be observed in ST. Only an improper ferroelastic phase transition from cubic (Pm3m) to a centrosymmetric tetragonal (I4/mcm) phase with a doubled primitive unit cell, associated with tilts of the O-octahedra in antiphase around [0 0 1] direction, occurs at cooling below the transition temperature, Ta 110 K [13–15]. The mode at the
1359-6454/$36.00 Ó 2009 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. doi:10.1016/j.actamat.2009.09.036
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Brillouin zone boundary (R point), softening as the temperature approaches Ta, is responsible for this structural transition in SrTiO3 [16]. The doubling of the lattice unit cell at the phase transition leads to a folding of the Brillouin zone, so that the R point is in the zone center, and hence six new zone-center phonons appear, whose progenitors were the zone-boundary phonons, one of them being infrared (IR) active [17]. Both Ta and the dielectric response of strontium titanate can be altered by dopant cations, occupying either Sr2+ (A) or Ti4+ (B) sites of the ABO3 perovskite lattice. When Sr2+ is substituted by larger Ba2+ or Pb2+ ions, the structural transition is suppressed due to the increase of the Goldschmidt tolerance factor t [4], whereas a ferroelectric phase transition, due to the cooperative displacements of the Ti4+ ions in the loosely packed oxygen octahedral, can be observed [18]. When Sr2+ is substituted by smaller ions like Ca2+, t decreases, Ta increases and elastic domains can be observed [4], and concomitantly a dielectric relaxation of the polar clusters [19] formed by the off-center dopant ions [20] can be induced. Moreover, while Sr-site doping can induce a ferroelectric phase transition or dielectric relaxation, the dielectric response of Ti-site doped ST is driven away from ferroelectric instability, just by decreasing the dielectric permittivity [18]. Manganese is an element of special interest, because its oxidation states include Mn2+ and Mn4+ and might isovalently substitute Sr2+ and Ti4+. In ST single crystals Mn has been shown to be incorporated into the perovskite lattice preferably as Mn4+ (i.e., on the Ti site) [14]. However, the estimated ratios of ionic radii of Mn2+ to Sr2+ and of Mn4+ to Ti4+, for 12-fold and 6-fold coordination respec2þ 4þ 4þ tively, are found to be equal: r2þ Mn =rSr ¼ rMn =rTi ¼ 0:88 [21]. Hence from the ionic size considerations, ST structure allows for the substitution with Mn cations on both Sr and Ti sites of the lattice. Indeed, the authors of the present work have previously reported that SrTiO3:Mn ceramics, prepared according to the chemical formula Sr1–xMnxTiO3 (SMnT) possess remarkably different dielectric and structural properties with respect to SrTi1–yMnyO3 (STMn) [4– 11,22]. Ti-site substitution was found to reduce the dielectric permittivity, reinforcing the quantum paraelectric behavior [22]. On the contrary, Sr-site substitution induces a notable low-frequency dielectric relaxation in the lowtemperature region, dependent on the frequency and Mn content [5]. This dielectric relaxation was attributed to the off-center displacements of small Mn2+ ions on large Sr sites of the highly polarizable ST lattice [5]. Moreover, the freezing of the electric dipoles formed by off-center Mn2þ Sr into the polar glass was found to trigger a spin freezing of Mn in SMnT [6], while no magnetic anomaly was observed in STMn. The microstructural characterization of SMnT has detected a MnTiO3 secondary phase above the solid solubility limit of x 0.03 in contrast to the monophasic STMn ceramics, characterized by a strong linear decrease of the lattice parameter with increasing Mn content up to y = 0.15 [7]. Qualitative studies of SrTiO3:Mn
ceramics by micro-Raman spectroscopy and electron diffraction within transmission electron microscopy have shown that Ta increases with the decreasing tolerance factor for SMnT and decreases with the increasing t for STMn ceramics [4]. However, these techniques give a rather local response from the material on a few grains, or even one single ceramic grain, and therefore a more macroscopic response is required. Moreover, to the best of the authors’ knowledge, no quantitative lattice vibrational studies with the temperature variation have been reported so far for SrTiO3:Mn ceramics, although vibrational modes and their temperature dependence are expected to be sensitive to the lattice site occupancy of dopant ions. Finally, while the lattice dynamics of undoped SrTiO3 are well known, there is currently a very limited number of works on the lattice dynamics of doped ST. On this background, in this work, SMnT and STMn ceramics are studied by low-temperature Fourier transform infrared (FTIR) and time-domain THz (TDT) spectroscopy techniques. The macroscopic effect of the lattice site substitution on the lattice dynamics and the structural transition is investigated and discussed based on ionic size considerations. 2. Experimental section Sr1–xMnxTiO3 and SrTi1–yMnyO3 ceramics were prepared by the conventional mixed-oxide method, as described in detail elsewhere [7]. Samples with x = 0.025 (about the solid solubility limit concentration) and y = 0.05 were chosen for a detailed analysis. Polished disk-shaped ceramic samples with a diameter of 8 mm and a thickness of 1 mm were used for the IR reflectivity studies. The spectra were obtained using a FTIR Bruker IFS 113 v spectrometer in the frequency range of 20– 650 cm1 (0.6–20 THz). A continuous flow He cryostat Optistat CF with polyethylene windows was used for measurements down to 10 K, and liquid-He cooled (1.5 K) Si bolometer was used as high-sensitive detector. To enhance the accuracy of the data in the far IR range, TDT transmission measurements were performed with the same samples, polished to a thickness of 49 lm. A custom-made TDT spectrometer, based on a femtosecond Ti:sapphire laser and using interdigited photoconducting switch for generation of THz pulses and electro-optic sampling scheme with [1 1 0] ZnTe crystal as THz detector, was used to obtain directly the complex dielectric response of the samples in the range from 7 to 53 cm1 (0.2–1.6 THz). A cryostat Optistat CF with mylar windows was used for measurements down to 10 K. Joint FTIR and THz data were fitted with the standard factorized damped harmonic oscillator model [23] in order to calculate the complex dielectric response function of the polar lattice modes. The dielectric permittivity measurements in the frequency range of 102– 106 Hz and temperature range of 10–300 K were performed in a He closed-cycle cryogenic system Displex ADP-Cryostat HC-2, using precision LCR meter HP 4284A.
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3. Results and discussion IR reflectivity spectra, obtained from 300 to 10 K and normalized to corresponding data in the THz range, are presented in Fig. 1a and b for SMnT and STMn ceramics, respectively. The spectra are similar to those of undoped ST [17], revealing three transverse optical modes: TO1, TO2, and TO4. TO1 is a soft mode, mostly related to Ti– O–Ti bending, whereas TO2 and TO4 modes are almost temperature-independent and related to Sr against TiO6 octahedra translations and Ti–O stretching, respectively [24]. The mode from the R point of the Brillouin zone (characteristic for undoped ST below Ta 110 K and related to the oxygen octahedra tilting) is also observed in the low-temperature FTIR spectra of SMnT and STMn ceramics, as shown in Fig. 1a and b and enlarged in Fig. 2a and b. However, for SMnT ceramics the R mode remains visible up to Ta 150 K and is more intense, compared to that of STMn ceramics, vanishing on heating above Ta 80 K. These results are in good agreement with previous studies of SrTiO3:Mn ceramics by electron diffraction within transmission electron microscopy and by Raman spectroscopy [4]. Therefore, the strong relationship between the structural transition and the lattice site, on which the Mn substitution occurs, is confirmed by three different techniques. The alteration of Ta in doped ST is attributed to modification of the Goldschmidt tolerance
factor, t, which is often used to express the stability of the perovskite lattice and is given by [4]. r A þ rO ð1Þ t ¼ pffiffiffi 2ðrB þ rO Þ where rA and rB stands for the average ionic radii of the Aand B-site respectively, and rO for the ionic radius of the ˚ [21]). For the coordination number oxygen (1.40 A 2þ ˚ Nc = 12, characteristic of the A-site, r2þ Sr is 1.44 A and r Mn ˚ , while for Nc = 6, characteristic of is estimated as 1.27 A 4þ ˚ ˚ the B-site, r4þ Ti is 0.605 A, and r Mn is 0.53 A [21]. For SMnT, t decreases with increasing Mn content x due to the smaller size of Mn2+ ions isovalently substituting Sr2+ ions. In such a way the average A-cation size becomes smaller, favoring tilting of the oxygen octahedra. Hence Ta increases, as observed. For STMn, the size of Mn4+ ions is smaller than that of isovalently substituted Ti4+ ions. Therefore, the average B-cation size decreases and tolerance factor increases with increasing Mn content y. Consequently, the octahedra tend to shrink rather than to tilt, which is reflected in the decrease of Ta, as observed. Thus, the difference in the appearance of the R mode in the IR spectra as a function of temperature, indicating the increase of Ta for SMnT and its decrease for STMn ceramics, strongly support the assumed occupancy of Mn ions onto two different sites in the perovskite lattice. Furthermore, it excludes the dominance of Mn accumulated at the grain boundaries, because if Mn segregates at the grain boundaries, R mode would appear from the quasi-undoped ST grains right below 110 K and no changes in Ta would be expected. There are also evident differences between THz reflectivity of SMnT and STMn ceramics, as shown enlarged in Fig. 2c and d for selected temperatures, implying a dissimilarity in phonon contribution to the dielectric response. In order to quantify the phonon parameters of SMnT and STMn ceramics, their THz and FTIR reflectivity spectra shown in Fig. 1 were fitted to the generalized-oscillator model with the factorized form of the complex dielectric permittivity [23]: pffiffiffiffiffiffiffiffiffiffiffi e ðxÞ 12 Y x2LOj x2 þ ixcLOj RðxÞ ¼ pffiffiffiffiffiffiffiffiffiffiffi with e ðxÞ ¼ e1 e ðxÞ þ 1 x2TOj x2 þ ixcLOj j
Fig. 1. IR reflectivity spectra (solid symbols) together with the data obtained by TDT transmission spectroscopy (open symbols) of Sr0.975Mn0.025TiO3 (a) and Sr0.95Mn0.05TiO3 (b) ceramics and their fits (lines) at selected temperatures.
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ð2Þ
where xTOj and xLOj denote the transverse and longitudinal frequencies of the jth polar phonon, respectively, and cTOj and cLOj are their corresponding damping constants. The high-frequency permittivity e1, resulting from the electronic absorption processes, was obtained from the room temperature frequency-independent reflectivity tails, above the phonon frequencies, and was assumed temperatureindependent. As also depicted in Fig. 1, the fit lines follow well the measurement results for both SMnT and STMn. Spectra of real e0 and imaginary e00 parts of the complex dielectric permittivity e*, calculated from the IR reflectivity fits and as directly obtained from the TDT transmission spectroscopy, are present in Figs. 3a and b and 4a and b for SMnT and STMn ceramics, respectively. Notice that
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Fig. 2. IR reflectivity spectra around the R mode (a and b) and THz reflectivity spectra (c and d) of Sr0.975Mn0.025TiO3 (a and c) and Sr0.95Mn0.05TiO3 (b and d) ceramics.
Fig. 3. Spectra of real e0 (a) and imaginary e0 0 (b) parts of the dielectric permittivity, calculated from the IR reflectivity fits of Sr0.975Mn0.025TiO3 ceramics together with data obtained from TDT transmission spectroscopy, at selected temperatures.
Fig. 4. Spectra of real e0 (a) and imaginary e00 (b) parts of the dielectric permittivity, calculated from the IR reflectivity fits of SrTi0.95Mn0.05O3 ceramics together with data obtained from TDT transmission spectroscopy, at selected temperatures.
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TDT data of e00 on the low-frequency soft mode wing (below 1 THz) are much higher for SMnT sample (Fig. 3b) than for the STMn one (Fig. 4b) and cannot be well fitted with the model of Eq. (2), just by assuming higher damping for the soft mode in SMnT, particularly at high temperatures. It appears that it is connected with the low-frequency dielectric relaxation which naturally shifts up in frequency on heating being thermally activated. Similar features in the THz e00 were also observed in KTaO3:Li crystals [25], in which the A-site substitution is widely accepted. Along with the effect on the appearance of low-frequency dielectric relaxation (as well as of R mode), the effect of Mn on the transverse optical modes of ST is also seen in Figs. 3 and 4. Whereas frequencies of TO2 and TO4 modes keep constant values of 176–177 and 547 cm1, respectively, polar soft TO1 mode is strongly affected by Mn, as well seen from Fig. 5a, where the soft mode frequency is plotted as a function of temperature. For both SrTiO3:Mn ceramics, TO1 modes continuously soften under cooling, as in the paraelectric phase of ferroelectric materials, but no soft mode condensation occurs. However, the soft mode frequency of STMn ceramics is higher than that of SMnT in the whole temperature range. Data points for the polar soft mode of undoped ST ceramics taken from Ref. [18] are also plotted in Fig. 5a for comparison. With respect to the undoped ST, the TO1 mode of STMn ceramics is stiffened in all the temperature range 10–
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300 K as well. On the other hand, the soft mode of SMnT ceramics behaves similarly to that of undoped ST above 200 K but is harder below this temperature. So, the effect of Mn on the polar soft TO1 mode of ST in the case of Sr-site substitution clearly differs from that of Ti-site substitution. Since TO1 mode in ST is known to be mostly related to Ti–O–Ti bending [24], breaking of Ti–O–Ti chains by substitution of highly polarizable Ti4+ ions with less polarizable Mn4+ ions is expected to harden the soft polar mode, as clearly seen from Fig. 5a for STMn ceramics. On the other hand, the TO1 mode in ST is known to be stiffened at low temperatures by the bias electric field [26]. In the case of SMnT ceramics, small Mn2+ ions are supposed to jump between symmetrical off-center positions, resulting in low-frequency polar dielectric relaxation at low temperatures [5]. The quasistatic (compared to the soft mode frequency) polarization, related to the off-center Mn2þ Sr ions and enhancing for decreasing temperature [5,10], perform a bias field action on the soft mode. Corresponding hardening of TO1 mode at low temperatures is clearly seen from Fig. 5a for SMnT ceramics, supporting once more the different incorporation schemes for SMnT and STMn. As can be seen from the far IR part of spectra presented in Figs. 3 and 4 and separately presented in Fig. 5, the stiffening of the soft mode corresponds to the lowering of THz permittivity values according to the Lyddane–Sachs–Teller relation D0j x ¼ const [27]. It should be noted also from Fig. 5b that the dielectric permittivity of STMn ceramics in the THz range matches well to that in the low-frequency range, indicating an absence of any dielectric relaxation from 102 to 1012 Hz. In the case of SMnT ceramics, as shown in Fig. 5b, the dielectric permittivity in the THz range does not exceed its low-frequency value as well, but shows the phonon contribution to the dielectric permittivity as a background for the low-frequency dielectric relaxation. Thus, the dielectric response of SMnT and STMn ceramics in a wide frequency and temperature range differs very much, and the main difference is the dielectric relaxation, where off-center Mn2þ Sr ions indeed play a key role. 4. Conclusions In conclusion, Sr1–xMnxTiO3 and SrTi1–yMnyO3 ceramics prepared by the conventional mixed-oxide method were studied as a function of temperature by FTIR and TDT spectroscopy. Lattice vibrational studies have shown that:
Fig. 5. Temperature dependence of the soft mode frequency xTO1 of SrTi0.95Mn0.05O3, Sr0.975Mn0.025TiO3, and SrTiO3 ceramics (a) and of the real part of the dielectric permittivity e0 of Sr0.975Mn0.025TiO3 and Sr0.95Mn0.05TiO3 ceramics at selected frequencies (b).
(i) Substitution of Sr by Mn increases Ta from 110 K up to 150 K in agreement with decreasing tolerance factor t, whereas substitution of Ti by 5% of Mn raises t and, therefore, decreases Ta to 80 K. (ii) Mn4+ incorporation on the Ti site breaks the highly polarizable Ti–O–Ti chains and increases the frequency of their lattice vibrations, i.e. stiffens TO1 mode in the whole temperature range.
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(iii) Incorporation of Mn2+ ions on the Sr sites and quasistatic polarization due to their off-central “freezing” at low temperatures causes stiffening of the polar soft mode below 200 K. Thus, the relationship between the soft lattice dynamics, octahedral tilt transition, and site occupancy for Mn ions in the perovskite lattice of studied SrTiO3:Mn ceramics is established, excluding the dominance of Mn segregations at the grain boundaries.
Acknowledgments The authors are thankful to Dr. Tatiana Ostapchuk for the help with IR characterizations. This work was funded by FCT, FEDER and European Network of Excellence FAME under the contract FP6-500159-1. References [1] Tagantsev AK, Sherman VO, Astafiev KF, Venkatesh J, Setter N. J Electroceram 2003;11:5. [2] Spaldin NA, Fiebig M. Science 2005;309:391. [3] Hill NA. J Phys Chem B 2000;104:6694. [4] Tkach A, Vilarinho PM, Kholkin AL, Reaney IM, Pokorny´ J, Petzelt J. Chem Mater 2007;19:6471 [and references therein]. [5] Tkach A, Vilarinho PM, Kholkin AL. Appl Phys Lett 2005;86: 172902.
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