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Structure–microstructure–dielectric tunability relationship in Mn-doped strontium titanate ceramics
Acta Materialia 53 (2005) 5061–5069 www.actamat-journals.com
Structure–microstructure–dielectric tunability relationship in Mn-doped strontium titanate ceramics Alexander Tkach, Paula M. Vilarinho *, Andrei L. Kholkin Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal Received 24 April 2005; received in revised form 20 July 2005; accepted 25 July 2005 Available online 12 September 2005
Abstract The influence of Mn incorporation into the Sr and Ti sites of SrTiO3 ceramics on the structure, microstructure and dielectric tunability is studied in this work. For both Sr1xMnxTiO3 and SrTi1yMnyO3 formulations, the lattice parameter, calculated from X-ray diffraction profiles, was found to decrease with increasing Mn content, but with different rates. Transmission electron microscopy analysis indicated that the solid solubility of Mn for Sr1xMnxTiO3 system is limited to x < 3%, while scanning electron microscopy analysis revealed a marked decrease of the grain size for SrTi1yMnyO3 ceramic samples. The temperature range where the dielectric constant is tunable was enlarged for Sr1xMnxTiO3 ceramics comparing with undoped SrTiO3. A higher tunability (70%) and quality factor (6000) around 75–100 K were obtained as well. It is suggested that the observed dielectric behaviour is closely related to the existence of polar microregions, formed by the introduction of Mn into the Sr site. In contrast, in SrTi1yMnyO3 ceramics a decrease of the dielectric tunability, following a reduction of dielectric permittivity was observed. 2005 Published by Elsevier Ltd on behalf of Acta Materialia Inc. Keywords: Electroceramics; X-ray diffraction; Scanning/transmission electron microscopy; Electrical properties
1. Introduction A strong dc-electric-field dependence of the dielectric permittivity makes strontium titanate (ST) an attractive material for applications in tunable electronic components, particularly in several microwave devices including filters and phase shifting elements in phased array antennas. Due to the high dielectric permittivity, these devices may be miniaturized and cost-effective. However, ST has limited application in the microwave electronic industry, since adequate tunability (change in the permittivity induced by a dc field) is achieved only below 80 K [1]. The temperature range of high tunability can be shifted towards high temperature by means of Sr site substitution by Ba, for example. Such a shift cor*
Corresponding author. Tel.: +351 234 370354; fax: +351 234 425300. E-mail address: [email protected] (P.M. Vilarinho).
responds to an induced ferroelectric phase transition in BaxSr1xTiO3 (BST) solid solutions at temperatures in the range of 0–400 K [2]. However, higher insertion loss and thermal instability of barium strontium titanate, continuously increasing with Ba concentration [3], impose serious restrictions on its application in phased array antennas. Hence, alternative ST-based systems, in which an anomaly of the dielectric response would be induced, are required. Recently, the authors reported a diffuse dielectric permittivity peak induced in ST by a Sr site Mn doping [4]. The temperature position and diffuseness of the peak were found to be controlled by the Mn content. The diffusivity of the peak implies a higher thermal stability of the dielectric behaviour of Sr1xMnxTiO3 (SMnT) compound, making Mn doped ST system a possible candidate for the above-mentioned applications. The earliest report on Mn-doped ST was published by Mu¨ller in 1959, in which the electron paramagnetic
1359-6454/$30.00 2005 Published by Elsevier Ltd on behalf of Acta Materialia Inc. doi:10.1016/j.actamat.2005.07.029
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resonance (EPR) of as-grown single crystal was investigated. The EPR spectrum obtained of ST doped with 0.01 wt.% of MnO2 was attributed to Mn4+ (3d3, S = 3/ 2) substitution of Ti4+ [5]. Later, EPR investigation of ST single crystal doped with 0.48 mol% of Mn provided evidence that the thermal annealing of such a crystal in a reducing atmosphere for several hours converts Mn4+ ions into the lower valence state ions such as Mn3+ (3d4, S = 2) and Mn2+ (3d5, S = 5/2) [6]. Oxygen vacancies were proposed as a charge compensation species for trivalent and divalent impurity ions in Ti4+ sites. More recently, EPR response of polycrystalline samples of ST with 1, 2.5, and 3 mol% of Ti substituted by Mn were measured between 120 and 300 K [7]. Lines attributed to Mn2+ ions at Ti sites were observed in the EPR spectra, in addition to those of Mn4+ ions at Ti sites [7]. Just a few studies have been devoted to the dielectric properties of Mn-doped ST. Popovic et al. [8] and Kulagin et al. [9] reported a continuous lowering of the dielectric constant in ST single crystals with addition of 0.05–0.10 wt.% of Mn, in the temperature range of 10–300 K. Regarding investigations on ceramic samples, (Sr13x/2Lax)(Ti1yMny)O3 system with x = 1.4 mol% and y = 0.1 mol% was studied by Iguchi and Lee [10]. The dielectric relaxation observed around 170 K was shown to consist of two relaxation peaks: one due to thermal motions of Ti4+ between potential minima produced by lattice distortions and another due to Mn4+ with activation energy somewhat smaller than that of Ti4+. The activation energies and the relative intensity of these relaxation processes were attributed to the difference in ionic radii of Mn4+ and Ti4+ and to the difference in the energies of formation of a strontium vacancy adjacent to Mn4+ and to Ti4+ [10]. More recently, Lemanov and co-workers [11] reported a dielectric relaxation in the SrTiO3:Mn ceramic system. Although the dopant site occupancy was not clearly stated in the text, the authors assume there was a Ti site preferential substitution for the Mn ion and that the observed relaxation is related to defects of the fMn2þ Ti –O g type. However, the oxygen post-sintering annealing treatment of the samples, conducted in the above mentioned work, should favour the oxidation of Mn ion to Mn4+ in the case of Ti site occupation, or at least to Mn3+, reducing the number of polar defects and, consequently, reducing the intensity of the anomaly. This was not observed in that study. In addition, the Mn solid solubility in ST ceramics was determined from the lattice parameter variation to be restricted to 5% [11]. These results are in contradiction to the recent report of the present authors, who studied a solid solubility of Mn4+ in SrTi1yMnyO3 ceramic system up to 10%, where no dielectric anomaly was observed [12]. Indeed, the analysis of the dielectric response of SrTi1yMnyO3 system according to the Barrett equation indicated that the system is driven away from the ferro-
electric instability [12]. On the other hand, the dielectric relaxation in ST:Mn similar to that reported by Lemanov and co-workers [11] was observed by the present authors for the Sr1xMnxTiO3 formulation, in which the Mn ions are considered to occupy the A site of the perovskite lattice [4]. To the authorsÕ best knowledge, no systematic research of Mn lattice site occupancy effect on the structure, microstructure and dielectric behaviour of ST ceramics as well as on the solid solubility limit of Mn in ST, either at the Sr or at Ti sites, has been undertaken. In order to clarify the effect of Mn doping on the structural, microstructural and dielectric properties of ST and the discrepancies found in the literature regarding the Mn effect on ST and to analyze the possible dependence of the properties on the lattice site substitution, both Sr1xMnxTiO3 (hereafter designated SMnT) and SrTi1yMnyO3 (hereafter designated STMn) ceramics were synthesized in this work. The effect of Mn and the lattice site occupancy on the structure, microstructure and dielectric tunability of Mn-doped ST ceramics are presented and their relationship is discussed. The crystallographic structure was analyzed by X-ray diffraction (XRD) technique, while scanning and transmission electron microscopy (SEM and TEM) together with energy dispersive spectroscopy (EDS) were employed for microstructural and local chemical characterization. The electric-field dependence of permittivity was explored to assess the possible application of these materials as tunable components.
2. Experimental procedure Ceramic samples were prepared by the conventional mixed oxide method. Reagent grade SrCO3, TiO2 and MnO2 were weighed according to the compositions SrTi1yMnyO3 (STMn) with y = 0, 0.01, 0.05, 0.10 and 0.15 and Sr1xMnxTiO3 (SMnT) with x = 0.005, 0.01, 0.02, 0.05, 0.10, 0.15 and 1, in which intentional stoichiometric variations of the Sr/Ti ratios are supposed to facilitate the incorporation of Mn ions either in A or B sites of the ST perovskite lattice. As follows from the formula, MnO2 contains Mn4+ ions, however it is known that Mn charge state can be reduced to Mn2+ by high temperature treatments and favourable conditions. In the case of SMnT, a favourable condition is the deficit of Sr, whose sites might be occupied with high probability with the Mn2+ ions according to charge compensation and ionic size considerations. To confirm that, an Mn(NO3)2 precursor originally containing Mn2+ ions was used for preparation of SMnT compositions with x = 0.0025 and 0.03. As will be shown later, the properties of these compositions do not deviate considerably from the general behaviour of the SMnT system prepared with MnO2.
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After ball-milling in alcohol for 5 h using teflon pots and zirconia balls in a planetary mill, the powders were dried, and then calcined at 1150 C for 2 h. The calcined powders were milled again for 5 h, to obtain powder particle size lower than 5 lm. Pellets of 10 mm in diameter were uniaxially pressed at 100 MPa and then isostatically pressed at 200 MPa. Sintering was performed in air at 1500 C for 5 h for all the samples, excluding the MnTiO3 samples, which were sintered at 1300 C for 2 h. In addition, Sr0.90Mn0.10TiO3 ceramics was sintered in air for 5 h at various temperatures from 1300 to 1500 C in order to examine the effect of the sintering temperature on the solid solubility of Mn in Sr1xMnxTiO3 system. Density of the ceramic samples, sintered at 1500 C for 5 h, ranging from 94.3% to 97.8% of the theoretical X-ray density, were measured by the ArchimedesÕs method, using diethyl phthalate as the immersion liquid. For MnTiO3 samples the measured density was 87%. The XRD measurements were performed at room temperature (Rigaku, CuKa radiation, Geigerflex C/ max-C series) in the range of 20–108 with a scanning rate of 1/min and a sampling step of 0.02. The lattice parameter was calculated by a least squares approach fitting the XRD data using WinPLOTR software. The microstructure of the ceramics was observed on polished and thermally etched sections using SEM/EDS (Hitachi S-4100). TEM/EDS analysis (Hitachi 9000) was carried out on sintered samples ground to approximately 30 lm thick and ion beam milled using a BAL-TEC Ion Mill (RES 100). For the measurements of a dc field dependence of the dielectric permittivity (tunability), sintered samples were polished to the thickness of 0.45 mm and gold electrodes were sputtered on both sides. Dielectric constant and loss were measured at a fixed temperature and a frequency of 10 kHz, as a function of the dc electric field in the range from 0 to 20 kV/cm, using a blocking circuit, a Precision LCR Meter (HP 4284A) and a high voltage dc power source (Glassman PS/EH10P10.0-22). For the dielectric tunability measurements at low temperatures in the range of 10–300 K, samples were cooled until the required temperature is reached and stabilized in a He closed cycle cryogenic system (Displex ADP-Cryostat HC-2) controlled by a digital temperature controller (Scientific Instrument Model 9650) with silicon diode thermometers.
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[4]. The observed relaxor-type dielectric behaviour was attributed by the present authors to the formation of electric dipoles and corresponding random fields due to the off-centre position of Mn2+ ions at Sr sites of highly polarisable SrTiO3 lattice [4]. The temperature dependence of the dielectric constant e 0 and of the dissipation factor tan d = e00 /e 0 at 10 kHz under the dc electric field up to 20 kV/cm is shown in Fig. 1(a) for SMnT ceramics with Mn content x = 0.03. It is evident that e 0 is mostly field dependent (or tunable) at the temperature of the maximum dielectric constant Te 0 m. The peak of the e 0 gradually decreases in value, broadens and shifts toward higher temperature for 20 K with the bias field increase. Due to the dielectric relaxation, induced by Mn-doping at Sr site of ST, the upper bound of the temperature range, in which e 0 is tunable, is expanded from 80 K for undoped ST to 150 K for SMnT. In addition, tan d decreases significantly with increasing dc bias field in the temperature range of 10– 100 K. Radio-frequency dielectric measurements of SrTi1yMnyO3 ceramic system under zero bias field showed a progressive decrease of the low-temperature dielectric permittivity with increasing amount of Mn [12]. As depicted in Fig. 1(b) and in contradiction to the interpretation of Lemanov et al. [11] no dielectric
3. Results Radio-frequency dielectric measurements of Sr1xMnxTiO3 ceramic system under zero bias field showed a diffuse low-temperature peak of the dielectric permittivity, shifting to higher temperatures with increasing measurement frequency and amount of Mn
Fig. 1. Variation of the dielectric constant e 0 and of dissipation factor tan d at 10 kHz with temperature for Sr0.97Mn0.03TiO3 (a) and SrTi0.95Mn0.05O3 (b) ceramics under different dc bias fields.
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anomaly was observed, when Mn ions were incorporated at Ti sites. Moreover, as reported by the authors of [12], the incorporation of Mn at the B site of the structure drives the system away from the ferroelectric instability. The bias field effect on the dielectric response of SrTi1yMnyO3 ceramic system exemplified by sample with y = 0.05 is presented in Fig. 1(b). The dielectric constant e 0 reveals a very weak dependence on the applied bias. Furthermore, tan d is rather insensitive to the bias field. In order to clarify these differences in the dielectric response observed between the two systems under study, SMnT and STMn, structural and microstructural studies were conducted. Fig. 2 presents the XRD profiles for the selected Sr1xMnxTiO3 samples. Ceramics with x 6 0.05 reveal a single cubic perovskite phase, similar to SrTiO3, shown in Fig. 2 for comparison. Second phase peaks start to appear in the XRD profiles at x = 0.10 and are clearly seen for x = 0.15. These features are consistent with MnTiO3 spectra, also shown in Fig. 2 for comparison.
The effect of the firing temperature on the solid solubility of Mn in Sr1xMnxTiO3 system was examined by XRD using samples of x = 0.10 after sintering in air for 5 h at various temperatures. The amount of MnTiO3 second phase was found to slightly increase as temperature decreased from 1500 to 1300 C. Therefore, one could conclude that, a high sintering temperature promotes a higher solid solubility value, but in rather limited range, as was also reported for Ce-doped BaTiO3 [13]. The lattice parameter a calculated from the XRD profiles of single-phase samples, is presented in Fig. 3 as a function of Mn content x. Increasing x leads to a decrease of a, which can be described by a linear Vegard ˚ . A decrease of the law with the slope da/dx 0.027 A lattice parameter with an increase in the Mn content can be expected from ionic size considerations. Mn2+ ionic radius value, extrapolated to the coordination number 12 from the values obtained by Shannon [14] for ˚ , which is smalsmaller coordination numbers, is 1.27 A 2+ ˚ ler than the value of 1.44 A for Sr . The observed variation is an indication of the substitution of Sr by Mn ions. Samples prepared from Mn(NO3)2 precursor do not change the slope of the line. The XRD profiles of SrTi1yMnyO3 ceramics exhibited a cubic crystallographic structure similar to undoped ST without any second phase for all Mn concentration range under study (y 6 0.15). The lattice parameter a decreases linearly with the increase of Mn ˚, content from y = 0 to 0.15 with the slope da/dy 0.1 A as also shown in Fig. 3. Such a behaviour of the lattice parameter might be an indication that Ti4+ with ionic ˚ is being mostly substituted by Mn4+ radius of 0.605 A ˚ , and not by the reduced with an ionic radius of 0.53 A ˚ ) or Mn2+ (0.83 A ˚ ) ions [14]. Mn3+ (0.645 A As can also be seen from Fig. 3, the lattice parameter of Sr1xMnxTiO3 system decreases with increasing Mn content at a lower rate than that of SrTi1yMnyO3, de-
Fig. 2. XRD profiles of sintered SMnT ceramics (* denotes MnTiO3 phase features).
Fig. 3. Lattice parameter of sintered SMnT and STMn ceramics as a function of Mn content.
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spite the fact that the ionic radii ratios Mn2+/Sr2+ and Mn4+/Ti4+ are rather similar. One may then suppose that besides the ionic size and charge, the lattice parameter is also dependent on the electronic structure and electron-orbital configuration of the dopant ions and may account for the higher solid solubility on the B site of the structure. Dense microstructures with an average grain size, varying in the range of 20–35 lm with no clear dependence on Mn content, were observed by SEM for Sr1xMnxTiO3 ceramics with x = 0.025–0.15, as exemplified in Fig. 4(a) and (b) for the samples with x = 0.01 and 0.03. No appreciable difference in the grain size was detected, when comparing these data with undoped ST, shown in Fig. 4(c). SEM micrographs of SrTi1yMnyO3 ceramics (shown for the sample with y = 0.05 in Fig. 4(d)), also reveal dense and homogeneous microstructures. However, a marked decrease of the average grain size to 0.7 ± 0.1 lm was observed for the STMn system. TEM bright field images of Sr1xMnxTiO3 ceramic samples with x = 0.02 and 0.05 are shown in Fig. 5(a) and (b). Their local structure was analyzed by electron diffraction (ED) and chemical composition by EDS.
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The energy dispersive spectra of SMnT and STMn ceramics are shown in Fig. 6(a) and (c). TEM micrographs of SMnT ceramics reveal slim grain boundaries and confirm also the predominance of the over-micron-sized grains, as observed by SEM. ED patterns of such grains reflect the perovskite-type cubic structure, as shown in the top insets in Fig. 5(a) and (b). Moreover, their energy dispersive spectra are close to that of SrTiO3 with an additional small (but detectable) Mn peak as presented in Fig. 6(a). No second phase was observed for Sr0.995Mn0.005TiO3 and Sr0.98Mn0.02TiO3 ceramic samples, apart from the small amorphous inclusions rich in Sr and Zr with trace amounts of Mn located at a few triple points (see Fig. 5(a) and bottom inset). Zr contamination might appear during the milling step. However, under-micronsized grains with a non-perovskite structure were clearly detected in SMnT ceramics with x = 0.03 and 0.05, as shown in Fig. 5(b) and the bottom inset for the Sr0.95Mn0.05TiO3 sample. The EDS spectra of this phase shown in Fig. 6(b) for Sr0.97Mn0.03TiO3 sample reveal almost equivalent peaks of Mn and Ti, suggesting the presence of MnTiO3 phase, which is known to have an ilmenite-type structure. TEM and the complementary
Fig. 4. SEM micrographs of Sr0.99Mn0.01TiO3 (a), Sr0.97Mn0.03TiO3 (b), SrTiO3 (c), and SrTi0.95Mn0.05O3 (d) ceramics.
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Fig. 6. Typical TEM-EDS spectra of grains of SMnT (a) and STMn (c) ceramics and of grain boundary crystalline phase, observed for SMnT ceramics with x P 0.03 (b).
4. Discussion Fig. 5. Bright-field TEM images of Sr0.98Mn0.02TiO3 (a), Sr0.95Mn0.05TiO3 (b), and SrTi0.85Mn0.15O3 (c) ceramics. Insets show ED patterns of SMnT ceramics for crystalline perovskite grains (in top corner) and for grain boundary phases (in bottom corner).
ED and EDS analyses of SMnT ceramics indicate that the formation of the MnTiO3 phase starts from x = 0.03. This value is thought to be the solid solubility limit of Mn in Sr site of ST, which is lower than the one obtained from XRD data and the value reported by Lemanov et al. [11]. Clean grain boundaries and solely perovskite grains of submicron size were observed by TEM in SrTi1yMnyO3 ceramics, whereas Mn incorporation into the lattice was confirmed by the evident peak in energy dispersive spectra of STMn grains, as shown in Figs. 5 and 6(c) for SrTi0.85Mn0.15O3 sample. The high solid solubility of Mn in Ti sites of ST was confirmed by TEM studies.
SrTiO3 is known to be a highly-packed perovskite structure of general formula ABO3 with a tolerance factor t = t1/t2 1. Fractional tolerance factors t1 and t2 describe the degree of packing of A or B ions in the perovskite lattice and are given by the following equation [15]: pffiffiffi 2ðrB þ rO Þ 2ðrA þ rO Þ and t2 ¼ ; ð1Þ t1 ¼ a a where rA and rB stand for the average ionic radii at the A and B sites, respectively, rO is the ionic radius of the oxygen, and a stands for the lattice parameter. The ionic radius of Mn2+ estimated for coordination number Nc = 12 (characteristic for the perovskite A site) 2þ 2þ 2þ ˚ ˚ is r2þ Mn ¼ 1:27 A, while rSr ¼ 1:44 A, i.e., rMn =r Sr ¼ 0:88 4þ 4þ ˚ rTi ¼ 0:605 A ˚ [14]. On the other hand, rMn ¼ 0:53 A, 4þ and r4þ =r ¼ 0:88 at N = 6, characteristic for the B c Mn Ti site. Hence, considering the charge and ionic radius, Mn ions can occupy both the A and B sites of the ST
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perovskite lattice. The results obtained in this work confirmed this prediction showing that the substitution by Mn occurred at both the A and B sites of the ST lattice and that the solid solubility limit of Mn on the ST lattice depends on the site of occupancy. In spite of the similar2þ 4þ 4þ ity of r2þ Mn =rSr and r Mn =rTi ratios, the solid solubility limit of Mn at the A site of ST was found to be lower than at the B site of structure. This result confirms earlier reports on EPR of Mn-doped ST crystals that indicated the Ti substitution by Mn [5,6]. In addition, the SMnT cell parameter was found to decrease almost four times slower than that of STMn with the increase of Mn content. Therefore, such sharp compression of the STMn cell, probably does not allow the Mn4+ ions to occupy the off-centre position at the Ti site of the SrTi1yMnyO3 system, whereas in the Sr1xMnxTiO3 system, Mn2+ ions are believed to flip between the off-centre equilibrium positions at the Sr sites, accounting for a low-temperature dielectric relaxation. Along with different structural changes induced by Mn doping at Sr and Ti sites (observed by XRD technique), the differences of the lattice site occupancy did also have a pronounced effect in the microstructure. The grain growth observed by SEM is different for the SMnT and STMn systems. As a result, one order of magnitude smaller average grain size was obtained for Ti site Mn-doped ST ceramics, similar to that for Ti site Mg-doped ST [16], while no strong difference in the average grain size was observed between undoped and Sr site Mn-doped ST. However, it should be remarked that this difference in the grain growth behaviour might be related to the variations of the Sr/Ti ratio [17]. This subject is currently being studied. Finally, TEM/EDS technique presented the main and unambiguous evidence of the difference in the lattice site occupancy between these two systems. In STMn ceramic samples, Mn incorporation into ST grains was evidenced by the absence of Sr-containing second phase and by the energy dispersive spectra of STMn ceramic grains in which a Mn peak was clearly observed. This confirms that Mn occupies the Ti site, but not the Sr site of ST [12]. For SMnT ceramics, as shown in Figs. 5 and 6(a), TEM equipped with the EDS analysis confirmed the incorporation of Mn into ST grains and the absence of any second crystalline phases for x 6 0.02. In particular, titanium oxide extra-phase, expected if Mn ions occupied B sites of the perovskite lattice, was not detected, supporting the assumption that Mn occupies Sr sites. Only samples with x > 0.02 revealed the presence of a second crystalline phase identified as MnTiO3 (see Figs. 5 and 6(b)). MnTiO3 is known to have an ilmenite-type structure [18], being stable at room pressure and temperature, although a transformation of MnTiO3 to a perovskite structure under certain conditions was also reported
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[19]. From the dielectric point of view, MnTiO3 behaves as an ordinary dielectric: its frequency-independent dielectric constant e 0 22, decreases with temperature with the slope of de/dT = 1.1 · 103 K1 and the dissipation factor tan d is about 0.007. Thus the MnTiO3 second phase can account for the decrease of the overall dielectric permittivity and the loss for SMnT system. So, the structural analysis supports quite well the results of the dielectric response of SMnT and STMn presented in this work and previously reported by the authors [4,12]. The dielectric behaviour observed for SMnT system is similar to that of relaxors and was attributed to the formation of electric dipoles and corresponding random fields due to the off-centre position of Mn2+ ions at Sr sites of highly polarisable ST lattice [4]. Thus, the decrease of e 0 with dc bias field can be explained by the stabilization of the local potential wells for the off-centre Mn2+ dipoles under applying external electric fields. Moreover, the dielectric loss vanishing with the field can also be explained by the alignment and stabilization of off-centre Mn2+ ions by applied field, reducing the contribution of domain wall dynamics to the losses. On the other hand, the sharp compression of the lattice cell, observed by XRD for the STMn system with increasing Mn content does not allow the Mn4+ ions to occupy the off-centre position at Ti sites. In addition, less polarisable Mn4+ ions substituting more polarisable Ti4+ ions, may also break Ti-Ti long-range interaction. This results in a strong reduction of the dielectric response without any dielectric anomaly in SrTi1yMnyO3 ceramics. Moreover, the ferroelectric instability was reported to be suppressed [12]. The main parameter for tunable electronic components is related to the dependence of the dielectric constant on applied electric field. Relative tunability can be calculated as follows: nr ðEÞ ¼ ½e0 ð0Þ e0 ðEÞ=e0 ð0Þ; 0
ð2Þ
where e (0) stands for the dielectric permittivity at zero field and e 0 (E) stands for the dielectric permittivity under applied bias field E. The dependence of the relative tunability nr for both Sr1xMnxTiO3 (x = 0.03) and SrTi1yMnyO3 (y = 0.05) ceramics on the bias field at fixed temperatures is represented in Fig. 7(a) and (b). Comparing Fig. 7 with Fig. 1, the rule ‘‘the higher the e 0 , the higher its tunability’’ is confirmed. Thus, the maximum nr value of 66.2% was obtained under the bias field of 20 kV/cm at 60 K for SMnT ceramics with Mn content x = 0.03. This is more than 20% higher than that of undoped ST ceramics. On the other hand, the maximum relative tunability of SrTi0.95Mn0.05O3 sample, obtained at 10 K under 20.5 kV/cm, is lower than 5%. These data are in close agreement with the suppression of the ferroelectric instability by Mn doping at Ti sites.
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A. Tkach et al. / Acta Materialia 53 (2005) 5061–5069 Table 1 Maximum relative tunability nmax and quality factor of a tunable component K, obtained at bias field Emax = 20 kV/cm for Sr0.97Mn0.03TiO3 and SrTiO3 ceramics T (K)
10 15 25 35 50 60 75 100 150 294
Maximum relative tunability nmax (%)
Quality factor of a tunable component K
SrTiO3
Sr0.97Mn0.03TiO3
SrTiO3
Sr0.97Mn0.03TiO3
76.8 75.2 70.1 – 46.9 – 21.1 7.1 2.1 0.2
22.0 27.8 – 50.9 65.0 66.2 58.0 35.9 8.7 0.4
77464.3 93392.0 70384.2 – 24222.1 – 678.3 221.7 27.0 0.1
1856.4 1222.1 – 120.5 353.3 994.6 3073.1 5899.4 102.1 0.2
range of 75–125 K. Due to this fact, SMnT ceramics can be used as a tunable component at liquid nitrogen temperature, where high temperature superconductors can be applicable as electrodes. Moreover, the figure of merit, Fm, of a phase shifter defined as the ratio of the phase shift, D/degree, in degrees to the insertion loss, LdB, in dB (Eq. (3)) is closely related to the QFTC of the ferroelectric component used in the phase shifter [3]: pffiffiffiffi Fm ¼ D/degree =LdB ¼ 6:6 K . ð4Þ Fig. 7. Variation of relative tunability nr at 10 kHz, calculated as nr (E) = [e 0 (0) e 0 (E)]/e 0 (0), with bias field for Sr0.97Mn0.03TiO3 (a) and SrTi0.95Mn0.05O3 (b) ceramics at different temperatures.
The low dielectric loss in the temperature range of the high tunability is an additional important factor for device applications. To characterize the capability of the ferroelectric component for practical applications in microwave engineering, the quality factor of a tunable component (QFTC) has to be known. QFTC, as a general parameter of a tunable component, combines both tunability and loss factor and is given by the following equation: K ¼ ðn 1Þ2 =½n tan dð0Þ tan dðEmax Þ; 0
0
ð3Þ
where n = e (0)/e (Emax) stands for the tunability of the component [3], and tan d (0) and tan d (Emax) are the loss factor under zero and maximum bias field, respectively. QFTC values were calculated for the SMnT sample with x = 0.03, as shown in Table 1 together with the maximum relative tunability nmax, and were compared with the corresponding parameters of undoped ST. The results obtained reveal that QFTC is one and a half order of magnitude higher for undoped ST than for Mn-doped ST at temperatures below 75 K, but the opposite situation is observed for the temperature
Following this equation, one can obtain a phase shift higher than 360 per 1 dB, if the ferroelectric component is characterized by K > 3000 [3]. The values obtained for Sr0.98Mn0.03TiO3 around 100 K satisfy this condition well, as expected from data shown in Fig. 1(a), where at 100 K the dielectric constant is still well tunable, and tan d is already low and rather insensitive to the bias field. In addition, the use of high temperature superconductors as device electrode material becomes possible in the vicinity of 90 K, supported also by the chemical and structural compatibility with ST-based materials [20]. Thus, low dielectric loss, high dielectric constant, and its dc field dependence combined with the low loss in high temperature superconductors such as YBa2Cu3O7 (YBCO) make the SMnT system promising for applications in high-quality-factor electrically tunable components for advanced microwave communications systems. On the other hand, a high thermal stability can be obtained due to high diffusivity of the dielectric constant peak of SMnT ceramics.
5. Conclusions Ceramic samples of Sr1xMnxTiO3 (0 6 x 6 0.15 and x = 1) and SrTi1yMnyO3 (0 6 y 6 0.15) systems were synthesized by conventional mixed oxide method, in which intentional stoichiometric variations allowed the
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incorporation of Mn ions at Sr and Ti sites of ST lattice, respectively. From the structural and microstructural point of view the following differences were observed: (1) The solid solubility limit for Mn at Sr site of ST was determined to be about 2%. Small grains of MnTiO3 were found by TEM/EDS to appear in the grain boundaries of Sr1xMnxTiO3 ceramics for higher Mn concentrations. On the other hand no second phases and a strong decrease of the lattice parameter with increasing Mn content were observed for SrTi1yMnyO3 ceramics. This fact points to the incorporation of Mn into the Ti sites of ST for concentrations as high as 15%. Consequently the solid solubility limit for Mn in B sites of ST is higher than for Mn in A sites of ST structure. (2) The grain size was found to differ markedly between Sr1xMnxTiO3 (variation from 20 to 35 lm) and SrTi1yMnyO3 ceramics (much finer grains from 0.6 to 0.8 lm were observed), without clear dependence on Mn content. From the dielectric point of view the following differences were observed: (1) A pronounced temperature- and frequency-dependent relaxation of the relaxor type for the SMnT system, as compared to a sharp reduction of the dielectric response without any dielectric anomaly observed for the STMn system. Although incorporation of Mn4+ into the Ti sites of the ST lattice (decreasing the fractional tolerance factor t2) indicates smaller B site packing degree and supposedly more favourable off-centre positions of these ions, the quantum fluctuations became more stable and the system was driven away from ferroelectricity. This is in line with the observed compression of the unit cell of STMn system with increasing Mn content that might not allow the Mn4+ ions to occupy the off-centre position at Ti sites. Also the substitution of more polarisable Ti4+ ions by less polarisable Mn4+ ions may break Ti–Ti longrange interaction. (2) As a consequence of the different dielectric response and due to the formation of polar dipoles, related to the introduction of Mn2+ into the Sr sites, the temperature range, where the dielectric constant is tunable, was enlarged up to 150 K, compared with undoped ST (80 K). In addition, a higher tunability (70%) and quality
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factor of a tunable component (6000) in the vicinity of liquid nitrogen temperature were obtained for SMnT ceramics. Hence Sr1xMnxTiO3 is a promising material for possible application as a phase shifter. In conclusion this work clarified that: (i) the dielectric response of ST doped with Mn depends on the dopant site occupancy; and (ii) the dielectric relaxation of the relaxor-type, observed in Mn-doped strontium titanate ceramics, was related to Sr site occupancy. Further detailed studies on the structure of SMnT ceramics as well as on the nature of the dielectric relaxation are ongoing.
Acknowledgement Alexander Tkach acknowledges FCT (Portuguese Foundation for Science and Technology) for financial support.
References [1] Saifi MA, Cross LE. Phys Rev B 1970;2:677. [2] Lemanov VV, Smirnova EP, Syrnikov PP, Tarakanov EA. Phys Rev B 1996;54:3151. [3] Vendik OG, Hollmann EK, Kozyrev AB, Prudan AM. J Supercond 1999;12:325. [4] Tkach A, Vilarinho PM, Kholkin AL. Appl Phys Lett 2005;86:172902. [5] Mu¨ller KA. Phys Rev Lett 1959;2:341. [6] Serway RA, Berlinger W, Mu¨ller KA, Collins RW. Phys Rev B 1977;16:4761. [7] Azzoni CB, Mozzati MC, Paleari A, Massarotti V, Bini M, Capsoni D. Solid State Commun 2000;114:617. [8] Popovic D, Sarvan M, Dojcilovic J, Kulagin N. Solid State Phenom 1998;61–62:183. [9] Kulagin N, Dojcilovic J, Popovic D. Cryogenics 2001;41:745. [10] Iguchi E, Lee KJ. J Mater Sci 1993;28:5809. [11] Lemanov VV, Smirnova EP, Sotnikov AV, Weihnacht M. Fiz Tverd Tela 2004;46:1402 [English translation: Phys Solid State 2004;46:1442]. [12] Tkach A, Vilarinho PM, Kholkin A. Ferroelectrics 2004;304:917. [13] Makovec D, Samardzˇija Z, Kolar D. J Solid State Chem 1996;123:30. [14] Shannon RD. Acta Crystallogr A 1976;32:751. [15] Lemanov VV. Ferroelectrics 1999;226:133. [16] Tkach A, Vilarinho PM, Kholkin A. Appl Phys A – Mater 2004;79:2013. [17] Tkach A, Vilarinho PM, Senos AMR, Kholkin AL. J Eur Ceram Soc 2005;25:2769. [18] Sohn J-H, Inaguma Y, Yoon S-O, Itoh M, Nakamura T, Yoon SJ, et al. Jpn J Appl Phys 1994;33:5466. [19] Ross NL, Ko J, Prewitt CT. Phys Chem Minerals 1989;16:621. [20] Vendik OG, Kollberg E, Gevorgian SS, Kozyrev AB, Soldatenkov OI. Electron Lett 1995;31:654.
Structure–microstructure–dielectric tunability relationship in Mn-doped strontium titanate ceramics Alexander Tkach, Paula M. Vilarinho *, Andrei L. Kholkin Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal Received 24 April 2005; received in revised form 20 July 2005; accepted 25 July 2005 Available online 12 September 2005
Abstract The influence of Mn incorporation into the Sr and Ti sites of SrTiO3 ceramics on the structure, microstructure and dielectric tunability is studied in this work. For both Sr1xMnxTiO3 and SrTi1yMnyO3 formulations, the lattice parameter, calculated from X-ray diffraction profiles, was found to decrease with increasing Mn content, but with different rates. Transmission electron microscopy analysis indicated that the solid solubility of Mn for Sr1xMnxTiO3 system is limited to x < 3%, while scanning electron microscopy analysis revealed a marked decrease of the grain size for SrTi1yMnyO3 ceramic samples. The temperature range where the dielectric constant is tunable was enlarged for Sr1xMnxTiO3 ceramics comparing with undoped SrTiO3. A higher tunability (70%) and quality factor (6000) around 75–100 K were obtained as well. It is suggested that the observed dielectric behaviour is closely related to the existence of polar microregions, formed by the introduction of Mn into the Sr site. In contrast, in SrTi1yMnyO3 ceramics a decrease of the dielectric tunability, following a reduction of dielectric permittivity was observed. 2005 Published by Elsevier Ltd on behalf of Acta Materialia Inc. Keywords: Electroceramics; X-ray diffraction; Scanning/transmission electron microscopy; Electrical properties
1. Introduction A strong dc-electric-field dependence of the dielectric permittivity makes strontium titanate (ST) an attractive material for applications in tunable electronic components, particularly in several microwave devices including filters and phase shifting elements in phased array antennas. Due to the high dielectric permittivity, these devices may be miniaturized and cost-effective. However, ST has limited application in the microwave electronic industry, since adequate tunability (change in the permittivity induced by a dc field) is achieved only below 80 K [1]. The temperature range of high tunability can be shifted towards high temperature by means of Sr site substitution by Ba, for example. Such a shift cor*
Corresponding author. Tel.: +351 234 370354; fax: +351 234 425300. E-mail address: [email protected] (P.M. Vilarinho).
responds to an induced ferroelectric phase transition in BaxSr1xTiO3 (BST) solid solutions at temperatures in the range of 0–400 K [2]. However, higher insertion loss and thermal instability of barium strontium titanate, continuously increasing with Ba concentration [3], impose serious restrictions on its application in phased array antennas. Hence, alternative ST-based systems, in which an anomaly of the dielectric response would be induced, are required. Recently, the authors reported a diffuse dielectric permittivity peak induced in ST by a Sr site Mn doping [4]. The temperature position and diffuseness of the peak were found to be controlled by the Mn content. The diffusivity of the peak implies a higher thermal stability of the dielectric behaviour of Sr1xMnxTiO3 (SMnT) compound, making Mn doped ST system a possible candidate for the above-mentioned applications. The earliest report on Mn-doped ST was published by Mu¨ller in 1959, in which the electron paramagnetic
1359-6454/$30.00 2005 Published by Elsevier Ltd on behalf of Acta Materialia Inc. doi:10.1016/j.actamat.2005.07.029
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resonance (EPR) of as-grown single crystal was investigated. The EPR spectrum obtained of ST doped with 0.01 wt.% of MnO2 was attributed to Mn4+ (3d3, S = 3/ 2) substitution of Ti4+ [5]. Later, EPR investigation of ST single crystal doped with 0.48 mol% of Mn provided evidence that the thermal annealing of such a crystal in a reducing atmosphere for several hours converts Mn4+ ions into the lower valence state ions such as Mn3+ (3d4, S = 2) and Mn2+ (3d5, S = 5/2) [6]. Oxygen vacancies were proposed as a charge compensation species for trivalent and divalent impurity ions in Ti4+ sites. More recently, EPR response of polycrystalline samples of ST with 1, 2.5, and 3 mol% of Ti substituted by Mn were measured between 120 and 300 K [7]. Lines attributed to Mn2+ ions at Ti sites were observed in the EPR spectra, in addition to those of Mn4+ ions at Ti sites [7]. Just a few studies have been devoted to the dielectric properties of Mn-doped ST. Popovic et al. [8] and Kulagin et al. [9] reported a continuous lowering of the dielectric constant in ST single crystals with addition of 0.05–0.10 wt.% of Mn, in the temperature range of 10–300 K. Regarding investigations on ceramic samples, (Sr13x/2Lax)(Ti1yMny)O3 system with x = 1.4 mol% and y = 0.1 mol% was studied by Iguchi and Lee [10]. The dielectric relaxation observed around 170 K was shown to consist of two relaxation peaks: one due to thermal motions of Ti4+ between potential minima produced by lattice distortions and another due to Mn4+ with activation energy somewhat smaller than that of Ti4+. The activation energies and the relative intensity of these relaxation processes were attributed to the difference in ionic radii of Mn4+ and Ti4+ and to the difference in the energies of formation of a strontium vacancy adjacent to Mn4+ and to Ti4+ [10]. More recently, Lemanov and co-workers [11] reported a dielectric relaxation in the SrTiO3:Mn ceramic system. Although the dopant site occupancy was not clearly stated in the text, the authors assume there was a Ti site preferential substitution for the Mn ion and that the observed relaxation is related to defects of the fMn2þ Ti –O g type. However, the oxygen post-sintering annealing treatment of the samples, conducted in the above mentioned work, should favour the oxidation of Mn ion to Mn4+ in the case of Ti site occupation, or at least to Mn3+, reducing the number of polar defects and, consequently, reducing the intensity of the anomaly. This was not observed in that study. In addition, the Mn solid solubility in ST ceramics was determined from the lattice parameter variation to be restricted to 5% [11]. These results are in contradiction to the recent report of the present authors, who studied a solid solubility of Mn4+ in SrTi1yMnyO3 ceramic system up to 10%, where no dielectric anomaly was observed [12]. Indeed, the analysis of the dielectric response of SrTi1yMnyO3 system according to the Barrett equation indicated that the system is driven away from the ferro-
electric instability [12]. On the other hand, the dielectric relaxation in ST:Mn similar to that reported by Lemanov and co-workers [11] was observed by the present authors for the Sr1xMnxTiO3 formulation, in which the Mn ions are considered to occupy the A site of the perovskite lattice [4]. To the authorsÕ best knowledge, no systematic research of Mn lattice site occupancy effect on the structure, microstructure and dielectric behaviour of ST ceramics as well as on the solid solubility limit of Mn in ST, either at the Sr or at Ti sites, has been undertaken. In order to clarify the effect of Mn doping on the structural, microstructural and dielectric properties of ST and the discrepancies found in the literature regarding the Mn effect on ST and to analyze the possible dependence of the properties on the lattice site substitution, both Sr1xMnxTiO3 (hereafter designated SMnT) and SrTi1yMnyO3 (hereafter designated STMn) ceramics were synthesized in this work. The effect of Mn and the lattice site occupancy on the structure, microstructure and dielectric tunability of Mn-doped ST ceramics are presented and their relationship is discussed. The crystallographic structure was analyzed by X-ray diffraction (XRD) technique, while scanning and transmission electron microscopy (SEM and TEM) together with energy dispersive spectroscopy (EDS) were employed for microstructural and local chemical characterization. The electric-field dependence of permittivity was explored to assess the possible application of these materials as tunable components.
2. Experimental procedure Ceramic samples were prepared by the conventional mixed oxide method. Reagent grade SrCO3, TiO2 and MnO2 were weighed according to the compositions SrTi1yMnyO3 (STMn) with y = 0, 0.01, 0.05, 0.10 and 0.15 and Sr1xMnxTiO3 (SMnT) with x = 0.005, 0.01, 0.02, 0.05, 0.10, 0.15 and 1, in which intentional stoichiometric variations of the Sr/Ti ratios are supposed to facilitate the incorporation of Mn ions either in A or B sites of the ST perovskite lattice. As follows from the formula, MnO2 contains Mn4+ ions, however it is known that Mn charge state can be reduced to Mn2+ by high temperature treatments and favourable conditions. In the case of SMnT, a favourable condition is the deficit of Sr, whose sites might be occupied with high probability with the Mn2+ ions according to charge compensation and ionic size considerations. To confirm that, an Mn(NO3)2 precursor originally containing Mn2+ ions was used for preparation of SMnT compositions with x = 0.0025 and 0.03. As will be shown later, the properties of these compositions do not deviate considerably from the general behaviour of the SMnT system prepared with MnO2.
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After ball-milling in alcohol for 5 h using teflon pots and zirconia balls in a planetary mill, the powders were dried, and then calcined at 1150 C for 2 h. The calcined powders were milled again for 5 h, to obtain powder particle size lower than 5 lm. Pellets of 10 mm in diameter were uniaxially pressed at 100 MPa and then isostatically pressed at 200 MPa. Sintering was performed in air at 1500 C for 5 h for all the samples, excluding the MnTiO3 samples, which were sintered at 1300 C for 2 h. In addition, Sr0.90Mn0.10TiO3 ceramics was sintered in air for 5 h at various temperatures from 1300 to 1500 C in order to examine the effect of the sintering temperature on the solid solubility of Mn in Sr1xMnxTiO3 system. Density of the ceramic samples, sintered at 1500 C for 5 h, ranging from 94.3% to 97.8% of the theoretical X-ray density, were measured by the ArchimedesÕs method, using diethyl phthalate as the immersion liquid. For MnTiO3 samples the measured density was 87%. The XRD measurements were performed at room temperature (Rigaku, CuKa radiation, Geigerflex C/ max-C series) in the range of 20–108 with a scanning rate of 1/min and a sampling step of 0.02. The lattice parameter was calculated by a least squares approach fitting the XRD data using WinPLOTR software. The microstructure of the ceramics was observed on polished and thermally etched sections using SEM/EDS (Hitachi S-4100). TEM/EDS analysis (Hitachi 9000) was carried out on sintered samples ground to approximately 30 lm thick and ion beam milled using a BAL-TEC Ion Mill (RES 100). For the measurements of a dc field dependence of the dielectric permittivity (tunability), sintered samples were polished to the thickness of 0.45 mm and gold electrodes were sputtered on both sides. Dielectric constant and loss were measured at a fixed temperature and a frequency of 10 kHz, as a function of the dc electric field in the range from 0 to 20 kV/cm, using a blocking circuit, a Precision LCR Meter (HP 4284A) and a high voltage dc power source (Glassman PS/EH10P10.0-22). For the dielectric tunability measurements at low temperatures in the range of 10–300 K, samples were cooled until the required temperature is reached and stabilized in a He closed cycle cryogenic system (Displex ADP-Cryostat HC-2) controlled by a digital temperature controller (Scientific Instrument Model 9650) with silicon diode thermometers.
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[4]. The observed relaxor-type dielectric behaviour was attributed by the present authors to the formation of electric dipoles and corresponding random fields due to the off-centre position of Mn2+ ions at Sr sites of highly polarisable SrTiO3 lattice [4]. The temperature dependence of the dielectric constant e 0 and of the dissipation factor tan d = e00 /e 0 at 10 kHz under the dc electric field up to 20 kV/cm is shown in Fig. 1(a) for SMnT ceramics with Mn content x = 0.03. It is evident that e 0 is mostly field dependent (or tunable) at the temperature of the maximum dielectric constant Te 0 m. The peak of the e 0 gradually decreases in value, broadens and shifts toward higher temperature for 20 K with the bias field increase. Due to the dielectric relaxation, induced by Mn-doping at Sr site of ST, the upper bound of the temperature range, in which e 0 is tunable, is expanded from 80 K for undoped ST to 150 K for SMnT. In addition, tan d decreases significantly with increasing dc bias field in the temperature range of 10– 100 K. Radio-frequency dielectric measurements of SrTi1yMnyO3 ceramic system under zero bias field showed a progressive decrease of the low-temperature dielectric permittivity with increasing amount of Mn [12]. As depicted in Fig. 1(b) and in contradiction to the interpretation of Lemanov et al. [11] no dielectric
3. Results Radio-frequency dielectric measurements of Sr1xMnxTiO3 ceramic system under zero bias field showed a diffuse low-temperature peak of the dielectric permittivity, shifting to higher temperatures with increasing measurement frequency and amount of Mn
Fig. 1. Variation of the dielectric constant e 0 and of dissipation factor tan d at 10 kHz with temperature for Sr0.97Mn0.03TiO3 (a) and SrTi0.95Mn0.05O3 (b) ceramics under different dc bias fields.
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anomaly was observed, when Mn ions were incorporated at Ti sites. Moreover, as reported by the authors of [12], the incorporation of Mn at the B site of the structure drives the system away from the ferroelectric instability. The bias field effect on the dielectric response of SrTi1yMnyO3 ceramic system exemplified by sample with y = 0.05 is presented in Fig. 1(b). The dielectric constant e 0 reveals a very weak dependence on the applied bias. Furthermore, tan d is rather insensitive to the bias field. In order to clarify these differences in the dielectric response observed between the two systems under study, SMnT and STMn, structural and microstructural studies were conducted. Fig. 2 presents the XRD profiles for the selected Sr1xMnxTiO3 samples. Ceramics with x 6 0.05 reveal a single cubic perovskite phase, similar to SrTiO3, shown in Fig. 2 for comparison. Second phase peaks start to appear in the XRD profiles at x = 0.10 and are clearly seen for x = 0.15. These features are consistent with MnTiO3 spectra, also shown in Fig. 2 for comparison.
The effect of the firing temperature on the solid solubility of Mn in Sr1xMnxTiO3 system was examined by XRD using samples of x = 0.10 after sintering in air for 5 h at various temperatures. The amount of MnTiO3 second phase was found to slightly increase as temperature decreased from 1500 to 1300 C. Therefore, one could conclude that, a high sintering temperature promotes a higher solid solubility value, but in rather limited range, as was also reported for Ce-doped BaTiO3 [13]. The lattice parameter a calculated from the XRD profiles of single-phase samples, is presented in Fig. 3 as a function of Mn content x. Increasing x leads to a decrease of a, which can be described by a linear Vegard ˚ . A decrease of the law with the slope da/dx 0.027 A lattice parameter with an increase in the Mn content can be expected from ionic size considerations. Mn2+ ionic radius value, extrapolated to the coordination number 12 from the values obtained by Shannon [14] for ˚ , which is smalsmaller coordination numbers, is 1.27 A 2+ ˚ ler than the value of 1.44 A for Sr . The observed variation is an indication of the substitution of Sr by Mn ions. Samples prepared from Mn(NO3)2 precursor do not change the slope of the line. The XRD profiles of SrTi1yMnyO3 ceramics exhibited a cubic crystallographic structure similar to undoped ST without any second phase for all Mn concentration range under study (y 6 0.15). The lattice parameter a decreases linearly with the increase of Mn ˚, content from y = 0 to 0.15 with the slope da/dy 0.1 A as also shown in Fig. 3. Such a behaviour of the lattice parameter might be an indication that Ti4+ with ionic ˚ is being mostly substituted by Mn4+ radius of 0.605 A ˚ , and not by the reduced with an ionic radius of 0.53 A ˚ ) or Mn2+ (0.83 A ˚ ) ions [14]. Mn3+ (0.645 A As can also be seen from Fig. 3, the lattice parameter of Sr1xMnxTiO3 system decreases with increasing Mn content at a lower rate than that of SrTi1yMnyO3, de-
Fig. 2. XRD profiles of sintered SMnT ceramics (* denotes MnTiO3 phase features).
Fig. 3. Lattice parameter of sintered SMnT and STMn ceramics as a function of Mn content.
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spite the fact that the ionic radii ratios Mn2+/Sr2+ and Mn4+/Ti4+ are rather similar. One may then suppose that besides the ionic size and charge, the lattice parameter is also dependent on the electronic structure and electron-orbital configuration of the dopant ions and may account for the higher solid solubility on the B site of the structure. Dense microstructures with an average grain size, varying in the range of 20–35 lm with no clear dependence on Mn content, were observed by SEM for Sr1xMnxTiO3 ceramics with x = 0.025–0.15, as exemplified in Fig. 4(a) and (b) for the samples with x = 0.01 and 0.03. No appreciable difference in the grain size was detected, when comparing these data with undoped ST, shown in Fig. 4(c). SEM micrographs of SrTi1yMnyO3 ceramics (shown for the sample with y = 0.05 in Fig. 4(d)), also reveal dense and homogeneous microstructures. However, a marked decrease of the average grain size to 0.7 ± 0.1 lm was observed for the STMn system. TEM bright field images of Sr1xMnxTiO3 ceramic samples with x = 0.02 and 0.05 are shown in Fig. 5(a) and (b). Their local structure was analyzed by electron diffraction (ED) and chemical composition by EDS.
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The energy dispersive spectra of SMnT and STMn ceramics are shown in Fig. 6(a) and (c). TEM micrographs of SMnT ceramics reveal slim grain boundaries and confirm also the predominance of the over-micron-sized grains, as observed by SEM. ED patterns of such grains reflect the perovskite-type cubic structure, as shown in the top insets in Fig. 5(a) and (b). Moreover, their energy dispersive spectra are close to that of SrTiO3 with an additional small (but detectable) Mn peak as presented in Fig. 6(a). No second phase was observed for Sr0.995Mn0.005TiO3 and Sr0.98Mn0.02TiO3 ceramic samples, apart from the small amorphous inclusions rich in Sr and Zr with trace amounts of Mn located at a few triple points (see Fig. 5(a) and bottom inset). Zr contamination might appear during the milling step. However, under-micronsized grains with a non-perovskite structure were clearly detected in SMnT ceramics with x = 0.03 and 0.05, as shown in Fig. 5(b) and the bottom inset for the Sr0.95Mn0.05TiO3 sample. The EDS spectra of this phase shown in Fig. 6(b) for Sr0.97Mn0.03TiO3 sample reveal almost equivalent peaks of Mn and Ti, suggesting the presence of MnTiO3 phase, which is known to have an ilmenite-type structure. TEM and the complementary
Fig. 4. SEM micrographs of Sr0.99Mn0.01TiO3 (a), Sr0.97Mn0.03TiO3 (b), SrTiO3 (c), and SrTi0.95Mn0.05O3 (d) ceramics.
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Fig. 6. Typical TEM-EDS spectra of grains of SMnT (a) and STMn (c) ceramics and of grain boundary crystalline phase, observed for SMnT ceramics with x P 0.03 (b).
4. Discussion Fig. 5. Bright-field TEM images of Sr0.98Mn0.02TiO3 (a), Sr0.95Mn0.05TiO3 (b), and SrTi0.85Mn0.15O3 (c) ceramics. Insets show ED patterns of SMnT ceramics for crystalline perovskite grains (in top corner) and for grain boundary phases (in bottom corner).
ED and EDS analyses of SMnT ceramics indicate that the formation of the MnTiO3 phase starts from x = 0.03. This value is thought to be the solid solubility limit of Mn in Sr site of ST, which is lower than the one obtained from XRD data and the value reported by Lemanov et al. [11]. Clean grain boundaries and solely perovskite grains of submicron size were observed by TEM in SrTi1yMnyO3 ceramics, whereas Mn incorporation into the lattice was confirmed by the evident peak in energy dispersive spectra of STMn grains, as shown in Figs. 5 and 6(c) for SrTi0.85Mn0.15O3 sample. The high solid solubility of Mn in Ti sites of ST was confirmed by TEM studies.
SrTiO3 is known to be a highly-packed perovskite structure of general formula ABO3 with a tolerance factor t = t1/t2 1. Fractional tolerance factors t1 and t2 describe the degree of packing of A or B ions in the perovskite lattice and are given by the following equation [15]: pffiffiffi 2ðrB þ rO Þ 2ðrA þ rO Þ and t2 ¼ ; ð1Þ t1 ¼ a a where rA and rB stand for the average ionic radii at the A and B sites, respectively, rO is the ionic radius of the oxygen, and a stands for the lattice parameter. The ionic radius of Mn2+ estimated for coordination number Nc = 12 (characteristic for the perovskite A site) 2þ 2þ 2þ ˚ ˚ is r2þ Mn ¼ 1:27 A, while rSr ¼ 1:44 A, i.e., rMn =r Sr ¼ 0:88 4þ 4þ ˚ rTi ¼ 0:605 A ˚ [14]. On the other hand, rMn ¼ 0:53 A, 4þ and r4þ =r ¼ 0:88 at N = 6, characteristic for the B c Mn Ti site. Hence, considering the charge and ionic radius, Mn ions can occupy both the A and B sites of the ST
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perovskite lattice. The results obtained in this work confirmed this prediction showing that the substitution by Mn occurred at both the A and B sites of the ST lattice and that the solid solubility limit of Mn on the ST lattice depends on the site of occupancy. In spite of the similar2þ 4þ 4þ ity of r2þ Mn =rSr and r Mn =rTi ratios, the solid solubility limit of Mn at the A site of ST was found to be lower than at the B site of structure. This result confirms earlier reports on EPR of Mn-doped ST crystals that indicated the Ti substitution by Mn [5,6]. In addition, the SMnT cell parameter was found to decrease almost four times slower than that of STMn with the increase of Mn content. Therefore, such sharp compression of the STMn cell, probably does not allow the Mn4+ ions to occupy the off-centre position at the Ti site of the SrTi1yMnyO3 system, whereas in the Sr1xMnxTiO3 system, Mn2+ ions are believed to flip between the off-centre equilibrium positions at the Sr sites, accounting for a low-temperature dielectric relaxation. Along with different structural changes induced by Mn doping at Sr and Ti sites (observed by XRD technique), the differences of the lattice site occupancy did also have a pronounced effect in the microstructure. The grain growth observed by SEM is different for the SMnT and STMn systems. As a result, one order of magnitude smaller average grain size was obtained for Ti site Mn-doped ST ceramics, similar to that for Ti site Mg-doped ST [16], while no strong difference in the average grain size was observed between undoped and Sr site Mn-doped ST. However, it should be remarked that this difference in the grain growth behaviour might be related to the variations of the Sr/Ti ratio [17]. This subject is currently being studied. Finally, TEM/EDS technique presented the main and unambiguous evidence of the difference in the lattice site occupancy between these two systems. In STMn ceramic samples, Mn incorporation into ST grains was evidenced by the absence of Sr-containing second phase and by the energy dispersive spectra of STMn ceramic grains in which a Mn peak was clearly observed. This confirms that Mn occupies the Ti site, but not the Sr site of ST [12]. For SMnT ceramics, as shown in Figs. 5 and 6(a), TEM equipped with the EDS analysis confirmed the incorporation of Mn into ST grains and the absence of any second crystalline phases for x 6 0.02. In particular, titanium oxide extra-phase, expected if Mn ions occupied B sites of the perovskite lattice, was not detected, supporting the assumption that Mn occupies Sr sites. Only samples with x > 0.02 revealed the presence of a second crystalline phase identified as MnTiO3 (see Figs. 5 and 6(b)). MnTiO3 is known to have an ilmenite-type structure [18], being stable at room pressure and temperature, although a transformation of MnTiO3 to a perovskite structure under certain conditions was also reported
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[19]. From the dielectric point of view, MnTiO3 behaves as an ordinary dielectric: its frequency-independent dielectric constant e 0 22, decreases with temperature with the slope of de/dT = 1.1 · 103 K1 and the dissipation factor tan d is about 0.007. Thus the MnTiO3 second phase can account for the decrease of the overall dielectric permittivity and the loss for SMnT system. So, the structural analysis supports quite well the results of the dielectric response of SMnT and STMn presented in this work and previously reported by the authors [4,12]. The dielectric behaviour observed for SMnT system is similar to that of relaxors and was attributed to the formation of electric dipoles and corresponding random fields due to the off-centre position of Mn2+ ions at Sr sites of highly polarisable ST lattice [4]. Thus, the decrease of e 0 with dc bias field can be explained by the stabilization of the local potential wells for the off-centre Mn2+ dipoles under applying external electric fields. Moreover, the dielectric loss vanishing with the field can also be explained by the alignment and stabilization of off-centre Mn2+ ions by applied field, reducing the contribution of domain wall dynamics to the losses. On the other hand, the sharp compression of the lattice cell, observed by XRD for the STMn system with increasing Mn content does not allow the Mn4+ ions to occupy the off-centre position at Ti sites. In addition, less polarisable Mn4+ ions substituting more polarisable Ti4+ ions, may also break Ti-Ti long-range interaction. This results in a strong reduction of the dielectric response without any dielectric anomaly in SrTi1yMnyO3 ceramics. Moreover, the ferroelectric instability was reported to be suppressed [12]. The main parameter for tunable electronic components is related to the dependence of the dielectric constant on applied electric field. Relative tunability can be calculated as follows: nr ðEÞ ¼ ½e0 ð0Þ e0 ðEÞ=e0 ð0Þ; 0
ð2Þ
where e (0) stands for the dielectric permittivity at zero field and e 0 (E) stands for the dielectric permittivity under applied bias field E. The dependence of the relative tunability nr for both Sr1xMnxTiO3 (x = 0.03) and SrTi1yMnyO3 (y = 0.05) ceramics on the bias field at fixed temperatures is represented in Fig. 7(a) and (b). Comparing Fig. 7 with Fig. 1, the rule ‘‘the higher the e 0 , the higher its tunability’’ is confirmed. Thus, the maximum nr value of 66.2% was obtained under the bias field of 20 kV/cm at 60 K for SMnT ceramics with Mn content x = 0.03. This is more than 20% higher than that of undoped ST ceramics. On the other hand, the maximum relative tunability of SrTi0.95Mn0.05O3 sample, obtained at 10 K under 20.5 kV/cm, is lower than 5%. These data are in close agreement with the suppression of the ferroelectric instability by Mn doping at Ti sites.
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A. Tkach et al. / Acta Materialia 53 (2005) 5061–5069 Table 1 Maximum relative tunability nmax and quality factor of a tunable component K, obtained at bias field Emax = 20 kV/cm for Sr0.97Mn0.03TiO3 and SrTiO3 ceramics T (K)
10 15 25 35 50 60 75 100 150 294
Maximum relative tunability nmax (%)
Quality factor of a tunable component K
SrTiO3
Sr0.97Mn0.03TiO3
SrTiO3
Sr0.97Mn0.03TiO3
76.8 75.2 70.1 – 46.9 – 21.1 7.1 2.1 0.2
22.0 27.8 – 50.9 65.0 66.2 58.0 35.9 8.7 0.4
77464.3 93392.0 70384.2 – 24222.1 – 678.3 221.7 27.0 0.1
1856.4 1222.1 – 120.5 353.3 994.6 3073.1 5899.4 102.1 0.2
range of 75–125 K. Due to this fact, SMnT ceramics can be used as a tunable component at liquid nitrogen temperature, where high temperature superconductors can be applicable as electrodes. Moreover, the figure of merit, Fm, of a phase shifter defined as the ratio of the phase shift, D/degree, in degrees to the insertion loss, LdB, in dB (Eq. (3)) is closely related to the QFTC of the ferroelectric component used in the phase shifter [3]: pffiffiffiffi Fm ¼ D/degree =LdB ¼ 6:6 K . ð4Þ Fig. 7. Variation of relative tunability nr at 10 kHz, calculated as nr (E) = [e 0 (0) e 0 (E)]/e 0 (0), with bias field for Sr0.97Mn0.03TiO3 (a) and SrTi0.95Mn0.05O3 (b) ceramics at different temperatures.
The low dielectric loss in the temperature range of the high tunability is an additional important factor for device applications. To characterize the capability of the ferroelectric component for practical applications in microwave engineering, the quality factor of a tunable component (QFTC) has to be known. QFTC, as a general parameter of a tunable component, combines both tunability and loss factor and is given by the following equation: K ¼ ðn 1Þ2 =½n tan dð0Þ tan dðEmax Þ; 0
0
ð3Þ
where n = e (0)/e (Emax) stands for the tunability of the component [3], and tan d (0) and tan d (Emax) are the loss factor under zero and maximum bias field, respectively. QFTC values were calculated for the SMnT sample with x = 0.03, as shown in Table 1 together with the maximum relative tunability nmax, and were compared with the corresponding parameters of undoped ST. The results obtained reveal that QFTC is one and a half order of magnitude higher for undoped ST than for Mn-doped ST at temperatures below 75 K, but the opposite situation is observed for the temperature
Following this equation, one can obtain a phase shift higher than 360 per 1 dB, if the ferroelectric component is characterized by K > 3000 [3]. The values obtained for Sr0.98Mn0.03TiO3 around 100 K satisfy this condition well, as expected from data shown in Fig. 1(a), where at 100 K the dielectric constant is still well tunable, and tan d is already low and rather insensitive to the bias field. In addition, the use of high temperature superconductors as device electrode material becomes possible in the vicinity of 90 K, supported also by the chemical and structural compatibility with ST-based materials [20]. Thus, low dielectric loss, high dielectric constant, and its dc field dependence combined with the low loss in high temperature superconductors such as YBa2Cu3O7 (YBCO) make the SMnT system promising for applications in high-quality-factor electrically tunable components for advanced microwave communications systems. On the other hand, a high thermal stability can be obtained due to high diffusivity of the dielectric constant peak of SMnT ceramics.
5. Conclusions Ceramic samples of Sr1xMnxTiO3 (0 6 x 6 0.15 and x = 1) and SrTi1yMnyO3 (0 6 y 6 0.15) systems were synthesized by conventional mixed oxide method, in which intentional stoichiometric variations allowed the
A. Tkach et al. / Acta Materialia 53 (2005) 5061–5069
incorporation of Mn ions at Sr and Ti sites of ST lattice, respectively. From the structural and microstructural point of view the following differences were observed: (1) The solid solubility limit for Mn at Sr site of ST was determined to be about 2%. Small grains of MnTiO3 were found by TEM/EDS to appear in the grain boundaries of Sr1xMnxTiO3 ceramics for higher Mn concentrations. On the other hand no second phases and a strong decrease of the lattice parameter with increasing Mn content were observed for SrTi1yMnyO3 ceramics. This fact points to the incorporation of Mn into the Ti sites of ST for concentrations as high as 15%. Consequently the solid solubility limit for Mn in B sites of ST is higher than for Mn in A sites of ST structure. (2) The grain size was found to differ markedly between Sr1xMnxTiO3 (variation from 20 to 35 lm) and SrTi1yMnyO3 ceramics (much finer grains from 0.6 to 0.8 lm were observed), without clear dependence on Mn content. From the dielectric point of view the following differences were observed: (1) A pronounced temperature- and frequency-dependent relaxation of the relaxor type for the SMnT system, as compared to a sharp reduction of the dielectric response without any dielectric anomaly observed for the STMn system. Although incorporation of Mn4+ into the Ti sites of the ST lattice (decreasing the fractional tolerance factor t2) indicates smaller B site packing degree and supposedly more favourable off-centre positions of these ions, the quantum fluctuations became more stable and the system was driven away from ferroelectricity. This is in line with the observed compression of the unit cell of STMn system with increasing Mn content that might not allow the Mn4+ ions to occupy the off-centre position at Ti sites. Also the substitution of more polarisable Ti4+ ions by less polarisable Mn4+ ions may break Ti–Ti longrange interaction. (2) As a consequence of the different dielectric response and due to the formation of polar dipoles, related to the introduction of Mn2+ into the Sr sites, the temperature range, where the dielectric constant is tunable, was enlarged up to 150 K, compared with undoped ST (80 K). In addition, a higher tunability (70%) and quality
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factor of a tunable component (6000) in the vicinity of liquid nitrogen temperature were obtained for SMnT ceramics. Hence Sr1xMnxTiO3 is a promising material for possible application as a phase shifter. In conclusion this work clarified that: (i) the dielectric response of ST doped with Mn depends on the dopant site occupancy; and (ii) the dielectric relaxation of the relaxor-type, observed in Mn-doped strontium titanate ceramics, was related to Sr site occupancy. Further detailed studies on the structure of SMnT ceramics as well as on the nature of the dielectric relaxation are ongoing.
Acknowledgement Alexander Tkach acknowledges FCT (Portuguese Foundation for Science and Technology) for financial support.
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