Preparation and characterization of Bi4–xPrxTi3O12 solid solutions 210 Klyndyuk A.I., Chizhova E.A., Poznyak A.I. Chimica Techno Acta. 2017. Vol. 4, No. 4. P. 210–217. ISSN 2409–5613 D O I: 1 0. 15 82 6/ ch im te ch /2 01 7. 4. 4. 01 A.I. Klyndyuk, E.A. Chizhova, A.I. Poznyak Belarus State Technological University 13a Sverdlova St., Minsk, 220006, Republic of Belarus Preparation and characterization of Bi 4–x Pr x Ti 3 O 12 solid solutions The Bi 4–x Pr x Ti 3 O 12 (BPT) solid solutions (x = 0.05, 0.10, 0.15) with small praseodymium content were prepared by solid-state method. Thermal, electric, and dielectric properties of BPT were studied. It was revealed that BPT titanates crystalize in аn orthorhombic structure and exhibit p-type semiconductivity. Dielectric constant of BPT increased, Curie temperature (T C ), electrical conduc- tivity and dielectric losses decreased, but lattice parameters and thermo-EMF coefficient remained practically unchanged with the increase of praseodymium content in layered Bi 4–x Pr x Ti 3 O 12 . It was determined that activation energy of direct current (DC) electrical conductivity and linear thermal expansion coef- ficient (LTEC) of BPT changes at ferroelectric (FE) → paraelectric (PE) phase transition. The activation energy and LTEC changed below and above T C from 1.08–1.56 eV to 0.45–0.86 eV and from (9.10–10.80)·10–6 K–1 to (13.12– 14.61)·10–6 K–1, respectively. The AC electrical conductivity studies of BPT il- lustrated short-range order with ionic translations assisted by small-polaron hopping. Keywords: layered bismuth titanates; dielectric constant; dielectric losses; electrical conduc- tivity; thermo-EMF; thermal expansion. Received: 20.10.2017; accepted: 15.11.2017; published: 25.12.2017. © Klyndyuk A.I., Chizhova E.A., Poznyak A.I., 2017 Introduction Bi4Ti3O12 belongs to the Aurivil- lius phase family Bi2An–1BnO3n+3, structure of which consists of alternated fluorite- like [Bi2O2] 2+ layeres and perovskite-like [An–1BnO3n+1] 2– blocks, where n – number of octahedral layers in the perovskite-like block [1]. This triple-layered (n  = 3) Au- rivillius phase is ferroelectric with high Curie temperature (TC  = 948 К  [2]). The possibility to preserve the ferroelectric properties within a wide temperature range lets us to consider layered bismuth titanate as a promising material for ra- dio-, acusto-, and optoelectronics; and thus can be used for production of optical displays, piezoelectric transducers, filters, capacitors, and different types of memory devices. So, for example, as a material for non-volatile memory devices the lantha- num-substituted Bi3.25La0.75Ti3O12 bismuth titanate is proposed, functional charac- teristics of which are better than of tradi- tional ferroelectrics, such as PbTi1–xZrxO3 or SrBi2Ta2O9 [3]. Crystal structure, physico-chemical, and functional properties of the solid so- lutions Bi4–xLnxTi3O12 (Ln  = rare-earth element) were studied in a number of pa- 211 pers [1, 3–8]. In [4] it was found that par- tial substitution of Bi by La in Bi4Ti3O12 leads to decrease of orthorhombic distor- tion degree of Bi4–xLaxTi3O12 solid solu- tions at x  ≤ 1.0, and at x > 1.0 they had tetragonal structure and were paraelec- trics. According to the [1, 4] results, at x ≤ 0.75 La3+ ions substitute Bi3+ in per- ovskite-like [Bi2Ti3O10] 2– blocks, and at x > 0.75 they can substitute Bi3+ ions in fluorite-like [Bi2O2] 2+ layers too, so for- mula of Bi4–xLaxTi3O12 solid solutions at x ≤ 0.75 and x > 0.75 should be written as [Bi2O2][Bi2–xLaxTi3O10] and [Bi2–yLayO2] [Bi2–x+yLax–yTi3O10], respectively. Authors of [5] established that partial substitution of Bi with La or Ce in Bi4Ti3O12 leads to the decrease of Curie temperature of cera- mics (TC  values for Bi3.5La0.5Ti3O12 and Bi3.5Ce0.5Ti3O12 samples were 798 and 813 K, respectively, both being lower than TC for Bi4Ti3O12 phase). At the same time, La2O3 addition to the layered bismuth ti- tanate improved its dielectric properties; by authors’ opinion, it was caused by de- creasing of oxygen vacancy concentration in ceramics. Partial substitution of Bi with Nd in Bi4Ti3O12 leads to the decrease of Curie temperature, dielectric losses, and electrical conductivity of Bi4–xNdxTi3O12 ceramic; and impoves its polarization properties because of reduction of bis- muth and oxygen vacancy concentra- tions [7]. In [8] the Bi4–yTbyTi3O12 phase formation in the powder mixtures of Bi2O3, Tb4O7, and TiO2 was investigated. It was found that partial replacing of Bi by Tb results in shrinking of unit cell of Bi4–yTbyTi3O12 solid solutions, decrease of orthorhombic distortion degree of their crystal lattice, and, as a consequence, leads to the decrease of temperature of FE → PE (ferroelectric → paraelectric) phase transition (up to ≈28 K for y = 0.4). In this work the results of investigation of crystal structure, thermal, electric, and dielectric properties of ceramic samples of Bi4–xPrxTi3O12 (BPT) solid solutions with small praseodymium oxide content (x ≤ 3.75 mol.%) are presented. Experimental Bi4–xPrxTi3O12 (x  = 0.00, 0.05, 0.10, 0.15) ceramic samples were prepared by solid-state reactions method from mix- tures of Bi2O3 (99.0%), Pr6O11 (99.0%), and TiO2 (99.5%) powders taken in ap- propriate stoichiometric ratios. Precur- sor mixtures were calcuned in air within temperature interval of 923–1223  K for 17 hours with some intermediate regrind- ings, according to procedure described in details in [6]. Identification of the samples was per- formed using X-ray diffraction analysis (XRD) (Bruker D8 XRD Advance dif- fractometer, Cu Kα radiation) and IR absorption spectroscopy (IR  Fourier- spectrometer Nexus of ThermoNicolet). Relative density (ρrel) of the samples was calculated as ρrel = (ρapp/ρXRD) · 100%, (1) where ρapp – apparent density, determined from the mass and dimensions of the samples; ρXRD – X-ray density. Thermal expansion of the samples was studied using DIL 402 PC (Netzsch) dilatometer within 290–1130 K with heat- ing-cooling rate of 1–5 K/min. Dielectric measurements were carried out in the tem- perature range 300–1090 K for frequencies between 100 Hz and 1 MHz using immit- tance meter E7–25. DC electrical conduc- titivy and thermo-EMF of sintered ceram- ics were studied within the temperature ranges of 470–1090  K and 780–1090  K, 212 respectively, according to the procedure described elsewhere [9]. Values of linear thermal expansion coefficient (LTEC, α) and activation energy of DC electrical con- ductivity (ЕA) of the samples were deter- mined from linear parts of Δl/l0 = f(T), and lgσDC = f(1/T) dependences, respectively. All measurements were performed in air. Results and discussion All Bi4–xPrxTi3O12 samples after fi- nal stage of annealing were found single phase within XRD reliability (Fig. 1), and crystallized in orthorhombic structure like parent compound Bi4Ti3O12 (space group B2cb) [10]. Lattice constants of Bi- 4TiO12 (a = 5.449(9) Å, b = 5.422(9) Å, and c = 32.85(4) Å) were in a close agreement with an earlier studies: 5.444(1), 5.413(1), and 32.858(1) Å [10], 5.4403, 5.4175, and 32.7862 Å [11], and 5.4438(1), 5.4105(1), and 32.8226(5) Å [8]. Lattice constants of Bi4–xPrxTi3O12 solid solutions were close to the Bi4Ti3O12 ones (for exam- ple, for Bi3.90Pr0.10Ti3O12 a = 5.449(7)  Å, b = 5.420(9) Å, and c = 32.80(3) Å), which agrees with the fact that sizes of substituting and substituted ions are close to each other (for C.N. = 6 according to [12] Bi3+ and Pr3+ ionic radii are 1.11 and 1.013 Å, respectively). It should be noted that 0014 reflec- tion in the Bi4–xPrxTi3O12 diffractograms was the most intensive, in contrast with 117 peak for Bi4Ti3O12. Other 00l peaks had higher intensity as well (I006/I117 ratio was equal to 0.6, 1.7, 3.0, and 2.8 for x = 0.00, 0.05, 0.10, and 0.15, respectively). This fact shows that partial substitution of Bi with Pr in Bi4Ti3O12 leads to the tex- turing of the samples. The nature of this phenomenon is not clear yet and will be studied in the future. Three absorption bands occurring at 810–818 cm–1 (ν1), 573–582 cm –1 (ν2), and 474 cm–1 (ν3) were observed in the absorption spectra of Bi4–xPrxTi3O12 pow- ders. According to [7, 11], these bands correspond to the stretching (ν1 and ν2) and bending (ν3) vibrations of Bi–O (ν1 and ν3) and Ti–O (ν2) bonds, respectively. The peak positions did not change with x increasing, so partial replacing of Bi by Pr in layered Bi4Ti3O12 did not affect practi- cally the metal-oxygen interactions in its crystal structure. The relative density values for Bi4–xPrx- Ti3O12 ceramics varied within 77–80% and increased with x, being essentially larger than for unsubstituted bismuth titanate (60%). These results show that addition of praseodymium oxide to the layered bismuth titanate improves its sin- terability. Note that according to the lit- erature data [6, 7] addition of lanthanum or neodymium oxides to the Bi4Ti3O12, on the contrary, had lowered its sinterability. On the temperature dependences of relative elongation an inflection point near 940–970  K was observed (Fig. 2). It is related to the FE → PE phase transition [8] and is accompanied by the increase of LTEC values of the samples (Table 1). Fig. 1. X-ray powder diffractograms (Cu Kα radiation) of Bi4–xPrxTi3O12 solid solutions 213 An inflection point, which was deter- mined as an intersection of linear parts of Δl/l0 = f(T) dependences at low (FE re- gion) and high temperatures (PE region), corresponds to the Curie temperature and decreases with x (Fig. 2, inset). It is in a good agreement with the literature data, according to which substitution of Bi with Ln in Bi4Ti3O12 leads to lowering of its Cu- rie temperature [4–8]. The LTEC values of Bi4–xPrxTi3O12 ti- tanates in FE state decreased, but in PE state increased with x (Table 1). The LTEC values in PE state can be explaned by an- harmonicity of metal-oxygen vibrations in disordered cationic sublattice of Pr3+-sub- stituted bismuth titanate Bi4–xPrxTi3O12. The LTEC values in FE state could be caused either by increase of dipole-dipole interactions or by decrease of oxygen and bismuth vacancy concentrations in the BPT. The first explanation is in con- trast with the fact that TC of Bi4–xPrxTi3O12 solid solutions decreases with x. So, the decrease of LTEC values of BPT ceramics in FE region is due to the decrease of the vacancy concentration in it [3]. Bi4–xPrxTi3O12 compounds are p-type semiconductors (Fig. 3), which confirms previous data [7, 14]. According to [7, 14] electrical conductivity of layered bis- muth titanate increases with tempera- ture [7,14] and thermo-EMF coefficient of Bi4Ti3O12 phase at high temperatures is positive [14]. Seebeck coefficient va- lues of BPT ceramics were close to each other (Fig. 3b), which corresponds to the isovalent character of substitution of Bi with Pr. But DC electrical conductivity of the samples decreased with x (Fig. 3a) due to the defect concentration decrease as was mentioned above. Near TC there is a change in the slope of linear sections at the Arrhenius plots lgσDC = f(1/T). Va- lues of activation energy of the samples’ DC electrical conductivity in PE region are essentially less than in FE one (Ta- ble  1). Similar results were obtained in [15] for Bi4Ti2Nb0.5Fe0.5O12 ceramics, acti- vation energy values of which were equal to 1.21  eV and 0.50 eV below and above TC, respectively (AC, ω = 10 5  Hz). Par- tial substitution of Bi with Pr in Bi4Ti3O12 increases EA of BPT in FE state and low- Table 1 Values of apparent activation energy of DC electrical conductivity (EA) and linear thermal expansion coefficient (α) of Bi4–xPrxTi3O12 titanates x EA, eV α·106, K–1 FE PE FE PE 0.00 1.08±0.02 0.86±0.02 10.80±0.06 13.12±0.02 0.05 1.24±0.01 0.46±0.02 09.62±0.01 14.61±0.02 0.10 1.23±0.02 0.45±0.01 09.31±0.01 13.53±0.01 0.15 1.56±0.04 0.49±0.01 09.10±0.01 13.48±0.01 Fig. 2. Temperature dependences of relative elongation of Bi4–xPrxTi3O12 sintered ceramics. Inset shows concentration dependences of TC 214 ers it in PE state (Table 1). Note that EA value of layered bismuth titanate below TC obtained in this work coincides with the data given in [14]: 1.0 eV for Bi4Ti3O12 ce- ramics. In the temperature dependences of di- electric constant of Bi4–xPrxTi3O12 titanates abrupt maxima near 930–940 K was ob- served (Fig. 4a). It was caused by FE → PE phase transition, and phase transition temperature (TC) lowered with increas- ing praseodymium content in the samples (Fig. 4d) and was close to the TC values determined from the Dl/l0 = f(T) depen- dences (Fig. 2, inset). Dielectric constant values of BPT ceramics increased with x, which was more prominent at high tem- peratures (Fig. 4a, c). Diеlectric losses of investigated samples increased with tem- perature and decreased when Pr concen- tration (Fig. 4b, e). Besides, on the tgd = f(T) dependences two anomalous regions were observed: near 760–820 K and 930– 940 K. The second anomaly is related to the FE → PE phase transition, but the first one is probably due to the oxygen vacancy movement out (migration) of the domain walls [16]. The values of Curie temperature of the samples are frequency independ- ent (Fig.  5a, b), which indicates that Bi4–xPrxTi3O12 phases are normal ferro- electrics [15]. When the testing frequency increased from 100 Hz to 100 kHz, the dielectric constant and dielectric losses of BPT ceramics decreased substantially due to the suppression of relaxing polarization at high frequencies. The dielectric constant of normal fer- roelectrics follows the Curie–Weiss law ε = С/(T – TΘ), (2) where C is Curie–Weiss constant and TΘ is Curie–Weiss temperature. The Curie– Fig. 3. Dependences of DC electrical conductivity (a) and thermo-EMF coefficient (b) of Bi4–xPrxTi3O12 samples vs temperature Fig. 4. Temperature (a, b) and concentration (c–e) dependences of dielectric constant (a, c), dielectric losses (b, e), and TC (d) of Bi4–xPrxTi3O12 ceramics (ω = 1 kHz) 215 Weiss plot for Bi3.85Pr0.15Ti3O12 phase at 100 kHz is shown in the Fig. 5c. The pa- rameters obtained from the linear fit are C = 1.63×105 K and TΘ = 821 K. The mag- nitude of Curie–Weiss constant is of the same order as of well-known displasive- type ferroelectrics, such as BaTiO3 (C  = 1.7·105 K [15]). The frequency dependences of AC electrical conductivity of Bi3.85Pr0.15Ti3O12 at various temperatures are given in Fig. 6. The frequency independent plateau at low frequencies is attributed to the long-range translational motion of ions contributing to DC conductivity (σDC) [17, 18]. At high frequencies (>104 Hz) the AC electrical conductivity shows ωn dependence which corresponds to the short-range transla- tion ion hopping [15, 18]. The frequency dependent AC electri- cal conductivity of BPT ceramics obeys Jonscher’s power law [19] at all tempera- tures σ(ω) = σ(0) + Aωn, (3) where σ(ω) is the total conductivity, σ(0) is the DC conductivity, A is the temper- ature-dependent constant which deter- mines the strength of polarizability, and n represents the degree of interaction between the mobile ions and the lattice around them [15, 19]. The values of n are less than one, which indicates that motion of charge carriers is translational [18, 20]. The shape of n vs. T dependence suggests hopping mechanisms of charge carriers [18, 19]. In case of small-polaron hop- ping, n increases with temperature, while for a large polaron hopping, n decreases with temperature. As shown in inset of Fig. 6, the values of n are less than 1 and are found to increase with temperature; Fig. 5. Temperature dependences of dielectric constant (a) and dielectric losses (b) of Bi3.85Pr0.15Ti3O12 at different frequencies. Inset (c) shows the inverse dielectric constant as a function of temperature at 100 kHz Fig. 6. Frequency dependences of AC electrical conductivity of Bi3.85Pr0.15Ti3O12 at different temperatures. Inset shows the variation of Jonscher’s power law parameters (n, A) as a function of temperature 216 hence we conclude that AC electrical con- ductivity arises mainly due to the short- range order translation hopping assisted by small-polaron hopping mechanism. Conclusions The Bi4–xPrxTi3O12 solid solutions (x = 0.05, 0.10, 0.15) with small substitution degree were synthesized and their ther- mal expansion, DC and AC electrical conductivity, dielectric constant and die- lectric losses were measured. The samples crystallized in orthorhombic structure and possessed p-type semiconductive and normal ferroelectric properties. Lattice constants and thermo-EMF coefficient of BPT were practically composition inde- pendent, but Curie temperature, electri- cal conductivity and dielectric losses de- creased with x. Activation energy of DC electrical conductivity and linear thermal expansion coefficient of Bi4–xPrxTi3O12 changed at the temperature of ferroelec- tric to paraelectric phase transition, and their values were 1.08–1.56 eV and 0.45– 0.86 eV, and (9.10–10.80)·10–6 K–1 and (13.12–14.61)·10–6 K–1 below and above Curie temperature, respectively. 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