Preparation and characterization of Bi4–xPrxTi3O12 solid solutions

Preparation and characterization of Bi4–xPrxTi3O12 solid solutions The Bi4–xPrxTi3O12 (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 (TC), electrical conductivity and dielectric losses decreased, but lattice parameters and thermo-EMF coefficient remained practically unchanged with the increase of praseodymium content in layered Bi4–xPrxTi3O12. It was determined that activation energy of direct current (DC) electrical conductivity and linear thermal expansion coefficient (LTEC) of BPT changes at ferroelectric (FE) → paraelectric (PE) phase transition. The activation energy and LTEC changed below and above TC 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 illustrated short-range order with ionic translations assisted by small-polaron hopping.


Introduction
Bi 4 Ti 3 O 12 belongs to the Aurivillius phase family Bi 2 A n-1 B n O 3n+3 , structure of which consists of alternated fluoritelike [Bi 2 O 2 ] 2+ layeres and perovskite-like [A n-1 B n O 3n+1 ] 2-blocks, where n -number of octahedral layers in the perovskite-like block [1].This triple-layered (n = 3) Aurivillius phase is ferroelectric with high Curie temperature (T C = 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 radio-, 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 lanthanum-substituted Bi 3.25 La 0.75 Ti 3 O 12 bismuth titanate is proposed, functional characteristics of which are better than of traditional ferroelectrics, such as PbTi 1-x Zr x O 3 or SrBi 2 Ta 2 O 9 [3].
Crystal structure, physico-chemical, and functional properties of the solid solutions Bi 4-x Ln x Ti 3 O 12 (Ln = rare-earth element) were studied in a number of pa-pers [1,[3][4][5][6][7][8].In [4] it was found that partial substitution of Bi by La in Bi 4 Ti 3 O 12 leads to decrease of orthorhombic distortion degree of Bi 4-x La x Ti 3 O 12 solid solutions at x ≤ 1.0, and at x > 1.0 they had tetragonal structure and were paraelectrics.According to the [1,4]   , respectively.Authors of [5] established that partial substitution of Bi with La or Ce in Bi 4 Ti 3 O 12 leads to the decrease of Curie temperature of ceramics (T C values for Bi 3.5 La 0.5 Ti 3 O 12 and Bi 3.5 Ce 0.5 Ti 3 O 12 samples were 798 and 813 K, respectively, both being lower than T C for Bi 4 Ti 3 O 12 phase).At the same time, La 2 O 3 addition to the layered bismuth titanate 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 Bi 4 Ti 3 O 12 leads to the decrease of Curie temperature, dielectric losses, and electrical conductivity of Bi 4-x Nd x Ti 3 O 12 ceramic; and impoves its polarization properties because of reduction of bismuth and oxygen vacancy concentrations [7].In [8] the Bi 4-y Tb y Ti 3 O 12 phase formation in the powder mixtures of Bi 2 O 3 , Tb 4 O 7 , and TiO 2 was investigated.It was found that partial replacing of Bi by Tb results in shrinking of unit cell of Bi 4-y Tb y Ti 3 O 12 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 Bi 4-x Pr x Ti 3 O 12 (BPT) solid solutions with small praseodymium oxide content (x ≤ 3.75 mol.%) are presented.
Identification of the samples was performed using X-ray diffraction analysis (XRD) (Bruker D8 XRD Advance diffractometer, Cu Kα radiation) and IR absorption spectroscopy (IR Fourierspectrometer Nexus of ThermoNicolet).
Thermal expansion of the samples was studied using DIL 402 PC (Netzsch) dilatometer within 290-1130 K with heating-cooling rate of 1-5 K/min.Dielectric measurements were carried out in the temperature range 300-1090 K for frequencies between 100 Hz and 1 MHz using immittance meter E7-25.DC electrical conductitivy and thermo-EMF of sintered ceramics were studied within the temperature ranges of 470-1090 K and 780-1090 K, respectively, according to the procedure described elsewhere [9].Values of linear thermal expansion coefficient (LTEC, α) and activation energy of DC electrical con-ductivity (Е A ) of the samples were determined from linear parts of Δl/l 0 = f(T), and lgσ DC = f(1/T) dependences, respectively.All measurements were performed in air.

Results and discussion
All Bi 4-x Pr x Ti 3 O 12 samples after final stage of annealing were found single phase within XRD reliability (Fig. 1), and crystallized in orthorhombic structure like parent compound Bi 4 Ti 3 O 12 (space group B2cb) [10].Lattice constants of Bi- It should be noted that 0014 reflection in the Bi 4-x Pr x Ti 3 O 12 diffractograms was the most intensive, in contrast with 117 peak for Bi 4 Ti 3 O 12 .Other 00l peaks had higher intensity as well (I 006 /I 117 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 Bi 4 Ti 3 O 12 leads to the texturing of the samples.The nature of this phenomenon is not clear yet and will be studied in the future.
The relative density values for Bi 4-x Pr x- Ti 3 O 12 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 sinterability.Note that according to the literature data [6,7] addition of lanthanum or neodymium oxides to the Bi 4 Ti 3 O 12 , 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).An inflection point, which was determined as an intersection of linear parts of Δl/l 0 = f(T) dependences at low (FE region) and high temperatures (PE region), corresponds to the Curie temperature and decreases with x (Fig. 2 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 contrast with the fact that T C of Bi 4-x Pr x Ti 3 O 12 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]. Bi 4-x Pr x Ti 3 O 12 compounds are p-type semiconductors (Fig. 3), which confirms previous data [7,14].According to [7,14] electrical conductivity of layered bismuth titanate increases with temperature [7,14] and thermo-EMF coefficient of Bi 4 Ti 3 O 12 phase at high temperatures is positive [14].Seebeck coefficient values 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 T C there is a change in the slope of linear sections at the Arrhenius plots lgσ DC = f(1/T).Values of activation energy of the samples' DC electrical conductivity in PE region are essentially less than in FE one (Table 1).Similar results were obtained in [15] for Bi 4 Ti 2 Nb 0.5 Fe 0.5 O 12 ceramics, activation energy values of which were equal to 1.21 eV and 0.50 eV below and above T C , respectively (AC, ω = 10 5 Hz).Partial substitution of Bi with Pr in Bi 4 Ti 3 O 12 increases E A of BPT in FE state and low-Table 1 Values of apparent activation energy of DC electrical conductivity (E A ) and linear thermal expansion coefficient (α) of Bi  ers it in PE state (Table 1).Note that E A value of layered bismuth titanate below T C obtained in this work coincides with the data given in [14]: 1.0 eV for Bi 4 Ti 3 O 12 ceramics.
In the temperature dependences of dielectric constant of Bi 4-x Pr x Ti 3 O 12 titanates abrupt maxima near 930-940 K was observed (Fig. 4a).It was caused by FE → PE phase transition, and phase transition temperature (T C ) lowered with increasing praseodymium content in the samples (Fig. 4d) and was close to the T C values determined from the Dl/l 0 = f(T) depen-dences (Fig. 2, inset).Dielectric constant values of BPT ceramics increased with x, which was more prominent at high temperatures (Fig. 4a, c).Diеlectric losses of investigated samples increased with temperature and decreased when Pr concentration (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 independent (Fig. 5a, b), which indicates that Bi 4-x Pr x Ti 3 O 12 phases are normal ferroelectrics [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 ferroelectrics follows the Curie-Weiss law ε = С/(T -T Θ ), (2) where C is Curie-Weiss constant and T Θ is Curie-Weiss temperature.The Curie- hence we conclude that AC electrical conductivity arises mainly due to the short-range order translation hopping assisted by small-polaron hopping mechanism.

Conclusions
The Bi 4-x Pr x Ti 3 O 12 solid solutions (x = 0.05, 0.10, 0.15) with small substitution degree were synthesized and their thermal expansion, DC and AC electrical conductivity, dielectric constant and dielectric 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 independent, but Curie temperature, electrical conductivity and dielectric losses de-creased with x.Activation energy of DC electrical conductivity and linear thermal expansion coefficient of Bi 4-x Pr x Ti 3 O 12 changed at the temperature of ferroelectric to paraelectric phase transition, and their values were 1.08-1.56eV and 0.45-0.86eV, and (9.10-10.80)•10 - K -1 and (13.12-14.61)•10 - K -1 below and above Curie temperature, respectively.AC electrical conductivity investigations illustrate short-range order ionic translation hopping assisted by small-polaron hopping mechanism.

Fig. 2 .
Fig. 2. Temperature dependences of relative elongation of Bi 4-x Pr x Ti 3 O 12 sintered ceramics.Inset shows concentration dependences of T C

Fig. 3 .
Fig. 3. Dependences of DC electrical conductivity (a) and thermo-EMF coefficient (b) of Bi 4-x Pr x Ti 3 O 12 samples vs temperature results, at x ≤ 0.75 La 3+ ions substitute Bi 3+ in perovskite-like [Bi 2 Ti 3 O 10 ] 2-blocks, and at x > 0.75 they can substitute Bi 3+ ions in fluorite-like [Bi 2 O 2 ] 2+ layers too, so formula of Bi 4-x La x Ti 3 O 12 solid solutions at x ≤ 0.75 and x > 0.75 should be written as [Bi 2 O 2 ][Bi 2-x La x Ti 3 O 10 ] and [Bi 2-y La y O 2 ] [Bi 2-x+y La x-y Ti 3 O 10 [4][5][6][7][8]in a good agreement with the literature data, according to which substitution of Bi with Ln in Bi 4 Ti 3 O 12 leads to lowering of its Curie temperature[4][5][6][7][8].The LTEC values of Bi 4-x Pr x Ti 3 O 12 titanates in FE state decreased, but in PE state increased with x (Table 1).The LTEC values in PE state can be explaned by anharmonicity of metal-oxygen vibrations in disordered cationic sublattice of Pr 3+ -substituted bismuth titanate Bi 4-x Pr x Ti 3 O 12 .