Detailed structure analyses on Cobalt doped PbTiO3 powders

The identification of the defects and secondary phases which significantly affect the material properties are of crucial importance. In this study, a systematic structure examination of PbTiO3 and cobalt doped PbTiO3 powder ceramics was carried out. X-ray diffraction (XRD), Fourier-transform infrared (FT-IR), Raman, and electron paramagnetic resonance (EPR) spectroscopies were applied along with nonsimultaneous thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The doped and undoped PbTiO3 materials were synthesized via a practical sol-gel route that takes place at 50 °C. The perovskite formation for both materials was verified. The dislocation density of cobalt doped PbTiO3 was found to be 0.0121 nm−2 while it was 0.00239 nm−2 for the undoped material. Besides, a strong strain effect was observed for cobalt doped PbTiO3 via XRD. This was attributed to the Co3O4 phase which was detected through EPR and FT-IR analyses. The formation of the Co3O4 phase during synthesis revealed the previously unexpected nonimproved ferroelectric behavior for cobalt doped PbTiO3. The dielectric constant and the dielectric loss (tan δ) of cobalt doped PbTiO3 were estimated as 1066 and 0.8370, respectively.


Sol-gel synthesis
The synthesis was carried out according to the method given by Odabasi [13]. Lead(II) acetate trihydrate (Pb(CH 3 COO) 2 .3H 2 O) was dissolved in glacial acetic acid at room temperature. Appropriate dopant precursor (Co(II) (NO 3 ) 2 .6H 2 O was also dissolved in this mixture. In another beaker, titanium isopropoxide (Ti(OCH(CH 3 ) 2 ) 4 ) was added to a mixture of glacial acetic acid and ethanol via a syringe. Two solutions were stirred at room temperature for around one h and then mixed. Vigorous stirring continued until a clear solution formed. Then, a mixture of citric acid and methanol was added to this solution. After a homogenous mixture was obtained, the temperature was raised up to 50 °C and heated for about one h. The cobalt doped material turned pink while the undoped material was off-white. All materials were calcined in two steps: Firstly, overnight at 100 °C and then at 650 °C for around three h with a heating rate of 50 °C/min.

XRD analysis
The crystal structure was characterized with a Rigaku Miniflex XRD instrument (with a CuK α , λ = 0.154 nm) between 20-80°. XRD patterns of both materials are given in Figure 1. The Miller indices of the main reflection planes (hkl) for PbTiO 3 perovskite structure are shown according to JCPDS card no. 01-077-2002. The perovskite structure with a tetragonal symmetry was obtained for both doped and undoped materials [14,15]. However, both homogenous and inhomogeneous strain effects are observed for the cobalt doped PbTiO 3 . The slight shifts from peak positions for (001), (002), (201), (112) planes point out homogenous strain while the broadened peaks at 22-23°, 32-33°, 53°, and 56° show inhomogeneous strain. A similar inhomogeneous strain pattern was recorded by Elbasset et al. [16] for cobalt doped PbTiO 3 and interpreted as either grain size or local disorder effect. On contrary, the formation of a monoclinic PbTi 3 O 7 phase (JCPDS card no. 00-021-0949), which was observed for the undoped PbTiO 3 around 28.9° and 34.6°, vanished upon cobalt doping [17,18]. Hence, the formation of this phase was also mentioned by Lee et al. [18] for PbTiO 3 powders synthesized via a similar sol-gel synthesis route. It was recorded that the formation of PbTi 3 O 7 phase could be eliminated via calcination temperatures above 600 °C for more than three h.
The average crystallite sizes were estimated -with the help of Scherrer equation using (101) base peak-as 34.2 nm and 17.6 nm for undoped and cobalt doped PbTiO 3 , respectively. The formation of defects as a result of cobalt doping may decrease the lattice parameters [19]. The lattice parameters were exploited from JPCDS Card Numbers via HighScore Plus software and compared with the calculated lattice parameters in Table. The difference between the expected (according to JCPDS Card Number) and calculated lattice parameters would result in phase transition temperature shifts like ±5 °C from Curie temperatures [10]. The dislocation densities were found as 2.39 × 10 -3 nm -2 and 1.21 × 10 -2 nm -2 for the undoped and cobalt doped PbTiO 3 with the help of the Williamson-Hall formula [20]. The very low dislocation density of the undoped PbTiO 3 is consistent with the similarly calculated lattice parameters. Moreover, the porosity of cobalt doped material was estimated. Bulk density (ρ b ) and X-ray density (ρ x ) were calculated as 4.504 g/cm 3 and 5.692 g/cm 3 according to the method given by Kumar et al. [11]. The porosity percentage (P%) was evaluated as 20% according to the following formula P% = [1-(ρ b / ρ x )] × 100.

Thermal analysis
Thermal analyses were carried out with a Mettler Toledo instrument under N 2 atmosphere with a flow rate of 40 mL/min. The thermogravimetric analyses (TGA) were carried out between 25 and 900 °C with a heating rate of 10 °C/min. The detailed TGA of cobalt doped PbTiO 3 was shown in Figure 2(a). In general, ceramics are quite stable at high temperatures [10]. As expected, the weight loss percentages were insignificant: 0.6% for undoped and 0.3% for cobalt doped PbTiO 3. as shown in Figure 2(a). It was already reported that PbTiO 3 ceramics decompose at temperatures higher than 900 °C [10]. Hence, PbO x phases are decomposing between the measured temperature ranges [21]. The relatively higher weight loss of undoped PbTiO 3 was attributed to the decomposition of the PbO 2 phase to PbO with the help of the first derivative of thermogravimetric (DTG) data as demonstrated in Figure 2(b). Hence, the uncalcined secondary phases like PbO 2 start to decompose around between 250-350 °C and as temperature increases, PbO phase forms. For cobalt doped sample, even though PbO 2 was not detected, other PbO x phases were identified [21]. Again, the decomposition of these phases ended up with PbO formation. The PbO x -related secondary phases cause the formation of cation and oxygen defects even if they are in minor amounts since they affect the ratio of Pb/Ti ion stoichiometry slightly.
The differential scanning calorimetry (DSC) measurements were conducted between 25 and 550 °C with a heating rate of 8 °C/min again under N 2 atmosphere. The Curie temperature at which the tetragonal crystal structure changes to the cubic phase is expected at 490 °C for PbTiO 3 [10]. However, the detected Curie temperature was around 480 °C for the undoped PbTiO 3 in Figure 3. A difference of 10 °C from the expected Curie temperature value was attributed to a lead deficient (V Pb

''
) PbTiO 3 material [22]. The formation of PbO x containing secondary phases would end up with such cation deficiencies within the perovskite structure. This will also cause the formation of oxygen vacancies (V O ) in order to balance the crystal charge compensation [21,23]. By this way two negatively charged holes created by cation vacancy should be balanced with 2 plus charged oxygen vacancy as shown in Eqn (1) where ⍉ corresponds to the defect-free crystal structure.
(1) Apart from the undoped PbTiO 3 , the Curie temperature vanishes for the cobalt doped PbTiO 3 in Figure 3. This phenomenon was also reported by Odabasi [13]. It might be related to the dislocation density that was estimated through the XRD analysis. The higher dislocation density may cause a decrease in detection limits for similar phase changes in the DSC analyses. Obviously, a counter exothermic peak at the expected Curie temperature is hindered as a result of cobalt doping. In order to resolve the spectrum, modulated DSC with a much slower heating rate should be applied [24]. Moreover, a bump between 150 and 250 °C followed by a sharp transition temperature around 305 °C was detected for the cobalt doped material. A similar trend at different temperatures was also observed for the undoped material. The bump of undoped and cobalt doped PbTiO 3 can be seen between 220 and 320 °C. The possible reason may be a Pb including secondary phase. The PbTi 3 O 7 phase which was detected via XRD is known to be stable at these temperatures and decompose around 700 °C [18]. Another possibility is the pyrochlore (Pb 2 Ti 2 O 6 ) phase which was mentioned by Lee et al. [18]. Even though the XRD patterns of Pb 2 Ti 2 O 6 were hard to detect around 30º, in the DSC analysis, the sharp peaks at 315, 305, and 257 °C clearly point out the transformation of the pyrochlore phase to the tetragonal PbTiO 3 [18]. Because cobalt ion was also incorporated into this pyrochlore phase, a slight shift in the observed temperature was observed for cobalt doped PbTiO 3 . Similar observations within the pyrochlore phase were reported for variously doped PbTiO 3 in literature [25][26][27][28].

FT-IR measurements
The FT-IR measurements were conducted at room temperature, between 450 and 4000 cm -1 via an ATR crystal Thermo Scientific instrument. Two main peaks at 503 and 880 cm -1 for the undoped PbTiO 3 are seen in Figure 4. These peaks were associated with Ti-O and Pb-O bonds, respectively [29][30][31]. The slight bump around 713 cm -1 , which could also be detected for cobalt doped PbTiO 3 , was attributed to six coordinated Ti 4+ ion octahedral complexes within the perovskite structure [15]. Especially, the undoped and cobalt doped materials have quite similar spectra.

EPR spectroscopy
X-Band (9.7 GHz) EPR spectroscopy of doped materials was measured with a Bruker EMX 081 type EPR spectrometer at room temperature. Simply, EPR spectroscopy deals with the interaction of electromagnetic radiation with the molecule's dipole moment, which arises from an unpaired electron in its orbital [32][33][34]. Principally, each paramagnetic ion in a certain environment has a characteristic signal. The Co 2+ ion has three unpaired electrons in its high spin d 7 state. The spin Hamiltonian for high spin Co 2+ is shown in Eqn.4 where ꞵ e is the Bohr magneton, B o is the applied external field, g is the g-factor or g tensor, S is the spin state, ꞵ n is the nuclear magneton, g n is the nuclear g-factor, I is the nuclear spin. A is the hyperfine interaction of the nucleus with the electronic spin and D is the zero-field splitting term that occurs from electron-electron dipole interaction of more than one unpaired electron containing system [33,34]. Since S is 3/2 and I is 7/2 for high spin Co 2+ ion, splittings in its EPR spectrum are expected.   [33,35]. Unfortunately, in Figure 5, the expected spectrum seems to vanish under the strong broad peak. A different measurement frequency rather than X Band may help to resolve this part.
In the literature, a similar broad peak was reported for Co 3 O 4 [36] which is obtained through the calcination of CoO between 600 and 700 °C [37]. Apparently, CoO phase was formed during sol-gel synthesis and later turned into Co 3 O 4 after calcination. This would result in less incorporation of Co 2+ ions into the perovskite structure. Moreover, the broadenings in the XRD spectrum and thermal analyses of the cobalt doped material most likely arouse from this complicated secondary phase. However, it should be noted that the amount of this phase must be quite low and therefore below the detection limits of XRD, since during the analyses, the spectrum related to Co 3 O 4 could not be exploited directly but just observed in terms of broadenings. Thus, Co 3 O 4 has a spinel structure where Co 3+ ions reside in the octahedral site, while Co 2+ ions reside in the tetrahedral sites [37,38]. Normally, bulk Co 3 O 4 was reported as antiferromagnetic at room temperature and Co 3 O 4 nanoparticles were reported as magnetic only at very low temperatures [39]. Therefore, a magnetic susceptibility measurement was carried out to verify the incorporation of Co 2+ ions into the perovskite structure. The magnetic susceptibility was compared with a copper doped PbTiO 3 , which was synthesized with a similar route [28], and shown in Figure 6. A Vibrating Sample Magnetometer (VSM) system was utilized for magnetic measurements at room temperature. Even though both materials have low magnetic behavior, when compared with copper doped PbTiO 3 , cobalt doped PbTiO 3 exhibits more ferromagnetic behavior. This may arise from the incorporation of cobalt ion into the PbTiO 3 perovskite structure. It should be noted that Co 3+ in the Co 3 O 4 phase was reported as diamagnetic due to its splitting in the spinel structure while the Co 2+ ions have a small contribution to spin-orbit coupling. However, the magnetic susceptibility of the cobalt doped PbTiO 3 material was found to be higher than Co 3 O 4 susceptibility as reported by Roth [38]. Therefore, this behavior was attributed to the incorporation of Co 2+ within the targeted structure.

Raman spectroscopy
Raman spectroscopy was applied to verify the secondary phases detected through all other methods. It was conducted with an InVia Qontor model Renishaw instrument at room temperature. Typical PbTiO 3 phonon transitions [15,[39][40][41][42][43][44][45][46] can be seen in Figure 7. After doping with cobalt, most of the transitions vanished or decreased drastically. The broadening of Raman lines and larger backgrounds for bulk ceramics were interpreted as an indication of disordered or amorphous structures [40].

Dielectric properties
Cobalt doped PbTiO 3 pellets (0.6010 cm radius and 0.771 mm thickness) were obtained under 12 MPa pressure at room temperature and sintered at 700 °C for two h. Then, the surface of the pellets was coated with gold (Au) via vapor deposition (sputtering) technique before electrical measurements. The undoped material was not dense enough to obtain a proper pellet. Capacitance (C) and dielectric loss (tan δ) measurements of doped material were taken with an LCRmeter (INSTEK LCR-816) at a frequency of 1 kHz at room temperature. The capacitance (C) was measured as 1389 pF and relative permittivity (dielectric constant) was calculated as 1066. Dielectric loss (tan δ) was estimated as 0.8370. The dielectric loss at 1 kHz and dielectric constant were reported as 0.09 and 96.8 for the undoped PbTiO 3 capacitors [49]. The doping has affected the material's properties according to the increased values. Hence, the existence of pyrochlore phases at surfaces is known to decrease the dielectric constant. High dielectric constant value verifies the pyrochlore-free characterization results for cobalt doped material. Besides, the parameters obtained in this study are in good agreement with the literature for doped and composite PbTiO 3 based ceramics [10,50,51]. Co 3 O 4 phase seems to enhance the dielectric constant. However, the existence of Co 3 O 4 phase is thought to be the reason for not obtaining a proper polarization-electric field (P-E) loop hysteresis. The distorted banana shape shows a current leakage within the material. A similar P-E behavior was also observed by Kumar et al. [11]. It is obvious that the formation of CoO during sol-gel synthesis should be inhibited or this phase should be eliminated from the material before calcination so that Co 3 O 4 phase can be avoided to overcome this problem.

Conclusions
The structural properties of the undoped and cobalt doped PbTiO 3 were investigated. Later these properties were used to interpret the nonferroelectric behavior of cobalt doped PbTiO 3 . PbO 2 , PbTi 3 O 7 , Pb 2 Ti 2 O 6 were detected for the undoped PbTiO 3 , while slight PbO x , Pb 2 (Ti x Co 2-x O 6 ) formations were observed for cobalt doped PbTiO 3 through XRD, Raman and thermal analyses. Additionally, Co 3 O 4 phase was detected through EPR and Raman spectroscopy. The vanishing Curie temperature of cobalt doped PbTiO 3 points out that a more sophisticated thermal analysis will be necessary to resolve the counter exothermic peak. The dielectric constant and dielectric loss for cobalt doped PbTiO 3 were estimated in good agreement with literature as 1066 and 0.8370, respectively.