Characterization and Rietveld Refinement of A-site deficient Lanthanum doped Barium Titanate

Journal of Alloys and Compounds 579 (2013) 473–484

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Characterization and Rietveld Refinement of A-site deficient Lanthanum doped Barium Titanate M. Ganguly a, S.K. Rout a,⇑, T.P. Sinha b, S.K. Sharma c, H.Y. Park d, C.W. Ahn e, I.W. Kim e a

Department of Applied Physics, Birla Institute of Technology, Mesra, Ranchi, India Department of Physics, Bose Institute, Kolkata, India c Department of Physics, ISM, Dhanbad, India d Electronics and Communication Semiconductor Applications, Ulsan College, Ulsan 680-749, Republic of Korea e Department of Physics and Energy Harvest-Storage Research Center, University of Ulsan, Ulsan 680-749, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 7 December 2012 Received in revised form 5 June 2013 Accepted 16 June 2013 Available online 27 June 2013 Keywords: Ceramics Ferroelectrics Solid state reaction Microstructure Dielectric response Luminescence

a b s t r a c t Ba1xLa2x/3TiO3 (0.00 6 x 6 0.10, in a step of 0.02) ceramics have been prepared through solid state reaction route. Structural studies suggested a transition in phase from tetragonal to cubic symmetry with increase in Lanthanum content. Rietveld Refinement technique employed for investigation confirmed the same. Photoluminescence study revealed introduction of structural disorder by means of A-site vacancies and displacement of M–O bond leading to shallow defects. Optical band gap value calculated from UV–Vis spectra decreased with increase in La concentration. A drastic decrease in grain size of undoped BT was observed with introduction of La through Scanning Electron Micrographs. Dielectric studies were performed and a gradual decrease in the Curie temperature with increase in La content in coherence with structural studies was observed along with pinching effect. Normal ferroelectric character was obtained for the composition x = 0.00 to x = 0.06 while relaxor like behavior was observed for composition x P 0.08. The composition x = 0.10 made a good Vogel–Fulcher fit. Inhomogeneity induced in the BT lattice due to 8% La doping is strong enough to make an onset of such behavior. PE hysteresis loop showed a regular decrease in remnant polarization and coercive field featuring similar relaxor like behavior. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Barium Titanate (BT) has been widely investigated and has experienced a renaissance in the past decade due to its long range of applications. It has its application in multilayer ceramic capacitors, electro mechanical system, electro optical system, pyroelectric detectors, piezoelectric actuators, MEMS, FeRAM devices, etc. [1]. Doped BT with rare earth elements has been an upcoming area of research for the past few decades and among them Lanthanum (La) has been a material of great importance. La behaves as a donor as it occupies the Ba site. Though undoped BT is electrically insulating, electrical resistance can be controlled through La doping. Even PTCR effect has been shown by donor doped BT [2,3]. Rare earth elements especially La doped BT has been mostly popular as a dielectric material in Ni-MLCC as these are effective in Vo trapping and controlling the microstructure enhancing the reliability [4–6]. Solubility of La into the BT lattice has been quite high as

compared to other rare earth elements resulting in a change in symmetry from tetragonal to cubic and a change in grain size which is again of importance [3,7,8]. Xinle et al. have also reported the same [9]. Beside this Aliouane et al. had been able to obtain relaxor behavior for more than 10% doping in charge compensated La doped BT. Structural distortion induced by La ions has been considered to be the reason behind [10]. Thus it has been well established that La doped BT is a perfect material to be used as a dielectric in capacitors with high dielectric constant, a stable capacitance value, long service life, low-loss factor, high insulation resistance, reduced Curie temperature, low voltage dependence and low temperature dependence of the dielectric constant over a wide temperature range [2–11]. BT has a perovskite structure (ABO3) where Ba occupies the A site and Ti the B site. La is incorporated at the A site where it behaves as a donor according to the equation:

Ba2þ ! La3þ þ e

ð1Þ

Creation of oxygen vacancies also takes place leading to semiconductive behavior, according to the equation: ⇑ Corresponding author. Tel.: +91 9471555277. E-mail addresses: [email protected], [email protected] (S.K. Rout). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.06.104

O2 ! 1=2 O2 þ 2e

ð2Þ

474

M. Ganguly et al. / Journal of Alloys and Compounds 579 (2013) 473–484 Calcined powders were pressed uniaxially under pressure 60 kg/cm2 to form disk shaped pellets of 10 mm diameter and 2 mm thickness with 5% polyvinyl alcohol as the binder. Sintering of undoped BT pellet was done at 1400 °C for 4 h while the doped pellets were done at 1400 °C for 10 h to obtain optimal shrinkage and compactness. Bulk densities calculated according to Archimedes Principle were found to be greater than 90% of their respective theoretical values. Microstructural study was done using a Scanning Electron Microscope (JSM-6390LV). Dielectric study of the sintered and densified pellets was done over a wide temperature range from 15 K to 573 K by HP4294A system. PE hysteresis loops were traced by applying a maximum electric field of 1 kV/mm at a frequency of 50 Hz. A Sawyer–Tower circuit at room temperature measured the ferroelectric PE hysteresis loops using a Radiant Technologies Ferroelectric Loop Tracer (Trek Model 609B).

The effect of Ln-substitution for Ba-ion can be expressed by Kröger–Vink notation as:

BaO þ Ln2 O3 ! BaBa þ 2LnBa þ V00Ba þ 4OO

ð3Þ 2+

Since the substitution at the A site is off valent (Ba being substituted by smaller La3+), so to obtain charge neutrality vacancy is created. Eq. (3) implies that for every two rare earth cation substitution at the A-site, three alkaline cations gets replaced creating one positively double charge vacancy, provided the charges are to be taken to the perfect lattice. This number of vacancy increases with increase in doping concentration [12,13]. La doped BT ceramics were prepared according to the equation:

ð1  xÞBaCO3 þ TiO2 þ ðx=3ÞLa2 O3 ! Ba1x La2x=3 TiO3 þ CO2

3. Results and discussion

ð4Þ Fig. 1 shows the XRD pattern of Ba1xLa2x/3TiO3 ceramic powders calcined at 1400 °C for 4 h. All the compositions show the reflections of single phase perovskite structure. The diffraction pattern of all composition within the 2h range of 44°–46° is shown in a magnified scale for clarity. The presence of (0 0 2), (2 0 0) peaks suggests tetragonal symmetry at room temperature for the compositions x 6 0.04. A Gaussian fit showing the presence of both the peaks is shown in inset. However merging of both the above peaks from the compositions x P 0.06 suggests cubic symmetry. Similar transition in phase with increase in La concentration had been reported by Valdez-Nava et al. [11] and Morrison et al. [12,14]. The disappearance of the (0 0 2) and (1 0 3) peaks for the compositions x P 0.06 basically suggests such transition in phase. Structural refinement was carried out for the compositions x = 0.00, 0.04, 0.06 and 0.10 using the Rietveld’s refinement program ‘‘Full Prof’’ and the final output is shown in Fig. 2a–d. From structural refinement it is confirmed that the compositions 0.00 6 x 6 0.04 belongs to tetragonal symmetry with space group P4mm while the composition 0.06 6 x 6 0.10 has cubic structure with space group Pm–3m at room temperature. The initial parameters are taken from the standard Wyckoff position table. The refinement produced satisfactory agreement factors and lattice parameters which are listed in Table 1. The radius of Ba ion is 1.6 Å for a state with co-ordination number 12 while that of La is 1.36 Å. Hence substitution of lower radii La at the higher radii Ba site results in shrinkage of cell volume (Table 1). A decrease in tetragonality takes place and for higher La concentration structural transition from tetragonal to cubic is hence quite probable. A theoretical study on the tolerance factor (t) which is unity for an ideal perovskites (cubic) can be done through the following equation [15]:

where x = 0.00, 0.02, 0.04, 0.06, 0.08, and 0.10. Excluding the composition x = 0.00, all the other compositions are non-stoichiometric, exhibiting A-site deficient perovskite type solid solutions according to the structural formula: Ba1xLa2x/3hx/3TiO3, where h denotes A-site vacancy in the perovskite structure [10]. Though much work has been done in the past years in A-site and B-site doping of BT there is no clear picture about the various doping mechanism and defect chemistry associated with the various types of doping. In particular the doping of La in BT has been widely debated. This attracted us towards this zone of research with a core aim to justify charge compensated A-site deficient Lanthanum doped Barium Titanate in all possible respect. 2. Experimental procedure Ba1xLa2x/3TiO3 (0.00 6 x 6 0.10) ceramics were prepared through solid state reaction technique from reagents BaCO3 (99% Pure, Merck, India Ltd.), TiO2 (99% Pure, Merck, India Ltd.) and La2O3 (99.99% Pure, Sigma–Aldrich, USA). Powders were mixed in an appropriate amount and grinded with distilled water in an agate mortar. The homogeneous mixture was milled in a FRITSCH ‘‘Pulverisette 5’’ planetary mill for 10 h with Zirconium balls (5 mm diameter) and then heated at 1200 °C for 12 h. The process was repeated followed by final calcination at 1400 °C for 4 h. Structural characterization was done by XRD using Cu Ka radiation from 15° to 80° with a step size of 0.02° and scanning rate of 1° per minute. Structural refinement was performed using a standard refinement program ‘‘Full Prof’’. The Fourier Transform Infra red spectra was recorded at room temperature by the standard KBr pellet technique in a Perkin Elmer Fourier Transform Infra Red Spectrophotometer (Spectrum 1000), Japan. Raman spectroscopic studies followed thereafter (Seki Technotron with excitation 514 nm). Photoluminescence study was done using a Hitachi Fluorescence Spectrophotometer (model-F2500) to study the luminescence efficiency of the doped ceramics. UV–Visual study was done to calculate the band gap using a Cary 5G (Varian, USA) spectrophotometer.

Ba 1-x La 2x/3 TiO3 Intensity (arbitrary units)

x =0.10

x=0.10

x =0.08

x=0.08 (211)

(101) (111) (200)

(100)

(201)

x=0.06

(202)

x = 0.06

Cubic

x = 0.04

(221) (310)

Tetragonal

x = 0.02

x=0.04 (002) (200)

x=0.02 x=0.02

(101) x=0.00 (100)

(111)

(211) (200) (002) (201)

(103) (202) (221) (310)

30

44.5

(002)

PCPDF (83-1880)

20

44.0

40

50

60

2 θ (in degree)

70

80

45.0

45.0 45.5 o 2 θ (in )

46.0

(200)

x = 0.00

45.5

46.0

2θ ( in degree)

Fig. 1. Room temperature XRD patterns of Ba1xLa2x/3TiO3 powders along with shifting and merging of peak of Ba1xLa2x/3TiO3 with increase in doping concentration.

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(a)

BaTiO3

(b)

Obs Cal Diff betn Obs and Cal BraggsPosition

20

30

40

50

60

70

Ba 1-x La 2x/3 TiO 3 (x=0.04)

80

20

30

2θ (in degree)

(c) Ba

1-x La 2x/3TiO 3

20

30

40

50

50

60

70

80

2 θ (in degree)

(d) Ba 1-x La 2x/3 TiO 3 (x=0.10)

Obs Cal Diff betn Obs and Cal Braggs Position

(x=0.06)

40

Obs Cal Diff betn Obs and Cal BraggsPosition

60

70

80

20

2 θ (in degree)

30

40

Obs Cal Diff betn Obs and Cal Braggs Position

50

60

70

80

2 θ (in degree)

Fig. 2. Observed (°), calculated (–) and residual X-ray powder diffraction pattern of Ba1xLa2x/3TiO3 compositions (a) x = 0.00 (b) x = 0.04 (c) x = 0.06 and (d) x = 0.10 revealed from Rietveld’s powder structure refinement analysis. Peak positions of the phases are shown at the base line as small marker. Positional parameters for the composition x = 0.00, 0.04 (P4mm) are: Ba/La at 1a(0, 0, 0), Ti at 1b(0.5, 0.5, 0.524), O1 at 1b(0.5, 0.5, 0.005), and O2 at 2c(0.5, 0, 0.427) and for the composition x = 0.06, 0.10 (Pm–3m) are: Ba/La at1a(0, 0, 0) Ti at 1b (0.5, 0.5, 0.5) and O at 3c(0, 0.5, 0.5).

Table 1 Results of Rietveld Refinement of X-ray diffraction data of Ba1xLa2x/3TiO3 measured at room temperature for the tetragonal region with space group P4mm for x = 0.00, 0.04 while for the cubic region with space group Pm–3m for x = 0.06, 0.10. Parameters Lattice parameter (a = b) (in Å) Lattice parameter (c) (in Å) Volume (in Å3) Rp Rwp Rexp Goodness of fit (v2)

x = 0.00

x = 0.04

x = 0.06

x = 0.10

3.99567

3.99532

3.99855

3.9950

4.02557 64.27 7.95 10.6 7.71 1.90

4.00734 63.968 6.60 8.85 6.81 1.69

3.99855 63.930 7.08 9.47 6.81 1.93

3.9950 63.76 7.05 9.92 6.71 2.19

RO þ RA t ¼ pffiffiffi 2ðRO þ RB Þ

ð5Þ

Ti—OvBT ¼ Ti—OvBLT

For Ba1xLa2x/3TiO3 the tolerance factor is given as

t¼

RO þ ð1  xÞR2þ þ ð2x=3ÞR3þ La pffiffiffi Ba 4þ 2ðRO þ RTi Þ

spectrum at room temperature is given in Fig. 3 while the inset shows shift in Ti–OI absorption peak w.r.t. La concentration. A strong absorption peak for pure BT powder is obtained at 524 cm1. This corresponds to the stretching normal vibration of Ti–OI octahedron as already reported [15–17]. Absorbance peaks for the same mode for compositions x = 0.02, 0.04, 0.06, 0.08 and 0.10 are obtained at 526 cm1, 528 cm1, 530 cm1, 534 cm1 and 537 cm1 respectively. Thus on La incorporation, a particular trend of shifting of peaks towards higher energy is observed. Since 2þ R3þ La < RBa so when La is incorporated at the Ba site cell parameters and the cell size decrease. This shortens the distance between Ti4+ ion and O2 ion, enhancing the bond strength. Hence they become stronger [18–20]. As explained by Jin et al. [18] according to the following equation:

ð6Þ

3þ 4þ Taking R2þ Ba ¼ 1:6 Å, RLa ¼ 1:36 Å, RTi ¼ 0:65 Å, and RO ¼ 1:4 Å, the values of tolerance factors obtained from Eq. (6) are 1.060, 1.056, 1.051, 1.046, 1.041 and 1.036 successively from x = 0.00 to x = 0.10. A successive decrease in the tolerance factor is noticed with increase in La concentration. This again reveals that on increasing the La content, the tetragonality in the prepared ceramics decreases. FTIR technique was employed to study the influence of additives and to determine the local symmetry in the ceramics. The

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiu f lTi—O u t Ti—OBT

lTi—O Ti—OfBLT

¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ti—OfBT Ti—OfBLT

ð7Þ

where BT = BaTiO3; BLT = Ba1xLa2x/3TiO3. The variation of Ti–O vibration is only related to force constant fTi–O. Smaller radius of La3+ ion than that of Ba2+ ion, results in the shortening of Ti–O bonds of BLT. Hence they become stronger than that of BT, so Ti—OfBLT > Ti—OfBT and Ti—OvBLT > Ti—OvBT [20]. Optical modes of cubic phase of BT transform according to the 3F1u + 1F2u irreducible representation. The detailed mechanism behind the origin of Raman spectra in tetragonal BT and doped cubic BT is given in our earlier works [21]. Fig. 4 shows the Raman spectra for the series of samples. Mode assignment shows appearance of A1(LO3) mode at 721 cm1, a feature of tetragonal BT,

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Ba1-x La 2x/3 TiO3

Ba 1-x La 2x/3TiO 3 x=0.10

x=0.08

%Transmittance

x=0.10 x=0.06

x=0.08

Intensity (arbitrary units)

x=0.04

x=0.02

Peak position (cm-1 )

x=0.00 540 536 532 528 524 0.00

0.02

0.04

0.06

0.08

x=0.06

cubic

Tetragonal

x=0.04

0.10

composition (x)

600

800

1000

1200

1400

1600

1800

2000

x=0.02

Wave number (cm -1) Fig. 3. FTIR spectra of Ba1xLa2x/3TiO3 at room temperature.

which arises for phonons propagating along the c axis. The E(LO) mode also arises simultaneously for phonons propagating in ab plane. In the tetragonal phase the separation between the A1 and E component of F1u mode is negligible. Presence of B1 mode at 305 cm1 along with overlapping of E(TO) and E(LO) is observable in 0.00 6 x 6 0.04. This overlapping may be due to random orientation of crystallites. This mode, arising from out-of-phase vibration of oxygen ions only, disappears from x P 0.06. This confirms the phase transition taking place, as observed from XRD study, from tetragonal symmetry (P4mm) to cubic symmetry (Pm–3m) [21– 23]. A similar feature had been observed by Dobal et al. [23] and Morrison et al. [14] with increase in the concentration of La in BT. The behavior of the soft mode frequency (A1 + E) in solid solutions is characterized by short range force constant, long range interactions and masses of the ions involved. A decrease in intensity and broadening in the A1(LO3), E(LO) mode is observed with subsequent increase in La content. This mode shows a second peak on the high frequency side, clearly noticeable in x P 0.06. It has been reported that mode frequency of this mode changes in a series of complex perovskites as a function of perovskite unit cell and with changes in ionic radii. Though neither A or B ions move in this vibration, the mode still reflects changes in the perovskite structure. In this mode only the oxygen ions move. Doping BT with La results in presence of different radii ions at the A-site. This results in formation of inequivalent oxygen octahedral that changes the spacing between A and B ions. A change in their bonding also takes place. Again a decrease in the B–O–B bond and respective bond angle also takes place. This modifies the short range Ti–O force constant, disturbing the Ti-(3d) and O-(2p) hybridization. Since Raman spectroscopy is very sensitive to instantaneous atomic shifts from regular sites, so mode frequency changes, resulting in appearance of extra peaks [24–26]. Asymmetric, broad, intense modes are obtained at 518 cm1 and centered at 219 cm1 [A1(TO)] are observed. A decrease in

A 1( TO1 )

A 1( TO2 ) B1 ,E(LO + TO) A 1 (TO 3 ) A 1 (LO3 ), E(LO) x=0.00

200

400

600

800

1000

Wave length (cm -1 ) Fig. 4. Raman spectra of Ba1xLa2x/3TiO3 at room temperature.

the intensity and broadening in the A1(TO3) mode at 518 cm1 is clearly noticeable in x P 0.06. This mode arises from the motion of the Ti and O1 ions against O2 and O3 ions, which are located in the perpendicular plane. This is also associated with change in the Ti–O–Ti bond [26]. Some anti-resonance effect (dip) is obtained at 180 cm1, similar to Dobal et al. [23]. This anti-resonance effect is basically an interference arising due to coupling between sharp A1(TO1 and TO2) modes. A sharp line at 160 cm1 is obtained in x = 0.00. This corresponds to vibration of Ba ions against TiO6 octahedron. This mode is noticed to soften in La doped compositions suggesting decrease in tetragonality [26]. The half width variation and peak positions for A1 and E(LO) modes (after Gaussian fit) w.r.t. composition is given in Table 2. Broadening in the modes is observed which may be due to introduction of disorder in the lattice of the doped samples due to La substitution. As already explained, size of the A-site cation modifies the movement of ‘‘O’’ atom along the B–O–B axis. B-site ordering decreases off-centering the Ti4+ ion. This disorder increases with increase in La content and hence the F.W.H.M. of this mode increases with La concentration. This may also lead to attainment of relaxor like behavior at higher La concentrations in the doped ceramics [24,25].

M. Ganguly et al. / Journal of Alloys and Compounds 579 (2013) 473–484 Table 2 Variation in FWHM and peak position of A1 mode of Ba1xLa2x/3TiO3 w.r.t x at room temperature. Composition

Peak position of A1(TO3) mode (cm1)

F.W.H.M. of A1(TO3) mode (cm1)

Peak position of A1(LO3), E(LO) mode (cm1)

F.W.H.M. of A1(LO3), E(LO) mode (cm1)

x = 0.00 x = 0.02 x = 0.04 x = 0.06 x = 0.08 x = 0.10

518 518 523 521 521 518

55.27 57.83 67.33 80.23 81.11 86.91

721 725 729 723, 835 727, 837 732, 840

23.65 28.08 31.74 21.40, 7.45 26.05, 7.61 33.61, 7.77

Thus the main features observed are broadening of all peaks especially at higher La concentrations. Decrease in intensity and disappearance of all tetragonal characteristic modes with increase in La content. Decrease in intensity though occurs yet doping with La does not thoroughly destroy the long range order of ferroelectric BT. La incorporation causes local distortion and breaks partially the translation symmetry in BT. Raman selection rules get relaxed and accordingly broadening in modes takes place. The microstructures of the polished and thermally etched sections of Ba1xLa2x/3TiO3 are shown in Fig. 5a and b. Disc/plate like morphology within large sized grains (50 lm) is obtained for undoped BaTiO3 (x = 0.00). These may be ferroelectric domains of BaTiO3 which are clearly visible at higher resolution. The doped ones are found to be dense with clearly visible grains of much smaller size than that of undoped BaTiO3 and with well defined grain boundary. With successive increase in La3+ ion content though average grain size increases from 2 lm in x = 0.02 to 5 lm in x = 0.10 but the homogeneity or regularity in grain shape decreases. Thus it can be considered that doping La in BaTiO3 inhibits grain growth but with increase in its content grain size

477

increases successively. Similar trend has been noticed in our earlier work [21,26–28] as well as in La doped BZT ceramic [29]. Rare earth ions have been known to suppress grain growth in perovskites [30,31]. No evidence of any secondary phase in obtained in any of the ceramics. Microstructural features are mainly governed by transport of matter during heating process. Solid state synthesis method starts with milling of raw materials, during which a reduction in average particle size occurs. Upon heat treatment, diffusion starts at the contact points of the particles which over prolonged sintering lead to formation of necks between grains. This is basically an elastic deformation due to the reduction in surface energy at the contact interface. During this period equilibrium is reached between the surface and interfacial tension. The formation of neck favors matter transport at long distances. Evolution of larger grains takes place disappearing the smaller ones [32]. For undoped BT formation of large grains at a sintering condition of 1400 °C for 4 h shows that this favors fast kinetics of interdiffusion and hence matter is transported at long distances leading to evolution of large grains. On La incorporation, an abrupt decrease in grain size suggests that the sintering condition allows slow kinetics of inter-diffusion at the contact points between the particles. Hence the compositions show smaller grains. But these grains have more or less regular morphologies showing uniform matter transport into the system. However on successive increase in La3+ ion content the grain sizes are observed to increase revealing an enhanced diffusion mechanism. Although larger grains are observed but an irregular shape is noticed. This may be due to variations in the kinetics of matter transport from boundary to boundary. In order to reduce surface free energy, atoms move from particles with smaller radii to those with larger radii resulting in irregular grains [33–35]. Dielectric behavior as a function of temperature for all the compositions at different frequencies is shown in Fig. 6 while the variation in dielectric loss at lower temperature are shown

Fig. 5. SEM micrographs of Ba1xLa2x/3TiO3.

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in insets. Dramatic decrease in Tc–t (cubic to tetragonal), while increase in Tt–o (tetragonal to orthorhombic) transition temperatures are observed with increase in La concentration. From x P 0.06 the Tc–t transition shifts below room temperature. Hence it can be concluded that the compositions with x 6 0.04 have a tetragonal symmetry while those with x P 0.06 have cubic symmetry at room temperature. This is in consistent with the room temperature XRD, Rietveld refinement and Raman data of the ceramics. Decrease in the value of dielectric constant is observed with increase in frequency. Higher values of DC at lower frequency may be due to space charge polarization [36]. Pinching effect [37] is observed where both Tc–t and Tt–o move towards each other with increase in La concentration. The Tc–t shifts towards lower value while Tt–o towards higher one and finally merge to give a broad transition. Such shifting of peaks is observed uptill x = 0.06 and in x P 0.08 merging of peaks results in broad transition. Frequency dependent dielectric behavior is observed only for

the compositions x = 0.08 and x = 0.10. The value of Tm of these two compositions shifts towards higher temperatures on increasing the frequency. Decrease in Tc–t with increase in La content has been well reported in literature [10,36,38]. This basically is an indication of a decrease in tetragonality. Replacing Ba2+ by La3+ results in A-site deficiency to maintain charge neutrality. This increases with increase in La content. Also shrinkage of unit cell takes place off centering the Ti4+ ion out of the octahedral site. Thus the coupling between the TiO6 octahedra weakens. This results in a strong decrease in Tc–t (Tc is directly related to the displacement of cation from the centre of the octahedral site to its position in the polar phase) [10,14,20,36,38]. Even similar effect has been observed in La doped Barium Zirconate Titanate [15]. It is a genuine fact that tetragonality, grain size and density have an impact on dielectric constant of ceramics. So the fall in the values of DC (at constant temperature and frequency) on

Fig. 6. Dielectric constant and dielectric loss as a function of temperature of Ba1xLa2x/3TiO3 at various frequencies.

M. Ganguly et al. / Journal of Alloys and Compounds 579 (2013) 473–484

479

Fig. 6 (continued)

introduction of La3+ ion in BT lattice can be attributed to the dramatic decrease in the grain size and tetragonality. Now on successive increase in La content, grain size is found to increase along with density, though tetragonality decreases. Thus as La content is successively increased, the DC behavior is mostly dominated by grain size and density rather than tetragonality. This may be the reason behind the respective increase in DC values with successive increase in La content. Dielectric stiffness (1/e) at 10 kHz for x = 0.08 and 0.10 (Fig. 7) are found to deviate from Curie–Weiss law. The Curie–Weiss temperature (T0) is found to be greater than Tm. This indicates a relaxor like behavior. The parameter DTm[(Tcw  Tm), Tcw denotes the deviation temperature of permittivity from C–W law, Tm is the dielectric maximum temperature] describes the degree of deviation from

Curie–Weiss law. The modified C–W law for such ceramics is given as:

1

e



1

emax

¼

ðT  T max Þc C

ð8Þ

where C and c are constants; emax is the maximum dielectric constant at the transition temperature Tmax. This law for the compositions x = 0.08 and x = 0.10 are plotted in Fig. 8. Linear relationships are observed. Slopes of the fitting curve give the values of relaxation strength or degree of diffuseness, c = 1.4 for x = 0.08 and c = 1.7 for x = 0.10. It is 1 for normal ferroelectrics following C–W law, 2 for ideal relaxor ferroelectrics (a quadratic dependency describes diffuse phase transition) [36,39]. The obtained values of c give a clear

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0.50 0.45

Ba1-xLa2x/3TiO3 (x=0.08) at 10kHz

0.7 0.6

0.40

0.5

1000/ε

0.35

1000/ε

Ba1-xLa2x/3TiO3 (x=0.10) at 10kHz

0.30 0.25

0.4

Tcw

0.3

0.20 0.2

Tcw 0.15

Tm

0.10 160

180

200

220

0.1

T0

240

260

280

300

Tm

T0

100 120 140 160 180 200 220 240 260 280 300

T ( o K)

T ( o K)

Fig. 7. Dielectric stiffness as a function of temperature for x = 0.10 and x = 0.08 at 10 kHz.

indication that the materials are disordered and transitions are of diffused type. The increase in the value of c indicates increase in diffusivity. In perovskites, relaxor like behavior is generally attributed when at least two different cations occupy the same crystallographic site. When smaller radii La3+ replaces Ba2+, it traps eight near neighbor oxygen and four more distant ones. A modification occurs which displaces the La ion out of the oxygen dodecahedron. The Ti4+ ion occupying the octahedral site also becomes off centered giving rise to differently polarized micro regions existing in macro regions, diluting the ferroelectric character. A site vacancy is also created leading to maintain charge neutrality at the same time. With increase in La content the vacancy also increases. Also the octahedral site and displacement in La ion is more and more affected. All such positional disorders induces compositional inhomogeneity resulting in diffused phase transition and relaxor like behavior and at higher doping concentration this behavior is prominent and not at lower concentration. Indication for such type of behavior is also evident from increase in half width of A1(TO3) mode of Raman spectra. It is worth mentioning that this structural disorder and formation of microdomains with local polarisation induced due to doping results in different local Curie points. [38–43]. The plot of log(t) vs (1/Tm) is shown in Fig. 9 for the composition x = 0.10. The nonlinear nature indicates that the data cannot be fitted with a simple Debye equation. To analyze the relaxation

features of the ceramics the experimental curve for the composition x = 0.10 were fitted using the Vogel–Fulcher formula [39]

t ¼ t0 exp



Ea kB ðT m  T f Þ

 ð9Þ

where t0 is the attempt frequency, Ea is the measure of activation energy, kB is the Boltzman constant and Tf is the freezing temperature. Tf is regarded as the temperature where the dynamic reorientation of the dipolar cluster polarization can no longer be thermally activated. The fitting curve for this composition is shown (Fig. 9). The fitting parameters are Ea = 0.033 eV, Tf = 151 K and t0 = 7.18  1011 Hz. The close agreement of the data with the V–F relationship suggests a relaxor like behavior for the composition. The various parameters obtained from temperature dependent dielectric study for the composition x = 0.08 and x = 0.10 at 10 kHz are listed in Table 3. It is clearly observable that the frequency dispersion obtained for the composition x = 0.08 is not much pronounced. The diffuseness obtained is not a good fit to Vogel–Fulcher law. Thus though an indication of relaxor like behavior is obtained at this composition yet it cannot be characterized as a typical relaxor. The PE hysteresis loops of pure and doped ceramics at room temperature for all the compositions are shown in Fig. 10. These hysteresis loops were measured using a maximum applied electric field of 1 kV/mm at a frequency of 50 Hz with temperature maintained at 20 °C. It is noticed that the hysteresis loops approach

-3.0

-3.0

Ba 1-x La 2x/3 TiO 3 (x=0.08)

Ba 1-x La 2x/3 TiO 3 (x=0.10) log[(1/ε)-(1/εmax)]

log[(1/ε)-(1/εmax)]

-3.5

-4.0

γ=1.4 -4.5

-3.5

γ =1.7

-4.0

-4.5

-5.0

-5.5

-5.0 0.8

1.2

1.6

log(T-Tmax)

2.0

1.0

1.5

2.0

log(T-Tmax)

Fig. 8. Plot of log(T  Tmax) vs log[1/e  1/emax] for x = 0.08 and x = 0.10 at 10 kHz.

2.5

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M. Ganguly et al. / Journal of Alloys and Compounds 579 (2013) 473–484

5.85

x=0.10

3

5.80 5.75 5.70 4

5

6

7

8

9

10

11

12

ln(υ) Fig. 9. 1/Tm as a function of measured frequency of Ba1xLa2x/3TiO3 for x = 0.10. The symbols are the experimental data points and the line is corresponding fitting to the Vogel–Fulcher relationship.

Table 3 Parameters obtained from temperature dependent dielectric study of Ba1xLa2x/3TiO3 (x = 0.08 and x = 0.10) at 10 kHz. Parameters

x = 0.08

x = 0.10

Tm To Tcw DTm (Tcw  Tm)

230.03 K 235.83 K 257.22 K 27.19 K 8263.52 1.4

172.68 K 199.25 K 247.93 K 75.25 K 10041.23 1.7

emax c

saturation under such an electric field. The values of the remnant polarizations obtained are 4.82 lC/cm2, 1.86 lC/cm2, 0.70 lC/ cm2, 0.42 lC/cm2, 0.31 lC/cm2 and coercive fields are 0.28 kV/ mm, 0.19 kV/mm, 0.07 kV/mm, 0.06 kV/mm, 0.05 kV/mm respectively from x = 0.00 to x = 0.08 while for x = 0.10 both the values dropped to almost zero giving a linear PE behavior. Clearly a steady decrease in the ferroelectric behavior of the samples is obtained with increase in amount of La doping. Loops are found to become slimmer with a regular decrease in the remnant polarization (Pr), loss decreases and a tendency towards a relaxor like character is noticeable for x P 0.08. A lower remnant polarization and smaller coercive field has been reported to be due to increased domain pinning by residual vacancies [39,44]. As already explained these prepared ceramics are A-site deficient. Hence under the applied electric field it is quite

x=0.10

x=0.08

x=0.06

x= 0.00 x= 0.02 x= 0.04 x= 0.06 x= 0.08 x= 0.10

2

5

x=0.04

0 x=0.02

-10 -1.0

-0.5

0.0

0.5

1.0

Electric Field (kV/mm) Fig. 10. P–E hysteresis loop of Ba1xLa2x/3TiO3(0.0 6 x 6 0.10).

300

400

x=0.00

Yellow

Violet

-5 Green

Polarization (μC/cm )

10

Ba1-x La 2x/3 TiO3

Ba 1-x La 2x/3 TiO 3

Blue

-1

(1/Tm)×10 (in K )

5.90

natural that these A-site vacancies (V00Ba ) along with some oxygen vacancies may hop to lower free energy sites such as domain walls and interfaces with electrodes. This weakens the defect mobility and contributes to domain pinning which decreases the remnant polarization and coercive field. With increase in doping content, vacancy increases resulting enhanced domain pinning effect which further decreases the remnant polarization and the coercive field. It has also been reported that the random electric field around the defects lowers the barrier energy required for domain nucleation, decreasing the coercive field [45]. Effect of grain size also cannot be neglected. It has been reported that larger grains have larger remnant polarization as domain walls among larger grains are easier to switch under external electric field [26]. In the prepared ceramics as La content increases grain size decreases, energy barrier and number of grain boundary increases. Now grain boundary is a low permittivity region having poor ferroelectricity. Hence remnant polarization decreases too along with successive increase in La content. The photoluminescence (PL) emission spectra of undoped and doped BT for all compositions were examined at room temperature irradiated with ultraviolet radiation at 250 nm with a 290 nm filter. The PL curves shown in Fig. 11 can be classified into five PL components, violet maximum below 418 nm, blue maximum below 448 nm, green maximum below 493 nm, yellow maximum below 577 nm, and red maximum below 657 nm, in allusion to the regions where the component’s maxima appear [46]. Undoped and doped compositions show peak at 400 nm, which broadens with increase in La content. Some small peaks at 451 nm, 470 nm, 484 nm, 494 nm are also observed. Clearly these materials belong to the zone of violet, blue and green maximum. Neither shifting of these peaks nor broadening is observed on doping and

Intensity (a.u.)

Expt value Tht Fit

5.95

500

600

Wave length (nm) Fig. 11. Room temperature photoluminescence emission spectra of Ba1xLa2x/3TiO3.

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even on increasing the concentration of La. Undoped BT has been reported to show photoluminescence peak in the UV region at 396 nm, which is quite close to our observations [47]. Luminescence of pure BT has generally been described to be due to self-trapped excitons [37,48]. The electronic band structure of BT has low-lying narrow conduction bands from Ti3+ 3d states and valence bands from O2 2p states [49]. The energy corresponding to 250 nm line makes a direct transition of electrons followed by some decay mechanism resulting in the luminescence. To exhibit room temperature PL, a system must have at least two types of differently charged clusters creating local polarization within the structure and/or some localized states existing in the band gap that directly affect the degree of order–disorder. The junction of these two conditions allows an easy trapping of electrons and holes during excitations causing PL emission. If a system is totally disordered, PL does not exist; also a totally ordered system does not exhibit PL [49–51].

Introduction of defect is a mechanism to create inhomogeneous charge distribution in cell and hence produce a disorder in the system. Displacement of A and B atoms together or separately increases this structural asymmetry. This induces destabilization of their atomic orbitals as well as of the oxygen orbital (Jahn–Teller effect) [52] leading to intermediate states in the band gap and hence reduction in the gap values [53,54]. When La is doped in BT, defect is introduced in the structure in the form of A-site vacancy. Modification in the position of A-site atom and B-site atom takes place simultaneously. Displacements in Ti–O, Ba–O/La–O bonds ensure Jahn–Teller effect. Presence of oxygen vacancies also leads to formation of distorted clusters as given below.

½BaO11  VxO  þ ½BaO12 x ! ½BaO11  VO  þ ½BaO12 0

ð10Þ

½BaO11  VO  þ ½BaO12 x ! ½BaO11  VO  þ ½BaO12 0

ð11Þ

x

0

½TiO6  þ ½TiO5  VxO  ! ½TiO6  þ ½TiO5  VO 

Fig. 12. Room temperature UV–Vis absorbance spectrum of Ba1xLa2x/3TiO3.

ð12Þ

M. Ganguly et al. / Journal of Alloys and Compounds 579 (2013) 473–484 x

0

½TiO6  þ ½TiO5  VO  ! ½TiO6  þ ½TiO5  VO 

ð13Þ

As a result relaxation takes place through numerous intermediate states within the band gap, self trapping of electrons and charge transfer via the complex clusters, giving rise to an emission band typical of multiphonon process. Similar PL emission spectra have been obtained in our earlier works [27,28]. If the degree of local order in a system is such that structurally inequitable sites can be distinguished by different types of electronic transitions then these different types of electronic transitions are linked to a specific structural arrangement. Red component represents the less energetic electronic transitions and are thus linked to states that are deeply inserted in the band gap. Conversely, the blue component, more energetic, is linked to shallow defects in the band gap. Since the prepared materials are found to belong to the zone of violet, blue and green maximum, it is clear that formation of shallow defects in the band gap of BT takes place with doping of La. The UV–Vis absorbance spectrum of La doped BT ceramics are shown in Fig. 12. The optical band gap energy (Egap) was estimated by the method proposed by Wood and Tauc [55] by the following equation:

ahm / ðhm  Egap Þn

ð14Þ

where a is the absorbance, h is the Planck constant, m is the frequency, Egap is the optical band gap and n is a constant associated with the different types of electronic transitions (n = 1/2, 2, 3/2 or 3 for direct allowed, indirect allowed, direct forbidden and indirect forbidden transitions, respectively) [34]. In this case, n = 2 as the transition is an indirect allowed one. The Egap values are evaluated by extrapolating the linear portion of the curve and are found to be 2.96 eV, 2.83 eV, 2.81 eV, 2.80 eV, 2.78 eV, 2.74eV for x = 0.00, 0.02, 0.04, 0.06, 0.08, and 0.10 respectively. A gradual decrease in the band gap values is observed as La content is increased successively. The band gap of pure BT in normal conditions lies between 3.2 to 3.5 eV. But here a lower value is obtained. This may be due to oxygen vacancies, lattice defects and/or local bond distortion which yield localized electronic levels in the band gap of the material [56]. The obtained Egap values for the La doped ceramics can be associated with the structural disorder introduced into the lattice due to creation of A-site vacancy, oxygen vacancies and distortions in the [TiO6] clusters. Structural defects form localised states in the band gap and inhomogeneous charge distribution allows trapping of electrons. This results in narrowing of band gap. Now these vacancies increase successively with increase in La content and result in formation of larger number of defects. Distortion in the TiO6 octahedron also increases along with. Hence band gap decreases further [53,54]. Again the electronic configuration of La is [Xe]6s25d1. The 5d sublevel lowers the conduction band in La doped compositions. It has been reported that the intermediary energy levels are composed of deep and shallow holes [57]. Deep holes (between the valence band and conduction band with small Egap values) are responsible for the green, yellow, orange and red PL emission at room temperature, while the shallow holes (between the valence band and conduction band with high Egap values) promote the violet and blue emissions [34,58]. Clearly, formation of shallow defects is the reason behind optical characteristics. 5. Conclusions Ba1xLa2x/3TiO3 (x = 0.02, 0.04, 0.06, 0.08, and 0.10) were prepared by solid state reaction route. Rietveld Refinement confirm a compositionally induced phase transition from tetragonal to cubic symmetry at the composition x = 0.06. The same is supported through FTIR, Raman spectroscopy and temperature dependent

483

dielectric study. FTIR study reveal distortion in the TiO6 octahedra and increase in Ti–O bond strength due to La substitution. Decrease in intensity, followed by a disappearance of the tetragonal Raman modes occur with successive La substitution. Presence of La is found to strongly modify the microstructure. An increase in grain size is observed with increase La content. Dielectric study shows normal ferroelectric character for the compositions x 6 0.06. A tendency towards relaxor like behavior is noticed at x = 0.08 with an increase in degree of relaxation strength at x = 0.10. V–F fit for the composition x = 0.10 confirms the same. Formation of microdomains with local polarizations could induce such behavior even at 8% La concentration. PE hysteresis loops show a successive decrease in remnant polarization and coercive field with successive La substitution. Domain pinning due to A-site vacancies reduces the remnant polarization and coercive field values. Photoluminescence emission spectra recorded at room temperature show structural distortion due to substitution and formation of oxygen and A site vacancies generate asymmetry and self-trapped excitons. UV– Visual study reveals a successive decrease in band gap values due to formation of shallow defects in the forbidden gap.

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