The Effect of Secondary Abnormal Grain Growth on the Dielectric Properties of La/Mn Co-Doped BaTiO3 Ceramics

La/Mn-codoped BaTiO3 systems, obtained by solid state reactions, were investigated regarding their microstructure characteristics and ferroelectric properties. Different concentrations of La2O3 were used for doping, ranging from 0.1 to 5.0 at% La, while a content of Mn was constant at 0.05 at%. For all samples sintered below the eutectic temperature (1332°C), a uniform microstructure was formed with average grain size from 1-3 μm. The appearance of secondary abnormal grains with (111) double twins, grains with curved or faceted grain boundaries were observed in La/Mn BaTiO3 ceramics after sintering at temperatures above the eutectic temperature. All sintered samples exhibited a high electrical resistivity. Better dielectric performances were obtained for low doped samples (0.1 at% La) sintered at 1350°C. For samples with La content above 1.0 at% a lower value in dielectric permittivity at higher sintering temperature is due to secondary abnormal grain growth, and to the presence of a non-ferroelectric phase rich in La. The Curie constant together with other dielectric parameters were also calculated .


Introduction
BaTiO 3 is one of the most useful ferroelectric materials because its attractive electrical properties can be tailored to meet strong demands for different electroceramic components.The variety of applications for which it is used arises from the possibility to design the electrical properties of ceramics by the addition of appropriate additive/dopants and to control the composition and microstructure through the sintering process [1][2][3][4][5].Grain growth behavior, electrical and dielectric properties such as permittivity and tangent loss are very sensitive to the type and amount of additive.Depending on the amount of dopants, the electrical resistivity can achieve a value as high value as 10 10 Ω m, measured in insulator ceramics, to 10 Ωm at room temperature in semiconductive ceramics.Besides niobium, yttrium and antimony, lanthanum is one of the very useful dopants that can be incorporated in BaTiO 3 [3,[6][7][8].Incorporation of a small content (0.20-0.30at%) La 3+ which replaces A sites in perovskite ABO 3 structure leads to a n-type BaTiO 3 semiconductor with a PTC effect and to significant change of the microstructure.Substitution of Ba 2+ with La 3+ requires the formation of negatively charged defects and possible compensation mechanisms exist _____________________________ *) Corresponding author: ljzivkovic@ef.ni.ac.yu depending on the La concentration regime: barium vacancies (VBa // ), titanium vacancies (VTi //// ) and electrons (e − ) [9,10].In heavily single (La) doped and donor-acceptor (La-Mn) co-doped BaTiO 3 ceramics, which are characterized by a small grain microstructure a high insulation resistance and life stability can be achieved.It has been reported that the lower donor concentration, named as grain growth inhibition threshold (GGIT) in La 2 O 3 doped ceramics occurs between 0.20 and 0.40 mol% [11].The appearance of secondary abnormal grains with (111) double twins, grains with curved or faceted grain boundaries are also common in BaTiO 3 ceramics.However, in addition to the various functions of additives, it has to be pointed out that the formation of a liquid phase above the eutectic temperature and the effect of Ba/Ti atomic ratio, may also influence the microstructural evolution.Sintering of TiO 2 -excess BaTiO 3 powder, which is often investigatied, is influenced by the BaTiO 3 -Ba 6 Ti 17 O 40 eutectic melt at 1332°C.In this case densification is assisted by a liquid-phase that becomes a predominant sintering mechanism at temperatures above the eutectic point [12].The microstructure features and electrical behavior are also related to intergranularly located "second phase" particles of La 2 Ti 2 O 7 that start to appear in samples with dopant levels higher than 0.8 at% La [6,12].
When small amounts of manganese (Mn), which acts as an acceptor dopant, are added to already donor doped ceramics, the PTC effect above the Curie temperature is enhanced and the life stability in ceramic multilayer capacitors is largely improved [13].Mn belongs to valence unstable acceptors which may take different valence states Mn 2+ , Mn 3+ or even Mn 4+ during the post-sintering annealing process.Mn 2+ is stable in a cubic phase above the Curie temperature and easily oxidized to the Mn 3+ state that is more stable in a tetragonal phase.Manganese as an additive, segregating at grain boundaries, can prevent exaggerated grain growth.For co-doped systems the formation of donor-acceptor complexes such as 2[La Ba • ]-[Mn Ti // ] prevents a valence change of Mn 2+ .Albertson et al. [14] reported that Mn 2+ ions could not be oxidized in the form of a donor-acceptor charge complex and stabilization of this ion depends on the donor concentration.
In this study La/Mn-doped ceramics with different amounts of La 2 O 3 were investigated regarding their microstructure and dielectric properties.Doped barium titanate samples were sintered at temperatures below and above the eutectic temperature.The effect of a secondary phase rich in La, together with the appearance of secondary abnormal grains on dielectric behavior in La-doped ceramics was also studied.

Experimental
The samples were prepared by a conventional mixed oxide sintering procedure starting from commercial BaTiO 3 powder (ELMIC BT 100, Ba/Ti=0.996±0.004,Rhone Poulenc) and reagent grade La 2 O 3 and MnO (Merck, Darmstadt) powders.The content of La 2 O 3 ranged between 0.10 and 5.0 at% La and the MnO content was kept constant 0.05 at% Mn.The specimens were denoted as 0.10La-BT for 0.10 at%La-doped BaTiO 3 and so on.Starting powders were ball milled in ethyl alcohol for 24 hours followed by drying at 200 o C for several hours.The powders were pressed into disks of 10 mm in diameter and 3 mm in thickness under 120MPa.The compacts were sintered at 1290 o , 1320 o and 1350 o C in air for two hours.The microstructures of as sintered or chemically etched samples were observed by scanning electron microscope JEOL-JSM 5300 equipped with EDS (QX 2000S) system.The average intercept length was obtained and then multiplied by 1.776 to estimate the average grain size [15].Prior to electrical measurements silver paste was applied on flat surfaces of specimens.Capacitance and tangents loss were measured using a HP 4276 LCZ meter in the frequency range 100 kHz-20 kHz.Variation of the dielectric constant with temperature was measured in the temperature interval from 20 to 180 o C .Electrical resistance was measured using the two points probe method with an electrometer Keithley 237-measuring unit.

Microstructure characteristics
The sintered densities varied from 70-80 % of theoretical density (TD), depending on the amount of additive, being higher for specimens sintered at 1350 o C. A densification retardation of La-doped BaTiO 3 can be attributed to the formation of a second phase rich in La, which is in accordance with literature La 2 Ti 2 O 7 [6] that effectively blocked the diffusion path during the initial stage of sintering.The evidence for a new phase is suggested through the analysis of corresponding EDS spectrum which clearly showed a region rich in La as can be seen in Fig. 1 for 1.0La-BT.Similar spectra, irrespective of temperature, were obtained for 0.5La-doped BaTiO 3 .It is worth saying that concentrations less than 1.0 wt% could not be detected by EDS attached to SEM, unless an inhomogeneous distribution or segregation of additive is present.EDS analysis did not reveal any content of Mn, thus an equal distribution of Mn through the specimen can be assumed.
Regarding microstructure features, even in low doped samples, such as 0.1La-BT, a small grained microstructure is observed, without any evidence for explicit bimodal grain growth.One of the possible reasons for the absence of a bimodal microstructure in these samples may be a high percentage of porosity.For a La 2 O 3 amount above the threshold value grain growth inhibition occurs and uniform and a fine-grained microstructure is formed, at 1290 o C and 1320 o C as illustrated in Fig. 2 with grain sizes ranging from 1.5 to 3 µm.
At 1350 o C the microstructure for specimens with 0.10 and 0.5 at% La is similar to the ones obtained for a lower sintering temperature, as illustrated in Fig. 3. On the other hand, the microstructural evolution in La-doped samples sintered at 1350 o C, especially for samples with 1.0 at% La (Fig. 4) was quite different from that observed in other samples.Regarding the temperature, the sintering process of La-doped BaTiO 3 may be separated into two different regions, the region below the eutectic (1332 o C) and above the eutectic temperature, bearing in mind that the eutectic temperature should be lower due to the presence of an additive.At 1350 o C liquid phase sintering, with in homogeneously distributed liquid phase, contributed to secondary abnormal grain growth, in contrast to the fine grain matrix.The fine grain matrix is related to the regions where grain boundaries are curved and normal grain growth occurs (Fig. 4a).The regions with long elongated grains and secondary abnormal grains with a domain structure are also evident in samples sintered at 1350 o C, as shown in Fig. 4b and Fig. 4c.
Similar microstructure characteristics, found in doped BaTiO 3 samples with 2.0 and 5.0 at% La sintered at 1350 o C, (Fig. 5) pointed out that secondary abnormal grain growth is closely related to sintering at temperatures near and above the eutectic temperature.
Abnormal grains, containing {111} twin lamellae, are observed randomly in very restricted regions, although the main part of the specimen shows a small sized microstructure.One of the peculiarities of microstructure features, observed in samples sintered above the eutectic temperature, are serrated features along grain boundaries and the appearance of a domain structure in secondary abnormal grains.Comparing the EDS spectra taken from the small grained regions and from the abnormal grains, it has been noticed that intensity of the Ti-peak is more pronounced in abnormal grains, suggesting that the abnormal grains contain more Ti than the small ones.Bearing in mind that the starting BaTiO 3 powder contained TiO 2 in excess, it is believed that fast precipitation of material through a Ti-rich liquid film at the grain boundaries leads to fast grain growth and to formation of secondary abnormal grains as suggested by [16].The grain size of secondary abnormal grains was in the range 10-30 µm.The phenomenon of fast and extensive grain growth that occurred in grains containing (111) double twins is called secondary abnormal grain growth in literature [16].Regarding the domain configuration, besides uniform and directional domain patterns (Fig. 4c), less defined domain patterns (Fig. 5) have also developed in abnormal grains.The estimated domain width from Fig. 4 and 5 is around 0.5 µm and the domain boundary layer ≈ 0.1 µm.According to Gaosheng and Roseman [9,17], the random domain structure is associated with a high resistance/dielectric behavior, which is found in our samples, and a well defined domain structure corresponds to low resistivity and a PTC effect.The formation of (111) double twins at higher sintering temperatures, T>1350 o C, was reported for 0.5 at% Nb-doped sample [8] and for pure BaTiO 3 in a narrow temperature range 1360 o C < T > 1370 o C.

Dielectric characteristics
Regarding electrical resistivity, all samples sintered at 1290 o -1350 o C, were yellow and dark yellow in color and are electrical insulators with the resistivity of 10 9 Ωm at room temperature.It is believed that the ionic compensation mechanism is exclusively involved and due to immobility of cation vacancies at room temperature the doped samples remain insulating.Also, in small grained microstructures, thickness of the grain boundary insulating layer becomes comparable to the size of grains and therefore the resistivity is very high [18].In addition, the effective carrier density is reduced owing to the presence of a Mn-acceptor.
Evaluation of dielectric properties were made by capacitance and dielectric loss measurements in the frequency range from 100 Hz to 20 kHz.According to the obtained results, the dielectric permittivity in La-doped samples, has a slightly higher value at low frequency and becomes nearly constant at frequencies greater than 3 kHz and the dielectric loss values (tan δ) are in the range 0.01-0.04[19].Variation of the dielectric constant as a function of temperature clearly manifested effects of additive content and microstructural composition (Fig. 6,7).Among the investigated samples, the highest value of dielectric permittivity at room temperature at 1 kHz (ε r =3100) and the greatest change with temperature was measured in 0.1La-BT sintered at 1350 o C that is characterized by a small grained and uniform microstructure.
The low dielectric constant in other samples can be attributed on one hand to a low density of samples, less than 80% TD, and on the other hand, to the formation of secondary abnormal grains (Fig. 4,5) that obviously lead to a decrease of dielectric permittivity.0.1% La/Mn BaTiO 3 0.5% La/Mn BaTiO 3 1.0%La/Mn BaTiO 3 2.0% La/Mn BaTiO 3 5.0% La/Mn BaTiO 3  Dielectric behavior of La-BaTiO 3 in the ferroelectric regime pointed out a rather significant influence of porosity and sintering temperature.In addition to these parameters, the role of the liquid phase and secondary phase is very complex.
La in at% ε r at 300K ε r at Tc The appearance of a new phase like La 2 Ti 2 O 7 and secondary abnormal grains at 1350 o C lead to a decrease in the dielectric constant.All specimens have a sharp phase transition and follow the Curie-Weiss law as illustrated in Fig. 8. for some of them.Data for other specimens were omitted for clarity although they have been used to calculate the Curie constant (C) and Curie temperature (T C ). Dielectric parameters are summarized in Tab.I.
The variations in dielectric constant in low and heavily doped La/Mn ceramics, sintered at the same temperature, are due firstly, to the different density of doped ceramics and secondly, to the presence of a La-rich phase and formation of secondary abnormal grains with a twin lamellae structure.Besides these two parameters, a third one must to be considered since we have La/Mn co-doped-ceramics.It is believed that variation of the oxidation state near the Curie point from Mn 3+ to Mn 2+ plays an important role in the resistivity of grain boundaries.The highest dielectric constant at room temperature was measured in 0.1%La-BaTiO 3 (ε r =3100), being as high as 4600 at Curie temperature.As can be seen from Tab. I. the Curie constant (C) increases with the increase of additive amount in La/Mn-modified BaTiO 3 ceramics and has an extrapolated Curie-Weiss temperature (T 0 ) down to very low temperature.

Fig. 4
Fig. 4 SEM images of 1.0La-doped BaTiO 3 sintered at 1350 o C (a) fine grain matrix, (b) and (c) abnormal grain growth with twin lamellae.SEM images are taken from the same specimen c)

Fig. 5
SEM images of La-doped BaTiO 3 sintered at 1350 o C (a) with 2.0 at% and (b) with 5.0 at% La.

Fig. 8 .
Fig. 8. Reciprocal values of ε r as a function of temperature.