Vol. 124 (2013) ACTA PHYSICA POLONICA A No. 1

La-doped and La/Mn-co-doped Barium Titanate CeramicsM.M. Vijatovi¢ Petrovi¢a,∗, J.D. Bobi¢a, R. Grigalaitisb, B.D. Stojanovi¢a

and J. Banysb

aInstitute for Multidisciplinary Research, University of Belgrade, Kneza Vi²eslava 1, Belgrade, SerbiabFaculty of Physics, Vilnius University, Sauletekio al. 9, Vilnius, Lithuania

(Received November 27, 2012; in nal form March 27, 2013)

Barium titanate ceramics doped with 0.3 mol.% lanthanum and co-doped with 0.3 mol.% lanthanum and0.05 mol.% manganese were investigated. The powders were prepared by a modied polymeric precursor methodbased on the Pechini process. The ceramics were obtained by sintering at 1300 C for 8 h. The inuence of dopantson structural changes and grain size reduction was analyzed. The presence of dopants inuenced the tetragonalityof the barium titanate crystal structure. Reduction of polygonal grain size with dopant addition was noticed.In the doped ceramics, characteristic phase transitions were shifted to lower temperatures in comparison withpure barium titanate. The dielectric permittivity value showed the tendency of a slight increase with lanthanumaddition and further increase with adding of manganese. La as a single dopant increased the diuseness of phasetransitions indicating the formation of a diuse ferroelectric material but in the co-doped ceramics the phasetransition diuseness decreased. The resistivity of the co-doped ceramics was higher than for lanthanum dopedceramics, indicating possible segregation of manganese at grain boundaries that inuenced the total resistivity ofthe material.

DOI: 10.12693/APhysPolA.124.155

PACS: 81.20.Ka, 81.40.Ef, 77.80.B, 72.20.i, 84.37.+q

1. Introduction

Over the years barium titanate (BaTiO3, BT) has beenexceedingly investigated due to its very interesting prac-tical applications in capacitors, thermistors, varistors, en-ergy converting systems etc. The perovskite structure ofbarium titanate (ABO3) has the capability to host dif-ferent sized ions and the properties of this material couldbe tailored by doping. Doping is possible at both A andB sites and it depends on the valence and radius of thesubstituting ions [1]. The barium titanate crystal struc-ture, microstructure and dielectric properties could bechanged by incorporation of very low concentrations ofdopants into its lattice. Some dopants shift BT transitiontemperatures or induce broadening of the εT curve andmany of them cause diuseness of the ferroelectric phasetransition [2, 3]. The degree of diuseness of the phasetransition could show a continuous change from classicalferroelectric to diuse ferroelectric or to relaxor, depend-ing on the type and concentration of dopant incorporatedin the BT lattice.Lanthanum as a donor dopant commonly substitutes

Ba in the lattice of barium titanate. Since La3+ hasa dierent valence than Ba2+ this change produces acharge imbalance and induces n-type semiconductivity.Therefore, charge compensation requires the productionof electrons, electron holes or titanium (VTi) and oxygen(VO) vacancies in the material, causing modication ofelectrical properties. On the other hand, manganese is

∗corresponding author; e-mail: miravijat@yahoo.com

believed to substitute Ti4+ and acts as an acceptor withunstable valence, from Mn2+, Mn3+ to Mn4+ dependingon the oxygen partial pressure [4]. Some theoretical andexperimental studies have been done on the valence ofmanganese that could exist in the BT lattice [57]. Ina reducing atmosphere Mn2+ is likely to be found butit converts into Mn4+ in oxidizing conditions [5]. In airprocessed samples Schwartz et al. found both Mn2+ andMn4+ with no traces of Mn3+. Usually manganese actsas an acceptor-type impurity at the grain boundaries.For co-doped systems the formation of donoracceptorcomplexes, such as 2[LaBa•][MnTi′′] prevents a valencechange of Mn2+ to Mn3+ and has a benecial eect on re-duction of the dissipation factor. Formation of the samecomplexes was also found in cases of co-doping of BTwith W/Mn and Nb/Mn [7].In this paper, the focus was on determining the roles of

an individual dopant, La and joint eects of two dopants,La/Mn on the properties of BaTiO3 ceramics that haveshown very interesting behavior. The crystal structurechange and a correlation between microstructure devel-opment and dielectric properties of the obtained dopedceramics are reported.

2. Experimental

A modied Pechini process was used to preparenanopowders of barium titanate doped with 0.3 mol.%of lanthanum (BTL) and barium titanate doped with0.3 mol.% of lanthanum and 0.05 mol.% of man-ganese (BTLM). Barium acetate (Ba(CH3COO)2),Alfa Aesar, 99.0102.0%), titanium iso-propoxide (Ti[OCH(CH3)2]4, Alfa Aesar, 99.995%), lanthanum nitrate(La(NO3)3·6H2O, Alfa Aesar, 99.99%) and manganese

(155)

156 M.M. Vijatovi¢ Petrovi¢ et al.

oxide (MnO2, Merck, 99.8%) were used as starting ma-terials. Solutions of titanium citrate and barium citratewere prepared using ethylene glycol (EG) and citric acid(CA) as solvents (M (metal ion):CA:EG=1:4:16). Aftermixing of the obtained citrate solutions lanthanum ni-trate salt and manganese oxide were added. The trans-parent yellow solution was heated up to 140 C until itchanged to a dark-brown glassy resin. Decomposition oforganic resin was performed at 250 C for 1 h and 300 Cfor 4 h, when a black solid mass was formed. Ther-mal treatment of the formed precursor was performed at500 C/4 h, 700 C/4 h, 800 C/2 h and 850 C/2 h [8, 9].Nanopowders were uniaxially pressed into disks 12 mm

in diameter at a pressure of 196 MPa. Sintering wasperformed in air at 1300 C for 8 h with a heating rate of10 C/min.X-ray diraction measurements were carried out in or-

der to determine the formed crystal structure and latticeparameters (Philips PW1710 diractometer). Scanningelectron microscopy (Tescan VEGA TS 5130MM) wasused to analyze the microstructure of obtained ceramics.Average grain size was determined from scanning elec-tron microscopy (SEM) micrographs using linear inter-cept technique. The density of barium titanate ceramicswas calculated geometrically. Samples were prepared forelectrical measurements by polishing and applying silverelectrodes on both sides of the samples. Dielectric mea-surements were carried out using a LCR meter (model4284 A, HewlettPackard).Impedance measurements of BTLM ceramics were car-

ried out at 500 C in the frequency range 42 Hz5 MHzusing a HIOKI 3532-50 LCR HiTester. To obtain con-tinuous metallic contacts, Pt paste was deposited on thepolished surfaces of the ceramics. Collected data wereanalyzed using the commercial software package Z-view.

3. Results and discussion

Barium titanate pure and doped nanopowders withround shaped particles ≈ 5070 nm were prepared bya soft chemical method, a modied Pechini process [3].Synthesized powders were used to prepare ceramic sam-ples by pressing into pellets. In accordance with previousresults sintering was performed at 1300 C for 8 h [3].X-ray diraction measurements (Fig. 1) showed forma-

tion of a tetragonal crystal structure in barium titanateceramics identied by the appearance of characteristicdiraction peaks (JCPDS les no. 05-0626). Lattice pa-rameters were calculated using the Lsucri program anda crystal structure change was evident. The tetragonality(c/a) of the BTL ceramic was 1.0102, while for BTLM itwas lower than 1.0078. The decrease of tetragonality no-ticed after addition of dopants suggested the formationof defects in the BT lattice. Adding both lanthanumand manganese in the structure led to the appearanceof many more irregularities in ion packaging and sta-bilization of the pseudo-cubic structure [10, 11]. La3+

substitutes a Ba ion in the barium titanate lattice and

Fig. 1. X-ray diractograms of BTL and BTLM sam-ples sintered at 1300 C/8 h.

doping commonly induced the formation of defects suchas vacancies VTi and VO and possibly VBa in low con-centrations [10]. It was expected that La3+ as a smallerion would stabilize the cubic structure as predicted byGoldschmidt's tolerance factor. Doping with manganeseis rather complex due to the tendency of Mn to changevalence. It was proposed that Mn ions with the valence2+, 3+, and 4+ partly replace Ti4+ ions [4] in the BTlattice. Some authors stated that substitution of Ti4+

with Mn2+ or Mn4+ can occur without any defect for-mation or with formation of one oxygen vacancy V”O.Oxygen vacancy generation can also be noticed duringsintering where oxygen loss can be observed [5]. For-mation of donoracceptor complexes, such as 2[LaBa•][MnTi′′] is also quite possible [7]. These changes of crys-tal structure can inuence the dielectric spectrum of thismaterial.

Fig. 2. Micrographs of barium titanate specimens sin-tered at 1300 C for 8 h: (a) BT, (b) BTL, and (c)BTLM.

La-doped and La/Mn-co-doped Barium Titanate Ceramics 157

SEM micrographs showed a ne grained microstruc-ture in both ceramics with a uniform grain size distri-bution and high percentage of porosity (Fig. 2b and c).Dierent authors have found high porosity in BT sam-ples doped with lanthanum prepared by various methods,as well [12]. The density of both materials was around85%. It was observed that lanthanum inhibits graingrowth in comparison with pure barium titanate (grainsize ≈ 2.5 µm, density 92%) prepared by the same chem-ical method (Fig. 2a). The grain size was ≈ 10.7 µm forthe BTL samples and ≈ 0.5 µm for the BTLM ceram-ics, indicating the inuence of manganese on the furtherdecrease of grains. The obtained grain size was muchsmaller than that obtained for a similar composition ofBT ceramics (14 µm) prepared by a solid-state reac-tion [12]. A more homogeneous microstructure with auniform grain size distribution was formed in the BTLMdoped samples. This was also noticed by other authors,where with the increase of Mn content the microstruc-ture became more homogeneous [13]. Mn segregation atgrain boundaries and therefore inhibition of grain growthis also possible. According to the obtained microstruc-ture, it was expected that the microstructure formed inBTLM ceramics could enable better dielectric propertiesof the material.

Fig. 3. Temperature dependence of dielectric constantin the frequency range 50 kHz1 MHz for (top) BT,(middle) BTL, and (bottom) BTLM ceramics.

The phase transitions that correspond to TC−T(cubic-tetragonal) and TT−O (tetragonal-orthorhombic)(Fig. 3) in the temperature range from 223448 K werenoticed. The phase transition TO−R (orthorhombic--rhombohedral) was not identied because it is below thetemperature range of our measurements [14].

TABLE

Transition temperatures, dielectric permittivity at roomand Curie temperature, dielectric losses and diusenessfactor for the BT, BTL and BTLM samples at 100 kHz.

SampleTC−T

[C]TT−O

[C]ε′(Troom) ε′(TC) tan δ γ

BT 120 14 805 1280 0.013 1.18

BTL 114 9 1595 1655 0.08 1.25

BTLM 115 12 1902 3609 0.06 1.15

The position of the TC−T peak in doped ceramics wasshifted to lower temperatures in comparison with 120 Cfound for pure BT (Table). Literature data also con-rmed that addition of lanthanum as a dopant in thebarium titanate lattice could cause a shift of the TC−Tphase transition to lower temperatures [12, 14]. Shiftingof phase transitions is commonly connected with a grainsize change and in the investigated case it is noticeablethat lanthanum and manganese reduced the grain sizeof ceramics (SEM micrographs). A possible explanationof these phenomena can also be found in defect chem-istry. A lanthanum ion with a radius r(La3+) = 1.17 Åchanges a barium ion with r(Ba2+) = 1.49 Å in a bariumtitanate lattice [15].Two types of compensation mechanisms are possible

for added concentrations of lanthanum. First, lanthanumdonor doping could occur via an electronic compensationmechanism where electrons are formed

BaBa → LaBa + e′. (1)Next, the ionic, titanium-vacancy compensation mech-anism is also possible for concentrations of La below0.3 mol.% and it is presented by

4BaBa + TiTi → 4LaBa + V′′′′Ti . (2)The presence of La on a Ba site makes the tetragonalstructure weaker and possible generation of Ti vacancies(VTi) destroys TiOTi linkages. This occurrence leadsto lowering of TC. On the other hand, manganese inBT can exist in three valence states r(Mn2+) = 0.80 Å,r(Mn3+) = 0.72 Å, and r(Mn4+) = 0.67 Å and it com-monly substitutes Ti4+ due to its small ionic radiusr(Ti4+) = 0.75 Å [15, 16]. Some authors proposed themodel where moving of manganese ions is always carriedon TiTi chains in the BT structure. The presence ofMn on a Ti site also leads to disrupting of TiOTi link-ages responsible for ferroelectricity and hence loweringof TC. Literature data have shown that sintering in airat high temperatures could induce the formation of oxy-gen vacancies and in some cases lead to the appearance ofsemiconducting behaviour [17]. The formation of oxygenvacancies and conduction electrons can occur accordingto the reaction

158 M.M. Vijatovi¢ Petrovi¢ et al.

2OO → O2 + 2V′′O + 4e′. (3)The loss of oxygen during sintering could enable stabi-lization of acceptor Mn2+ ions and in combination withformed oxygen vacancies shift the TC. On the other hand,during cooling, stabilization of Mn4+ or Mn3+ is possiblewith a decrease of V′′O concentration and therefore againa Curie point shift. If stable donoracceptor complexes2[La•Ba][Mn′′Ti] are formed, the Mn2+ ion can no longerbe oxidized into Mn3+ or Mn4+ [7].Phase transitions TC−T for BTL and BTLM ceramics

were located at almost the same temperature. This couldindicate that lanthanum inuenced shifting of the Curiepoint but added concentrations of manganese did not af-fect its additional movement. The temperature positionsof phase transitions did not change with frequency buta certain frequency dispersion of dielectric permittivitywas noticed especially for BTL.The characteristic TC−T peak for pure barium titanate

was sharp (Fig. 3a). The shape of this phase transitionin doped samples was found to be broader, indicatingthe eect of dopants on dielectric properties. This is es-pecially evident for BTL ceramics. A certain degree ofphase transition diuseness can be noticed from the pre-sented diagrams. Therefore, the modied CurieWeisslaw was used to determine the degree of diuseness of thebarium titanate phase transition. The modied CurieWeiss law was presented by the following equation:

1/ε− 1/εmax = (T − Tmax)γ/C, (4)where γ and C are modied constants, εmax is the di-electric permittivity maximum at the transition temper-ature Tmax (TC−T) [18]. The constant γ gives informationabout the character of the phase transition and it variesbetween 1 in the case of a classical ferroelectric and 2 forrelaxor materials. The constant γ represents the slopeof the graph between ln(1/ε− 1/εmax) and ln(T − Tmax)and is an indicator of the diuseness or disorder of thephase transition (Fig. 4). The results obtained for dif-fuseness for BT, BTL and BTLM ceramics are presentedin Table. It can be noticed that lanthanum increased the

Fig. 4. ln(1/ε−1/εmax) and ln(T−Tmax) plots for BT,BTL, and BTLM ceramics at 100 kHz.

diuseness of the barium titanate phase transition butadding of manganese caused its decrease.

Dielectric permittivity values have also changed withdopant addition. In pure BT, the dielectric permittivitywas 805 at room temperature and 1280 at the Curie tem-perature. The results given in Table indicate a slight in-crease in dielectric permittivity with lanthanum additionand a further increase with adding of manganese. Theexistence of smaller grains in doped samples could be apossible reason for the dielectric constant increase [19].This is in agreement with the study performed by Arltet al. [20] where BT samples with a grain size ≈ 0.5 µmpossessed the highest dielectric constant value. Theseauthors consider that a plausible explanation is based onthe size of the ferroelectric domain structure. When thegrain size decreases, the size of the ferroelectric 90 do-mains will become smaller and hence the material willhave more but smaller domains.

Since the permittivity of BT ceramics is considered tobe the sum of a volume contribution and domain wallcontribution, the increase in permittivity of ne-grainedBT can be explained primarily by a large number of 90

domain walls per unit volume [20]. It can be assumedthat very low concentrations (up to 0.6 mol.%) of lan-thanum are completely incorporated in the BT latticeas proposed by other authors [19], inuencing the dielec-tric constant increase. The dielectric loss factor showedlower values for BTLM ceramics in comparison with BTL(Table). These results are in agreement with the fact thatmanganese is usually added with donor dopants in orderto reduce the dissipation factor [7, 21].

Regarding electrical resistivity, in pure barium titanateprepared by the same processing methods, the electricalresistivity was > 108 Ω cm, indicating BT insulating be-haviour. The addition of dopants into barium titanateusually leads to the appearance of semiconducting be-haviour and in some cases appearance of the positivetemperature coecient of resistivity (PTCR) eect.

The resistivity of a material depends on transport pro-cesses that occur in grains and grain boundaries. Freecarrier motion is often inuenced by the grain size andgrain size distribution, secondary phases, nature andplace of impurities, crystal lattice disorder, defects, etc.[2224]. Both doped ceramics (BTL and BTLM) werebluish, indicating the formation of n-type semiconduct-ing materials [25, 26]. Electrical resistivity of the BTLceramics at room temperature was 1.3× 105 Ω cm and itwas lower than 1.1 × 107 Ω cm obtained for the BTLMceramics.

Adding of lanthanum and manganese led to a grain sizereduction and formation of defects (VTi and VO vacan-cies, as well as La/Mn defect complexes [7, 10]). First,doping only with lanthanum induced the formation offree electrons and possibly VTi and VO that decreasedthe resistivity of the material. In BTLM ceramics man-ganese acted as an acceptor on Ti-sites and it was be-lieved to segregate at the grain boundaries.

La-doped and La/Mn-co-doped Barium Titanate Ceramics 159

Fig. 5. The complex impedance plane plot (Z′′−Z′) at500 C for BTLM ceramic.

In order to see the contribution of grain and grainboundary resistivity of the BTLM ceramic, an impedancespectrum at 500 C is given in Fig. 5. Looking veryclosely, one depressed semicircle indicates possible over-lapping of two semicircular arcs. The low frequency semi-circle corresponds to the grain boundary contributionand the high frequency semicircular arc is attributed tothe grain contribution. The equivalent circuit consistedof two parallel RC elements connected in series and theZ-view tting software was used to evaluate the values ofRg (grain resistivity) and Rgb (grain boundary resistiv-ity). The chosen equivalent circuit was indicated as suit-able for the investigated ceramics due to very ne ttingof experimental results. The grain boundary resistivitywas found to be much higher than grain resistivity.

Fig. 6. Variation of imaginary parts of impedance (Z′′)and modulus (M ′′) with frequency at 500 C for BTLMceramic.

In order to complement and verify the data obtainedby tting of the impedance complex plane plot (Z ′′−Z ′)the Z ′′f and M ′′f spectroscopic plots were used forthe extraction of grain and grain boundary resistivitiesof the BTLM material (Fig. 6) [27, 28]. Z ′′ spectra domi-nated by the grain boundary response and the Debye-like

peak can be used to estimate Rgb and Cgb. The obtainedRgb = 2.3× 106 Ω cm and Cgb = 0.5 nF are values simi-lar to those obtained from the complex impedance spec-trum (Fig. 5) conrming the high contribution of grainboundary resistivity to the total resistivity of the mate-rial. M ′′ spectra are dominated by the grain responseand Rg and Cg could be calculated from it. Since M ′′

spectra did not show a Debye-like peak for the impedancemeasurements at 500 C, these results were not obtained.The resistivity increased with manganese addition and

it can be attributed to a drop of carrier concentrationsince manganese could act as an electron trap at the grainboundaries. On the other hand, Mn3+ and Mn4+ aremore reducible than Ti4+ and therefore electrons weretrapped at these sites. The manganese concentrationwas very low and the hopping motion of trapped elec-trons from one Mn site to another was almost impossible.Thus, the conduction electrons were eectively localizedat these Mn sites, which resulted in a drop of the car-rier concentration and increase of the BTLM ceramicsresistivity. In order to see the appearance of the PTCReect measurements of resistivity vs. temperature are inthe scope of future investigations.

4. Conclusion

Barium titanate doped with La and co-doped withLa/Mn was prepared by a soft chemical method.A tetragonal structure was formed in both ceramics butlattice parameters for the BTLM ceramics showed stabi-lization of the pseudo-cubic structure due to formation ofmore irregularities in the BT lattice. A grain size reduc-tion in doped samples was noticed in comparison withpure BT ceramics (≈ 2.5 µm). The phase transitionsin doped ceramics were shifted to lower temperaturesshowing the well known inuence of dopants on dielectricspectra of barium titanate. The dielectric permittivityincreased slightly with lanthanum addition and further-more with adding of manganese. Co-doping with La/Mnincreased the dielectric constant and made the transitionsmore prominent. Therefore, the diuseness or disorder ofthe phase transition in the BTL was higher than in theBTLM, suggesting stabilization of the classical ferroelec-tric shape of the phase transition with manganese addi-tion. The resistivity of the BTLM ceramics at room andthe Curie temperature was higher than for the BTL ce-ramics, showing the inuence of doping with manganeseon electron carrier decrease. Impedance analysis provedthe high contribution of the grain boundary resistivity inthe total resistivity of BTLM ceramics, indicating possi-ble segregation of manganese at grain boundaries.Future work may lead to the development of novel ma-

terials with higher dielectric permittivity and a homoge-neous but porous structure that can possibly nd appli-cation in humidity sensors.

Acknowledgments

The authors gratefully acknowledge the Ministry ofEducation, Science and Technological Development of

160 M.M. Vijatovi¢ Petrovi¢ et al.

the Republic of Serbia for the nancial support of thiswork (project III45021) and COST MP0904.

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