Ferroelectric Phase Transition in Cu-doped BaTiO3 Crystals

The paper examines the ferroelectric-paraelectric phase transition in Cu:BaTiO3 single crystals. Using thermo-currents and dielectric measurements, we found for pure samples the phase transition temperature to be close to Curie temperature (Tc=120°C) with an anisotropy of the dielectric constantε.The lowering of temperature Tm, corresponding to the maximum of pyroelectric signal, with the increase of concentration of impurities is confirmed by measurements of ε. Moreover, the ε(T) curves show progressive broadening as well as higher peak with the increase of impurity concentration, and the transition becomes more and more diffuse. The same effect occurs when the samples are subjected to a laser irradiation (λ=5145Ǻ). During annealing, crystals lose copper and Tm is thereafter observed to increase again. BaTiO3 is a displacive ferroelectric (having high permittivity k-values) which undergoes a first order phase transition. Ions substitutions have an effect on the lattice dynamics attributed to charge transfer to Ti and a lowering of elastic forces in BaTiO3. Kewwords: Ferroelectric transition; BaTiO3 single crystals; Thermo-currents; Dielectric constant. © 2009 JSR Publications. ISSN: 2070-0237 (Print); 2070-0245 (Online). All rights reserved. DOI: 10.3329/jsr.v1i2.1876


Introduction
The remarkable impact of Fe, Co and Mn concentrations on the ferromagnetic and ferroelectric properties is attributed to the effective exchange coupling interaction between oxygen vacancies and the doping ions [1,2].It is also known that the unusual dielectric properties of BaTiO 3 make this ferroelectric compound at ambient temperature (Curie temperature T c =120°C) an important material for electrical engineering.Its photorefractive character is very useful in nonlinear optics for holographic storage [3].Indeed BaTiO 3 , SrTiO 3 and CaTiO 3 , the best-known perovskites of the ABO 3 type, find their way in the manufacture of multi-layer capacitors, thermistors and also transducers for their piezoelectricity [4,5].The ferroelectric materials are promising for microwave applications (capacities with strong permittivity, tunable filters and resonators) and modulators of frequency due to their strong permittivity (high-k) and their weak dielectric losses.The large variation of the dielectric permittivity at ambient temperature also proves to be an important characteristic.Indeed the reliability, effectiveness and improvement of these electronic devices are related to the structure and microstructure of thin-layersferroelectric materials [6].This paper examines the effect of Cu concentration on the ferroelectric properties such as the temperature of the phase transition and the dielectric constant in barium titanate (BaTiO 3 ).

Materials and methods
The entire measurements [7] of the thermo-currents and the dielectric permittivity presented here are developed in the Laboratory of Dielectrics of the University of Dijon where the single crystals are prepared using the method of KF flow [8].Crystallization was made during the cooling of a molten mixture of barium carbonate (BaCO 3 ) and titanium oxide (TiO 2 ) in a bath of potassium fluoride (KF).The crystals have the shape of triangular thin blades whose thickness varies between 6x10 -5 m and 15x10 -5 m according to the rate of doping.Known as butterfly wings with a surface area of about 0.5cm 2 , the ferroelectric axis c is generally perpendicular to the large face.
The samples doped with copper look yellow, becoming darker and darker, with the percentage x of impurities.Doping is carried out during the growth of crystallization.For doping with copper, Cu(NO 3 ) 2 ,3H 2 O is added and then TiO 2 subtracted, whereas doping with iron, Fe 2 O 3 is added and then TiO 2 subtracted.The operation is controlled by spectrophotometric analysis.Temperature T of the crystal rises linearly (with a rate b=dT/dt) through the ferroelectric-paraelectric phase transition and the current i p leads to the pyroelectric coefficient: f = dP/dT = i p /Sb.It is linked to a global polarization P, higher than the spontaneous polarization P s which is a characteristic of ferroelectric materials.This is due to the other kinds of polarization taking place in the sample: dipolar, and by space charge.At T=T m there occurs a maximum in the thermal variation of the current i M ; this maximum of i M corresponds to an inflection point in the curve of the polarisation versus temperature.The measurement of dielectric constants is performed using an impedence meter 1613-A, an audio-frequency oscillator of the type 1311-A, an amplifier and a detector of zero type 1232-A.Two very thin electrodes in silver paste are deposited on the crystal.The cooling of the oven is achieved by means (electric floodgates) of a temperature regulated cold nitrogen jet directed onto the tube of a steel cell on which the sample is located within the tube at 4mm from the sample.Temperature is measured by means of a chromel-alumel thermocouple and the rate of heating is programmed.Temperature accuracy is then about 0.5°C [7].

Results and Discussion
The samples, made of single domain c by a field E=500V/cm, are assembled according to one of the directions a or c for measurement.Let us call T M as the temperature at which the maximum of ε occurs.It corresponds to the passage of the quadratic to cubic symmetry phase.For the Cu-doped crystals, T M decreases according to concentration as shown on Fig. 1.We observe in Fig. 2 that on increasing the Cu concentration in the crystal, the value of ε at maximum increases whereas the ε(T) curve appears to exhibit a broader peak.Measurements of the thermo-currents provide results consistent with these data.The temperature corresponding to maximum of permittivity is observed to practically coincide with that of the maximum of thermo-current i p .We therefore identify both quantities, T M and T m .For two pure crystals from the same bath and assembled according to a and c axis, equal values of T M ≈120°C are measured (Fig. 1) and an anisotropy of ε is observed.The dielectric constant measured in the crystallographic a-direction, ε a (T) is larger than ε c (T) at any temperature (Fig. 2).The ratio ε a /ε c decreases from 16 at ambient temperature to 2 at the transition temperature and reaches its lowest value (1.1) in the non-polar phase.Merz [9] had found, for very thin crystals with a single field, a value ε a /ε c =20 and noted that this factor takes the value 1 at the transition at Curie point (120°C).Thus, the anisotropy is the largest in the region of the ambient temperatures.
The calculated difference ∆T between the temperatures T m for the pure crystal and the doped crystals, varies linearly with the percentage of Cu from x≈0.3 (Fig. 3).A broadening and flattening of the maximum of ε(T) and a shift of the Curie temperature T c have also been observed in Fe-doped BaTiO 3 ceramics [10,11].Similar effects also exist in the potassium tantalite niobate [12] and in several oxides with perovskite structure: Nb:KTaO 3 , Cu,La:PbTiO 3 [13].After annealing at 1300K for 150 hours, there is a loss of copper (% Cu becomes: y~0.55x) and temperature T m increases in Cu:BaTiO 3 crystals (Fig. 3).For a concentration y=0.2%, ∆T=12°C.Oxygen vacancies are created by heating, the gap is reduced as a result, and the Curie temperature increases T c [1].From ε -1 (T), Ouedraogo et al. [11] determined the Curie constants as 13×10 4 K in ferroelectric phase and 25×10 4 K in paraelectric phase (i.e., a ratio of 2 for 1.6% Fe:BaTiO 3 ).These values are not affected when the samples are subjected to a laser irradiation (λ=5145Ǻ).Photo-thermo-currents show a more diffuse transition with the discontinuity of dielectric constant ε a (T) less acute and a lowering of the temperature T M .These results are confirmed by Fe:BaTiO 3 with high percentages of doping (iron penetrates the samples more easily than copper) (Fig. 4).Irradiation influences the transition, the domains structure and thus the ferroelectric properties [1].The existence of an electric field induces a variation of the optical index in photo sensitive crystals which is the case during laser irradiation where charges transports take place.It is possible to suppress the phase transition at strong concentrations in the case of La: BaTiO 3 .The effect of La was observed by Raman diffusion, for which an additional mode appears with an increase of intensity with the concentration of La [14].The study confirmed the increase in conductivity under reduction treatment as due to the exponential increase in Cu 2+ defects.Substitution induces a transfer of charges of oxygen towards titanium (partial reduction of titanium) [15].Chen et al. [16] also concluded that Cu doping might be responsible for the metallic behaviour observed in Cu-doped BaTiO 3 .There is thus an influence of heterovalent substitutions on the dynamics of the lattice and conduction in perovskites [15].
For percentages higher than 0.3%, the determination of concentrations of various ions made it possible to deduce that the most probable site for the Cu 2+ ion is the Ti 4+ site [7].This results in a very marked Jahn-Teller effect with a distortion of the lattice [7].For high concentrations, the disorder increases by doping, giving a decrease in the activation energy W. In reduced crystals, W drops to zero when the conductivity increases.This could be understood on the basis of correlated electron hopping or of band broadening [18].
Displacements of ions are associated with a particular mode of vibration.In the Cochran theory of the soft mode, this mode is linked to the order parameter (the polarization) which in BaTiO 3 transforms like the irreducible representation T 1u of the high temperature symmetry group.The cancellation of an eigen-frequency of vibration of the solid at temperature T c corresponds to the disappearance of the force to which the atoms are subjected, that drives them to the equilibrium position in high temperature phase.The solid becomes "soft" (i.e.unstable) for this mode.A stability can be found only when the system is moving to a new phase.
The existence of the soft mode is characteristic of structural transitions [19].In the case of BaTiO 3 , the transition is displacive and the thermal variation of spontaneous polarization presents a discontinuity.In any case, the various models must take into account the modifications in the couplings between electrons and vibrations if the doping ion is located in the titanium site.This situation induces local symmetry changes and modifications of the couplings [20] and thus affects the properties of BaTiO 3 [21].
In view of miniaturization in microelectronics, perovskite materials with very high permittivity are interesting candidates to replace dielectrics currently used in the development of Metal/Insulator/Metal (MIM) capacitors.High dielectric constant porcelain composition contains mainly BaTiO 3 , but also contains by weight 1 to 5% of LaTiO 3 and 2 to 5% of Ta 2 O 5 providing a high dielectric constant of at least 3000.The maximum of ε is found when the size of grains is about 700 nm when BaTiO 3 is quadratic, i.e. ferroelectric and is thus not derived from the phase transition quadratic-cubic [22].
The origin and the microstructure of the material are important in the dielectric behaviour of barium titanate.T C and ε decrease with the size of grains when the diffusivity increases until it disappears for grains of about 100 nm [23,24].
Taking the polarization P r associated with the vectorial representation T 1u of the group O h (cubic phase) as the order parameter (following the Landau theory of second order phase transitions), it appeared in fact experimentally that P r exhibits a discontinuity at the transition point : the transition is then known to be of first-order [25], but close enough of second order to be still described by a Landau-type approach.

Conclusion
Experimentally, the Curie temperature T c can be determined by measuring the pyroelectric current i p or the dielectric permittivity ε versus temperature: T c is identified with the temperature T m of the maximum of i p and to the temperature T M of the maximum of the dielectric permittivity ε.The cubic-quadratic transition of BaTiO 3 is influenced by Cu and other dopings; T M decreases and transition becomes more diffuse for high rates of doping agents.Transition could disappear for very large quantities of inserted ions, such as Cu, Fe, La and Co.
The measured variations of ε -1 (T) are in agreement with the thermodynamic approach of Devonshire [26] for a first-order transition, with the Curie constants doubling when the system passes from the ferroelectric phase to the paraelectric phase in BaTiO 3 .These dielectric properties of barium titanate could be affected by several different parameters such as temperature, coating process, grains size, etc.The excellent opto-electrical properties (high-k, and large pyroelectric and non linear coefficients) of barium titanate make this ferroelectric eminently suitable for applications such as capacitors, pyroelectric detectors and nonlinear applications.However, many potential applications for BaTiO 3 require a thin-film form rather than a bulk crystal.

Fig. 1 .
Fig. 1.Variation of T M (dashed line) and T m (full line) against Cu concentration.