Ceramics Li0.98-xTa1.004-x/5NixO3 (LTN) samples have been prepared, for  										  x{0;0.01;0.02;0.05;0.08;0.15;0.2}.  										  Ac impedance measurements were carried  										  out in the frequency range 1Hz-1MHz  										  and from 300°C to 900°C. The influence  										  of Ni doping on dc conductivity was  										  depicted. Through the ac impedance,  										  dielectric constant was deduced and  										  two dielectric relaxations have been  										  clearly identified at high  										  temperature. The low frequency  										  relaxation is attibuted to the space  										  charges, and the high frequency one to  										  ferroelectric dipoles. Systematic fits  										  have been done using the Cole-Cole  										  model. It’s appears for identified  										  relaxations, that at the temperature  										  close to Tc, the dispersion step Δ ε  										  is maximum ; the relaxation time τ is  										  thermally activated and also present  										  an extremum at Tc. The influence of Ni  										  doping on these relaxations, specially  										  the relaxation of the space charges  										  was examinedCeramics Li0.98-xTa1.004-x/5NixO3 (LTN) samples have been prepared, for x{0;0.01;0.02;0.05;0.08;0.15;0.2}. Ac impedance measurements were carried out in the frequency range 1Hz-1MHz and from 300°C to 900°C. The influence of Ni doping on dc conductivity was depicted. Through the ac impedance, dielectric constant was deduced and two dielectric relaxations have been clearly identified at high temperature. The low frequency relaxation is attibuted to the space charges, and the high frequency one to ferroelectric dipoles. Systematic fits have been done using the Cole-Cole model. It’s appears for identified relaxations, that at the temperature close to Tc, the dispersion step Δ ε is maximum ; the relaxation time τ is thermally activated and also present an extremum at Tc. The influence of Ni doping on these relaxations, specially the relaxation of the space charges was examined

Aboubakar, M.

Bennani, F.

English

Ceramics Li0.98-xTa1.004-x/5NixO3 (LTN) samples have been prepared, for x{0;0.01;0.02;0.05;0.08;0.15;0.2}. Ac impedance measurements were carried out in the frequency range 1Hz-1MHz and from 300°C to 900°C. The influence of Ni doping on dc conductivity was depicted. Through the ac impedance, dielectric constant was deduced and two dielectric relaxations have been clearly identified at high temperature. The low frequency relaxation is attibuted to the space charges, and the high frequency one to ferroelectric dipoles. Systematic fits have been done using the Cole-Cole model. It’s appears for identified relaxations, that at the temperature close to Tc, the dispersion step Δ ε is maximum ; the relaxation time τ is thermally activated and also present an extremum at Tc. The influence of Ni doping on these relaxations, specially the relaxation of the space charges was examined.Ceramics Li0.98-xTa1.004-x/5NixO3 (LTN) samples have been prepared, for  										  x{0;0.01;0.02;0.05;0.08;0.15;0.2}.  										  Ac impedance measurements were carried  										  out in the frequency range 1Hz-1MHz  										  and from 300°C to 900°C. The influence  										  of Ni doping on dc conductivity was  										  depicted. Through the ac impedance,  										  dielectric constant was deduced and  										  two dielectric relaxations have been  										  clearly identified at high  										  temperature. The low frequency  										  relaxation is attibuted to the space  										  charges, and the high frequency one to  										  ferroelectric dipoles. Systematic fits  										  have been done using the Cole-Cole  										  model. It’s appears for identified  										  relaxations, that at the temperature  										  close to Tc, the dispersion step Δ ε  										  is maximum ; the relaxation time τ is  										  thermally activated and also present  										  an extremum at Tc. The influence of Ni  										  doping on these relaxations, specially  										  the relaxation of the space charges  										  was examine

Revues Scientifiques Marocaines

M. J. CONDENSED MATTER                                   VOLUME 9,  NUMBER 1                                JULY  2007 Study of the Dielectric Relation and the Doping Effect on Conductivity in Lithium Tantalate  M. Aboubakara, F. Bennanib,* aDépartement de Physique, Faculté des Sciences, LGPE, BP 133 Kénitra, Morocco e-mail : aboumahamoudou@yahoo.fr bDépartement de Physique, Faculté des Sciences, LPMC, BP 133 Kénitra, Morocco e-mail : f_bennani@yahoo.fr *  Corresponding Author.  Ceramics Li0.98-xTa1.004-x/5NixO3 (LTN) samples have been prepared, for x∈  { }0;0.01;0.02;0.05;0.08;0.15;0.2 . Ac impedance measurements were carried out in the frequency range 1Hz-1MHz and from 300°C to 900°C. The influence of Ni doping on dc conductivity was depicted. Through the ac impedance, dielectric constant was deduced and two dielectric relaxations have been clearly identified at high temperature. The low frequency relaxation is attibuted to the space charges, and the high frequency one to ferroelectric dipoles. Systematic fits have been done using the Cole-Cole model. It’s appears for identified relaxations, that at the temperature close to Tc, the dispersion step εΔ is maximum ; the relaxation time τ  is thermally activated and also present an extremum at Tc. The influence of Ni doping on these relaxations, specially the relaxation of the space charges was examined.  Keywords : Impedance spectroscopy, dc conductivity, relaxation, space charge  I. INTRODUCTION       Lithium tantalate (LT) has in recent years been the object of intensive study, because of its large optic non-linearities and corresponding device potential in fields of electrooptic modulation, parametric oscillator, harmonic generator, etc…      LT compound is ferroelectric at room temperature. It’s uniaxial at all temperature, with only a single structural phase transition para-ferroelectric at about 660°C, the precise value depending on the Li/Ta ratio [1] and which is close to a second order one.     Although LT does not have the perovskyte structure, his  ferroelectric structure  belongs to the space group R3c and can be considered as a superstructure of the α-Al2O3 corundum structure with Li+ and Ta5+ cations along the c-axis [2].     According to Abrahams and Keve [3], ferroelectricity in this compound is due to the displacements of both the Ta and Li ions within the octaedric sites.    LT is well-known to be narrow range non stœchiometric compound, the solid solubility range extends from about 46 to 50.4% mole Li2 O at room temperature[4]. The Curie temperature Tc decrease linearly with decreasing Li2O concentration [1], [5], and with varying the doped oxide concentration [6], [7], [8].        Different defect models were proposed to account for the non-stœchiometry. Jebbari et al [9] studied the vacancy model in LT and found that the Li-sites vacancy model seems more probable than the Nb-sites vacancy model. The oxygen vacancy model was eliminated because he cannot describe the density variation in function of the composition [7].      Several studies have reported on the change in electrical conductivity of the Lithium tantalate solid solutions[10], [11], [12], [13], [14]. By complex impedance spectroscopy Huanosta et al [11] studied the variation of the conductivity as function of the stœchiometry in Li1-5xTa1+xO3 compounds. The LT non-stœchiometric samples present different kind of intrinsic defects to maintain the charge neutrality and therefore can be easily doped by the extrinsic cations , in this way various studies have been performed [15], [6], [16]. Ravez et al [15] studied the relaxor ferroelectric ceramics Li1-xCax(Ta1-xZx)O3 (x=0.15 and x=0.20), they identified a ferroelectric relaxation and determined his relaxation parameters . The relaxation frequency fr is minimum and the dispersion step is maximum at Tc. When x increase a higher conductivity associated with a lower activation energy and a decrease of the magnitude of fr at Tc are observed, while the unit cell volume become larger, Tc lower and the transition more diffuse. Joo et al [16] studied the crystallographic and dielectric properties of LiTaO3-based non-stœchiometric solid solutions substituted by trivalent ions (Li1-x M3+x/3 )Ta03, M=Al, Cr, Fe, In and x≤0.20. They showed that all phases obtained are ferroelectric at room temperature, the Curie temperature Tc decreases when the composition deviates from LiTaO3. Tc has been related to the size of the M3+ cations and to the axial ratio c/a. Torii et al [6] studied the evolution of  Tc as a function of doping in (Li1-xM2+x/2 x/2)TaO3 with M=Zn, Ni, Mg, Ca. They 9     77    ©2007  The Moroccan Physical and Condensed Matter Society 78                                                               M. ABOUBAKARa, F.BENNANI                                                     9 showed that the change of the Curie temperature was found to be closely related with the c/a ratio of the hexagonal cell. The determination of complex impedances and dielectric permittivities allows one to access to conductivity and dielectric constant and eventually to couple them.       In the present paper we study the modification of the electric properties particularly the dc conductivity with Ni doping and the influence of doping in the dielectric properties    II. EXPERIMENTAL       The samples used in our studies were prepared starting from Li2CO3 carbonate, Ta2O5 and NiO oxydes. The mentioned reagents weighted and mixed in the required amounts according to the following chemical reaction.    (0.49-x/2)Li2CO3+(0.502-x/10)Ta2O5+xNiO         Li0.98-xTa1.004-x/5NixO3+(0.49-x/2)CO2   Then they were fired in three steps of 10 hours; first at 600°C, 800°C and 1000°C. The obtained powder was crushed in the agate mortar and pelletized as disks of 13mm of diameter and about 1mm thickness. The pellets were then pressed under 2500 bars pressure, and finally sintered at 1200-1300°C for 4 hours.  Silver electrodes were deposited on the pellet surface by painting. The painted pellets were placed in a furnace at 600°C for 2hours. Ceramics were characterised by X-ray diffraction with a Philips  PW 1729 diffractometer using the Cu Kα wave-length, the spectre obtained were pure.  Ac impedance measurements were carried out using an impedance analyser ‘’Solartron1260’’ in the frequency range 1Hz-1MHz and from 300°C to 900°C .     All the prepared compositions were analysed in the Service Central d’Analyse of CNRS at Vernaison. The formulae obtained are reported in Table1. The number of vacancies was calculated by subtraction of the amount of cation sites, considering a main substitution mechanism 5Li++Ta5+↔5Ni2+. The errors in the formulae obtained were estimated to be about 0.8% for Li, 0.1% for Ta, and 0.5% for Ni.                %Ni              Experimental formulae                   Proposed formulae                                              0               Li0.98Ta1.004O3              [Li        0.98Ta0.004  0.016][Ta]O3                1               Li0.97Ta1.002Ni0.01O3              [Li0.97Ta0.002Ni0.01  0.018][Ta]O3                  2               Li0.96TaNi0.02O3              [Li      0.96Ni0.02  0.02][Ta]O3                  5               Li0.93Ta0.994Ni0.05O3              [Li0.93Ni0.044  0.026][Ta0.994Ni0.006]O3                8               Li0.9Ta0.988Ni0.08O3               [Li0.9Ni0.068  0.032][Ta0.988Ni0.012]O3               15              Li0.83Ta0.974Ni0.15O3              [Li0.83Ni0.124  0.046][Ta0.974Ni0.026]O3                20              Li0.78Ta0.964Ni0.2O3               [Li0.78Ni0.164  0.056][Ta0.964Ni0.036]O3 Table 1. Chemical formulae obtained by analysis and proposed formulae according to Iyi et al model [17], where [.] represents the cations sites and  denotes the vacancies.   III. ELECTRICAL STUDY      It was found that is most convenient to plot the Impedance diagram results at each temperature in the complex plan plot Z’’vs Z’ ; typical result is shown in Fig.1.  0,0 2,0x104 4,0x104 6,0x104 8,0x104 1,0x1050,07,0x1031,4x1042,1x1042,8x1043,5x1044,2x1044,9x1040,0 5,0x106 1,0x107 1,5x107 2,0x1070,02,0x1064,0x1066,0x1068,0x1061,0x1070 500 1000 1500 2000 2500 3000 3500 40000400800120016002000(b) LTN2%670°C640°CZ"Z'103Hz 102Hz1MHz1Hz(a) LTN2%400°CZ"Z'(c) LTN2%900°CZ"Z' FIG. 1 : Impedance diagram of  2%Ni doped LT measured at fixed temperatures  At the temperature well below the Curie temperature (~ 660 °C), the impedance data for Li0.98-xTa1.004-x/5NixO3 take the form of single arc and appears to pass through the origin as shown in Fig1-a, this arc represents the bulk response of our 9                                                Study Of The Dielectric Relaxation And The Doping …                                     79 sample. The low frequency effect such as a spike, could be attributed to polarisation at the electrode-sample interfaces involving that conduction are ionic and charge carriers are probably the Li ions because of its relative feeble mass. However evidence for such polarisation behaviour, associated with ionic conduction is found in the materials which contain vacancies as our cases.      In the region of the Curie temperature, Fig.1-b ; data is regarded as comprising two semi-circles. The additional semi-circle is due to the polarisation mechanisms such as charge storage in the spontaneous polarisation processes responsible for the ferroelectricity [11].    At the higher temperature the arcs are incomplete at the high frequencies as shown in Fig.1-c, this is due to the lack of data caused by the frequency limit of the ‘’Solartron1260’’.     The value of dc resistance R is given by the low frequency intercept of the semi-circle on the real Z’ axis. The impedance data for Li0.98-xTa1.004-x/5NixO3 were analysed in this way and values of dc resistance are determined.     In order to examine the influence of Ni doping on dc conductivity of our samples we have represented at fixed temperatures its evolution with Ni content in Fig.2. Three domains of variation of dc conductivity appear, separated by two critical concentration of Ni: x=0,02 and x=0,15.   0 5 10 15 20 2510-710-61x10-51x10-410-310-2T =500°CT =400°CT =300°C σ dc(Ω-1m-1)% Ni FIG. 2 : Evolution of dc conductivity with Ni content at fixed temperature  For x 0,02 ; assuming that for this range of doping, the Ta ions in excess occupy the Li-sites [18] ; the O-octaedra of elementary cell containing Ta ions in Li-sites would smaller than that the O-octaedra filled by the Li ions (R≤Ta = 0.64Å < RLi = 0.76 Å). This deranges the structure and complicates the motion of Li ions; consequently we obtain the decrease of dc conductivity. For 0,02 x≤ 0,15; dc conductivity increases with Ni content. Indeed, there are no Ta ions in Li-sites [18], the number of vacancies increases with the introduction of Ni, also Li and Ni ions have approximately the same ionic radius (R≤Ni = 0.69 Å) ; the motion of both Li and Ni ions contributes simultaneously to conduction, and increases the dc conductivity. When the Ni concentration reaches to x=0,15; an insulating behaviour arises and dc conductivity decreases. So, we think that for high concentration of defects, the charge density at the grain boundaries and at the electrode-sample interface becomes so elevated that the blocking effect of these interfaces takes away the thermal diffusion of charge carriers.  IV. DIELECTRIC STUDY     In Fig.3 we have represented the temperature dependence of the dielectric permittivity for two compositions.   700 800 900 1000 1100 120001x1032x1033x1034x1035x103LTN2 (2%Ni)LTN1 (1%Ni)Tc=930 KTc=938 Kε (1 KHz)T (K) FIG. 3 : The temperature dependence of the dielectric permittivity for two compositions.  These curves present an extremum at the ferro-paraelectric temperature transition Tc. For high temperatures we observe the increase of the permittivity, this is due to the ionic conductivity component of the permittivity.  From the electrical response of our samples through the ac impedance, we have deduced their dielectric response. We have represented the frequency dependence of the real part of the dielectric constant in Fig.4. In the range of frequencies (1Hz-1MHz), and for all Ni compositions, two dielectric relaxations were clearly identified at high temperature.    FIG. 4 : The frequency dependence of the dielectric constant of the data for 15%Ni, The solid line represents the fit , circle and triangle are the 80                                                               M. ABOUBAKARa, F.BENNANI                                                     9 experimental data presented respectively at 400°C and 700°C    Regarding typical domain of associated frequencies, we have attributed the high frequency relaxation to the ferroelectric dipole such as the movement of the Li+ ions [19]; and the low frequency relaxation is due to the localisation of the free charges at the metal-dielectric interface and eventually at the grain boundaries. Indeed free carriers are stored at the metal-dielectric interface and at the grain boundary leading to a space charge, this effect is well known in number of dielectric materials [20]; the application of an electric field breaks the symmetry of this space charge and the macro-dipole is thus created.    In order to identify physical mechanisms associated to these relaxations, we have realised fitting curves using the following formula:                      ( )( )( ) ( ) ( )∑= −−−∞+⎟⎠⎞⎜⎝⎛+⎥⎦⎤⎢⎣⎡⎟⎠⎞⎜⎝⎛+Δ+=2,1 1211,2sin212sin1iiiiiiiiiiiαααωτπαωτπαωτεεωε                                        Which represents the real part of the response as sum of the two dielectric relaxations according to the Cole-Cole model (i=1 points to the relaxation of the space charge, and i=2 to the relaxation of the ferroelectric dipole). Fig.4 shows for instance the fitting curves for two compositions.     From these fittings, the relaxation parameters have been determined, typical temperature dependence of these parameters are presented in Fig.5&6. It appears for identified relaxations that at the temperature close to Tc, the dispersion stepiisi ∞−=Δ εεε ; i=1,2 ; is maximum involving that the relaxor force is maximum.  500 600 700 800 900 1000 110001x1052x1053x1054x1055x1056x1057x105LTN2%LTN1%Δε1T(°K)  FIG. 5 : Evolution of the dispersion step of the relaxation of the space charge with the temperature for two studied concentrations of Ni.  The relaxation time of the relaxation of the ferroelectric dipole is thermally activated and also presents an extremum at Tc as consequence of the ferroelectric soft mode vibration of the Li+ ions [19], which can oscillate from one side to another of an octahedral face.  0,9 1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8-6-5-4-3-2-1LTN2%LTN1%Logτ21000/T(°K-1)     FIG. 6 : Evolution of the relaxation time of the dipole with the inverse of temperature for two studied concentrations of Ni.   In order to  examine the influence of Ni doping on dielectric properties particularly the dielectric relaxation associated to space charges, we have represented at fixed temperature (T=700°C) the evolution of its relaxation time with doping in Fig.7.   0 5 10 15 200,000,050,100,150,200,250,300,35 T=700°Cτ 1%Ni FIG. 7 : Evolution of the relaxation time of the space charge with Ni content at fixed  temperature.  It appears that Ni introduction slows down linearly this relaxation; also at this temperature, dc conductivity increases linearly with Ni content as depicted in Fig.8. Indeed, the ions experience the combined influence of the field which tends to accumulate the charges at interfaces, and thermal diffusion which tends to oppose this accumulation. So we think that the Ni introduction increases linearly the charge density at the metal-dielectric 9                                                Study Of The Dielectric Relaxation And The Doping …                                     81 interface and eventually at the grain boundaries, through the dc conductivity; this reinforces the macro-dipole and renders its reorientation more difficult and therefore slower.   0 5 10 15 200,00100,00150,00200,00250,00300,00350,00400,0045 T=700°Cσ dc(Ω-1m-1)%Ni FIG. 8 : Evolution of dc conductivity with Ni content at fixed temperature   One notices that the evolution of dc conductivity with Ni content at T=700°C is not rigorously linear; in the small concentration of Ni (x  0,02), we have a considerable deviation from a linear law as a consequence of the substitution of the Li ions by the Ta ions and related phenomena discussed earlier in the electrical study paragraph. ≤ V. CONCLUSION      In the region of the ferroelectric Curie temperature, complex impedance plot shows evidence for an additional semi-circle associated with the capacitance of the ferroelectric polarisation processes. According to the substitution mechanism described by Iyi[17], for %Ni≤2%, Ta ions in excess and Ni ions occupy the vacants Li ions sites ; dc conductivity decrease with Ni content. Beyond 2%, Ni ions occupy simultaneously the vacants Li and Ta ions sites, dc conductivity increases with Ni content, until at15%, we have the decrease of dc conductivity. Two dielectric relaxations have been clearly identified at high temperature. The low frequency relaxation is attributed to the space charge and the high frequency one to ferroelectric dipole. Systematic fits have been done. It appears for identified relaxation that the relaxation time is thermally activated and presents an extremum at Tc; also at Tc the relaxor force is maximum. At T=700°C, the Ni doping slows down the relaxation of the space charge.       BIBLIOGRAPHY  [1]-Y. Fujino, H. Tsuya and K. Sugibuchi, Ferroelectrics 2, 113 , 1971 [2]-M. E. Lines and A. M. Glass ‘’Principles and applications of ferroelectrics and related materials’’, clarendon press. Oxford, 1979. [3]-S.C. Abrahams and E. T. Keve ; ferroelectrics, 2, 129, 1971 [4]- Paul M., Tabuchi M., A.R. West, chem. matter,1997, 9, 3206. [5]-Ballman A.A., H.J.Livinstein, C.D.Cupio, and H.J.Brown, Amer. Ceram. Soci., 1967, 50, 657. [6]-Y. Torii, T. Sekiya and T.Yamamoto, Kei Koyabachi, and Yoshihiro Abe, Mat. Res. Bull ;V18, pp1569-1574,1983. [7]-Iyi N., K. Kitamura, F. Izumi, J.K. Yamamoto, T. Hayachi, H. Asano, S. Kimura, J.of solid state chem.101(1992) 340 [8]-Abrahams S.C. and Marsh P., Acta cryst. 1986, B42, 61. [9]- S. Jebbari, F. Bennani, A. Jennane and N. Masaif, Ferroelectric letters 32 :1-16, 2005 [10]-R. L. Barns and J. R. Carruthers, J.appl Crystallog V3, P395, 1970. [11]-A. Huanosta and A. R. West, J. apply phys 61 (12), 1987. [12]-D. C. Sinclair and A. R. West Physical review B,V39, N18,1989 [13]-Tomeno I., S. Matsumura, Phys. Rev, B38 (1988)606. [14]-D. Ming, J.M.Reau, J. Ravez, J. Gitae, and P. Hagenmuller, J. solid state chem., 1995, 116, 185  [15]-J.Ravez, J. M. Reau, H. B. In and M. Maglione, ferroelectrics, 1992, vol127,pp149-154. [16]-G. Joo, J. Ravez, P. Hagenmuller, Revue de chimie minerale, t22, p18,1985. [17]- Iyi, N. Kitamura, K. Yajima, Y. Kimura, S.furukawa, Y. and Sato, M., J.of solid state chem.,1995,   118,148. [18]- F. Bennani, E. Husson, Journal of European Ceramic Society,2001. [19]-J.Ravez, G. T. Joo, M. Dong et J.M. Reau ; phys. Stat. Sol146, K71(1994). [20]- A.K. Joncher, Dielectric relaxation in solids (Chelsea, New York, 1983).                82                                                               M. ABOUBAKARa, F.BENNANI                                                     9  

Study of the Dielectric Relation and the Doping Effect on Conductivity in Lithium Tantalate

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Study of the Dielectric Relation and the Doping Effect on Conductivity in Lithium Tantalate

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