Structural and dielectric study of the Na0.5Bi0.5TiO3–PbTiO3 and K0.5Bi0.5TiO3–PbTiO3 systems Omar Elkechai, Pascal Marchet, Philippe Thomas, Michel Manier and Jean-Pierre Mercurio* L aboratoire deMate�riaux Ce�ramiques et T raitements de Surface, URA CNRS no 320, Faculte� des Sciences, 123, Avenue Albert-T homas, 87060 L imoges Cedex, France A study of the Na0.5Bi0.5TiO3–PbTiO3 and K0.5Bi0.5TiO3-PbTiO3 systems has been carried out using X-ray diffraction, differential scanning calorimetry and dielectric measurements.The limits of the rhombohedral (Na0.5Bi0.5TiO3-rich side) and tetragonal (PbTiO3-rich side) solid solutions have been determined, as well as the evolution of their lattice parameters as a function of composition and temperature. Ceramic materials have been prepared by natural sintering (1090–1220 °C; 0.5 h) of powders obtained by solid-state reaction (900–1000°C; 20 h) of the corresponding oxides and carbonates. The dielectric permittivities of these materials have been measured in a wide frequency range for temperatures between 20 and 800 °C.The results showed that they all are ferroelectric at room temperature and exhibit either a diffuse, probably second-order phase transition, or a sharp, firstorder- like phase transition from the ferroelectric to the paraelectric state, depending on the composition.Lead titanate–zirconate ceramics (PZT) are currently the most The powders were pressed into discs of diameter 10 mm and thickness 1 mm and were sintered in air at 1090–1220 °C, used ferroelectric materials in the field of piezoelectric applications.A noticeable feature of these materials is the occurrence depending on the composition. The thermal cycle consisted of heating at 5°C min-1 to the highest temperature (dwelling of a morphotropic phase boundary (MPB) in the phase diagram which separates tetragonal and rhombohedral ferro- time 30 min) followed by natural cooling to room temperature in the oven. Samples with 90–95% of the theoretical density electric regions.Solid solutions with compositions close to the MPB present the best electromechanical properties. were obtained. After polishing the major faces, they were coated with a low-temperature silver or gold paste fired at In the research of new complex systems with interesting piezoelectric characteristics, care should be taken that thermal 600°C for 10 min, aged overnight at 100 °C and left for 24 hours at room temperature before the measurements.and time stability of the properties is connected with the value of the Curie temperature of the components. Previous works Low-frequency dielectric measurements were carried out between room temperature and 1000°C (at increasing and have shown that in most cases this corresponds to the situation where the MPB is located between tetragonal and rhombo- decreasing temperature) at chosen frequencies in the range 10 kHz–5 MHz using a HP 4194A impedance analyser.hedral phases.1 Up to now, results concerning systems other than PZTs have only been devoted to either Na0.5Bi0.5TiO3–(Sr,Ba)TiO3 or the low-lead range of Na0.5Bi0.5TiO3–PbTiO3.2–6 A study Results was therefore conducted on the whole Na0.5Bi0.5TiO3–PbTiO3 Structural characterisation (NBT–PT) and K0.5Bi0.5TiO3–PbTiO3 (KBT–PT) systems, which are both likely to present MPBs.Actually, at room At room temperature, NBT is rhombohedral (a=0.3891 nm, temperature NBT is rhombohedral (Tc#320 °C), KBT is tetra- a=89.6°) whereas KBT and PT are tetragonal with the gonal (Tc#380 °C) as is PT (Tc#490 °C).7 This paper presents following lattice parameters; KBT, a=0.3918 nm, c= the results concerning the preparation of some materials 0.3996 nm; PT, a=0.3899 nm, c=0.4155 nm.belonging to this family as well as their thermal and dielectric properties. NBT–PT system. XRD data showed two (Na0.5Bi0.5)1-xPbxTiO3 solid solution ranges; rhombohedral on the sodium-rich side and tetragonal on the lead-rich side, Experimental separated by a biphasic region for 0.10<x<0.18.This result is somewhat different from that obtained by Takenaka et al. Polycrystalline compounds were prepared by solid-state reaction of the corresponding oxides or carbonates. Stoichiometric and recently by Park and Hong, who did not find a biphasic range, but found a morphotropic phase boundary at x= mixtures of reagent grade TiO2, Bi2O3, Na2CO3, K2CO3 and PbO (or PbCO3) were mixed thoroughly and calcined in 0.13–0.14 between rhombohedral and tetragonal symmetry at room temperature.4,6 The observed discrepancy of the present alumina crucibles between 900 and 1000°C for 20 h.Further calcinations were necessary to achieve complete reaction.results with respect to other works could be attributed to the different preparation processes, especially the annealing con- Phase characterisation and phase boundary limits were determined by X-ray diffraction (XRD) with a Siemens D5000 ditions. Fig. 1 shows the evolution of the room-temperature lattice parameters as a function of the composition as well as diffractometer (graphite monochromator, Cu-Ka radiation). The lattice parameters were refined by a least-squares method.the c/a ratio within the tetragonal range. As expected, they increase with x according to the increase of the mean ionic The structural evolution of the compounds with temperature was observed using a high-temperature X-ray (HTXRD) radii which change from 0.134 nm (Na,Bi) to 0.147 nm (Pb).Nevertheless, in the tetragonal region, the c parameter increases attachment (Anton Parr HTK10) working between room temperature and 1000°C. Differential scanning calorimetry more strongly than a, leading to a change of the tetragonality from 1.02 (x=0.18) to 1.06 (x=1). This result is slightly (DSC) analyses were performed in air using a Netzsch STA 409 DSC device.different from the parent system NBT-KBT in which the J. Mater. Chem., 1997, 7(1), 91–97 91Fig. 3 shows the thermal evolution of the lattice parameters of tetragonal (Na0.5Bi0.5)1-xPbxTiO3 solid solutions with x= 0.30, 0.50, 0.70 and 1. In each case, increasing temperature causes the tetragonal lattice parameters to vary in opposite senses: i.e.up to the transition temperature a increases and c decreasesalmost monotonously. Nevertheless, the way in which the parameters change between room temperature and the transition temperature is dependent on the composition. On the left-hand side of the tetragonal domain, a and c reach almost the same value corresponding to the cubic lattice parameter at the transition temperature as shown in Fig. 3 (left-hand side). On the right-hand (PbTiO3-rich) side the thermal variation of the lattice parameters at the transition shows a strong discontinuity, which is clearly visible when the lead content is higher than 0.5. Fig. 1 NBT–PT system. Room-temperature lattice parameters vs. composition. KBT–PT system. As expected, XRD data showed that KBT and PT give rise to a full range solid solution with tetragonal symmetry.The evolution of the room-temperature lattice par- tetragonal distortion does not vary greatly with the composiameters of (Na0.5Bi0.5)1-xPbxTiO3 solid solutions as a function tion within the tetragonal domain.8 of composition is given in Fig. 4. As Pb is substituted for Variable-temperature XRD experiments were performed on (Na,Bi), the a parameter of the tetragonal cell decreases slightly several samples belonging to both single-phase domains but the c parameter increases strongly increases, both quasi- (rhombohedral and tetragonal) up to temperatures well above linearly, leading to a more and more distorted unit cell: the the Curie temperature.tetragonality, c/a, changes from 1.02 (KBT) to 1.06 (PT).The thermal evolution of lattice parameters and volume for The thermal evolution of the lattice parameters of some Na0.5Bi0.5TiO3 and (Na0.5Bi0.5)1-xPbxTiO3 (x=0.03, 0.05 and selected compositions of the system are given in Fig. 5. The 0.09 are presented in Fig. 2 as examples of rhombohedral solid behaviour is similar to that observed for the tetr solid solutions. In the temperature range studied, the parameters solutions of the NBT–PT system; the higher the lead content, show a very slight monotonic increase with increasing tempera- the steeper the discontinuity of the lattice parameters (cf.ture, and there are no anomalies at temperatures close to the Fig. 3). expected structural phase transitions. This is because the lowtemperature rhombohedral unit cell is not strongly distorted Dielectric properties with respect to the prototype high-temperature cubic cell.This result is similar to that observed previously in the NBT–KBT The dielectric properties of the ceramic materials were measured at several frequencies between 1 kHz and 10 MHz. In system.8 Fig. 2 NBT–PT system: rhombohedral range. Lattice parameters and cell volume vs.temperature for (Na0.5Bi0.5)1-xPbxTiO3 : x=0 (a), 0.03 (b), 0.05 (c) and 0.09 (d). 92 J. Mater. Chem., 1997, 7(1), 91–97Fig. 3 NBT–PT system: tetragonal range. Lattice parameters and cell volume vs. temperature for (Na0.5Bi0.5)1-xPbxTiO3: x=0.30 (a), 0.50 (b), 0.70 (c) and 1 (d). In the tetragonal range, the sharpness of the permittivity maximum becomes more and more pronounced as the lead content is increased.KBT–PT system. In contrast to the previous system, KBT–PT does not exhibit any lack of miscibility over the whole composition range. As a consequence, one expects that the dielectric properties, especially the thermal variations of the dielectric permittivities, will have continuous behaviour from KBT to PT. Fig. 7 shows the thermal variations of the dielectric permittivities and losses at 1 MHz of compositions within the KBT–PT system.As observed already for the tetragonal compositions of the NBT–PT system, the curves e(T ) show a progressive evolution from a diffuse maximum (KBT) to a very sharp one (PT). Fig. 4 KBT–PT system. Room-temperature lattice parameters vs. composition. Discussion general, the results showed no significant frequency dispersion of either dielectric permittivity or loss. For clarity, only the The dielectric permittivities of NBT and KBT exhibit broad maxima at 320 and 380 °C respectively, undoubtedly connected results obtained at 1 MHz will be given hereafter.with the transition towards the cubic paraelectric state.7 In contrast,PT is known to present asharp permittivity maximum NBT–PT system.The thermal variations of the dielectric permittivities and losses at 1 MHz of some compositions of at 490°C. As the compositions at the extremes of the systems under investigation show well established ferroelectric behav- the NBT–PT system are given in Fig. 6A (x=0, 0.03, 0.08 and 0.09—rhombohedral) and B (x=0.19, 0.20, 0.30 and 0.60— iour at room temperature, the discussion of the thermal evolution of the structural and dielectric characteristics over tetragonal).They clearly show different behaviour according to the symmetry: rhombohedral low-lead content materials the whole composition ranges will be conducted in terms of the phenomenological approach developed by Devonshire exhibit a diffuse character whereas a sharp evolution of the permittivity is present in tetragonal lead-rich materials.after the Ginzburg–Landau theory of phase transitions.9 A main consequence of this theory is that the thermodynamical The rhombohedral solid solutions exhibit the same behaviour as similar materials in the NBT–KBT system.8 The nature of the phase transition of a ferroelectric material can be derived qualitatively from the evolution of the reciprocal temperature of the maximum permittivity is lower than that for pure NBT, but is almost composition independent along permittivity below and above the transition temperature, Tt.For a so-called first-order phase transition, the ratio r of the the rhombohedral range. J. Mater. Chem., 1997, 7(1), 91–97 93Fig. 5 KBT–PT system. Lattice parameters vs.temperature for (K0.5Bi0.5)1-xPbxTiO3: x=0.05 (a), 0.20 (b), 0.40 (c) and 0.90 (d ). slopes of the reciprocal permittivities, e-1(T<Tt)/e-1(T>Tt), over the whole solubility range for NBT–PT and for x>0.20 should be ca. 4, whereas it should be only 2 for a second-order in the case of KBT–PT. The Curie constants calculated at high phase transition. In addition, for a second-order phase trans- temperature are in the range 2–2.5×105 K, as observed comition, the Curie temperature is very close to the temperature, monly for one-dimensional ferroelectrics (e.g. perovskites, tung- T0, extrapolated from the high temperature variations of e-1, sten bronzes).When the lead content increases to PT the whereas this extrapolation always leads to a T0 value less than thermal variations of the permittivities change gradually from Tc for a first-order phase transition.a diffuse to a non-diffuse character (Fig. 6B and 7), as do the The overall properties of the materials under investigation slope ratios r of the reciprocal permittivities which change are strongly dependent on their crystal structures, either from 2.5 to 4 (Fig. 8). After Devonshire, this behaviour should rhombohedral for the (Na,Bi)-rich side of NBT–PT, or tetra- correspond to a progressive evolution from a diffuse, second- gonal for both the lead-rich side of NBT–PT and the whole order to a sharp, first-order phase transition as the composi- composition range of KBT–PT. The discussion will be focused tions approaches PT. This assumption should be supported separately on the properties of materials with (i) rhombohedral by the thermal evolution of the lattice parameters.At low lead and (ii) tetragonal structures. contents, the transition from the tetragonal to the cubic symmetry occurs gradually by a mere convergence of the (i ) Rhombohedral solid solutions: (Na0.5Bi0.5)1-xPbxTiO3 lattice parameters a and c towards the value of the parameter (0<x0.09) of the cubic unit cell at the transition temperature.So, there is no volume change at the transition. However, for x>0.40, A careful examination of the thermal evolutions of the permitthere is a discontinuous change of symmetry at the transition tivities show a composition dependent weak shoulder on the temperature leading to an increase in cell volume.This change low-temperature side of the peak. In NBT, this shoulder was becomes larger as x increases. The volume variation, which assigned initally to a ferroelectric–antiferroelectric transition.2 Recently, Park and Hong refined this sequence and proposed changes from -0.07×10-3 (NBT–PT) to -0.4×10-3 nm3 a diffuse phase transition zone in the temperature range (PT), is characteristic of a first-order phase transition.In 240–280 °C for x<0.1.6 In the present work, the temperatures addition, DSC experiments carried out at increasing and at which this shoulder appears decreases from ca. 230 °C for decreasing temperatures have shown very well defined peaks NBT to ca. 150 °C for the upper limit of the rhombohedral corresponding to the transition temperatures.With a heating/ domain. This result is similar to that already observed for cooling rate of 20°C min-1, the DSC curves exhibit thermal NBT–ST solid solutions where the ferroelectric–antiferroelec- peaks at the transition temperatures with a thermal hysteresis tric transition temperature decreases monotonically as the DT#-15 °C, as observed already in pure PT.These peaks strontium content increases.5 As a consequence, Devonshire are even stronger and sharper when the lead content is calculations cannot be applied to the low-temperature side of increased. The energy involved in the phase transitions can be the permittivity peak. calculated accurately only for compositions where x>0.30. These calculations show that the energies increase linearly up (ii) Tetragonal solid solutions: (Na0.5Bi0.5 )1-xPbxTiO3 to x=0.70 and then increase more rapidly as x approaches ( 0.18<x1) and (K0.5Bi0.5)1-xPbxTiO3(0x1) unity.This behaviour is very similar to that of the increase in volume change at the transition calculated from the lattice These tetragonal solid solutions show similar behaviour. The phenomenological calculation can be carried out successfully parameters. 94 J. Mater. Chem., 1997, 7(1), 91–97Fig. 6 NBT–PT system. Permittivity and loss vs. temperature for (Na0.5Bi0.5)1-xPbxTiO3 . A, rhombohedral range: x=0 (a), 0.03 (b), 0.08(c) and 0.09 (d). B, etragonal range: x=0.19 (a), 0.20 (b), 0.30 (c) and 0.60 (d). In the tetragonal domain, the more striking feature is the of the Curie temperatures as observed, for example, in BT–ST or BT–PT.However, in both cases, the Curie temperatures evolution of the temperature of the phase transition from tetragonal to cubic symmetry. As the tetragonal compositions show a maximum value at ca. 70–80 mol% lead, in close agreement with HTXRD and DSC experiments (Fig. 9). This seem to behave as regular solid solutions (e.g.KBT–PT solid solutions follow Vegard’s law), one expects a linear variation unusual behaviour could be connected with the respective J. Mater. Chem., 1997, 7(1), 91–97 95Fig. 7 KBT–PT system. Permittivity and loss vs. temperature for (K0.5Bi0.5)0.51-xPbxTiO3: x=0 (a), 0.09 (b), 0.18 (c), 0.30 (d), 0.40 (e), 0.50 (f ), 0.60 (g) and 0.80 (h). polarizabilities of lead and (Na,Bi) or (K,Bi).Up to ca. 60 Conclusion mol% lead, the influence of increasing lead content dominates Ferroelectric to paraelectric phase transitions in the NBT–PT over the effect of decreasing the (Na,Bi) or (K,Bi) content and and KBT–PT systems were studied as a function of composi- the Curie temperature increases, whereas above 60 mol% lead, tion and temperature. The nature of the transition was dis- the decrease of (Na,Bi) or (K,Bi) content is not compensated cussed in terms of the phenomenological derivation of the by the increase in Pb content and the Curie temperature decreases to that of PT.Ginzburg–Landau theory. The diffuse character decreases 96 J. Mater. Chem., 1997, 7(1), 91–97Fig. 9 Transition temperatures vs. composition from HTXRD (%, increasing T ) and DSC (', increasing T ; #, decreasing T ) for (a) Fig. 8 Direct and reciprocal permittivities of two selected NBT–PT NBT–PT and (b) KBT–PT solid solutions showing (a) second-order (x=0.19) and (b) first-order (x=0.60) phase transitions 3 C. S. Tu, I. G. Siny and V. H. Schmidt, Phys. Rev. B, 1994, 49, strongly as the lead content increases in both systems. A study 11550. of the thermal evolution of the polarisation coupled with 4 T. Takenaka, K. Sakata and K. Toda, Ferroelectrics, 1990, 106, variable-temperature optical microscopy examinations of these 375. materials is now in progress in order to confirm the above 5 K. Sakata and Y. Masuda, Ferroelectrics, 1974, 7, 347. statements. 6 S-E. Park and K. S. Hong, J. Appl. Phys., 1996, 79, 383. 7 G. A. Smolenskii, V. A. Isupov, A. I. Agranovskaia and N. N. Krainik, Fiz. T vergodo T ela, 1960, 2, 2982. References 8 O. Elkechai, M. Manier and J. P. Mercurio, Phys. Status Solidi A, in the press. 1 B. Jaffe, W. R. Cook and H. Jaffe, Piezoelectric Ceramics, Academic 9 A. F. Devonshire, Philos. Mag., 1949, 40, 1040. Press, London, 1971. 2 B. Vakhrushev, V. A. Isupov, B. E. Kvyatkovsky, N. M. Okuneva, I. P. Pronin, G. A. Smolenskii and P. P. Syrnikov, Ferroelectrics, 1985, 63, 153. Paper 6/02148D; Received 27thMarch, 1996 J. Mater. Chem., 1997, 7(1), 91–97 97