Polarization and acoustic properties of barium-modified lead-free potassium–sodium niobate ceramics

. Results of polarization and polarization switching in samples prepared by solid phase synthesis of the (1-х )(K 0.5 Na 0.5 )(Nb 0.93 Sb 0.07 )O 3 – x BaTiO 3 + 0.5mol%MnO 2 system at x = 0.01, 0.02, and 0.04 (KNNS7– x BT) studied over a wide range of temperatures are reported. The observed features of the dielectric nonlinearity at the ferroelectric and structural phase transitions in the materials are described. The dielectric response data of the samples of different contents of the admixture in the KNNS7– x В T ceramics are compared with the behaviour of their elastic properties.


INTRODUCTION
* Lately, because of the toxicity of lead and lead compounds, the interest in ferroelectric materials without lead has grown considerably.Potassium-sodium niobate (KNN) -a solid solution of ferroelectric KNbO 3 and anti-ferroelectric NaNbO 3 -is one of such materials.The most interesting for application is the (K 0.5 Na 0.5 )NbO 3 compound on the morphotropic phase boundary [1][2][3] the ferroelectric phase transition (PT) in which proceeds at Т С  670 K. Below that temperature KNN has a tetragonal crystal lattice and a structural tetragonal-toorthorhombic PT proceeding at Т o-t  470 K.
The KNN modified by a small amount of admixture is known for a substantial change of its properties [2][3][4][5][6][7].A detailed study of structural transformations at the lowtemperature PT and around Т С [8] unravelled the behaviour of lattice parameters in the (K x Na 1-x )NbO 3 compounds with the temperature.A conclusion of * Corresponding author, Maija.Antonova@cfi.lu.lv different phases coexisting in the modified ceramics over a wide range of temperatures improving the piezoelectric properties of the material is drawn on the basis of the obtained results.A lower Т С and broadening of the PT in KNN is also achieved by admixtures isovalent to Nb.In the (K 0.5 Na 0.5 )(Nb 0.93 Sb 0.07 )O 3 (KNNS7) compound Т С ≈ 550 K and Т o-t ≈ 410 K.A very small admixture of barium efficiently modifies electrical properties.Studies [5,6] point to improved electromechanical parameters of KNN modified by addition of BaTiO 3 or BaZrO 3 .Regardless of the numerous studies of lead-free ferroelectric ceramics a series of questions related to relaxation processes of polarization at structural PTs remain unanswered.
This study aims at presenting a completer overview of the effects of admixtures such as BaTiO 3 on the properties of the KNN system by examining over a wide range of temperatures the behaviour of the dielectric, elastic, and polarization characteristics in the (1-x)(K 0.5 Na 0.5 ) (Nb 0.93 Sb 0.07 )O 3 -xBaTiO 3 + 0.5mol%MnO 2 (KNNS7-xBT) ferroelectric ceramics system of x = 0.01, 0.02, and 0.04.
The dielectric response in weak fields (linear response) was studied with an E7-15 RLC measuring device, in medium and strong fields by analysis of polarization loops obtained at frequencies 1-10 Hz on a modified Sawyer-Tower circuit.
The velocity of longitudinal acoustic waves was measured by resonance techniques in bar samples 12-18 mm long and 1.5-2 mm thick.The electrodes were applied to the large faces after which the samples were polarized under the applied field E = 10 kV/cm at T ≈ 370 K and cooled to room temperature T room under the field.

EXPERIMENTAL RESULTS
The behaviour of a number of parameters of the KNNS7-xBT ceramics at different concentrations of the modifying admixture of BaTiO 3 is illustrated in Figs 1, 2, and 3.As seen from Fig. 1a, two anomalies are apparent on the temperature curve of the dielectric permittivity ε΄(Т): a step and a maximum.The step-like anomaly on the ε΄(Т) curve in the compound without BT is known to indicate the range of the structural PT from the orthorhombic to the tetragonal phase (Т o-t ) at heating the sample [6].The temperature range of the step-like anomaly in ostensibly pure KNNS7 extends over 380 to 410 K and in the case of KNNS7-0.01BT over 350 to 370 K (Fig. 1a, insert).The maximum on the ε΄(Т) curve exhibits the temperature of the ferroelectric PT at Т С ≈ 540 K.The temperature Т С is about 10 degrees lower compared with the data of [6]; the average temperature of the step-like range on the ε΄(Т) curve (regarded as the temperature of the structural PT) dropped by almost 30 degrees.Thus, at the BT concentration of x = 0.01 in particular the temperature Т o-t considerably decreases.
The remnant polarization P r (T) as a function of temperature in the range of Т o-t is presented in Fig. 1b.The data are obtained from polarization loops at frequency 10 Hz and Е ≈ 18 kV/cm (the insert of Fig. 1b illustrates polarization loops under different fields E max at one of the studied temperature points).The step-like anomaly here is also seen as a noticeable reduction of the P r (T) curve within the range from Т ≈ 340 to Т ≈ 370 K.The shape of the loops has a pronounced slope from where it follows that the ratio is small (around 0.5).On the one hand, such behaviour may indicate a considerable coercive field, which is consistent with the polarization data of the compound without BT [6] and a substantial reduction of P r (T) at T < 330 K with the decrease of the temperature.For that reason the maximum values of polarization are not reached at Е мах used in the present study.On the other hand, in this case the decrease of polarization may be related to structural inhomogeneity of the material because of the presence of BT.
The behaviour of the velocity v(T) and the coefficient of absorption α(Т) of the longitudinal acoustic wave in the range of temperatures including Т o-t and Т C is illustrated in Fig. 1c.The pattern and the magnitude of the v(T) and α(Т) curves are rather typical of ceramics [9][10][11].The temperature Т ≈ 370 K at the minimum velocity of the acoustic wave determines the temperature of the structural PT.The v(Т) curve has a fracture in the range of Т ≈ 340 K where α(Т) has a minor additional maximum.Since the domain structure in ferroelectric materials has a substantial effect on the elastic properties, the observed fracture might be related to the change of the domain state of the sample when at heating the PT temperature is being approached.It is in good agreement with the behaviour of P r (T) at Т ≈ 340 K: the drop of the magnitude of P r (Fig. 1b).
The behaviour of the parameters in compounds of higher contents of BaTiO 3 (Fig. 2 and Fig. 3) allows of the following conclusion: in KNNS7-0.02BTТ C continues shifting to lower temperatures (Т C ≈ 500 K), approximately by 40-45 degrees with regard to KNNS7-0.01BT.The range of the structural PT is also reduced from Т ≈ 340 K to Т ≈ 370 K while broadening of the ferroelectric and structural PTs is increased.A 'tail' of ε΄(Т) at lower temperatures in KNNS7-0.02BTappears at a frequency of 100 Hz (Fig. 2a), which is most likely related to the growth of the contribution to conductivity with the increasing content of BT.The value of ε΄(Т) is elevated in the whole range of temperatures studied.However, the dispersion of ε΄ (the difference of its values at 1000 and 100 Hz) in KNNS7-0.02BT is distributed over the whole range of temperatures whereas in the KNNS7-0.01BTcompound the dispersion of ε΄(Т) is pronounced only around Т C .
The behaviour of the dielectric response in KNNS7-0.02BT at weak fields is in good agreement with the data on polarization (Fig. 2b).The magnitude of the remnant polarization at the same Е мах substantially increases (almost three times at room temperature) compared to KNNS7-0.01BT.The same follows from the shape of polarization loops (Fig. 2b, insert), showing that the ratio P r /Р мах also increases (~0.6).The features of polarization switching in the compound point to the formation of a more homogeneous polarization under the applied field and to a stable domain state emerging in the material.As seen from Fig. 2b, the range of temperatures of dropping Р r (Т) or the step on the Р r (Т) curve around Т o-t extends from Т ≈ 320 K to Т ≈ 350 K.
The presence of homogeneous polarization in the KNNS7-0.02BTcompound is also displayed by the behaviour of the elastic properties.The minimum of v(T) is distinctly observed at Т ≈ 350 K (Fig. 2c), which is the temperature of the maximum of α(Т).The fracture on the v(T) curve below Т o-t is located at 330 K. Comparison to the behaviour of the remnant polarization Р r (Т) curve (Fig. 2b) on which the step starts around Т ≈ 320 K suggests that it is related to the change of the KNN-0.02Badomain structure approaching the PT temperature as in the case of KNNS7-0.01BT.
Substantial differences in the behaviour of the presented parameters are observed in KNNS7-0.04BT (Fig. 3).The high-temperature 'tail' of ε΄(Т) at a frequency of 100 Hz practically levels the maximum at the ferroelectric PT (Fig. 3a), indicating a considerable contribution of conductivity.The dielectric permittivity at 1000 Hz reveals a strongly broadened maximum of ε΄(Т) at 450 K -a further decrease of the mean temperature of the PT by about 45-50 degrees compared with the KNNS7-0.02BTcompound.The low-frequency dispersion of ε΄ here is increased over the whole range of temperatures including the low-temperature bounds (Fig. 3a, insert).Instead of a step on the ε΄(Т) curve at the structural PT observed in compounds of smaller BT concentrations here the anomaly appears as a smooth bent around 295 K, which might suggest the end of PT beginning at Т ≈ 260 K or even a lower temperature.Most likely the reason of the strongly broadened PTs in KNNS7-0.04BT is a considerable heterogeneity of the material as evidenced by the behaviour of polarization (Fig. 3b).In the given case a noticeable decline of Р r (Т) begins at Т ≈ 260 K and continues within a relatively wide range of temperatures (approximately up to 50 degrees).The polarization loops at T ≈ 270 K presented in the insert of Fig. 3b have a glass-like shape as in disordered ferroelectric relaxors [12,13], which means it does not saturate at increasing the applied field Е max .
The conclusion about a substantial heterogeneity of the KNNS7-0.04BTcompound is consistent with v(T) and α(T) presented in Fig. 3c, from where it follows that the velocity of the acoustic wave is lower compared with other compounds while its minimum is practically like a plateau extending from 300 to 320 K.The absorption curve α(T) of relatively higher α values is monotonously declining over the whole range of temperatures.Such substantial broadening of the anomalies is observed in ferroelectric relaxors [9][10][11].

CONCLUSIONS
The study of the effects of BaTiO 3 admixtures on the properties of the KNNS7 ferroelectric ceramics revealed the following:  The growth of the BT content broadens and shifts to lower temperatures the low-frequency maximum of ε΄(Т) corresponding to the ferroelectric PT by ~10 degrees at x = 0.01, by ~55 degrees at x = 0.02, and by ~100 degrees at x = 0.04. The mean temperature Т о-t of the step-like anomaly on the ε΄(Т) curve determining the structural PT decreases with the growth of BT content by ~30 degrees at x = 0.01, by ~40 degrees at x = 0.02, and by ~120 degrees at x = 0.04. The growth of the BT content increases the conductivity of the material.

KNN-0.04Ba
 The range of temperatures of a noticeable decrease of the remnant polarization Р r (Т) around Т о-t substantially extends with the growth of the BT content the pattern of polarization loops transforming them to the shape characteristic of ferroelectric relaxors. In polarized KNNS7 samples containing BT an anomaly displayed at heating by a minimum of the acoustic wave velocity v(T) distinctly indicates the temperature Т о-t of the structural PT in the KNNS7-0.01ВTcompound Т о-t ≈ 370 K, in KNNS7-0.02ВTТ о-t ≈ 350 K, and in the KNNS7-0.04ВTcompound the v(T) minimum is substantially broadened to a plateau ranging from 300 to 320 K indicating a substantial heterogeneity of the material's structure at BT content of x = 0.04. It can be deduced that the most favourable polarization and elastic properties are exhibited by the compound of x = 0.02.