Raman spectroscopy, dielectric properties and phase transitions of Ag0.96Li0.04NbO3 ceramics

Materials Research Bulletin 65 (2015) 123–131

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Raman spectroscopy, dielectric properties and phase transitions of Ag0.96Li0.04NbO3 ceramics Adrian Niewiadomski a, * , Antoni Kania a , Godefroy E. Kugel b , Mustapha Hafid c , Dorota Sitko d a

A. Chelkowski Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland LMPOS, University of Metz and Supelec Metz, 2 rue E. Belin, Metz 57070, France LPGC Dept. of Physics BP 133, Faculty of Science, Ibn Tofail University, 14000 Kenitra, Morocco d Institute of Physics, Pedagogical University, ul. Podchorazych 2, PL 30-084 Krakow, Poland b c

A R T I C L E I N F O

A B S T R A C T

Article history: Received 7 July 2014 Received in revised form 5 November 2014 Accepted 1 January 2015 Available online xxx

Silver lithium niobates Ag1
xLixNbO3 are promising lead free piezoelectrics. Good quality Ag0.96Li0.04NbO3 ceramics were obtained. Dielectric and DSC studies showed that, in comparison to AgNbO3, temperatures of phase transitions slightly decrease. Dielectric studies pointed to enhancement of polar properties. Remnant polarisations achieves value of 0.6 mC/cm2. Maximum of e(T) dependences related to the relaxor-like ferroelectric/ferrielectric M1–M2 transition becomes higher and more frequency dependent. Analysis of Raman spectra showed that two modes at 50 and 194 cm
1 exhibit significant softening. Low frequency part of the Raman spectra which involve central peak and soft mode were analysed using two models. CP was assumed as relaxational vibration and described by Debye function. The slope of temperature dependences of relaxational frequency g R(T) changes at approximately 470 and 330 K, indicating that slowing down process of relaxational vibrations changes in the vicinity of partial freezing of Nb-ion dynamics Tf and further freezing at ferroelectric/ferrielectric phase transition. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: Ceramics Differential scanning calorimetry (DSC) Dielectric properties Ferrielectricity Raman spectroscopy Lattice dynamics

1. Introduction Silver niobate-tantalate AgNb1
xTaxO3 (ATN) and silver-lithium niobate Ag1
xLixNbO3 (ALN) solid solutions, are promising candidates for high-permittivity microwave dielectrics and leadfree piezoelectrics, respectively. AgNb1
xTaxO3 ceramics (x  0.47) exhibit in the microwave region simultaneously high (e  400) and temperature stable (De/e = 0–0.04 for
40–60  C) dielectric permittivity and relatively high quality factor (Q  f = 860 GHz), and additionally, at application temperatures a lack of dielectric dispersion for the broad frequency range from 1 kHz up to approximately 100 GHz [1–4]. Dispersion observed in the submillimetre region is related to the relaxational mode [1]. Contribution of this mode to dielectric susceptibility is an origin of a broad maximum of the low frequency and microwave e(T) dependences in the vicinity of the M2–M3 phase transition [5] and appearance of applicable dielectric properties [6,7]. Spectroscopic

* Corresponding author. Tel.: +48 323591581. E-mail address: [email protected] (A. Niewiadomski). http://dx.doi.org/10.1016/j.materresbull.2015.01.047 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.

studies of ATN samples from the whole concentration range showed that the strength, frequency and temperature range of the relaxational component appearance depend strongly on the Nb/Ta ratio [1,3,8,9]. These results allowed us to connect the relaxational mode with Nb/Ta ion dynamics [1,9] and predict an appearance of structural disorder in the antiferroelectric Nb/Ta ion displacement arrangement [3,10]. These assumptions were recently confirmed experimentally by structural investigations [11,12]. Similarly to PbZn1/3Nb2/3O3–PbTiO3 or PbMg1/3Nb2/3O3–PbTiO3 solid solutions exhibiting “giant piezoelectricity”, the structural disorder may also play a crucial role in the appearance of relatively good, as for lead-free materials, piezoelectric properties of Ag1
xLixNbO3 solid solutions. Piezoelectric coefficients and electromechanical coupling factors reached values: d31 = 170 pC/ N and k31 = 0.71 for Ag0.9Li0.1NbO3 single crystals [13] and d33 = 210 pN/C for Ag0.914Li0.086NbO3 ones [14]. The origin of structural disorder and its influence on polar ordering of the particular phases and nature of phase transitions in silver niobate based materials are also important questions from basic point of view. The disorder phenomenon is most significantly seen in pure silver niobate AgNbO3 (AN).

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Silver niobate undergoes the complex sequence of phase transitions: 852KC 634KO2 !660KT ! 626KO1 !

M1

340KM2 !540KM3 !

!

where M1, M2, M3 and O1, O2 are phases with orthorhombic symmetry in rhombic and parallel orientation, respectively. T and C denote the tetragonal and cubic phases, respectively [9,10,13–16]. The high temperature M3–O1, O2–T and T–C phase transitions, detectable by diffraction, differential scanning calorimetry DSC and dielectric investigations, are related to oxygen octahedron tiltings [11,12,17–21]. Additionally, the displacements of the Ag and Nb ion appear at O2–T and M3–O1 transitions, respectively [21]. On the contrary, the low temperature M1–M2 and M2–M3 transitions are hardly detectable by diffraction or thermal methods. They are observed as diffuse maxima of e(T) dependences. The average symmetry of all M phases determined by diffraction experiments is orthorhombic with the Pbcm space p p group and 2ac  2ac  4ac unit cell (ac – lattice parameter of pseudo-perovskite unit cell), and antiferroelectric array of the Nb and Ag ion displacements [11,21,22]. Combined X-ray, neutron and

electron diffraction and extended x-ray absorption fine structure (EXAFS) distinguished between average symmetry determined from diffraction methods and local symmetry deduced from the diffuse scattering patterns (TEM) and EXAFS profiles. This study pointed also to very weak structural modifications connected with phase transitions between M phases [11,12]. Evolution of the Nb ion dynamics is related to changes of the local symmetry. In the O phases the Nb ions are randomly displaced along eight (111)C directions which gives on average the ideal positions. Below the M3–O1 phase transition the Nb ions become partially ordered with two average anti-parallel displacements along [11 0]C direction. Locally they still occupy these eight positions but two of them are preferred. With decreasing temperature occupancy probabilities for the remaining six sites decrease and vanish below the freezing temperature Tf = 448 K [11]. Below Tf long range order of [11l]c (0  l  1) Nb ion displacement into anti-polar array appears. However, two site disorder is still remained, i.e., these displacements are ordered in Nb chains along the orthorhombic c axis while disordered in chains perpendicular to this axis. It means that the M2 and M3 phases are disordered antiferroelectric ones. The freezing of Nb ion dynamics was confirmed by nonlinear dielectric

Fig. 1. SEM micrographs of Ag0.96Li0.04NbO3 ceramics. (a) Images of the polished surface (backscattered electrons), (b) images of polished and then thermally etched surface (secondary electrons), and (c) the X-ray patterns of Ag0.96Li0.04NbO3 ceramics (Cu Ka radiation). Disks mark LiNbO3 phase.

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[30,31]. With the increase of Li content maximum of e(T) dependences related to the ferroelectric M1–M2 transition increases significantly and become more frequency dependent while M2–M3 transition increase slightly. Both M1–M2 and M2–M3 phase transitions slightly shift towards lower temperatures while M3–O1 transition slightly towards higher temperatures [31]. For x from the range 0.05–0.06 a morphotropic phase boundary is observed. At room temperature the ALN solid solutions with x  0.06 exhibit ferroelectric phase of rhombohedral [32] or orthorhombic symmetry and remnant polarisation of order of 40 mC/cm2 [14]. In spite of the fact that these compounds are potentially applicable only a few papers are published. Raman scattering study of Ag0.96Li0.04NbO3 ceramics in wide temperature range and finding temperature evolution of the central peak CP and its correlation with the Nb-ion dynamics and phase transitions are the main aims of the present paper. This composition seems to exhibit this phenomenon most pronouncedly. There is a lack of systematic Raman studies of ALN compounds. Only one paper [33] presents the Raman spectra of ALN solid solutions with x equal to 0.05, 0.1, 0.15, 0.2 and 0.3 recorded at 83 and 293 K.

Fig. 2. Temperature dependences of the real e0 part of the electric permittivity and dielectric loss tan d (1 kHz to 1 MHz) of Ag0.96Li0.04NbO3 (ALN4) and AgNbO3 (AN). Inset show the tan d(T) dependence in the vicinity of freezing temperature Tf.

studies [23]. The Ag ion displacements create also an antiferroelectric array [10,11,19] and together with Nb dipolar subsystem create one resultant antiferroelectric state. Structural and electrical studies showed that weak ferroelectric M1 phase [4,24–29] appear as a result of a slight modification of the antiferroelectric state. However, for most diffraction experiments the crystal structure of the M1 phase was refined for the centrosymmetric Pbcm space group, which exclude ferroelectricity. This contradiction was solved by recent convergent beam electron diffraction (CBED) experiment for single domain region of AgNbO3 [27]. Authors identified the space group as noncentrosymmetric Pmc21. Moreover, analyses of synchrotron and neutron powder diffraction data allowed them to determine crystal structure of the M1 phase as ferrielectric. The ferrielectric state appears as a result of non-equivalent antiparallel displacements of both Nb and Ag ions along the b orthorhombic axis. The ferrielectric nature of the M1 phase was earlier proposed in paper [4]. Usually lithium ions substituted host A-cations in the ABO3 perovskite lattice, occupy the off-centre positions and create additional electric dipole moments. Therefore, one can expect that substitution of Ag ions by Li ones should strongly influence polar properties of AgNbO3. Structural and dielectric studies of Ag1
xLixNbO3 solid solutions showed that up to x = 0.05 they exhibit the same sequence of phase transitions and the same crystal structure and electric order of particular phases as AgNbO3

Fig. 3. (a) Hysteresis loops at selected temperatures and (b) temperature dependences of remnant polarization on cooling for Ag0.96Li0.04NbO3 ceramics.

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measured using triangular voltage signal method [34] for frequency 5 Hz and strength of electric field up to 15 kV/cm. Raman experiments were carried out for a 90 scattering configuration. A Spex 1401 double monochromator with photoncounting system was used. The argon laser line of 514.5 nm and 200 mW power was used as excitation beam. 3. Results and discussion 3.1. Sample characterization

Fig. 4. Heat flow for Ag0.96Li0.04NbO3 ceramics on heating and cooling. Arrows mark broad bumps of the DSC curves.

Dense and light yellow-green Ag0.96Li0.04NbO3 ceramics were obtained. XRD tests showed that obtained ceramics exhibit mainly perovskite phase of the orthorhombic symmetry. Fig. 1 presents the back-scatter electron image of polished cross-section and secondary electron image of polished and then thermally etched cross-section. Room temperature diffraction pattern is also included. The SEM images point to homogenous perovskite phase with well developed grains of sizes up to 7 mm. However similarly as in the previous paper [33], the X-ray diffraction pattern contains small second phase peaks attributed to LiNbO3. The amount of this secondary phases is approximately 1 mol%. It means that Li concentration in the perovskite phase of investigated Ag0.96Li0.04NbO3 ceramic is smaller than nominal. Because LiNbO3 exhibit high Curie temperature (TC = 1410 K) and its dielectric permittivity slightly and monotonically increases in the temperature range from 0 to 1000 K [35], one can expect no detectable modification of the e(T) dependence of studied ALN solid solution. 3.2. Dielectric and thermal properties

2. Experimental Ag0.96Li0.04NbO3 (ALN4) ceramics were obtained by solid state reaction method. For calcinations and sintering processes the samples were placed in double alumina crucible in which an oxygen atmosphere was maintained. Starting reagents were Ag2O (99.5%), Li2CO3 (99%) and Nb2O5 (99.99%). At first step silver niobate and lithium niobate were prepared. The mixture of Ag2O and Nb2O5, and Li2CO3 and Nb2O5 weighted in molar ratio were thoroughly milled, pressed and calcined for 3 h at 1123 K and 923 K, respectively. Calcination products were crushed and milled. The powder taken in appropriate proportion were carefully mixed, pressed and sintered at 1313 K for 2 h and again powdered, pressed into pellets and finally sintered at 1353 K for 3 h. More details concerning ceramics technology and their characterization are given in Ref. [31]. The scanning electron microscope JSM-4410 was used for studies of ceramic morphology and distribution of individual elements. Two types of images were done. In order to study morphology of the secondary phases the back-scatter electron images of polished cross-sections were taken. Secondary electron images of polished and then thermally etched, at 975  C for 5 min, cross-sections allowed to determine morphology and grain sizes. Room temperature X-ray powder tests were carried out with PANanalytical’s Empyrean diffractometer (filtered Cu Ka radiation, V = 40 kV, I = 100 mA). Differential scanning calorimetry (DSC) experiments were carried out using the DSC 200 F3 Maia apparatus (Netzsch). Measurements were done in the temperature range: 150–800 K on heating and cooling at a rate of 5 and 10 K/min. Plates for dielectric measurements were cut from ceramics, polished and electroded with silver paste. The experiments were performed in temperature range 120 K–900 K using HP 4192 Impedance Analyser for electric field of frequency range 1 kHz–1 MHz and of strength of about 20 V/cm. Current hysteresis loops were

Temperature dependences of the real part of dielectric permittivity e0 and dielectric loss tand of Ag0.96Li0.04NbO3 measured on cooling are plotted in Fig. 2. The e0 (T) and tan d(T) dependences of pure AgNbO3 are included for comparison. The e0 (T) functions exhibit similar shape to those observed for AgNbO3, however maxima or anomalies related to phase transitions are higher and observed at slightly shifted temperatures. The broad and strongly frequency dependent local maximum related to the ferroelectric (ferrielectric)–antiferroelectric M1–M2 phase transition is observed at 321 K (1 MHz). The diffuse and frequency independent maximum related to transition between the

Fig. 5. Raman spectra of Ag0.96Li0.04NbO3 ceramics at selected temperatures.

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temperatures another anomaly seen as a bump of e0 (T) function is detected near 160 K. Nature of this anomaly is not recognized yet. Dielectric losses exhibit two distinct anomalies: the broad and frequency dependent maximum below M1–M2 phase transition and the jump of tan d(T) dependences at M3–O1 transition. The weak and broad bump of tan d(T) function visible near 440 K (Inset in Fig. 2) is related to the freezing of Nb-ion dynamics. Similarly as in pure AN this anomaly is detected only for frequencies higher than 100 kHz. However, in comparison to AN this maximum is significantly broader what suggests that the freezing process happens in wider temperature range and may also explain a lack of e0 (T) anomaly. It points to higher structural disorder in studied ALN4 ceramics than in pure AN. Dependences of polarization versus electric field P(E), calculated from current hysteresis loops, measured at different temperatures along with temperature dependence of remnant polarization Pr are shown in Fig. 3. The values of remnant polarization Pr and spontaneous polarization Ps are approximately 10 times higher than measured for AN [24]. It is seen that polarization appears at approximately 325 K and increases with temperature decreasing typically for disordered ferroelectrics or ferroelectric relaxors. However, this increase become faster below 160 K which together with anomaly of dielectric permittivity indicate that at this temperature region a further ordering of dipoles connected with the Ag/Li and Nb ion displacements and enhancement of ferroelectric properties take place. Fig. 4 presents the temperature dependences of heat flow (DSC) for investigated ceramics measured on heating and cooling processes at a rate of 10 K/min. On cooling, the DSC curve exhibits two pronounced exothermic peaks at 627 K and 668 K related to the M3–O1 and O2–T phase transitions, respectively. On heating only one endothermic peak, connected with the M3–O1 transition, is observed at 681 K. Such large thermal hysteresis (60 K) of the M3–O1 transition was also detected by dielectric studies [31]. Additionally to these distinct peaks, two broad bumps of the DSC curves appear both on cooling near 290 K and 440 K and on heating near 290 K and 460 K. The appearance of these additional bumps is repeatable. It seems that they are related to the Nb-ion freezing (Tf) and appearance of ferroelectric state (M1–M2). Measurements of hysteresis loops pointed to much higher value of spontaneous polarisation than in pure AN. It means that substitution of the Ag

Fig. 6. Temperature dependences of phonon wavenumbers (top) and dampings of some representative bands (bottom) of Ag0.96Li0.04NbO3.

antiferroelectric M2 and M3 phases appears at 529 K. The sharp jump related to the antiferroelectric–paraelectric M3–O1 phase transition is observed at 625 K. Above this temperature the e0 (T) functions obey the Curie Weiss low e0 (T) = C/(T
T0). The slope of the 1/e'(T) dependence changes at approximately 670 K (Fig. 2) indicating the O2–T phase transition. Dispersion appearing at high temperatures is related to increase of electric conductivity and appearance of two-layered capacitor [36,37]. The e0 (T) dependences do not clearly indicate the freezing process at Tf. At low

Fig. 7. Low-frequency part of the Raman spectra of Ag0.96Li0.04NbO3 ceramics at selected temperatures.

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Fig. 8. Fitting examples for two selected temperatures and models: I (top), II (middle), and III (bottom).

ions by Li ones causes significant increase of dipole-dipole interaction and therefore one can expect bigger changes of entropy during both Tf and M1–M2 processes. It should be said here that authors had some doubts concerning these additional DSC anomalies but we show these results in order to inspire other researchers to perform thermal studies of silver niobate based materials using more precise calorimetric equipment. 3.3. Raman Studies 3.3.1. Temperature evolution of phonon lines Raman spectra for wavenumber range 0–800 cm
1 of Ag0.96Li0.04NbO3 ceramics recorded at different temperatures covering the whole range of the M phases are shown in Fig. 5. At temperatures higher than 400 K opacity of samples significantly

increases and they became completely opaque at approximately 600 K. Therefore, the Raman intensities were normalized. The Raman spectra are very similar to those observed for pure AgNbO3 [8,38]. It is worthy to notice here that even at 10 K the line widths of the Raman lines are much bigger than those observed for highly structurally ordered antiferroelectrics like isostructural NaNbO3 [39] or PbMg1/2W1/2O3 [40], which points to presence of structural disorder in studied ALN ceramics. The observed spectra show an ordinary temperature evolution i.e., with the increase in temperature most of Raman lines broaden and shift towards lower frequencies. For lines at 50 and 192 cm
1 significant softening is observed. For more precise quantitative picture an analysis of Raman spectra is required. According to fluctuation-dissipation theory the Raman intensity for the Stoke’s part of spectrum is as follow:

A. Niewiadomski et al. / Materials Research Bulletin 65 (2015) 123–131 Table 1 Residual sum of squares (SSR) and coefficients of determination R2 for all three models for chosen temperatures. 373 K

473 K

Model I

SSR R2

8747.3 0.99462

2483.9 0.99881

Model II

SSR R2

1727.7 0.99894

1791.0 0.99914

Model III

SSR R2

2164.6 0.99867

1783.1 0.99915

IN ðv; TÞ ¼ ½nðv; TÞ þ 1 x00 ðv; TÞ

(1)

where n(v,T) is the Bose–Einstein population factor and x00 is imaginary part of dielectric susceptibility. For further analysis the Raman spectra were divided into two ranges: the low frequency part (below 60 cm
1) and medium and high frequency part (above 60 cm
1). The later part is related to phonon modes and therefore the model of independent damped oscillators was used to deconvolute the Raman spectra and to determine the intensity, frequency, and linewidth of particular lines. According to this model the intensity of Raman spectrum can be written in the form:   P Si v2i Gi v (2) IN ðv; TÞ ¼ ½nðv; TÞ þ 1 2 i ðv2i
v2 Þ þv2 G2i

129

h i 2  SB vð nðv; TÞ þ 1Þ G0 þ gd = v2 þ g 2 INðv; TÞ ¼  2  2 2 2 2 v21
v2
vd2 þgg 2 þ v2 G0 vd2 þgg 2

(3)

where SB is normalization factor, G 0 is damping of phonon mode, g is the characteristic frequency of supplementary relaxational excitation (CP), d2 is coupling strength between CP and phonon mode and v1 is a renormalized mode frequency in high-frequency limit. Inversion of g is a relaxation time. In the second and third models the central component was assumed to be connected with relaxational vibrations. Therefore, the scattering intensity function consisting the classical Debye relaxation and damped oscillator functions were used: ( ) X Si v2i Gi v SR g R v þ (4) IN ðv; TÞ ¼ ½nðv; TÞ þ 1 2 2 2 2 2 v2 þ g 2R i ðv
v Þ þ v G i

i

where SR is relaxation strength, g R is characteristic relaxation frequency (inverse relaxation time g R = 1/t) [43,44]. In fact, our analyses were performed for broader frequency range (0–130 cm
1) therefore for above scattering functions (Eqs. (3) and (4)) the responses of several damped oscillators were added (Eq. (2)). Fitting results of the Raman spectra recorded at temperatures 373 and 473 K using these three models are shown in Fig. 8. The residuals SSR and R2 of fits are listed in Table 1. For all models the satisfactory fits were achieved. However, more detail examination points that model I omits some component near 38 cm
1. In addition, temperature dependence of fitted CP and SM parameters exhibit abrupt changes while observed Raman spectra evolve gradually.

where Si, vi and g i are strength, frequency and damping of i-th mode, respectively. Temperature dependences of calculated values of frequencies and dampings of phonon modes are presented in Fig. 6. They show that with temperature increasing the frequencies decrease while the damping constants increase. Frequency of modes observed at 10 K at 192.6 cm
1 and 50 cm
1 decrease to 147 cm
1 and 40 cm
1 at 573 K, respectively.

3.3.2. Central peak and soft mode. Raman spectra for frequency range 0–125 cm
1 are shown in Fig. 7. Qualitatively they exhibit very similar temperature evolution as observed in AgNbO3. It is seen that with temperature increasing the lowest frequency mode at 50 cm
1 softens significantly and its line width increase. Simultaneously intensity of central peak (CP) increase approaching the M2–M3 phase transition and slightly decrease above this transition. Moreover, below this transition CP increase while quasi-soft mode (SM) decreases, which may suggest an intensity transfer between these components. The low frequency (0–60 cm
1) part of spectra, which consists of central peak and distinct vibrational band at 50 cm
1, was fitted using three models:

I Model of coupled relaxational and phonon modes representing

central peak and soft mode, respectively. II Model of uncoupled relaxational and damped oscillator modes

with additional phonon-like excitation at approximately 38 cm
1of temperature variable frequency. III Model of uncoupled relaxational and damped oscillator modes with additional phonon-like excitation of temperature stable frequency at approximately 38 cm
1. In the first model we used the following scattering-response function [41,42]:

Fig. 9. Relaxational frequency g R and integral intensity of central peak (top), and wavenumbers and phonon dampings (bottom) calculated with model II.

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with increase of temperature while dampings of these modes increases. It should be pointed here that central component of Raman spectra, especially at high temperatures, may have different physical origin. However, taking into account that our earlier Raman studies of AgNbO3 and AgNb1
xTaxO3 single crystals showed similar temperature evolution of CP and that the same models used allowed us to connect appearance of CP with niobium dynamics, which was later confirmed by structural studies, we believe that present results are reliable. Therefore we may related changes of the slope of g R(T) function at 470 and 330 K to partial freezing of niobium dynamics at Tf and further freezing at antiferroelectric–ferroelectric phase transition M1–M2, and thus contribution of the relaxational mode to dielectric susceptibility as responsible for broad maximum of dielectric permittivity at M2– M3 phase transition. 4. Conclusions

Fig. 10. Relaxational frequency g R and integral intensity of central peak (top), and wavenumbers and phonon dampings (bottom) obtained by fitting spectra with model III.

Moreover, damping of phonon mode and relaxational time determined using this model show nonphysical temperature evolution i.e., for the broad temperature ranges damping of the soft mode is close to zero and relaxational time increases with the increase of temperature. Therefore, we abandoned this model and assumed that it is unfit to the system. In models II and III we added phonon mode between CP and SM (appr. 38 cm
1) not only because of fitting improvement, but also because of highly probable its presence. Our earlier Raman study of AgNbO3 single crystals [8] showed that at least up to 200 K well resolved phonon line at approximately 30 cm
1 is observed. In addition, at 10 K this line is twice broaden than line at 50 cm
1. Substitution of 4% of the Ag ions by Li ones causes further structural disorder in already disordered AN lattice and consequently broadening of phonon modes. It could be a reason of its difficult detection. Because nature of this additional line was not fully recognized yet and it is not known whether it is lattice mode or excitation caused by lattice impurities or defects [43], we used models with temperature variable (II) and constant (III) frequency. Fitting results for model II and III, namely temperature dependences of relaxational frequency and strength, and frequencies and dampings of phonon modes, are plotted in Figs. 9 and 10, respectively. The results obtained for both models are very similar. The relaxational frequency g R increase with the rise in temperature. The slope of the g R(T) function changes at approximately 330 and 470 K. Relaxational strength (integrated intensity) increase with increasing temperature achieving maximum near 500 K. For both models the frequencies of phonon modes decrease

Good quality Ag0.96Li0.04NbO3 ceramics, consisting of homogenous perovskite phase and small amounts of secondary LiNbO3 phase have been obtained. Dielectric and thermal DSC studies showed that, in comparison to pure AgNbO3, temperatures of structural phase transitions slightly decrease. Additionally, dielectric investigations pointed to enhancement of polar properties. Within the ferroelectric/ferrielectric M1 phase remnant polarisation reaches value of 0.6 mC/cm2 while 0.04 mC/cm2 for AN. Moreover, local maximum of e(T) dependences related to the relaxor-like M1–M2 phase transition becomes higher and more significantly frequency dependent. Analysis of the Raman spectra showed that most of the lattice modes exhibit ordinary behaviour, i.e. their frequencies decreases with increase of temperature while dampings increase. Two modes at 50 and 194 cm
1 show significant softening. Low frequency part of the Raman spectra which involve central peak and soft mode were analysed using two models. CP was assumed to be related to relaxational vibrations described in the models by classical Debye function. Both models gave very similar results. The relaxational frequency g R increase with the rise in temperature. The slope of the g R(T) dependences changes at approximately 470 and 330 K indicating that slowing down process of the relaxational vibrations changes in the vicinity of partial freezing of Nb-ion dynamics Tf and further freezing at antiferroelectric–ferroelectric(ferrielectric) phase transition M1– M2. Moreover, temperature dependences of strength of the relaxational mode show maximum in the vicinity of M2–M3 phase transition. These features allowed as to connect the temperature evolution of central peak with changes of niobium dynamics. References [1] A.A. Volkov, B.P. Gorshunov, G. Komandin, W. Fortin, G.E. Kugel, A. Kania, J. Grigas, High-frequency dielectric spectra of AgTaO3–AgNbO3 mixed ceramics system, J. Phys.: Condens. Mat. 7 (1995) 785–793. [2] M. Valant, D. Suvorov, New high-permittivitty AgNb1
xTaxO3 microwave ceramics: part II dielectric characteristics, J. Am. Ceram. Soc. 82 (1999) 88–93. [3] J. Petzelt, S. Kamba, E. Buixaderas, V. Bovtun, Z. Zikmund, A. Kania, V. Koukal, J. Pokorny, J. Polivka, V. Pashkov, G. Komandin, A.A. Volkov, Infrared and microwave dielectric response of the disordered antiferroelectric Ag(Ta,Nb)O3 system, Ferroelectrics 223 (1999) 235–246. [4] A. Kania, Dielectric properties of Ag1
xAxNbO3 (A: K Na and Li) and AgNb1
xTaxO3 solid solutions in the vicinity of diffuse phase transitions, J. Phys. D: Appl. Phys. 34 (2001) 1447–1455. [5] A. Kania, AgNb1
xTaxO3 solid solutions–dielectric properties and phase transitions, Phase Transit. 3 (1983) 131–140. [6] M. Valant, D. Suvorov, C. Hoffmann, H. Sommariva, Ag(Nb,Ta)O3-based ceramics with suppressed temperature dependence of permittivity, J. Eur. Ceram. Soc. 21 (2001) 2647–2651. [7] D. Klement, M. Spreitzer, D. Suvorov, Suppressed temperature dependence of the resonant frequency of a AgNb0.5Ta0.5O3 composite vs. single phase ceramics, J. Eur. Ceram. Soc. 34 (2014) 1537–1545.

A. Niewiadomski et al. / Materials Research Bulletin 65 (2015) 123–131 [8] A. Kania, K. Roleder, G.E. Kugel, M.D. Fontana, Raman scattering, central peak and phase transitions in AgNbO3, J. Phys. C: Solid State Phys. 19 (1986) 9–20. [9] M. Hafid, G.E. Kugel, A. Kania, K. Roleder, M.D. Fontana, Study of the phase transition sequence of mixed silver tantalate-niobate (AgTa1
xNbxO3) by inelastic light scattering, J. Phys.: Condens. Mat. 4 (1992) 2333–2345. [10] A. Kania, J. Kwapulinski, Ag1
xNaxNbO3 (ANN) solid solutions: from disordered antiferroelectric AgNbO3 to normal antiferroelectric NaNbO3, J. Phys.: Condens. Mat. 11 (1999) 8933–8946. [11] I. Levin, V. Krayzman, J.C. Woicik, J. Karapetrova, T. Proffen, M.G. Tucker, I.M. Renney, Structural changes underlying the diffuse response in AgNbO3, Phys. Rev. B 79 (104113) (2009) 14. [12] I. Levin, J.C. Woicik, A. Llobet, M.G. Tucker, V. Krayzman, J. Pokorny, I.M. Renney, Displacive ordering transitions in perovskite-like AgNb1/2Ta1/2O3, Chem. Mater. 22 (4987) (2010) 9. [13] A. Saito, S. Uraki, H. Kakemoto, T. Tsurumi, S. Wada, Growth of lithium doped silver niobate single crystals and their piezoelectric properties, Mat. Sci. Eng. B-Solid 120 (2005) 166–169. [14] D. Fu, M. Endo, H. Taniguchi, T. Taniyama, S. Koshihara, M. Itoh, Piezoelectric properties of lithium modified silver niobate perovskite single crystals, Appl. Phys. Lett. 92 (172905) (2008) 3. [15] J. Kuwata, K. Uchino, S. Nomura, Dielectric and piezoelectric properties of 0.91Pb(Zn1/3Nb2/3)O3–0.09PbTiO3 single crystals, Jpn. J. Appl. Phys. 21 (1982) 1298–1302. [16] S.E. Park, T.R. Shrout, Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals, J. Appl. Phys. 82 (1997) 1804–1811. [17] M. Lukaszewski, M. Pawelczyk, J. Handerek, A. Kania, On the phase transitions in silver niobate AgNbO3, Phase Transit. 3 (1983) 247–258. [18] M. Verwerft, D. Van Dyck, V.A.M. Brabers, J. Van Landuyt, S. Amelinckx, Electron-microscopic study of the phase transformations in AgNbO3, Phys. Status Solidi A 112 (1989) 451–466. [19] A. Kania, An additional phase transition in silver niobate AgNbO3, Ferroelectrics 205 (1998) 19–25. [20] A. Kania, A. Niewiadomski, S. Miga, I. Jankowska-Sumara, M. Pawlik, Z. Ujma, J. Koperski, J. Suchanicz, Silver deficiency and excess effects on quality, dielectric properties and phase transitions of AgNbO3 ceramics, J. Eur. Ceram. Soc. 34 (2014) 1761–1770. [21] P. Sciau, A. Kania, B. Dkhil, E. Suard, A. Ratuszna, Structural investigation of AgNbO3 phases using X-ray and neutron diffraction, J. Phys.: Condens. Mat. 16 (2004) 2795–2810. [22] J. Fabry, Z. Zikmund, A. Kania, V. Petricek, Silver niobium trioxide, AgNbO3, Acta Crystallogr. C 56 (2000) 916–918. [23] S. Miga, A. Kania, J. Dec, Freezing of the Nb+5 ion dynamics in AgNbO3 studied by linear and nonlinear dielectric response, J. Phys.: Condens. Mat. 23 (2011) 4. [24] A. Kania, K. Roleder, M. Lukaszewski, The ferroelectric phase in AgNbO3, Ferroelectrics 52 (1984) 265–269. [25] D. Fu, M. Endo, H. Taniguchi, T. Taniyama, M. Itoh, AgNbO3: a lead-free material with large polarization and electromechanical response, Appl. Phys. Lett. 90 (252907) (2007) 3.

131

[26] A. Kania, K. Roleder, Ferroelectricity in AgNb1
xTaxO3 solid solutions, Ferroelectrics Lett. 2 (1984) 51–54. [27] M. Yashima, S. Matsuyama, R. Sano, M. Itoh, K. Tsuda, D. Fu, Structure of ferroelectric silver niobate AgNbO3, Chem. Mater. 23 (2011) 1643–1645. [28] M. Yashima, S. Matsuyama, Origin of the ferrielectricity and visible light photocatalytic activity of silver niobate AgNbO3, J. Phys. Chem. C 116 (2012) 24902–24906. [29] L. Li, M. Spreitzer, D. Suvorov, Unique dielectric tunability of Ag(Nb1
xTax)O3 (x = 0-0. 5) ceramics with ferrielectric polar order, Appl. Phys. Lett. 104 (182902) (1829) 4. [30] V.B. Nalbandyan, B.S. Medviediev, I.N. Belyaev, The AgNbO3–LiNbO3, Inorg. Mater. 16 (1980) 1239–1243. [31] A. Kania, S. Miga, Preparation and dielectric properties of Ag1
xLixNbO3 (ALN) solid solutions ceramics, Mater. Sci. Eng. B 86 (2001) 128–133. [32] D. Fu, M. Endo, T. Taniguchi, M. Taniyama, S. Koshihara, Ferroelectricity of Lidoped silver niobate (Ag,Li)NbO3, J. Phys.: Condens. Mat. 23 (75901) (2011) 7. [33] H.U. Khan, I. Sterianou, S. Miao, J. Pokorny, I.M. Reaney, The effect of Lisubstitution on the M-phases of AgNbO3, J. Appl. Phys. 111 (24107) (2012) 9. [34] M.A.P. Jubindo, M.J. Tello, J. Fernandez, A new method for measuring the ferroelectric behaviour of crystals at low frequencies, J. Phys. D: Appl. Phys. 14 (1981) 2305–2320. [35] I. Tomeno, S. Matsumura, Elastic and dielectric properties of LiNbO3, J. Phys. Soc. Jpn. 56 (1987) 163–177. [36] R. Stumpe, D. Wagner, D. Bauerle, The influence of bulk and interface properties on the electric transport in ABO3 perovskite, Phys. Status Solidi A 75 (1983) 143–154. [37] P. Lunkenheimer, V. Bobnar, A.V. Pronin, A.I. Ritus, A.A. Volkov, A. Loidl, Origin of apparent colossal dielectric constants, Phys. Rev. B 66 (52105) (2002) 4. [38] M.D. Fontana, G.E. Kugel, A. Kania, K. Roleder, Central peak and soft mode in AgNbO3, Jpn. J. Appl. Phys. 24 (Suppl. 24–2) (1985) 516–518. [39] S.J. Lin, D.P. Chiang, Y.F. Chen, C.H. Peng, H.T. Liu, J.K. Mei, W.S. Tse, T.R. Tsai, H.P. Chiang, Raman scattering investigations of the low-temperature phase transition of NaNbO3, J. Raman Spectrosc. 37 (2006) 1442–1446. [40] A. Kania, E. Jahfel, G.E. Kugel, K. Roleder, M. Hafid, A Raman investigation of the ordered complex perovskite PbMg0.5W0.5O3, J. Phys.: Condens. Mat. 8 (1996) 4441–4453. [41] T. Riste, E.J. Samuelsen, K. Otnes, J. Feder, Critical behaviour of SrTiO3 near the 105 K phase transition, Solid State Commun. 9 (1971) 1455–1458. [42] S.M. Shapiro, J.D. Axe, G. Shirane, T. Riste, Critical Neutron Scattering in SrTiO3 and KMnF3, Phys. Rev. B 6 (1972) 4332–4341. [43] W. Fortin, G.E. Kugel, J. Grigas, A. Kania, Manifestation of Nb dynamics in Raman, microwave and infrared spectra of the AgTaO3–AgNbO3 mixed system, J. Appl. Phys. 79 (1996) 4273–4282. [44] E. Bouziane, M.D. Fontana, M. Ayadi, Study of the low-frequency Raman scattering in NaNbO3 crystal, J. Phys.: Condens. Mat. 15 (2003) 1387–1395.