Role of dopant induced defects on the properties of Nd and Cr doped PZNT single crystals

Materials Science and Engineering B 185 (2014) 60–66

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Role of dopant induced defects on the properties of Nd and Cr doped PZNT single crystals B. Srimathy a , R. Jayavel a , Indranil Bhaumik b , S. Ganesamoorthy c , A.K. Karnal b , P.K. Gupta b , J. Kumar a,∗ a

Crystal Growth Centre, Anna University, Chennai 600025, India Laser Materials Development and Devices Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, India c Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India b

a r t i c l e

i n f o

Article history: Received 9 September 2013 Received in revised form 20 January 2014 Accepted 29 January 2014 Available online 13 February 2014 Keywords: Crystal growth Dopants Defects Dielectric Ferroelectric

a b s t r a c t The dielectric, ferroelectric and piezoelectric properties of rare-earth Nd-doped lead zinc niobate–lead titanate (Nd:PZNT) and transition metal Cr-doped lead zinc niobate–lead titanate (Cr:PZNT) single crystals have been investigated. Formation of pure perovskite phases was confirmed from powder X-ray diffraction. Lattice parameters have been obtained by Rietveld refinement. Both Nd and Cr doped crystals showed a diffuse phase transition and frequency dispersive phenomena supporting the enhancement in relaxor behavior. The variation in the values of dielectric constant (εr ), AC conductivity and piezoelectric charge coefficient (d33 ) due to the creation of oxygen vacancies and lead vacancies as a result of doping is investigated in detail. Hysteresis behavior of the doped crystals is explained by considering the possible switching mechanism of domains and the formation of defect dipoles. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Lead-based perovskite ferroelectrics are widely used in multilayer capacitors, micro-electro mechanical systems (MEMS) and integrated devices [1–5]. Most of these applications demand materials with excellent dielectric and ferroelectric properties. Among the lead-based perovskites, single crystals of lead zinc niobate–lead titanate Pb(Zn1/3 Nb2/3 )O3 –PbTiO3 (PZNT) grown at morphotropic phase boundary (MPB) composition (x = 9 mol%) are widely used in high performance piezoelectric actuator and transducer materials owing to their high piezoelectric coefficient (d33 ) and electromechanical coupling factor (k33 ) [6,7]. Though these crystals have excellent d33 and k33 , they have very low values of mechanical quality factor (Qm < 50) which limit the application of these materials in high power devices. Moreover, a thirst is always there to attain a higher piezoelectric coefficient in order to increase their efficiency for device applications. Fortunately, one significant advantage of lead-based materials is that their properties can be modified by doping. Depending upon their valence and site occupancy in the PZNT unit cell, the dopants may be classified as donors (higher valent element occupying a lower valent site), acceptors (lower valent element occupying a higher valent site) and isovalent

∗ Corresponding author. Tel.: +91 44 2235 8329. E-mail addresses: [email protected], [email protected] (J. Kumar). 0921-5107/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2014.01.018

substituents (both the substituting element and the occupying site have same valency). Defects would be created according to their site replacement. These defects are known to affect the properties of piezoelectrics prominently. In general, donor dopant occupying the A-site induces cationic defects [8,9] in the ABO3 lattice and the acceptor-type dopants occupying the B sites enhance the oxygen vacancies [10,11]. Such differential behavior may impart varied effects on the polarization behavior through interaction with the domain walls [12,13]. So far, among acceptor dopants Mn, Co, Fe and Ce has been extensively reported [14–16] whereas investigation on A-site modifications has been reported in Pb-based ceramics [17,18] but not in PZNT single crystals. In this paper, the role of defects due to doping of neodymium and chromium on the dielectric, ferroelectric and piezoelectric properties of 0.91Pb(Zn1/3 Nb2/3 )O3 –0.09PbTiO3 (PZNT 91/9) crystals is reported. 2. Materials and methods PZNT 91/9 crystals with dopants 1 mol% Nd and 1 mol% Cr were grown using high temperature solid solution (flux) technique as described elsewhere [19,20]. High purity chemicals PbO, TiO2 , ZnO and Nb2 O5 and for dopants Nd2 O3 and Cr2 O3 were selected as starting materials. PZNT and flux (PbO) were mixed in the ratio of 55:45 mol%. The mixture was transferred to a platinum crucible and sealed with platinum lid. This in turn was kept inside an alumina

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Fig. 2. (a) XRD pattern of undoped and doped PZNT crystals. (b) Observed, calculated and difference XRD profiles for Nd:PZNT.

Fig. 1. As grown (a) PZNT 91/9 crystals, (b) Nd:PZNT 91/9 crystals and (c) Cr:PZNT 91/9 crystals.

crucible and sealed with alumina lid to repress lead evaporation at high temperatures. The temperature was increased at a rate of 150 ◦ C/h till 1200 ◦ C/h and maintained for 24 h to achieve homogenization of the melt solution. After this soaking period, spontaneous nucleation was initiated by slow cooling process at a rate of 2.5 ◦ C/h till 950 ◦ C/h and afterwards at a rate of 100 ◦ C/h to reach room temperature to avoid the formation of pyrochlore phase. The obtained crystals were leached out of the flux using hot concentrated nitric acid. Fig. 1a–c shows the pictures of as grown crystals of undoped, Nd-doped and Cr-doped PZNT, respectively. Undoped crystals were brown in color and Nd-doped and Cr-doped crystals were brownish yellow and dark brown in color, respectively. Compositional analysis of the chemical elements was carried out by inductively coupled plasma optical emission (ICP-OES) spectrometer Optima 3000XL (PERKIN ELMER). Phase formation and structural analysis was studied using X-ray diffraction (RIGAKU GEIGER FLEX). Rectangular pieces of crystal of dimension ∼5 mm × 5 mm × 1 mm were electroded with silver paste and subjected to dielectric measurements at different temperatures and frequencies using an automated HP4194A IMPEDANCE ANALYZER. The measurements were carried out during cooling at a rate of 1 ◦ C/min. Ferroelectric hysteresis loops were obtained by a

RADIANT made 609A LOOP TRACER. For P–E loop studies, the samples were immersed in silicone oil bath to prevent arcing during measurement. Piezoelectric charge coefficients (d33 ) were measured by PIEZOTEST made d33 METER with an applied static force of 0.25 N for 10 s. 3. Results and discussions 3.1. Composition analysis and X-ray diffraction Crystals were collected from top and bottom portions of the crucible. They were crushed and subjected to compositional analysis, twice, by inductively coupled plasma (ICP) using standard mixtures of HNO3 and HF. Crystals obtained from the top portion were Pb rich whereas crystals from the bottom were concurrent with the theoretical values. As far as Zn is concerned, its composition was comparable to the starting composition. The concentration of Ti was found to be more in Cr-doped crystals. Also composition determination in terms of dopants was performed and it was observed that the dopant concentrations were slightly lower than the starting composition because of segregation. From ICP analysis it was confirmed that the crystals from the bottom portion closely constituted the initial composition and those crystals were used for further characterization. Single crystals were ground into a fine powder and XRD was carried out with 2 value ranging from 20◦ to 70◦ , using a step size of 0.01◦ and a counting time of 3 s. Fig. 2a shows the powder

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Table 1 Lattice parameters determined via Rietveld Refinement. Crystal

Lattice parameters Rhombohedral

Tetragonal

PZNT

a = 4.054 A˚ ˛ = 89.95◦

a = 4.033 A˚ c = 4.061 A˚

Nd:PZNT

a = 4.051 A˚ ˛ = 89.94◦

a = 4.031 A˚ c = 4.059 A˚

Cr:PZNT

a = 4.049 A˚ ˛ = 89.93◦

a = 4.028 A˚ c = 4.057 A˚

diffraction patterns of undoped, Nd and Cr doped crystals. Formation of pure perovskite phases was confirmed from the X-ray diffraction pattern. Absence of secondary phases showed that Nd and Cr have good solubility in PZNT. Rietveld refinement of the X-ray powder diffraction data was performed with FULLPROF [21] program for quantitative analysis. Scaling factors and lattice parameters were refined simultaneously. For PZN-PT (91/9), various reports have been published on the coexistence of phases near MPB. Zhang et al. [19] has shown that the rhombohedral R3m and tetragonal P4mm phases coexist in PZNT (91/9) whereas Cox et al. [22] has reported the presence of monoclinic phases Pm and Cm. Also, Uesu et al. [23] ended up with cubic lattice with space group Pm3m on refinement of PZNT (91/9). Hence, in the present work, several models were tested: all the samples were assumed to be single phase rhombohedral R3m, single phase monoclinic Pm, single phase monoclinic Cm, phase mixtures between rhombohedral R3m–tetragonal P4mm and single phase cubic Pm3m. Fig. 2b shows the observed, calculated and difference profiles obtained for Nd:PZNT. The fit between the observed and calculated profiles was quite good confirming the coexistence of rhombohedral R3m and tetragonal P4mm phases in undoped and doped PZNT. The results of the refined lattice parameters are given in Table 1. 3.2. Dielectric and AC conductivity studies Fig. 3a–c shows the variation of dielectric constant and dielectric loss as a function of temperature for undoped, Nd and Cr doped PZNT crystals. The dielectric, ferroelectric and piezoelectric properties of undoped and doped crystals are listed in Table 2. Fig. 3 clearly demonstrates the relaxational behavior characterized by diffuse dielectric peak and shift in the temperature of dielectric maximum (Tm ) with increasing frequency which is common in A(B B )O3 perovskites that occurs as a result of short range non-stoichiometric order between the B-site ions [24,25]. Nd3+ has ionic radii of 1.04 A˚ which is almost close to that of Pb2+ having ionic radii of 1.32 A˚ [26]. Hence, the probability of Nd entering the Pb site, i.e. A-site is higher [27,28]. So Nd ions act as donor dopants and effectively compensate

Table 2 Dielectric constant ε , Curie temperature TC , remnant polarization Pr , coercive field Ec , piezoelectric coefficient d33 of 0 and 1 mol% Nd-doped and Cr-doped 0.91Pb(Zn1/3 Nb2/3 )O3 –0.09PbTiO3 single crystals. Crystal

ε

TC (◦ C)

Pr (␮C/cm2 )

Ec (kV/cm)

d33 (pC/N)

PZNT Nd:PZNT Cr:PZNT

46,500 47,110 31,755

179 156 186

18.2 25.6 21.4

7.1 6.3 9.3

2160 2450 1778

Enhancement in the piezoelectric coefficient (d33 ) and dielectric constant (εr ) of PZNT single crystals by doping neodymium (Nd) is reported for the first time. The influence of defects created due to donor (Nd) and acceptor (Cr) dopants on structural, dielectric, ferroelectric and piezoelectric properties of PZNT single crystals have been studied in detail.

Fig. 3. Dielectric spectra of (a) PZNT crystals, (b) Nd:PZNT crystals and (c) Cr:PZNT crystals.

the charge imbalance by producing lead vacancies [29] and reducing the oxygen vacancy concentration according to the following equation: Pb2+

Nd3+ −→NsPb +

1  V 2 Pb

(1)

But B-site doping in PZNT is quite complicated, because dopant ion can either act as an acceptor or a donor depending upon whether it replaces the Nb5+ or Zn2+ or Ti4+ site [30]. So mixed properties are expected due to the distribution of the doped ions at the B-site. Thus, the competition between Nd and Cr to occupy the lattice sites in PZNT increases the disorderness in the material

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leading to large dielectric relaxation as observed from the strong diffuse dielectric peaks of the doped crystals. The dielectric constant (ε ) shows strong dispersion at higher frequencies which is attributed to the vibration of the domain walls and due to the existence of polar nanodomains that contributes to an order–disorder type paraelectric–ferroelectric phase transition which is common in ABO3 perovskite structures. The dopants have a significant effect on the Curie temperature TC which reduces for Nd:PZNT and increases for Cr:PZNT which can be explained as follows: in the case of undoped PZNT, any particular ion in the lattice experiences the same average field induced by other ions in the lattice. Hence, domains are formed during ferroelectric phase transition when the ion in the lattice gets shifted. In contrast, PZNT doped with Nd and Cr can be considered as a system with more disorderness where random field distribution created by dopant ions and intrinsic lattice vacancies changes TC . Also the oxygen octahedra undergo large distortion due to the stress created by the substitution of the dopants which is also a reason for the change in TC . Reduction of oxygen vacancies in Nd:PZNT facilitates the movement of domain walls and this motion of domain walls can be the cause for higher value of ε . When Cr preferentially occupies the B-site, oxygen vacancies would be produced and these defects have a tendency to cluster at the domain boundaries providing pinning sites for polarization [31]. Particularly the 180◦ domain motion is disturbed. In addition, these vacancies form complex dipolar defects with dopants which remain as point defects in the domain walls or interfaces and impose an energy barrier against migration which lowers ε [32]. This is consistent with the reports by Priya et al. [16] and Kobour et al. [33] where an acceptor doping like Mn increases the diffuseness of ferroelectric to paraelectric transition as well as induces the effect of pinning on the domain dynamics which decreases the dielectric constant and piezoelectric coefficient. Increase in TC shows that addition of Cr induces ‘hard’ characteristics in the crystal. It was found that dielectric loss (tan ı) was around 0.5% and 0.38% for Nd and Cr doped crystals, respectively, whose values are slightly increased when compared with undoped crystal (0.28%). The dielectric loss of doped crystals showed large frequency dispersion similar to the behavior of dielectric constant. The dependence of the dielectric permittivity of diffuse phase transition (DPT) ferroelectrics on temperature above the TC differs from the Curie–Weiss law over a wide temperature range. Uchino et al. [34] proposed an empirical expression to describe the diffuseness of the phase transition: 1 1 (T − Tm ) = + ε εm 2εm ı2

(2)

where ε and ε m are the dielectric constant and its maxima, respectively, ␥ is diffusivity and ␦ is the diffuseness parameter. A linear fit was obtained when log(1/ε − 1/ε m ) was plotted with log(T − Tm ) at various frequencies for undoped and doped samples. Fig. 4a and b shows the variation of log(1/ε − 1/ε m ) with log(T − Tm ) at 100 Hz and 500 kHz for Nd:PZNT crystals from which  ranged from 1.33 to 1.89 with frequency. Similarly, for PZNT the value of diffusivity varied from 1.33 to 1.69 and for Cr:PZNT from 1.34 to 1.84 with frequency ranging from 100 Hz to 500 kHz. The enhanced value of  confirms that Nd and Cr doping induces the relaxational behavior in the crystals. The conductivity of dielectric materials are influenced by the ordered motion of weakly bound charge carriers in the presence of an external field and dominated mainly by the type of charge carriers. This conduction process in turn affects the piezoelectric, ferroelectric and dielectric properties of the material. Further, the electrode effect (unwanted parasitic impedance caused by the accumulation of ions on the electrode surface) and the defects play a major role in the conduction behavior of a system. Thus, study of

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Fig. 4. Variation of log(1/ε − 1/εm ) with log(T − Tm ) for Nd:PZNT crystals at (a) 100 Hz and (b) 100 kHz.

electrical conductivity with frequency and temperature provides an insight on the role of defects to the conduction process. The conductivity in a complex perovskite system PZNT can be due to the defects in the system, i.e. oxygen vacancies, lead vacancies, dipolar defect pairs (lead and oxygen) and space charges [35,36]. Out of these, the defect whose activation energy is minimum would dominate the conduction process. Fig. 5a–c shows the variation of AC conductivity as a function of inverse of temperature for different frequencies for undoped and doped PZNT crystals. AC conductivity was calculated using the equation: ac = ωεo ε tan ı

(3)

where ω is the angular frequency and εo is the vacuum permittivity. AC conduction in disordered solids like PZNT is an increasing function of frequency and the same trend is followed in the case of doped PZNT crystals also, thus, endorsing the hopping model of conduction mechanism [37]. At higher frequencies, large jump probability exists and hence, hopping of ions backwards and forwards takes places between pairs of localized states in the crystal giving larger AC conductivity. At lower frequencies there was a large dispersion in AC conductivity whereas at higher frequencies the conductivity curves merge together supporting the disordered

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Fig. 5. Variation of AC conductivity with temperature at various frequencies for (a) PZNT crystals, (b) Nd:PZNT crystals and (c) Cr:PZNT crystals. (d) Variation of AC conductivity with frequencies for undoped and doped PZNT crystals at 180 ◦ C.

nature of the PZNT system. Variation in AC conductivity with frequency for doped crystals compared to the undoped counterpart at 180 ◦ C is shown in Fig. 5d. In lead based perovskite system, oxygen vacancy is the most dominant mobile charged defect. Activation energy of oxygen and lead vacancy conduction in perovskite system are ∼1 eV and ∼1.4 eV, respectively. Thus, due to the presence of more oxygen vacancies in Cr:PZNT, it exhibits higher conductivity. In the stoichiometric composition of the acceptor-doped material like Cr:PZNT, the acceptor centers are charge-compensated by oxygen vacancies. Here the charged acceptor centers serve as deep hole traps so that conduction by electrons becomes dominant thus resembling the phenomena of n-type conductivity as in the case of a electronic semiconductor [38]. The higher activation energy of lead vacancies in Nd doped PZNT makes it less conductive. Below the temperature of the dielectric maxima Tm , AC conductivity of the material shows almost a linear behavior. The activation energy was calculated for the region  below Tm by fitting the Arrhenius equation:(4)ac = 0 exp

3.3. P–E hysteresis studies Fig. 6 shows the high field (P–E) hysteresis for undoped, Nd and Cr doped PZNT crystals measured at 20 Hz and 15 kV/cm. The augmentation of Pb vacancies intensifies the effect on the domain

−Ea kB T

Higher activation energy for Nd:PZNT (0.29 eV @ 500 kHz) explains less hopping of ions in this donor doped system. The activation energy eventually decreases for Cr:PZNT (0.21 eV @ 500 kHz) thus exhibiting higher conductivity than the undoped one (activation energy for undoped PZNT crystal was 0.24 eV @ 500 kHz). For both undoped and doped samples activation energy decreases with frequency which is consistent with the previous reports [39].

Fig. 6. Hysteresis behavior of undoped and doped PZNT crystals.

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movement due to the elongation of the unit cell in the direction of polarization causing a depinning effect resulting in higher Pr in Nd:PZNT [40]. Eva-Maria Anton et al. [41] proposed four domain switching mechanisms for the underlying electromechanical interactions in a material: simple switching, grain boundary induced switching, negative poling mechanism and domain reswitching mechanism. Presently, addition of Nd exhibits simple switching mechanism where the energetically preferred fraction of domains align with the direction of the applied electric field, i.e. the domains whose local dipolar energy is greater than the coercive energy will switch causing the remnant polarization to increase and coercive field (Ec ) to decrease. The increase in Pr for Cr:PZNT can be explained by the domain reswitching mechanism, i.e. when external field is applied the first switching event takes place and the second reversing switching event is a result of the subsequent piezoelectric effect dominated behavior. The domains surrounding a central one of opposite polarity transform a fraction of their mechanical energy into electrical via the piezoelectric effect in directions that do not necessarily coincide with the applied field. When the local electromechanically induced electric field is greater in magnitude than the externally applied one, a second reversing switching event is induced. The domain reswitching mechanism is confined to a small area of ferroelectric domains and large electric fields. Increase in Ec is attributed to the defect dipoles formed by the oxygen vacancies (Cr3+ –V..0 ) [42]. When an external field is applied those defect dipoles that are parallel to spontaneous polarization elongate in an effort to decrease their electrostatic energy at a rate given by its local piezoelectric properties. Defect dipoles, antiparallel to spontaneous polarization, attempt to contract at the same rate of elongation. The spatial interlocking of both types of defect dipoles leads to a state of stress where the elongating dipoles are in compression and the contracting dipoles are in tension. This non-trivial state of stress increases the Ec of Cr:PZNT. Thus, oxygen vacancies play significant role in controlling the domain motion.

3.4. Piezoelectric studies To carry out piezoelectric measurements, the crystals were oriented along (0 0 1) crystallographic direction using Laue back reflection camera and poled at room temperature by applying an electric field of 12 kV/cm for 30 min. A static force of 0.25 N was applied on the electroded crystal for about 10 s to obtain the value of d33 . From Table 2, it is seen that the piezoelectric charge coefficient (d33 ) for Nd:PZNT is 2450 pC/N which is slightly high when compared to its undoped (2160 pC/N) counterpart. This is due to the fact that when the concentration of the donor impurity is nearer to that of the acceptor impurity originating from lead vacancies, most of the holes from the acceptor level are compensated by electrons from the donor level. This increases the bulk resistivity of the material and hence, the samples can be poled to a higher electric field wherein the domains are also easily reoriented resulting in higher d33 . Moreover, it is known that 60–70% of the total piezoelectric coefficient is due to domain movement (which is extrinsic contribution) and 30–40% is due to the dimensional change of the unit cell (which is intrinsic contribution) [43,44]. From the investigations of dielectric and ferroelectric properties of Nd:PZNT, it is obvious that the domain movement is enhanced compared to PZNT due to the creation of defects paving the way for high d33 . The increment of oxygen vacancies and the presence of intrinsic lead vacancies introduce space charge and thereby an internal bias field in Cr:PZNT which inhibits the domain motion. The increase of space charge reduces the resistivity and hence, d33 of Cr:PZNT.

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4. Conclusions Single crystals of Nd and Cr doped PZNT were successfully grown by self flux technique. Increase in the value of piezoelectric coefficient (2450 pC/N) and dielectric constant due to Nd doping has been reported in PZNT crystals which occurs as a result of enhanced domain motion thus, making them effective material for actuator applications. Domain pinning mechanism causes a decrease in ε and d33 for Cr:PZNT exhibiting ‘hard’ characteristics. Hysteresis of Nd:PZNT exhibits a simple switching mechanism whereas the oxygen vacancy related dipolar defects plays a major role in the polarization switching behavior of Cr:PZNT.

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