Synthesis and Characterization of Li-Modified AgTaO3

HIDAYAT ULLAH KHAN1 and YASEEN IQBAL2,3

1.—Institute of Physics and Electronics, University of Peshawar, Peshawar 25120, Pakistan.2.—Materials Research Laboratory, Institute of Physics and Electronics, University of Peshawar,Peshawar 25120, Pakistan. 3.—e-mail: dryaseeniqbal@yahoo.co.uk

(LixAg1�x)TaO3 (0 £ x £ 0.15) ceramics were fabricated by a mixed-oxide solid-state sintering route. Compositions with x £ 0.1 sintered into single-phasecompounds and could be indexed according to the rhombohedral, R�3c or R3cstructure. For x > 0.1, x-ray diffraction revealed second-phase formation,probably LiTaO3 and Ag2Ta8O21. It was noticed that the metastable and stablephase solubility limits for Li in AgTaO3 were x< 0.05 and 0.10< x< 0.15,respectively. Raman and dielectric measurements confirmed the existence of aferroelectric state in the compositions with x ‡ 0.05. This triggering of ferro-electricity supports the premise that substitution of relatively smaller ionsincreases the displacement of the A-site cation. The transition temperaturewas found to increase as a function of increasing Li concentration. Low-tem-perature (<100 K) Raman spectra and electron diffraction indicated thatLi-doped AgTaO3 ceramics seem to adopt new tilt systems.

Key words: Ferroelectricity, phonon dynamics, phase transition, quantumparaelectric, dielectric response

INTRODUCTION

AgTaO3 (AT) has not been well investigated becauseof difficulties in its synthesis.1 The main problems inits processing are rapid Ag decomposition at highsintering temperature (�1603 K) and cracking ofsintered pellets. To avoid or minimize these problems,AgTaO3 and Ta-rich compositions are fired using ele-vated (6.25 atm to 13 atm) oxygen pressures2,3 andlow ramp rates (1 K/min to 1.5 K/min).

The phase-transition temperatures of AgTaO3 arestill not settled due to the coexistence of phases, butthe ones generally accepted4 are shown in Fig. 1. Thephase-transition sequence starts from the high-temperature (>770 K) symmetric cubic (C) struc-ture, which transforms to tetragonal (T) phase at�770 K, followed by the monoclinic (M4) phase below�694 K. The M4 phase exists in a narrow tempera-ture range and transforms to the rhombohedralphase at �667 K.

Francombe and Lewis1 reported the paraelectricorthorhombic multiple-cell perovskite-type structure

for polycrystalline AgTaO3 (AT) at 293 K with latticeparameters ao = co = 3.931 A, bo = 3.914 A, andb = 90�21¢. This symmetry and these lattice parame-ters were also confirmed latter.5 Another study6

reported rhombohedral symmetry for AT having lat-tice parameters a = 3.925 A and a = 89�38¢.Lukaszewski et al.7 proposed two possible spacegroups, R3m (C3v

5 ) and R�3m (D3d5 ) for single crystals of

AgTaO3 grown by the molten salt method. Subsequentstudies of single-crystal AT8,9 reported a rhombohe-dral symmetry, whereas the latter study9 preferablydescribed the crystal symmetry of AT as acentric R3cat room temperature. By analyzing temperature-dependent Raman scattering from single-crystal AT,Kugel et al.4 also reported an additional phase tran-sition upon heating at 170 K, and cooling at 120 K.They proposed that this phase transition may beassociated with the occurrence of ferroelectricity. Arecent micro-Raman and dielectric study of ATceramics3 reported the formation of rhombohedral(R�3c) structure with aR = 5.5758 A and aR = 59.44� atroom temperature, in contrast to the ferroelectricstate proposed in earlier investigations.

Li is considered to be a highly polarizable ionexhibiting its typical off-centering behavior in the(Received December 9, 2013; accepted April 23, 2014)

Journal of ELECTRONIC MATERIALS

DOI: 10.1007/s11664-014-3219-x� 2014 TMS

cuboctahedral interstice of the perovskite struc-ture.10 Li+ substitution for Ag+ in AgTaO3 has,therefore, been reported to induce a ferroelectricstate with Pr � 15 lC/cm2 (�180 kV/cm) at 77 K for12% Li, underlying the transition2,11

R�3c a�a�a�ð Þ ! R3c a�a�a� þ displacementð Þ;

where R�3c is a quantum paraelectric or incipientferroelectric version of the rhombohedral structure.The Glazer notation, a�a�a�, of the associated tilt-ing scheme indicates that the oxygen octahedra areequally oriented along x-, y-, and z-axes with anangle other than 90�. With Li substitution on theA-site, the hidden ferroelectricity is unlocked due tothe displacement of Li+ cations. R3c is thus thespace group of the same ferroelectric structure withthe same Glazer notation but with an extra factorfor the ionic displacement. This phenomenon, ingeneral, is also consistent with the premise12 thatdoping of small-radius ions on the A-site increasesits shift from the mean position. The occurrence ofthis ferroelectric state has stimulated furtherresearch interest in these materials. The purpose ofthis study is to investigate Li-modified AT compo-sitions to explore their potential for possibleferroelectric applications.

EXPERIMENTAL PROCEDURES

All the reagents were first dried according to theirspecific requirements, i.e., Ta2O5 (99.99+%; Sigma-Aldrich) at 1173 K for 12 h, Ag2O (99.9+%; Sigma-Aldrich) at 373 K for 8 h, and Li2CO3 (99.9+%;Sigma-Aldrich) at 373 K for 8 h. (LixAg1�x)TaO3

(0 £ x £ 0.15) batches were prepared by mixingcarefully weighed stoichiometric ratios of Ag2O,Ta2O5, and Li2CO3. The batches were mix-milled indistilled water using alumina medium in disposablemill jars for 24 h, employing horizontal ball milling.The dried mixture was calcined at 1348 K to 1378 Kfor 12 h in a sealed zirconia tube connected to anoxygen cylinder through a pressure control valve.The oxygen atmosphere is necessary to avoid/mini-mize loss of Ag and Li, and the flowing oxygenatmosphere allows the flow of CO2 gas out of thetube through the bubble chamber. Breaking of Li–Oand Ag–O bonds begins at 573 K, leading to loss ofLi and Ag; therefore, the greater the oxygen pres-sure, the lower such loss will be. The reacted pow-ders were then hand-milled using a pestle andmortar system and uniaxially pressed into 2-mm- to3-mm-high, 8-mm-diameter pellets at �50 MPa.The green pellets were isopressed using a cold iso-static press at 200 MPa for 2 min. This was followedby sintering at 1523 K to 1573 K for 3 h at a lowramp rate of 1 K/min in flowing O2. This procedureenabled processing of single-phase ceramics with novisible cracks/spots, suitable for further character-ization. A STOE PSD x-ray diffractometer (XRD)supported by WinXpow software (version 1.06; STOE

& Cie GmbH, Darmstadt, Germany) was used atscanning speed of 0.2�/min to determine the phasepurity and perform structural analyses of calcinedpowders and crushed sintered pellets. The micro-structure of as-polished pellets was examined usinga JEOL 6400 scanning electron microscope (Tokyo,Japan), equipped with an energy-dispersive spec-trometer (EDS, Links System; Oxford Instruments,Oxford, UK). Electron diffraction patterns andbright-field images of samples were recorded using aPhilips EM 430 ST transmission electron micro-scope (Eindhoven, The Netherlands) operating at300 kV. Raman spectroscopy of polished disc sur-faces was carried out using a Renishaw InViaRaman spectrometer (Renishaw Plc., UK) equippedwith a Peltier-cooled charge-coupled device (CCD)detector. The excitation source used was a contin-uous-wave Ar laser (20 mW, 514.5 nm). Data wererecorded in the wavenumber range from 50 cm�1 to1000 cm�1 at temperatures ranging from 83 K to700 K. Spectra were analyzed using Igor Pro soft-ware (IGOR Pro version 6.0; WaveMetrics, Inc.,Lake Owego, USA). The opposite surfaces of thepellets were gold-coated to measure the tempera-ture-dependent dielectric response using E4980A(10 K to 320 K) and HP 4284A (320 K to 850 K)LCR meters (Agilent, USA). P–E loops wereobtained from the electric current induced in aspecimen subjected to a periodical field using asinusoidal waveform. Integration of the currentwith respect to time yielded the electric charge,enabling calculation of the polarization in terms ofsurface charge density.

Fig. 1. Structural phase transitions in AgTaO3.

Khan and Iqbal

RESULTS AND DISCUSSION

Phase and Microstructural Analysis

XRD patterns of crushed sintered pellets of (LixAg1�x)TaO3 (LAT) compositions with 0 £ x £ 0.15(Fig. 2) could be indexed according to the rhombo-hedral R�3c or R3c symmetry. No second-phase for-mation was observed in the compositions withx £ 0.1 within the detection limit of the in-houseXRD facility even at a slow scanning speed of�0.2�/min. Due to the onset of the solubility limit,low-intensity XRD peaks, probably due to LiTaO3,Ag2Ta8O21, and Ta2O5 (marked as ‘‘*,’’ ‘‘U,’’ and ‘‘W’’in Fig. 2), could be observed in the XRD patterns for

the compositions with 0.1< x< 0.15. Li+ substitu-tion [with rLi � 1.25 A for coordination number(CN)12 derived from Ref. 13] for Ag+ (withrAg = 1.445 A for CN1213) caused slight shift of XRDpeaks of LAT ceramics towards relatively higher 2hvalues (Fig. 2, inset). Consequently, the volume ofthe unit cell decreased linearly as a function ofincreasing Li concentration (Fig. 3), thus obeyingVegard’s law.14

Visual examination of scanning electron micros-cope (SEM) images (Fig. 4) indicated that themicrostructure of Li-substituted samples was por-ous but more contiguous in comparison to the pureAgTaO3 samples. The maximum relative densityachieved for these samples was �90% (Table I).Although various microregions, e.g., for the samplewith x = 0.15, could not be distinguished, EDSanalysis revealed three types of spectrum on thebasis of the Ag peak intensity. The elemental com-position of microregion ‘b’ with slightly brightercontrast than ‘c’ appeared close to that of the majorLAT phase. Similarly, microregion ‘c’ with slightlydarker contrast appeared to be the Ag-deficientLiTaO3 phase, and the bright areas around thepores were Ag-rich, indicating Li loss at high sin-tering temperatures.

Raman Spectroscopy

Raman spectra of sintered samples with x £ 0.1 inthe temperature range 93 K to 238 K are shown in

Fig. 2. Room-temperature XRD traces for (LixAg1�x)TaO3 (0 £ x £ 0.15) ceramics showing the main perovskite phase XRD peaks for x< 0.15indexed according to Joint Committee on Powder Diffraction Standards (JCPDS) card no. 72-1398. Minor phases: * LiTaO3 (JCPDS card no.87-2461); U: Ag2Ta8O21 (JCPDS card no. 21-1344); w: Ta2O5 (JCPDS card no. 35-1193).

Fig. 3. Unit cell volume as a function of Li concentration.

Synthesis and Characterization of Li-Modified AgTaO3

Fig. 5a–c. For the composition with x = 0 (Fig. 5a),the phonon at �543 cm�1 almost disappeared withincreasing temperature from 93 K to 238 K, accom-panied by the emergence of a broad band at�235 cm�1 at �238 K (same for this mode). Addi-

tionally, the following changes were observed as afunction of increasing Li concentration:

(i) For x = 0.05, theRamanmodesarrowedat�93 cm�1

and�266 cm�1 almost vanished at�123 K (Fig. 5b).

Fig. 4. SEM micrographs from as-polished surface of (LixAg1�x)TaO3 ceramics for (a) x = 0, (b) x = 0.05, (c) x = 0.10, and (d) x = 0.15;(e), (f), and (g) EDS spectra recorded from microregions a, b, and c, respectively, of the sample with x = 0.15.

Table I. Observed variation in relative density with calcination and sintering temperatures

Composition (LixAg12x)TaO3 Calcination (K) Sintering (K) % Theoretical density

x = 0 1348 (12 h) 1523 (3 h) �90x = 0.05 1358 (12 h) 1523 (3 h) �89x = 0.10 1368 (12 h) 1543 (3 h) �87.5x = 0.15 1378 (12 h) 1573 (3 h) �87

Khan and Iqbal

(ii) For x = 0.1, the broad bands at �150 cm�1 and�268 cm�1, marked with arrows, became dif-fused and could be scarcely seen at 173 K. Theencircled sharp peak might be a spectrometerfault (Fig. 5c).

These observations confirmed a structural phasetransition (SPT) below �200 K.

As shown in Fig. 5d, at x ‡ 0.05, the TO1 mode(�81 cm�1) disappeared whereas extra modesappeared at �95 cm�1 and �267 cm�1. These pho-non dynamics clearly indicate that Li dopinginduced a structural phase transition in AgTaO3.These changes were also accompanied by broaden-ing of the whole spectrum at both temperatures, i.e.,293 K and 93 K. Further broadening of the spectra,particularly the one for the A1g + Eg mode at�239 cm�1, was apparent, which seems to be due to

increased disorder on the A-site with an increase inLi concentration (Fig. 5e). Presumably, this dis-torted lattice may cause coalescence of the modes at�95 cm�1 and �106 cm�1 into a single mode at�112 cm�1.

The simplest possible Li-induced phase transi-tion, suggesting a loss of the center of symmetry,seems to be R�3c fi R3c. Such a transition ispurely ferroelectric in nature and difficult to detectby XRD since there is no associated ferroelasticdistortion of the cell, which accompanies, e.g., thecubic fi tetragonal transition in BaTiO3. How-ever, Raman is sensitive to the onset of purelyferroelectric transitions since the selection rulesare relaxed due to the breakage of an inversioncenter, thereby activating five E and three A1

modes.3 To further investigate these transitions,

Fig. 5. Raman spectra of sintered pellets of (LixAg1�x)TaO3 as a function of increasing temperature for (a) x = 0, (b) x = 0.10, and (c) x = 0.15,and phonon dynamics at (d) 93 K and (e) 293 K as a function of increasing Li content.

Synthesis and Characterization of Li-Modified AgTaO3

electron diffraction patterns and bright-fieldtransmission electron microscopy (TEM) imageswere recorded as a function of temperature andcomposition.

Electron Diffraction

Figure 6 shows a montage of the room-temperatureh100ip and h110ip (where ‘‘p’’ denotes pseudocubic)

Fig. 6. (a) Room-temperature (RT) bright-field (BF) TEM image, (b) corresponding h100ip and (c) h110ip ZADPs for (LixAg1�x)TaO3 (x = 0); (d)RT BF TEM image, (e) corresponding h100ip and (f) h110ip ZADPs for (LixAg1�x)TaO3 (x = 0.05); (g) RT BF TEM image, (h) correspondingh100ip and (i) h110ip ZADPs for (LixAg1�x)TaO3 (x = 0.10).

Fig. 7. Electron diffraction patterns recorded from AgTaO3 at 25 K along (a) h100ip and (b) h110ip axes.

Khan and Iqbal

zone axis diffraction patterns (ZADPs) along withcorresponding bright-field (BF) TEM images of theobserved domains in (LixAg1�x)TaO3. These pat-terns are qualitatively similar, indicating that thereis no cell multiplication as a function of increasingLi concentration; however, a visible change in theaverage width of the so-called ferroelectric domainscan be noticed in the corresponding BF TEM ima-ges. This width (�20 nm for x = 0, increasing to�25 nm for x = 0.05) indicates the transition from a

quantum paraelectric to ferroelectric state. Thistriggering of ferroelectricity is strongly supportedby the subtle disappearance of the room-tempera-ture phonon mode A1g + Eg (Fig. 5d). The 1/2{ooe}-type reflections visible in the h100ip and h110ipZADPs recorded for the composition with x = 0(Fig. 7) indicate the suggested in-phase tilting(a�b�c+) of oxygen octahedra. Additionally, thesereflections are inconsistent with the Pawełczykphase diagram,15 i.e., with purely R3c or R�3c

Fig. 8. Relative permittivity and dielectric loss for (LixAg1�x)TaO3 ceramics for (a) x = 0, (b) x = 0.05, (c) x = 0.10, and (d) x = 0.15, and (e)comparison of their relative permittivities at 1 MHz.

Synthesis and Characterization of Li-Modified AgTaO3

symmetry. In the first instance, these reflectionspossibly seem to pertain to the orthorhombic (Pbcm)structure; however, further work is required toreach a decisive conclusion about their nature.

Dielectric Properties

Figure 8 shows the variation of the relative per-mittivity (er) and dielectric loss (tan d) with Li con-tent (x) at 1 MHz for LAT samples recorded at 50 Kto 850 K. It was difficult to precisely resolve thetransition temperature due to the broadening of theanomaly for the x = 0 composition, but it seemed tooccur at some temperature from 40 K to 100 K. Theobserved diffusion of the peak was in clear contrastto the ones appearing for compositions with x > 0.Additionally, the Curie temperature (Tc) wasobserved to increase from �116 K at x = 0.05 K to�250 K at x = 0.15. These er peaks corresponded toa sharp increase or peaking of the tan d curves atalmost the same temperatures.

The behavior of AgTaO3 seems to be analogous tothat of SrTiO3 (ST), for which large quantum-mechanical fluctuations disrupt the ferroelectricnature in favor of a paraelectric state. Like inST,16,17 the permittivity of AT increased as a func-tion of decreasing temperature for a wide range offrequencies (1 kHz to 1 MHz) and finally saturatedat a high value (>180). Thus AT, like ST,18,19

exhibits a quantum paraelectric behavior which canbe induced to adopt a ferroelectric state by chemicalsubstitution as reported for ST.20,21 This impliesthat functional properties of both ST20–22 and ATare highly susceptible to dopants.

Ferroelectric Properties

In this study, hysteresis data were recorded atroom temperature (i.e., in the paraelectric state)

and were not expected to reveal ferroelectric hys-teresis. As shown in Fig. 9, the composition withx = 0 demonstrated a linear response, typical of aquantum paraelectric or incipient ferroelectric. Asthe Li concentration was increased, there was evi-dence of conductivity contributing to the polariza-tion loops. Further work is required to investigatethe polarization field loops below TC to establish thenature of LAT as a ferroelectric with switchablepolarization.

CONCLUSIONS

Pure AgTaO3 is essentially a quantum paraelec-tric or incipient ferroelectric in which a ferroelectricstate is unlocked by the substitution of smaller andrelatively more polarizable Li for Ag ions. Based onthe electron diffraction patterns, AgTaO3 seems toadopt orthorhombic (Pbcm) structure at low tem-peratures (25 K), which is inconsistent with thePawełczyk phase diagram; however, further studiesare required to investigate the symmetry changesthrough the transition and to analyze polarization–field loops below 100 K.

ACKNOWLEDGEMENT

The authors acknowledge the partial financialsupport extended to one of the authors by the Uni-versity of Peshawar, Pakistan.

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Fig. 9. Room-temperature ferroelectric loops for (LixAg1�x)TaO3

(0 £ x £ 0.10) ceramics.

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Synthesis and Characterization of Li-Modified AgTaO3