Complex GdSc1âxInxO3 Oxides - American Chemical Society

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Complex GdSc1−xInxO3 Oxides: Synthesis and Structure Driven Tunable Electrical Properties V. Grover,† R. Shukla,† D. Jain,† S. K. Deshpande,‡ A. Arya,§ C. G. S. Pillai,† and A. K. Tyagi†,* †

Chemistry Division, ‡UGC-DAE Consortium for Scientific Research, and §Materials Science Division, Bhabha Atomic Research Centre, Mumbai 400085, India ABSTRACT: Detailed structural and electrical investigations were carried out on a GdSc1−xInxO3 (0.0 ≤ x ≤ 1.0) series. The solubility of In3+ in GdScO3 could be increased by 20 mol % by changing the synthesis route. GdScO3, a technologically important dielectric material, could be synthesized at a temperature as low as 850 °C. An orthorhombic modification of GdInO3 could be stabilized by 20 mol % Sc3+ substitution, which is otherwise known to prefer hexagonal polymorphs. Nonpreference of trigonal bipyramidal coordination by Sc3+ led to the existence of a very narrow biphasic field. A very interesting observation is the existence of same nominal composition (GdSc0.1In0.9O3) in two different modifications (orthorhombic and hexagonal) in the biphasic region. Careful Raman spectroscopic studies highlighted the fact that the local polyhedral coordination of the Sc3+/In3+ undergoes an abrupt change as the phase relations evolve from orthorhombic to hexagonal phase field. The trend in the net shrinkage observed on the powder compacts matched well with the theoretical X-ray density calculated for various nominal compositions. A careful control of the composition, and consequently, the optimization of the structure, led to tuning of electrical behavior for the system GdSc1−xInxO3 from a conventional dielectric (up to x = 0.8) to a classical relaxor ferroelectric, for the nominal composition GdSc0.9In0.1O3. KEYWORDS: rare earth scandates, perovskites, relaxor, X-ray diffraction, Raman spectroscopy

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INTRODUCTION The rational design of advanced materials assumes an understanding of the correlation between basic crystal chemistry and simple structure−property relationships. Implicit in such a relationship is an understanding of the underlying connection between chemical composition and crystal structure in solid-state materials. The compounds with the nominal formula ABO3 are perhaps the most versatile class of materials in terms of these composition−structure relationships. Various groups have been working on this aspect, and different approaches have been adopted to explain these relationships on factors such as ionic radii (radius ratio rules), bond ionicities, etc.1 The compounds with the nominal formula ABO3 are most generally synonymous with perovskite lattice. However, there is a plethora of structures adopted by ABO3 compositions (depending on various factors) such as corundum, bixbyite, ilmenite, perovskites, and several interesting hexagonal phases. There is lucid review written by Giaquinta et al.1 describing structural variations observable in this class of compounds. Generally, an ideal perovskite is characterized by corner sharing BO6 octahedra wherein a larger A cation occupies the bigger cuboctahedral site. However, if A-cation is smaller than what is required to form ideal perovskite structure (as in the case of REScO3), BO6 octahedra tilt, which results in two of the RE− O distances becoming much larger, and this, in turn, decreases the coordination for A-cation and also decreases the space group symmetry. Hence, the coordination polyhedra for the lanthanide cation in REScO3 can be best described by 8 instead of 12.2 On the contrary, the B site is not much affected by this distortion. © 2012 American Chemical Society

The rare-earth scandates, REScO3, which are known to crystallize in orthorhombic modification, are being investigated for various applications, which become all the more interesting due to their composition driven structure variations. For example, they have been reported as high-temperature protonic conductors.3,4 They are also high-dielectric-constant materials with very wide band gaps. The development of these RE scandate single crystals as substrates has enabled the growth of highquality films of a variety of ferroelectric,5−8 multiferroic9−11 and superconducting12 perovskites; the uniform strain that can be achieved in sufficiently thin commensurate epitaxial films on these RE scandates has allowed their ferroelectric properties to be tuned.13 At the structural front, it has been elucidated by Giaquinta et al.1 that if the radius ratio rA/rB further decreases, there is a possibility of various other structures; low symmetric hexagonal modifications given by space group P63cm and P63mmc are few of them. This hexagonal structure is only known for trivalent A cations, which may be In or a small rare-earth ions (Gd−Lu, Sc). It is reported that the B cation can be Al, Ga, In, Mn, Fe, 1:1 Cu/ Ti, 2:1 Cu/V, or 3:1 Cu/Mo.14−20 REInO3 (RE = Eu−Ho, Y), which can be prepared at atmospheric pressure, are also hexagonal (a ∼ 6.3 Å, c ∼ 12.3 Å, Z = 6) and possess the noncentrosymmetric space group P63cm and the LuMnO3 structure. The In3+ ions are located in 5-fold-coordinated trigonalReceived: March 22, 2012 Revised: May 10, 2012 Published: May 18, 2012 2186

dx.doi.org/10.1021/cm300908h | Chem. Mater. 2012, 24, 2186−2196

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bipyramidal sites, and the Ln3+ ions are 7-fold coordinated.21 HoInO3 and YInO3 transform to cubic solid solutions with the Ctype rare-earth oxide structure above ∼1000 °C. TbInO3, GdInO3, and EuInO3 transform to a much denser orthorhombic perovskite at high pressure, but DyInO3, HoInO3, and YInO3 first transform to pseudo-hexagonal orthorhombic phases at ∼20 kbar/1000 °C. No hexagonal form of ErInO3 could be prepared at 950−1050 °C and 0−34 kbar. Structurally, rare-earth indates are interesting because it is easier to explain existence of Mn3+ in trigonal bipyramidal (TBP) coordination (based on Jahn−Teller distortion),22 but it is not very common for In3+ to occupy TBP coordination. Hexagonal AMnO3 compounds have been of interest as multiferroics because these compounds are ferroelectric and magnetically ordered.23 Rare-earth indates, REInO3, as well other compounds belonging to the hexagonal ABO3 structure have evoked a lot of interest in recent years owing to the possibility of geometric ferroelectricity in these materials.24 Hexagonal indates are known to exist in two polymorphic forms, paraelectric (space group P63mmc) and ferroelectric (space group P63cm). The transition from ferroelectric to paraelectric form is a displacive one. Other than the structural point of view, GdScO3 has held tremendous interest of materials chemists due to its high dielectric constant and associated applications, whereas GdInO3 is postulated to be a good geometric ferroelectric material. The difference between the two structures primarily originates from difference in rA/rB radius ratios, which lead to distortion of A−O polyhedra. Even in REInO3 structures, there is a possibility of two polymorphs that are related by small displacive changes with different electrical properties. All these factors make GdScxIn1−xO3, an extremely interesting system for exploring the structural variations (brought about by variation in radius ratio) and the accompanying changes in electrical properties. Also, since geometric ferroelectricity is attributed to the presence of non-centrosymmetric atomic arrangement of crystal, which is caused merely by ionic size effects. Hence, it would be of interest to observe the existence or absence of such an effect by changing average ionic radii at the B-site. Hence, the present work attempts to explain the phase and structure evolution on progressing from a distorted orthorhombic perovskite (GdScO3) to a hexagonal perovskite-related structure (GdInO3). The various compositions are thoroughly characterized by X-ray diffraction (XRD), thermomechanical analysis, and electrical characterizations. Raman spectroscopy, because of its ability to probe local distortions, has been employed to study the subtle structural variations occurring throughout the series. It has been attempted to relate the observed electrical behavior to the structure of the system.

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of glycine as fuel. The combustion reactions were carried out in the fueldeficient stoichiometry wherein the oxidant-to-fuel (O/F) ratio was kept at 1:1.5 (Stoichiometric O/F ratio for this reaction is 1:3.33). The combustion proceeds at an acidic pH, and no special efforts were made to control the pH in the present study. The solutions were dehydrated to highly viscous liquids (gels). At this stage, the temperature was raised to 250 °C. The viscous liquid then swelled and auto-ignited, with a rapid evolution of large volume of gases to produce voluminous powders. The powders were annealed at 850 °C in static air for 6 h. Characterization. The two sets of products so obtained were characterized by powder X-ray diffraction using monochromatized Cu Kα radiation on a Panalytical Xpert Pro. Silicon was used as external standard. The patterns were refined using Rietveld refinement, and the lattice parameters were calculated. Raman spectroscopic measurements were carried out on a micro/ macro-Raman spectrometer (LABRAM-1, France) using a 488 nm line of an Ar+ ion laser for excitation. The scattered Raman signal was collected using a single monochromator spectrometer equipped with a Peltier-cooled charge-coupled device (CCD) detector in the backscattering geometry. Samples were used in the form of pellets, and a laser line was focused on a flat surface of the sample using an optical microscope (Olympus BX-40, 50× objective lens) connected to the spectrometer. The spectra recorded were averaged out of 50 scans with a time interval of 2 s and a resolution of 2 cm−1. All the densification studies were carried out in a vertical thermomechanical analyzer (TMA, SETARAM, Model SETSYS 1600 TMA) in a near zero load mode. The cylindrical green pellets (8 mm diameter, ∼375 mg) were mounted on the TMA, and the shrinkage profiles were recorded in a helium atmosphere (15 mL/min) from 35 to 1350 °C (20 °C/min) followed by isothermal soak period of 1 h. Samples were cooled to 35 °C (20 °C/min). True shrinkage profiles were obtained after correcting for the thermal expansion on the same sintered samples. Dimensional densities of the samples were measured before and after the shrinkage experiments. Theoretical densities of the prepared compositions GdSc1−xInxO3 were calculated using the refined lattice parameters. The microstructure of the pellet of a representative composition was investigated using an AIS 210 scanning electron microscope (Mirero Inc., Korea). The pellet was coated with gold before performing the microstructural studies. The dielectric properties of the GdInxSc1−xO3 compounds were measured over a temperature range from 25 to 300 °C and a frequency range of 10 Hz to 5 MHz using a Novocontrol Alpha-AN impedance analyzer (Novocontrol Technologies GmbH, Germany) equipped with a Quatro nitrogen gas cooling/heating system. Samples in the form of cylindrical pellets were sandwiched between two gold-plated electrodes in a parallel plate capacitor configuration. Samples were coated with silver paste for proper electrical contact.

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RESULTS AND DISCUSSION

Structural Characterization. (1) Solid State Synthesized Products. The entire GdSc1−xInxO3 series heated at 1200 °C followed by 1300 °C was thoroughly characterized by X-ray diffraction (XRD). The XRD pattern (not shown in the manuscript) of the end member GdScO3 matched well with that reported for orthorhombic GdScO3, space group Pnma (JCPDS Card No. 79-0577). It was observed that, on substituting 10 mol % In3+, the orthorhombic lattice was retained. The trend was continued until 60 mol % In3+ substitution, beyond which a biphasic region of a orthorhombic GdScO3-type phase and a hexagonal GdInO3-type phase was observed, which continued to persist until 90 mol % In3+. Pure GdInO3 was found to possess hexagonal structure given by space group P63cm. Previously, a similar phase relation between orthorhombic GdFeO3 and GdInO3 had been reported by Kuo et al.,25 wherein they observed a solubility of 25 mol % of In3+ into GdFeO3. The solubility of In3+ into GdFeO3 lattice had a pronounced effect on

EXPERIMENTAL SECTION

AR (analytical reagent) grade powders of Gd2O3, Sc2O3, In2O3, and glycine were used as the starting reagents. Two synthetic approaches were adopted to investigate the differences in phase relations and solubilities. Solid State Synthesis. Herein, the stoichiometric amounts of the reactants (viz. Gd2O3, Sc2O3, and In2O3) required for various nominal compositions in the GdSc1−xInxO3 (0.0 ≤ x ≤ 1.0) system were weighed. The different nominal compositions were ground, pelletized, and heated at 1200, 1300, and 1400 °C for 24 h each. All of the three heatings were performed with intermittent grindings to obtain better homogeneity in the reaction mixture. Gel Combustion Synthesis. To obtain different compositions of GdSc1−xInxO3 (0.0 ≤ x ≤ 1.0), stoichiometric amounts of reactants were dissolved as nitrates and were made to undergo combustion in presence 2187



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Figure 2. Lattice parameters of the nominal compositions in GdSc1−xInxO3 (0.0 ≤ x ≤ 1.0) with increase in In3+ content.

Figure 1. XRD patterns for representative compositions GdScO3, GdSc0.4In0.6O3, and GdInO3.

Table 2. Bond Lengths of the BO5 Polyhedra Obtained from the Rietveld Refinements

electrical properties of these compositions. To explore the possibility to extend the phase width of single-phase compound, the samples were further heated at 1400 °C. However, the XRD analysis on the samples heated at 1400 °C showed that the samples containing In3+ have decomposed into Gd2O3, Sc2O3, and two types of ABO3 phases (orthorhombic and hexagonal). (2) Gel Combustion Synthesized Products. It has been observed that the phase relations can be altered depending upon the synthesis route followed.26 To investigate the similar effect on the present series, the GdSc1−xInxO3 system was synthesized by gel combustion route, followed by annealing at 850 °C. The XRD analysis revealed GdScO3 and GdInO3 to possess orthorhombic and hexagonal lattices, respectively, as was observed in solid state synthesis. To the best of our knowledge, this is the lowest temperature at which GdScO3 and its derivatives have been synthesized. As mentioned earlier, the scandates are materials that are highly in demand for their interesting electrical properties. Therefore, the synthesis of these technologically important materials at lower temperature is significant. The indexed XRD patterns for GdScO3 and GdInO3 are shown in Figure 1. Interestingly, now the single-phasic orthorhombic phase field could be observed up to as high as 80 mol % In3+ substitution. The nominal composition containing 90 mol % In3+, however, was biphasic, with the major phase being a

B−O Bond Lengths (Å)

a

a

composition

B−Oap1

GdInO3 GdIn0.9Sc0.1O3

2.1383 2.1408

B−Oap2

B−Oax1b

B−Oax2

B−Oax3

2.1167 2.1190

2.1466 2.1428

2.1393 2.1355

2.1396 2.1358

ap: apical. bax: axial.

Figure 3. Schematic depicting the release of lattice strain on Sc3+ introduction in GdInO3 lattice.

Table 1. Crystallographic Data and Structure Refinement Parameters of the GdSc1−xInxO3 (0.0 ≤ x ≤ 1.0) System molecular formula molecular weight space group unit cell params a (Å) b (Å) c (Å) vol. (Å3) density (calcd) (g/cm3) refinement profile goodness-of-fit (χ2) Rp Rwp RF

GdScO3 250.2 Pnma

GdSc0.8In0.2O3 264.18 Pnma



5.7523 (2) 5.7738 (1) 5.7918 (2) 5.8153 (1) 7.9348 (2) 7.9675 (2) 7.9912 (4) 8.0221 (4) 5.4867 (2) 5.5007 (3) 5.5094 (2) 5.5235 (2) 249.95 (1) 253.04 (2) 254.99 (1) 257.67 (1) 6.64 6.93 7.24 7.53 Rietveld refinements (Fullprof-2K) (Rodriguez-Caravjal, 2000) pseudo-Voigt 3.11 2.52 2.71 2.19 12.5 11.3 13.7 13.2 18.4 14.9 17.1 17.1 16.6 10.2 8.12 7.09 2188


GdSc0.1In0.9O3 313.08 Pnma (6%)

P63cm (94%)

GdInO3 320.07 P63cm

5.8334 (3) 8.0449 (1) 5.5326 (2) 259.64 (4) 7.83

5.8429 (17) 8.0649 (19) 5.5426 (16) 261.18 (13) 7.94

6.3359 (2) & 12.3527 (5) 429.46(2) 7.26

6.3472 (1) & 12.3400 (1) 430.54 (2) 7.40

3.06 12.8 16.5 9.26

1.44 11.1 15.1 12.1

4.45

2.43 8.63 11.6 9.43



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Figure 4. Typical fitted patterns of the representative nominal compositions GdSc0.2In0.8O3 and GdSc0.1In0.9O3. Figure 6. Net linear shrinkage with In3+ content in GdSc1−xInxO3 (0.0 ≤ x ≤ 1.0).

Table 3. Equilibrium Ground State Cohesive Properties Obtained by Density Functional Theory Calculations on Hexagonal GdInO3 (P63cm), GdScO3 (P63cm), Orthorhombic GdInO3 (Pnma), and GdScO3 (Pnma) Structures theoretically calculated lattice parameters (Å) cmpd (space group) GdInO3 (P63cm (185)) GdScO3 (P63cm (185)) GdInO3 (Pnma (62)) GdScO3 (Pnma (62))

a

b

c

vol. (Å )

cohesive energy (eV/ atom)

6.4034

6.4034

12.4046

440.49

−9.5895

6.2608

6.2608

12.3122

417.94

−8.1015

5.8956

8.2201

5.5736

270.11

−8.2357

5.7834

7.9906

5.5003

254.19

−9.8826

3

Figure 7. SEM image for the nominal composition GdSc0.2In0.8O3.

phase observed in combustion-synthesized GdSc0.1In0.9O3 is miniscule, compared to that synthesized by solid state route. It should be noted that, as compared to solid state route, the

Figure 5. Measured shrinkage profiles for nanocrystalline GdSc1−xInxO3 (0.0 ≤ x ≤ 1.0) powder compacts as a function of time/temperature (after thermal expansion correction).

hexagonal solid solution and a small amount of orthorhombic phase. It should be noted that the amount of orthorhombic-type

Figure 8. Raman spectrum for different nominal compositions in the series GdSc1−xInxO3 (0.0 ≤ x ≤ 0.8). 2189



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pure GdInO3 to Sc3+ substituted GdInO3. Ideally, as also observed in the orthorhombic phase field, substitution of the larger In3+ by smaller Sc3+ should cause all the B−O bonds to decrease, consequently reflecting the same in the trends observed in lattice parameters. However, the refined data revealed (Table 2) that while planar (or axial) B−O bonds decrease in length as expected, the vertical (or apical) bond length increases. This trend in bond lengths was reflected in the lattice parameter values as well. The a (or b) cell parameter decreases, while the c parameter increases on proceeding from GdInO 3 to GdSc0.1In0.9O3 (majority hexagonal phase). This apparently happens to release the lattice strain produced on introduction of Sc3+ at the trigonal bipyramidal B-site. The schematic diagram depicting the same is shown in Figure 3. The Rietveld refinement on the nominal composition GdSc0.1In0.9O3 was performed by assuming the orthorhombic phase to be scandium-rich and hexagonal phase to be indiumrich. It would be reasonable to expect that the lattice parameters of the orthorhombic phase of this biphasic composition would be closer to the terminal orthorhombic phase (x = 0.8) formed by scandium-rich single-phasic compositions. However, it was observed to be much higher than expected. Subsequently, further detailed refinement studies were performed, and it was found that best fit was observed when both orthorhombic and hexagonal phases were assumed to have same composition, which is identical to the nominal composition, that is, GdSc0.1In0.9O3. A biphasic Rietveld refinement revealed the presence of 6% phase as orthorhombic and 94% phase hexagonal with identical composition (GdSc0.1In0.9O3) Hence, it appears that, in present scenario, the same conglomerate of ions exists in two different modifications. To the best of our knowledge, Sc3+ does not prefer trigonal bipyramidal (TBP) coordination (the Bcation in hexagonal ABO3 has TBP coordination) in ABO3-type compounds; plausibly, this is the factor behind the segregation of small amount of orthorhombic phase at 90 mol % In3+ containing composition. This way, some Sc3+ retains the preferred 6coordinated site. Thus, the phase separation is driven by nonpreference of Sc3+ for trigonal bipyramidal configuration. The typical fitted patterns of the representative nominal compositions GdSc0.2In0.8O3 (single-phasic orthorhombic) and GdSc0.1In0.9O3 (biphasic) are shown in Figure.4. On proceeding from GdScO3-type structure to GdInO3-type structure, the coordination number of A-site cation decreases from 8 to 7, whereas for B-cation it decreases from 6 to 5. It is the apparent stability of In3+ in both octahedral and distorted TBP surroundings that ultimately results in such high solubility (80 mol %) of In3+ in orthorhombic lattice. Vice versa, the fact that Sc3+ does not prefer such a distorted TBP environment leads to very limited solubility of Sc3+ (∼10 mol %) in the hexagonal lattice. Reduction of cell volume of GdSc0.1In0.9O3 compared to GdInO3 concludes the incorporation of Sc3+ in the hexagonal lattice of GdInO3. The unstability of Sc3+ in a trigonal-bipyramidal coordination, which is the feature of B-site of hexagonal GdInO3-type structure, was explored by first principle calculation. The energies of GdScO3 were calculated assuming both Pnma (orthorhombic) and P63cm (hexagonal) structures. For comparison, the energies of GdInO3 were also calculated for both the modifications. The projector augmented wave (PAW) potential plane-wave based density functional theory calculations were performed on hexagonal GdInO3 (P63cm) and GdScO3 (P63cm) structures and orthorhombic GdInO3 (Pnma) and GdScO3 (Pnma) structures using the generalized gradient approximation

Figure 9. Raman spectra obtained for nominal compositions GdScxIn1−xO3 (0.8 ≤ x ≤ 1.0); the peaks due to minor orthorhombic phase in x = 0.9 are encircled.

combustion synthesized GdSc0.1In0.9O3 contains the orthorhombic phase in minority. Hence, the phase width of single-phasic solid solution could be considerably extended by altering the synthesis route. In the soft chemical processes (such as the gel combustion adopted in present study), mixing is achieved at a atomic level, which, in turn, decreases the diffusion length for the reacting species in the reaction mixture (solution). This maintains the stoichiometry and homogeneity at an atomic level, and hence, there is an enhanced probability of obtaining a single phase product with homogeneous composition. This contrasts with the conventional solid state synthesis wherein the reacting ions have to diffuse through solids, which is further made difficult as the reaction proceeds and the thickness of product layer increases. Hence, this increased solubility of In3+ into GdScO3 can be explained based on the fact that, in case of gel combustion, the precursor is a gel wherein atomistic level mixing of the constituent ions (Gd3+, Sc3+, In3+) is present, which ensures much better homogeneity, faster kinetics, shorter diffusion path, and, in turn, higher solubility, and better probability of obtaining a single-phasic product. Such extension of solubility limits has been observed earlier, also in our group, in the CeO2−Sc2O3 system.26 Phase diagrams can be altered, and metastable phases can also be stabilized by varying the synthesis route, which has a bearing crystallite size, too.27 Further, under ambient conditions, hexagonal modification is the stable polymorph for GdInO3, whereas the orthorhombic modification for GdInO3 could be synthesized only at high pressure and high temperature.21 An interesting feature of the present work is the existence of GdSc0.2In0.8O3 nominal composition in orthorhombic phase. Hence, it is noteworthy, that the orthorhombic phase of GdInO3, which is otherwise metastable, could be stabilized at room temperature by Sc3+ substitution. Rietveld refinement was performed on all the compositions in the series. The refinement results are summarized in Table 1. It is observed that all the three lattice parameters of the orthorhombic phase increase with increase in In3+ content (Figure 2). The trend can be explained by increase in average cationic radii on substituting In3+ at Sc3+ site. The cationic radii of Sc3+ and In3+ in 6-fold coordination are 0.75 Å and 0.80 Å, respectively.28 An interesting observation is the variation of bond lengths and/or lattice parameters of the hexagonal phase on proceeding from 2190



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Figure 10. Temperature dependence of the dielectric constant, ε′, at several frequencies for GdSc1−xInxO3 (0.0 ≤ x ≤ 1.0).

(core radii) and reconstruct the exact valence wave function with all nodes in the core region. The PAW potentials used in this study are those provided in the VASP database (version 5.2). For Gd, the standard potential that treats semicore s and p states as valence states was used, while for Sc and In, Sc_sv and In_d potentials were applied, which treat semicore s and d states as valence states, respectively. For oxygen, the standard PAW

(GGA) for the exchange-correlation potential, as parametrized by Perdew−Burke−Ernzerhof.29 The “Vienna ab-initio simulation package” (VASP)30,31 was used, which solves the Kohn− Sham equations using a plane wave expansion for the valence electron density and wave functions. The interactions between the ions and electrons are described by the projector augmented wave (PAW) potentials,32,33 which use smaller radial cutoffs 2191



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abrupt change at x = 0.90 (946 °C) compared to x ≤ 0.80 (989 °C), and then, it remains nearly the same for x = 1.0 (945 °C). This trend in the onset temperature can be taken as a signature of structural phase transformation from orthorhombic (x ≤ 0.80) to hexagonal (0.9 ≤ x ≤1). It is interesting to note that if we observe the trends in theoretical densities as calculated from refined lattice parameters (Table 1), it remarkably matches the trend in net shrinkage observed in the thermomechanical analysis. As is observed with thermomechanical studies, the densities calculated from refined parameters increase with increases in In3+ content and reaches maximum at 80 mol % In3+ doping. Beyond that, it decreases with the biphasic composition showing the minimum calculated density. This shows the phase purity and the superior quality of the powder samples. The scanning electron microscopy (SEM) image for representative composition GdSc0.2In0.8O3 was recorded and presented in Figure 7. It shows the sample to be dense, and some of the grains were fused. The smaller grains have joined to give larger grains, which is quite evident from the given SEM image. The density was found to be ∼90% of the theoretical density. Phase Evolution as Studied by Raman Spectroscopy. It has been well documented and known that Raman spectroscopy is a highly sensitive and simple laboratory scale technique to detect local, as well as subtle, structural variations in the materials. To further explore the local structure in the present series, all the nominal compositions were subjected to Raman spectroscopic studies. It has been reported35 that the primitive unit cell of the cubic perovskite contains one formula unit of ABO3 giving rise to 15 vibrational modes at the zone center, from which none is Raman active. In the orthorhombic rare-earth scandates, on the other hand, the four REScO3 formula units consisting of 20 atoms per unit cell giving rise to 60 vibrational modes, as determined from factor group analysis, out of which 24 modes are Raman-active, which are given by 7Ag + 5B1g + 7B2g + 5B3g.35 Chopelas36 has done a detailed study on Raman specectroscopy of ABO3 orthorhombic compounds and has deduced that the A cation plays the dominant role in determining the Raman shift. It has been reported that even though all the 24 modes are theoretically predicted to be Raman-active for orthorhombic ABO3-type composition, to date none of the perovskites has shown all the modes. Since the undistorted perovskite has no Raman active mode, obviously, the number of Raman active bands will depend on amount of distortion and this determines the number of modes as well as their intensities exhibited by a particular ABO3 composition. Figure 5a shows the Raman spectrum of GdScO3, as observed in the present series. The ambient Raman spectrum and the different mode frequencies of GdScO3 in the present study agree quite well with the values reported in the literature.35 It should be noted that, in orthorhombic symmetry, the B cation occupies a site of inversion symmetry, and thus, the Raman spectra of these perovskites will not have modes that are due to B cation translations or vibrations of the octahedral unit.36 However, the volume change brought about by the substitution of the B-cation or the distortion imbibed by the structure due to B-site substitution will significantly affect the Raman shifts. A recent site symmetry analysis showed that, out of 24 Raman active modes shown above, 6 modes, which are the lowest frequency modes in Ag (2 modes), B1g (2 modes), B2g, and B3g (1 mode each), are due to A-site translations, while the rest are due to oxygen motions (bending, stretching, and tilting). In the present study, since the substitutions are being made at the B-

Figure 11. Frequency−temperature behavior (Vogel−Fulcher plot) for GdSc0.1In0.9O3.

potential as provided in the database was used. Our calculations are fully converged with respect to size of the basis set (kinetic energy cutoff, Ecutoff = 500 eV) and the number of k-points in the irreducible Brillouin zone (IBZ = 126 and 180 for hexagonal and orthorhombic structures, respectively). The hexagonal structures contained 30 atoms/unit cell, while the orthorhombic structures contained 20 atoms/unit cell. We optimized all internal degrees of freedom, viz., a, b/a, c/a and ionic positions for all the four structures. The ions were relaxed into their local minima such that the residual forces on each ion was less than 1 meV/Å. The respective energies calculated are tabulated in Table 3, which clearly show that Sc3+ is less stable in a GdInO3-type hexagonal modification. The table also predicts that In3+ should not have any strong preference for the orthorhombic structure (space group: Pnma). However, the observation of various indium-rich compositions in this modification definitely emphasizes the stabilization bestowed upon them by Sc3+ substitution. It is well-known that sinteractivity of electroceramics is a key issue for their applications. High sintered densities are important with respect to electrical behavior of the material. For example, it is always desired to have as high density as possible to negate the effects of any porosity in the system on its electrical properties. For this purpose, the densification behavior of the green pellets of these nominal compositions was investigated by a thermomechanical analyzer. The measured shrinkage profiles for asprepared GdSc1−xInxO3 (0.0 ≤ x ≤ 1.0) powder compacts as a function of time/temperature after thermal expansion correction are shown in Figure 5. Maximum shrinkage of ∼22% is observed in case of GdSc0.2In0.8O3. On the other hand, pure GdScO3 shows only 9.5% linear shrinkage and thus is found to have poor sinterability. Net shrinkage for further compositions in GdSc1−xInxO3 (0.80 ≤ x ≤ 1.0) decreases marginally. The net linear shrinkage with increasing indium content is depicted in Figure 6. The insets in the figure show the onset temperature of densification and the fractional theoretical densities of the compositions in both the green state and the sintered state. It is clear that incorporation of larger In3+ ions in the orthorhombic GdScO3 lattice introduces distortion, which facilitates atomic movement at surface as well as bulk of the particles, and, in turn, lowers the onset temperature of sintering as well as enhances the net densification.34 In the case of GdSc0.1In0.9O3, as mentioned earlier, traces of orthorhombic phase along with majority hexagonal phase perhaps result into relatively lower shrinkage. Careful observation of onset temperature of shrinkage reveals an 2192



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Figure 12. Temperature dependence of the dielectric loss tangent (tan δ) for GdSc1−xInxO3 (0.0 ≤ x ≤ 0.8).

cation, Gd3+, is same throughout the series GdSc1−xInxO3. Figure 8 shows the Raman spectrum for the series GdSc1−xInxO3 (0.0 ≤

site, this implies that these low-frequency modes in various symmetries should not shift significantly because the A-site 2193



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tions. At first instance, it would be assumed that there is a gradual transition of average polyhedral coordination from 6 to 5 on moving from the Sc3+-rich region to the In3+-rich region. A conclusive inference cannot be drawn from XRD studies alone, as it gives an average information. The Raman spectra were fairly uniform from GdScO3 to GdSc0.2In0.8O3, except for the shifts observed in modes corresponding to oxygen motion. After Raman studies, which are more sensitive to local changes, it becomes evident that the change from 6-coordination to 5coordination is obviously not gradual but abrupt. This is further emphasized by the fact that the modes corresponding to the Asite cation (here Gd3+) do not undergo any shifts. Considering that the coordination polyhedral of Gd3+ decreases from 8 to 7 on proceeding from an orthorhombic to a hexagonal structure, a coexistence of hexagonal microdomains in orthorhombic lattice would have been manifested as significant changes in the bands associated with Gd3+ vibrations. Further, the hexagonal signature appears only in 90 mol % In3+-containing composition. Since Raman spectroscopy has been proven to be a versatile and reliable tool for detecting even subtle local structural distortion in the lattice, it is reasonable to believe that it would have provided the signal of hexagonal domains had they been present in the GdSc1−xInxO3 (x ≤ 0.8). Dielectric Studies. As mentioned earlier, rare earth scandates are known to have very high dielectric constants and are technologically important dielectric materials. The curiosity to understand the changes in the electrical behavior of these nominal compositions brought about by doping In3+ in GdScO3 is a motivation to probe these materials by AC impedance studies. This is further important in wake of the fact that Tohei et al.24 have predicted GdInO3 to be a geometric ferroelectric using first principle calculations. The temperature dependence of the dielectric constant (relative permittivity) ε′ at several frequencies for samples with x = 0.0, 0.2, 0.4, 0.8, 1.0 are shown in Figure 10. The relative permittivity increases rapidly (near T = 150 °C for 10 Hz), showing a step-like feature, and then again increases sharply at higher temperatures when conductivity and electrode polarization effects are likely to dominate. The value of ε′ increases from about 30 to around 250−300 near the step before increasing further at T > 250 °C. However, for x = 0.9 and 1.0 (i.e. the nominal compositions GdSc0.1In0.9O3 and GdInO3), the values of ε′ were much smaller and the behavior of ε′ in these two compounds is altogether different from the other compounds. While the GdInO3 sample shows a broad relaxor-like peak near 120 °C (Figure 10e), the x = 0.9 sample shows a feature indicative of a peak near 150 °C, which is merged into the large increase in ε′ at higher temperatures possibly due to conductivity and electrode contribution (Figure 10d). The arrow in Figure 10d indicates the variation in the peak position with increasing frequency. In fact, the qualititative behavior of ε′ in x = 0.9 is suggestive of a classical relaxor ferroelectric, where the frequency dispersion at higher temperatures is less than that for temperatures below the peak, resulting in the curves coming somewhat closer together above T = 200 °C. Further analysis of the temperature dependent on the frequency of this relaxation showed that the relaxation follows a Vogel−Fulcher-type behavior of the form

Figure 13. Variation of Tm with frequency for various nominal compositions in GdSc1−xInxO3 (0.0 ≤ x ≤ 0.8).

Table 4. Variation of Activation Energy (Ea) as Obtained from Arrhenius Equation with Composition no.

composition

Ea (eV)

1 2 3 4 5

GdScO3 GdSc0.8In0.2O3 GdSc0.6In0.4O3 GdSc0.4In0.6O3 GdSc0.2In0.8O3

0.77 0.80 0.74 0.71 0.73

x ≤ 0.80), that is, for the orthorhombic phase field. It is quite evident that modes at 110, 131, and 157 cm−1 are almost constant in accordance with the discussion stated above. The modes at 113 and 115 cm−1 are too weak to be observed. However, the mode at 223 cm−1, which should have been constant, as is described by Chopelas et al.,36 is found to shift to lower frequencies, implying that it might not be pure Gd-vibrations and that there might be some mixing of oxygen vibrations. Further, on observing other modes (e.g. the modes at 452, 373, 321 cm−1), it is clearly shown (Figure 8), that they are undergoing red-shift; that is, a softening of Raman modes is taking place. It has been reported that the Raman frequencies depend significantly on unit cell volume. On substituting In3+, as is reported in Table1, the cell parameters (a, b, and c) increase, and hence, the unit cell volume is increasing, which implies that the Raman shifts should decrease. This is in accordance with what is being observed in the present case. The same trend with constancy of Raman shifts for Gd dependent vibrations and decrease in Raman shift on oxygen dependent modes is observed until 80 mol % In3+ doping. Interestingly, in 90 mol % In3+ containing composition (i.e., GdSc0.1In0.9O3), the Raman spectra contains an altogether different set of peaks, with only traces of orthorhombic type modes, which completely disappear in GdInO3. Figure 9 shows the comparison of Raman spectra obtained for nominal compositions GdScxIn1−xO3 (0.8 ≤ x ≤ 1.0). The peaks corresponding to orthorhombic modification are marked. This further corroborates the XRD observation that the nominal composition GdSc0.1In0.9O3 is biphasic with hexagonal majority phase and trace amount of orthorhombic modifications. Based on Raman analysis, it would be interesting to discuss the mechanism of changes in local polyhedral coordination of B site cation on proceeding from orthorhombic (B is octahedrally coordinated) to hexagonal (B is TBP coordinated) modifica-

f = f0 exp(−E /kB(Tm − TVF))

(1)

where f 0 is a prefactor, kB is the Boltzmann constant, TVF is the Vogel−Fulcher temperature, and E is the activation energy. The result of fitting eq 1 to the frequency−temperature behavior is 2194



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shown in Figure 11. The best-fit values obtained were f0 (attempt frequency) =289 Hz, E = 0.01 eV and TVF = 424 K. This further suggests a relaxor-like behavior for the x = 0.9 composition. In the Vogel-Fulcher (V−F) relation, as given in eq 1, f 0 is a fitting parameter, usually associated with the attempt frequency of the dipoles, or the Debye frequency. The unusually low value of f 0 can be explained as follows: The value of attempt frequency is associated with the size of the polar regions and the interaction between them. If the size of the polar regions is large, the value of f 0 should be smaller. Also, the stronger the interaction between the polar regions, the smaller is the value of f 0. In most known relaxors, f 0 is typically 109 to 1013 Hz or so. In our opinion, the small value of f 0 in the present system is probably due to both larger polar regions and stronger interaction. Since the high temperature data is dominated by conductivity effects (thus hiding the loss peaks), relatively small range available for fitting the V−F relation. It perhaps precludes more precise determination of the V−F parameters. However, our analysis points to the fact that the composition with 90 mol % In3+ fraction follows V−F behavior and not Arrhenius behavior, which, in turn, is followed by the compositions with lower In3+ content. The comparison of the present study with relaxation dynamics of known and reported relaxor materials become a little difficult due to the small set of data points available for V−F analysis. In an earlier section, it has been shown that nominal compositions GdSc0.1In0.9O3 and GdInO3 have hexagonal lattices. In addition, the GdSc0.1In0.9O3 shows presence of a small amount of orthorhombic phase. The results of the dielectric study therefore suggest that the sample progresses from a normal dielectric to a relaxor-like behavior in GdSc1−xInxO3 series. The presence of Vogel−Fulcher behavior for GdSc0.1In0.9O3 at low frequencies, such as that in a classic relaxor ferroelectric, and the absence of frequency dispersion of Tm in the GdInO3 sample suggest that the electrical (relaxor) properties could indeed be tailored by controlling the substituent (here Sc3+) in the hexagonal lattice. It is known that YInO3 and YMnO3 exist in a ferroelectric form (belonging to space group P63cm) and a paraelectric form (belonging to space group P63mmc).37 Despite the fact that XRD patterns show that GdInO3 synthesized in present study belong to the noncentrosymmetric space group P63cm (similar to ferroelectric polymorph of YInO 3 and YMnO3 ) and the geometric ferroelectricity has been predicted in GdInO3,24 it was not observed in the present study. On the other hand, small amounts of Sc3+ substitution in this structure (non-centrosymmetric GdInO3), apparently cause the bond lengths to alter in such a way that it is yet not ferroelectric but introduces a relaxor-like behavior in the GdSc0.1In0.9O3. The temperature dependence of the dielectric loss tangent (tan δ) revealed the presence of peaks due to dielectric relaxation (Figure 12) in all compositions. The value of Tm, the temperature at the tan δ peak, shifts to higher values with increasing frequency. This behavior is similar for all compositions except GdSc0.1In0.9O3 and GdInO3, where, again, the behavior of the loss tangent is relaxor-like. For GdSc0.1In0.9O3, there is a change in magnitude of the Tm peak along with the dispersion with increasing frequency, which is expected for a relaxor ferroelectric. However, for GdInO3, there is a change in magnitude of Tm though there is no frequency dispersion, thus showing that it is not a classical relaxor ferroelectric material. For all compositions having orthorhombic structure in GdSc1−xInxO3 system, that is, the nominal compositions with x ≤ 0.8, the variation of Tm with frequency f was found to obey the Arrhenius relation.

f = f0 exp( −E /kBTm)

(2)

where f 0 is a prefactor, kB is the Boltzmann constant, and E is the activation energy. One such typical curve is depicted in Figure 13 for the nominal composition. This suggests that the relaxations in compounds with x ≤ 0.8 are due to the thermally activated hopping of charge carriers. The activation energy varied between 0.7 eV to 0.8 eV with the indium content, as presented in Table 4. The value of activation energy is too high for polaronic conduction, and oxygen ion hopping may be the dominant mechanism for conduction and relaxation in these compounds. The activation energy shows an almost decreasing trend as indium content increases, suggesting an increase in the oxygen disorder as increasing amount of indium is incorporated in the lattice. It is pertinent to discuss here that the electrical properties of a material are influenced by the presence of heterogeneity in the system. The presence of different phases and the accumulation of charges at interface between two phases may lead to Maxwell−Wagner polarization behavior, which is exhibited in the tan δ curves as well as in the behavior of real part of permittivity (ε′) with frequency. However, the comparison of these for GdSc1−xInxO3 for x ≥ 0.8 and x = 0.9 indicate that interfacial effects are not significant in the present system. Hence, we believe that the observed dielectric behavior is intrinsic in origin. Hence, the dielectric studies revealed that it has indeed been possible to tailor the electrical properties of GdSc1−xInxO3 series. The entire orthorhombic phase field produced dielectrics with high dielectric constants, whereas pure hexagonal GdInO3 depicted a broad relaxor-like peak, though not a true ferroelectric. Classical relaxor-like behavior could be observed in GdSc0.1In0.9O3 composition. The major phase in this particular composition is a hexagonal GdInO3-type phase, with small amount of Sc3+ as a substituent. This shows how the fine control of composition leads to very subtle changes in structure, which manifest in a range of electrical properties observed in this series. These studies also strengthen our observation that, in this particular series, the abrupt structural changes are brought about at nominal composition GdSc0.1In0.9O3, which corroborates the observations made by XRD, Raman, and thermomechanical studies.

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CONCLUSIONS The GdSc1−xInxO3 (0.0 ≤ x ≤ 1.0) complex oxides were explored by structural and electrical investigations. The single-phasic orthorhombic phase field in this system could be increased by 20 mol % in the combustion synthesized products as compared to the solid state synthesized compositions. In the orthorhombic region, lattice parameters increased on In3+ substitution in GdScO3, whereas the lattice contracted in the a−b plane, while expanded in the c direction on substituting Sc3+ in GdInO3. Raman spectroscopy revealed that the structural changes are rather abrupt at GdSc0.1In0.9O3 instead of showing a gradual change across the series. The net thermal shrinkage observed on green pellets of various nominal compositions increased with increase in In3+ content, reached a maximum at GdSc0.2In0.8O3, and then decreased marginally in accordance with calculated theoretical densities. The highlight of the study is the shift of electrical behavior of the system from normal dielectric (for x ≤ 0.8) to classical relaxor ferroelectric for GdSc 0.1 In 0.9 O 3 composition and relaxor-type behavior of GdInO3. Other than producing a candidate for a potential lead-free relaxor, this study shows how by subtle changes in structure brought about by 2195



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(22) Goodenough, J. B.; Kafalas, J. A.; Longo, J. M. Preparation Methods in Solid State Chemistry; Hagenmuller, P., Ed.; Academic Press: New York, 1972. (23) Van Aken, B. B.; Palstra, T. T. M.; Filippetti, A.; Spaldin, N. A. Nat. Mater. 2004, 3, 164. (24) Tohei, T.; Moriwake, H.; Murata, H.; Kuwabara, A.; Hashimoto, R.; Yamamoto, T.; Tanaka, I. Phy. Rev. B 2009, 79, 144125. (25) Kuo, D. H.; Huang, K. C. Ceram. Int. 2008, 34, 1503. (26) Shukla, R.; Arya, A.; Tyagi, A. K. Inorg. Chem. 2010, 49, 1152. (27) Abdala, P. M.; Craievich, A. F.; Fantini, M. C. A.; Temperini, M. L. A.; Lamas, D. G. J. Phys. Chem. C 2009, 113, 18661. (28) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. (29) Perdew, J. P.; Burke, K.; Wang, Y. Phys. Rev. B 1996, 54, 16533. (30) Kresse, G.; Furthmüller, J. Comput. Mater. Sci. 1996, 6, 15. (31) Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, 11169. (32) Blöchl, P. E. Phys. Rev. B 1994, 50, 17953. (33) Kresse, G.; Joubert, J. Phys. Rev. B 1999, 59, 1758. (34) Hausner, H. H. Powder Metallurgy in Nuclear Engineering; Am. Soc. Metals: Materials Park, OH, 1958; p 1 (35) Chaix-Pluchery, O; Kreisel, J. J. Phys.: Condens. Matter 2009, 21, 175901. (36) Chopelas, A. Phys. Chem. Miner. 2011, 38, 709. (37) Abrahams, S. C. Acta Crystallogr. 2001, B57, 485.

carefully controlling the compositions, the electrical properties of the GdScO3 /GdInO3 could be tuned to obtain the desired functionality.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 0091-22-2559 5330. Fax: 0091-22-25505151, E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Department of Atomic Energy’s Science Research Council (DAE-SRC) is acknowledged for supporting this work vide sanction no. 2010/21/9-BRNS/2025 dated 7-12-2010. Thanks are due to Dr. S. N. Achary for his constructive comments.

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