Thin Film Synthesis and Structural Characterization of a New

Preferred Polymorph in the RE2Ti2O7 (RE=La-Y) Family ... Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Ave,...
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DOI: 10.1021/cg900556d

Thin Film Synthesis and Structural Characterization of a New Kinetically Preferred Polymorph in the RE2Ti2O7 (RE=La-Y) Family

2009, Vol. 9 4546–4554

S. Havelia, S. Wang, K. R. Balasubramaniam, and P. A. Salvador* Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh, Pennsylvania 15213 Received May 22, 2009; Revised Manuscript Received August 12, 2009

ABSTRACT: A new polymorph was found for all RE2Ti2O7 (RE = La-Y) compounds during their growth as epitaxial films on SrTiO3(110) substrates. Detailed structural characterization using X-ray and electron diffraction revealed that the new polymorph adopts an orthorhombic crystal structure with the space group Ammm and unit cell parameters (for La) of a=7.89, b=5.54, c=13.45 A˚; the refined lattice parameters decreased monotonically with decreasing size of the RE cation. Though these parameters are similar to those of the known monoclinic (110)-layered perovskite polymorph, the structures are not related by simple distortive transformations. While both of these polymorphs can be formed as epitaxial films on SrTiO3(110) substrates at T ∼ 900 C, the new polymorph is favored in conditions where diffusion is more limited between pulses. This implies that the new polymorph is kinetically favored during nucleation over the (110)-layered perovskite phase, in agreement with the smaller electroneutral unit of the new polymorph.

1. Introduction Epitaxial stabilization is the process in which phase (or polymorph) selection during thin film growth is controlled by interactions of the film nuclei with the underlying substrate.1,2 Essentially, the process is understood in that the thermodynamics of the nucleation process can be used to direct the synthesis of a specific phase owing to substrate interactions.1,3,4 Of course, such phase selection is determined by both thermodynamic and kinetic factors,1,5,6 though the latter are not well understood. The general assumption used during epitaxial stabilization is that the system has appropriate kinetics to select the lowest energy nucleation event. In this paper we report on the synthesis and structural characterization of a new polymorph (for which there is no bulk structural analogue) in the RE2Ti2O7 family that is stabilized, relative to the two known polymorphs, by both thermodynamic and kinetic factors during epitaxial thin film growth. Epitaxial stabilization has been used to synthesize metastable compounds having specific structural features for a range of different compositional families.1,5,7,8 In spite of the impressive range of compositions for which epitaxial stabilization has been successful, most of the work has been focused on densely packed crystal structures, including the large number of oxides that have been formed as metastable perovskites.7-11 Phases having dense, relatively simple crystal structures, such as perovskites, are likely to have fewer kinetic barriers to nucleation during thin film growth than competitive phases having less dense or more complex layered structures. As such, both thermodynamics and kinetics can favor the stabilization of metastable perovskites as thin films.1,4,7,8 Much less work has focused on systems in which thermodynamic and kinetic factors are working against each other; this paper discusses such a system. Metastable phases adopting complex or open layered structures cannot be easily synthesized using other common techniques, such as high pressure synthesis routes.12-14 In such *Corresponding author. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 09/08/2009

cases, epitaxial stabilization provides an attractive alternative to their synthesis. We have been interested in the growth of thin films of layered compounds of complex oxides, such as the hexagonal REMnO3 family and the (110)-layered perovskite RE2Ti2O7 (RE = rare earth) family.3,12,15,16 Though both of these families are known to be high-temperature ferroelectrics, only a limited number of compositions are known to adopt these crystal structures.3,17,18 Epitaxial stabilization has been successful at increasing the number of compositions known to adopt the layered, hexagonal YMnO3 structure.1,12 We also recently showed that RE2Ti2O7 (RE = Gd, Sm) compounds could be synthesized in the metastable monoclinic (110)layered perovskite (herein called the (110)-LP) structure as epitaxial thin films.16 In the course of that investigation, we discovered a new polymorph that competes in stability as a thin film with both the pyrochlore and the (110)-layered perovskite. Whether the new polymorph or the (110)-LP polymorph is observed is largely dependent on growth parameters that affect both the thermodynamic and kinetic factors. Growth parameters are, of course, well-known to play an important role in the phase and orientation selection during the growth of complex oxide films.19-23 In general, orientation and phase selection can be treated using the same theoretical thermodynamic and kinetic framework. A wellknown example involves the growth of c-axis oriented YBCO (YBa2Cu3O7-x) thin films (and other related layered cuprate superconductors) on (100)-oriented perovskite crystals.19,22 Though the (001) orientation is expected to be the lowest energy orientation for YBCO (to avoid the increased energy associated with in-plane variant boundaries of (100) oriented YBCO), a significant amount of out-of-plane diffusion is required to obtain c-axis growth (while much less is required for a-axis growth).19,21,22 Temperature, oxygen pressure, plasma dynamics, and the substrate surface were found to influence the growth orientation; that is, the deposition parameters determined whether the films grew in the thermodynamically favored (00l) orientation or the kinetically favored (h00) orientation.19,20,22 r 2009 American Chemical Society

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Kinetically favored phases are known to form in a number of material systems.24-26 In 1897, Ostwald formulated his now famous Step Rule, which states that the crystal phase that nucleates is not necessarily the one that is thermodynamically stable at that temperature and pressure but generally the metastable phase that is closest in free energy to the parent phase.27 The metastable (kinetically favored) phase is observed when the nucleation barrier for its formation is significantly lower than that of the thermodynamically favored phase.24,25 This concept can be used to understand the orientation selection for YBCO (described above) or the (110)-LP polymorph (described below) versus temperature, as well as the phase selection between the new polymorph described in this paper and the known (110)-LP polymorph for RE2Ti2O7 compounds. RE2Ti2O7 compounds are known to exist as two different polymorphs in the bulk: the pyrochlore and the (110)-LP phase, depending on the rare earth cation.13,14 The larger rare earth titanates, RE=La or Nd, are stable in the (110)-layered perovskite structure, while the smaller rare earth titanates, RE=Sm-Y, are stable in the pyrochlore structure.13,14,28,29 The (110)-layered perovskites are important materials with a range of useful properties, including ferroelectricity, second harmonic generation, and photocatalytic activity.30-35 The pyrochlore compounds are generally used as dielectrics, solid electrolytes, and refractory materials.14,36-38 The different rare earth titanates have been synthesized in their bulk stable form using a variety of synthesis techniques.14,39-45 We have already been able to synthesize the different RE2Ti2O7 compounds in both these polymorphs: the (110)LP phase on SrTiO3(110) crystals and the pyrochlore on (Y, Zr)O2(100) crystals.15,16 Growth of (00l) oriented (110)-layered perovskites is known to be kinetically limited; high temperatures (900 C) were required to obtain (001) oriented growth of both stable and metastable compounds.15-17 While exploring the process space, we synthesized the entire rare earth titanate family in a new polymorphic structure. In this paper, the effects of different deposition parameters on the phase competition between this new polymorph and the two known polymorphs are discussed. Detailed structural characterization of the new polymorph is also described. A schematic diagram, based on experimental observations, of the bulk formation energy of the three polymorphs vs RE cation size is also developed for the RE2Ti2O7 family. 2. Experimental Details RE2Ti2O7 films were deposited on single crystal substrates (Crystal GmbH, Germany) of SrTiO3(110), MgO(111), and YSZ(100) (cubic (Y,Zr)O2) crystals. The substrates were ultrasonically cleaned in acetone and ethanol for 5 min each, and attached to a heater plate using silver paste. Stoichiometric ceramic targets were prepared using standard solid-state synthesis methods.14,40 Films were deposited by pulsed laser deposition (PLD), as described elsewhere.46 The films were grown under a dynamic oxygen environment (maintained from 1 to 50 mTorr O2) to a thickness ≈ 100 nm (based on experimentally determined growth rates) and were cooled down to room temperature under a static oxygen pressure of 200 Torr. The crystalline nature (phase, crystalline quality, and epitaxial relationship) of the films was characterized using both Rigaku (Rigaku, Japan) and a Philips X’Pert (Philips Analytical, Netherlands) X-ray diffractometers equipped with Cu-KR radiation; normal θ-2θ scans were carried out on the former, while ω and φ scans were carried out on the latter. The Rigaku diffractometer was operated at 35 kV and 25 mA, with a step size of 0.05 and a 2 s count time. The X’Pert was operated at 45 kV and 40 mA, with an incident beam lens to provide parallel optics. Beam size, divergence,

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Figure 1. XRD patterns of Nd2Ti2O7 films deposited at oxygen pressures of (a) 1 mTorr, (b) 5 mTorr, (c) 10 mTorr, (d) 25 mTorr, and (e) 50 mTorr on SrTiO3(110) substrates. The film peaks for both the (110)-LP (m) and the γ phase (o) are marked with (00l). The primary substrate peaks are marked ‘s’. and noise were limited using a 2  2 mm2 antiscatter slit, a 0.27 Soller slit, and a graphite monochromator. The deposition rates were determined using X-ray reflectivity (XRR) on the X’Pert diffractometer as described elsewhere.47 Transmission electron microscopy (TEM) was carried out in both diffraction and imaging modes using a Tecnai F20 TEM at 200 kV. Samples used for the TEM study were prepared as cross sections using conventional techniques: by cutting, gluing, mechanical polishing, and then argon-ion milling until perforation occurred. Convergent beam electron diffraction (CBED) analysis was carried out to help determine the space group of the new polymorph.48

3. Results and Discussion Effects of Growth Parameters on Phase Selection. Figure 1 presents the X-ray diffraction (XRD) patterns for 100 nm thick (nominally) Nd2Ti2O7 films deposited at five different dynamic oxygen pressures ; p=(a) 1 mTorr, (b) 5 mTorr, (c) 10 mTorr, (d) 25 mTorr, and (e) 50 mTorr ; all deposited on SrTiO3(110) substrates at a substrate temperature of 900 C using a laser energy density of 2J/cm2 and a laser pulse frequency of 3 Hz. The average deposition rates (in A˚/ pulse) for films whose diffraction patterns are shown in Figure 1 were (a) 0.16, (b) 0.14, (c) 0.11, (d) 0.09, and (e) 0.07 (determined using XRR on other films grown in identical conditions). Because Nd2Ti2O7 adopts the (110)-layered perovskite structure in the bulk and the (110)-oriented perovskite substrate provides a structurally similar surface for growth, one expects the (001)-oriented, epitaxial films of the stable (110)-LP polymorph to be observed in such experiments. Indeed, peaks from the (metastable) pyrochlore polymorph were not seen in any of the X-ray scans. Surprisingly though, two distinct families of (00l) reflections are observed with slightly different values for the lattice parameter c, as described below. The film whose pattern is shown in Figure 1a is the only film for which all peaks can be indexed to the (00l) reflections of the stable (110)-LP polymorph.16 Films whose X-ray scans are shown in Figure 1b-d exhibit two families of (00l) peaks: the first from the stable (110)-LP polymorph (marked as m) and the second from peaks that appear at slightly lower angles to the l = 2n peaks of the stable polymorph (marked as o). The film that was deposited at 50 mTorr exhibits (Figure 1e) X-ray peaks only arising from the second family of reflections. This second family of peaks corresponds to the (00l) family of a new polymorph that has a

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Figure 2. XRD patterns for Nd2Ti2O7 films deposited at laser energy density of E.D. =(a) 1 J/cm2, (b) 2 J/cm2, and (c) 4 J/cm2 on SrTiO3(110) substrates. The film peaks for both the (110)-LP (m) and the γ phase (o) are marked with (00l). The primary substrate peaks are marked ‘s’.

Figure 3. XRD patterns for Nd2Ti2O7 films deposited at laser pulse frequencies of ν = (a) 1 Hz, (b) 3 Hz, and (c) 10 Hz on SrTiO3(110) substrates. The film peaks for both the (110)-LP (m) and the γ phase (o) are marked with (00l). The primary substrate peaks are marked ‘s’.

slightly larger lattice parameter along the c-axis than the (110)-LP polymorph; for simplicity we will refer to this third polymorph in the RE2Ti2O7 family as the γ phase. It is important to emphasize that at higher background oxygen pressures (lower deposition rates) the second family of peaks increases in relative intensity while the first family of peaks decreases in relative intensity. Several underlying factors are simultaneously affected by an increase in the background growth pressure. Higher oxygen pressures lead to an increase in the activity of oxygen, a decrease in the instantaneous growth rate, and an increase in the number of collisions in the plasma, resulting in lower kinetic energies of the arriving species. Therefore, Nd2Ti2O7 films were deposited at different laser energies and laser pulse frequencies, to determine the primary factor in phase selection. Figure 2 shows XRD patterns for Nd2Ti2O7 films deposited at three different energy densities ; E.D. = (a) 1J/cm2, (b) 2J/cm2, and (c) 4J/cm2 ; all deposited on SrTiO3(110) substrates at a substrate temperature of 900 C in a dynamic oxygen pressure of 5 mTorr and using a laser repetition rate of 3 Hz. The average deposition rates (in A˚/pulse) for the films shown in Figure 2 were (a) 0.06, (b) 0.09, and (c) 0.21, respectively. All films were deposited to ∼ 100 nm thickness. Varying the energy density changes both the instantaneous supersaturation and the kinetic energy of arriving species, while the oxygen activity is fixed. At lower laser energy densities (Figure 2a), the films tend to form the γ polymorph. On increasing the energy density, which is equivalent to increasing the supersaturation and the kinetic energy of the adsorbed species on the substrate surface, the amount of the γ phase decreases while the amount of the (110)-LP phase increases. At 2J/cm2 and 4J/cm2 (Figure 2b,c), both phases are observed in the X-ray patterns, though the (110)-LP phase increases in intensity at the higher laser energy density (Figure 2c). This observation argues that the oxygen activity is not a primary factor in phase formation. Figure 3 presents XRD patterns for Nd2Ti2O7 films deposited at three different laser pulse frequencies ; ν=(a) 1 Hz, (b) 3 Hz, and (c) 10 Hz ; all deposited on SrTiO3(110) substrates at a substrate temperature of 900 C in a dynamic oxygen pressure of 5 mTorr and using a laser energy density of 2J/cm2. All films were deposited for 7200 pulses to give a

nominal thickness of ∼100 nm. Changing the laser pulse frequency does not affect any of the three factors discussed above; however, the time allowed for processes to occur prior to the arrival of new material is directly affected, which can be most closely connected to variations in the kinetic energy of arriving species. The figure indicates that at lower repetition rates, when the adsorbed species have the most time to diffuse (and nucleate/grow) on the substrate surface before the arrival of the next pulse, the films tend to form the (110)LP phase. On increasing the laser pulse frequency, which is equivalent to decreasing the time for adsorbed species to diffuse and nucleate before the arrival of the next pulse, the amount of the (110)-LP phase decreases while the amount of the γ phase increases. At 3 Hz, both phases are observed in the X-ray pattern (Figure 3b). At a repetition rate of 10 Hz, the X-ray patterns showed peaks only from the γ phase. Substrate temperature is another deposition parameter that is closely connected to the kinetics and thermodynamics of phase nucleation and could influence the phase selection. It has already been shown that the growth of the (110)-LP phase is kinetically limited and high temperatures (900 C) are required for its growth (particularly in this orientation with the larger c-axis out-of-plane).15-17 Depositions at lower substrate temperatures led to X-ray amorphous films.15 Unfortunately, depositions could not be made at higher substrate temperatures (due to chamber restraints) but one would expect that higher substrate temperatures would enhance surface diffusion and lead to the nucleation of the thermodynamically stable, kinetically challenged (110)-LP phase. These collected observations implicate kinetics associated with diffusion and nucleation processes ; occurring on the time scale from milliseconds to seconds ; as the determining factor for phase selection, which can be explained in the following fashion. The (110)-LP phase is the thermodynamically stable phase that is kinetically difficult to form (in the (001)-orientation). Therefore, this phase is found in conditions that provide sufficient kinetics to allow for the complex structure to form. At low oxygen pressures, even though the growth rates are modestly higher than at higher pressures, the arriving species have more kinetic energy and can form the kinetically challenged thermodynamically favored phase. At high oxygen pressures, the growth rates decrease along

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with the kinetic energy of the arriving species, which favors the thermodynamically metastable kinetically preferred γ phase. Similarly, by fixing the plasma parameters and only changing the time between pulses, decreased rearrangement times favor the kinetically preferred phase while increased times favor the thermodynamically preferred phase. On changing the laser energy densities, the kinetic energy of the arriving particles and the growth rate both change; therefore, lower energy densities favor the kinetically preferred phase while at higher energy densities both phases nucleate. Metastable phases can also form because they are known to better accommodate nonstoichiometry or point defects.49,50 Higher concentrations of antisite (and other point) defects can be introduced by high-energy arriving species in PLD. Our observations indicate that the metastable γ phase is formed in conditions where the arriving species have less energy (at low laser energies and high oxygen pressures). Therefore, the formation of the γ phase cannot be explained by higher concentration of point defects caused by PLD parameters. Processing parameters can also cause nonstoichiometry, even in compounds having no volatile components;51,53 however such compositional (relative) and structural changes owing to such nonstoichiometry are measurable with electron dispersive spectroscopy (EDS) and X-ray diffraction, respectively. EDS (carried out on a scanning electron microscope) indicated that the two phases have the same composition and that the compositions were not a function of the processing parameters. Furthermore, X-ray diffraction measurements showed that the lattice parameters of the two phases were independent of processing parameters. Also, there are no reports in the literature that describe any other phases competing with the (110)-LP phase (besides the pyrochlore) as a function of small compositional or stoichiometric deviations from the A2B2O7 parent. The most consistent interpretation, which also agrees with the structural characteristics of the new polymorph described later, is that there is a kinetic preference to its formation at early times in the nucleation process; that is, to say the γ phase is formed because it is the kinetically favored phase during PLD. To investigate further the occurrence of the γ phase, films were deposited on MgO(111), a substrate that is not selective for phase formation and therefore should favor the thermodynamically stable phase, and YSZ(100), which should favor pyrochlore nucleation from an interfacial energy perspective (epitaxial stabilization). Figure 4 shows XRD patterns for RE2Ti2O7 films: RE = Nd, La deposited on MgO(111) (Figure 4a,b) and YSZ(100) (Figure 4c,d) substrates. Figure 4a,b shows that both RE = Nd, La nucleate in the γ phase on MgO(111) substrates. This substrate should favor the (110)-LP phase, but we observe the kinetically preferred γ phase. This indicates that, for these rare earth cations, the c-axis oriented γ polymorph forms not through an epitaxial stabilization process but through a kinetic preference during nucleation versus the c-axis (110)-LP phase. (The in-plane diffraction scans showed no in-plane texture for the (001)oriented γ phases on MgO(111) substrates). These observations demonstrate that the two phases are stable in identical deposition conditions. In other words, the formation of a particular phase is not the result of process dependent variations in composition or defect concentrations. Figure 4c,d show that, on YSZ(100) substrates, La nucleates in the bulk stable (110)-LP phase while Nd is epitaxially

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Figure 4. XRD patterns of (a) Nd2Ti2O7 and (b) La2Ti2O7 films, deposited on MgO(111) substrates, and (c) Nd2Ti2O7 and (d) La2Ti2O7 films, deposited on YSZ (100) substrates. The film peaks for both the (110)-LP (m) and the γ phase (o) are marked with (00l). The primary substrate peaks are marked ‘s’. The inset shows the zoomed in pattern for the Nd2Ti2O7 pyrochlore film deposited on YSZ(100).

stabilized in the pyrochlore form. The inset shows the magnified XRD pattern for the Nd2Ti2O7 film around the (400) YSZ substrate peak and one can clearly see the (800) pyrochlore peak from the Nd2Ti2O7 film. This is the first time that Nd2Ti2O7 has been synthesized in the pyrochlore phase and will be reported elsewhere. The fact that the La compound forms in the (110)-LP phase indicates that kinetics are not an issue in phase formation on YSZ(100) substrates (under these conditions); the absence of pyrochlore peaks for the La compound indicates that the pyrochlore is more metastable for the La compound than for the Nd compound. It is interesting that the phase formed by the RE = La compound is substrate dependent, arguing that the kinetics of nucleation is substrate dependent. This observation can be understood if diffusion on MgO(111) is inhibited as compared to that on YSZ(100). It is possible, though there is no direct support for this, that atomistic diffusion on MgO(111) is slower than on YSZ(100), which would support our observations. A similar macroscopic argument could be as follows. MgO(111) substrates are significantly rougher than the YSZ(100) substrates.20 Rougher surfaces are known to have lower surface diffusion.19,20 Therefore, one can expect that surface diffusion on MgO(111) substrates would be significantly lower than that on YSZ(100) substrates. Lower surface diffusion would result in the growth of the kinetically preferred phase for RE = La, Nd as observed in Figure 4. Compositional Stability and Structural Characterization. Figure 5a-f shows the XRD patterns for the RE2Ti2O7 films, RE = (a) La, (b) Nd, (c) Sm, (d) Gd, (e) Dy, and (f) Y, deposited at 900 C on SrTiO3(110) substrates. The films were deposited at a high oxygen pressure (50 mTorr O2), a laser energy density of 2J/cm2, and a laser pulse frequency of 3 Hz, to facilitate the growth of the γ phase. The average deposition rates (in A˚/pulse) for the films shown in Figure 5 were (a) 0.06, (b) 0.07, (c) 0.08, (d) 0.07, (e) 0.09, and (f) 0.06, respectively. All films were deposited to ∼100 nm thickness. The patterns indicate that all compositions adopted the

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γ phase as (00l)-oriented films; peaks arising from the γ phase are marked with the corresponding (00l) indices. The only other major peaks observed are the (hh0) peaks of the substrate (marked with ‘s’). The peak locations marked with an ‘*’ in Figure 5 indicate the locations of the most intense pyrochlore peaks: note their absence in all patterns but (e) and (f), where pyrochlore peaks were observed. The presence of the pyrochlore peaks indicate that the γ and the (110)-LP phases have now become metastable with respect to the pyrochlore phase and the destabilization energy (ΔpyflpΔGfv) is becoming too large to overcome using SrTiO3(110) substrates.3 In order to determine the crystal structure of the γ phase, a TEM analysis was undertaken. Figure 6 shows the results of a cross-sectional TEM study from a γ phase Sm2Ti2O7 film. Selected area electron diffraction (SAED) patterns taken from the film along the [001], [110], and [110] zone axes of the substrate are shown in Figure 6, panels a, b, and c, respectively. These SAED patterns show that the film is epitaxial with the SrTiO3(110) substrate. The patterns can be indexed as [100], [010], and [001] zone axes of the film and the epitaxial relationship can be written as {001}film {110}subs:Æ010æfilm Æ110æsubs. The SAED patterns show that, in contrast to the monoclinic (110)-layered perovskite structure,16 the angle between the three principle directions is 90, indicating that the γ phase has an orthorhombic unit cell. The absence of odd (00l) peaks, as observed in X-rays, is corroborated in the SAED patterns. The film [100] zone axis

Figure 5. XRD patterns for RE2Ti2O7, RE = (a) La, (b) Nd, (c) Sm, (d) Gd, (e) Dy, and (f) Y, films deposited on SrTiO3(110) substrates. The film peaks are marked with (00l) while the primary substrate peaks are marked ‘s’. Weak pyrochlore peaks (note the log scale) have been marked with an “*”. The pyrochlore peaks are observed only for (e) Dy and (f) Y.

SAED pattern (Figure 6a) indicates that only peaks corresponding to (kþl=2n) are observed in the diffraction pattern and that the lattice parameter of the γ phase is aγ ≈ 2ap, where ap corresponds to the simple cubic perovskite lattice parameter of ≈ 3.7-3.9 A˚. The SAED patterns along the [010] and [001] axes indicate respectively that only peaks corresponding to (h0l: l=2n) and (hk0: k=2n) are observed in any diffraction pattern. The lattice parameters of the γ phase are bγ ≈ 21/2ap ≈ 5.36 - 5.54 A˚ and cγ ≈ 13.25-13.45 A˚, which is similar to but slightly larger than c(110)-LP.42 From the reflection conditions obtained in the SAED (and XRD) patterns, it is not possible to determine the space group of the structure unambiguously. CBED patterns from the film can help us determine the diffraction point group and the potential space group.48 Whole field and bright field CBED patterns were taken from the film along the [001], [110], and [110] zone axes of the substrate (see Supporting Information). All three whole field patterns showed 2 mm symmetry. The bright field CBED patterns showed an absence of G-M lines. From the CBED patterns and the fact that the crystal structure is orthorhombic, one can deduce from crystallographic tables that the diffraction point group is mmm.48 The absence of G-M lines in the bright field CBED patterns indicates that the space group does not have any nonsymmorphic (screw axis or glide planes) point groups.48 From the above conditions and using the International Crystallography Tables, one can show that the γ phase has a space group Ammm.54 X-ray diffraction was used to verify the epitaxy and to determine the lattice parameters of each of the different phases. The in-plane epitaxy of the γ phase films was determined by registering azimuthal φ-scans and by comparing the locations in φ-space of SrTiO3 substrate and the γ phase RE2Ti2O7 reflections. Figure 7a shows the φ-scan registered from the {111} reflection of the SrTiO3 substrate for the Nd2Ti2O7 film, which serves as a reference for the orientation of the substrate on the goniometer. Two peaks separated by 180 in φ are observed, as expected. Figure 7b-g shows the φ-scans from the {204} reflections of the RE2Ti2O7 γ phase for RE = (b) La, (c) Nd, (d) Sm, (e) Gd, (f) Dy, and (g) Y. Each of the {204} reflections observed in these scans were aligned with the {111} reflections of the SrTiO3. It should be noted that the φ scans of the film and substrate were acquired at different 2θ and ψ (the sample tilt) angles. The angles used for the different scans are shown in Figure 7. For all of the film patterns, two peaks separated by 180 in phi space are observed. The presence of two peaks in the {204} phi scans agree with expectations from films having the Ammm orthorhombic structure. The epitaxial relationship between all films and their substrate is given by

Figure 6. Selected area electron diffraction (SAED) patterns taken along the (a) [100], (b) [010], and (c) [001] zone axes from the Sm2Ti2O7 γ phase film.

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{001}film {110}subs:Æ010æfilm Æ110æsubs, which is in agreement with the SAED patterns from the Sm2Ti2O7 films. This is the same epitaxial relationship as we observed for the growth of the (110)-LP phase, indicating that the two structures are similar.16 The lattice parameters for the different RE2Ti2O7 compounds were determined using our X-ray observations and the Unit Cell program,55 which utilizes a least-squares refinement algorithm to fit the collected observations to the unit cell geometry. In the range of 10-90, 2θ values from six different families of peaks ({204}, {024}, {011}, {013}, {211}, and {404}) were used as inputs for the Unit Cell program. The films were assumed to be relaxed and the effect of peak shifting owing to strain was neglected in refining the lattice parameters. Table 1 summarizes the refined lattice parameters and cell volume as obtained in this work. The inplane lattice parameters do not match those of the substrate, confirming that the films are relaxed from the coherently strained situation. The lattice parameters of the γ phase are very similar to those of the (110)-LP phase, even though the space groups are different.16 The question arises as to whether or not they are related by distortive or displacive transformations, such as octahedral tilting. The space group of a tilted structure can be derived as a subgroup of an untilted aristotype structure.18 Maximal group/subgroup relationship can be determined by identifying the symmetry elements that are lost owing to an operation of a particular tilt.18 Levin et al. carried out an in-depth analysis of the symmetry classification of the layered perovskite-derived AnBnX3nþ2 structures (the RE2Ti2O7 compounds belong to this family).18 According to their analysis, the Ammm (space group for the γ phase) and the Cmcm (the aristotype of the (110)-layered perovskite structure) cannot be related by a combination of octahedral tilts.18 This indicates that the γ and the (110)-LP phases are not related by a simple distortive transformation.

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Our results demonstrated that RE2Ti2O7 (RE = La-Y) films can be synthesized in the γ phase using epitaxial stabilization of thin films, with the caveat being that kinetics play an extremely important role. Because the γ phase has a space group of Ammm with similar lattice parameters to the (110)-LP phase of space group P21, the asymmetric unit of the former must be smaller than that of the latter (owing to the mirror versus screw axis symmetry perpendicular to the [001]). From another point of view, the absence of the odd (00l) peaks for the Ammm structure indicates an identical plane of atoms (i.e., (002)) is located exactly half way between planes separated by ≈ 13.3 A˚ (d001) (i.e., (001)).56 This is not true for the P21 structure, for which all (00l) peaks are observed. In PLD, the normal nucleation unit is one electroneutral unit having the appropriate structure/orientation for the phase/epitaxy of interest.7,8 The above analysis indicates that such an electroneutral unit in the γ phase is 1/2 the c-axis length, that is, ∼6.65 A˚, while such an electroneutral unit for the (110)-LP phase is the full c-axis length, that is ∼13 A˚. Diffusion normal to the surface requires considerable kinetic energy,8,17,21 which is much higher than diffusion in the plane of the film. The smaller appropriate electroneutral unit of the (001)-oriented γ phase makes it kinetically simpler to form as compared to the (001)-oriented (110)-LP phase. For the smaller rare-earths (Y-Sm), it should be noted that both the γ and (110)-LP phases are formed through epitaxial stabilization relative to the stable pyrochlore phase. Figure 8 shows a high resolution TEM (HRTEM) image of the Sm2Ti2O7 film substrate interface whose TEM analysis was discussed above. The image shows an abrupt interface between the film and the substrate indicating no appreciable diffusion. In the absence of atomic positions, it was not possible to simulate the HRTEM image; however, the stacking is markedly different from that of the (110)-LP phase.16 This indicates that the γ phase is indeed a new polymorph and the systematic absences in the X-ray and electron diffraction patterns did not arise from defects (stacking faults, APB’s etc.) in the (110)-LP phase. The unit cell of the γ phase and the asymmetric electro-neutral unit (based on the above discussion) at half the full ‘c-axis’ length has been marked in the HRTEM image. Another question that arises is whether or not the γ phase is thermodynamically less stable than the (110)-LP phase over the entire rare earth series, or does the γ phase become thermodynamically more stable for the smaller rare earth titanates (keeping in mind the pyrochlore is the most stable phase for the smaller rare earths)? To answer this question, depositions were made at low laser pulse frequencies (0.1 Hz) for RE = Sm, Gd, and Dy. Depositions made at such low frequencies will provide sufficient time for surface diffusion and should result in the nucleation of the more thermodynamically stable phase. At depositions made at 0.1 Hz, the Sm2Ti2O7 and Gd2Ti2O7 films were single-phase (110)layered perovskites while the Dy2Ti2O7 film showed X-ray peaks only from the γ phase (see Supporting Information).

Figure 7. XRD φ-scans of the (a) {111} reflections of the SrTiO3(110) substrate and (b-g) {204} reflections of RE2Ti2O7 films: RE = (b) La, (c) Nd, (d) Sm, (e) Gd, (f) Dy, and (g) Y. 2θ and ψ values for each case are listed.

Table 1. Lattice Parameters and Cell Volumes for the Different γ Phase RE2Ti2O7 Compoundsa orthorhombic a (A˚) b (A˚) c (A˚) volume (A˚3) a

La2Ti2O7

Nd2Ti2O7

Sm2Ti2O7

Gd2Ti2O7

Dy2Ti2O7

Y2Ti2O7

7.89 5.54 13.45 589.34

7.73 5.48 13.41 568.05

7.68 5.45 13.35 558.77

7.66 5.42 13.30 552.17

7.52 5.40 13.29 541.28

7.40 5.36 13.26 525.94

The lattice parameters were refined using Unit cell program (see text).

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Figure 9. Schematic diagram of bulk formation energy of the three different RE2Ti2O7 polymorphs as a function of the ionic radii of the rare earth cations. The red line represents the pyrochlore, the blue line represents the (110)-LP, while the green line represents the γ phase. The suggested crossover points are shown by black dashed lines (I), (II), and (III). The bulk stable phases are shown by black boxes. The metastable phases synthesized in this work are shown by blue, green, and red boxes for the (110)-LP, γ, and pyrochlore phase, respectively. Figure 8. HRTEM image of the Sm2Ti2O7 γ phase film substrate interface. The unit cell and the asymmetric electroneutral unit has been marked.

This strongly supports the idea that the γ phase is thermodynamically more stable phase than the (110)-LP phase for Dy2Ti2O7, while it is the thermodynamically less stable phase for Sm2Ti2O7 and Gd2Ti2O7 (both are less stable then the pyrochlore in the bulk). There is a general lack of thermodynamic data on the RE2Ti2O7 family of compounds.14,29 Helean et al. calculated the heat of formation of the bulk stable RE2Ti2O7 compounds using drop solution calorimetry.29 Unfortunately, the enthalpy data is insufficient to make any quantitative predictions about the relative stability (which requires entropy data as well) of the two bulk stable polymorphic phases. On the basis of the bulk stability, one can make a schematic of the free energy for the two stable polymorphs in the RE2Ti2O7 family. In order to complete this diagram, one would like to include the new γ polymorph as well. Figure 9 shows a schematic of the bulk free energy for the RE2Ti2O7 family. The red line represents the pyrochlore phase while the blue line represents the (110)-LP phase, with the crossover between the ionic radii of Sm and Nd, as shown by a black dashed line (I). The green line represents the new γ phase. The suggested crossover points for the γ phase are shown by black dashed lines (II) and (III). On the basis of our observations that the (110)-LP phase did not form for the smaller rare earth titanates (Dy and Y), even at extremely low laser repetition rates, the crossover in stability between these two polymorphs is suggested as line (II). Note they are both metastable relative to the pyrochlore. The crossover point represented by line (III), where the γ phase becomes more thermodynamically stable than the pyrochlore phase, is speculative, since we could not directly test the thermodynamic/kinetic stability of the pyrochlore versus the γ polymorph. On the basis of the fact that La2Ti2O7 could not be epitaxially stabilized in the pyrochlore structure, even on YSZ(100), we suggest that line III may exist between Nd and La. Still, the absence of a La2Ti2O7 pyrochlore could be explained simply by a large divergence in the

stability lines of the pyrochlore and the (110)-LP phases, with the γ polymorph being observed because of kinetic preferences. It should be mentioned that the authors were not able to find any similar structure in crystallographic databases. What is exciting about the observations in this work is that thin film synthesis offers the possibility of not only designing metastable crystals of well-known phases using the thermodynamic principles of epitaxial stabilization, such as has been done for many perovskites, but also realizing completely new structures that are kinetically stabilized owing to the special, constrained equilibriums that occur during nucleation, which can be controlled to promote specific polymorph formation. In this work, the complex layered structure of the known polymorph presents a kinetic barrier to growth, resulting in the formation of a new polymorph that is unknown using a wide-array of other synthetic techniques. It is likely that other compounds that adopt the (110)-layered perovskite structure might be synthesized in this new phase by controlling the growth parameters to capture the kinetically preferred polymorph. More detailed diffraction analysis, perhaps at a synchrotron, will be useful in determining the atomic positions to have a better understanding of the crystal structure. Knowledge of the crystal structure will also help in further understanding the nucleation and growth of this kinetically favored phase. It would also be interesting to look at structure-property relationships of this new phase, as layered compounds are known to have interesting properties. 4. Conclusions RE2Ti2O7 films, RE = La-Y, were synthesized as a new polymorph (which we call the γ polymorph) on SrTiO3(110) substrates. The γ polymorph has an orthorhombic crystal structure with a space group of Ammm. The lattice parameters are similar to those of the monoclinic (110)-LP polymorph, though the structures are not related by simple distortive transformations. In spite of the lattice parameter similarity, the γ phase has a smaller asymmetric (and electro-neutral) unit and hence is kinetically preferred to the (110)-LP

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)

)

polymorph during nucleation as a thin film by PLD. The films were epitaxial on SrTiO3(110) and the relationship between the substrate and films was found to be {001}film {110}subs:Æ010æfilm Æ110æsubs. For RE = Gd, Sm, Nd, and La, the γ phase is kinetically preferred to a (001)-oriented (110)-layered perovskite. For RE = Y, Dy, Gd, and Sm, the γ phase is epitaxially stabilized relative to the pyrochlore. For RE=Y and Dy, the γ phase is thermodynamically more stable than the (110)-LP phase.

Acknowledgment. This work was supported by the CBC program of the National Science Foundation (NSF): Award number CHE-0434567 and made use of facilities supported by the MRSEC program of the NSF: Award number DMR0079996. Supporting Information Available: Details of space group determination and compositional stability for the RE2Ti2O7 compounds. This material is available free of charge via the Internet at http:// pubs.acs.org.

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