Article pubs.acs.org/cm
Site Dependency of the High Conductivity of Ga2In6Sn2O16: The Role of the 7‑Coordinate Site Karl Rickert,† Ashfia Huq,§ Saul H. Lapidus,∥ Allison Wustrow,† Donald E. Ellis,‡ and Kenneth R. Poeppelmeier*,† †
Department of Chemistry and ‡Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, United States § Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *
ABSTRACT: The 6-coordinated cation site is the fundamental building block of the most effective transparent conducting oxides. Ga2In6Sn2O16, however, maintains 4-, 6-, 7-, and 8-coordinated cation sites and still exhibits desirable transparency and high conductivity. To investigate the potential impact of these alternative sites, we partially replace the Sn in Ga2In6Sn2O16 with Ti, Zr, or Hf and use a combined approach of density functional theory-based calculations, X-ray diffraction, and neutron diffraction to establish that the substitution occurs preferentially on the 7-coordinate site. In contrast to Sn, the empty d orbitals of Ti, Zr, and Hf promote spd covalency with the surrounding oxygen, which decreases the conductivity. Pairing the substitutional site preference with the magnitude of this decrease demonstrates that the 7-coordinate site is the major contributor to conductivity. The optical band gaps, in contrast, are shown to be site-independent and composition-dependent. After all 7coordinate Sn has been replaced, the continued substitution of Sn results in the formation of a 7-coordinate In antisite or replacement of 6-coordinate Sn, depending on the identity of the d0 substitute.
1. INTRODUCTION Transparent conducting oxides (TCOs) are used as transparent electrodes in renewable energy production (specifically solar cells) and advanced electronic devices (flat panel displays and touch screens).1−3 The demand for commercial TCOs is currently met by In2O3- and SnO2-based materials, but higher conductivities and a reduced indium content are desired.4 Much of the reported research on n-type TCOs focuses on either optimizing existing materials or doping known TCO host materials.5 In contrast, comparatively few design principles are known for pursuing new TCO materials. Those that do exist focus almost exclusively on composition, seeking new materials in the combined phase spaces of elements that are already employed in successful TCOs.6 Few structure-based design principles have been elucidated for these ubiquitous materials. One of the most developed structural concepts for seeking new TCOs is to maximize the density of octahedrally coordinated cation sites, as this improves the density of corner and edge sharing octahedra that form conductive pathways in the crystal structure.7,8 This theory, however, is based upon empirical data from the top-performing TCOs that exclusively contain 6-coordinate cation sites (with the exception of cubic Cd2SnO4, which has a 4-coordinate site) in their rock salt (CdO), rutile (SnO 2), or bixbyite (In2 O3 ) structures. Furthermore, the type of 6-coordinate site can impact the © 2015 American Chemical Society
performance of a material, as in the case of In2O3, for which the doping limit is dependent on the regular octahedral site (25% of the cation sites).9,10 Higher and lower coordination sites are omitted from this concept in part because alternative TCO structures are typically intricate, containing multiple distinct coordination sites (often 4- and 8-coordinate). Such complexity makes the differentiation between desirable and nondesirable sites difficult. For example, Ga3−xIn5+xSn2O16 (GITO), where 0.3 ≤ x ≤ 1.6, contains 4-, 6-, 7-, and 8- coordinate cation sites and has recently demonstrated a competitive room-temperature conductivity of as high as 1888 S cm−1, but the origin of this conductivity has yet to be determined.11−14 Similarly prepared 10% tin-doped indium oxide (ITO) bulk samples have a reported conductivity of ∼3600 S cm−1.15 Unlike ITO, the Sn in the GITO system is a structural element, stabilizing the local coordination environments that form the bulk of the crystal structure. Altering the tetravalent-to-trivalent ratio of GITO does not produce a solid solution like in most doped materials, but GITO is conductive, suggesting that Sn acts as a dopant. Herein, we report the partial substitution of Sn in Ga2In6Sn2O16 with Ti, Zr, or Hf and the structures and Received: September 25, 2015 Revised: November 10, 2015 Published: November 11, 2015 8084
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Table 1. Crystallographic and Acquisition Parameters of the Structures of Ga2In6Sn2−xMxO16, Obtained from Combined Refinements of Synchrotron X-ray and TOF Neutron Diffraction Data −1
formula weight (g mol ) temp (K) crystal system space group a = b (Å) c (Å) α = β = γ (deg) volume (Å3) Z ρcalc (g cm−3) X-ray θ range (deg) no. of reflections collected X-ray neutron χ2 Rwp (total) RF2 (XRD) RF2 (ND)
M = Ti, x = 0.1
M = Ti, x = 0.3
M = Ti, x = 0.7
M = Zr,a x = 0.8
M = Hf,a x = 0.7
1314.65 300 tetragonal I41/a 11.1684(1) 10.0280(1) 90 1250.82(2) 4 6.98 1.5−23
1300.49 300 tetragonal I41/a 11.1593(1) 10.0153(2) 90 1247.21(4) 4 6.93 1.5−23
1272.18 300 tetragonal I41/a 11.1445(1) 9.9920(2) 90 1240.99(4) 4 6.81 1.5−23
1297.78 300 tetragonal I41/a 11.1697(0)b 10.0471(0) 90 1253.51(0) 4 6.88 1.5−23
1363.97 300 tetragonal I41/a 11.1719(0) 10.0474(0) 90 1254.04(0) 4 7.22 1.5−23
2031 14759 3.112 0.0662 0.0643 0.1121
2554 12924 3.471 0.0593 0.0402 0.1050
2308 12623 4.123 0.0622 0.0353 0.1109
2115 28725 4.961 0.0833 0.0477 0.0589
2132 13538 3.531 0.0676 0.0490 0.0420
a This material is not single-phase; there are minor secondary phases present. bThe values with a listed error of (0) do have an error, but it is an error that is one order of magnitude smaller than what is measurable by the instruments.
Laboratory. All data were collected at 300 K, and the M = Ti samples had additional collections performed at 100 K. The central wavelength used was 1.066 Å. Another portion of each sample was packed into a cylindrical Kapton capillary (inner diameter of 0.8 mm), and both ends were sealed with Q Compound (Apiezone). Synchrotron X-ray diffraction (XRD) data were collected at 11-BM on the Advanced Photon Source at Argonne National Laboratory with a wavelength of 0.459 Å at the same temperatures that were used for the neutron data. Structural determinations used these data in joint Rietveld refinements via EXPGUI using the General Structure Analysis System (GSAS).18,19 One of the five pellets of each sample was removed after the second firing, and a second pellet was removed after the 500 °C reduction. Each pellet was ground in an agate mortar and pestle with ∼10% silicon (Alfa Aesar, 99.9985%) by mass, packed onto a flat glass slide, and subjected to Cu radiation in an Ultima IV (Rigaku) X-ray diffractometer. Scans were taken with 2θ beginning at 10 and ending at 70. The nonreduced patterns were the input used to calculate lattice parameters via whole pattern fitting (MDI Jade 2010). Silicon was used as an internal standard to correct for the instrumental offset. 2.3. Thermogravimetric Analyses. As-fired samples of Ga2In6Sn2−xTixO16 (x = 0, 0.1, 0.3, or 0.7) were ball milled with tungsten carbide media at 600 rpm for 3 min, placed on Al2O3 pans, and used for thermogravimetric analyses (TGA) in a TGA Q50 instrument (TA Instruments) under a 5% hydrogen atmosphere (argon balance). The heating profile that was used was the same as that of the reduction steps. 2.4. Computational Methodology. The structural energies and electronic properties were calculated using spin-restricted density functional theory (DFT) employed through the Vienna ab initio simulation package (VASP).20−23 Plane augmented wave (PAW) pseudopotentials were used with Perdew−Burke−Ernzerhof (PBE) exchange correlation functionals.24 A 2 × 2 × 2 k-point grid and an energy cutoff of 400 eV were used in initial structural relaxations. The final grid was augmented (up to 5 × 5 × 5) to guarantee adequate energy precision to distinguish between differing site configurations. Full 104-atom tetragonal unit cells were used, with varying amounts and locations of Ti, Zr, and Hf substitutions. For full details about the unit cells used and substitution procedures, see the Supporting Information. To reduce computational costs, structural relaxations were performed with tin and indium d electrons in the pseudopotential core. Some static calculations on relaxed cells were performed with indium d electrons in the valence space to obtain more
properties of the resulting solid solutions. A combination of experimental structural characterization and energy of formation computations demonstrate that these substitutions are site-directed, which, combined with the property measurements, establishes a site−property relationship. Although such substitutional studies have never been performed before on GITO to the best of our knowledge, similar substitutions have been conducted on monoclinic phases that also populate the Ga−In−Sn−O phase space.16,17
2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Sample Preparation. Samples of Ga2In6Sn2−xMxO16 (M = Ti, Zr, or Hf; x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, or 0.7, and x = 0.8 for M = Zr) were prepared by mixing stoichiometric quantities of Ga2O3 (Alfa Aesar, 99.99%), In2O3 (Alfa Aesar, 99.99%), SnO2 (SigmaAldrich, 99.9%), and MO2 (M = Ti, Zr, or Hf; Sigma-Aldrich, 99.9, 99, or 98%, respectively) via a Fritsch planetary ball mill using agate media at 600 rpm for four cycles of 15 min, with 5 min pauses between cycles. Each sample was pressed into a 2.54 cm cylindrical pellet under 20000 psi, buried in sacrificial powder in nested alumina crucibles, and heated to 1300 °C at a rate of 5 °C min−1. The temperature was held at 1300 °C for 28 h before being decreased to 25 °C at a rate of 5 °C min−1. Each pellet was then ball milled again using tungsten carbide media at 600 rpm for 3 min, pressed into five 13 mm cylindrical pellets under 16000 psi, and subjected to the same preparation and heating cycle. These pellets were placed on beds of sacrificial powder in alumina boats and subjected to two separate reductions. The first was heated to 400 °C at a rate of 5 °C min−1, held at 400 °C for 7 h, and cooled to room temperature at a rate of 5 °C min−1. The second reduction had the same heating profile but had a maximal temperature of 500 °C. Both reductions took place under a 5% hydrogen atmosphere (argon balance). 2.2. X-ray and Neutron Diffraction. Samples of Ga2In6Sn2−xTixO16 (x = 0.1, 0.3, or 0.7), Ga2In6Sn2−xZrxO16 (x = 0.8), and Ga2In6Sn2−xHfxO16 (x = 0.7) were prepared as described above but were not subjected to the reduction treatments. Instead, they were reground in the ball mill using tungsten carbide media at 600 rpm for 3 min. A portion of each sample was placed in a cylindrical (6 mm diameter) vanadium can, and time-of-flight (TOF) neutron diffraction (ND) data were collected at the POWGEN beamline at the Spallation Neutron Source at Oak Ridge National 8085
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Figure 1. Neutron (a, c, and e) and X-ray (b, d, and f) refinements at 300 K of Ga2In6Sn2−xTixO16 for x = 0.1 (a and b), x = 0.3 (c and d), and x = 0.7 (e and f). The Y-axes are normalized intensity, and the x-axes are d spacing (neutron) and Q (X-ray). Insets show the goodness of fit at high Q values. nuclear site. Thus, QWS and QB are defined as Z − NWS and Z − NB, where Z is the number of “valence electrons” included in the pseudopotential as described above and N is the integrated charge by either method. QWS and QB measure the same quantity by different methods and thus provide a useful comparative interpretation scheme. Relaxed geometric structures and electronic charge densities were visualized using VESTA.25 2.5. Conductivity and Band Gap Determinations. The sheet resistance of each pellet was measured after the second firing and both reductions at five different locations on each face at room temperature
accurate total energy values and to assess possible effects on spectroscopic properties. Although quantitative energies changed somewhat, energy ordering with respect to doping and ordering of structures was essentially unaltered. The valence states represented in the underlying atomic pseudopotentials were thus O 2s2sp4, Ti 3p63d34s14p0, Ga 3d104s24p1, Zr 4s24p64d25s2, In 5s25p1, Sn 5s25p2, and Hf 5p65d26s2. Net ionic charges were measured by a spherical volume integration about each site, characterized by radii, RWS, and by Bader’s topological atoms method, defined by zero-charge flux surfaces around each 8086
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Chemistry of Materials using a four-point probe (model 280PI, Four Dimensions, Inc.). Sample dimensions and masses were taken at each step to apply geometry and thickness corrections, and the sheet resistance was converted to bulk conductivity.26 Additionally, a porosity correction following the Bruggeman symmetric medium model was applied to each result, assuming that the air in the pellet acts as an insulating phase.27 The diffuse reflectance of each pellet was measured from 250 to 800 nm at room temperature using a Lambda 1050 UV−vis spectrophotometer with an integrating sphere attachment (PerkinElmer) after the second firing and the 400 °C reduction. Background spectra of compacted polytetrafluoroethylene were recorded. Optical band gaps were calculated by converting diffuse reflectance into Kubelka−Munk notation and approximating the optical band gap as the intersection of a linear extrapolation of the band edge and a linear extrapolation of the background.28−30 The full details for the conductivity and optical band gap calculations are provided in the Supporting Information.
Table 3. Differences in Energies of Formation for Ga2In6Sn2−xMxO16 (relative to GITO) Obtained with a 3 × 3 × 3 k-Point Grida no. and coordination of substituted sites
3. RESULTS AND DISCUSSION 3.1. Structural Determinations. The acquisition and crystallographic parameters for Ga2In6Sn2−xTixO16 (x = 0.1,
formation energy (eV)
x
8
4
7
6
M = Ti
M = Zr
M = Hf
0.25 0.25 0.25 0.25 0.25 0.25 0.5 0.5 0.5 0.75 0.75 0.75
1 1 0 0 0 0 0 0 0 0 0 0
0 0 1 1 0 0 0 0 0 0 0 0
0b 0 0b 0 1 0 2 1 0 3 1 0
0 0b 0 0b 0 1 0 1 2 0 2 3
−7.16 −6.87 −7.31 −7.47 −7.93 −7.78 −7.87 −7.88 −7.72 −7.70 −7.81 −7.75
−9.53 −9.21 −7.89 −8.03 −9.84 −9.06 −9.68 −9.46 −9.10 −9.56 −9.35 −9.10
−11.34 −11.03 −10.09 −10.04 −11.82 −11.03 −11.69 −11.54 −11.27 −11.55 −11.46 −11.27
a
A negative value denotes an exothermic substitution. For reference, the energy of formation of GITO is calculated to be −605.041 eV. The most energetically favored values for each combination of M and x are underlined. bThe non-Sn atom that is displaced from the 4- or 8coordinate sites as a result of the M substitution is relocated onto this site, replacing Sn.
GITO. Although the Ga2In6Sn2−xMO16 with M = Ti consists of a single phase, M = Hf and M = Zr compounds have secondary phases that suggest the substitution limit was slightly surpassed. The secondary phases are a combination of In2O3 and Ga0.5In1.5O3 that exist in mass percents of 0.2 and 3.4%, respectively, for M = Hf and 1.4 and 5.4%, respectively, for M = Zr. Given these secondary phases, more appropriate chemical formulas for these materials would be Ga2.0In5.9Sn1.4Hf0.7O16.1 and Ga2.0In5.8Sn1.3Zr0.9O16.1. The cation contents of the updated formulas are used in the Rietveld refinements, but the updated oxygen content is not used, as the difference is too small to be observable with these techniques. The M = Ti XRD patterns have unusual peak shapes that are likely a result of either a bimodal distribution of grain size or strain that is introduced into the system by replacing Sn with the comparatively small Ti.31 This unusual peak shape is accounted for in the Rietveld refinements by refining the shape parameters of two phases that maintain identical structural parameters. The impact of the different sizes of M can also be tracked by cell volume, as provided in Figure 2, which is similar to the variations displayed by the a and c parameters (provided in the Supporting Information). The system follows Vegard’s law when M is the smaller Ti, as a unit cell contraction is observed up to the solubility limit (x = 0.7) and the volume is static once the limit
Figure 2. Cell volumes for Ga2In6Sn2−xMxO16, with M = Ti (blue), M = Zr (red), or M = Hf (green), obtained from laboratory XRD (circles) and the most energetically favored DFT calculations (triangles). x > 0.7 is beyond the solubility limit for M = Ti.
0.3, or 0.7) at 300 K (100 K parameters are provided in the Supporting Information), Ga 2 In 6 Sn 1 . 2 Zr 0 . 8 O 1 6 , and Ga2In6Sn1.3Hf0.7O16 are listed in Table 1. Patterns exhibiting joint Rietveld refinements of the synchrotron XRD and TOF ND for Ga2In6Sn2−xTixO16 (x = 0.1, 0.3, or 0.7) at 300 K are provided in Figure 1 (100 K, Ga2In6Sn1.2Zr0.8O16, and Ga2In6Sn1.3Hf0.7O16 patterns are provided in the Supporting Information). Atomic positions and thermal parameters are provided in the Supporting Information for each refinement. Each material is isostructural with the parent structure of Table 2. Experimentally Determined Site Occupancies at 300 K Wyckoff position
coordination number
M = Ti, x = 0.1
M = Ti, x = 0.3
M = Ti, x = 0.7
M = Zr, x = 0.8
M = Hf, x = 0.7
4a 4b 16f
8 4 7
100% In 100% Ga 22.5% Zr 77.5% In
100% In 100% Ga 82.5% In 17.5% Hf
6
100% In 100% Ga 7.5% Ti 92% In 0.5% Sn 25% Ga 33% In 42% Sn
100% In 100% Ga 8% Ti 92% In
16f
100% In 100% Ga 2.5% Ti 92% In 5.5% Sn 25% Ga 33% In 42% Sn
9.5% Ti 25% Ga 33% In 32.5% Sn
25% Ga 42.5% In 32.5% Sn
25% Ga 40% In 35% Sn
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is surpassed.32,33 The volumes for M = Hf or Zr, however, remain approximately constant as a result of the similarity of their ionic radii with that of Sn (within 0.03 Å).31 The site occupancies, listed in Table 2, that are obtained from the Rietveld refinements are of great import. The M = Ti materials allow precise tracking of the substitution with increasing x. The substitution occurs first on the 7-coordinate site, replaces all of the Sn on this site, and then begins replacing Sn on the 6-coordinate site. The solubility limit is reached when ∼25% of the 6-coordinate Sn is replaced. Although only a single structure has been determined for M = Zr and for M = Hf, they establish a new substitution behavior: the 7-coordinate site remains the preferred site, but Zr and Hf continue to substitute onto the 7-coordinate site once all of the Sn has been replaced. The 7-coordinate site thus partially becomes an In antisite, and the displaced In is forced onto the 6-coordinate site to replace Sn. Ti, Zr, and Hf are traditionally considered to demonstrate similar chemical tendencies, but different site occupancies in a given system have been previously reported.34 Despite the two alternative behaviors that are present after the complete replacement of Sn on the 7-coordinate site, the 7-coordinate site is the universally preferred substitution site in these systems. Even with the high-quality synchrotron XRD and ND data, however, many of the atoms have similar X-ray and/or neutron scattering cross sections and are therefore difficult to differentiate.35,36 M = Hf in particular suffers from this challenge, as the refinement for the same saturation behavior as observed for M = Ti (see the Supporting Information) produces an Rwp of 7.11%. Although this value is similar to the Rwp reported here (6.76%), the saturation models satisfy the criteria for applying Hamilton’s R ratio test, which demon-
Figure 3. Conductivity values for Ga2In6Sn2−xMxO16, where M = Ti (blue), M = Zr (red), or M = Hf (green) (a) post-500 °C reduction (triangles) and post-400 °C reduction (circles) and (b) postfiring (squares). Except for Ti, all of the samples with x > 0.3 had no measurable conductivity after firing. The dashed line is at x = 0.32 (7coordinate Sn saturation point).
Figure 4. Valence charge density (electrons per cubic angstrom) around the 7-coordinate site for (a) Sn at x = 0.0 and for (b) M = Ti, (c) M = Zr, and (d) M = Hf at x = 0.25. Colors denote different levels, which are nested inside each other. 8088
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Figure 5. GITO structure in the [2 −1 0] direction showing the corner- and edge-sharing lattice of 6-coordinate sites (left) and 7-coordinate sites (right), with 4-coordinate sites colored black, 6-coordinate sites/polyhedra colored green, 7-coordinate sites/polyhedra colored blue, and 8coordinate sites colored white.
Figure 6. Atom-resolved PDOS for Ga2In6Sn2−xMxO16 with (a) M = Ti, (b) M = Zr, or (c) M = Hf and x = 0.25 with all substitution occurring on the 7-coordinate site. The Fermi energy is set to 0 eV, and the Sn and In core d states are suppressed.
formation energies are listed in Table 3. As can be observed from these values, the replacement of a 4- or 8-coordinate site (4b or 4a, respectively) is not favored. Both experiment and
strates that the difference in these values is statistically significant.37 DFT structural energy calculations are performed to corroborate the experimental findings. The relative 8089
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Figure 7. Optical band gaps of Ga2In6Sn2−xMxO16, where (a) M = Ti, (b) M = Zr, or (c) M = Hf, before (squares) and after (circles) hydrogen reduction at 400 °C.
theory concur that the 7-coordinate site is the preferred replacement site in the Ga2In6Sn2−xMxO16 systems and that M = Ti saturates the 7-coordinate Sn and then moves to the 6coordinate site whereas the heavier elements substitute on only the 7-coordinate site. 3.2. Structure−Conductivity Relationship. The 7coordinate site has been established as a preferred substitution site, and two alternative oversaturation behaviors have been determined. The property characterization of these materials provides the singular opportunity to elucidate a structure− property relationship. The conductivity, pre- and postreduction, as a function of increasing x, is provided in Figure 3. No structural alterations or decompositions occur as a result of the reduction process, as shown by powder XRD of reduced samples (provided in the Supporting Information). The most obvious trend that can be observed from the conductivity data is that the conductivity decreases with increasing x for all systems. Additionally, the conductivity increases dramatically after reduction. If Sn were only a structural element, then the latter trend would not be apparent, as a reduction would not radically increase the carrier concentration or mobility. It is reasonable to conclude that Sn is in fact functioning as both a dopant and a structural element in GITO, as has been previously hypothesized.38 Although the dopant nature accounts for the improved conductivity postreduction, as the reduction increases the anion deficiencies that make the tetravalent cation a more effective dopant, it does not explain why the other tetravalent cations show such a remarkable decrease in conductivity. It is true that Sn is widely considered to be the best dopant in In2O3-based systems, but using Ti as a dopant has produced comparable conductivity values.15 Unlike
Figure 8. Spectra of Ga2In6Sn1.9Hf0.1O16 (a) before and (b) after reduction with the Y-axes in Kubelka−Munk notation. These show the extrapolations (red) used to calculate the optical band gap (intersection).
other systems in which tetravalent cations are purely dopants, the structural nature of the tetravalent cations in GITO allows Ti, Zr, and Hf to form “covalent necks” with the three closest O atoms, as shown in charge density isosurfaces provided in Figure 4. Sn, despite having a higher electronegativity, does not form these necks, although there is a distortion toward the Sn in the 0.04 e/Å3 contour. The empty d orbitals of Ti, Zr, and Hf are the cause of these necks, as they lie near the Fermi energy and promote spd covalency, whereas the Sn d orbitals are filled and corelike. Despite the overall complexity of the GITO parent structure, the local coordination environment expands or contracts to accommodate the substitute atom. This allows Ti to form the covalent necks despite the disparity in its size relative to Sn, as the average M−O bond distance of the three closest O atoms decreases by an average of 0.13 Å when Sn is replaced with Ti. Interestingly, the bond length to the seventh oxygen increases to 3.22 Å, meaning that Ti has altered the 7-coordinate site of Sn to a 6-coordinate site (see the Supporting Information for complete localized bonding). These covalent necks mix the metal conduction states with the oxyanion states, leading to electron scattering and thus the observed increased resistance. This bonding mechanism, however, impacts the conductivity only if the 7-coordinate site plays a role in the conduction pathway, which would 8090
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thus carrier generation. If the carrier generation is normalized with mass loss, then there must be a second factor that inhibits conductivity to account for the difference in relative changes. The second factor is being partially attributed here to the formation of covalent necks and their impact on the conduction pathway and thus the carrier mobility. For comparative purposes, it should be noted that the asfired Ga2In6Sn2O16 (x = 0.0) ceramics have a conductivity (15 ± 7 S cm−1) higher than that originally reported (2 S cm−1) but lower than what would be anticipated from recent results (∼300 S cm−1).11,14 Synthetic differences are likely the cause of the discrepancies between these values. The samples reported here were slowly cooled to room temperature, which allowed anion vacancies to repopulate; previous reports quenched the samples and did not allow such repopulation to occur. Quenching would produce a more conductive sample, and after the samples had undergone a reduction procedure similar to what is reported for the quenched samples, our conductivity values were slightly higher, but similar. This small difference is likely a result of the difficulty inherent in standardizing the reduction step. 3.3. Composition−Band Gap Relationship. The partial density of states (PDOS) of each material at x = 0.25 is provided in Figure 6. Examination of the top-most valence band (VB) energies and the bottom-most conduction band (CB) energies shows that a first O-to-O weak absorption is present at the Γ point in all of the materials and is largely a result of a long tail on the antibonding portion of the oxygen sp band. Weak band tails in the intergap region are frequently attributed to disorder, but in the case presented here, it is believed to follow from the extreme range of oxygen environments within the crystal structure. The CB features vary systematically with atomic number, which modifies the principal VB to CB transitions and offers the possibility of tuning the optical absorption properties by varying the composition. Specifically, compared to the Ti4+ 3d04s04p0 configuration, the Zr4+ 4d05s05p0 configuration reveals CB states starting ∼3.5 eV above EF that are somewhat broadened. This is as expected, leading to a ligand to metal band gap that is ∼1 eV higher than that of the Ti-substituted system. Compared to the Ti and Zr CB features, the Hf CB features are further shifted to a higher energy and broadened. The dispersion around the top of the VB and the bottom of the CB, in contrast, is remarkably constant regardless of the identity of M. The experimentally measured optical band gaps of the prereduction and post-400 °C reduction samples are presented in Figure 7. The post-500 °C reduction samples are dark brown or gray and are unsuitable for band gap calculations. Both the pre- and post-400 °C reduction samples are pale yellow/green. A general trend is that the reduced samples have greater optical band gaps than their as-fired counterparts, which can be explained by a Burstein−Moss shift, as the reduction improves the conductivity and therefore the concentration of charge carriers.40 Examples of pre- and postreduction absorption curves are provided in Figure 8 and illustrate the Burstein− Moss shift (see the Supporting Information for additional absorption curves). A second trend is the increasing band gap as x increases and is likely caused by an increase in the ionic nature of the material. This also agrees with other reported TCO systems.41 Semiconductor systems typically have a fundamental band gap that is inversely correlated with the concentration of the substitute, but as seen here, the combination of a Burstein−Moss shift and the increasing
expand upon the previous exclusive focus on the 6-coordinate site. The structural data establish the 7-coordinate site as the preferred substitution site, which then links the trends in conductivity from x = 0.0 to 0.3 directly to the Sn composition on the 7-coordinate site (complete replacement occurs at x = 0.32). All systems lose >50% of their conductivity in this range, suggesting that the 7-coordinate site is a dominant factor in the conductivity. The 6- and 7-coordinated cation sites form two distinct corner- and edge-sharing lattices with regular (6coordinate) and irregular (7-coordinate) octahedra in the extended GITO crystal structure. It is likely that transient electrons prefer one of these lattices, as each lattice would have a unique local energy landscape. As shown in Figure 5, these lattices are interwoven and approximately equal. The conductivity shows a >50% decrease when only the 7coordinate site is modified, suggesting the 7-coordinate lattice is the preferred route for electrons. This is not, however, to say that the 7-coordinate site is the exclusive origin of conductivity. If this were so, then there would be no further decrease in conductivity for M = Ti once the 7-coordinate site is saturated, but such a decrease is observed. This conclusion is supported by a report that correlates the properties of GITO with its Ga/ In content.14 Alternative Ga/In compositions alter the 6coordinate site and exhibit decreasing conductivity with increasing Ga content, thereby demonstrating that the 6coordinate site also contributes to the conductivity. For example, a change in x from 1 to 0.3 in Ga3−xIn5+xSn2O16 decreases the conductivity from 1241 S cm−1 (from this study) to ∼700 S cm−1, whereas a smaller change in x, from 0.0 to 0.3, in Ga 2 In 6 Sn 2−x M x O 16 results in a larger decrease in conductivity, resulting in values of ∼550 S cm−1. The differing magnitudes of the change in conductivity as a function of composition demonstrate that the 7-coordinate Sn is the dominant factor in governing conductivity, compared to the 6coordinate site. This finding is particularly important, as the focus has historically been on 6-coordinate sites to improve the conductivity of TCOs. Using the aforementioned octahedral site density to conductivity link, fully optimized (i.e., thin film) GITO is predicted to have a conductivity of ∼4000 S cm−1, but this may be an underestimate given the previously unconsidered impact of the 7-coordinate site.8 A similar property site dependence has previously been reported for Mn3Ta2O8, which is isostructural with GITO, as the electronic transitions across the band gap are governed by the 4- and 7-coordinate sites.39 Oddly, the differing oversaturation behavior of Ti does not appear to be reflected in the conductivity. There is a noteworthy increase in conductivity after the 400 °C reduction for x = 0.4 for M = Zr and Hf that would correspond to the first formation of the In antisite, but the lack of a corresponding increase in either of the other conductivity sets suggests this may not be significant. Alterations in the conduction pathway impact carrier mobility, but a decrease in conductivity could also be a result of a decreased carrier concentration. Thermogravimetric analyses (see the Supporting Information) demonstrate that the mass loss during the 500 °C reduction is inversely correlated with x and thus conductivity, but the mass loss after 400 °C does not display any clear relationship with either conductivity or composition. Furthermore, the relative changes in mass between the two reduction steps do not correlate with the relative changes in conductivity. This mass loss can tentatively be assigned to the creation of oxygen vacancies and 8091
DOI: 10.1021/acs.chemmater.5b03790 Chem. Mater. 2015, 27, 8084−8093
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Chemistry of Materials ionic character of a material appears to negate this.42,43 In these systems, the concentration of the substitution is directly correlated with the band gap and inversely correlated with the conductivity.
International Institute for Nanotechnology. A portion of this research was performed at POWGEN at Oak Ridge National Laboratory’s Spallation Neutron Source and was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Use of 11-BM on the Advanced Photon Source at Argonne National Laboratory (ANL) was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357. This work made use of the J. B. Cohen X-ray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR-1121262) at the Materials Research Center of Northwestern University. A portion of this work was supported by the Northwestern University Keck Biophysics Facility and a Cancer Center Support Grant (National Cancer Institute Grant CA060553). We thank Michael Holland (NU) for helpful discussions.
4. CONCLUSION The Sn in Ga2In6Sn2O16 is partially replaced to form Ga2In6Sn2−xMxO16 (M = Ti, Zr, or Hf) with x ≤ 0.7 (M = Ti or Hf) or x ≤ 0.8 (M = Zr). A combined approach with synchrotron XRD, TOF ND, and DFT-based calculations demonstrates that such a substitution occurs preferentially on the 7-coordinate site. The 6- and 7-coordinate sites each form continuous networks in the extended crystal structure, but the magnitude of an inverse relationship between the substitution level and conductivity shows that the 7-coordinate site is the major, but not exclusive, contributor to conductivity. This extends previous reports that focused exclusively on 6coordinate sites.7,8 The decrease in conductivity is a result of covalent necks that form for M = Ti, Zr, and Hf but are not present when x = 0. The covalent necks result from the roles of the tetravalent atoms as both a structural element and a dopant and the d0 nature of the replacements. Once the Sn on the 7coordinate site has been fully replaced, the M = Ti system begins to replace Sn that is present on a 6-coordinate site, but the M = Zr and Hf systems continue to substitute on the 7coordinate site, displacing In to the 6-coordinate site and generating an In antisite. Optical band gap measurements are presented and show a shift that can be attributed to a Burstein− Moss effect and a general increase that is attributed to the increasing ionicity of the materials. Furthermore, the nature of the conduction band is determined to be dependent on the identity of the substitute.
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ABBREVIATIONS TCO, transparent conducting oxide; GITO, Ga3−xIn5+xSn2O16; ITO, tin-doped indium oxide; TOF, time-of-flight; ND, neutron diffraction; XRD, X-ray diffraction; GSAS, general structure analysis system; DFT, density functional theory; VASP, Vienna ab initio simulation package; PAW, planeaugmented wave; PBE, Perdew−Burke−Ernzerhof; TGA, thermogravimetric analyses; PDOS, partial density of states; VB, valence band; CB, conduction band
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b03790. DFT geometry details, conductivity calculation details, optical band gap calculation details, 100 K Rietveld refinements for M = Ti, 300 K Rietveld refinements for M = Zr and Hf, atomic positions and thermal factors, DFT-based bond lengths and ionicities, alternative oversaturation Rietveld refinement for M = Hf, reduced PXRD, TGA data, optical band gap spectra, and uncorrected conductivity data (PDF) Crystallographic data (CIF)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS K.R. acknowledges that this material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant DGE-1324585. K.R. and K.R.P. gratefully acknowledge additional support from Department of Energy Basic Energy Sciences Grant DE-FG0208ER46536. A.W. gratefully acknowledges support from the Ryan Fellowship and the Northwestern University (NU) 8092
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