Evolution of the Local Structure within Chromophoric Mn–O5 Trigonal

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Evolution of the Local Structure within Chromophoric Mn−O5 Trigonal Bipyramids in YMn1−xInxO3 with Composition

Soham Mukherjee,† Hasitha Ganegoda,‡,§ Abhinav Kumar,†,∥ Somnath Pal,†,⊥ Carlo U. Segre,‡ and D. D. Sarma*,†,# †

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bengaluru 560012, India CSRRI & Department of Physics, Illinois Institute of Technology, Chicago, Illinois 60616, United States

‡

Inorg. Chem. Downloaded from pubs.acs.org by TU MUENCHEN on 07/23/18. For personal use only.

S Supporting Information *

ABSTRACT: We have investigated the local environment around Mn3+ and In3+ ions in YMn1−xInxO3 chromophores to understand the origin of the intense blue color for small values of x in these solid solutions. While X-ray diffraction results provide an average description of the trigonal bipyramidal (TBP) units about Mn/In atoms with five oxygens surrounding the cation, the X-ray absorption near edge structure (XANES) as well as extended X-ray absorption fine structure (EXAFS) of these materials clearly suggest the presence of two different TBP environments, one of which is similar to MnO5 TBP in YMnO3. EXAFS in conjunction with first-principles calculations show that replacing larger In3+ ions by smaller Mn3+ ones additionally gives rise to another TBP strongly distorted along the axial direction, expanding one of the axial Mn−O bonds by ∼11%. The relative fraction of these two environments changes in close agreement with the global stoichiometry with the elongated TBP, therefore, being dominant in the regime of the low Mn content. This local structural difference is responsible for the intense, but relatively narrow, absorption feature in the red-yellow region of the absorption spectrum, and hence YMn1−xInxO3 appears blue for small Mn dopings. This distortion is relatively less abundant in Mn-rich compositions, and therefore, such compositions appear black, controlled by the wide absorption feature of the trigonal bipyramid coordination with Mn−O bond lengths that are essentially the same as those in YMnO3, covering the entire visible range. The chromophore properties are, thus, governed by the ratio of these two MnO5 TBP environments, one with a characteristic optical absorption giving it a blue color and the other absorbing over the entire visible range.

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INTRODUCTION Hexagonal YMn1−xInxO3 has recently emerged as a synthetic pigment showing intense blue coloration1 with better durability and lower environmental hazards compared to existing commercially available blue pigments like cobalt blue (CoAl2O4), Han blue (BaCuSi4O10), Prussian blue (Fe4[Fe(CN) 6 ] 3 ), ultramarine (Na 7 Al 6 Si 6 O 24 S 3 ), Persian blue ((Na,Ca) 8 (AlSiO 4 )(S, SO 4 , Cl) 1 − 2 ), and azurite [Cu3(CO3)2(OH)2]. This discovery in the YMn1−xInxO3 solid solution is rather surprising since the two stoichiometric, isostructural, and homovalent end-members, namely, YMnO3 and YInO3, are not chromophores with YMnO3 which is black in color and YInO3 which is colorless. The Mn3+/In3+ ions in these solid solutions reside in trigonal bipyramidal cages of five O2− ions. This is a rather unusual coordination for Mn3+, as it mostly prefers an octahedral coordination.2,3 At room temperature, both hexagonal and orthorhombic forms of YMnO3 and YInO3 can be stabilized under controlled reaction conditions.2,4−6 Although both compounds assume the typical ABO3 formula unit of a perovskite structure, only the orthorhombic phase is related to the perovskite structure, as © XXXX American Chemical Society

it assumes the centrosymmetric mmm point group symmetry. The hexagonal structure typically assumes 6mm point group symmetry which is noncentrosymmetric (γ = 120°) and, hence, is not related to the perovskite structure.5 Both crystal structures (a) hexagonal and (b) orthorhombic ABO3 are shown schematically in Figure 1, viewed along a (top panel) and c (bottom panel) directions. Hexagonal ABO3 (Figure 1a) when viewed along a/b direction shows a typical ABAB... type layered structure consisting of alternating planes of corner shared B-O5 TBPs and A ions. The hexagonal arrangement becomes evident when viewed along the c direction. Orthorhombic ABO3 (Figure 1b), on the other hand, is composed of a three-dimensional network of distorted corner shared B-O6 octahedra, with A atoms residing in the cavities in between them. The unit cell edges for both the structures have been marked in blue lines. In the hexagonal structure, Mn3+/In3+ ions reside in a TBP (trigonal bipyramidal) environment of five O2− ions, Received: April 12, 2018

A

DOI: 10.1021/acs.inorgchem.8b00997 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Crystal structures of (a) hexagonal and (b) orthorhombic ABO3 (A = purple, B = gray, O = red). Crystal field splitting diagram of Mn3+ (d4 system) in a (c) D3h and (d) Oh environment. For D3h, e′ → a′ is a symmetry-allowed transition.

defined long-range symmetry, various types of local distortions around different sites within the solid solution can be strikingly different from each other,27 none of which may scale with smoothly varying lattice parameters with the composition.12 In this work, using XAFS as the local structural tool, we have addressed why and how distortions within the MnO5 trigonal bipyramidal units are responsible for both the intense blue coloration for low Mn content and the evolution of the coloration to black with an increasing Mn composition in the YMn1−xInxO3 solid solutions. Experimental Details and Data Reduction. We have synthesized solid solutions of YMn1−xInxO3 following standard solid-state reactions, using Y2O3, Mn2O3, and In2O3 as the starting materials. The composition was varied by changing the Mn/In ratio where x assumes values 0.00, 0.10, 0.25, 0.50, 0.60, 0.70, 0.75, and 0.95 in the following reaction: 0.5·Y2O3 + 0.5·(1 − x)Mn2O3 + 0.5·xIn2O3 = YMn1−xInxO3. Because of the hygroscopic nature of Y2O3, it was first heated at 900 °C for 12 h, weighed under hot conditions, and mixed in stoichiometric ratio with Mn2O3 and In2O3. The thoroughly ground samples were then pelletized and sintered at 1200 °C for 12 h. The pellets were then subjected to a second heat treatment at 1300 °C for 24 h to ensure better homogeneity and to suppress chances of creating antisite disorder.28 Room temperature powder X-ray diffraction measurements using Cu Kα radiation were performed on the samples with the help of Siemens and Philips diffractometers. Since these systems can also be stabilized in a centric orthorhombic symmetry,3 we used a small step size of 0.02° to track the presence of any splitting of the Bragg peaks occurring due to orthorhombic distortions, or any additional impurity phases. The noise level was reduced by employing a slow scan speed of 0.2°/min. Lattice parameters for the YMn1−xInxO3 series were extracted by fitting of the XRD profiles using the GSAS and EXPGUI programs.29 XAFS measurements were performed at both Mnand In-K edges at the bending magnet lines of Sectors 10 and 12 at the Advanced Photon Source (APS), Argonne National Laboratory. All experiments were performed at a low temperature of 25 K, so the thermal contributions may be eliminated and the extracted Debye−Waller factor determined from the analysis of the EXAFS (extended X-ray absorption fine structure) data would be largely static, originating from a spread of various bond lengths inevitable in such disordered structures.30 Since a few of our samples have low Mn concentrations, we collected XAFS signals in the fluorescence

whereas in the orthorhombic structure, they occur in an octahedral environment of six O2− ions. This difference between the distributions of O2− ions about Mn3+ ions creates very different crystal field splitting of the Mn 3d orbitals. While the point group symmetry of the MnO6 octahedron in a cubic perovskite is Oh, the local structure of MnO5 in YMnO3 has been discussed7 in terms of D3h point group symmetry ignoring small distortions in bond lengths that make this assignment only approximate. In the following, we have adopted the same convention. We compare the energy level crystal field splitting diagram for the 3d orbitals of Mn3+ in the two different local geometries: TBP (Figure 1c) and octahedral (Figure 1d), respectively. Mn3+ (d4 configuration) is a Jahn−Teller active ion in an octahedral environment,8 i.e., the centrosymmetric orthorhombic phase, but remains Jahn−Teller inactive in a TBP environment,9 i.e., the noncentrosymmetric hexagonal phase. The origin of the intense blue coloration in the case of dilute Mn compositions in YMn1−xInxO3 has been associated1 with the crystal field splitting of Mn d-orbitals in a TBP environment of O2− ions with the lowest-energy excitation from e′ → a′ (see Figure 1c) being a dipole-allowed transition and appearing in the red-yellow region of the visible spectrum. Within this existing interpretation, increasing Mn content in the solid solution leads to interactions between different MnO5 TBPs, resulting in broad absorption features covering the entire visible range for larger Mn content and, thereby, appearing black in color. However, reported1 absorption spectra, while showing some evidence of broadening, suggest the growth of additional features with increasing Mn content that cannot be accounted for only by the broadening of features present at low Mn content. Displacement of atoms from their ideal lattice sites has a strong influence on the energy structure of a material.10 Such local distortions11 about different atoms, if not ordered in a periodic manner, are largely ignored by techniques like X-ray diffraction which probe long-range crystal structures, since Bragg scattering averages over many unit cells. On the other hand, a large number of experiments on solid solutions in recent times12−16 have emphasized the importance of the local structure in contrast to the global one probed by diffraction techniques. Techniques such as X-ray absorption fine structure spectroscopy17−19 (XAFS), neutron scattering,20−22 pair distribution functions23,24 (PDF), and molecular dynamics simulations25,26 (MD) have been found to be effective in studying such local structural information. Beneath a wellB

DOI: 10.1021/acs.inorgchem.8b00997 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

dashed line following the peak near 60°. This is a consequence of larger In3+ ions substituting for smaller Mn3+ ions, thereby increasing the average lattice parameters, a and c, which are plotted in Figure 2b; both a and c show a nearly linear increase with the composition, x, across the entire series in accordance with the well-known Vegard’s Law. It is interesting to note here that the lattice parameter expansion in these compounds with x is significantly more pronounced for the c cell parameter as compared to a or b cell parameters. The results obtained from the Rietveld fits are given in Table S1 and the fits are shown in Figure S2 of the Supporting Information. The collection of XANES spectra from standard samples with Mn in different valence states, namely, 0, 2, 3, and 4 oxidation states, represented by Mn metal, MnO, Mn2O3, and MnO2, respectively, is shown in Figure 3a. These plots make it evident that there is a substantial chemical shift for every unit change in valency, thereby providing an easy identification of the valence state by the energy position of the leading edge of the XANES plot. The XANES plot of YMnO3 with Mn3+, included in Figure 3a, matches well at the leading edge with that of Mn2O3. The closely overlapping XANES leading edge of all compounds in the series YMn1−xInxO3 is in agreement with the expectation that the valency of the Mn ions cannot change when Mn3+ in YMnO3 is replaced by In3+ ions. Mn-K edge XANES clearly shows a gradual evolution of the peak shape with increasing In3+ incorporation (Figure 3b) with a number of isosbestic points being evident. The changes in the Mn-K XANES, related to the Mn p-related partial density of states, reflect changes in the local electronic structure arising most prominently from changes in the first coordination shell, with decreasing contributions from coordination shells with increasing distance from the central Mn ion. The first coordination shell around Mn is composed of five oxygen atoms, necessarily in a noncentrosymmetric arrangement. In fact, the pre-edge feature appearing at ∼6541 eV (see inset to Figure 3b), common to all, is a characteristic of Mn ions in a noncentrosymmetric environment.33,34 The second coordination shell around Mn ions consists of Y ions for all compositions. The third coordination shell around Mn3+ ions consists of Mn ions only for YMnO3, and it is reasonable to expect that, in the extreme dilute limit of x = 0.95, the third coordination shell of a central Mn3+ site will almost always have only In3+ ions. For other x-values in between these two extremes, the third coordination shell around Mn3+ will on an average have a combination of Mn and In ions dictated by the composition. The presence of the isosbestic points in the XANES spectra suggests a specific description of the evolution of the XANES line-shapes for these intermediate compositions with changing x as the weighted average35,36 of XANES of the two end compositions, namely, x = 0.0 and x = 0.95. This approach is also supported by the earlier findings reported in the literature37−40 that the electronic structures of substituted transition metal oxides are often accurately described as a linear combination of the valence spectra of the two endmembers. We have validated this hypothesis by carrying out least-squared error fit of the XANES data for intermediate xvalues over a wide range of energy (−30 to 100 eV about E0) for the entire composition window, and the resulting fit is compared with the experimental data for a representative composition, x = 0.5, in Figure 3c. The results of this fitting procedure for all compositions are shown in Figure S3 of the Supporting Information. Here we note that the only adjustable parameter used in this fitting procedure is the relative weights

mode. Respective elemental foils were measured as the reference during the data collection at different elemental edges for each measurement. Mn is known to exist in different valence states. In order to ascertain the Mn valence state in each compound, we measured XAFS of standard oxides of Mn, namely MnO for Mn2+, Mn2O3 for Mn3+and MnO2 for Mn4+. Self-absorption corrections to the Mn-K fluorescence data were performed using the Booth algorithm.31 Two component linear combination fits to the Mn-K XANES (X-ray absorption near edge structure) data were performed over the range −30 to 100 eV about the edge position, using YMnO3 and YMn0.05In0.95O3 as the reference systems in Athena.32 We also performed linear combination fits to the χ(k) data over the range 2.0−14.0 Å−1. Similar linear combination fits to the χ[μ(E)] and χ(k) data were also performed for the In-K edge in each sample. We have analyzed EXAFS data for both Mn-K and In-K edges for the first (O atoms), second (Y atoms), and third (Mn and In) near neighbor coordination environments using Artemis.32 We first estimated the amplitude reduction factor (S02) values for Mn from YMnO3 (maximum Mn concentration) and In from YMn0.05In0.95O3 (maximum In concentration) environments. The S02 values were kept identical for all compositions during simultaneous fitting of both edges. From the knowledge of global symmetry, we used hexagonal YMnO3 and YInO3 systems with Mn and In absorbers for the model FEFF calculations. Details of the EXAFS fitting procedure are given in the Supporting Information. We also wanted to have a theoretical estimate of local distortions occurring within an O5 TBP cage when a Mn3+ ion replaces a larger In3+ ion in the limit of dilute Mn doping. So, we have performed first-principles density functional calculations on ∼8% Mn doped YInO3 (1/12 doping in a hexagonal system) to have a composition as close to the real system (YMn0.05In0.95O3) as possible.

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RESULTS AND DISCUSSION X-ray diffraction data for all compounds in the YMn1−xInxO3 solid solutions investigated here are shown in Figure 2a. The results from Rietveld refinement reveal that these systems exhibit hexagonal symmetry (P63cm). No impurity phase or any feature related to the orthorhombic phase could be observed. The systematic shift of the reflection peaks to smaller angles with increasing In content, x, is clearly visible, particularly for the high-angle peaks, as illustrated with the

Figure 2. (a) X-ray diffraction patterns and (b) evolution of unit cell parameters (a/b = red triangles, c = blue circles) for the YMn1‑xInxO3 solid solutions showing that the systems stabilize in hexagonal symmetry. Unit cell volume expansion due to In3+ incorporation at the Mn3+ sites is predominantly reflected in the increment of the c cell parameter compared to a/b cell parameters. C

DOI: 10.1021/acs.inorgchem.8b00997 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. (a) Mn ions in YMnO3 assume +3 valence state. (b) Overlay XANES plots of the Mn-K edge in YMn1−xInxO3 solid solutions showing Mn to be in +3 valence state for all x-values. Distinct isosbestic points are observed for the series, suggesting a mixture of two environments.34 Inset reveals the characteristic noncentrosymmetric (TBP in this case) pre-edge feature, arising due to electronic transition to the unoccupied d states. (c) LCF (linear combination fit, in red) to Mn-K edge XANES (in black ○) and the corresponding difference spectra (in blue) for x = 0.5 sample.

Table 1. Relative Contributions of the Two TBP Geometries Obtained from Linear Combination Fits to the Normalized χ[μ(E)] and χ(k) Dataa nominal In (x) XANES Norm(χ[μ(E)]) EXAFS [χ(k)]

x = 0.00

x = 0.10

x = 0.25

x = 0.50

x = 0.60

x = 0.70

x = 0.75

x = 0.95

ref ref

0.091 ± 0.007 0.10 ± 0.02

0.239 ± 0.01 0.25 ± 0.03

0.476 ± 0.011 0.51 ± 0.03

0.604 ± 0.011 0.63 ± 0.02

0.697 ± 0.010 0.72 ± 0.02

0.775 ± 0.009 0.78 ± 0.02

ref ref

a

The numbers agree well with the nominal composition.

Table 2. Quantification of How O5 TBP Cages Distort When Mn3+ Ions Replace In3+ Ions in the Dilute Limita MnO5 TBP in YMnO3 [EXAFS] MnO15

bond dist (Å)

1.848 axial Mn−O1 axial Mn−O2 1.882 equatorial Mn−O3 1.966 equatorial Mn−O4 2.118 equatorial Mn−O5 2.118 InO5 TBP in YMn0.9In0.1O3 [EXAFS]

MnO5 TBP in YMn0.05In0.95O3 [EXAFS] MnO25

MnO5 TBP in YMnO3 [XRD]

bond dist (Å)

axial Mn−O1 1.879 axial Mn−O2 2.087 equatorial Mn−O3 1.976 equatorial Mn−O4 1.976 equatorial Mn−O5 2.087 InO5 TBP in YMn0.05In0.95O3 [EXAFS]

InO25

bond dist (Å)

InO15

bond dist (Å)

axial In−O1 axial In−O2 equatorial In−O3 equatorial In−O4 equatorial In−O5

2.078 2.083 2.087 2.116 2.116

axial In−O1 axial In−O2 equatorial In−O3 equatorial In−O4 equatorial In−O5

2.086 2.090 2.098 2.121 2.121

bond dist (Å) axial Mn−O1 1.855 axial Mn−O2 1.873 equatorial Mn−O3 1.973 equatorial Mn−O4 2.106 equatorial Mn−O5 2.106 InO5 TBP in YInO3 [XRD] bond dist (Å) axial In−O1 axial In−O2 equatorial In−O3 equatorial In−O4 equatorial In−O5

2.089 2.093 2.097 2.127 2.127

a

Details of structural parameters of all the four different TBP arrangements obtained from EXAFS analyses are given for proper comparison.

possibly indicative of less pronounced differences in the first coordination shell about In compared to that for the Mn in the two distinct local environments. XANES data at the In-K edges of the intermediate compositions are also describable as linear combinations of spectral features of the two extreme compositions; the fits to the data and the difference spectra are given in the Supporting Information (Figure S5). Turning to the analysis of the EXAFS region, we first note that the analysis of the YMnO3 (x = 0.0) sample yields essentially the same results as known from XRD, establishing five oxygens surrounding the central Mn absorber with bond distances, as shown in Table 2. Likewise, the EXAFS analysis of the signal for the most dilute Mn sample, namely, x = 0.95, suggests again a trigonal bipyramidal coordination around Mn3+, but the bond distances turn out to be distinctly different from those of the MnO5 TBP in YMnO3. The five Mn−O

of the spectra of the two end-members. We view such a restrictive fit providing such good descriptions of experimental results for all intermediate values of x between 0.0 and 0.95 as a strong validation of this approach. In addition, we note that the fitted values for the relative spectral weights correspond to an independent estimate of the composition of each intermediate compound. These estimated compositions, obtained from the fits, show good agreement with the nominal compositions, as shown in Table 1. Thus, we have two different MnO5 geometries occurring in these systems, one in a Mn-rich environment, resembling that of pure YMnO3, and other one in an In-rich environment similar to that of YMn0.05In0.95O3. The In-K edge XANES data shown in Figure S4 of the Supporting Information present a case similar to that of the Mn-K XANES data (Figure 3b), though with less prominent changes across the composition range. This is D

DOI: 10.1021/acs.inorgchem.8b00997 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry distances obtained from EXAFS analysis are given in Table 2; we have also included the five In−O distances obtained from XRD analysis (PDF 70-133) for YInO3 in Table 2. In order to understand the differences in Mn−O bond distances between YMnO3 and YMn0.05In0.95O3, we first note that the primary difference between MnO5 and InO5 TBPs in the respective pure compounds is a marked elongation of the two apical M− O bonds, namely, changing from 1.86 Å for Mn−O to ∼2.09 Å for In−O. In contrast, two of the equatorial M−O distances vary considerably less with an average difference of ∼0.03 Å, and the shorter equatorial M−O distance varies by ∼0.1 Å, as against an average difference of ∼0.2 Å for the apical bonds. This explains the much larger change in the c-parameter compared to a, b parameters across the series, shown in Figure 2b. YMn0.05In0.95O3 represents the dilute doping limit of the smaller Mn3+ ion at the larger In3+ ionic site of YInO3 with long In−O apical bonds. Thus, it becomes evident that the Mn-dopant will try to collapse the local Mn−O bond distances to those of YMnO3 to maximize stability from bonding, but the lattice strain caused from such a local collapse would tend to increase the energy. These two opposing trends lead to the optimal Mn−O apical distances in this limit that are in between those of YMnO3 and YInO3, as can be seen in Table 2. The two distinct InO5 local polyhedral environments obtained from independent analysis of x = 0.1 and x = 0.95 are listed in Table 2. It is clear from the comparison in this table that the doping of the larger In3+ ion at the smaller Mn3+ site, as in YMn0.9In0.1O3, forces the local In−O5 bond lengths to be nearly the same as in the opposite limit of YMn0.05In0.95O3. Thus, the bond length differences in the InO5 polyhedra for the two end-members considered here are much less pronounced compared to those observed for MnO5 bond lengths within the MnO5 polyhedra. This also explains why we find significant changes in the Mn-K edge XANES across the series (Figure 3b) while such changes are much less obvious for the In-K edge XANES shown in Figure S3. Our attempt to fit the EXAFS data of compounds with intermediate compositions (0.0 < x < 0.95) with a single MnO5 polyhedron failed to provide physically reliable fits. Prompted by the additive nature of the XANES region already discussed, we explored the possibility of describing experimental results of these intermediate compositions as a linear combination of the data of the end-members, namely, x = 0.0 and 0.95, with the relative weights being the only adjustable parameter. This approach is found to provide a very accurate description of the EXAFS oscillations for all intermediate compounds, as shown in Figure 4a. The relative weights provide an independent estimate of the composition, x, for each compound, and these are listed in Table 1, showing a remarkable agreement with the nominal values (Figure 4b) as well as those obtained from XANES establishing the validity of this approach. So, as already stated, we analyzed the EXAFS data for x = 0.0 and x = 0.95 compositions which are presented in Figure S6 of the Supporting Information. Using these as end-members, the evolution of the EXAFS data for all intermediate compounds, shown in Figure S7 of the Supporting Information, can be described by the simultaneous presence of two distinct MnO5 local polyhedra with only their relative abundances changing with the composition, x (Table 1). We analyzed the EXAFS data obtained at the In-K edge in the same way and found once again that these EXAFS oscillations can also be described as a linear combination of the extreme compositions (x = 0.1 and 0.95) with the relative

Figure 4. (a) Experimental k2-weighted χ(k) data for Mn-K edge (symbols) and the corresponding fits (dark red lines) for intermediate compositions of the YMn1−xInxO3 solid solutions (x = 0.10, 0.25, 0.50, 0.60, 0.70, and 0.75) obtained from a linear sum of k2-weighted χ(k) data for pure YMnO3 and YMn0.05In0.95O3. For comparison, x = 0.00 (YMnO3) and x = 0.95 (YMn0.05In0.95O3) are added as references used for linear combination fits. (b) Relative fraction of the two types of MnO5 TBPs, MnO15, x = 0.0 (blue ▲), and MnO25, x = 0.95 (blue △), and two types of InO5 TBPs, InO15, x = 0.95 (red ○), and InO25, x = 0.1 (red ●), occurring in YMn1−xInxO3 solid solutions.

weights in conformity with the nominal compositions. The relative abundances of each type of polyhedra, namely, two of the MnO5 and two of the InO5 polyhedra, obtained from the above-mentioned description of EXAFS oscillations as a linear combination of those of the end-members are plotted as a function of the composition, x (Table 1), in Figure 4b. As expected from the data presented in Table 1, a linear variation is observed for each polyhedron with the composition. A schematic representation of the distribution of oxygen atoms in the two types of MnO5 TBP is shown in Figure S8 of the Supporting Information. The above discussion on changes in XANES and EXAFS data across the solid solution provides a natural explanation of the spectacular changes in optical properties observed in these materials.1 It is known that YInO3 is a wide band gap, colorless insulator; in absence of any transition metal ion, it does not have any color-imparting d−d transition. On the other hand, YMnO3 absorbs all visible colors, thereby appearing black. On doping YInO3 with small amount of Mn, a blue coloration is observed that intensifies with progressively larger amount of Mn doping. Published UV−vis absorption spectra show1 that the dilute doping of Mn in YInO3 introduces two absorption features within the wide band gap of YInO3. The intense absorption band, appearing in the red-green range of the visible spectrum (∼2−2.5 eV), has been assigned to Laporte selection rule allowed e′ → a′ transition within the Mn-d levels. This absorption band is separated from a weaker absorption band appearing with a peak near 3.5 eV with a pronounced dip in the absorption in the blue region, imparting the characteristic blue color to the compound.1 Both of these absorption features are due to the presence of Mn in the composition, since these do not exist in pure YInO3 (see Figure 4 of ref 1). The evolution of the low-energy absorption band with an increasing Mn content is primarily to make the band progressively broader, most probably due to band structure effects with more Mn−Mn hopping interactions being established. In contrast, the high-energy absorption feature shows a more rapid change with increasing Mn content, shifting systematically to lower energy and increasing in the oscillator strength. This down-shift of the absorption feature, starting from the UV E

DOI: 10.1021/acs.inorgchem.8b00997 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

and 1.93 Å, and an equatorial Mn−O bond length of 2.04 Å; these numbers are close to the MnO bond distances calculated for YMnO3, namely, 1.85, 1.88, and 2.03 Å, respectively, thereby supporting the existing interpretation based on a minor evolution of the local MnO5 structure. We also performed similar calculations for even higher (50%) Mn content and found very similar results, namely, two apical bond distances of 1.92 Å and three equatorial Mn−O distances of 1.98, 2.10, and 2.10 Å. This apparent contradiction between our experimental findings of a MnO5 polyhedron being distinctly different from that in YMnO3 and results of the above-mentioned first-principles calculations prompted us to explore a lower regime of Mn content with the use of the same first-principles calculations. This line of thinking is further supported by two other observations. First, our experimental results show that the strongly distorted MnO5 polyhedra dominate only in the low-concentration limit and the more usual MnO5 TBP species, similar to those in YMnO3, rapidly grow in number with an increasing Mn content. The second observation is that the brilliant blue coloration exists only up to 10% Mn content and the color changes qualitatively rather abruptly for higher concentrations, as evident from Figure 3 of the original publication.1 Therefore, we carried out a firstprinciples calculation of a composition with ∼8% Mn content; interestingly, at this concentration level we find geometryoptimized results for MnO5 TBP from our calculations are drastically different from those obtained from calculations with higher Mn contents. In this low Mn limit, the MnO 5 polyhedron is characterized by two very different apical Mn− O distances of 1.88 and 2.13 Å and three equatorial distances of 1.96, 1.96, and 1.99 Å, in very good agreement with our results obtained from EXAFS, thereby establishing unequivocally the strongly distorted, local MnO5 polyhedra, dominating the compositions with low Mn content and being unrelated to the MnO5 TBP of YMnO3, as the real origin of the brilliant blue color.

region, wipes out the dip in the absorption spectrum in the blue region for samples with low Mn content, thereby making the sample absorb over the entire visible range at higher Mn content and making the sample appear black. These changes are evident in Figure 4 of ref 1 and can be roughly visualized as linear evolution of spectral features between the two endmembers, namely, YMn0.05In0.95O3 with the pronounced dip in absorbance in the blue region and YMnO3 absorbing over the entire visible range. Thus, these results are similar to and, therefore, can be understood in terms of the XANES and EXAFS results presented here. At the low Mn limit (large x), YMn1−xInxO3 is primarily characterized by the strongly distorted MnO5 polyhedra (Figure S7) which are essentially isolated from each other and thereby result in a relatively narrow band in the red-yellow region of the visible spectrum arising from e′ → a′ transitions. These MnO5 TBPs in this dilute Mn limit are strongly distorted with a long Mn−O axial distance and are characterized by an absorption feature in the UV region (∼3.5 eV), separated from the low-energy feature by a pronounced low-absorbance region. With an increasing Mn-content, more such local MnO5 clusters form, thereby intensifying these absorption bands from the distorted MnO5 TBP and, as a consequence, intensifying the blue color. At the same time, absorption features characteristic of the less distorted MnO5 TBP, akin to those in YMnO3, also begin to grow in number, providing states within the absorbance gap around the blue. However, the pronounced gap-like structure is not significantly removed until these less distorted MnO5 TBPs begin to dominate for x ≤ 0.5, making the sample appear black. We stress here that EXAFS and XANES results presented here unambiguously establish the presence of two very different types of MnO5 coordinations in YMn1−xInxO3 solid solutions with their relative abundances controlled by the global stoichiometry. This is in sharp contrast with all other reports1,21 on this family of compounds where the discussion has been invariably based on a single MnO5 trigonal bipyramidal coordination, with the local geometry evolving with the composition. This arises from the implicit assumption that the global structural parameters, such as the average lattice parameters determined by any diffraction technique, also uniquely determine the local structure at the atomic lengthscale. This has already been proven12 to be wrong in describing the evolution of the local structure with the composition in a solid solution. This differing interpretation between the existing literature and our work presented here has an important bearing on understanding the remarkable optical properties of these samples, since we attribute the strongly distorted MnO5 polyhedra, present in relative abundance in the low Mn limit of compositions, as being responsible for the color. In the existing interpretation, it is considered that the usual MnO5 TBP in YMnO3 distorts to some extent and the dilution of Mn content leads to the narrowing of the optical features, forming a gap in the absorption feature and, thereby, leading to the blue coloration. Thus, it becomes important to understand the real origin of the intense coloration in terms of the correct understanding of the microscopic local structure, if possible, by employing some independent means. Such an understanding of the microscopic local structure was indeed attempted in the past by carrying out detailed first-principles theoretical calculations.1 The calculation performed for a composition with a 25% Mn content showed a MnO5 polyhedron with two Mn−O apical bond distances of 1.91

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CONCLUSIONS We have investigated the local structure−property correlation in recently discovered inorganic pigments YMn1−xInxO3 to understand the origin of their intense blue color. From lowtemperature XANES and EXAFS data, we find that the Mn local environments throughout the solid solution range consist of two distinct MnO5 trigonal bipyramidal cages. The first MnO5 TBP contains a long Mn−O axial bond that is created when smaller Mn3+ ions replace larger In3+ ions in an O5 TBP cage, similar to the axial In−O distances in InO5 TBP of YInO3. The second MnO5 TBP consists of two rather similar Mn−O axial bonds, as found in pure YMnO3. Our results show that the relative abundances of these two types of MnO5 TBPs vary essentially linearly with the composition, with the elongated and distorted MnO5 TBP being dominant in the dilute Mn doping limit. This highly distorted MnO5 TBP can be related to the prominent absorption feature1 in the redyellow region of the absorption spectrum with a pronounced dip in the blue region, consistent with the color of these samples. The systematic shift of the UV absorption feature (at ∼3.5 eV for x = 0.95, still below the band gap of YInO3) toward lower energy with increasing Mn content, that can be seen in Figure 4 of ref 1, can be attributed to the contribution of absorption features of the second MnO5 TBP that grows in abundance with increasing Mn content, filling in the prominent dip in absorbance in the blue region, consistent with the F

DOI: 10.1021/acs.inorgchem.8b00997 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

(5) Yakel, H. L. On the structures of some compounds of the perovskite type. Acta Crystallogr. 1955, 8, 394−398. (6) Salvador, P. A.; Doan, T.-D.; Mercey, B.; Raveau, B. Stabilization of YMnO3 in a Perovskite Structure as a Thin Film. Chem. Mater. 1998, 10, 2592−2595. (7) Cho, D. Y.; Kim, J. Y.; Park, B. G.; Rho, K. J.; Park, J. H.; Noh, H. J.; Kim, B. J.; Oh, S. J.; Park, H. M.; Ahn, J. S.; Ishibashi, H.; Cheong, S. W.; Lee, J. H.; Murugavel, P.; Noh, T. W.; Tanaka, A.; Jo, T. Ferroelectricity Driven by Y d0-ness with Rehybridization in YMnO3. Phys. Rev. Lett. 2007, 98, 217601. (8) Alonso, J. A.; Martínez-Lope, M. J.; Casais, M. T.; FernándezDíaz, M. T. Evolution of the Jahn−Teller Distortion of MnO6 Octahedra in RMnO3 Perovskites (R = Pr, Nd, Dy, Tb, Ho, Er, Y): A Neutron Diffraction Study. Inorg. Chem. 2000, 39, 917−923. (9) van Aken, B. B.; Meetsma, A.; Palstra, T. T. M. Hexagonal YMnO3. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2001, C57, 230−232. (10) Bertini, L.; De Sole, A.; Gabrielli, D.; Jona-Lasinio, G.; Landim, C. Macroscopic fluctuation theory. Rev. Mod. Phys. 2015, 87, 593− 636. (11) Cohn, J. L.; Neumeier, J. J.; Popoviciu, C. P.; McClellan, K. J.; Leventouri, T. Local lattice distortions and thermal transport in perovskite manganites. Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 56, R8495−R8498. (12) Mukherjee, S.; Nag, A.; Kocevski, V.; Santra, P. K.; Balasubramanian, M.; Chattopadhyay, S.; Shibata, T.; Schaefers, F.; Rusz, J.; Gerard, C.; Eriksson, O.; Segre, C. U.; Sarma, D. D. Microscopic description of the evolution of the local structure and an evaluation of the chemical pressure concept in a solid solution. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 224105. (13) Balzarotti, A.; Czyzyk, M.; Kisiel, A.; Motta, N.; Podgòrny, M.; Zimnal-Starnawska, M. Local structure of ternary semiconducting random solid solutions: Extended x-ray-absorption fine structure of Cd1‑xMnxTe. Phys. Rev. B: Condens. Matter Mater. Phys. 1984, 30, 2295−2298. (14) Frenkel, A.; Stern, E. A.; Voronel, A.; Qian, M.; Newville, M. Buckled crystalline structure of mixed ionic salts. Phys. Rev. Lett. 1993, 71, 3485−3488. (15) Mikkelsen, J. C.; Boyce, J. B. Atomic-Scale Structure of Random Solid Solutions: Extended X-Ray-Absorption Fine-Structure Study of Ga1‑xInxAs. Phys. Rev. Lett. 1982, 49, 1412−1415. (16) Pong, W. F.; Mayanovic, R. A.; Bunker, B. A.; Furdyna, J. K.; Debska, U. Extended x-ray-absorption fine-structure studies of Zn1‑xMnxSe alloy structure. Phys. Rev. B: Condens. Matter Mater. Phys. 1990, 41, 8440−8448. (17) Wernet, P.; Nordlund, D.; Bergmann, U.; Cavalleri, M.; Odelius, M.; Ogasawara, H.; Näslund, L. Å.; Hirsch, T. K.; Ojamäe, L.; Glatzel, P.; Pettersson, L. G. M.; Nilsson, A. The Structure of the First Coordination Shell in Liquid Water. Science 2004, 304, 995− 999. (18) Khan, A. H.; Dalui, A.; Mukherjee, S.; Segre, C. U.; Sarma, D. D.; Acharya, S. Efficient Solid-State Light-Emitting CuCdS Nanocrystals Synthesized in Air. Angew. Chem., Int. Ed. 2015, 54, 2643− 2648. (19) Pradhan, J.; Mukherjee, S.; Khan, A. H.; Dalui, A.; Satpati, B.; Segre, C. U.; Sarma, D. D.; Acharya, S. Two-Dimensional Hybrid Organohalide Perovskites from Ultrathin PbS Nanocrystals as Template. J. Phys. Chem. C 2017, 121, 6401−6408. (20) Finney, J. L.; Hallbrucker, A.; Kohl, I.; Soper, A. K.; Bowron, D. T. Structures of High and Low Density Amorphous Ice by Neutron Diffraction. Phys. Rev. Lett. 2002, 88, 225503. (21) Li, J.; Sleight, A. W.; Subramanian, M. A. Determination of the Local Environment of Mn3+ and In3+ in the YInO3−YMnO3 Solid Solution, Which Exhibits an Intense Blue Color. Chem. Mater. 2016, 28, 6050−6053. (22) Louca, D.; Egami, T.; Brosha, E. L.; Röder, H.; Bishop, A. R. Local Jahn-Teller distortion in La1‑xSrxMnO3 observed by pulsed neutron diffraction. Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 56, R8475−R8478.

YMnO3 absorption spectrum and rendering the sample black. The chromophoric properties of YMn1−xInxO3 can thus be approximately described by the ratio of these two MnO5 TBP environments, since this ratio closely follows the global composition of the YMn1−xInxO3 samples, one rendering it blue and the other black.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00997.

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Tables of crystal structure refinement data and corresponding fits, XAFS data reduction, linear combination fits to the XANES and EXAFS data, distortions within a Mn−O5 TBP, and first-principles density functional calculations (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Carlo U. Segre: 0000-0001-7664-1574 D. D. Sarma: 0000-0001-6433-1069 Present Addresses

§ CTS Corp., 479 Quadrangle Drive Suite E, Bolingbrook, IL 60440, United States. ∥ Department of Applied Physics, Hongkong Polytechnic University, Hong Kong. ⊥ Metal Power Analytical (India) Pvt. Ltd. C-46 and 47, First Floor, Raj Industrial Complex, Military Road, Marol, Andheri (East), Mumbai-400 059, India. # Also at Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bengaluru-560064, India.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank CSIR and DST of Government of India, NSF, and DFG for funding this investigation. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357. Support for author C.U.S. was provided in part by the National Science Foundation under Grant DMR-086935. D.D.S. thanks Jamsetji Tata Trust for support.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b00997 Inorg. Chem. XXXX, XXX, XXX−XXX