Chromophore in Ternary Barium Oxides as a Strategy toward

Mar 18, 2016 - ... exhibit a turquoise/green color. The investigation indicates that the tetrahedral Mn5+O4 is stabilized in hosts containing barium a...
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Stabilization of a Tetrahedral (Mn5+O4) Chromophore in Ternary Barium Oxides as a Strategy toward Development of New Turquoise/ Green-Colored Pigments Sourav Laha, Subramani Tamilarasan, Srinivasan Natarajan,* and Jagannatha Gopalakrishnan* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: An experimental investigation of the stabilization of the turquoise-colored chromophore Mn5+O4 in various oxide hosts, viz., A3(VO4)2 (A = Ba, Sr, Ca), YVO4, and Ba2MO4 (M = Ti, Si), has been carried out. The results reveal that substitution of Mn5+O4 occurs in Ba3(VO4)2 forming the entire solid solution series Ba3(V1−xMnxO4)2 (0 < x ≤ 1.0), while with the corresponding strontium derivative, only up to about 10% of Mn5+O4 substitution is possible. Ca3(VO4)2 and YVO4 do not stabilize Mn5+O4 at all. With Ba2MO4 (M = Ti, Si), we could prepare only partially substituted materials, Ba2M1−xMn5+xO4+x/2 for x up to 0.15, that are turquoise-colored. We rationalize the results that a large stabilization of the O 2p-valence band states occurs in the presence of the electropositive barium that renders the Mn5+ oxidation state accessible in oxoanion compounds containing PO43−, VO43−, etc. By way of proof-of-concept, we synthesized new turquoise-colored Mn5+O4 materials, Ba5(BO3)(MnO4)2Cl and Ba5(BO3)(PO4)(MnO4)Cl, based on the apatiteBa5(PO4)3Clstructure.

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Sr10(VO4)6F2.15,16 We have shown that in the isostructural Ba3(P1−xMnxO4)2−palmierite solid solution series, the green color of the end member Ba3(MnO4)2 could be tuned to turquoise/blue colors by varying the P/Mn ratio.17 For x ≤ 0.25, the samples are turquoise/blue, and for x ≥ 0.50, samples are green. Jiang et al. described intense turquoise- and greencolored brownmillerite oxides of the composition Ba2In2−xMnxO5+x, where again tetrahedral Mn5+O4 is the chromophore responsible for the characteristic color.18 More recently, Medina et al explored the intense turquoise color of apatite-type Ba5Mn3−xMxO12Cl (x = 0−3.0) (M = V, P) compounds, which display high near-infrared (NIR) reflectance and have potential to be environmentally benign inorganic “cool pigments”.19 The foregoing instances of turquoise-colored materials suggest that (i) tetrahedral Mn5+O4 is a new chromophore that imparts blue/turquoise/green colors to oxide hosts containing tetrahedral oxoanions such as PO43− and VO43− and (ii) stabilization of this chromophore requires the presence of electropositive alkaline earth metal atoms especially barium. Accordingly, we explored the stabilization of Mn5+O4 in other alkaline earth metal hosts containing tetrahedral VO43− and TiO44−/SiO44− units, viz., A3(VO4)2 (A = Ba, Ca, Sr) and Ba2MO4 (M = Ti, Si). Our results, which are reported herein, show that the tetrahedral Mn5+O4 chromophore is indeed stabilized in all the cases where barium is present and to a limited extent in the presence of strontium.

INTRODUCTION Turquoise is a naturally occurring mineral of chemical composition CuAl6(PO4)4(OH)8·4H2O that displays a unique color between blue and green, corresponding to a narrow wavelength window of 490−500 nm.1−3 The term turquoise denotes not only the mineral but also this special color itself. Turquoise mineral is valued as a gemstone from ancient times, finding use for ornamental purposes in jewelry and art and to a lesser extent as a pigment material also.4 Although several members of the turquoise group minerals of the general formula A0−1B6(PO4)4−x(PO3OH)x(OH)8·4H2O (A = Cu, Zn, Fe2+, Ca and B = Al, Fe3+) (0 ≤ x ≤ 2) are known,5 none of them are turquoise in color. To our knowledge, the only other naturally occurring material displaying turquoise color is moolooite, having the composition CuC2O4·xH2O (0 ≤ x ≤ 1).6 Considering the rarity and demand for inorganic materials displaying turquoise color for their probable application as “cool pigments”, there is a current interest in chemically synthesizing new turquoise-colored solids. Thus, cobalt- and nickel-doped willemite (Zn2SiO4), vanadium-doped zircon, Y/ Tm-substituted CuAl 2 O 4 , and nickel-doped hibonite (CaAl12O19) are some of the turquoise/blue-colored synthetic pigments reported in recent times.7−11 Li1.33Ti1.66O4 spinel is a novel turquoise pigment where the color arises from the intervalance Ti3+ → Ti4+ charge transfer.12 Compounds containing tetrahedral Mn5+O4 are known to exhibit a turquoise blue to dark green color, for example, Ba3(MnO4)2 (bright green),13 Ba5(MnO4)3Cl (green),14 Mn5+substituted Ca2(MO4)Cl (M = P, V, As) (green-blue), and © XXXX American Chemical Society

Received: December 26, 2015

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DOI: 10.1021/acs.inorgchem.5b02957 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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EXPERIMENTAL SECTION

Synthesis. Oxides corresponding to nominal compositions A 3 (V 1 − x Mn x O 4 ) 2 (A = Ba, Ca, and Sr), YV 1 − x Mn x O 4 , Ba2M1−xMnxO4+x/2 (M = Ti and Si), Ba5(BO3)(MnO4)2Cl, and Ba5(BO3)(PO4)(MnO4)Cl samples were prepared by a conventional ceramic method. Stoichiometric mixtures of BaCO3, SrCO3, CaCO3, Y2O3 (predried at 900 °C overnight), V2O5, TiO2, SiO2, H3BO3, (NH4)2HPO4, NH4Cl, and MnC2O4·2H2O were progressively heated from 680 to 1050 °C with several intermittent grindings. Products were checked by powder X-ray diffraction (PXRD). Single-phase products were obtained for x = 0, 0.10, 0.20, 0.25, 0.50, and 0.75 in the case of Ba3(V1−xMnxO4)2, x = 0.10 in the case of Sr3(V1−xMnxO4)2, and x = 0.10 and 0.15 in the case of Ba2M1−xMnxO4+x/2 (M = Ti and Si). Characterization. PXRD patterns were recorded employing a Philips X’pert diffractometer (Ni-filtered Cu Kα radiation, λ = 1.5418 Å). PXRD data for Rietveld refinement of the structures were collected at room temperature by means of the same diffractometer in the 2θ range 5−120° with a step size of 0.02° and step duration of 50 s. The PXRD patterns were refined with the program GSAS-EXPGUI.20,21 Lattice parameters, scale factors, background (Fourier polynomial background function), pseudo-Voigt (U, V, W, and X), and isothermal temperature factors (Uiso) were refined. Thermal parameters were constrained to be the same for different atoms occupying the same sites. PXRD patterns were simulated with the help of the program POWDERCELL.22 Iodometric titration on representative cases was carried out to determine the oxidation state of manganese. The diffuse reflectance spectra for all the powdered samples were recorded on a Perkin-Elmer Lambda 750 UV−vis double-beam spectrometer over the spectral region of 250−1250 nm. The diffuse reflectance data were converted to the Kubelka−Munk function employing the equation23

Figure 1. Color of different Ba3(V1−xMnxO4)2 powder samples.

F(R ) = (1 − R )2 /2R = α /S where R is the reflectance and α and S denote absorptivity and scattering factor, respectively. A Bruker X-band EMX spectrometer was employed to record the EPR spectra in the interval of 2000−5000 Gauss. For characterization of pigment quality of the samples, room-temperature solid-state emission spectra were recorded in a Perkin-Elmer, U.K., model no. LS55, and CIE-1931 chromaticity coordinates were calculated from the emission spectra employing the gocie.exe program from http:// www.geocities.com/krjustin/gocie.html. Scanning electron microscope (SEM) images and energy-dispersive X-ray spectra (EDX) data were recorded on an Ultra 55 field emission scanning electron microscope (Carl Zeiss).

Figure 2. PXRD patterns of different Ba3(V1−xMnxO4)2 samples.

spectra (Figure S2) show that A3(V1−xMnxO4)2 (0 < x ≤ 1.0 for A = Ba and 0 < x ≤ 0.10 for A = Sr) are single-phase solid solutions. PXRD patterns of Ba3(V1−xMnxO4)2 (0 ≤ x ≤ 1.0) indicate that all the members of the solid solution are isostructural, adopting the hexagonal (R3̅m) palmierite structure of the parents Ba3(MnO4)2 and Ba3(VO4)2.13,24,25 The structure consists of discrete VO43− tetrahedra (slightly distorted to C3v symmetry) along with two crystallographically distinct Ba2+ atoms that are six and ten coordinated by oxygens. The variation of the cell parameters of Ba3(V1−xMnxO4)2 members with x is consistent with the lattice parameters of the parent oxides, Ba3(VO4)2 and Ba3(MnO4)2, also reflecting the small difference in the ionic radii of V5+ (0.355 Å) and Mn5+ (0.33 Å).26 The variations of unit cell parameters (a, c, cell volume) as a function of x (manganese content) are shown in Figure 3. We refined the crystal structure of a representative member, Ba3(V0.75Mn0.25O4)2, on the basis of end members’ crystal structure. We present the refinement results in Figure 4 and Table 1, and the crystal structure is drawn in Figure 5. Similarly, Sr3(V0.90Mn0.10O4)2 is isostructural with Sr3(VO4)2,25,27 as shown by the refinement of PXRD data (see Figure S3 and Table S1). To determine the oxidation state of manganese in the solid solutions, room-temperature electron paramagnetic resonance (EPR) spectra of selected compounds were recorded. EPR data of Ba3(V0.75Mn0.25O4)2 (g = 1.97) are in close agreement with the data reported for tetrahedral Mn5+ in different oxides,14,18,28,29 revealing the presence of Mn5+O4 in

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RESULTS AND DISCUSSION A3(V1−xMnxO4)2 (A = Ba, Ca, and Sr). In an effort to explore the stabilization and color of the tetrahedral Mn5+O4 chromophore in oxide hosts, we first investigated the formation of A3(V1−xMnxO4)2 (A = Ba, Ca, and Sr) and YV1−xMnxO4 solid solutions. We could readily prepare the single-phase solid solutions A3(V1−xMnxO4)2 (0 < x ≤ 1.0) for A = Ba and (0 < x ≤ 0.10) for A = Sr by a conventional ceramic route. The Ba3(V1−xMnxO4)2 members display turquoise (x = 0.10, 0.20, and 0.25) to green and dark green (x ≥ 0.50) color (Figure 1). The color and its variation are similar to Ba3(P1−xMnxO4)2.17 Sr3(V0.90Mn0.10O4)2 also exhibits a turquoise color (Supporting Information, Figure S1). Attempts to synthesize Sr3(V1−xMnxO4)2 (x > 0.10), Ca3(V1−xMnxO4)2 (x > 0), and YV1−xMnxO4 (x > 0) resulted in mixtures of SrMnO3 + Sr3(VO4)2, CaMnO3 + Ca3(VO4)2, and YMnO3 + YVO4, respectively. The PXRD patterns of the single-phase Ba3(V1−xMnxO4)2 solid solutions are presented in Figure 2. For comparison, the PXRD patterns of the parent oxides are also shown. PXRD together with the SEM images and EDX B

DOI: 10.1021/acs.inorgchem.5b02957 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Unit cell parameters of Ba3(V1−xMnxO4)2 solid solutions: (bottom) a and c and (top) cell volume. Figure 5. Crystal structure of Ba3(V0.75Mn0.25O4)2 showing isolated V/ MnO4 tetrahedra.

Figure 4. Rietveld refinement of Ba3(V0.75Mn0.25O4)2. Observed (o), calculated (red line), and difference (bottom blue line) profiles are shown. The vertical bars indicate Bragg reflections (|).

the solid solutions (see Figure S4). Iodometric titrations also show that manganese is present in the 5+ state in Ba3(V1−xMnxO4)2 samples. The optical (UV−vis) absorption spectra of Ba3(V1−xMnxO4)2 (Figure 6), which compare well with those

Figure 6. UV−visible spectra of different Ba3(V1−xMnxO4)2 powder samples.

Table 1. Crystallographic Data for Ba3(V0.75Mn0.25O4)2a atom

site

x

y

z

Uiso (Å2)

occupancy

Ba1 Ba2 V/Mn O1 O2

3a 6c 6c 6c 18h

0.0 0.0 0.0 0.0 0.493(1)

0.0 0.0 0.0 0.0 0.507(1)

0.0 0.795(1) 0.592(1) 0.671(1) 0.232(1)

0.028(1) 0.021(9) 0.030(1) 0.085(8) 0.033(3)

1.0 1.0 0.75/0.25 1 1

Space group R3m ̅ : a = 5.775(1) Å, c = 21.350(1) Å. Reliability factors: Rp = 6.21%, Rwp = 9.02%, RF2 = 13.21%, χ2 = 6.33. Bond lengths (Å): Ba(1)− O(2) = 2.780(1) (×6); Ba(2)−O(1) = 2.639(1); Ba(2)−O(2) = 2.943(9) (×6); Ba(2)−O(2) = 2.817(9) (×3); V/Mn−O(1) = 1.686(9); V/Mn− O(2) = 1.695(8) (×3). a

C

DOI: 10.1021/acs.inorgchem.5b02957 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry of Ba3(P1−xMnxO4)2, reveal the origin of color and its change across the series from turquoise to green to dark green (Figure 1 and Figure S5). As Ba3(VO4)2 (x = 0) has a large band gap of 3.8 eV,30 practically it has no absorption in the entire visible region. On the other hand, Ba3(MnO4)2 (x = 1.0) has a strong absorption in the entire visible region with two maxima, around 450 and 600 nm, leaving a shallow minimum in the green region (∼500 nm), which causes in its dark green color. For small amounts of manganese substitution (x = 0.10), we observe two distinct peaks, at ∼350 and ∼ 660 nm, that leave a valley of almost no absorption between ∼450 and ∼510 nm, which imparts the turquoise color to the samples. As the value of x increases along the Ba3(V1−xMnxO4)2 solid solution, the two peaks (∼350 and ∼660 nm) become stronger and broader, forcing the valley between the two peaks to become gradually narrower and sharper, which causes the tuning of color from turquoise to green to dark green. These color changes are also seen in the CIE chromaticity diagram (Figure S5). Similar color changes have been reported for tetrahedral Mn5+O4 in various host lattices.14,15,17−19 The absorption spectra of Ba3(V1−xMnxO4)2 are similar to the spectra of Ba3(P1−xMnxO4)2 and other Mn5+O4 doped into PO43−/VO43− and other hosts,14−19,31,32 consisting essentially of two strong absorption bands centered at ∼350 and ∼660 nm. There are additional features on the low-energy side of the 660 nm band. For an ideal tetrahedral 3d2: Mn5+O4 (Td) geometry, only one d−d transition corresponding to 3A2 → 3 T1(F) is allowed. For the low-symmetry (C3v) Mn5+O4 however transitions to both 3T1(F) and 3T2(F) could become allowed; additional transitions to spin-forbidden 1A1 and 1E states also could become allowed.15,16 Accordingly, we could assign the main band at ∼660 nm to 3A2 → 3T1(F), 3T2(F) transitions with the additional features arising from transitions to 1A1 and 1E states (Figure S6). The high-energy broad band at ∼350 nm most likely arises from ligand to metal charge transfer.15,18 The representative assignment of the transitions is presented for Ba3(V0.50Mn0.50O4)2 in Figure S6. The UV− visible spectrum of Sr3(V0.9Mn0.1O4)2 is similar to that of its barium analogues (Figure S1). Mn5+O4 could not be stabilized in the single phase in Ca3(VO4)2 as well as in other orthovanadates such as YVO4 under our experimental conditions. Ba2M1−xMnxO4 (M = Ti and Si). We investigated the stabilization of Mn5+O4 in two other barium-containing tetrahedral oxoanion hosts, viz., Ba2TiO4 and Ba2SiO4. For nominal “Ba2Ti1−xMnxO4”, we could prepare single-phase materials for x = 0.10 and 0.15 that are turquoise in color (Figure 7). For x > 0.15, we could not prepare single-phase materials (Figure S2). Both the members are isostructural with the α′-Ba2TiO4 modification (Figure S7),33 crystallizing in an orthorhombic (P21nb) structure consisting of discrete TiO4 tetrahedra. We could refine the structure of the x = 0.1 member from the PXRD data on the basis of the α′-Ba2TiO4 structure model (Figure 8 and Table 2).33 Considering that Mn5+ replaces Ti4+ in these materials, the composition could be formulated as either Ba2Ti1−xMnxO4+x/2 (I) or Ba2‑x/2Ti1−xMnxO4 (II). On the basis of the refinement results (Figure 8, Figure S8, Table 2, and Table S2), we consider that formula I is more likely to represent the composition of the materials. The crystal structure of Ba2Ti0.90Mn0.10O4.05 is drawn in Figure 9. The EPR spectrum of Ba2Ti0.90Mn0.10O4.05 (g = 1.97) supports the presence of a tetrahedral Mn 5+ O 4 chromophore.14,18,28,29 The optical absorption spectra (Figure

Figure 7. Color and UV−visible spectra of manganese-substituted Ba2TiO4 powder samples.

Figure 8. Rietveld refinement of Ba2Ti0.90Mn0.10O4.05. Observed (o), calculated (red line), and difference (bottom blue line) profiles are shown. The vertical bars indicate Bragg reflections.

7) are similar to those of Ba3(V1−xMnxO4)2, corresponding to a distorted tetrahedral Mn5+O4 chromophore. Ba2TiO4 exists in two modifications, orthorhombic (P21nb) α′-Ba2TiO4 and monoclinic (P21/n) β-Ba2TiO4; the former is the superstructure of the latter stabilized by slow cooling.33,34 Ba2Ti1−xMnxO4+x/2 (0 < x ≤ 0.15) however are formed only with an α′-Ba2TiO4 structure. Similarly we prepared nominal “Ba2Si1−xMnxO4” phases for x = 0.10 and 0.15. For x > 0.15, we could not prepare singlephase “Ba2Si1−xMnxO4” materials (Figure S9). Both are turquoise colored (Figure 10) single phases adopting the Ba 2SiO4 structure. 35,36 Refinement of the structure of Ba2Si1−xMnxO4 for x = 0.10 on the basis of the Ba2SiO4 structure model (Pmcn, a = 5.805 Å, b = 10.200 Å, c = 7.499 Å) (Figure 11 and Table 3) shows that the Mn-substituted material is isostructural with Ba2 SiO 4.35,36 Here again comparison of the refinement results (Figure 11 and Figure S10, Table 3 and Table S3) based on Ba2Si1−xMnxO4+x/2 (I) or D

DOI: 10.1021/acs.inorgchem.5b02957 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Crystallographic Data for Ba2Ti0.90Mn0.10O4.05a

a

atom

site

x

y

z

Uiso (Å2)

occupancy

Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ti1/Mn1 Ti2/Mn2 Ti3/Mn3 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12

4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a

0.746(2) 0.757(3) 0.789(2) 0.260(2) 0.278(2) 0.237(2) 0.781(6) 0.758(2) 0.751(7) 0.525(9) 0.997(9) 0.808(9) 0.801(9) 0.511(9) 0.981(8) 0.677(9) 0.812(9) 0.532(9) 0.995(9) 0.809(9) 0.684(9)

0.279(1) 0.612(1) 0.946(1) 0.168(1) 0.502(1) 0.834(1) 0.095(1) 0.427(1) 0.760(1) 0.073(3) 0.071(3) 0.171(3) 0.057(3) 0.401(3) 0.416(3) 0.502(3) 0.390(3) 0.749(2) 0.726(3) 0.836(2) 0.723(4)

0.075(1) 0.078(1) 0.074(1) 0.200(1) 0.200(1) 0.201(1) 0.085(3) 0.084(2) 0.085(3) 0.153(5) 0.188(8) 0.059(6) −0.064(6) 0.143(7) 0.194(6) 0.054(6) −0.068(6) 0.202(7) 0.145(9) 0.062(4) −0.064(7)

0.045(2) 0.022(1) 0.029(2) 0.024(2) 0.031(2) 0.024(2) 0.028(5) 0.010(4) 0.037(6) 0.022(9) 0.020(9) 0.083(9) 0.039(9) 0.015(9) 0.089(5) 0.040(9) 0.046(9) 0.012(9) 0.047(6) 0.033(9) 0.040(9)

1.0 1.0 1.0 1.0 1.0 1.0 0.9/0.1 0.9/0.1 0.9/0.1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Space group P21nb: a = 6.096(2) Å, b = 23.004(1) Å, c = 10.552(1) Å. Reliability factors: Rp = 3.11%, Rwp = 4.64%, RF2 = 11.71%, χ2 = 3.65.

Figure 10. Color and UV−visible spectra of manganese-substituted Ba2SiO4 powder samples.

manganese-substituted Ba2SiO4 samples also correspond to a distorted tetrahedral Mn5+O4 chromophore. On the basis of the foregoing results and also in light of the literature reports,14,16−19,31,32 we find that tetrahedral Mn5+O4 is stabilized in oxide hosts containing PO43−, AsO43−, VO43−, SiO44−, TiO44−, and In3+O4/2 oxoanions together with electropositive alkaline earth metal cations, especially barium. We also find that complete series of solid solution replacing the entire oxoanion by Mn 5+ O 4 tetrahedra occurs only with Ba5(V1−xMnxO4)3Cl, Ba5(P1−xMnxO4)3Cl, Ba3(P1−xMnxO4)2, and Ba3(V1−xMnxO4)2, whereas partial substitution occurs with hosts containing SiO44−, TiO44−, and In3+O4/2, suggesting that isovalent Mn5+ → P5+/V5+ substitution is more favorable than aliovalent Mn5+ → Si4+/Ti4+/In3+ substitution. Also the

Figure 9. Crystal structure of Ba2Ti0.90Mn0.10O4.05 (without interstitial oxygens) showing isolated Ti/MnO4 tetrahedra.

Ba2‑x/2Si1−xMnxO4 (II) models suggests that formula I is likely to correspond to the actual composition of the material. The crystal structure of Ba2Si0.90Mn0.10O4.05 is drawn in Figure 12. The EPR spectrum of Ba2Si0.90Mn0.10O4.05 (g = 1.97) suggests the presence of tetrahedral Mn5+O4 in the solid solutions.14,18,28,29 Color and UV−visible spectra (Figure 10) of E

DOI: 10.1021/acs.inorgchem.5b02957 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 11. Rietveld refinement of Ba2Si0.90Mn0.10O4.05. Observed (o), calculated (red line), and difference (bottom blue line) profiles are shown. The vertical bars indicate Bragg reflections. Figure 12. Crystal structure of Ba2Si0.90Mn0.10O4.05 (without interstitial oxygens) showing isolated Si/MnO4 tetrahedra.

presence of barium definitely seems to play a crucial role in the formation of extensive solid solution with considerable Mn5+ content. One possibility is that barium plays a crucial role by providing a large band gap to the host. For example, the band gap of Ba3(VO4)2 is 3.8 eV,30 but this possibility could be discounted because YVO4, having the same band gap, does not stabilize Mn5+O4. What appears to be important is not just the band gap but the extent of stabilization of the O 2p-valence band vis-à-vis the redox energies of various oxidation states of manganese. The O 2p-valence band stabilization is known to increase with increasing electropositive character of the alkali/ alkaline earth metal atoms in transition metal oxides, enabling access to higher oxidation states of metal atoms.37 While a detailed examination of the DFT results on the band structure would reveal the extent of O 2p-valence band stabilization in various oxides containing VO43−,30 the highly covalent nature of tetrahedral V−O bonds and its consequent reduction in V− O bond length is an experimental indicator of the O 2p-valence band stabilization in the presence of strong electropositive atoms. Thus, the average V−O bond length in Ba3(VO4)2 (1.649 Å)25 is noticeably shorter than that (1.709 Å)38 in YVO4, although both the oxides have the same band gap (3.8 eV).30 Accordingly, we suggest that the strong electropositive character of barium together with appropriate oxoanions such as PO43−/VO43− that facilitate isovalent substitution of Mn5+O4

favors stabilization of the latter chromophore, imparting tunable turquoise/blue/green colors to the hosts. To provide a proof-of-concept support to the foregoing rationalization, we synthesized the following new turquoisecolored Mn 5+ O 4 materials: Ba 5 (BO 3 )(MnO 4 ) 2 Cl and Ba5(BO3)(PO4)(MnO4)Cl. Both are isostructural with the parent Ba5(PO4)3Cl and Ba5(MnO4)3Cl apatites,14,39−43 stabilizing the turquoise-colored MnO43− chromophore (Figure 13 and Figure S11). The formation of these materials also shows the adaptability of the apatite structure for BO33−/ MnO43−/PO43− exchange.44,45 Significantly, we could not prepare the corresponding Sr- and Ca-apatite analogues, again stressing the importance of barium for the stabilization of the tetrahedral Mn5+O4 chromophore.

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CONCLUSION Our investigation of the stabilization of tetrahedral Mn5+O4 in various tetrahedral oxoanion hosts containing alkaline earth metals, A3(VO4)2 (A = Ba, Sr, Ca), Ba2MO4 (M = Ti, Si), and YVO4 shows that the tetrahedral Mn5+O4 is stabilized in hosts containing barium and to a lesser extent strontium, resulting in turquoise-colored materials that could be of significance as pigments for this rare color. The investigation also identifies the role of chemical composition and electronic structure factors that promote the stabilization of tetrahedral Mn5+O4.

Table 3. Crystallographic Data for Ba2Si0.90Mn0.10O4.05a atom

site

x

y

z

Uiso (Å2)

occupancy

Ba1 Ba2 Si/Mn O1 O2 O3

4c 4c 4c 4c 8d 4c

0.25 0.25 0.25 0.25 0.021(2) 0.25

0.086(1) 0.695(1) 0.422(1) 0.572(1) 0.352(1) 0.413(1)

0.161(1) −0.009(1) 0.228(1) 0.306(1) 0.309(2) 0.017(2)

0.027(1) 0.020(1) 0.022(2) 0.035(5) 0.043(4) 0.047(5)

1.0 1.0 0.91(1)/0.09(1) 1.0 1.0 1.0

Space group Pmcn: a = 5.817(1) Å, b = 10.219 (1) Å, c = 7.512(1) Å. Reliability factors: Rp = 3.28%, Rwp = 4.61%, RF2 = 11.16%, χ2 = 4.26. Ba(1)− O(1) = 2.923(1) (×2); Ba(1)−O(2) = 2.890(11) (×2); Ba(1)−O(2) = 3.010(13) (×2); Ba(1)−O(3) = 2.674(19); Ba(2)−O(1) = 2.680(17); Ba(2)−O(1) = 2.761(17); Ba(2)−O(2) = 2.811(12) (×2); Ba(2)−O(2) = 2.731(12) (×2); Ba(2)−O(3) = 2.885(15); Si/Mn−O(1) = 1.637(20); Si/Mn−O(2) = 1.597(12) (×2) ; Si/Mn−O(3) = 1.593(20). a

F

DOI: 10.1021/acs.inorgchem.5b02957 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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Figure 13. Color and UV−visible spectra of (a) Ba5(BO3)(MnO4)2Cl and (b) Ba5(BO3)(PO4)(MnO4)Cl.

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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.5b02957. SEM and EDX analysis, additional PXRD patterns, UV− vis spectra, Rietveld refinement data, CIE chromaticity diagram, and CIE coordinates (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S. Natarajan). *E-mail: [email protected] (J. Gopalakrishnan). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.G. is grateful to the National Academy of Sciences, Allahabad, India (NASI), for the award of a Senior Scientist Fellowship, and S.N. to DST for the award of a J.C. Bose National Fellowship.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b02957 Inorg. Chem. XXXX, XXX, XXX−XXX