A Cubic Non-Centrosymmetric Mixed-Valence Iron Borophosphate

Article pubs.acs.org/crystal

A Cubic Non-Centrosymmetric Mixed-Valence Iron Borophosphate− Phosphite Hooman Yaghoobnejad Asl,† Ronetta Morris,‡ T. Thao Tran,§ P. Shiv Halasyamani,§ Kartik Ghosh,∥ and Amitava Choudhury*,† †

Department of Chemistry and ‡Department of Chemical and Biological Engineering, Missouri University of Science and Technology, Rolla, Missouri 65409, United States § Department of Chemistry, University of Houston, 112 Fleming Building, Houston, Texas 77204-5003, United States ∥ Department of Physics, Astronomy and Materials Science and Center for Applied Science and Engineering, Missouri State University, Springfield, Missouri 65897, United States S Supporting Information *

ABSTRACT: A first member of a new family of metal borophosphate−phosphite Fe1.834IIFe0.166IIIB0.5[PO3(OH)]0.8(HPO3)2.033 has been successfully synthesized by using the boric acid−phosphorous acid flux. The compound crystallizes in the cubic crystal system in a non-centrosymmetric space group I4̅3d (No. 220) with unit cell parameters of a = 21.261(3) Å, and Z = 48, featuring a very condensed network of FeO6 octahedra and disordered phosphate-phosphite moieties with mixed valency of iron. The compound contains a novel fundamental building unit (FBU), a cyclic borophosphatephosphite ring which is further connected to form a propeller like partial anionic structure. Metal polyhedra also form a propellerlike structure through edge-sharing and are further connected to the partial anionic structure to form the three-dimensional structure. Thermal analyses, infrared and Mössbauer spectroscopy, magnetic and second harmonic generating (SHG) measurements using 1064 nm radiation have been performed on this compound. SHG measurements indicate that the compound has an efficiency approximately equal to α-SiO2.

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phate by the Kniep group,19 a large number of borophosphates with both open framework and condensed network featuring a plethora of oligomeric borophosphate anionic species have been reported.20−27 Some of these borophosphates show useful optical properties.28−32 Other combination of polyanions such as borosilicate and borosulfates are also emerging with fascinating structural chemistry and potential applications as optical materials.33−36 There is a huge inventory of synthetic approaches that can be used based on the target material. For example, to synthesize open-framework materials especially in borophosphate family generally low temperature hydrothermal,25,26 ionothermal,22,24 or low-melting flux such as boric acid21,27 can be used. On the other hand a high temperature solid state ceramic method30−32 or flux-based synthesis23 has been used to form relatively condensed networks. Recently, we have used low melting phosphorus acid flux to synthesize iron fluoro phosphate of tavorite structure type, LiFePO4F, an important cathode for Li-ion battery.37 It is well-known that phosphites often get oxidized to phosphates under certain reaction conditions and

INTRODUCTION Research in the area of polyanion-based materials (borate, silicate, phosphate, and sulfate) has intensified in the recent years due to a variety of applications that these materials can offer. When combined with transition metals these polyanionbased materials can serve as alkali-metal ion insertion electrodes.1 A large variety of porous solids can be formed with metal phosphates or phosphites using different templates, which are good for catalysis, sorption, and separation applications.2−4 Many transition metal sulfates have the tendency to form structures, such as Kagome lattice, that exhibit novel magnetic property related to geometrically frustrated magnetic systems.5,6 Borates and phosphates have the tendency to crystallize in non-centrosymmetric structure type giving rise to nonlinear optical (NLO) properties.7−12 Some of the most well-known NLO materials come from the phosphate [KH2PO4 (KDP), KTiOPO4 (KTP)] and borate [β-BaB2O4 (BBO), LiB3O7 (LBO)] families.13−17 Often polyanions can combine to form a mixed polyanionic species; for example, borate and phosphate can polymerize through P−O−B linkages, and the resulting borophosphate has been the subject of intense research due to its rich structural chemistry and the varieties of application.18 Since the first report of zeolitic metal borophos© XXXX American Chemical Society

Received: August 2, 2015 Revised: November 13, 2015

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DOI: 10.1021/acs.cgd.5b01106 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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temperatures.38−40 We therefore wanted to explore what kind of oligomeric partial anionic structure can be formed if a H3PO3−H3BO3 reactive binary flux is used. There can be several intriguing possibilities, e.g., the nature of the borophosphite or borophosphate or borophosphate-phosphite anionic species; formation of a condensed network as opposed to a more open-framework type structure; the effect of difference of coordination of phosphite, phosphate, and borate in dictating the crystallization in centro- or non-centrosymmetric space group. In these regards, we explored the multicomponent system FeCl3·6H2O−LiOH·H2O−H3BO3−H3PO3 to identify new phases. This exploration has yielded a new non-centrosymmetric cubic structure of the composition, Fe1.834IIFe0.166IIIB0.5[PO3(OH)]0.8(HPO3)2.033 (I), where Fe is in mixed oxidation state, and condensation of borate (BO45−), phosphate (PO43−), and phosphite (HPO32−) polyanions create a complex disordered borophosphate-phosphite three-dimensional (3-D) partial anionic structure. In this article we report the synthesis, crystal structure, Mössbauer spectroscopy, and magnetic and nonlinear optical properties of the title compound.

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EXPERIMENTAL SECTION

Synthesis. In a typical synthesis 0.9511 g of FeCl3·6H2O (3.52 mmol), 1.2371 g of H3PO3 (15.08 mmol), 0.6145 g of LiOH·H2O (14.64 mmol), and 3.0914 g of H3BO3 (50 mmol) were mixed and ground in air in an agate mortar, and the resulting homogeneous mixture was transferred to a Teflon beaker, which was then placed in a 23 mL capacity Parr reaction vessel. The reactor was heated for 4 days at 200 °C, and after this period the reactor was removed from the oven and allowed to cool naturally. The product, composed of shiny blackish cuboctahedral crystals, was washed with hot water several times to remove the excess boric acid and allowed to dry in air. Single-Crystal X-ray Structure Determination. The crystal structure of the shiny dark crystal was solved from single-crystal intensity data sets collected on a Bruker Smart Apex diffractometer with monochromated Mo Kα radiation (λ = 0.7107 Å). The data sets were collected at room temperature using SMART41 software employing a scan of 0.3° in ω with an exposure time of 10 s/frame; the cell refinement and data reduction were carried out with SAINT,42 while the program SADABS42 was used for the absorption correction. The structure was solved by direct method using SHELX-9743 and difference Fourier syntheses. Full-matrix least-squares refinement against |F2| was carried out using the SHELXTL-PLUS43 suite of programs. Structure of the title compound was solved in the I43̅ d (No. 220) space group. The positions of two Fe atoms, four P atoms, and seven O atoms were easily located from the difference Fourier maps and refined isotropically with good thermal displacement parameters. Three oxygen atoms and one B atom were subsequently located, and refinement yielded high thermal parameters. Therefore, fractional occupancies of three oxygen atoms O2, O5, and O6 and one boron (B1) atom were then refined to yield refined values of occupancies as 0.70(1), 0.71(1), 0.58(1), and 0.47(2), respectively. These values were then fixed to the closest value obtained from the refinement as 70% for O2 and O5, 60% for O6, and 50% for B1. Partial occupancies on the oxygen atoms attached to P atoms (which are fully occupied) clearly meant the presence of disordered phosphite-phosphate groups. This disorder can be described as 40% phosphate and 60% phosphite, respectively, for P1; 30% phosphate and 70% phosphite for P2, respectively, as shown schematically in Figure 1a,b. The ratio of phosphate and phosphite has been deduced by solving the linear equations, 4x + 3y = 1.7, x + y = 0.5 for P1 and 4x + 3y = 3.3, x + y = 1 for P2, where x and y represent fractions of phosphate and phosphite, respectively. Note that partially occupied oxygen atoms (O2, O5, and O6) are linked to B atom as part of polymeric P−O−B networks (Figure 1c), while the remaining percentage of O2, O5, and O6 atoms pointing toward the vacant boron site form terminal P−O−H groups as indicated by their long P−O distances (Table 3). After anisotropic refinement, two new peaks (1.75 and 0.9 e/Å3)

Figure 1. Schematic showing the disorder of phosphate and phosphite in P1 centered (a) and P2 centered (b) polyhedra. Also shown in (c), the role of P−O−B linkages associated with two phosphate units in forming the borophosphate network. Note P1 is located on a special position with a site occupancy factor (SOF) of 0.5. appeared in the difference electron-density map approximately 1.5 Å away from P4 and P3, respectively. These peaks were assigned as phosphite hydrogen; however, refinement resulted in negative isotropic thermal parameters with high P−H distances of approximately 1.5 Å. Therefore, a partial oxidation of phosphite to phosphate was considered, and these peaks were refined as oxygen atoms (O11 and O12 close to P4 and P3, respectively) yielding approximately 30 and 20% occupancies for O11 and O12, respectively, thus giving rise to disordered phosphite-phosphate groups. Note O11 is located on a 3-fold symmetry; therefore, the site occupancy (SOF) is only 10%. These terminal oxygen atoms attached to P4 and P3 were assigned as terminal P−O−H groups though the P−O distances (P3−O12 = 1.43(2) and P4−O11 = 1.53(2) Å) are less than ideal for a protonated P−O bond distance. However, these P−O distances are not very meaningful as they are refined from very low electron density and represent an average of P−O and P−H distances. Taking reference from Mössbauer spectroscopy (presence of 8.22% of Fe3+, which was approximated to 8.25% for charge balance) a composition of Fe1.834IIFe0.166IIIB0.5(HP1O3)0.3(HP1O4)0.2(HP2O3)0.7(HP2O4)0.3 (HP3O3)0.8(HP3O4)0.2(HP4O3)0.233(HP4O4)0.100 can be reached. Thus, the final charge-balanced formula can be written as Fe1.834IIFe0.166III B0.5[PO3(OH)]0.8(HPO3)2.033. None of these hydrogen atoms could be located from the difference electron density map, and hence they were not taken into account for refinement. The analysis of hydrogen corroborates well with the theoretical amount. The analysis of boron employing ICP-OES was in close agreement with the theoretical value (experimental 1.29%, theoretical 1.59%). The presence of the −OH group was also evident in the IR-spectrum of the compound. It is to be noted here that there were no indication of satellite peaks, twinning, and also the presence of any diffuse peak that would indicate a superstructure. Disordered phosphate-phosphite and the mixed valency of iron have been observed previously by Rojo’s group in an organically templated compound, (C4N2H12)[FeII0.86FeIII1.14(HPO3)1.39(HPO4)0.47(PO4)0.14F3].44 Crystal data and refinement parameters for Fe1.834II Fe0.166 III B0.5 [PO3 (OH)] 0.8 (HPO3)2.033, I, are given in Table 1. The final atomic coordinates and the isotropic displacement parameters and selected interatomic distances are listed in Tables 2 and 3. Further details of the crystal structure can be B



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optical emission spectrometry (ICP-OES) technique. The sample was dissolved in high purity HNO3 for the analysis, and a high purity boric acid solution was used as a standard. ICP-OES could not detect the presence of any lithium in the solution of the sample. Thermogravimetric Analysis. Thermogravimetric analysis of the sample was done using a TA Instruments Q50 TGA from room temperature up to 800 °C with a heating rate of 10 °C·min−1 in N2 atmosphere. IR and Mö ssbauer Spectroscopy. IR spectra were obtained using a Thermo Nicolet Nexus 470 FT-IR spectrometer on KBr pellets in the wavenumber range of 400 to 4000 cm−1. 57Fe Mössbauer experiments were performed in transmission geometry at room temperature using a conventional constant acceleration spectrometer. The data were collected using a 57Co (50 mCi) gamma-ray source embedded in a Rh matrix. Velocity calibration and isomer shifts are given with respect to alpha-Fe foil at room temperature. The Mössbauer data were analyzed by Lorentzian line fitting using RECOIL software.45 Physical Property Measurements. Magnetic susceptibility of I was measured at 0.5 T (1 T = 10 000 Oe) after zero-field cooling over the temperature range 1.8−300 K, and isothermal magnetization at 2 K up to an applied field of 5 T was measured with a Quantum Design SQUID magnetometer. Powder second-harmonic generation (SHG) measurements were performed on a modified Kurtz-nonlinear optical (NLO) system using a pulsed Nd:YAG laser with a wavelength of 1064 nm. A detailed description of the equipment and methodology has been published.46 The compound I was ground and sieved into distinct particle size ranges (90 μm). Relevant comparisons with known SHG materials were made by grinding and sieving crystalline α−SiO2 and LiNbO3 into the same particle size ranges. The SHG, i.e., 532 nm light, was collected in reflection and detected using a photomultiplier tube. No index matching fluid was used in any of the experiments.

Table 1. Crystal Data and Structural Refinement Parameters for Compound I compound

I

empirical formula formula weight temperature (K) wavelength (Å) crystal system, space group unit cell dimensions volume (A3) Z, calculated density absorption coefficient F(000) limiting indices reflections collected/unique refinement method data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) absolute structure parameter largest diff. peak and hole

B0.5Fe2H2.83O9.3P2.83 356.51 296(2) 0.71073 cubic, I4̅3d a = b = c = 21.261(3) Å 9610(3) 48, 2.957 Mg/m3 4.220 mm−1 8363 −28 ≤ h ≤ 28, −28 ≤ k ≤ 28, −28 ≤ l ≤ 28 56585/1994 [R(int) = 0.0516] full-matrix least-squares on F2 1994/127/162 1.168 R1 = 0.0210a, wR2 = 0.0544b R1 = 0.0213, wR2 = 0.0546 0.008(14) 0.475 and −0.268 e·A−3

R1 = Σ ||Fo| − |Fc||/Σ|Fo|. bwR2 = {Σ[w(F02 − Fc2)2]/Σ[w(F02)2]}1/2, w = 1/[σ2(FO)2 + (aP)2 + bP] where P = [FO2 + 2FC2]/3; a = 0.0295 and b = 14.5559 for I. a

obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany, (fax: (49) 7247−808−666; e-mail: crysdata@fiz.karlsruhe.de) by quoting the depository number CSD-430438 for Fe1.834IIFe0.166IIIB0.5[PO3(OH)]0.8(HPO3)2.033, I. Hydrogen and Boron Analysis. The analysis of hydrogen was carried out in a modified carbon, hydrogen, and nitrogen (CHN) analyzer which involves the combustion of materials in ultrapure oxygen at 990 °C and analyzing the product gases (in this case H2O) before and after the removal from the analyzer. The sample was weighed in a N2 glovebox, and the calibration was done using benzoic acid. The percentage of hydrogen found from this analysis (0.94%) was slightly higher than the theoretical value (0.8%). The analysis of boron was carried out by employing inductively coupled plasma

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RESULTS AND DISCUSSION Synthesis. Phosphorous acid, H3PO3, is a low melting solid (73.6 °C), and its potential as a reactive flux has been explored for the first time in combination with another well-known flux, boric acid (H3BO3). However, compared to boric acid, phosphorous acid (also known as phosphonic acid, IUPAC

Table 2. Positional Coordinates and Equivalent Isotropic Displacement Parameters for Ia

a

atom

Wyckoff

occupancy

x/a

y/b

z/c

Ueq [Å2]

Fe1 Fe2 P1 P2 P3 P4 B1 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12

48e 48e 24d 48e 48e 16c 48e 48e 48e 48e 48e 48e 48e 48e 48e 48e 48e 16c 48e

1 1 1 1 1 1 0.5 1 0.7 1 1 0.7 0.6 1 1 1 1 0.3 0.2

0.50134 0.57552 1/2 0.61643 0.49464 0.42876 0.65180 0.51769 0.55779 0.66152 0.57696 0.57183 0.65749 0.50056 0.46929 0.55787 0.43502 0.38728 0.45444

0.32864 0.38231 1/4 0.44087 0.37483 0.42876 0.55251 0.30609 0.23155 0.40792 0.39709 0.48346 0.48834 0.31099 0.37061 0.40923 0.39628 0.38728 0.41363

0.52563 0.40632 0.38922 0.54635 0.67749 0.42876 0.48127 0.42905 0.34597 0.58957 0.50642 0.58443 0.50590 0.70775 0.61079 0.67831 0.49266 0.38728 0.71526

0.013 0.0142 0.0187 0.0173 0.0132 0.0106 0.0126 0.0163 0.0352 0.0254 0.0202 0.0335 0.024 0.0206 0.0181 0.0293 0.0153 0.0355 0.0525

U(eq) = 1/3rd of the trace of the orthogonalized U Tensor. C



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Crystal Structure. The compound I crystallizes in a cubic system with a non-centrosymmetric space group of I4̅3d. The asymmetric unit contains 19 non-hydrogen atoms (2Fe, 4P, 1B, and 12O) including one partially occupied B and 5 partially occupied oxygen atoms, besides hydrogen atoms that belong to disordered phosphite groups as well as terminal P−O−H groups (Figure 2).

Table 3. Selected Interatomic Distances and Angles for the Coordination Polyhedra of Ia atom−atom

distances (Å)

atom−atom−atom

angles (deg)

Fe1−O8 Fe1−O10 Fe1−O1 Fe1−O7#1 Fe1−O7#2 Fe1−O4 Fe2−O3#3 Fe2−O1 Fe2−O10#4 Fe2−O8#4 Fe2−O4 Fe2−O9#3 P1−O1 P1−O1#5 P1−O2#5 P1− O2 P2−O3 P2−O4 P2−O5 P2−O6 P3−O12 P3−O7 P3−O8 P3− O9 P4−O11 P4−O10#6 P4−O10 P4 O10#4 B1−O6 B1−O2#7 B1−O5#4 B1−O9#4

2.130(2) 2.132(2) 2.137(2) 2.137(2) 2.153(2) 2.207(2) 2.001(2) 2.091(2) 2.099(2) 2.135(2) 2.152(2) 2.273(3) 1.510(2) 1.510(2) 1.584(4) 1.584(4) 1.501(2) 1.514(2) 1.541(4) 1.588(4) 1.434(16) 1.507(2) 1.520(2) 1.530(2) 1.527(14) 1.530(2) 1.530(2) 1.530(2) 1.466(7) 1.489(7) 1.491(7) 1.636(6)

O1−P1−O1#5 O1−P1−O2#5 O1#5−P1−O2#5 O1−P1−O2 O1#5−P1−O2 O2#5−P1−O2 O3−P2−O4 O3−P2−O5 O4−P2−O5 O3−P2−O6 O4−P2−O6 O5−P2−O6 O12−P3−O7 O12−P3−O8 O7−P3−O8 O12−P3−O9 O7−P3−O9 O8−P3−O9 O11−P4−O10#6 O11−P4−O10 O10#6−P4−O10 O11−P4−O10#4 O10#6−P4−O10#4 O10−P4−O10#4 O6−B1−O2#7 O6−B1−O5#4 O2#7−B1−O5#4 O6−B1−O9#4 O2#7−B1−O9#4 O5#4−B1−O9#4

111.79(16) 108.85(15) 109.14(16) 109.14(16) 108.85(15) 109.0(3) 114.23(13) 110.22(17) 108.34(17) 106.09(17) 113.08(17) 104.4(2) 109.2(7) 110.2(6) 112.01(13) 104.0(9) 110.62(14) 110.51(13) 107.60(8) 107.60(8) 111.27(8) 107.60(8) 111.27(8) 111.27(8) 109.9(4) 108.9(4) 111.5(4) 111.8(4) 104.8(4) 109.9(4)

Figure 2. Asymmetric unit of I showing the atom labeling scheme. The thermal ellipsoids are given at 40% probability. Only atoms that appear in the asymmetric unit are labeled.

There are 48 such units present in the unit cell. All the four P atoms are part of disordered hydrogen phosphate-phosphite [(HPO42−)x(HPO32−)1−x] units, P1 and P4 are also located in special positions related to 2-fold (P1) and 3-fold (P4) symmetry, respectively. The B atom is 50% occupied in its atomic site and adopts a distorted tetrahedral coordination with one long B−O bond. All the oxygen atoms are covalently connected to the phosphorus atoms, and they are further connected to Fe and B through P−O−Fe and P−O−B linkages. Six oxygen atoms are also tricoordinated; besides being connected to P, they (O1, O4, O7, O8, and O10) bridge between two Fe atoms or between Fe and B (O9) atoms. Three oxygen atoms (O2, O5, and O6) exclusively participate in bridging between P and B atoms; thus oxygen atoms that participate in P−O−B linkages create a complex macroanionic framework, in which disordered phosphate-phosphite and borate moieties are corner-shared. However, one of the phosphate−phosphite units [(HP4O42−)0.1(HP4O32−)0.233] does not participate in this macroanionic network and remains isolated. The complex anionic networks can be described in terms of a secondary building unit approach. B1O4 and disordered [(HP2O42−)0.3(HP2O32−)0.7] moieties are alternately corner-shared to form a cyclic six-membered ring, which is decorated by disordered phosphate−phosphites [(HP3O 4 2− ) 0.2 (HP3O 3 2− ) 0.8 ] and [(HP1O42−)0.2(HP1O32−)0.3] from the top and bottom of the plane of the six-membered ring through the corner-shared O9 and O2 atoms, respectively (Figure 3a). Such building units are then further connected through the O2 atom of the decorated phosphate/phosphite (P1) units to three more six-membered rings and extends in three dimensions like a propeller. The absence of the center of inversion arises because of the blades (six-membered rings) of the propeller-like building units point in the same direction, toward the diagonal of the cube (Figure 3b). The cyclic borophosphate-phosphite unit is reminiscent of 6□: < 6□> (□ signify tetrahedron and < ···>

Symmetry transformations used to generate equivalent atoms: #1 y + 1/4, −x + 3/4, −z + 5/4; #2 −y + 3/4, x − 1/4, −z + 5/4; #3 −z + 5/4, −y + 3/4, x − 1/4; #4 z, x, y; #5 −x + 1, −y + 1/2, z + 0; #6 y, z, x; #7 −x + 5/4, z + 1/4, −y + 3/4. a

systematic name) is more interesting in terms of the chemistry; H3PO3 is a good reducing agent, and also it can disproportionate around 200 °C to form H3PO4 and PH3. Therefore, a combination of boric acid and phosphorous acid flux may serve as a unique reactive medium for reactions where depending upon the temperature and oxidation state of the starting transition metal salt a variety of materials can be synthesized. In our exploration we have used FeIII salt, FeCl3·6H2O, and a higher temperature to create a borophosphate/phosphite anionic structure and also examined how such phosphate-phosphite chemistry affects the oxidation state of iron. Li(OH)·H2O was also added into the reaction mixture to act as a base to deprotonate the acids and as well as incorporate Li into the structure. However, both single-crystal structure solution and ICP-OES analysis confirm the absence of Li in the structure. The reaction has yielded pure phase material of the composition Fe1.834IIFe0.166IIIB0.5[PO3(OH)]0.8(HPO3)2.033 as evident by the perfect match between the experimental and simulated powder pattern generated from the atomic coordinates obtained from singlecrystal X-ray data (Supporting Information, Figure S1). Reduction of starting Fe3+ salt to Fe2+ has been achieved because of the concomitant oxidation of phosphite to phosphate at higher temperature. D



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Figure 3. (a) Polyhedral representation of the six-membered ring borophosphate−phosphite fundamental building unit (FBU). Also shown in the figure the additional phosphate−phosphite units (ball and stick representation) that decorates the FBU. (b) The partial anionic structure showing the connectivity between the FBUs to form a propeller-like structure. (c) The connectivity between the FeO6 octahedron showing how the edgeshared six-membered Fe−O−Fe rings are further connected to form a propeller like substructure.

signify ring motif) fundamental building unit (FBU) found in many borophosphate materials.20 All the P−O distances (see Table 3) are in good agreement with the reported borophosphate or pure phosphate-based compounds.26 P−O distances associated with partially occupied oxygen atoms (P1−O2 = 1.584, P2−O5 = 1.541, and P2−O6 = 1.588 Å) are a little longer as expected for partial protonation. Three of the B−O distances, 1.466(10)−1.491(10) Å, are in a typical range for borophosphates;21 however, the distance of 1.636(9) (B1−O9) is considerably longer than normal. Fe1 and Fe2, on the other hand, adopt distorted octahedral geometry surrounded by six oxygen atoms. The Fe−O bond lengths are in the range of 2.130(2)−2.207(2) Å [d(Fe1−O)av = 2.149(3) Å] and 2.001(2)−2.273(3) Å [d(Fe2−O)av = 2.125(3) Å] for Fe1 and Fe2, respectively. The Fe−O distances are in good agreement with those reported for related compounds as in LiFePO4 where Fe is in the +2 oxidation state.47 There is, however, a slight difference in the average bond distances of the two Fe sites. The slightly shorter average Fe −O distance for Fe2 may indicate presence of some Fe3+ in that site as required for a charge balanced composition of I. Bond valence sum (BVS) calculation48 assuming Fe in 2+ oxidation state also suggest higher bond valence sum for the Fe2 site (BVS = +2.13) compared to Fe1 site (BVS = +1.95), which may also indicate presence of some Fe3+ in Fe2 site as was also evident in the Mössbauer spectra. Like the phosphates, FeO6 octahedra also form complex networks of Fe−O−Fe linkages, in which Fe1 and Fe2 octahedra are alternately edge shared through O1−O4 and O8−O10 edges of Fe1 and Fe2, respectively, to form a 6-membered cyclic ring. This cyclic ring sits exactly on top of the borophosphate-phosphite ring and the center of the ring is capped by the disordered [(HP4O42−)0.1(HP4O32−)0.233] group, which is not part of borophosphate−phosphite macroanionic network, as mentioned earlier. Each six-membered Fe−O−Fe ring is further connected to three such rings through the O7−O7 edge of the Fe1 octahedra and extends like a propeller in three dimensions, similar to the borophosphate− phosphite network (Figure 3c). The borophosphate−phosphite and Fe−O−Fe propellers are fused together through the participation of O7 and O8 oxygen atoms of the decorated phosphatephosphite unit, [(HP3O42−)0.2(HP3O32−)0.8], to form the 3-D structure. Such a complex network between the FeO6, PO4, BO4, and HPO3 create a very condensed and non-centrosymmetric structure of I (Supporting Information, Figure S2).

Figure 4. TGA of I showing the difference in behavior for finely ground powder and crystals.

Thermogravimetric Analysis. Figure 4 shows the TGA of the as synthesized crystals along with the finely ground sample after drying in a vacuum oven at 50 °C for 24 h. The powder sample exhibits some gradual mass loss of 1.2% up to 530 °C presumably due to the removal of adsorbed water and some hydroxyl groups. After 530 °C the sample shows mass gain which is assigned to oxidation of phosphite to hydrogen phosphate due to O2 impurities in the nitrogen gas flow. Such conversion of phosphite to phosphate and the subsequent weight gain during the TGA have been observed before.49 To eliminate the effect of adsorbed water we have also carried out TGA of the sample by taking crystals without grinding them into fine powder, and as expected it does not show the weight loss from the beginning and a weight loss of 0.4% can be observed between 470 to 530 °C similar to the powder sample. However, the phosphite to phosphate conversion takes place around the same temperature but at a much faster rate. Weight gain due to oxidation of phosphite to phosphate is more pronounced in the crystal than the powder, which may be due to different oxygen impurity levels in the two experiments. The definitive support of the conversion of phosphite to phosphate has been found in IR spectrum of the heated residue from the TGA sample pan (vide infra). Infrared Spectroscopy. The FT-IR spectrum of the vacuum-dried powder sample shows the dominant stretching modes of PO3 and PO4 groups as overlapping peaks in the region from 900 to 1300 cm−1 and the corresponding bending E



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indicates that the doublet is slightly asymmetrical, the low velocity peak has a broader width, the fwhm of the low and high velocity peaks being 0.60(3) and 0.53(2) mm/s, respectively. Broadness of the experimental line width, the presence of two crystallographically independent Fe-sites, and the asymmetric nature of the spectrum give an indication that it should be fitted with three quadrupolar doublets. Accordingly, two doublets with a high isomer shift and large quadrupole splitting and a minor one with a smaller isomer shift and a smaller quadrupole splitting, buried in the low velocity peak, have been fitted with Lorentzian line fitting. Figure 6 shows the experimental Mössbauer spectrum recorded for I along with three fitted components and the calculated spectrum from the sum of the three fitted doublets. The characteristic fitted parameters, isomer shift (δ), and the quadrupole splitting (Δ) yielded from this refinement are given in Table 4. The spectra thus display

Figure 5. FTIR spectra of I from finely ground sample (dried in vacuum), freshly ground sample inside a N2-filled glovebox, and heattreated crystals of I.

modes at 550 cm−1 (Figure 5). The characteristic P−H bond stretching mode appears around 2500 cm−1. Signature of strong stretching modes of hydroxyl (−O−H) from terminal P−OH as well as from adsorbed water is evident from a broad peak centered around 3350 cm−1, and a weak peak at 1600 cm−1 is due to the bending mode of adsorbed water. From the weak intensity of the bending modes of water, it can be concluded that amount of adsorbed water in the vacuum-dried sample is negligible, and in fact for the IR spectrum of single crystals freshly ground inside N2 filled glovebox, the bending mode peak of water at 1600 cm−1 is almost absent. This confirms that the broad peak between 3000−3600 cm−1 in the freshly ground crystals is due to the −O−H groups associated with the terminal P−OH groups. The IR spectrum of the heat-treated sample after TGA clearly indicates the absence of the −P−H and −O−H stretching modes around 2500 and 3000−3600 cm−1, respectively, confirming the conversion of phosphites and hydrogen phosphates to phosphate. Mössbauer Spectroscopy. A Mössbauer spectroscopic study was carried out in order to investigate the oxidation states of iron in I. At first the obtained Mössbauer spectrum (Figure 6) appears to contain single quadrupolar doublet; however, a careful inspection of the general profile of the spectrum

Table 4. Values of Fit Parameters for Isomer Shift (δ), Quadrupole Splitting (Δ) and Site population for I sites

isomer shift, δ (mm/s)

Δ (mm/s)

site populations (%)

Doublet 1 Doublet 2 Doublet 3

1.25(3) 1.24(2) 0.298(4)

2.314(16) 1.785(21) 0.757(16)

47.3(4) 44.4(5) 8.22(4)

two doublets with almost an identical isomer shift of 1.25(3) and 1.24(3) mm/s and quadrupole splitting of 2.31(2) and 1.78(2) mm, respectively, corresponding to two different octahedral sites with Fe2+ in high spin state and a third doublet with isomer shift of 0.298(4) mm/s and a Δ of 0.75(2) mm can be assigned to Fe3+ in high-spin state that substitutes some percentage of the Fe2+ sites. The isomer shift and quadrupole splitting values are consistent with high spin Fe(II) and Fe(III) ions in octahedral coordination.50 The relative quantities of Fe2+ and Fe3+ calculated from the relative spectral areas indicated a predominant presence of Fe2+ (91.7%) and a minor substitution of Fe3+ (8.22%). This 8.22% percent of Fe3+ corresponds to the presence of 0.164 Fe3+ ions in the composition, which is in good agreement with the crystallographically derived formula of Fe1.834IIFe0.166IIIB0.5[PO3(OH)]0.8(HPO3)2.033 from charge neutrality consideration, where percentage of Fe3+ is rounded to 8.25%. It appears from the site population of doublet 1 and doublet 2, 47.7, and 44.7%, respectively, that substitution of Fe2+ by Fe3+ probably occurs unequally in the two crystallographic Fe sites (Fe1 and Fe2). Based on the BVS (Fe2 = +2.13) calculation doublet 2 can be tentatively assigned to Fe2. The octahedral distortion analysis also supports assignment of doublet 2 with smaller quadrupole splitting with less distorted Fe2-centered octahedron. The distortions of the octahedra can be quantified by an angular variance (σ2) analysis.51 The mean values of octahedral angular variance is defined as 12

σθ̅ 2(oct)

=

∑i = 1 (θi − 90°)2

11 where θi refers to the octahedral (cis-) angles. It was found that the values of σ2θ(oct.) for Fe1O6 and Fe2O6 are 111.67 and 52.01, respectively. This shows that Fe2 is less distorted and should have smaller quadrupole splitting. The quadrupole interactions in iron compounds are related to the electric field gradient at the nucleus, spin-states, and distortion geometry. Therefore, the quadrupole splitting (QS) value of Fe2+ in the high-spin octahedral site is a measure of distortion.52

Figure 6. Mössbauer spectra of I collected at room temperature and corresponding fit of the experimental data into three doublets and the calculated fitted curve. F



Article

Magnetic Measurements. The magnetic measurements were performed on powdered sample to confirm the oxidation states of Fe and to verify if there is any magnetic phase transition. The temperature dependence of the magnetic susceptibility and inverse susceptibility of I measured in an applied field of 5000 Oe from 2 K to room temperature under zero-field cooled (ZFC) condition are given in Figure 7. The asymptotic

Figure 8. Temperature dependence of χMT for I, in black filled squares, measured in an applied field of 0.5 T. Inset shows the isothermal magnetization obtained at 2 K (black filled circle) and 5 K (filled red circles) for I.

produced less and less separation between ZFC and FC curves and the FC-ZFC divergence is completely absent at 1000 Oe. On further increasing the applied field to 10 000 Oe the antiferromagnetic cusp is also flattened out (inset of Figure 7). The isothermal magnetization (M) measured at temperatures below the transition (2 K) and above the transition (5 K) show saturation moments of 2.5 and 2.4 NμB/Fe, respectively, at the highest applied field of 5 T (inset of Figure 8). M−H measurement at 5 K does not show any hysteresis and M−H curve at 0−5 T at both 2 and 5 K display initially a linear increase up to 1 T and then a nonlinear increase in magnetization with no sign of saturation at 5 T of applied field. The disappearance of antiferromagnetic (AF) transition in higher applied fields (in χM(T) plots, inset Figure 7) points toward the field-induced nature of the AF transition; however, the nonsigmoidal nature of the M−H curve at 2 K makes it a nonobvious case for a field induced metamagnetic transition. Probably one needs to measure M−H at even lower temperature to see the field-induced transition from an AF state to a ferro-/ferrimagnetic state. SHG Measurements. Since I crystallizes in the NCS space group I4̅3d, we investigated the SHG property. Powder SHG measurements using 1064 nm radiation revealed a SHG efficiency of approximately 1 × α−SiO2 in the 45−63 μm particle size range.

Figure 7. Temperature dependence of the molar magnetic susceptibility (1 Fe), χM, of I (filled squares) measured in an applied field of 0.5 T. The temperature dependence of 1/χM (1 Fe) (empty squares) for I with a Curie−Weiss law fit from 100 to 300 K (shown in red). Inset, at top left shows the ZFC-FC plots of I at various applied field.

nature of the χM(T) plot between 4−300 K indicate paramagnetic behavior of the compound in that temperature range and a peak at 2.6 K indicate an onset of antiferromagnetic ordering below 2.6 K. Accordingly, the plot of thermal variation of inverse susceptibility, χM−1(T), shows linear behavior between 4−300 K, and the linear fit of the data in the range 150−300 K according to Curie−Weiss law yields a Curie constant of 3.77 emu K/mol and a Curie−Weiss constant (θp) of +1.74(1) K, respectively. The experimental average effective magnetic moment, μeff (experimental) observed per iron ion was 5.49 μB. This can be explained by taking into account the experimental magnetic moment of 5.45 μB per Fe2+ ion, and theoretical magnetic moment of Fe3+ as 5.92 μB [assuming that g = 2 and S = 5/2] and considering the presence of 8.25% of Fe3+ as determined from the Mössbauer study. The observed magnetic moment of 5.45 μB per Fe2+ ion is in good agreement with Fe2+ in high spin (S = 2) octahedral coordination, which is often higher than the spin-only value (4.9 μB) and ranges between 5.1−5.5 μB due to contribution from orbital magnetic moment.53 The small positive value of the Weiss constant reflects the weak ferromagnetic interactions between the Fe centers in the paramagnetic region. Accordingly, temperature variation of χMT shows a very sluggish increase in χMT upon cooling from 3.79 emu K mol−1 at 300 K to 3.87 emu K mol−1 at 40 K consistent with the small positive Weiss constant, after which χMT starts to decrease, and there is a sharp fall below 20 K to a value of 0.85 emu K mol−1 at 2 K (Figure 8). The low-temperature magnetic behavior was further studied by dc magnetic susceptibilities obtained in the field cooled (FC) and zero field cooled (ZFC) conditions (inset of Figure 7). The ZFC and FC magnetic susceptibilities measured at 10 and 50 Oe show irreversibility below 2.8 K and a clear peak at 2.6 K for the ZFC data indicating the onset of an antiferromagnetic ordering. Increasing the applied field to various higher fields

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CONCLUSIONS A combination of boric acid and phosphorous acid has been used as a unique medium to synthesize a non-centrosymmetric complex 3D mixed valent Fe-borophosphate-phosphite, Fe1.834IIFe0.166IIIB0.5[PO3(OH)]0.8(HPO3)2.033. This compound is the first member of a new family of solid that contains phosphite, phosphate and borate as part of the partial anionic structure and exhibit SHG efficiency similar to α-SiO2 in the 45−63 μm particle size range. The compound also serves as an example of complexities of phosphite−phosphate disorder and the ubiquity of redox reactions in HBO3−H3PO3 flux, which can be exploited as a route to produce new materials with complex mixed polyanions. With respect to structure and chemistry, this family of solids will offer many new possibilities where interplay of disordered phosphite−phosphate moieties, oxidation states of metal and their influence on crystallographic centricities will determine their future application, for example, in alkali ion batteries or in nonlinear optics. G



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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.cgd.5b01106. Experimental powder X-ray diffraction, and a view of the 3D structure along the c-axis (PDF) Accession Codes

CCDC 1437582 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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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 T.T.T. and P.S.H. thank the Welch Foundation (Grant E-1457) for support. A.C. thanks to UM Research board and Materials Research Center (MRC) of Missouri S&T for financial support. The authors gratefully acknowledge Dr. Vaclav Petricek’s insightful comments and evaluation of the structure solution, as well as for examining the presence of any diffuse scattering, twinning, and satellite peaks.

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REFERENCES

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A Cubic Non-Centrosymmetric Mixed-Valence Iron Borophosphate

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