Communication pubs.acs.org/IC
Topochemical Oxidation of Perovskite KCoF3 to a K2PtCl6 StructureType Oxyfluoride Rajamani Nagarajan,* Shahzad Ahmad, and Poonam Singh Materials Chemistry Group, Department of Chemistry, University of Delhi, Delhi 110007, India S Supporting Information *
Anion-deficient oxyfluoride K2CoO2−xF4 (x = 1) with a K2PtCl6type structure has been identified as the product from this reaction. Synthesis and characterization details are described in the Supporting Information. The transformation of cubic perovskite KCoF3 to a product possessing K2PtCl6-type structure is exemplified by comparing the powder X-ray diffraction (PXRD) pattern before and after its reaction with H2O2 (Figure 1a). The pale-pink color of KCoF3
ABSTRACT: Oxyfluoride, K2CoOF4 with K2PtCl6 structure, has been stabilized from a perovskite-based KCoF3 complex by a simple oxidative anion insertion topochemical reaction. Similar structural transformations have been observed for Ni- and Mn-containing systems. The generation of such unusual compounds, by soft chemical methods, strengthens the efforts toward materials discovery. aterials discovery was most often directed toward finding solutions to problems of energy conversion, storage and transmission, cleaner environment, and health.1 If such a study generates novel compounds with metals in unusual oxidation states, their properties will enhance and expand the chemical understanding of such elements and their compounds. Interest in high oxidation states of transition metals is not only of fundamental academic importance but also for industries because they find extensive use as oxidants, catalysts, and luminescent materials.2 The condensed-phase syntheses of fluorides containing transition-metal ions (especially the first row) in higher oxidation states are quite complex and challenging. While the hightemperature reactions usually employed in ceramic preparations often result in a thermodynamically stable product, lowtemperature topochemical reaction schemes have been demonstrated to produce unusual and kinetically stable products.3 Utilizing a mobile cation or anion, one can design and generate products by such topochemical reactions. A majority of the studies on the synthesis of oxyfluorides are dealing with the reaction of either metal oxides with metal fluorides or metal oxides with a fluorinating agent.4 A reverse strategy using a single-source mixed-metal fluoride precursor for the oxidative insertion of anions is uncommon. Our own experience with the oxidation of KZnF3 and KSnF3 with H2O2 has given rise to two important conclusions.5 In the case of KZnF3, F-doped ZnO2 resulted, implying oxidation of the ligand, viz., oxide to peroxide.5a On the other hand, heavily F-ion-doped (21.2%) SnO2 along with the simultaneous change in the oxidation of the central metal (Sn2+ to Sn4+) occurred upon reaction of KSnF3 with H2O2.5b These two results suggest the possibility of the formation of oxyfluorides from KMF3 (a coordination complex, dissociating into K+ with MF3− units in suitable solvents6) under suitable oxidizing conditions if the central metal ion can exist in variable oxidation states. With this motivation, oxyfluoride synthesis has been attempted by the one-step reaction of perovskite-structured KCoF3 with a H2O2/methanol mixture.
M
© XXXX American Chemical Society
Figure 1. (a) PXRD patterns of KCoF3 (1) and the product from the oxidation of KCoF3 with H2O2 (2). (a) Digital pictures of the sample before and after oxidation. Core-level spectra of (b) Co 2p and (c) K 2p of the oxidized sample from KCoF3. Insets: Core-level spectra of O 1s and F 1s in parts b and c, respectively. (d) FTIR spectrum along with the Raman spectrum (inset) of the oxidized sample.
changes to orangish-brown, suggestive of a change in the oxidation state of the Co ion. Qualitative and quantitative estimations of the elemental composition of the sample have been realized by the combined use of inductively coupled plasma (ICP), X-ray photoelectron spectroscopy (XPS), wet-chemical, and F-ion selective electrode techniques. From the ICP analysis, 27.58 wt % K and 17.93 wt % Co are found to be present in the sample. A survey XPS spectrum is presented in Figure S1 (Supporting Information), from which the presence of K, Co, O, and F are Received: July 30, 2015
A
DOI: 10.1021/acs.inorgchem.5b01694 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry deduced. The core-level spectrum of Co 2p consists of two main bands at 784.0 and 801.0 eV (2p1/2 and 2p3/2 separated by 17 eV; Figure 1b). These features match closely with the XPS data reported for Co4+-containing SrCoO3 in which a similar higher binding energy (BE) is observed.7a The slight deviation in the BE values in our case may be due to a change in the electronegative environment due to the presence of F around Co. Additionally, two satellite peaks are observed at positions of 6 eV higher than the main bands. The separation between the main bands of Co 2p3/2 and 2p1/2 has been related to the possible spin state of the Co ion. The separation of 17 eV between these two main peaks indicates a higher number of unpaired electrons for Co than that in paramagnetic Co2+ complexes (16 eV) and diamagnetic Co3+ (15 eV) complexes.7b It is a well-documented fact that the highspin Co ion exhibits strong satellite peaks at higher BE (5−6 eV) from the main peaks. Dupin et al.7c have claimed that the observance of satellite peaks at 5 eV is mainly due to the existence of Co in the 4+ oxidation state. In fact, for low-spin Co cases, only a main peak with little evidence of a satellite peak on the higher energy side (typically 9 eV) is observed routinely.7d In the corelevel spectra of F 1s and K 2p3/2, the peaks are observed at positions of relatively higher BEs of 691.8 and 297.8 eV, respectively (Figure 1c). This indicates the presence of these ions with the transition-metal ion in the higher oxidation state. In addition to these elements, two bands are observed at 532.5 and 535.2 eV in the core-level spectrum of O 1s. These are slightly higher than those observed for Co2+- and Co3+-containing oxides and match with the values reported for O 1s (∼535 eV) in SrCoO3, in which Co exists in 4+ oxidation state.7a The F-ion content, using ion-selective electrode experiments, suggests a reduction in the F content from the expected value of 6 (based on K2MF6 stoichiometry). Further, when the Rietveld refinement of the PXRD pattern was carried out using GSAS software8 considering 24e, 4a, and 8c sites being exclusively occupied by F, Co, and K ions, respectively, the cubic K2CoF6 [space group Fm3̅m (No. 225)] structure did not result in a satisfactory fitting (Supporting Information, Figure S2). Hence, in the subsequent refinement cycles, the presence of O along with F at the 24e site is considered while the F stoichiometry is fixed at 4 (corresponding to the determined amount of 34 ± 0.8 wt % from ion-selective electrode experiments). The occupancy of O alone was then verified by keeping the occupancy (0.6666) and thermal parameters of F (0.025), along with the thermal parameter of O, constant (Uiso fixed at 0.025). The occupancy of O was then found to be 0.1670, indicating anion vacancies. Further attempts to vary the amount of O have led to unstable refinements. After taking these facts into consideration, satisfactory fit was found between the experimental and theoretical X-ray diffraction patterns (Figure 2), with a cubic lattice constant of a = 8.1199 (3) Å. The crystal data, structure refinement parameters, and position parameters are summarized in Tables S1 and S2 (Supporting Information). Bands observed at around 482 and 744 cm−1 in the Fourier transform infrared (FTIR) spectrum of our sample are the fingerprint bands of the K2PtCl6 structure (Figure 1d).9a Broad bands near 580 and 1065 cm−1 signify the Co−O vibration modes.9b,c In the Raman spectrum, bands at 123, 408, 478, and 656 cm−1 are observed. The band at 478 cm−1 is relatively broader, suggesting the overlap of two or more bands. An expanded plot of the Raman spectrum (100−620 cm−1) is shown in Figure S3 (Supporting Information). Deconvolution of the band near 478 cm−1 reveals the presence of two bands at 468 and 480 cm−1. Additionally, a weak band is also present at 540 cm−1. While the bands at 408, 468, and 656 cm−1 represent the
Figure 2. Rietveld refinement of the PXRD pattern of oxidized product from KCoF3. Observed, calculated (profile matching), and difference profiles are given respectively as red, green, and pink lines and the Bragg positions as black vertical lines. Inset: crystal structure of the oxidized product.
fingerprint bands of the K2PtCl6 structure type, the bands at 123, 480, and 540 cm−1 are attributed to Co−O vibrational modes.9d−f It is worth noting that A2CoF6 (A = K, Rb, Cs) complexes possessing K2PtCl6-type structure are obtained by the highpressure fluorination of A2CoCl4 (A = Rb, Cs) or by the reaction of KF and CoF2 under high fluorine pressures (up to 250 bar).10 They are pale-orange solids and highly hygroscopic in nature. In contrast, K2CoOF4 is orangish-brown, showing no appreciable mass loss in the thermogravimetric trace until 300 °C (Supporting Information, Figure S4). This observation negates the presence of water and suggests a change in the ligand composition in our sample. In the UV−visible diffuse-reflectance spectrum, an intense band near 250 nm, followed by a broad absorption in the 400− 1100 nm range, is observed (Figure 3a). While the band in the UV region signifies the ligand-to-metal charge transfer (LMCT), d−d transitions appear in the visible region. The shift in the charge-transfer band position from 270 nm for the A2CoF6 (A = Rb, Cs) to 248 nm in our system unequivocally suggests
Figure 3. (a) UV−visible absorption spectrum. (b) Deconvoluted emission spectrum at λex = 350 nm. (c) Plot of χM versus temperature of K2CoOF4 at an applied field of 5000 Oe. The inset in part c shows the fitting of the magnetic data to extract the Weiss constant. B
DOI: 10.1021/acs.inorgchem.5b01694 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry modulation of the ligand-field strength caused by the presence of O in addition to F.11 Considering a high-spin Co4+ (d5 system) in our case, all transitions from the ground 6S state to the excited quartet ligand-field 4G state are, in principle, forbidden according to the spin and Laporte selection rules. Statistical disordering of F and O atoms and vacancies occupying (with equal probability) all of the vertices of the hexacoordinated polyhedron may lead to irregular vibronic coupling. Such an arrangement may be facilitating the magnetic coupling of electronic spins of nextnearest-neighbor atoms.12 Additionally, the Laporte selection rule can be relaxed by bonding with O 2p orbitals through the 3d orbital of the magnetically coupled Co pair.11 Hence, the intense peak observed at 248 nm in the UV−visible absorption spectrum is attributed to LMCT via t1g(π) → t2g(π*)/eg(σ*) transitions, and the broad band is attributed to magnetically coupled Co4+ (6A1)−Co4+ (6A1) pairs simultaneously excited to a quartet ligand-field 4G state.13 In the photoluminescence spectrum, emission peaks at 412, 440, and 467 nm are observed for K2CoOF4 corresponding to 4G (4E, 4T2, 4T1) → 6S (6A1) transitions of Co4+ (Figure 3b).14 A magnetization measurement of the sample down to 4 K at an applied field of 5000 Oe is presented in Figure 3c. Variation of the magnetic susceptibility with temperature above 100 K obeys the Curie−Weiss law, yielding θ of 75 K, and suggests ferromagnetic interaction between CoIV ions. At room temperature (300 K), the molar magnetic susceptibility of K2CoOF4 is 10.86 × 10−3 emu·Oe−1·mol−1, giving a magnetic moment of 5.08 μB. In the literature, a debate still exists to provide a satisfactory explanation of the observed magnetic behavior of A2CoF6 (A = Rb, Cs). The existence of Co4+ in a low spin−high spin equilibrium or the presence of a magnetic impurity were cited as reasons for the anomalous magnetic behavior of these systems.10 We believe that the presence of a small amount of magnetic impurity in our preparation may be lowering the effective magnetic moment from the spin-only value of 5.92 μB (Co4+ d5; Figure 3c). Neutron diffraction experiments on these samples are being planned to determine the anion composition accurately and to comprehend its effect on the magnetic behavior. To substantiate our proposition that transformation of perovskite fluorides to the K2PtCl6 structure is feasible for central metal ions having variable oxidation states, a reaction of KNiF3 and KMnF3 with H2O2 under identical conditions are carried out. PXRD patterns, FTIR, and Raman spectra of the products bear a close resemblance to the data obtained for K2CoOF4 (Supporting Information, Figures S5 and S6). Because perovskite is a prototype system for understanding many physical phenomena, its transformation to a K2PtCl6-type structure opens up the possibility for generating new compounds and new chemistry essential for materials discovery. Also, the synthetic barrier, such as the requirement of hazardous HF or F2 gas, intricate setups, and high-pressure conditions, existing for the realization of oxyfluorides have been crossed.4 Uncovering new phenomena existing in these compounds can be the natural consequence, thus increasing their technological capabilities. Caution! As the reactions involve the use of strong oxidizers, extreme care must be taken to perform in a f ume hood.
■
■
Details of synthesis and characterization, survey XPS spectrum, Rietveld refinement trial, expanded portion of the Raman spectrum with deconvolution, thermogravimetric trace and crystallographic data of K2CoOF4 including the CIF, PXRD patterns of oxidized products from KMF3 (M = Mn, Ni), and their FTIR, Raman spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors thank the University of Delhi, DST (SB/S1/PC-08/ 2012), DU-DST PURSE Grant, for funding this research. S.A. and P.S. thank the UGC, Government of India, for D. S. Kothari and SRF fellowships, respectively. Useful discussions with Professors Uma and Pavan Mathur, Department of Chemistry, are gratefully acknowledged.
■
REFERENCES
(1) Shoemaker, D. P.; Hu, Y.- J.; Chung, D. Y.; Halder, G. J.; Chupas, P. J.; Soderholm, L.; Mitchell, J. F.; Kanatzidis, M. G. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 10922−10927. (2) Lv, L.; Chen, Z.; Liu, G.; Huang, S.; Pan, Y. J. Mater. Chem. C 2015, 3, 1935−1941. (3) Hayward, M. A. Semicond. Sci. Technol. 2014, 29, 064010. (4) (a) Leblanc, M.; Maisonneuve, V.; Tressaud, A. Chem. Rev. 2015, 115, 1191−1254. (b) Tsujimoto, Y.; Yamamura, K.; Muromachi, E. T. Appl. Sci. 2012, 2, 206−219. (5) (a) Ahmad, S.; Kharkwal, M.; Govind; Nagarajan, R. J. Phys. Chem. C 2011, 115 (20), 10131−10139. (b) Kumar, V.; Govind, A.; Nagarajan, R. Inorg. Chem. 2011, 50, 5637−5645. (6) Muller, B. G. Angew. Chem., Int. Ed. Engl. 1987, 26, 1081−1097. (7) (a) Saitoh, T.; Mizokawa, T.; Fujimori, A.; Takeda, Y.; Takano, M. Surf. Sci. Spectra 1999, 6, 302−312. (b) Frost, D. C.; McDowell, C. A.; Woolsey, I. S. Chem. Phys. Lett. 1972, 17, 320−323. (c) Dupin, J. C.; Gonbeau, D.; Benqlilou-Moudden, H.; Vinatier, Ph.; Levasseur, A. Thin Solid Films 2001, 384, 23−32. (d) Shivakumara, C.; Hegde, M. S. Proc. Indian Acad. Sci., Chem. Sci. 2003, 115, 447−457. (8) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS); Los Alamos National Laboratory Report LAUR 86-748;Los Alamos National Laboratory: Los Alamos, NM, 2004. (9) (a) Badachhape, R. B.; Hunter, G.; Mccory, L. D.; Margrave, J. L. Inorg. Chem. 1966, 5, 929−931. (b) Busca, G.; Guidetti, R.; Lorenzelli, V. J. Chem. Soc., Faraday Trans. 1990, 86, 989−994. (c) Chertihin, G. V.; Citra, A.; Andrews, L.; Bauschlicher, C. W., Jr J. Phys. Chem. A 1997, 101, 8793−8802. (d) Milne, J. B.; Moffett, D. M. Inorg. Chem. 1976, 15, 2165−2169. (e) Martens, W.; Frost, R. L.; Williams, P. A. J. Raman Spectrosc. 2003, 34, 104−111. (f) Xia, H.; Wan, Y.; Assenmacher, W.; Mader, W.; Yuan, G.; Lu, L. NPG Asia Mater. 2014, 6, e126. (10) (a) Quail, J. W.; Rivett, G. A. Can. J. Chem. 1972, 50, 2447−2450. (b) Grannec, J.; Sorbe, P.; Portier, J. C. R. Seances Acad. Sci., Ser. C 1976, 283, 441−444. (11) (a) Brisdon, A. K.; Holloway, J. H.; Hope, E. G.; Townson, P. J.; Levason, W.; Ogden, J. S. J. Chem. Soc., Dalton Trans. 1991, 3127−3132. (b) Allen, G. C.; Warren, K. D. Inorg. Chem. 1969, 8, 1902−1904. (12) Borghi, E.; Occhiuzzi, M.; Foresti, E.; Lesci, I. G.; Roveri, N. Phys. Chem. Chem. Phys. 2010, 12, 227−238. (13) He, Y. P.; Miao, Y. M.; Li, C. R.; Wang, S. Q.; Cao, L.; Xie, S. S.; Yang, G. Z.; Zou, B. S.; Burda, C. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 1−9. (14) Lever, A. B. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier: Amsterdam, The Netherlands, 1984.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01694. C
DOI: 10.1021/acs.inorgchem.5b01694 Inorg. Chem. XXXX, XXX, XXX−XXX
