Layered hydroxysalts derive their crystal structure from mineral brucite. Nickel and cobalt based hydroxysalts with the composition M3(OH)4(NO3)2 crystallize in trigonal crystal system. On careful examination of the PXRD patterns, it is found that Ni3(OH)4(NO3)2 crystallizes in hexagonal crystal system while Co3(OH)4(NO3)2 crystallizes in rhombohedral crystal system. This clearly demonstrates that the crystal structures in layered hydroxysalts are mainly dictated by the nature of the cation occupying the preferential site within the layer

Ramesh, T.N.

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Inorganic Chemistry Communications 12 (2009) 832–834Contents lists available at ScienceDirectInorganic Chemistry Communicationsjournal homepage: www.elsevier .com/locate / inocheObservation of polytypism among the hydroxysalts of Ni(II) and Co(II)T.N. Ramesh *Department of Chemistry, Central College, Bangalore University, Bangalore 560 001, Indiaa r t i c l e i n f o a b s t r a c tArticle history:Received 31 May 2009Accepted 29 June 2009Available online 3 July 2009Keywords:Layered compoundsChemical synthesisX-ray diffractionCrystal structurePolytypism1387-7003/$ - see front matter  2009 Elsevier B.V. Adoi:10.1016/j.inoche.2009.06.032* Tel.: +91 080 2296 2354.E-mail address: adityaramesh77@yahoo.comLayered hydroxysalts derive their crystal structure from mineral brucite. Nickel and cobalt based hydrox-ysalts with the composition M3(OH)4(NO3)2 crystallize in trigonal crystal system. On careful examinationof the PXRD patterns, it is found that Ni3(OH)4(NO3)2 crystallizes in hexagonal crystal system whileCo3(OH)4(NO3)2 crystallizes in rhombohedral crystal system. This clearly demonstrates that the crystalstructures in layered hydroxysalts are mainly dictated by the nature of the cation occupying the prefer-ential site within the layer. 2009 Elsevier B.V. All rights reserved.Layered metal hydroxides, layered hydroxysalts and layereddouble hydroxides have drawn considerable attention of materialschemists due to its potential application in catalysis, electrochem-istry, as precursors to nanoparticulate oxides, ion exchange reac-tions, etc. [1–6]. Divalent metal hydroxides, layered hydroxysaltsand layered double hydroxides derive their structure from brucite,a mineral form of Mg(OH)2 [7–9]. The structure of layered hydrox-ysalts comprise of a stacking of hydroxyl deﬁcient positivelycharged layers with the composition [M(OH)2x]x+ (M = metalion). The structure incorporates anions ðAn ¼ Cl; NO3 ; SO24 ;CO23 Þ in the interlayer region for charge neutrality with the result-ing composition [M(OH)2x](An)x/n]. The commonly observed val-ues of ‘x’ for hydroxysalts are 0.5, 0.67 and 1.0, respectively [10,11].The structure of hydroxyl-rich phases with compositionM3(OH)4A2, and M2(OH)3A crystallize in mineral brucite [12]. Incase of nickel, cadmium and magnesium based hydroxysalts, themetal ion occupies octahedral interstitial site, while in zinc and co-balt based hydroxysalts, the metal ion occupies both octahedraland tetrahedral interstices within the layer. The interlamellar dis-tance in these layered hydroxysalts are determined by the size ofthe interlayer anion and commonly varies from 6.9 to 9.2 Å (forMg, Ni, Co, Zn and Cd systems) [13,14]. We have recently reportedCo3(OH)4(NO3)2 phase in which all the Co2+ ions occupy octahedralsites within in brucite-like layers [15]. Rajamathi and Kamath havereported the basic salt of nickel with the composition Ni3(OH)4(NO3)2 [16]. In all the above compounds, the layers are stackedalong the c-crystallographic direction and depending on thearrangement of stacking of layers with respect to each other, differ-ll rights reserved.ent type of polytypes can be formed [17]. In contrast to the polyty-pism investigated in layered double hydroxides, layered metalchalcogenides and carbides, polytypism in layered hydroxysaltsis less studied [7,18]. In this paper we have examined the polyty-pism in layered hydroxysalts especially in nickel hydroxynitrateand cobalt hydroxynitrate.The basic salts of Ni(II) and Co(II) were synthesized by followingthe procedure reported in literature [6,19]. The urea hydrolysis ofNi(II) salt was carried out at 160 C and Co(II) salt at 150 C.The products were characterized by powder X-ray diffraction(PXRD) using a Philips powder X-ray diffractometer (Cu Ka source,k = 1.5418 Å). Data were collected at a scan rate of 2hmin1 with2h steps of 0.05. Infrared spectra were recorded using a NicoletModel Impact 400D FTIR spectrometer (KBr pellets, 4 cm1 resolu-tion). Thermogravimetry was carried out using a Mettler ToledoModel 851e TG/SDTA system (heating rate 10 C min1). All thesamples were heated to 800 C for 2 h and the weight loss was ac-counted. All the samples were subjected to wet chemical analysisas described elsewhere [15].DIFFaX simulations studies were carried out using DIFFaX code.The details about the PXRD simulations have been explicitly de-ﬁned in the literature [15].The PXRD pattern of the products obtained from nickel nitrateand cobalt nitrate melts are shown in Fig. 1a and c, respectively.The chemical composition of these two samples were found tobe Ni3(OH)4(NO3)2 and Co3(OH)4(NO3)2, respectively. Thermogravi-metric analysis of nickel hydroxynitrate and cobalt hydroxynitratesamples shows weight loss of 38.8% and 33.2%, respectively. Wetchemical analysis of cobalt hydroxynitrate sample shows theabsence of Co3+ content in the sample. Infrared spectra of nickelhydroxynitrate and cobalt hydroxynitrate samples shows sharpFig. 1. The PXRD patterns of (a) observed nickel hydroxynitrate, (b) DIFFaXsimulated nickel hydroxynitrate, (c) observed cobalt hydroxynitrate and (d) DIFFaXsimulated cobalt hydroxynitrate samples.Fig. 2. DIFFaX simulated PXRD patterns of (a) 1H, (b) 2H2, (c) 3R1 and (d) 3R2polytypes of nickel hydroxynitrate, respectively.T.N. Ramesh / Inorganic Chemistry Communications 12 (2009) 832–834 833band in the higher wave number region (3625 cm1and3606 cm1) due to stretching vibration of OH groups. A weak bandat 3300–3310 cm1 arises due to hydrogen bonded OH groups ofadsorbed water molecules. In the mid wave number (1000–1600 cm1) we observe strong absorption bands at m1:1310 cm1; m3: 990 cm1 and m4: 1503 cm1 and have been as-signed to the strongly bound nitrate group while the fourth (m2:1385 cm1) arises due to the interaction of the sample with theKBr in the pellet. The large splitting Dm = 193 cm1, between them4 and m1 vibrational modes of the nitrate ion in the infrared spec-trum of nickel hydroxynitrate reveals that one of the oxygen atomsof the nitrate group is directly coordinated to the metal ion [20,21].In case of hexagonal layered compounds, we observe Braggreﬂections due to (i) 00l planes; (ii) hk0 planes; (iii) h0l/0kl planesand (iv) hkl planes [22]. The PXRD pattern of both the samples dis-play two successive reﬂections at 6.98 Å and 3.46 Å, respectivelycould be indexed to 00l planes. The hk0 reﬂections provide infor-mation about the a-parameter of the samples. The metal to metaldistance (a-parameter) within the layer for nickel hydroxynitrateand cobalt hydroxynitrate are 3.13 Å and 3.18 Å, respectively. Thedifferences in these values arise due to the variation in the metalto oxygen bonding within the layer. If we carefully examine thePXRD patterns of these samples the peak positions in the mid-2hregion of 30–50 (2.7–1.8 Å) differ from each other. The reﬂectionsin the mid-2h could be indexed to h0l/0kl reﬂections and can beused as indicators to identify the speciﬁc stacking sequence of lay-ers with respective to each other. This provides direct evidencethat the two samples crystallize in two different polytypes [23].Polytypism in the case of layered double hydroxides is extensivelystudied [24,25]. The nature of the cations within the layer and theinterlayer anions dictate the stacking sequence of layers in layereddouble hydroxides [9,23]. To the best of our knowledge there areno reports on the polytypism in layered hydroxysalts. In order todifferentiate the polytypes in layered hydroxysalts, DIFFaX codewas used. It enables us to simulate the PXRD patterns of layeredstructure. In order to simulate the PXRD patterns we need to de-scribe the crystal structure in terms of stacking individual layerson top of each other. There are no reports of the single crystalstructure data of these compounds. In such instances, the atomicposition coordinates were taken from Ni(OH)2 and Co(OH)2 crystalstructures, respectively and the z-coordinates were modiﬁed in or-der to maintain the M–O bond distance [26]. We have describedthe overall structure of the layered compound in terms of the pack-ing of anions. We used the symbols A, B and C to represent hydro-xyl ion positions and symbols a, b and c to represent cationpositions. The metal hydroxide slab, [M(OH)2] can be representedby the symbol AbC or more simply as AC. Pure 1H polytype com-prises of a hexagonal stacking sequence of AC AC AC-layers. We fol-low the Bookin and Drits nomenclature to represent different typeof stacking sequences [27]. The number ‘1’ denotes the layeredperiodicity and ‘H/R’ stands for the hexagonal/rhombohedral stack-ing. Different type of polytypes was generated by using distinctstacking sequences as described in the literature [15]. Structureswith three-layer periodicity having the rhombohedral symmetrycan be generated by only two sequences:(i) AC CB BA AC—3R1; (ii) AC BA CB AC—3R2.The 3R1 polytype generates only prismatic interlayer sites,while the 3R2 generates only octahedral interlayer sites. The onlyway to distinguish one polytype from the other is to examine therelative intensities and the peak positions of different h0l/0klreﬂections. Similar observations have been reported in case oflayered double hydroxides [23]. Fig. 2 shows the simulated PXRDpatterns of 1H, 2H2, 2H3, 3R1 and 3R2 polytypes for nickel hydro-xynitrate. We have also simulated the PXRD patterns of 1H, 2H2,2H3, 3R1 and 3R2 polytypes for cobalt hydroxynitrate. The peakpositions in the DIFFaX simulated PXRD patterns of the 1H poly-type matches well with the observed PXRD pattern of nickelhydroxynitrate and could be indexed to 101, 102 and 103 reﬂec-tions in the mid-2h region (see Fig. 1a and b). The broadening ofh0l reﬂections in the nickel hydroxynitrate can arise due to thepresence of structural disorder [28]. The peak positions in the sim-ulated PXRD pattern for 3R2 polytype matches well with the ob-served PXRD pattern of cobalt hydroxynitrate. The reﬂectionswere indexed to 101, 104 and 107 planes (see Fig. 1c and d). Thesimulated powder X-ray diffraction patterns of 1H and 3R2 poly-typic modiﬁcations show similar peak positions in the low anglewhile the differences arises for h0l/0kl reﬂections. We have shownqualitatively that the peak positions and the relative intensities ofh0l/0kl reﬂections in the PXRD patterns of nickel hydroxynitrateand cobalt hydroxynitrate samples provides information aboutthe nature of polytypes. The misﬁt in the relative intensities ofthe peaks between the observed and simulated PXRD patternscould be due to the exclusion of the interlayer species during theDIFFaX simulations.In summary, polytype selectivity can be exercised by the choiceof the metal ion and the bonding between the metal-anion. For theﬁrst time, we have classiﬁed polytypes in layered hydroxysaltsusing PXRD simulation studies and conclude that nickel834 T.N. Ramesh / Inorganic Chemistry Communications 12 (2009) 832–834hydroxynitrate crystallizes in 1H polytype, while cobalt hydrox-ynitrate crystallizes in 3R2 type of polytype, respectively.AcknowledgmentsT.N.R. thanks the Council of Scientiﬁc and Industrial Research,GOI for the award of a Research Associate Fellowship and T.N.Rthanks P.V.K. for providing laboratory facilities to carry out re-search work.References[1] F. Cavani, F. Triﬁro, A. Vaccari, Catal. Today 11 (1991) 173.[2] P.V. Kamath, M. Dixit, L. Indira, A.K. Shukla, V.G. Kumar, N. Munichandraiah, J.Electrochem. Soc. 141 (1994) 2956.[3] S.P. Newman, W. Jones, New J. Chem. 105 (1998) 115.[4] M. Meyn, K. Beneke, G. Lagaly, Inorg. Chem. 32 (1993) 1209.[5] P. Rabu, S. Angelov, P. Legoll, M. Belaiche, M. Drillon, Inorg. Chem. 32 (1993)2463.[6] P. Gallezot, M. Prettre, Bull. Soc. Chim. France (1969) 407.[7] H.R. Oswald, R. Asper, Preparation and Crystal Growth of Materials withLayered Structures, vol. 1, D. Reidel Publishing Company, Dordrecht, 1977, p.71.[8] M. Louer, D. Louer, D. Grandjean, D. Weigel, Acta Crystallogr. B 29 (1973) 1696.[9] V.A. Drits, A.S. Bookin, in: Crystal Structure and X-ray Identiﬁcation of LayeredDouble Hydroxides, Nova Science, New York, 2001, p. 39.[10] A.K. Carrado, A. Kostapapas, S.L. Suib, Solid State Ion. 26 (1988) 77.[11] A.F. Wells, in: Structural Inorganic Chemistry, Oxford University Press, Oxford,1979.[12] K. Petrov, N. Zotov, E. Mirtcheva, O. Gracia-Martinez, R.M. Rojas, J. Mater.Chem. 4 (1994) 611.[13] L. Markov, K. Petrov, V. Petrov, Thermochim. Acta 106 (1986) 283.[14] K. Petrov, N. Zotov, O. Garcia-Martinez, R. Rojas, J. Solid State Chem. 101 (1992)145.[15] T.N. Ramesh, M. Rajamathi, P.V. Kamath, J. Solid State Chem. 179 (2006) 2386.[16] M. Rajamathi, P.V. Kamath, J. Power Sources 70 (1998) 118.[17] R. Prasad, O.N. Srivastava, J. Appl. Crystallogr. 4 (1971) 516.[18] A.R. Verma, P. Krishna, in: Polymorphism and Polytypism in Crystals, Wiley,New York, 1966.[19] M. Rajamathi, P.V. Kamath, Int. J. Inorg. Mater. 3 (2001) 901.[20] F. Portemer, A. Delahaye-Viadal, M. Figlarz, J. Electrochem. Soc. 139 (1992)671.[21] S.D. Ross, in: Inorganic Infrared and Raman Spectra, McGraw-Hill, London,1972, p. 2463.[22] T.N. Ramesh, R.S. Jayashree, P.V. Kamath, Clays Clay Miner. 51 (2003) 570.[23] S.P. Newman, W. Jones, P.O. Connor, D.N. Stamires, J. Mater. Chem. 12 (2002)153.[24] G.S. Thomas, M. Rajamathi, P.V. Kamath, Clays Clay Miner. 52 (2004) 693.[25] A.V. Radha, C. Shivakumara, P.V. Kamath, Clays Clay Miner. 53 (2005) 520.[26] C. Greaves, M.A. Thomas, Acta Cryst. B 42 (1986) 51.[27] A.S. Bookin, V.A. Drits, Clays Clay Miner. 41 (1993) 551.[28] C. Delmas, C. Tessier, J. Mater. Chem. 7 (1997) 1439.

Observation of polytypism among the hydroxysalts of Ni(II) and Co(II)

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Observation of polytypism among the hydroxysalts of Ni(II) and Co(II)

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