J. MATER. CHEM., 1991, 1(3), 461-468 46 1 Thermal Decomposition of Cobalt(i1) Acetate Tetrahydrate studied with Time-resolved Neutron Diffraction and Thermogravimetric Analysis Robin W. Grimes*a and Andrew N. Fitchb a Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London WlX 4BS, UK Department of Chemistry, University of Keele, Keele, Staffordshire ST5 5BG, UK The thermal decomposition of cobalt acetate tetrahydrate has been studied using time-resolved powder neutron diffraction. By using selectively deuterated samples, the loss of water or the breakdown of the acetate group can be identified by following the decrease in the incoherent background of the diffraction pattern as the hydrogen atoms are lost. The results suggest that by 150 "C dehydration is complete and a glass-like phase is formed.Crystallization of this anhydrous acetate occurs at 200 "C. Further heating initiates a two-stage decomposition of the anhydrous acetate terminated by the formation between 275-310 "C of a tetrahedrally co- ordinated cubic zinc blende form of COO. This transforms at 310°C to a rock-salt structure. The neutron diffraction data have been complemented by thermogravimetric and chemical analyses from which we have been able to propose some possible intermediate decomposition products and suggest an explanation for the formation of the unusual zinc blende form of COO. Keywords: Cobalt oxide; Cobalt acetate; Thermogravimetric analysis; Polymorph; Neutron diffraction 1.Introduction The thermal decomposition of cobalt acetate is of particular interest as it can lead to the formation of three polymorphs of COO. The most common form exhibits the rock-salt (sodium chloride) crystal structure (space group Fm3m). The two additional polymorphs have the cubic zinc blende (Fa34 and hexagonal wurtzite (P6,mc) structures.' With respect to co- ordination, the difference between the polymorphs is that in the zinc blende and wurtzite forms both oxygen and cobalt ions are tetrahedrally co-ordinated, whereas in the sodium chloride structure they are octahedrally co-ordinated. The zinc blende and wurtzite forms of COO can be synthe- sized via the decomposition of cobalt(I1) acetate, cobalt(I1) butanoate or cobalt@) ~ctanoate.'-~ In all cases, the decomposition proceeds either in ~)acuo~-~or under an inert atmosphere such as nitrogen'.* or argon.3 Particularly careful temperature control is necessary since the zinc blende and wurtzite structures are only formed in the range 290-310 "C, whereas the rock-salt material forms above 320 "C.After they are quenched to room temperature, the tetrahedrally co-ordinated oxides are completely stable in air. In the first study of COOpolymorphs, Redman and Stew- ard,' using X-ray diffraction, were able to identify zinc blende and wurtzite forms of COO as the decomposition products of cobalt(I1) acetate. In more recent work,, transmission electron microscopy (TEM) techniques were used to establish the morphology of the small zinc blende and wurtzite crystallites formed as the end products of acetate decomposition.The morphology of the polymorphs could be distinguished by TEM and the low-energy planes were identified in each case. For the cubic zinc blende material, which forms at the lowest temperature, the particles take on a characteristic 'tetrahedral' shape. The hexagonal wurtzite polytype, formed at slightly higher temperatures, exhibits 'lath'-shaped crystallites, one end of which is flat, the other pointed. The 'lath'-shaped crystallites were always associated with small clusters of the zinc blende polymorph. It was suggested that growth faults between the zinc blende crystallites would provide low-energy nucleation sites for wurtzite crystallites.The structural study was complemented by lattice energy calculation^,^ which were used to predict cohesive energies and relative permittivities for different cobalt oxide structures. It was found that the total lattice energy favoured the rock- salt structure over both the wurtzite and the zinc blende polymorphs. These theoretical and experimental observations suggest that details of the thermal decomposition process particular to carboxylates are responsible for the formation of a less stable tetrahedral co-ordinated form of COO. The thermal decomposition of anhydrous cobalt@) acetate has been investigated using thermogravimetric analysis.' Edwards and Hayward reported that the acetate decomposed to COO at ca. 350 "C.In addition to the thermal decompo- sition, these workers measured the diffuse visible spectrum of the anhydrous acetate and noted that it was typical of a material with an octahedral co-ordination of metal ions. This hypothesis was supported by measurements of the magnetic moment of the anhydrous material, which were characteristic of an octahedrally co-ordinated high-spin cobalt(I1) ion. Since on average there are only two acetate ligands per cobalt ion, to achieve octahedral co-ordination, the acetate bonding must be bidentate and is probably polymeric. The co-ordination of cobalt ions in cobalt(I1) acetate tetra- hydrate is also octahedral.6 However, in this material the cations are surrounded by four water molecules and by two oxygen ions from unidentate acetate groups. Hence, the way in which the acetate groups co-ordinate around cobalt ions in the acetate tetrahydrate must be different from in the anhydrous material. Cobalt(I1) acetate tetrahydrate has been studied using elec- tron ionization spectroscopy.' In addition, the mass spectrum of the basic form of cobalt(I1) acetate, tetracobalt(I1) hexa- acetate oxide Co,O(Ac),, has also been investigated.* In both cases, the majority gas-phase species were clearly +[Co,O(Ac),]+, [Co,O(Ac),] and the molecular ion [Co,O(Ac),] +.This was interpreted as indicating that the hexaacetate oxide is a stable gas-phase Although the structure of tetracobalt(I1) hexaacetate oxide has not been determined, the structure of the related material Zn,O(Ac), is well known.g It has an analogous structure to J.MATER. CHEM., 1991, VOL. 1 form of COO, a second CO(CH&O~)~.~H~Osample was heated in the neutron beam until just after the single peak was observed. From this temperature, the sample was quenched rapidly, thereby providing a method of synthesising the material, monitored in situ in the neutron beam. This material was subsequently analysed by powder X-ray diffrac- tion and the resulting pattern is shown in Fig. 2. Although the diffraction peaks are broad, presumably a consequence of the small particle size,4 the pattern was easily indexed and the calculated lattice parameter of 4.544(3)A is in good agreement with previous value^.'^^ Above 3 10 "C the diffrac- tion profile is consistent with a rock-salt COO structure.' Thus, the results presented in Fig.1 suggest that by 295 "C the starting material has decomposed to an oxide and that at 310 "C a phase transition occurs between the zinc blende and rock-salt oxide structures. 10000 -111 9000 -8000 -7000 -6000 v)c.5 5000 0 4000 3000 2000 1000 0 Fig. 3 shows a neutron diffraction profile from the sample in which the acetate ligands were deuterated but the water molecules were not. Since deuterium does not create the large incoherent scattering that hydrogen does, the diffracted inten- sity is now discernible above the background. Nevertheless, the background caused by the hydrogen of the water is still considerable and these results show quite clearly that between 100 and 155 "Cthere is a complete loss of water.Also, within this temperature range, as the water is lost, new diffraction peaks are observed in addition to those from the starting material. This suggests that an alternative hydrated acetate structure can be formed which presumably has less than four water molecules. We have noted that there is a considerable difference between the incoherent scattering profiles reported in Fig. 1 and those in Fig. 3; there is also a change in the coherently Fig. 2 X-Ray powder diffraction pattern of the zinc blende form of COO synthesised in the neutron beam and taken on a Philips PW 1050 diffractometer with Cu-Kcc radiation. The calculated lattice parameter is 4.544(3)A.The small peak at 44" is aluminium (200) from the sample plate h I I I I 1 I I 1 6oo 10 20 30 40 50 60 70 80 281" Fig.3 Neutron diffraction profiles of CO(CD,CO~)~*4H20decomposition as a function of heating diffracted profiles. This is due to the different scattering lengths of hydrogen and deuterium, which results in different relative intensities of the diffraction peaks but does not affect the angular distribution. If we now consider in detail the diffraction profiles between 190 and 295 "C it is apparent that a number of different anhydrous forms of cobalt acetate are formed before complete decomposition to the oxide. This is an important observation to which we shall refer in our discussion of subsequent data.We also note that the formation of the zinc blende form of COO can clearly be seen to begin at 270 "C although decompo- sition of the acetate is not complete until 295 "C. In addition, the increase in the diffraction intensity from the zinc blende phase occurs in parallel with a decrease in the diffraction intensity from the last and most well defined of the anhydrous acetate phases (we shall refer to this highest-temperature anhydrous phase as the HT anhydrous phase). As in Fig. 1, the zinc blende to rock-salt transition is observed to occur at 310 "C. In Fig.4 by considering the reduction in the background for the diffraction profiles of decomposing CO(CH,CO~)~ 4D20, we are able to investigate the temperature dependence of acetate loss.The results show that acetate loss begins at 230 "C and is complete by 295 "C as it was in Fig. 1. These temperatures also correspond to the range in stability of the HT anhydrous phase. More detailed analysis of Fig. 4 reveals that there is an initial slow loss of acetate which is accompanied by an increase in the diffraction intensity from the HT anhydrous phase. More rapid acetate loss occurs as the diffraction intensity of the HT anhydrous phase decreases, with zero intensity occurring at the same temperature as the minimum in the incoherent background. Thus, both the formation and the decomposition of the HT anhydrous phase seem to be associated with acetate loss. This observation would be consistent with the HT anhydrous phase being a product of acetate decomposition intermediate between CO(CH~CO~)~and COO.We shall test this hypothesis in sections 3 and 4. The last feature in Fig. 4 to which we wish to draw attention is the apparent lack of any sharp Bragg peaks at a temperature of ca. 170 "C. A similar situation occurs in Fig. 1 and 3 but it is obscured by adjacent data at higher and lower tempera- tures. The feature is more visible in Fig. 4 because of a sudden small drop in the incoherent background which occurs at 170 "C. This single isolated reduction in the background is a typical consequence of sample movement in the neutron beam J. MATER. CHEM., 1991, VOL. 1 and is not due to any physical or chemical change in the sample.The last sample (Fig. 5) has deuterated acetate and water ligands. Consequently, there is very little incoherent scattering and hence a very low background. This results in clear diffraction profiles. Therefore, having determined from the data presented in Fig. 1, 3 and 4 the temperatures at which the acetate and water ligands are lost, the detailed effect that such chemical changes have on the diffraction profiles can be realised in Fig. 5. To capitalise on this further, the data in Fig. 5 have been reformulated in such a way that the diffrac- tion profiles are viewed in the reverse direction to the previous figures, that is in order of descending temperature (ie. the highest temperature profile is now at the back of the plot).The data have also been partitioned into three overlapping temperature ranges: (a) 252-25 "C;(b) 295-167 "C; (c) 376-273 "C. Together these constitute Fig. 6. It is important to be aware that the diffraction intensities of each set of profiles in Fig. 6 have been normalised to the largest diffraction peak of each set. As such, the three sets of profiles (a),(b) and (c) no longer connect in an obvious way. However, this transform- ation has the advantage that more of the detailed diffraction data is discernible, particularly in the high-temperature pro- files which have lower absolute intensities. In describing the data of Fig. 6, we can also summarise the results of the neutron diffraction experiments as a whole. As heating proceeds, there is little change in the diffraction profile until 155 "C when the intensity drops almost to zero.The relatively small modulations in diffraction-peak heights that precede 155 "C might justifiably be attributed to movements of the material in the sample holder. The extra diffraction peaks that were noted during the decomposition of CO(CD~CO~)~*4H20 were absent with this last fully deuter- ated sample. Fig. 6(b)begins just after the water ligands have been lost and during the period in which there are no well defined diffraction peaks. The broad undulations of the diffraction intensity that are seen between 160 and 200 "C are typical of a material in a glassy phase. Beyond 200 "C, the broad profile transforms into a well defined crystalline pattern.This anhy- drous phase rapidly reduces in intensity as the HT anhydrous phase begins to appear at 250 "C and as acetate ligands start to devolve. The maximum intensity of the HT anhydrous phase occurs at 270 "C. In Fig. 6(c), at 273 "C, the diffraction intensity of the HT anhydrous phase is falling and the zinc blende form of COO I I I I I I I 1 €jo0 10 20 30 40 50 60 70 80 2810 Fig. 4 Neutron diffraction profiles of Co(CH,COJ2 *4Dz0decomposition as a function of heating J. MATER. CHEM., 1991, VOL. 1 Fig. 5 Neutron diffraction profiles of CO(CD,CO~)~*4D20decomposition as a function of heating is starting to appear. The maximum for the zinc blende structure occurs at 310 "C after which COO quickly takes on the rock-salt form. This result is supported by previous work1l4 and is consistent with a solid-state phase transition.We note that during the decomposition of all four differently deuterated samples, no neutron diffraction evidence was found for the formation of the wurtzite form of This is in fact not a particularly surprising result as the wurtzite form of COO is not always observed during decomposition. It has been suggested that wurtzite formation requires overlapping zinc blende structured crystallites4 and these may well not be present in sufficient density to permit the formation of wurtzite in the relatively short time before the rock-salt form of COO becomes overwhelmingly favoured. 3. Thermogravimetric Analysis Thermogravimetric analysis (TG) was carried out under flow- ing argon using a Stanton balance.While the sample was heated at 1 "C min-' up to 500 "C the weight was monitored continuously. As the final product of heating was COO, the total weight lost during the experiment implied that the starting material had a molecular weight of 279.3. This corresponds to a material with excess surface water or acetic acid equivalent to 1.35 water molecules per formula unit. The thermogravimetric data were also used to calculate the weight loss over 5 min intervals. This resulted in the difference thermogravimetric profile for cobalt@) acetate tetrahydrate decomposition shown in Fig. 7. From the diagram it is immediately apparent that the decomposition proceeds in two stages.The first weight loss (54% of the total) occurs between 20 and 150 "C. In the neutron diffraction study, the equivalent temperature range corresponds to the loss of the hydrogen associated with water ligands. The second decomposition stage occurred between 230 and 315 "C (46% of the total weight loss). By comparison with the neutron diffraction results this decrease in sample weight can be related to the loss of the acetate hydrogen. The important point that we can now make is that the weight changes determined from TG are consistent with the hypothesis that the loss of hydro- gen associated with either the water or the acetate implies the loss of the whole ligand. In addition the results suggest that if there is excess acetic acid left over from the sample prep- aration, this is lost with the water.Lastly, we note that decomposition temperatures determined from the thermograv- imetric and diffraction experiments disagree by k10 "C. This is expected since somewhat different temperature gradients will be in evidence across the samples in the different experi- mental set-ups. Between 150 and 230 "C the thermogravimetric data (see Fig. 7) show that the sample is stable to decomposition. However, at 200 "C the diffraction data suggest that the sample changes from being glass-like to crystalline. Therefore, we may conclude that the phase change at 200 "C is physical and not driven by a decomposition process. In Fig. 7, it can be seen that the decomposition of the anhydrous acetate (between 230 and 315 "C) occurs in two steps.The first stage between 230 and 270°C accounts for 28% of the acetate weight loss. The second process starts at 250 "C,overlapping slightly with the first, and finishes at 315 "C with the formation of COO. From a comparison with diffraction data, it would seem logical to suggest that the first stage of acetate loss results in the formation of the HT anhydrous phase (discussed in section 2.2 while commenting in Fig. 3-5). On the basis of previous data concerning the decomposition of cobalt(I1) a~etate,~we suggest that the formula of the intermediate decomposition HT anhydrous phase is CO~(CH&O~)~O Ctetracobalt(I1) hexaacetate oxide]. As such, we would expect its formation to occur with the loss of 25% of the total acetate weight.Although this is slightly lower than the experimental value for first-stage anhydrous acetate decomposition (28%) the discrepancy could easily be accounted for if just 4% of the hexaacetate oxide had decomposed to COO (see Table 1). This possibility is not inconsistent with diffraction results since in Fig. 6(c) we noted that the initial formation of COO occurred very early in the stability range of the HT anhydrous phase. Nevertheless, on the basis of these data alone we cannot rule out alternative explanations for the discrepancy. For example, the incorporation into the HT anhydrous phase of basic cobalt acetate fragments such as those observed in mass-spectroscopic studies.'** However, it is not clear if these fragments would be stable in the solid state.The thermogravimetric data in the temperature range 20- 150°C suggest that during the loss of water and/or excess acetate intermediate products are also formed. In section 2.2 we commented that extra peaks observed during the decompo- sition of CO(CD&O~)~ *4H20 might be associated with the formation of a secondary hydrous phase. If such a material did form, the thermogravimetric results would be consistent with the formation of CO(CH~CO~)~*2H20 at 100 "C. We note that an analogous phase is known for zinc.'0b However, on the basis of our results this suggestion must regarded as simply speculative. 4. Carbon/Hydrogen Analysis Additional support for our hexaacetate oxide model of anhy- drous acetate decomposition has been obtained through CHN analysis.The results presented in Table 1 are for a partially decomposed anhydrous acetate sample which was prepared in a tube furnace, under flowing argon, from cobalt(I1) acetate 295 2 16?\ 1 I 20 J. MATER. CHEM., 1991, VOL. 1 tetrahydrate heated at 1 "C min-' to 260 "C. By weighing the sample before and after CHN analysis a molecular weight of 143.3 was determined for the partially decomposed acetate. This value implies that the decomposition of the acetate has proceeded to 33%, that is slightly beyond the first stage of acetate loss. Therefore, from the diffraction results we can assume that some CO~(CH~CO~)~O has decomposed to COO.By fitting a combination of these two compounds to yield an average molecular weight of 143.3 the hypothetical carbon/ hydrogen content of the mixture can be calculated. The results of this process, presented in Table 1, show an excellent agree- I I I 1 I I 40 60 80 281" J. MATER. CHEM., 1991, VOL. 1 (c1 /_nT I I I I 80 I 20 I II 40 60 Fig. 6 Neutron diffraction profiles of Co(CD,C0,),*4D20 decomposition as a function of inverse heating. (a) 252-25 "C;(b)295-167 "Cwhere G-L is the glass-like phase and HT is the HT anhydrous phase; (c) 376-273 "C where ZB is the zinc blende COO phase and RS is the rock- salt COOphase Tl°C 20 50 100 150 200 250 300 350 I I I I I I I I 80 7 60 .-c E \F rn' E 40 CJ).-P 20 I\.-._._.-.:-.-.-.-..-.--: I I '.\. I I 1 I I 1 \I0 0 30 60 90 120 150 180 210 240 270 300 330 Fig. 7 Difference thermogravimetric profile determined for the decomposition of cobalt(1x) acetate tetrahydrate under flowing argon at a heating rate of 1 "Cmin-'. (a) Loss of water; (b)loss of acetate ment with the experimental CHN analysis. It is gratifying intermediate decomposition products for cobalt@) acetate that such a simple model is consistent with diffraction, thermo- tetrahydrate. Initially, water and/or excess acetate is lost gravimetric and CHN results. resulting in a glassy-phase anhydrous acetate. A glassy phase is able to form since, as explained in the Introduction, there 5.Summary is a complete change in the co-ordination of the cobalt ion as a consequence of dehydration. When it is heated further, From the complementary use of diffraction and thermogravi- the glassy phase first crystallises and then begins to decom- metric techniques we have been able to identify a number of pose. We suggest that the anhydrous acetate decomposition J. MATER. CHEM., 1991, VOL. 1 Table 1 Analysis of hexaacetate oxides as partial decomposition products of anhydrous cobalt acetate C/H analysis Co(Ac),Co,(Ac),Omodel fitted to a weight loss of 28% 0.96 Co,(Ac),O+0.16 COO experimental values for a partially decomposed acetate sample model fitted to a molecular weight of 143.4 0.89 Co4(Ac),0+0.42 COO occurs via CO~(CH~CO~)~Oand that this is a stable, crystal- line, intermediate.This result is consistent with diffraction, thermogravimetric and chemical analyses. Further heating to 290 "C results in the total decomposition of the oxyacetate to form the zinc blende form of COO.This unusual form of COO transforms at 310 "C to the rock-salt polymorph. Previous theoretical calculations4 suggest that the rock-salt form of COO is more stable than the zinc blende. We wish, on the basis of our present observations, to offer one possible explanation for the formation of the zinc blende polymorph. By inference from experimental structures of similar hexaacet- ate oxides, it would seem that our basic acetate intermediate decomposition product is composed of structural units of CO~(CH~CO~)~Oin which the four cobalt ions tetrahedrally co-ordinate a central oxygen ion and are themselves tetra- hedrally co-ordinated by oxygen.During the final decompo- sition, this structure might provide a stable nucleus which would lead to a tetrahedrally co-ordinated zinc blende form of COO rather than necessitate a complete change in co- ordination in forming the octahedrally co-ordinated rock-salt structure. We are currently attempting to determine the structure of the intermediate decomposition product by using powder X-ray diffraction techniques. molecular wt. per Co ion C (%) 177.0 27.1 151.5 23.8 148.4 23.3 143.3& 0.2 22.2& 0.3 143.4 22.4 H (YO) weight loss (YO) 3.4 0 3.0 25 2.9 28 2.9& 0.3 33 2.8 33 We thank the Institut Laue-Langevin for the provision of the neutron beam time on DlB, the Department of Chemistry, University of Keele for the CHN analysis and Richard Catlow for useful discussions. References 1 M.J. Redman and E. G. Steward, Nature, (London) 1962, 193, 867. 2 M. J. Redman, Ph.D. Thesis, Royal Society of Chemistry, 1961 (available at City University Library, London, UK). 3 A. N. Fitch, R. W. Grimes and C. R. A. Catlow, I.LL Report 88CA03G, 1987, p. 149. 4 R. W. Grimes and K. P. D. Lagerlof, J. Am. Ceram. SOC., 1991, 74, 270. 5 D. A, Edwards and R. N. Hayward, Can. J. Chem., 1968, 46, 3443. 6 J. N. van Niekerk and F. R. L. Schoening, Acta Crystallogr., 1953, 6, 609. 7 G. C. DiDonato and K. L. Busch, Inorg. Chem., 1986, 25, 1551. 8 J. Charalambous, R. G. Copperthwaite, S. W. Jeffs and D. E. Shaw, Inorg. Chem. Acta, 1975, 14, 53. 9 H. Koyama and Y. Saito, Bull. Chem. SOC.Jpn., 1954,27, 112. 10 R. W. G. Wykoff, Crystal Structures, John Wiley, New York, 2nd edn., 1968, vol. 5, (a)p. 336, (b)p. 329. Paper 1/00661 D; Received 12th February, 1991