The pyrosynthesis of strontium zincate

J. inorg,nucl.Chem., 1966,Vol.28, pp. 2801to 2809. Pm'gamonPrmsLtd. Printedin NorthernIreland

THE PYROSYNTHESIS OF STRONTIUM ZINCATE H. G. MCADIE Department of Physical Chemistry, Ontario Research Foundation, Toronto 5, Ontario, Canada

(Received 1 March 1966; in revised form 31 May 1966) Al~traet--As the preliminary stages in the pyrosynthesis of strontium zincate, the thermal decompositions of strontium acetate hemihydrate and of zinc acetate dihydrate have been examined. Decomposition occurs following fusion of the anhydrous salts, liberating the organic residue and producing strontium carbonate and zinc oxide respectively. Evidence was obtained for the limited formation of zinc oxyacetate prior to the fusion of anhydrous zinc acetate, and the thermal decomposition of this salt has also been studied. The pyrosynthesis of strontium fine,ate from an equimolar mixture of strontium carbonate and zinc oxide, under 1 atm of carbon dioxide, is suggested to occur by crystallization from a melt of strontium oxide and zinc oxide which results following decomposition of the strontium carbonate above 1250°C. INTRODUCTION

STRONTIUMzincate was first prepared ~1)by the thermal decomposition of a mixture of strontium carbonate and zinc oxide under nitrogen at 1000°C, and later ~2) by the pyrolytic decomposition of a mixture of strontium and zinc acetates, eventually heating to 1150°C under oxygen. The crystal structure of strontium zincate C2--4) consists of chains of ZnO2~ tetrahedra which form a series of waved layers with Sr2+ located interstitially between these layers. Two crystalline modifications exist, designated 0c- and/~.~2), having basically the same structure but sufficient differences in the axial ratio b/a to produce distinctions between the X-ray diffraction patterns at 20 > 80°. The ~-modification is found on slow cooling from about 1150°C, whereas the metastable fl-modification appears on rapid cooling and slowly reverts to the ~-modification at room temperature. The relatively minor differences between the two modifications are not of significance in the present study, which is concerned with investigating the mechanism of the pyrosynthesis of strontium zincate from a mixture of strontium and zinc acetates. EXPERIMENTAL AR grade materials were used throughout this work without further purification. Debye-Scherrer powder patterns were obtained using CuK~ radiation. Simultaneous DTA-TGA experiments were carried out on undiluted samples of 25 or 100 mg initial weight, using a Mettler Recording Vacuum Thermoanalyser. The heating rates and sample atmosphere employed are indicated on the individual figures. Studies of the acetate decompositions by DTA were also carried out on samples diluted to 20 per cent by weight in ignited ~-alumina under conditions of a dynamic nitrogen atmosphere. The sample holder used for this purpose and general features of the equipment have been described elsewhere.~s~ High-tmnperature DTA studies of the SrO-ZnO system were carded out with an R. L. Stone Company Model KA-W apparatus using platinum cup sample holders. tx~ R. ~2~H. c8~R. c4~H. ~5~H.

Scxaou~R, Angew. Chem. 66, 461 (1954). G. SCHNImmOand R. HOl,eE, Z. anorg, allg. Chem. 312, 87 (1961). Hot,PE, Angew. Chem. 71, 457 (1959). G. SCrINmtrNQand R. HoPPE, Naturwissenschaften 47, 467 (1960). G. McADm, Can. J. Chem. 44, 1373 (1966). 2801

2802

H.G. RESULTS

AND

McADm DISCUSSION

Decomposition of strontium acetate hemihydrate The simultaneous DTA-TGA of strontium acetate hemihydrate is shown in Fig. 1 together with the DTA of the diluted sample under dynamic atmosphere conditions. Sr(CH3COO) a . ~taHzO CRUCIBLE TYPE S A M P L E HOLDER NITROGEN A T M O S P H E R E HEATING R A T E 4 ° C . m l n "1

Sr(CH3CO0) 2 . ~ H20

WEIGHT LOSS : FOUND 4 1 5 ~ THEORETICAL 419 Sr(CH3COO) 2

T

I I00

I 300

2o0

400 \

//

500

Id

_t2 . o

cc u_ WEIGHT LOSS : FOUND 28.21% THEORETICAL 28.23%

Sr(CH~,COO) 2 . ~2 HzO DYNAMIC NITROGEN ATMOSPHERE 2 0 % SAMPLE IN ALUMINA HEATING RATE 7 " C . m i ~ = *or

,oo

2oovv

3ooV

TEMPERATURE

FIG.

.

......

1

('C)

1.

Dehydration commences at about 180°C and proceeds in a single step, as far as weight loss is concerned; however, the loss of 0.5 mole of water appears to be accompanied by two endothermal processes. In the dehydration of polyhydrates it is common to find stepwise loss of water, the resolution of these steps being controlled by the prevailing ambient water vapour pressure. However, in the present work on strontium acetate hemihydrate, atmospheric conditions are such as to maintain extremely low ambient water vapour pressures, and it is therefore unlikely that the doublet endotherm represents separation of two modes of bonding of water molecules within the strontium acetate lattice on the basis of different dissociation pressures. Unfortunately, the

The pyrosynthesis of strontium zineate

2803

lattice structures of strontium acetate hemihydrate and the corresponding anhydrous acetate required for further interpretation have not been elucidated, only the powder patterns being available3 e) RVaINSH~IN and IAKERSON(7) have reported an initial slow loss of about two-thirds of the water of hydration followed by a rapid expulsion of the remainder at about 200°C. Consistent with this, DTA thermograms of samples diluted in alumina show a broad endotherm followed by a second, sharp endotherm. However the present TGA data do not support the concept of a two-stage type of water loss, there being no evidence of an inflection point on the weight-loss curves. Present data suggest that the hemihydrate lattice may undergo some form of general reorientation before dehydration can proceed at a marked rate. The endothermal fusion of the anhydrous strontium acetate occurs at 321 ° :k I°C (peak temperature), with no other enthalpic or weight changes being observed between dehydration and fusion. Based on the ratio of the areas under the dehydration and fusion endotherms, the heat of fusion of strontium acetate is estimated to be 4.0 -4- 0.5 kcal mole -1. Following fusion weight loss again commences at about 350°C. In the undiluted DTA-TGA sample rapid endothermal decomposition commences at 430°C, liberating the constituents of acetone, and yielding rhombic strontium carbonate as the ultimate product above 500°C. The liberation of acetone is endothermal initially, however thermal degradation of the liberated organic fragment gives rise to considerable exothermal activity superimposed upon the decomposition process. It was observed that this exothermal activity commenced about 80° lower when the sample was diluted in alumina, and this is ascribed to catalysis of the organic decomposition by the hot alumina surface. Increasing the dynamic atmosphere flow rate 5-10 fold eliminated most of this latter exothermal activity. Decomposition of strontium acetate hemihydrate is thus suggested to follow the process: Sr(CH3COO) ~ • ½ HgO (s) --~ Sr(CH3COO)~ (s) -+ Sr(CH3COO) 2 (1) ~ SrCO 3 (s) + + ½ H20 (g) CH3COCH3 (g) Decomposition o f zinc acetate dihydrate

Studies of the thermal decomposition of zinc acetate dihydrate have been reported by isothermal ca) and dynamic tg) methods, indicating this decomposition to be somewhat more complex than that of strontium acetate hemihydrate. Figure 2 shows the simultaneous DTA-TGA of undiluted zinc acetate dihydrate. The loss of two molecules of water occurs in a single step under a dry nitrogen atmosphere, giving a weight loss in good agreement with the theoretical, and the X-ray diffraction patterns of samples taken before and after this weight loss correspond to the dihydrate
2804

H . G . McAl)m

Zn (CH3COO~z • 2H20 CRUCIBLE TYPE SAMPLE HOLDER NITROGEN ATMOSPHERE HEATING RATE 4° C. rnin.-'

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I 200 TEMPERATURE

I

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300 (" C )

FIG. 2.

of the anhydrous salt. Evidence was obtained from other DTA experiments not illustrated here that the dehydration of zinc acetate dihydrate will occur with the intermediate production of the monohydrate under appropriate ambient water vapour pressures. The anhydrous zinc acetate continues to lose weight graduatly following dehydration and this is ascribed to slow sublimation of the sample. The rate of weight loss begins to increase noticeably above 180°C, without an accompanying enthalpic effect being detected in either undiluted (Fig. 2) or diluted (Fig. 3) samples. In this temperature range visual microscopy shows the crystals of anhydrous zinc acetate to swell, sublime, and the sublimate to appear as octahedra characteristic of zinc oxyacetate.(m This formation of the oxyacetate has been reported c13.m to occur by the process: 4Zn(CHaCOO)= (s) --~ Zn40(CHaCOO)6 (s) + (CHaCO)~O (g), u=) H. KOYAMAand Y. SAITO, Bull. chem. Soc. Japan 27, 112 (1954). u=) V. AUGER and I. RoBt,, C.r. hebd. Sdan¢. Acad. Sci., Paris 178, 1546 (1924). u,) H. D. HAR~T and F. STAWNOW,Z. anorg, allg. Chem. 301, 267 (1959).

The pyrosynth~is of strontium zinc,ate

2805

and volatilization of small amounts of acetic anhydride supplements any weight loss due to sublimation resulting in an increased rate of weight loss. Unfortunately, thermodynamic data are not available for zinc oxyacetate on which to base an estimate of the accompanying enthalpy change. On further heating, a doublet endotherm is encountered with peak temperatures of 252°C 4- I°C and 258°C -q- I°C. P~DERSENcIS~has reported the melting point of zinc acetate to be 244°C, but present work cannot confirm this value. If zinc acetate dihydrate is dehydrated isothermally at low temperature (ca. 60°C) a single fusion endotherm is obtained with a reproducible peak temperature of 258°C. Comparison of peak areas for the dehydration and fusion endotherms suggests that heat of fusion of zinc acetate to be about 4.5 kcal mole-1. The peak temperature of the first endotherm in the fusion doublet agrees within l°C of the reported melting point of zinc oxyacetate.~12'z8) The relative magnitude of this peak suggests that a measurable quantity of zinc oxyacetate can form below 250°C, and some support for this was found from X-ray diffraction data. If the zinc acetate sample was sealed into the X-ray capillary so that volatile materials could not escape, the diffraction pattern of the sample heated to 305°C clearly showed both zinc acetate and zinc oxyacetate. On the other hand, if the volatile materials were allowed to escape, only zinc acetate could be detected at 270°C. There is also TGA evidence to indicate that zinc oxyacetate decomposes more rapidly in this temperature range than does zinc acetate, so that the latter X-ray results are not surprising. It therefore appears that the doublet endotherm may be ascribed first to the fusion of some zinc oxyacetate formed in pre-fusion decomposition of zinc acetate and, subsequently, to the fusion of the remaining undecomposed zinc acetate. Fusion of the zinc acetate is accompanied by a rapid decrease in the rate of weight loss and, visually, by vigorous bubbling in the melt. This, coupled with the sublimation occurring at lower temperatures, may account for the somewhat higher than theoretical weight loss obtained. The predominant process occurring during this weight loss appears to be Zn(CHaCOO) ~ (I) ---*ZnO (s) + CHaCOCH ~ (g) + CO s (g), with some contribution from zinc oxyacetate according to the reaction Zn40(CHsCOO)6 (1) --+ 4ZnO (s) + 3CH3COCH a (g) + 3COz (g). Thus, on the basis of products and rates of weight loss it is not possible to separate these two processes. The final product in either case is a highly porous form of zinc oxide. Under conditions of the simultaneous DTA-TGA experiment (Fig. 2), the enthalpy change accompanying decomposition of the zinc acetate-zinc oxyacetate melt is not dearly defined, but appears as a broad endotherm with a peak temperature of about 380°C. The decomposition endotherm is better resolved in diluted samples of both zinc acetate and zinc oxyacetate (Fig. 3), and the subsequent exothermal activity again is ascribed to decomposition of the liberated organic upon the surface of the hot alumina diluent. The structural identifications shown in Fig. 3 were obtained from Debye-Scherrer patterns of samples packed into open capillaries, which were heated in the DTA sample, withdrawn at the appropriate temperature, and sealed. (Is) J. PEDERSEN, Z. Elektrochem. 20, 328 (1914).

2806

H . G . McAom

20% SAMPLES DILUTED IN ALUMINA DYNAMIC NITROGEN ATMOSPHERE HEATING RATE $ ° C rnin."1

Zn(CH~COOz).2HzO

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Zn(CH~C(X:Ot

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TEMPERATUR£ ('C.)

FIG. 3.

Under these conditions detectable quantities of zinc oxide are present at 305°C, and by 350°C the sample is mainly zinc oxide with some acetate remaining. In contrast, zinc oxyacetate at 350°C has completely decomposed to the oxide. The thermal decomposition of zinc acetate dihydrate may then be represented as follows: Zn(CH3COO)2.2HzO (s) --* / primary

)" Zn(cH'cOO)' (1")

Zn(CH3COO)2 (s)

+

, ZnO (s) + CO~ (g)

Zn,O(eH~eO0), (s) ~ Zn,O(CH~CO0),(1)

2H20 (g)

+ CH,COCH8 (g)

+ CHsCO)~O (g) IAKERSON(16) has postulated the thermal decomposition of strontium and zinc acetates to proceed by similar mechanisms in the solid phase. The present work shows that decomposition takes placefrom the melt in each case, and that the decompositions are dissimilar both in mechanism and in the final products. No evidence for the formation of zinc carbonate has been found at any stage of the decomposition of zinc acetate. (le~V. I. lAKER,SON, Isv.hkad. Nauk SSSR, OtdeL Khim. Nauk 6, 1003 (1963).

pyrosynthesis of strontiumzinc.ate

The

2807

The formation o f stronium zincate

The interaction of strontium carbonate and zinc oxide at high temperatures produces strontium zincate with the elimination of carbon dioxide. Thus, whether one reacts a mixture of the acetates, which individually decompose as described above, or strontium carbonate and zinc oxide directly, the mechanism of the formation of

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TYPE

SAMPLE HOLDER

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DIOXIDE ATMOSPHERE

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COOLING BATES IO% rnin:'

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TEMPERATURE

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strontium zincate should be the same. Rate of reaction, of course, will depend upon the particle size and intimacy of the reactants. The simultaneous DTA-TGA of these reactants under carbon dioxide alone, and in combination, is given in Fig. 4. Strontium carbonate undergoes the rhombic to hexagonal transition at 926°C, in agreement with LANDER(17) who reported this transition to occur at 924°C with an enthalpy change of approximately 4.7 kcal mole -z. Under 1 atm CO= and at 10°C (1~) j. j. LA~a)ER,J. Am. chem. Soc. 73, 5794 (1951).

2808

H.G.

McADm

rain-t heating rate, decomposition of strontium carbonate begins about 1260°C and is complete by 1360°C. If the sample is then allowed to cool from about 1400°C an exothermal weight gain is observed, associated with surface recarbonation of the strontium oxide. This reaction only proceeds to about 50 per cent completion because of the impedance of the carbonate product layer formed and the constantly decreasing sample temperature. The extent of the back reaction is confirmed by the relative areas under the rhombic to hexagonal transition peaks, the area obtained on cooling being 52 per cent of that on heating. The X-ray diffraction pattern of the residue removed at 800°C was predominantly rhombic strontium carbonate with some lines also attributable to strontium oxide. Other workersc18.m have reported lower decomposition temperatures for strontium carbonate, but the partial pressure of CO2 has not been specified. The peak temperature of the rhombic ~ hexagonal transition occurs about 50° lower during programmed cooling than during programmed heating at the same rate. This has been found for a number of similar solid I ~ solid II transitions (m) and may be associated with the kinetics of the transformation and possible supercooling. The report of VAUGHANand WIEDEMANN(gl) that the peak temperature of this transition is the same regardless of the direction in which it is observed, could not be confirmed in the present work. Zinc oxide is thermally inert up to about 1280°C, when a small loss in weight becomes measurable. This is due to volatilization of the zinc oxide and the accompanying enthalpy change is below the limit of detection of the instrument. Cooling from 1450°C arrests this volatilization and no differences were observed in the X-ray diffraction pattern of the sample removed at 800°C. Equimolar mixtures of zinc oxide and strontium carbonate were prepared in a number of ways to ensure intimate mixing, the most satisfactory method being to mull the constituents in 99.95 ~o ethanol followed by low-temperature drying and grinding in an agate mortar. On heating samples of such a mixture a 10°C rain-t under 1 atm of CO~, no change is observed below the strontium carbonate phase transition. On further heating, carbon dioxide is lost at about 1200°C--about 60° lower than for pure strontium carbonate--and the accompanying endothermal activity is somewhat erratic. The rapid weight loss portion of the TGA curve corresponds approximately to that expected for decomposition of strontium carbonate. An inflection point on the weight-loss curve corresponds to completion of the loss of carbon dioxide and further weight loss appears to be due to volatilization of zinc oxide. On cooling from 1450°C a small exotherm at 1213°C is associated with limited recarbonation of the strontium oxide. This process is arrested at 1170°C by a very sharp exotherm and no further weight change was detected below this temperature. On cooling, exposed strontium oxide recarbonates to a very limited extent, arrested by the major exothermal process. A further small exotherm appears at I ll0°C, although no weight change is detectable at this temperature, and at 872°C the small amount of strontium carbonate initially produced undergoes the hexagonal to rhombic transition. The X-ray diffraction pattern of the residue removed below 800°C is (xB) M. W. GOH~N and R. J. ROBINSON,Analytiea chim. Aeta 30, 234 (1964). (tg) L. WALTER-L~vYand J. LA~FC~, C.r. hebd. S~anc. Acad. ScL, Paris 260, 3617 (1965). (~o) H. G. McADm, unpublished results. tit) H. P. VAUGHANand H. G. W m D ~ - A ~ , Vacuum Microbalance Techniques 4,1 (1964) (publ. 1965).

The pyrosynthesis of strontium zinc,ate

2809

predominantly that of strontium zincate with small but detectable amounts of strontium carbonate and zinc oxide being present. The residual zinc oxide is expected to be approximately equivalent to the strontium oxide made unavailable to reaction through recarbonation to strontium carbonate. When a similar mixture of strontium carbonate and zinc oxide is heated at 10°C rain -1 in an oxygen atmosphere, decomposition of the strontium carbonate produces a measurable weight loss at 800°C. The process is complete by 1050°C after which a small and gradual weight loss is ascribed to volatilization of zinc oxide. Superimposed on the broad decomposition endotherm of the strontium carbonate is a sharp endotherm at 940°C associated with the phase transition in the undecomposed material. There is a further well-defined endotherm in the region 1250--1280°C which was previously masked by the higher decomposition temperature of strontium carbonate under 1 atm of carbon dioxide. This process likely contributed to the erratic record in this latter case, because of shrinkage of the sample and changing thermal contact with the holder. Complementary DTA studies on equimolar mixtures of strontium and zinc oxides support the suggestion that this mixture fuses above 1250°C. On cooling from about 1400°C a very sharp exotherm is observed in the region 1220-1160°C. The exact temperature appears to depend upon the maximum temperature achieved during the heating cycle and the initial degree of mixing between the reactants, among other factors. Below this temperature no further changes are observed and the X-ray diffraction pattern of the product corresponds to that for strontium zincate. On reheating strontium zincate produced from the direct interaction of SrO and ZnO in oxygen, an endotherm is observed at 1258-1260°C. Subsequent cooling of the sample produces an exotherm in the region 1220-1180°C Comparison of the data obtained under carbon dioxide and under oxygen suggest that strontium zincate is formed by crystallization from a fused mixture of strontium and zinc oxides. The thermal effects accompanying the fusion-reclystallization process appear to be reversible, allowing for the temperature differences normally encountered for a given transition between heating and cooling, c2°) Other chal acteristics of the fused salt have not been established, although it seems reasonable to expect the individual oxides to be present above the fusion temperature.

Acknowledgements--The author wishes to thank Mr. H. P. VAUOHANand Mr. H. HoErn~, Mettler Instrument Corporation, Princeton, N.J., for their kindness in performing the simultaneous DTATGA work describedin this paper. Grateful acknowledgementis also made to Dr. T. R. INOP.AaAM, Department of Mines and Technical Surveys, for helpful discussions. This research was sponsored by a research grant to the Ontario Research Foundation from the Province of Ontario received through the Department of Economics and Development.