Summary

In material science and condensed matter research solid state decomposition

studies are of paramount importance and a lot of research has been done in this field in

the past few decades. Such studies have brought to light a good deal of information

concerning the kinetics and mechanism of solid state reactions. A number of

theoretical models have been proposed to substantiate and interpret the results of these

studies.

Studies on solid state reactions have shown that crystal imperfections of

various kinds do play a very significant role in deciding the reactivity of solids.

Processes such as irradiation, precompression, crushing, doping and addition of

impurities increase the lattice imperfections in solids. This will profoundly influence

the formation and growth of reaction nuclei in the solid matrix and thereby enhance

the reactivity.

Considerable amount of work has been done on the thermal decomposition of

alkali and alkaline earth metal bromates.1-12 It was of interest therefore to extend the

studies to other bromates. Nickel bromate, yttrium bromate and neodymium bromate

were taken for the investigations in a program of comparative study of the thermal

decomposition behaviour of metal bromates.

Thermal decomposition reactions are very sensitive to the presence of

impurities.13-18 Impurities can produce vacancies19 or act as electron traps20 and can

affect the thermal decomposition reaction. It was of interest therefore to study the

variation of the decomposition behaviour of barium bromate, nickel bromate, yttrium

bromate and neodymium bromate due to the addition of intentional impurities.

All bromates required for the studies (barium bromate, strontium bromate,

magnesium bromate, zinc bromate, cadmium bromate, nickel bromate, yttrium

bromate and neodymium bromate) were synthesized by the method of Bancroft and

Gesser.1,21-24 Crystals of bromates containing intentional impurities were prepared by

slow evaporation of solutions containing the host bromate and the one used as

impurity in the mole fraction range 10-3 to 10-1.

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The untreated barium bromate, yttrium bromate, nickel bromate and

neodymium bromate and samples of barium bromate containing NaBrO3, KBrO3,

Mg(BrO3)2, Sr(BrO3)2, Zn(BrO3)2 , Cd(BrO3)2 , Ni(BrO3)2 , Nd(BrO3)3 , Y(BrO3)3 , KBr,

SrBr2 as intentional impurities, samples of nickel bromate containing Zn(BrO3)2 ,

Nd(BrO3)3 , Y(BrO3)3 as intentional impurities, samples of neodymium bromate

containing Ni(BrO3)2 , Ba(BrO3)2 as intentional impurities and samples of yttrium

bromate containing Ni(BrO3)2 , Ba(BrO3)2 as intentional impurities were subjected to

dynamic thermogravimetric analysis. 52 samples were used in all in the form of fine

powder (200-240 mesh). Some samples were subjected to XRD studies with a view to

get information concerning the samples containing the impurities.

Analysis of the TG curves shows that the addition of impurities do not alter the

pattern of the TG curve to any significant extent though it does alter the initial

temperature of decomposition(Ti), final temperature(Tf) and peak temperature(Ts)

characteristic of the decomposition. In general lowering of Ti, Tf and Ts is observed in

samples containing impurities.

Earlier studies have shown that the thermal decomposition of barium bromate

is a first order process19 and hence the kinetic parameters viz., energy of activation(E),

frequency factor(Z) and entropy of activation(ΔS) were calculated using the Coats-

Redfern25 , the Freeman-Carroll26 and the Horowitz-Metzger27 methods in the form

applicable to first order process. For nickel bromate, neodymium bromate and yttrium

bromate and for all the samples containing intentional impurities the kinetic

parameters were calculated by the above methods. The E and Z values obtained by

the three methods show good agreement within ±10 percent in all cases.

XRD studies indicate no new phase formation in samples containing

intentional impurities except in two cases.

Thermal decomposition of barium bromate occurs with an initial loss of water

of hydration followed by the decomposition of anhydrous bromate to bromide. In the

case of bromates of Ni, Nd and Y loss of water of hydration occurs in two steps

followed by the decomposition of bromate into bromide and oxide.

The thermal stability of barium bromate decreased on adding the impurities. In

the case of samples of barium bromate containing intentional impurities the effect of

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lowering of activation energy was the highest in the case of barium bromate

containing Sr(BrO3)2 and the effect decreased in the order

Sr2+ > Cd2+ > Na+ > Mg2+ > K+ > Zn2+

and for barium bromate samples containing Ni(BrO3)2 , Nd(BrO3)3 , Y(BrO3)3 as

impurities, the lowering of activation energy was in the order

Nd3+ > Y3+ > Ni3+

The higher susceptibility of the bromates containing the intentional impurities

towards thermal decomposition is attributed to the presence of lattice defects

generated. The extent of the lattice defects depends on the valency, size, surface

charge density and concentration of the added impurity. In general the intensity of the

lattice defects increases with increase in concentration of the impurity. The

enhancement of the decomposition in the presence of the added Br− ion is due to

eutectic formation between Br− and BrO3− as proposed by Jach.28

In the case of Nd, Y and Ni bromates the decomposition yields oxides which,

as is well known, are compounds with very good catalytic properties. So the

enhancement of decomposition may also be due to the catalyzing effect of the oxide

apart from the lattice defects and eutectic formation.

Addition of zinc bromate and yttrium bromate to nickel bromate decreases the

activation energy while addition of neodymium bromate increases the activation

energy. This may be probably due to the greater thermal stability of neodymium

bromate. Addition of nickel bromate and barium bromate to yttrium bromate decreases

the activation energy of decomposition of yttrium bromate. Z and ΔS were also

lowered in the samples containing impurities.

The mechanism of decomposition was established by following the non-

isothermal method by Sestak and Berggren29 and Satava30. For the decomposition of

barium bromate, neodymium bromate and yttrium bromate fairly good correlation was

obtained for the “Contracting Cube Model”31

1 - (1 - α)1/3 = kt

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where α is the fractional decomposition and k is the rate constant. This correlation

suggests that the rate controlling process is a phase boundary reaction assuming

spherical symmetry. For nickel bromate the data fit well with other mechanistic

equations also. However, the activation energy obtained with the contracting cube

model is in better agreement with the experimental results obtained by the Coats-

Redfern method.25 Thus it is concluded that the decomposition of nickel bromate also

follows the contracting cube equation. It is also found that the activation energies

calculated for the samples containing intentional impurities using the above model

fairly agree with those obtained by non-mechanistic equations. These findings

conclusively prove that the presence of impurities, though does tune the rate of

decomposition, does not alter the mechanism of decomposition of barium bromate,

nickel bromate, neodymium bromate and yttrium bromate.

. . ………………………………………. References :

1. Bancroft G. M and Gesser H.D. , J. Inorg. Nucl. Chem., 27, 1545 (1965).

2. Erdey L. , Simon J. and Gal S., Talanta., 15, 653 (1968).

3. Arnikar H.J and Patnaik S.K, J. Univ. Poona Sci. Technol., 50, 163-70 (1977).

4. Nair S.M.K, Malayil Koshykunju, Daisamma Jacob P.,Thermochim. Acta., 141,

61-80 (1989).

5. Nair S.M.K and James C. Thermochim. Acta., 96(1), 27-36 (1985).

6. Diefallah, EL.H.M., Basahl, Suliman N, Obaid Abdullah Y, J. Radioanal. Nucl.

Chem. 109(1) 177-81(1987).

7. Solymosi. F. and Bansagi T., J. Phys. Chem., 74, 15 (1970).

8. Bhattamisra. S.D and Mohanty S.R, Indian J. Chem. 15A, 350 (1977).

9. Bhattamisra. S.D and Mohanty S.R, Inorg. Nucl. Chem., 39(12), 2103-09 (1977).

10. Mohanty S.R. and Satapathy M., Radio Chem., Radioanal. Lett., 37, 345 (1979).

11. Jena B., Mohanty S.R and Satapathy M., Indian J. Chem., 19A, 1139 (1980).

12. Das B.C., Patnaik D., J. Them. Anal .and Calorimetry, vol 61, 879-883 (2000).

13. Bhatta D., Sahoo M.K, Jena B., Thermochim Acta, 132, 7-13 (1988).

14. Das B.C, Sethi K.K, J. Teach. Res. Chem., 6(2), 41-46 (1999).

x

15. Sahu K.K., Bose S., Bhatta D., React. Kinet. Catal. Lett, 52(1), 149-54 (1994).

16. Bhatta D., Mishra S, Sahu K.K, J. Them. Anal., 39(3), 275-80 (1993).

17. Shoba U.S. and Udupa M.R, Proc. Natl. Symp. Them. Anal., 9th 395-8 (1993).

18. Shoba U.S. and Udupa M.R, Thermochim Acta., 242(1-2), 215-21 (1994).

19. Delbeeq C.J, Hayes W. and Yuster P.H, Phys. Rev., 121, 1043 (1961).

20. Johnson P.D. and Williams F.E., Phys. Rev., 117, 964 (1960).

21. Nair S.M.K. and Daisamma Jacob P, Thermochim Acta, 179, 273-280 (1991).

22. Abbasi, A, Eriksson L, Acta Crystallogr. Ser. E. , 62, p.m. 126 (2006).

23. Abbasi, Alireza; Badiei, Alireza., Iran J. Chem. Chem. Eng, Research note,

Vol 26, No 4 (2007).

24. A. Chatterji, K.N. Chattopadhyay, D. Neogy, P. Paul, Reba Chatterjee,

S. Chatterjee., Journal of Magnetism and Magnetic materials, 271, 1-8 (2004).

25. Coats A. W and Redern J.P, Nature, 201, 68 (1964).

26. Freeman E.S. and Carroll B., J. Phys. Chem., 62,394 (1958).

27. Horowitz H.H. and Metzger. G., Anal. Chem., 35,1464 (1963).

28. Jach J., Reactivity of Solids(Ed. De Boer J.H.) Elsevier Publication Co.,

Amsterdam, p. 334 (1961).

29. Sestak J. and Berggren G., Thermochim Acta, 3,1 (1971).

30. Satava V., Thermochim Acta, 2,423 (1971).

31. M. Avrami, J.Chem. Phys., 7,103 (1939).

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