Lanthanide oxalates, decompositions

While there have been many non-isothermal studies of the decompositions of lanthanide oxalates, fewer detailed kinetic investigations have been reported. The anhydrous salts are difficult to prepare. La, Pr and Nd oxalates decompose [1097] to the oxide with intervention of a stable oxycarbonate, but no intermediate was detected during decomposition of the other lanthanide oxalates. The product CO disproportionates exten-... 

The a—time curves for the vacuum decomposition at 593—693 K of lanthanum oxalate [1098] are sigmoid. Following a short induction period (E = 164 kJ mole-1), the inflexion point occurred at a 0.15 and the Prout—Tompkins equation [eqn. (9)] was applied (E = 133 kJ mole-1). Young [29] has suggested, however, that a more appropriate analysis is that exponential behaviour [eqn. (8)] is followed by obedience to the contracting volume equation [eqn. (7), n = 3]. Similar kinetic characteristics were found [1098] for several other lanthanide oxalates and the sequence of relative stabilities established was Gd > Sm > Nd > La > Pr > Ce. The behaviour of europium(III) oxalate [1100] is exceptional in that Eu3+ is readily reduced... 

The decompositions of the light lanthanide [81] oxalates (613 to 713 K), were generally similar to that described above for the lanthanum salt. The relative stabilities of these six salts, and also of the normal and basic carbonates, is Gd > Sm > Nd > La > Pr > Ce. With few exceptions, the intermediates (identified by composition and infi-ared measurements) were ... 

As intermediate products of thermal decomposition of hydrated R carbonates or oxalates, oxide carbonates R2O2CO3 are obtained for practically all Rs (Petru et al. 1966, Sawyer et al. 1972). Several metastable phases with defined crystal structures are formed as a function of temperature, before hexagonal (type-II) R2O2CO3 appears, with crystal structure (Christensen 1970, Attfield and Ferey 1989) [La202C03 P6 /mmc 407.55(6), 1595.7(1)] related to that of the hexagonal oxides of the early lanthanides. For R=La, Pr, Nd, Sm, and Gd, these oxide carbonates are relatively stable and can be isolated as single... 

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