Thermal decomposition of solid state compounds of lanthanide and yttrium benzoates in CO 2 atmosphere

Solid-state Ln-Bz compounds, where Ln stands for trivalent lanthanides and Bz is benzoate have been synthesized. Simultaneous thermogravimetric and differential thermal analysis in a CO2 atmosphere were used to study the thermal decomposition of these compounds.

In the present study, as an extension of the work of ref. 25, benzoates of trivalent lanthanides and yttrium (III) were investigated by using simultaneous thermogravimetry and differential thermal analysis (TG-DTA) in a CO 2 atmosphere.The results of this study are discussed in connection with those found for previously study involving these compounds in an air atmosphere.

Experimental
The preparation of the solid state compounds of trivalent lanthanides and yttrium (III) with Bz, as well as, the determination of hydration water, metal-lic ions and ligand contents, the same procedure were used as previously described in Ref. 25.
The Simultaneous TG-DTA curves were obtained with thermal analysis system model SDT 2960 from TA Instruments.The purge gas was a CO 2 with flow of 100 mL min -1 .A heating rate of 20 ºC min -1 was adopted with samples weighing about 7 mg.Alumina crucibles were used for TG-DTA.

Results and Discussion
The analytical and thermoanalytical (TG) data were obtained in air atmosphere, Table 1 Ref. 25.These results establish the stoichiometry of these compounds, which are in agreement with the Ln(Bz) 3 .nH 2 O general formula, where Ln represents trivalent lanthanides or yttrium, Bz is benzoate and n= 2.5 (Nd), 2 (La, Ce, Pr, Sm), 1.5 (Eu, Gd), 1(Tb), 0.5 (Dy) and 0 (Ho to Lu and Y).
The simultaneous TG and DTA curves of the compounds are shown in Fig. 1.These curves exhibit mass loss in steps and endothermic peaks corresponding to these losses or due to physical phenomenon, Table 2.
For the lanthanum to dysprosium compounds, the first mass loss (range: 130 -90ºC), associated to endothermic peaks at 120 -80 ºC, is ascribed to the dehydration, which occurs in a single step.No influence of the atmosphere (CO 2 or air) was observed in the dehydration process of these compounds.
Once dehydrated, the anhydrous compounds are stable up to 350 ºC (La to Sm), 375 ºC (Eu), 385 ºC (Er, Tm) and 400 ºC (Gd to Ho, Yb, Lu, Y), showing that the anhydrous compounds are more stable in CO 2 than in an air atmosphere.Above these temperatures the TG -DTA curves show mass losses in steps characteristic of each compound and the general aspect of these curves allows the division into the four groups; first group: La, Pr, Nd; second group: Ce, Sm, Gd, Tb; Third group: Eu and fourth group: Dy to Lu and Y. Thus the feature of each of these compounds are discussed on the base of their similar thermal profiles.

Lanthanum, praseodymium and neodymium compounds
The TG-DTA curves of the lanthanum and praseodymium as representative of these compounds are shown in Fig. 1.The first mass loss up to 130 ºC (La) and 120 ºC (Pr, Nd) with the corresponding endothermic peaks at 120 ºC (La) and 110 ºC (Pr, Nd) is due to the release of 2.5H 2 O (Nd) and 2H 2 O (La, Pr).
After their dehydration the anhydrous compounds are stable up to 350 ºC, and the endothermic peaks at 250 ºC (La), 245 ºC (Pr) and 240 ºC (Nd) are atributed to the reversible crystalline phase transition, in agreement with the thermal decomposition of these compounds in an air atmosphere [25].
The thermal decomposition of the anhydrous compounds occurs in three (La, Pr) and four (Nd) steps.The first one corresponding to endothermic peaks is attributed to thermal decomposition of these compounds and tests with hydrochloric acid solution on samples heated up to the final temperature of this step, showed in all cases, the presence of carbonaceous residue and of CO 2 , indicating the formation of carbonate or a derivative of carbonate, as intermedi-ate.The formation of carbonate or derivative carbonate and carbonaceous residue, undoubtedly was favoured by CO 2 used as purge gas.The for-mation of intermediate derivative of carbonate had been observed during the thermal decomposition of other lanthanide compounds [26,27].
The second step corresponding to endothermic peaks is attributed to the thermal decomposition of the intermediate and carbonaceous residue to lanthanide dioxycarbonate [26,27].The last (La, Pr) or the two last (Nd) steps corresponding to endothermic peaks are attributed to the thermal decomposition of the dioxycarbonate to the respective oxide, La 2 O 3 , Pr 6 O 11 , Nd 2 O 3 .
The thermal decomposition of the anhydrous compounds takes place in two steps.The first one accompanied by a small endothermic peak is attributed to the thermal decomposition of these compounds with the formation of a mixture of carbonaceous residue and carbonate or a derivative of carbonate, as intermediate.
The last mass loss corresponding to the endothermic peaks is attributed to the thermal decomposition of the carbonaceous residue and of the intermediate to the respective oxide; CeO 2 , Sm 2 O 3 , Gd 2 O 3 and Tb 4 O 7 .

Europium compound
The TG-DTA curves of the europium compound are shown in Fig. 1.The first mass loss up to 110 ºC with the corresponding endothermic peak at 105 ºC is due to the dehydration with loss of 1.5H 2 O.
The anhydrous compound is stable up to 375 ºC and the endothermic peak at 245 ºC is attributed to the reversible crystalline phase transition [25].
The thermal decomposition of the anhydrous compounds takes place in three steps with the two last overlapping ones.The first step corresponding to endothermic peak is attributed to the thermal decomposition of the compound with the formation of a mixture of carbonaceous residue and a derivative of carbonate, as intermediate.
The two last overlapping mass losses corresponding to the endothermic peaks are attributed to the thermal decomposition of the carbonaceous residue and of the intermediate to the respective oxide, Eu 2 O 3 .

Dysprosium to lutetium and yttrium compounds
The TG-DTA curves of the holmium and ytterbium as representative of these compounds are shown in Fig. 1. these curves show that all the compounds were obtained in the anhydrous state and stable up to 385 ºC (Eu, Tm) 400 ºC (Dy, Ho, Yb, Lu) and 420 ºC (Y) and the endothermic peak at 270 ºC (Dy), 275 ºC (Lu) and 280 ºC (Ho, to Yb, Y) is ascribed to the reversible crystalline phase transition [25].
The thermal decomposition of these compounds occurs in three (Ho to Lu) or four (Dy, Y) steps.The first one corresponding to endothermic peaks is attributed to the thermal decomposition of these compounds with the formation of an intermediate derivative of carbonate and carbonaceous residue.Above 620 ºC (Dy to Yb) and 630 ºC (Lu, Y) the mass losses corresponding to endothermic peaks are attributed to the thermal decomposition of the intermediate derivative of carbonate followed by the carbonaceous residue, with formation of the respective oxide, Ln 2 O 3 (Ln= Dy to Lu, Y).For these compounds the thermal decomposition of the intermediate derivative of carbonate occurs before of the carbonaceous residue, due to the decreasing thermal stability of these intermediates with increasing atomic number of the lanthanide ions, as had already been observed [28].For this reason the large endothermic event between 850 -1100 ºC attributed to the thermal decomposition of the carbonaceous residue is not observed for the light lanthanide compounds since the carbonaceous residue is evolved before the thermal decomposition of the intermediate derivative of the carbonate.