Thermal decomposition behaviour of lanthanum(III) tris-tartrato lanthanate(III) decahydrate

Article abstract of DOI:10.1023/B:JTAN.0000042170.86978.b5

Lanthanum(III) tris-tartrato lanthanate(III) decahydrate, La[La(C ₄H₄O₆)₃]·10H₂O has been synthesized and characterized by elemental analysis, IR, electronic spectral and X-ray powder diffraction studies. Thermal studies (TG, DTG and DTA) in air showed a complex decomposition pattern with the generation of an anhydrous species at ～170°C. The end product was found to be mainly a mixture of La₂O₃ and carbides at ～970°C through the formation of several intermediates at different temperature. The residual product in DSC study in nitrogen at 670°C is assumed to be a similar mixture generated at 500°C in TG in air. Kinetic parameters, such as, E*, ΔH, ΔS, etc. obtained from DSC are discussed. IR and X-ray powder diffraction studies identified some of the decomposition products. The tentative mechanism for the thermal decomposition in air of the compound is proposed.

Full text of DOI:10.1023/B:JTAN.0000042170.86978.b5

Journal of Thermal Analysis and Calorimetry, Vol. 78 (2004) 227–237
THERMAL DECOMPOSITION BEHAVIOUR OF
LANTHANUM(III) TRIS-TARTRATO
LANTHANATE(III) DECAHYDRATE
N. Deb*
Department of Chemistry, North Eastern Regional Institute of Science and Technology,
Nirjuli 791109, Itanagar, Arunachal Pradesh, India
(Received August 10, 2003; in revised form November 6, 2003)
Abstract
Lanthanum(III) tris-tartrato lanthanate(III) decahydrate, La[La(C4H4O6)3]⋅10H2O has been synthesized
and characterized by elemental analysis, IR, electronic spectral and X-ray powder diffraction studies.
Thermal studies (TG, DTG and DTA) in air showed a complex decomposition pattern with the genera-
tion of an anhydrous species at ~170°C. The end product was found to be mainly a mixture of La2O3 and
carbides at ~970°C through the formation of several intermediates at different temperature. The residual
product in DSC study in nitrogen at 670°C is assumed to be a similar mixture generated at 500°C in TG
in air. Kinetic parameters, such as, E*, ∆H, ∆S, etc. obtained from DSC are discussed. IR and X-ray pow-
der diffraction studies identified some of the decomposition products. The tentative mechanism for the
thermal decomposition in air of the compound is proposed.
Keywords: IR, lanthanum tartrato complex, thermal decomposition, X-ray powder diffraction
Introduction
Many workers [1–4] have studied tartrates of transition and non-transition metals of
different types. Structure determinations have been made on numerous crystalline
tartrate complexes and coordination patterns have been reviewed[5, 6]. The tartrate co-
ordination in crystal structure of (NMe4)2[MoO2(C4H4O6)2]EtOH⋅1.5H2O has been in-
vestigated [7]. Gel grown pure and mixed rare earth tartrates [8] of yttrium and samar-
ium are found to be decomposed to their stable oxides. The carboxylate precursor,
magnesium tris(maleato)ferrate(III)dodecahydrate has found [9] to decompose to dif-
ferent intermediates i.e. α-Fe2O3 and MgO through the formation of magnesium
maleate. Further, magnesium ferrite, MgFe2O4 is generated through a solid–solid reac-
tion of MgO and α-Fe2O3 at 600°C, a temperature much lower than the ceramic
method. The complexes of 2,3-dimethoxy benzoates of heavy lanthanides and yttrium
of the type Ln(C9H9O4)3⋅nH2O are studied [10], where the complexes of Tb(III),
*
Author for correspondence:E-mail: nidhubandeb@yahoo.com
1388–6150/2004/ $ 20.00
Akadémiai Kiadó, Budapest
© 2004 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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Dy(III), Ho(III), Er(III), Tm(III) and Y(III) are found to produce the oxides of the re-
spective metals at 1273 K in air through the formation of anhydrous salts, whereas
Yb(III) and La(III) are directly decomposed to the oxides. The carboxylate coordina-
tion compounds of iron, nickel and zinc with malates are reported [11] to produce
α-Fe2O3, NiO and ZnO as end products in air through the formation of anhydrous mal-
ates, malonates, oxoacetates (iron and nickel compounds) and hydroxocarbonate (zinc
compound) as intermediates. We have synthesized and studied extensively the thermal
behaviour of carboxylato complexes with oxalate as ligand of the type
M[M(C2O4)n]⋅xH2O [12–16] (where M=same metal), M3[La(C2O4)3]⋅xH2O [17, 18]
(where M=Li, Na and K), M1[M2(C2O4)2]⋅xH2O[19] (where M1=Co or Cd, M2=Ni) and
M[La(C2O4)3]⋅H2O [20] (where M=Cr or Co), where various interesting decomposition
phenomena are observed leading to different products. Although, the oxalato com-
plexes of various types are reviewed [21] and studied, the carboxylato complexes with
tartrate as ligand are less reported. Although there had been a report [22] on tartrato
complexes of the type M[M(tart)n]⋅xH2O (where M=Co(II)), and keeping in mind the
formation of different intermediates and end products and their possible catalytic appli-
cability the paper reports the synthesis, characterization and thermal decomposition of
lanthanum(III)
La(III)[La(III)(C4H4O6)3]⋅10H2O in air and nitrogen media. The tentative mechanism
tris-tartrato
lanthanate(III)
decahydrate,
for the decomposition in air has also been proposed.
Experimental
All the chemicals used were of AR grade purity. We followed the same preparative
method adopted for oxalato complexes [14–18]. Lanthanum nitrate was converted to
chloride by digesting repeatedly with concentrated hydrochloric acid. The chloride free
hydroxide was then produced by washing several times with distilled water. It was
dissolved in glacial acetic acid and heated on a steam bath. A white compound was
precipitated by dropwise addition of a saturated solution of tartaric acid. The compound
was separated from solution and washed several times with distilled water and air dried
at ambient temperature, then stored in a desiccator. Water and metal contents were
determined thermogravimetrically and gravimetrically [23] by known methods.
Microanalytical data were recorded with a Carlo Erba 1108 elemental analyser. IR
spectra of the compound and the intermediates were recorded from 200 to 4000 cm–1 in
nujol mulls with a Perkin Elmer 883 spectrophotometer. Diffuse reflectance spectrum
was measured with Shimadzu UV-240 spectrophotometer using BaSO4 as the reference
material. TG, DTG and DTA profiles were recorded in static air at a heating rate of 10°C
min–1 using Shimadzu DT 30B thermal analyzer. DSC was carried out in dynamic
nitrogen up to 670°C using Perkin Elmer PC series DSC-7 at a heating rate of 10
and 15°C min–1. The kinetic parameters of the phase transformation processes were
determined after integration of the peaks. Powder X-ray diffraction (XRD) of the
compound and decomposition products was obtained using a Geigerflex microprocessor
controlled automated Rigaku (Japan) X-ray diffractometer system D/Max IIIC with
SASG accessory.
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Results and discussion
The complex, La(III)[La(III)(C4H4O6)3]⋅10H2O is obtained as white powder and is
insoluble in water or in common organic solvents. However, it decomposes in strong
acid or alkali. The water analysis revealed that the compound contains ten molecules
of water. Analytical and microanalytical results were consistent with the proposed
molecular formula of the complex. As vs. the calculated composition (in mass%) of
La, 30.79; C, 15.98; H, 3.58; H2O, 19.97, the analysis showed La, 30.43; C, 16.30;
H, 3.62; H2O, 19.35. Selected bands in the IR spectrum of the complex are included
in Table 1. The chelating character of the carboxylate (tartrato) group as well as the
presence of coordinated and crystal water was concluded on the basis of earlier stud-
ies [1, 3, 15, 22, 24]. The electronic spectrum of the solid sample showed the bands
around 27.390 and 46.500 cm–1 which are due to L→M charge transfer and
intraligand π→π* transition, respectively. The XRD pattern (Table 2) of the com-
pound indicates its crystalline nature.
Table 1 Selected bands in the IR spectrum of La(III)[La(III)(C4H4O6)3]⋅10H2O
IR bandsa/cm–1
Assignments
2400–4000 (b)
νsy(O–H)+νasy(O–H) of water or hydrogen bonding
ν(O–H) of free hydroxyl of tartrato group
(Centered at 3420)
1300–1720 (mb)
(centered at 1600)
νasy(C=O) and/or δsy(H–O–H)
1380 (S)
1080 (m), 1110 (m)
710 (s)
νsy(C–O)+ν(C–O) and/or δ(O–C=O)
Alcoholic C–O stretching vibration
Coordinated water
640 (s)
ν(La–O) and/or ν(C–C) or water of crystallisation
ν(La–O) and/or ν(C–C)
560 (s)
480 (vs)
Ring deformation and/or ν(O–C=O)
ν(O–C=O)
400 (vs)
305 (m)
Out-of-plane bending
as – small; vs – very small; m – medium; S – strong; b – broad
The thermal profiles (TG, DTG and DTA) of La[La(C4H4O6)3]⋅10H2O
at 10°C min–1 in air are shown in Fig. 1. The TG curve shows a continuous mass loss
from room temperature to 1000°C with several breaks indicating the formation of
several intermediates. An inclined slope upto 170°C with a mass loss of 20% (calcu-
lated, 19.97%) indicates the elimination of all ten molecules of water. DTG change
between room temperature and 180°C and an endothermic peak in DTA in the
range 53–162°C (∆Tmin=78°C) with a small shoulder peak at 146°C are due to the de-
hydration step. The shoulder at 146°C may be due to removal of coordinated water
molecules. The anhydrous form loses mass of 2% upto 260°C, which apparently indi-
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231
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Fig. 1 TG, DTG and DTA curves of La(III) [La(III)(C4H4O6)3]⋅10H2O in air at 10°C min–1
cates the formation of La[La(C4H4O6)2.85 and led to the cumulative mass loss of 22%
(calculated value 22.43%). The intermediate is not stable in TG and decomposed to
subsequent products at 353°C through partial breaking of C4H4O62– anion of the an-
hydrous moiety. The partial rupture of the bidentate group with evolution of CO(g)
and CO2(g) is also attributed in other studies [15, 17]. An endotherm in the
range 250–300°C (∆Tmin=280°C) in DTA may be responsible for the initiation of rup-
ture of tartrato group and possible rearrangement. The IR bands of the residue iso-
lated from the compound preheated to 250°C in furnace are found mostly in similar
with the precursor, which give credence to the retention of coordinated tartrato
groups. Further, another distinct slope in TG from 260 to 353°C with 39% cumula-
tive mass loss apparently suggests that the intermediate is either an oxalate of lantha-
num, La[La(C2O4)3] (calcd. mass loss, 39.94%); or, a mixture of 1/2La2O3,
and 1/2La2(C4H4O6)x (where x=3) (calcd. mass loss, 41.93%). The difference be-
tween the observed and calculated mass loss to get the residue may be due to the pres-
ence of free unoxidised C [28, 29] and it compounds in trace. Corresponding to the
above changes, a sharp change in the range 260–360°C in DTG and an exotherm of
∆Tmax of 400°C followed by an overlapping endotherm of ∆Tmin of 420°C in DTA are
displayed.
As the powder XRD data (Table 2) of the pyrolysed product at ~355°C in furnace
does not coincide with the data obtained [15] for lanthanum oxalate, the possibility of
its formation can be ignored. In the case of cobalt tartrate complex, however, the exis-
tence of mixture of cobalt oxalate and oxide was proposed [22] at 360°C through the
formation of cobalt meso oxalate at ~220°C. The powder XRD data of the product iso-
lated at ~355°C confirmed [25] the presence of La2O3 along with traces of LaCO,
La2C3 and C (carbon). The carbon formed due to disproportionation [18] of the evolved
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233
CO(g) during the rupture of C4H4O62– groups partially could react even at the low tem-
perature, producing [15] carbide (La2C3) and oxycarbide (LaCO). The remaining mis-
matched d-values in the XRD data of the mixture species at 355°C might be the possi-
ble rearranged products of 1/2La2(C4H4O6)x (where x=3 or <3). The IR band at
1300–1720 cm–1 that contains the strong feature at 1580 cm–1 and associates with other
features at 1385 (s), 1130 (s), 1082 (m), 940 (s), 860 (m), 780 (ms), 720 (s), 440 (ms),
400 (s), 376 (vs), 355 (vs) and 340 (vvs) cm–1 further substantiates the retention of
bonds for characteristic vibration for tartrate groups. Terms written within brackets
symbolise the strengths of IR peaks (for details refer Table 1). Some of the bands are
for the species of lanthanum [17, 30] formed at 355°C in furnace. Like earlier study
[17] observed in the case of Na3[La(III)(C2O4)3]⋅8H2O, the rapid conversion of an exo-
thermic change at 400°C to an endotherm at 420°C and subsequently to a broad
exotherm at ~454°C are exhibited in the DTA of this study. These are attributed [26] to
the partial decomposition of tartrate groups with the evolution of CO(g), and exother-
mic conversion of a part of CO(g) to CO2(g) at 400°C. The separation of 1/2La2O3 and
1/2La2(C4H4O6)3 from one mole of tartrate and their rearrangement took place at
420oC; and their subsequent decomposition manifested exothermically in accordance
with 454°C. The observed mass loss in TG in the range of 353–428°C may be due to
decomposition of La2(C4H4O6)x(s) with the release of gaseous products and the
solid–solid interaction of the intermediates formed at ~353°C. A part of solid carbon
generated at 353°C may be oxidised to CO(g) and CO2(g), which may also lead to mass
loss. Further, a change in DTG (410–510°C) which corresponds to a steep slope in TG
in the range 428–500°C attaining 51% cumulative mass loss suggests that the product
may be a mixture of 1/2La2O3 with 1/2La2(C2O4)3, the oxalate (calcd. mass loss,
51.91%) or LaC4H2O5, the meso oxalate [22] (calcd. mass loss 52.13%) from one mole
of the tartrate. The exotherm in DTA displayed at 454°C is due to the decomposition of
1/2La2(C4H4O6)3, which was formed at 353°C. The XRD data (Table 2) of the
pyrolysed greyish black product at 500°C confirmed the existence of La2O3 along with
a trace of La2C3 and LaCO. But the XRD data does not match with reported [15] data of
lanthanum oxalate; instead a few d-values coincided with the d-values of lanthanum
oxycarbonate, La2O(CO3)2. This suggests that the oxalate as soon as it formed as inter-
mediate immediately decomposed to oxycarbonate, which is reported to be produced
[15] at ~494°C from the precursor La[La(C2O4)3]⋅9H2O. Unlike earlier study
[4, 22, 24], the possibility of formation of lanthanum meso oxalate, LaC4H2O5 as inter-
mediate could not be observed. In this context, the dioxycarbonate, La2O2CO3 may also
be formed through the trapping [28] of CO2(g) with a part of porosive La2O3. Some
mismatched d-values gives the credence to the formation of dioxycarbonate,
La2O2CO3, which also might be produced directly from the decomposition of lantha-
num oxalate (transient in this study), or from the slow conversion of a part of the
oxycarbonate within 500°C. However, our earlier study [15] reported it to be formed at
526°C. The medium to small IR bands at 1640 (s), 1600 (m), 1360–1550 (centered at
1386), 845 (m), 765 (s), 720 (s), 400 (vs), 340 (vvs) and 310 (vvs) cm–1 of the isolated
species at 500°C could be attributed due to the presence of CO23– group [18] and other
products. The product at 500°C gave effervescence with hydrochloric acid, which indi-
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cates the existence of carbonate. Further, the continuous mass loss in TG even after
500°C suggests the slow conversion of oxycarbonate and dioxycarbonate to the oxide,
La2O3, where the earlier generated La2O3 may act as catalyst. We isolated the
pyrolysed product at 805°C and its XRD patterns (Table 2) confirmed the presence of
La2O3, La2C3, LaCO and La. The absence of d-values of oxy and dioxycarbonate indi-
cates the complete transformation to oxide within 805°C. The multiple IR absorption
bands observed at 1640 (s), 1386 (m), 1360 (m), 1085 (vs), 765 (vs), 645 (mS), 580
(mS), 400 (vs), 375 (vs), 340 (vvs), 310 (vvs) cm–1, having similarity to the bands of
the product obtained at 500°C further substantiates the presence of the oxide, carbide
and oxycarbide of lanthanum. An endotherm in the DTA in the range of 823–860°C
(∆Tmin at 840°C) and the mass loss till a TG break at 865°C with cumulative loss of
67.5% may be due to the solid–solid reaction of the products contemplated at 805°C,
which in turn, may be converted to some other products with continuous mass loss upto
the limit of the scanning at 1000°C. Eventually, the 78% mass loss at 1000°C gives the
hint of the tendency of formation of metallic lanthanum. Prior to the formation of resi-
due at 1000°C, we isolated the pyrolysed species at ~970°C in furnace, and the XRD
data (Table 2) still confirmed the presence of La2O3, La2C3, LaCO and a trace of La.
The DSC profile of the compound in nitrogen at 10°C min–1 is shown in Fig. 2.
The profile displayed three endothermic peaks. We enlarged the endotherms for ki-
netic analysis by changing the heat flow axis. The first endotherm between 52.12
to 166.68°C is accounted for the dehydration step, followed by another endothermic
peak in the range 251.62–318.48°C may be corroborating to the partial breaking of
the tartrato groups. The study suggests that the anhydrous compound be formed both
in nitrogen and air approximately at similar temperature. The kinetic parameters
evaluated after integration of the peaks is included in Table 3. The high activation en-
ergy (E*) for the second step indicative [15] of the slow fragmentation of anhydrous
Fig. 2 DSC curves of La(III) [La(III)(C4H4O6)3]⋅10H2O in nitrogen at 10°C min–1
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DEB: LANTHANUM(III) TRIS-TARTRATO LANTHANATE(III)
species. Further, another flat endotherm between 424.28 to 557.71°C indicates fur-
ther rapid decomposition of the intermediates with low E* as compared to second
step. The 50.71% mass loss at the end of the scanning upto 670°C may be due to the
formation of similar product formed at 500°C in TG in air. From the Table 3 the val-
ues of change of enthalpy (∆H) and entropy (∆S) seem to be dependent on the area of
the endotherms of the respective changes. The low value of ∆H and ∆S of the 2nd en-
dothermic peaks supports the partial breaking of the C4H4O62– moiety, whereas com-
paratively the large value of ∆H and ∆S of last peaks indicates the rapid decomposi-
tion of the intermediates to oxide and oxycarbonate/dioxycarbonate through the com-
plete rupture of all the metal–ligand bonds.
The foregoing results suggest the following tentative decomposition scheme in air.
room temp. – 170°C
~260°C
La[La(C4H4O6)3]⋅10H2O→La[La(C4H4O6)3](s)+10H2O(g)→
~353°C
La[La(C4H4O6)x](s) (where 2<x<3)+1CO(g)→
1/2La2O3(s)+1/2La2(C4H4O6)x(s)(where x=3 or <3)+LaCO(s)(trace)+
428–500°C
La2C3(s)(trace)+C(trace)+mCO(g)+nCO2(g)→
1/2La2O3(s)+La2O(CO3)2(s)/La2O2CO3(s)+
~805°C
LaCO(s)+La2C3(s)+pCO(g)+qCO2(g)+rC2H4(g)→
~970°C
La2O3(s)+La2C3(s)+LaCO(s)+La(s) (trace)+xCO(g)+yCO2(g)→
La2O3(s)(trace)+La2C3(s)(trace)+ LaCO(s)(trace)+La(s)(trace)
* * *
The author is thankful to Dr N. Sen Sarma, Dibrugarh University, Assam for recording the DTA-TG
profile, Dr S. D. Baruah of Regional Research Laboratory, Jorhat, Assam for DSC profile and Head,
RSIC, CDRI, Lucknow for elemental analysis.
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