Low-temperature heat capacity and thermodynamic properties of crystalline [Re2(Ala)4(H2O)8] (ClO4)6 (Re = Eu, Er; Ala = alanine)

Article abstract of DOI:10.1016/S0040-6031(02)00062-X

[Re₂(Ala)₄(H₂O)8] (ClO₄)₆ (Re = Eu, Er; Ala = alanine) were synthesized, and the low-temperature heat capacities of the two complexes were measured with a high-precision adiabatic calorimeter over the temperature range from 80 to 370 K. For [Eu₂(Ala)₄(H₂O)₈] (ClO₄)₆, two solid-solid phase transitions were found, one in the temperature range from 234.403 to 249.960 K, with peak temperature 243.050 K, the other in the range from 249.960 to 278.881 K, with peak temperature 270.155 K. For [Er₂(Ala)₄(H₂O)₈] (ClO₄)₆, one solid-solid phase transition was observed in the range from 270.696 to 282.156 K, with peak temperature 278.970 K. The molar enthalpy increments, ΔH_m, and entropy increments, ΔS_m, of these phase transitions, were determined to be 455.6 J mol^-1, 1.87 J K^-1 mol^-1 at 243.050 K; 2277 J mol^-1, 8.43 J K^-1 mol^-1 at 270.155 K for [Eu₂(Ala)₄(H₂O)₈] (ClO₄)₆; and 4442 J mol^-1, 15.92 J K^-1 mol^-1 at 278.970 K for [Er₂(Ala)₄(H₂O)₈] (ClO₄)₆. Thermal decompositions of the two complexes were investigated by use of the thermogravimetric (TG) analysis. A possible mechanism for the thermal decomposition is suggested.

Full text of DOI:10.1016/S0040-6031(02)00062-X

Thermochimica Acta 390 (2002) 31±37
Low-temperature heat capacity and thermodynamic properties
of crystalline [Re2(Ala)4(H2O)8](ClO4)6
(Re ˆ Eu, Er; Ala ˆ alanine)
Beiping Liua,b, Zhi-Cheng Tanb,*, Dashun Zhanga, Zhaodong Nanb,
Lixian Sunb, Fen Xub, Xiaozhen Lana, Ping Liangc, Xiaomei Yuc
aDepartment of Chemistry, Changde Normal College, Changde 415000, China
bThermochemistry Laboratory, Dalian Institute Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
cState Key Laboratory of Fine Chemical, Dalian University of Technology, Dalian 116012, China
Received 11 December 2001; received in revised form 7 February 2002; accepted 10 February 2002
Abstract
[Re2(Ala)4(H2O)8](ClO4)6 (Re ˆ Eu, Er; Ala ˆ alanine) were synthesized, and the low-temperature heat capacities of the
two complexes were measured with a high-precision adiabatic calorimeter over the temperature range from 80 to 370 K. For
[Eu2(Ala)4(H2O)8](ClO4)6, two solid±solid phase transitions were found, one in the temperature range from 234.403 to
249.960 K, with peak temperature 243.050 K, the other in the range from 249.960 to 278.881 K, with peak temperature
270.155 K. For [Er2(Ala)4(H2O)8](ClO4)6, one solid±solid phase transition was observed in the range from 270.696 to
282.156 K, with peak temperature 278.970 K. The molar enthalpy increments, DHm, and entropy increments,DSm, of these
phase transitions, were determined to be 455.6 J molÀ1, 1.87 J KÀ1 molÀ1 at 243.050 K; 2277 J molÀ1, 8.43 J KÀ1 molÀ1 at
270.155 K for [Eu2(Ala)4(H2O)8](ClO4)6; and 4442 J molÀ1, 15.92 J KÀ1 molÀ1 at 278.970 K for [Er2(Ala)4(H2O)8](ClO4)6.
Thermal decompositions of the two complexes were investigated by use of the thermogravimetric (TG) analysis. A possible
mechanism for the thermal decomposition is suggested. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: Alanine rare earth compounds; Heat capacity; Thermal decomposition; Adiabatic calorimeter; TG analysis
1. Introduction
and determined their crystal structures because of
the role of these complexes in studies of biological
systems and the interesting properties. Rare-earth ions
have peculiar physiological and biochemical activities
in animals, and their effects of anti-in¯ammation
and anti-cancer have been gradually recognized [4].
Nearly 200 kinds of solid complexes of rare-earth
compounds with L-amino acids have been prepared
and studied in the last decade.
Solid complexes of rare-earth compounds with
L-amino acids have been extensively investigated in the
last 20 years due to their signi®cant biological effects
[1]. For instance, Dao and co-workers [2,3] have
prepared and characterized lanthanide compounds
with amino acids and holmium with L-a-alanine,
In order to improve the processes of chemical synth-
esis of these complexes and carry out relevant appli-
cation and theoretical research, the thermodynamic
* Corresponding author. Tel.: ‡86-411-4379215;
fax: ‡86-411-4691570.
E-mail address: tzc@dicp.ac.cn (Z.-C. Tan).
0040-6031/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ( 0 2 ) 0 0 0 6 2 - X

32
B. Liu et al. / Thermochimica Acta 390 (2002) 31±37
properties of these substances are needed both in rare-
earth science and technology. However, the thermody-
namic data of these complexes have not been measured
previously.
2.2. Adiabatic calorimetry
Heat capacity measurements were carried out in a
precision automatic adiabatic calorimetric system
described in detail previously [5]. Brie¯y, it is an
adiabatic calorimeter with intermittent energy inputs
and temperature equilibrium after each input. The
calorimeter consists mainly of a sample cell, an
adiabatic (or inner) shield, a guard (or outer) shield,
two sets of differential thermocouples and a high
vacuum can. Liquid nitrogen was used as the cooling
medium. The evacuated can was kept within ca.
1 Â 10À3 Pa during the heat capacity measurements
so as to eliminate the heat loss owing to gas convec-
tion. Six pairs of copper±constantan differential ther-
mocouples (connected in series) were used to detect
the temperature difference between the sample cell
and the inner shield. Likewise, six pairs of differ-
ential thermocouples (connected in series) were
installed between the inner and the outer shields.
When the temperature of the sample cell increases
due to heating, the differential thermocouples detect
the temperature differences. These signals are used to
control the current through the heater wounded on the
inner and outer shields. Both shields were heated
under the control of the signals and kept at the
same temperatures as the sample cell. The sample
cell was a gold-plated copper container with an
internal volume of about 6 cm3. A Y-shaped gold-
plated copper vane was placed in the sample cell to
promote the heat conduction between the sample and
the cell. The temperature of the cell was determined
by a miniature platinum resistance thermometer
tightly inserted in a copper sheath that was silver-
soldered at the bottom of the sample cell. The calori-
metric data were automatically collected by use of
the Data Acquisition/Switch Unit (Model: 34970A,
Agilent, USA) and processed on line by a computer.
The sample mass of [Eu2(Ala)4(H2O)8](ClO4)6 and
[Er2(Ala)4(H2O)8](ClO4)6 used for the heat capacity
measurements are 1.9043 and 2.6361 g, which are
equivalent to 1.359 and 1.841 mmol, based on their
corresponding molar mass 1400.96 and 1431.58
g molÀ1, respectively. In order to demonstrate the
repeatability of the experimental results, two series
of heat capacity measurements were performed
under the same conditions for [Er2(Ala)4(H2O)8]-
(ClO4)6.
Heat capacity is one of the fundamental thermody-
namic properties of substances and closely related to
the energetic structure, and is sensitive to the variations
in other properties of substances. In the present work,
the low-temperature heat capacities of [Re2(Ala)4-
(H2O)8](ClO4)6 (Re ˆ Eu, Er; Ala ˆ alanine) have
been measured over the temperature range from 80
to 370 K. A possible mechanism of thermal decom-
position of the two complexes are proposed on the basis
of thermogravimetric (TG) data.
2. Experimental
2.1. Sample preparation
Rare-earth oxides (Eu2O3, Er2O3, 99.9%), perchlo-
ric acid (99%) and L-a-alanine were used to prepare
the calorimetric experimental samples in accordance
with the method described in [3]. First, the rare earth
oxides were dissolved in perchloric acid to obtain
aqueous solutions of the rare-earth perchlorates, then
after mixing the aqueous solutions with L-a-alanine at
the mole ratio 1:3, the mixed solutions were stirred
(in a 80 8Cwater bath) for 6 h. The mixed solutions
were concentrated by evaporation, and the solutions
weresubsequentlycooledand®ltered.The®ltrateswere
placed into a desiccator with P2O5 until crystalline
products were separated out from the solutions. The
crystals were ®ltered out and washed with absolute
alcohol for three times. Finally, the collected crystals
were desiccated in a dryer until the mass of the crystals
became constant.
The molecular compositions and structures of the
two complexes have already been reported in [3]. The
actual contents of the rare-earth ions in our complex
samples were determined by EDTA titrimetric ana-
lysis. The experimental results are very close to the
theoretical contents of rare-earth metals in the sam-
ples calculated according to the molecular composi-
tions and structures of the complexes. The results of
EDTA titrimetric analysis demonstrated that the che-
mical purities of the two samples are all higher than
99.8%.

B. Liu et al. / Thermochimica Acta 390 (2002) 31±37
33
Fig. 1. Experimental molar heat capacities Cp,m of [Eu2(Ala)4(H2O)8](ClO4)6 as a function of temperature.
Prior to the heat capacity measurements of the solid
complex samples, the reliability of the calorimetric
apparatus was veri®ed by heat capacity measurements
of the reference standard material a-Al2O3. The devia-
tions of our calibration results from the recommended
values reported by Ditmars of the National Bureau of
Standards [6] are within Æ0.2% in the temperature
range of 80±400 K.
Fig. 2. Experimental molar heat capacities Cp,m of [Er2(Ala)4(H2O)8](ClO4)6 as a function of temperature.

34
B. Liu et al. / Thermochimica Acta 390 (2002) 31±37
2.3. TG/DTG analysis
3. Results and discussion
3.1. Heat capacity
The TG measurements of the two samples were
carried out by a TG analyzer (Model: TGA/
SDTA851e, Metter- ToLedo, Switzerland) under high
purity nitrogen (99.999%) with the ¯ow rate of
60 ml minÀ1. The mass of the sample used for TG
analysis was 1.0134 and 1.6351 mg for [Eu2(Ala)4-
(H2O)8](ClO4)6 and [Er2(Ala)4(H2O)8](ClO4)6, res-
The experimental molar heat capacities of the two
solid complex of rare-earth compound with L-amino
acid are shown in Figs. 1 and 2, and tabulated in
Tables 1 and 2, respectively. The molar heat capacities
are ®tted to the following polynomials in reduced
temperature (X), by means of the least square ®tting.
pectively. The heating rate was 10 8Cmin À1
.
Table 1
The experimental molar heat capacities of [Eu2(Ala)4(H2O)8](ClO4)6 (molar mass: M ˆ 1400:96 g molÀ1
)
T (K)
Cp (J KÀ1 molÀ1
)
T (K)
Cp (J KÀ1 molÀ1
)
T (K)
Cp (J KÀ1 molÀ1
)
79.931
81.198
639.58
644.93
654.53
673.85
692.85
712.57
732.84
752.95
772.38
791.05
810.61
830.49
848.02
864.69
881.20
897.11
915.50
933.53
947.61
962.03
978.40
994.16
1010.06
1025.76
1044.59
1060.41
1078.37
1096.41
1116.30
1134.89
1152.36
1166.26
1181.97
1203.27
1230.63
1255.74
189.706
192.511
195.262
197.995
200.700
203.388
206.055
208.708
211.338
214.104
216.909
219.481
222.045
224.596
227.078
229.221
231.013
232.727
234.403
236.273
238.095
239.779
241.415
243.050
244.688
246.325
247.976
249.960
252.247
254.584
256.856
259.124
261.364
263.602
265.817
268.000
1277.87
1292.79
1307.91
1323.54
1337.41
1348.91
1365.81
1377.84
1393.91
1407.35
1413.93
1421.57
1426.53
1434.46
1445.02
1450.56
1457.93
1459.25
1469.26
1482.70
1498.31
1511.68
1540.04
1578.31
1560.69
1537.76
1527.21
1521.19
1538.29
1554.10
1568.96
1588.70
1611.01
1644.90
1677.85
1718.45
270.155
272.312
274.436
276.686
278.881
281.078
283.255
285.403
287.545
291.948
294.519
297.130
299.701
302.263
304.805
307.370
309.909
312.403
314.935
317.437
319.961
322.442
325.235
328.299
331.386
334.455
337.532
340.571
343.571
347.000
349.494
352.087
354.597
357.052
359.266
361.413
1790.99
1759.60
1700.18
1625.21
1600.70
1614.43
1630.15
1640.25
1651.58
1661.33
1670.02
1678.45
1683.72
1690.33
1692.42
1701.28
1704.54
1709.81
1714.81
1723.20
1728.78
1733.74
1741.77
1750.38
1759.33
1768.03
1775.67
1781.47
1791.74
1802.55
1816.25
1833.96
1850.76
1861.12
1876.11
1892.04
83.580
87.099
90.494
93.783
97.247
100.877
104.409
107.851
111.209
114.494
117.714
120.873
123.977
127.029
130.034
132.994
135.915
138.796
141.640
144.449
147.481
150.734
153.942
157.114
160.245
163.344
166.401
169.425
172.419
175.384
178.318
181.222
184.093
186.955

B. Liu et al. / Thermochimica Acta 390 (2002) 31±37
35
Table 2
The experimental molar heat capacities of [Er2(Ala)4(H2O)8](ClO4)6 (molar mass: M ˆ 1431:58 g molÀ1
)
T (K)
Cp (J KÀ1 molÀ1
)
T (K)
Cp (J KÀ1 molÀ1
)
T (K)
Cp (J KÀ1 molÀ1
)
T (K)
Cp (J KÀ1 molÀ1
)
80.797
83.075
85.300
87.477
89.610
91.703
93.758
95.779
97.766
99.724
101.653
103.555
105.430
107.283
109.113
110.922
112.710
114.479
115.901
117.639
119.359
121.062
122.748
124.421
126.079
127.724
129.353
130.968
132.573
134.400
136.447
138.844
141.582
144.285
422.10
439.92
453.50
466.90
479.98
494.20
507.02
516.22
524.78
537.23
548.58
555.79
564.03
572.69
582.07
591.78
601.64
617.68
620.88
622.66
630.60
640.77
644.81
652.06
657.09
665.48
673.22
677.35
687.03
695.33
701.64
715.83
726.18
735.06
146.958
149.604
152.222
154.814
157.382
159.927
162.457
164.407
165.784
167.941
170.842
173.717
176.565
179.384
182.175
184.939
187.679
190.394
193.085
195.766
198.458
201.156
203.826
205.974
209.058
211.598
214.081
216.560
219.038
221.515
223.117
223.967
224.805
225.628
749.12
764.53
774.61
780.28
791.04
803.29
804.14
820.02
830.22
839.72
849.32
866.55
880.17
895.04
908.64
919.57
934.16
953.24
964.45
976.38
981.92
990.62
1002.51
1013.11
1024.64
1039.18
1046.88
1060.61
1073.59
1076.93
1088.64
1096.17
1099.18
1104.16
226.450
228.052
230.433
232.697
234.265
236.234
238.225
240.173
242.121
244.088
246.038
247.981
249.917
251.844
253.764
255.676
257.579
259.475
261.364
263.247
265.122
266.988
268.847
270.696
272.533
274.363
275.934
277.583
278.970
281.006
282.156
283.900
285.682
287.453
1108.55
1116.74
1127.28
1132.72
1134.62
1140.16
1143.68
1146.31
1149.53
1155.07
1157.52
1161.75
1170.63
1176.10
1182.72
1189.09
1191.34
1195.22
1199.88
1209.30
1216.83
1220.12
1225.97
1242.93
1253.92
1291.13
1419.36
1760.18
2928.74
1605.97
1298.65
1288.91
1301.66
1313.47
289.205
292.289
293.425
296.281
298.770
301.857
304.929
307.988
311.039
314.069
317.061
320.055
323.032
325.992
328.935
331.860
334.768
337.657
340.527
343.377
346.203
349.005
351.775
354.502
357.186
359.827
362.468
367.489
370.303
372.900
375.456
378.095
1323.40
1319.60
1328.16
1324.43
1322.34
1313.04
1308.99
1311.63
1322.25
1333.96
1342.60
1348.24
1360.06
1369.94
1382.18
1394.64
1411.48
1422.62
1436.71
1455.15
1470.80
1495.30
1516.04
1531.85
1546.19
1557.32
1574.00
1586.30
1594.50
1599.18
1603.43
1606.21
For solid complex [Eu2(Ala)4(H2O)8](ClO4)6, over
the temperature range of 79±234 K:
For solid complex [Er2(Ala)4(H2O)8](ClO4)6, over
the temperature range of 80±271 K:
Cp;mꢀJ KÀ1 molÀ1
†
Cp;mꢀJ KÀ1 molÀ1
†
ˆ 1060:1169 ‡ 452:4446 X ‡ 76:6044 X2
ˆ 879:0532 ‡ 450:3532 X ‡ 17:9161 X2
‡ 18:4897X 3 À 93:0282 X4 À 57:2763 X5 (1)
À 138:9800 X3 À 71:7120 X4 ‡ 93:8387 X5 (3)
where X ˆ ꢀT À 156:5†/77.5, and T is the absolute
temperature. The correlation coef®cient of the ®tted
curve, R2 ˆ 0:9997.
where X ˆ ꢀT À 176†/95, R2 ˆ 0:9997.
Over the temperature range of 282±378 K:
Over the temperature range of 278±362 K:
Cp;mꢀJ KÀ1 molÀ1
†
Cp;mꢀJ KÀ1 molÀ1
†
ˆ 1383:9620 ‡ 237:2587 X ‡ 189:7223 X2
ˆ 1728:2331 ‡ 100:7472 X ‡ 23:7906 X2
À 1:2947 X3 À 4:9575 X4 ‡ 53:4252 X5
where X ˆ ꢀT À 320†/42; R2 ˆ 0:9992.
À147:4266 X3 À124:4560 X4 ‡66:3432 X5 (4)
(2)
where X ˆ ꢀT À 330†/48; R2 ˆ 0:9968.

36
B. Liu et al. / Thermochimica Acta 390 (2002) 31±37
From Figs. 1 and 2, it can be seen that the heat
thermodynamic function relationship as [7], DtrsHm
ˆ
capacities of the samples of [Eu2(Ala)4(H2O)8](ClO4)6
and [Er2(Ala)4(H2O)8](ClO4)6 increase with increasing
temperature in a smooth and continuous manner from
79 to 234 K, and from 278 to 362 K for [Eu2(Ala)4-
(H2O)8](ClO4)6, from 81 to 271 K, and from 282 to
378 K for [Er2(Ala)4(H2O)8](ClO4)6, respectively. No
phase transition or thermal anomaly was observed in
these temperature ranges. Therefore, the two samples
are stable in the above ranges. However, the thermal
anomalies were found in the temperature ranges from
234 to 278 K for [Eu2(Ala)4(H2O)8](ClO4)6 and from
271 to 282 K for [Er2(Ala)4(H2O)8](ClO4)6, respec-
tively. The peak temperatures of these thermal anoma-
lies were found to be 243.050 K, 270.155 K for
[Eu2(Ala)4(H2O)8](ClO4)6; 278.970 K for[Er2(Ala)4-
(H2O)8](ClO4)6, respectively. Two series of heat capa-
city measurements carried out in the regions of thermal
anomaly (see Figs. 1 and 2) may verify that the thermal
anomalies are reversible and repeatable. No fusion
phenomenon was observed in the sample after the heat
capacity measurements were completed. Thus, these
thermal anomalies may be ascribed to solid±solid phase
transitions in the investigated complex. So far no study
on the phase transitions of these complexes has been
reported in literature.
460 J molÀ1, DtrsSm ˆ 1:87 J KÀ1 molÀ1 at 243.050 K;
DtrsHm ˆ 2280 J molÀ1, DtrsSm ˆ 8:43 J KÀ1 molÀ1 at
270.155 K for [Eu2(Ala)4(H2O)8](ClO4); DtrsHm
ˆ
4440 J molÀ1
278.970 K for [Er2(Ala)4(H2O)8](ClO4), respectively.
,
DtrsSm ˆ 15:92 J KÀ1 molÀ1
at
3.2. The results of TG/DTG analysis of the
complexes
The TG/DTG curves of the two solid complexes of
rare-earth compounds with alanine are shown in Figs. 3
and 4, respectively. It can be seen from the TG/DTG
curves that the thermal decompositions takes place in
three stops. According to the mass loss in each step,
possible mechanisms for the thermal decompositions
are as follows
[Eu2(Ala)4(H2O)8](ClO4)6
104À203 ꢁ
9:82%ꢀ10:28%†
C
!
‰Eu2ꢀAla† ŠꢀClO4†
4
6
259À304 ꢁ
24:18%ꢀ25:44%†
C
!
Eu2ꢀClO4†
6
319À422 ꢁ
28:53%ꢀ27:30%†
C
!
2 EuCl3
[Er2(Ala)4(H2O)8](ClO4)6
The molar enthalpies, DtrsHm, and the entropies,
DtrsSm, of the phase transitions of the two complexes
can be derived from the heat capacity data according to
104À190ꢁC
9:27%ꢀ10:05%†
!
‰Er2ꢀAla† ŠꢀClO4†
4
6
Fig. 3. TG/DTG curve of [Eu2(Ala)4(H2O)8](ClO4)6 under nitrogen atmosphere.

B. Liu et al. / Thermochimica Acta 390 (2002) 31±37
37
Fig. 4. TG/DTG curve of [Er2(Ala)4(H2O)8](ClO4)6 under nitrogen atmosphere.
227À312 ꢁ
25:80%ꢀ24:91%†
C
References
!
2 ErꢀClO4†
3
[1] B.S. Guo, Chinese Rare Earth 20 (1999) 64.
312À412ꢁC
28:34%ꢀ29:97%†
!
2 ErCl3
[2] J. Legendziewcz, T. Glowiak, E. Huskowska, C.N. Dao, Rare
Earths Spectroscopy, World Scienti®c, Singapore, 1985,
p. 146.
The mass-loss (in %) in the brackets are calculated
theoretical values.
[3] H.D. Zeng, K.Z. Pan, J. Struct. Chem. 11 (1992) 393.
[4] R.W. Den, Z.S. Zhang, Chinese Rare Earth 8 (1987) 36.
[5] Z.C. Tan, G.Y. Sun, Y. Sun, A.X. Yin, W.B. Wang, J.C. Ye,
L.X. Zhou, J. Therm. Anal. 45 (1995) 59.
Acknowledgements
[6] D.A. Ditmars, S. Ishihara, S.S. Chang, G. Bernstein, E.D.
West, J. Res. Nat. Bur. Stand 87 (1982) 159.
The authors gratefully acknowledge the National
Nature Science Foundation of China for ®nancial
support to this work under Grant No. 20073047.
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