A thermal study of several lanthanide triflates

Article abstract of DOI:10.1016/S0277-5387(98)00158-2

Five lanthanide triflates, Ln(TfO)₃·nH₂O, where TfO^-=CF₃SO₃^-, Ln=La³, Nd³, Sm³, Gd³ and Yb³, and n=9 and 13, have been prepared and the thermal decomposition processes of these triflates up to 600°C were characterized by means of TG, DTA, XRD. The thermal studies have shown almost all the lanthanide triflates prepared in this study to exist as a stable nonahydrate. During the stepwise dehydration processes, it was found that mono-, di-, tri-, penta-, and heptahydrates were formed. Decompositions were found to be exothermic, and calcinations of these triflates at 600°C resulted in the formation of the corresponding LnF₃. Crystal systems of the trifluorides thus obtained were hexagonal for La, Nd and Sm trifluorides, whereas those of Gd and Yb were found to be orthorhombic. The volatile decomposition products at 600°C were identified by MS, and it was revealed that the over all reaction scheme for the thermal decomposition proceeds as follows: Ln(OTf)₃→LnF₃+3SO₂+CO₂+CF ₃OCF₃.

Full text of DOI:10.1016/S0277-5387(98)00158-2

Polyhedron Vol[ 06\ No[ 19\ pp[ 2514Ð2520\ 0887
Þ 0887 Elsevier Science Ltd
Pergamon
All rights reserved[ Printed in Great Britain
9166Ð4276:87 ,08[99¦9[99
\
PII] S9166Ð4276"87#99047Ð1
A thermal study of several lanthanide tri~ates
Naohisa Yanagihara\ꢀ Shin Nakamura and Masayoshi Nakayama
Department of Materials Science and Engineering\ Teikyo University\ 0!0 Toyosatodai\ Utsunomiya\
219\ Japan
"Received 5 March 0887^ accepted 7 April 0887#
Abstract*Five lanthanide tri~ates\ Ln"TfO#2=nH1O\ where TfO−ꢁCF2SO2−\ LnꢁLa2¦\ Nd2¦\ Sm2¦\ Gd2¦
and Yb2¦\ and nꢁ8 and 02\ have been prepared and the thermal decomposition processes of these tri~ates up
to 599>C were characterized by means of TG\ DTA\ XRD[ The thermal studies have shown almost all the
lanthanide tri~ates prepared in this study to exist as a stable nonahydrate[ During the stepwise dehydration
processes\ it was found that mono!\ di!\ tri!\ penta!\ and heptahydrates were formed[ Decompositions were
found to be exothermic\ and calcinations of these tri~ates at 599>C resulted in the formation of the cor!
responding LnF2[ Crystal systems of the tri~uorides thus obtained were hexagonal for La\ Nd and Sm
tri~uorides\ whereas those of Gd and Yb were found to be orthorhombic[ The volatile decomposition products
at 599>C were identi_ed by MS\ and it was revealed that the over all reaction scheme for the thermal
decomposition proceeds as follows] Ln"OTf#2:LnF2¦2SO1¦CO1¦CF2OCF2[ Þ 0887 Elsevier Science Ltd[
All rights reserved
Keywords] lanthanide tri~ates^ thermal process^ stable nonahydrate[
———————————————————————————————————————————————
INTRODUCTION
obtained in studies of the thermal decomposition of
several other lanthanide tri~ates[
Tri~uoromethanesulfonic acid\ commonly known
as tri~ic acid "TfOH#\ was _rst reported in 0843 by
Haszeldine and Kidd ð0Ł[ Since then\ there has been
a rapid growth in the chemistry of tri~ate and its
derivatives\ and a great variety of metal complexes
containing the coordinated tri~uoromethanesulfonate
"tri~ate# have been synthesized ð1Ð3Ł[ Interest in these
compounds as a potential use in both inorganic and
organic synthons stems from speci_c properties of
the tri~ate anion\ e[g[\ the low basicity and the poor
coordinating ability ð4Ł[ A recent striking example in
this area is the use of lanthanide tri~ates as Lewis acid
catalysts in organic synthesis ð6Ł[ Until now\ however\
it appears that most of the interest in the derivatives
has centered on the chemistry involving transition
metals\ and that little attention has been paid to the
lanthanide tri~ates[ Moreover\ a survey of the litera!
ture revealed only a few studies ð6Ð09Ł on the thermal
decomposition of lanthanide compounds[
EXPERIMENT
Preparation of lanthanide tri~ates
Tri~uoromethanesulfonic acid "TfOH# was used as
received[ Lanthanide oxides were of 88[4) pure
"Wako Pure Chemical\ Japan#[ Prior to use the oxides
were freshly ignited at 799>C in quartz crucibles[ On
the basis of reports in the literature ð01\ 02Ł\ an excess
of the corresponding sesquioxide was added to a 49)
aqueous solution of TfOH\ and the mixture was boiled
and _ltered to remove the unreacted oxide[ The water
was then removed from the _ltrate under reduced
pressure until the hydrate was formed[ The hydrates
were dried in a rotary vacuum evaporator at 79>C for
0 h[ The drying was not prolonged in order to main!
tain the same dehydration conditions\ and the tri~ates
thus obtained were used for the thermal analysis stud!
ies[ The IR spectra of the hydrated tri~ates that were
used initially were prepared as KBr disks and recorded
with the aid of a Shimadzu FTIR 3299[
Recently\ we reported a study on the thermal
decomposition of lanthanum tri~ate ð00Ł[ In our con!
tinuing investigations\ we report\ here\ the results
The lanthanide content and the water content of
the each starting tri~ates were determined by titration
ꢀ Author to whom correspondence should be addressed[
2514

2515
N[ Yanagihara et al[
with a standard EDTA solution and TGA experi! curve is denoted as T0max\ T1max\ and so on[ In addition\
ments\ respectively[ Found] La\ 07[4[ Calc for the superscript s in the peak temperature means that
ð"LaC2F8S2O8#=8H1OŁ] La\ 07[5)[ Found] Nd\ 08[0[ the peak is poor or a shoulder[ The reaction tem!
Calc for ð"NdC2F8S2O8#=8H1OŁ] Nd\ 08[9)[ Found] peratures along with the WL obtained from both TG
Sm\ 08[8[ Calc for ð"SmC2F8S2O8#=8H1OŁ] Sm\ 08[7)[ and DTA curves are summarized in Table 0[
Found] Gd\ 19[7[ Calc for ð"GdC2F8S2O8#=8H1OŁ] Gd\
TG studies revealed that the decomposition of tri!
19[4)[ Found] Yb\ 19[4[ Calc for ð"YbC2F8S2O8#= ~ates occurred in two main stages[ The _rst stage
02H1OŁ] Yb\ 19[2)[
involved a stepwise dehydration to the anhydrous
state\ followed by the decomposition of the tri~ates
to stable _nal products[ As shown in Table 0\ the
observed WL values that were found in both dehy!
dration and decomposition processes are in good
agreement with the calculated values[ In conjunction
with the TG curves\ it is obvious that the multiplicity
of the endothermic peaks corresponds to each dehy!
dration step\ and the exothermic peaks correspond to
the decomposition "see Fig[ 1#[ Rather complex peaks
observed in the decomposition temperature region
may be explained by the postulate that the decompo!
sition process also occurs step by step[
The tri~ates began to lose water of hydration from
31Ð53>C giving two or three clearly de_ned steps prior
to reaching the anhydrous state[ The corresponding
to the anhydrous state\ beginning around 089Ð119>C
were not quite horizontal\ and suggests that a slow
decomposition occurs[ Finally\ more rapid decompo!
sitions at 259Ð309>C resulted in a sharp weight loss to
a continuous horizontal plateau[ After the complete
decompositions\ however\ some of the TG curves tend
to increase and the weight increases\ which suggests a
probable slow oxidation of the _nal product[ These
oxidations are anticipated from reexaminations\ in
which when the same experiments were carried out in
a dynamic atmosphere of N1\ the resulting _nal pla!
teaus on the TG curves remained quite horizontal
until a temperature reached 599>C[
As shown in Table 0\ the number of hydrated water
molecules in almost all starting tri~ates were found to
be 8\ except for Yb tri~ates[ These stoichiometries of
the hydrated water molecules are consistent with the
results of X!ray structural studies of the hydrates of
lanthanide tri~ates\ ðLn"H1O#8ŁðTOfŁ2^ "LnꢁLa\ Nd\
Gd\ Ho and Lu# ð04\ 05Ł and with those analogous
compounds such as Tb"H1O#8"BrO2#2 ð06Ł and non!
aquasamarium"III# bromide!0\3!dioxane adduct\ e[g[\
Sm"H1O#8=Br2"C3H7O1#1 ð07Ł[ In the present study\
although the number of hydration molecules varies in
each tri~ate\ it is of interest to note that all the tri~ates
lose the 5 hydrated water molecules at the _rst dehy!
dration step\ which corresponds to the strongest endo!
thermic peak in the DTA curves[ On the basis of the
IR measurements of the starting hydrated Ln tri~ates\
it was suggested ð8Ł that there exist two types of dis!
Thermal analysis
TG and DTA curves were obtained on heating up
to 599>C\ using a Shimadzu DT!39[ The heating rate
was 4>C min−0 in a static atmosphere of air[ A small
portion "ca[ 04Ð19 mg# of sample was used for the TG\
and the highly sintered a!Al1O2 was the thermally inert
reference material used for the DTA[
Using copper"II# sulfate pentahydrate as an exter!
nal standard\ a computational integration of the endo!
thermic area on the DTA curves of each Ln tri~ate
was applied to estimate roughly an enthalpy change
for the process encountered throughout the dehy!
drations[
Analysis of solid and `aseous products obtained by the
calcination
The calcined solid was fused with NaOH\ and dis!
solved in a minimum of 5 m HCl\ diluted\ and the
lanthanide content was determined by titration with
a standard EDTA solution ð03Ł[ The oxygen content
in the calcined product was not determined[ Found]
La\ 60[9[ Calc for LaF2] La\ 69[8)[ Found] Nd\ 60[3[
Calc for NdF2] Nd\ 60[6)[ Found] Sm\ 61[0[ Calc for
SmF2] Sm\ 61[4)[ Found] Gd\ 62[9[ Calc for GdF2]
Gd\ 62[3)[ Found] Yb\ 63[7[ Calc for YbF2] Yb\
64[1)[
Powder di}ractograms of the calcination products
were obtained by means of a Shimadzu XD!509 with
a source of Cu Ka radiation[ For identi_cation
purposes\ di}raction patterns ð"I:I9# vs d spacing "A#Ł
of the products were matched with the relevant data
on JCPDS cards[
Ä
Mass spectra "MS# of a gas phase decomposition
products were obtained with a JEOL A V494W in the
electron impact positive ion mode[
RESULTS AND DISCUSSION
Dehydration and decomposition processes
While Fig[ 0 shows the simultaneous DTA and TG tinct water molecules in the tri~ates[ The IR bands\
curves for La tri~ate as a typical example of the ther! attributed to the OH stretching frequency\ which
mal analysis of lanthanide tri~ates\ DTA curves for appeared around 2589 and 2549 cm−0 may be due to
all the tri~ates studied are shown in Fig[ 1[ For con! very weakly held water while the broad OH band
venience\ each weight loss "WL# steps monitored on around 2349 cm−0 may be due to more strongly hydro!
the TG curve is designated in Roman Numerals\ and gen!bonded water in the complexes[ Therefore\ it is
the corresponding peak temperature on the DTA reasonable to conclude that the aforementioned 5

A thermal study of several lanthanide tri~ates
2516
Fig[ 0[ TG and DTA curves for lanthanum tri~ate[
water molecules which were liberated at relatively low the initial decomposition temperature tends to
temperature regions are those that are not strongly decrease gradually with an increase in atomic number
bonded to the salts[
of the lanthanide[
In the subsequent series of the stepwise dehydration
processes\ a variety of hydrated intermediates were
formed "see Table 0#[ However\ the most stable Solid decomposition products
hydrate formed as an intermediate was the dihydrated
species for the La\ Pr\ Nd\ and Sm tri~ates[ Mono and
In an e}ort to con_rm the products remaining after
trihydrates were found to be stable for Gd and Yb decomposition at 599>C in a static atmosphere of air\
tri~ates\ respectively[ The observations of the series of the products were subjected to an XRD study[ In
intermediates also con_rm that there exist di}erent order to identify the solid products\ the relevant
types bonded water molecules in the lanthanide tri~! JCPDS cards were employed[ The JCPDS cards used
ate\ and they play a special role in the coordination of for the veri_cation were 21Ð372\ 8Ð305\ 01Ð681\ 01Ð
the ligands with the lanthanide metal[ Similar _ndings 677 and 23Ð091 for the tri~uorides of La\ Nd\ Sm\ Gd
were reported by Rillings et al[ ð8Ł\ during the thermal and Yb\ respectively[ In a typical result\ the XRD data
study of the lanthanide salts[ They reported that Pr\ of the La and Yb samples are compared with the
Sm and Er tri~uoroacetates show mono!\ sesqui! and reference data of LaF2 and YbF2\ respectively\ in
dihydrate intermediates while the corresponding pen! Table 1[ As shown in Table 1\ it is clear that both the
ta~uoropropionates show sesqui! and dihydrate inter! XRD data match very well\ and this is the case for
mediates during their dehydrations[
the rest of the tri~ates[ Furthermore\ since no peaks
In addition\ the roughly estimated enthalpy values corresponding to lanthanide oxy~uorides were found
ðDHdehyd[ "kJ mol−0#Ł for the total dehydration process on the XRD pro_les\ it is reasonably safe to conclude
of each Ln tri~ate are] 012 "La#^ 014 "Nd#^ 020 "Sm#^ that all the lanthanide tri~ates prepared in this study
024 "Gd# and 014 "Yb#[ These values are within the are converted almost completely to the corresponding
range characteristic of dehydration processes ð09Ł[
tri~uorides at 599>C[ This conclusion is also sub!
In Table 0\ it is also worth noting that the initial stantiated by the quantitative analysis of the each
temperature of the dehydration process increases as tri~uoride "see Experimental section#[
the atomic number of the lanthanide increases thus
When the calcinations of the tri~ates were carried
re~ecting the lanthanide contraction and decreasing out at higher than 699>C\ almost all of them decom!
base strength of the elements[ In contrast to this tend! posed gradually to a mixture of oxy~uoride and oxide\
ency\ however\ the decomposition behavior of the tri! and _nally at 0999>C\ they were all converted into
~ates is somewhat more complicated[ It appears that the Ln1O2[ These observations are consistent with the

2517
N[ Yanagihara et al[
Fig[ 1[ DTA curves for lanthanide tri~ates "see text for roman numerals#[
results reported by Rillings et al[ ð8Ł[ During a study forms at atmospheric temperature\ respectively[ These
of the thermal decomposition of several lanthanide results are consistent with our observations\ except
per~uoroacetates\ they found that LnF2 and LnOF for the crystal system of the SmF2[ It is known ð08\
"LnꢁPr\ Sm and Er# are formed around 349>C and 11Ł that samarium tri~uoride is dimorphic occurring
699>C upon the decomposition of the corresponding as a high!temperature phase "b!SmF2\ hexagonal# and
tri~uoroacetates and penta~uoroacetates[ In addition\ low!temperature phase "a!SmF2\ orthorhombic#\ and
they reported that the decomposition of the ~uoride the latter is substantially more stable than the former[
to oxy~uoride is a hydrolysis reaction[ Thus\ there is Thus\ it is noteworthy that the b!SmF2 can be easily
a little chance of the formation of LnOF under the obtained upon the thermal decomposition of the cor!
experimental conditions of this study[
responding Sm tri~ate[
On the other hand\ based on the JCPDS cards\ the
tri~uorides of La\ Nd and Sm are hexagonal\ whereas
GdF2 and YbF2 are found to be orthorhombic and Volatile decomposition products and overall reaction
isostructural with YF2[ It was reported ð08Ð10Ł that
the tri~uorides of the rare earths of La to Nd and
When the tri~ates were pyrolyzed at a slightly
those of Sm to Lu are thought to occur in the hex! higher temperature at the end of the decomposition\
agonal form and the orthorhombic form\ as stable it was found that volatile products are evolved[ The

A thermal study of several lanthanide tri~ates
Table 0[ Thermal analysis data for lanthanide tri~ates
2518
Weight loss ")#
dehydration decomposition
Temperature ">C#
Num[ of dehydration
Ln
La
Stage Num[
I
range
peak
in each stage
total
8
obsd[
calcd[
obsd[
calcd[a
31½094
T0maxꢁ79
T0max0sꢁ84
T1maxꢁ064
T2maxꢁ310
T0maxꢁ89
T1maxꢁ003
T2max0sꢁ051
T2max1sꢁ066
T3max0sꢁ394
T3max1sꢁ305
T0maxꢁ82
T1maxꢁ033
T2maxꢁ072
T3max0sꢁ273
T3max1sꢁ392
T3max2sꢁ301
T0maxꢁ81
T1maxꢁ059
T2maxꢁ083
T3maxꢁ268
T4maxꢁ302
T0maxꢁ83
5
0
1
03[5
05[6
10[4
03[3
05[7
10[6
II
III
I
II
III
045½077
399½316
37½093
095½018
040½089
64[1
62[7
Nd
Sm
5
0
1
03[6
06[2
10[6
03[2
05[6
10[4
8
8
IV
275½311
62[3
62[2
I
II
III
IV
41½092
020½045
057½086
263½307
5
0
1
02[6
07[5
10[2
03[1
05[5
10[2
61[0
60[5
63[2
61[6
61[0
62[00
Gd
Yb
I
II
III
IV
V
I
II
III
IV
V
40½092
028½057
079½192
252½274
394½319
53½097
008½038
045½188
236½264
319½329
5
1
0
02[7
07[5
19[7
03[0
07[7
10[0
8
5
3
2
02[7
10[6
16[4
01[5
10[0
16[3
T1maxꢁ032
T2maxꢁ077
T3maxꢁ252
T4maxꢁ315
02
aCalculated as LnF2[
MS analysis of the gases showed that three peaks at This reaction mechanism is di}erent from those that
m:z 33\ 53 and 74 with relatively high intensity are have been proposed by Rillings et al[ ð8Ł and others
present in the spectra "see Fig[ 2#[ The _rst two peaks ð6\ 12Ł[ They studied the thermal decomposition of
are easily assigned to be molecular ion peaks of CO1 Pr and Sm tri~uoroacetates in the temperature range
and SO1\ respectively\ and each of them has isotopic from 164 to 299>C[ In this study\ on the basis of IR
peaks at m:z 34 and 55[ The peak at m:z 74 can be analysis\ they suggested that tri~uoroacetic anhydride
considered as a fragment ion peak that was generated exists as an intermediate during the decomposition in
as a result of the cleavage from the molecular ion\ the _rst stage\ in the next stage the anhydride decom!
ðCF2OCF2Ł ¦\ which is probably unstable and easily poses to tri~uoroacetyl ~uoride\ carbonyl di~uoride
=
decomposes during the ionization process in the MS and carbon monoxide[ Accordingly\ they reported the
analyzer[ And the ðCF2Ł¦ peak at m:z 58 arises sub! pyrolysis of the tri~uoroacetates to proceed as]
sequently from the ðCF2OŁ¦[ Thus\ the probable path!
1Ln"CF2CO1#2:1LnF2¦2CF2COF¦2CO1¦2CO
way of the fragmentation can be rationalized as]
The main di}erence in the thermal decomposition of
=
−CF2
the tri~uoroacetacs and tri~ates can be found in the
gaseous products^ i[e[\ CF2COF and CF2OCF2[
Although it is di.cult to compare both mechanisms
since the gases formed during pyrolysis are usually
very complicated\ the di}erence between the mech!
anisms and the major gaseous products might be
attributable to the counter anion in these compounds\
and also to the temperatures at which decomposition
occurs[
ðCF2OCF2Ł ¦ 004
=
=
ðCF2OŁ¦ "m:z 74# 0−0O4 ðCF2Ł¦ "m:z 58#
Consequently\ the present studies revealed that the
general thermal decomposition reaction for the lan!
thanide tri~ates proceed according to an overall reac!
tion scheme as follows]
Ln"CF2SO2#2:LnF2¦2SO1¦CO1¦CF2OCF2

2529
N[ Yanagihara et al[
Table 1[ A comparison of X!ray powder di}raction data of "A# LaF2
and "B# YbF2
Referencea\c
Ä
Observed
Ä
h k l
d "A#
I:I9
d "A#
I:I9
Dd:Aýb
"a# LaF2
9 9 1
0 0 9
0 0 0
0 0 1
2 9 9
0 0 2
2 9 1
1 1 0
1 1 2
2 9 3
0 0 4
2[567
2[485
2[120
1[469
1[963
1[913
0[796
0[634
0[338
0[265
0[278
49
39
099
04
44
54
39
14
07
05
01
2[576
2[592
2[126
1[463
1[967
1[915
0[797
0[636
0[349
0[265
0[250
35
20
099
8
41
48
20
19
04
00
6
9[998
9[996
9[995
9[993
9[993
9[991
9[990
9[991
9[990
9
−9[917
"b# YbF2
9 9 0
0 9 0
9 1 9
0 0 0
1 0 9
0 1 0
9 9 1
1 1 0
0 0 1
0 2 0
2 9 0
9 1 1
1 2 9
2 0 0
0 1 1
1 0 1
9 3 9
2 1 0
0 3 0
2 2 0
1 2 1
2[601
2[509
2[283
2[076
1[715
1[361
1[105
1[925
0[885
0[805
0[766
0[744
0[718
0[798
0[667
0[633
0[585
0[531
0[424
0[333
0[300
08
59
51
099
55
16
01
04
19
39
15
04
21
07
03
12
01
06
01
01
05
2[619
2[504
2[287
2[080
1[716
1[363
1[108
1[925
0[887
0[807
0[766
0[747
0[718
0[798
0[668
0[634
0[586
0[531
0[424
0[333
0[301
06
53
47
099
50
14
09
04
06
25
14
01
16
04
00
11
09
06
09
00
05
9[997
9[994
9[993
9[993
9[990
9[991
9[992
9
9[991
9[991
9
9[992
9
9
9[990
9[990
9[992
9
9
9
9[990
aJCPDS 21!372[
bDdꢁdobsd[−dref[
cJCPDS 23!091[
[
Fig[ 2[ Mass spectra of the volatile products obtained by the pyrolysis of lanthanum tri~ate[

A thermal study of several lanthanide tri~ates
2520
Acknowled`ements*N[ Y[ would like to express his grateful 01[ Solomon\ R[ G[ and Kochi\ J[ K[\ J[ Chem[ Soc[\
thanks to Eno Science Foundation for the _nancial support[
Chem[ Commum[\ 0868\ 441[
02[ Fosberg\ J[ H[\ Spaziano\ V[ T[\ Bala!
subramanian\ T[ M[\ Liu\ G[ K[\ Kinsley\ S[ A[\
Duckworth\ C[ A[\ Poteruca\ J[ J[\ Brown\ P[ S[
and Miller\ J[ L[\ J[ Or`[ Chem[\ 0876\ 41\ 0906[
REFERENCES
0[ Haszeldine\ R[ N[ and Kidd\ J[ M[\ J[ Chem[ Soc[ 03[ Catalano\ E[\ Bedford\ R[ G[ and Silveria\ V[ G[\
0843\ 3117[
J[ Phys[ Chem[ Solids\ 0858\ 29\ 0502[
04[ Harrow_eld\ J[ M[\ Keper\ D[ L[\ Patrick\ J[ M[
and White\ A[ H[\ Aust[ J[ Chem[\ 0872\ 25\ 372[
05[ Santos\ C[ O[ P[ and Castellano\ E[ E[\ Inor`anica
Chimica Acta\ 0874\ 009\ 72[
1[ Senning\ A[\ Chem[ Rev[\ 0854\ 54\ 274[
2[ Howells\ R[ D[ and McCown\ J[ D[\ Chem[ Rev[\
0866\ 66\ 58[
3[ Lawrance\ G[ A[\ Chem[ Rev[\ 0875\ 75\ 06[
4[ Stang\ P[ J[ and White\ M[ R[\ Aldrichimica Acta\ 06[ Gallucci\ J[ C[\ Gerkin\ R[ E[ and Reppart\ W[ J[\
0872\ 05\ 04[
Cryst[ Struct[ Chem[\ 0871\ 00\ 0030[
5[ Kawada\ A[\ Mitamura\ S[ and Kobayashi\ S[\ J[ 07[ Barries\ J[ C[ and Nicoll\ G[ Y[ R[\ Inor`anica
Chem[ Soc[\ Chem[ Commun[\ 0882\ 0046[
Chimica Acta\ 0874\ 009\ 36[
6[ Roberts\ J[ E[\ J[ Am[ Chem[ Soc[\ 0850\ 72\ 0976[ 08[ Zalkin\ A[ and Templeton\ D[ H[\ J[ Am[ Chem[
7[ Paul\ R[ C[\ Bains\ M[ S[ and Ghotra\ J[ S[\ Indian
J[ Chem[\ 0853\ 6\ 403[
8[ Rillings\ K[ W[ and Roberts\ J[ E[\ Thermochimica
Acta\ 0863\ 09\ 174[
09[ Hussen\ G[ A[!H[ and Ismail\ H[ M[\ Bull[ Chem[
Soc[ Jpn[\ 0883\ 56\ 174[
Soc[\ 0842\ 64\ 1342[
19[ Thoma\ R[ E[ and Brunton\ G[ D[\ Inor`[ Chem[\
0855\ 4\ 0826[
10[ Weigel\ F[ and Scherrer\ V[\ Radiochem[ Acta\
0856\ 6\ 39[
11[ Oftedal\ I[\ Z[ Phys[ Chem[ B\ 0818\ 4\ 160[
00[ Yanagihara\ N[\ Nakamura\ S[ and Nakayama\ 12[ Corbett\ P[ J[ and Whittle\ E[\ J[ Chem[ Soc[\ 0852\
M[\ Chem[ Lett[\ 0884\ 444[
2136[