Structural studies on the phase transition of rubidium formate

Article abstract of DOI:10.1016/S0040-6031(98)00327-X

The thermal phase transition of rubidium formate, observed at 363 K, was studied by means of thermal analysis and X-ray diffraction method. The transition was found to be a reversible first-order transition. In the phase transition, the orthorhombic phase (Pca2₁) with lattice constants a=9.229(1), b=4.630(2) and c=7.408(2) A, was transformed to the monoclinic phase (P2₁/m) with one of a=4.6552(4), b=4.6528(7), c=7.5170(6) A and γ=97.610(6)°. The values of the enthalpy, ΔH_tr, and entropy change, ΔS_tr, for the transition were 0.42±0.02 kJ mol^-1 and 1.16±0.05 J K^-1 mol^-1, respectively. Although the relative positions of rubidium atoms hardly changed in the phases before, and after, the transition, the orientation of formate ions differed in both the phases. The values of ΔH_tr and ΔS_tr seem to reflect a slight change of structure in the orientation of formate ions.

Full text of DOI:10.1016/S0040-6031(98)00327-X

Thermochimica Acta 318 (1998) 39±43
1
Structural studies on the phase transition of rubidium formate
2
,a,*
b
Yoshio Masuda , Wataru Morita , Akihiko Yahata , Yasuhiko Yukawa
c
a
a
Department of Environmental Science, Graduate School of Science and Technology, Niigata University, Niigata 950-21, Japan
b
Department of Chemistry, Faculty of Science, Niigata University, Niigata 950-21, Japan
c
Nikki Chemical Co., Ltd., Niitzu 956, Japan
Received 26 June 1997; accepted 13 January 1998
Abstract
The thermal phase transition of rubidium formate, observed at 363 K, was studied by means of thermal analysis and X-ray
diffraction method. The transition was found to be a reversible ®rst-order transition. In the phase transition, the orthorhombic
Ê
phase (Pca2
(
1
) with lattice constants aˆ9.229(1), bˆ4.630(2) and cˆ7.408(2) A, was transformed to the monoclinic phase
Ê
tr
P2 /m) with one of aˆ4.6552(4), bˆ4.6528(7), cˆ7.5170(6) A and ꢀˆ97.610(6)8. The values of the enthalpy, ÁH , and
� 1 � 1 � 1
1
entropy change, ÁStr, for the transition were 0.42Æ0.02 kJ mol and 1.16Æ0.05 J K mol , respectively. Although the
relative positions of rubidium atoms hardly changed in the phases before, and after, the transition, the orientation of formate
ions differed in both the phases. The values of ÁHtr and ÁStr seem to re¯ect a slight change of structure in the orientation of
formate ions. # 1998 Elsevier Science B.V.
Keywords: Rubidium formate; Thermal analysis; Thermal phase transition; X-ray diffraction method
1
. Introduction
thermal phase transitions observed before melting of
these formates.
Thermal properties of alkali metal formates have
Ferloni et al. [4] reported the values of enthalpy and
entropy changes for alkali metal formates; however,
they have not mentioned the structural changes of the
phases obtained before, and after, the phase transi-
tions.
Hashimoto et al. [5±9] studied the phase transitions
of alkali metal formates by means of thermal analysis
and powder X-ray diffraction measurement and deter-
mined the lattice parameters of both the phases before,
and after, the transitions of these formates. However,
they could not determine the crystal structure of each
phase.
been studied by Shishido and Masuda [1±3]. These
formates decomposed through two stages. In the ®rst
stage, the formates decomposed to form the respective
carbonates and oxalates, and the oxalates thus formed
decomposed to carbonates and carbon monoxide in
the second stage. Although they described the decom-
position process and the atmospheric effect on the
decomposition in detail, they scarcely referred to the
*
1
Corresponding author.
Dedicated to Professor P.K. Gallagher in honor of his 70th
birthday.
The crystal structures for both phases of cesium
formate were determined by Masuda et al. [10] using
the Rietveld analysis [11,12] of their X-ray powder
2
Department of Environmental Science, Faculty of Science,
Niigata University, Niigata 950-21, Japan
0040-6031/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved
P I I S 0 0 4 0 - 6 0 3 1 ( 9 8 ) 0 0 3 2 7 - X

4
0
Y. Masuda et al. / Thermochimica Acta 318 (1998) 39±43
diffraction pro®les. They also discussed the relation-
ship between the values of the enthalpy and entropy
changes and the structural changes for the phase
transition.
the sample chamber was purged with dry nitrogen gas,
and the measurements were carried out at a heating
�
1
rate of 5 K min under a closed system. The instru-
ment was calibrated with enthalpy changes for the
In this paper, the crystal structure of rubidium
formate before transition is determined by means of
the single-crystal X-ray diffraction method, and that
after the transition is determined with the help of the
Rietveld method [11,12] by analyzing the X-ray pow-
der diffraction pro®les. The relationship between
enthalpy (ÁH ) and entropy (ÁS ) changes and the
phase transitions of potassium nitrate (T ˆ400.9 K,
tr
�
1
ÁH ˆ5.4 kJ mol
)
and potassium perchlorate
tr
�
1
(T ˆ572.7 K, ÁH ˆ13.8 kJ mol ) [13].
tr
tr
2.3. Determination of the crystal structure.
A 0.5Â0.4Â0.3 mm specimen was sealed in a glass
capillary, and measurements were performed on a
Rigaku AFC5R diffractometer with graphite mono-
tr
tr
structural changes for the phase transition is discussed
brie¯y.
Ê
chromated MoK radiation (ꢁˆ0.7107 A). The data
a
were collected using !� 2ꢂ scan technique to a max-
imum 2ꢂ value of 70.08. A total of 591 re¯ections were
collected, and 323 of the re¯ections, having intensities
three times stronger than the standard ones, were used
for the structural analysis. The observed intensities
were corrected for the Lorentz-polarization absorption
effects. The liner absorption coef®cient, ꢃ, was
2
. Experimental
2
.1. Materials
Rubidium formate was directly synthesized from
formic acid and saturated aqueous solution of rubi-
dium carbonate. After slow evaporation of the mix-
ture, a residual crystalline material was obtained. This
compound was recrystallized from distilled water and
identi®ed by means of TG and FT-IR measurements.
The compound was so hygroscopic that it was kept in a
desiccator under vacuum and dried in an oven above
�
1
153.6 cm and the transmission factors were in the
interval 0.86±1.00.
Powder X-ray diffraction data were collected using
a Rigaku Geiger¯ex RAD-3R diffractometer equipped
with a high-temperature sample holder. The sample
was set on a platinum plate in a grove pack ®lled with
dry nitrogen gas. The data were collected in the 20<
2ꢂ<708 range with a step-scan width of 0.028 and ®xed
time (10 s) counting procedure in vacuo (ca.
3
73 K for one week before measurement.
The crystallized rubidium formate obtained from
distilled water was a monohydrate, so the hydrate was
heated over 373 K to obtain an anhydrous rubidium
formate. This compound was recrystallized from
methanol, and single crystals of rubidium formate
were obtained.
�
1
Ê
10 Pa mm. CuKꢄ radiation (ꢁˆ1.5418 A) was
monochromatized at the opposite side using a graphite
monochrometer.
2
.2. Thermal analyses
3. Results and discussion
TG and DTA were simultaneously recorded on a
The DTA curves are shown Fig. 1. Although there
was no weight change on the TG at temperatures
below the melting point (446 K), an endothermic peak
was con®rmed at 363 K. Fig. 2 shows the powder
X-ray diffraction patterns measured before, and after,
the endothermic peak. The different diffraction pro-
®les indicate that the endothermic peak corresponds
to a phase transition. When the samples were heated to
403 K and then cooled gradually to 293 K, an exother-
mic peak was recognized at just below the temperature
of the endothermic peak, 341 K. This phenomenon is
Rigaku Thermo¯ex TG8101D in vacuo (ca. 102 Pa).
Nearly 10 mg of the sample were weighed into a
platinum crucible, and a-alumina was used as a
reference material. The measurement was performed
�
1
at a heating rate of 5 K min . The enthalpy changes
for phase transition (ÁH ) and melting (ÁH ) were
tr
fus
obtained from the DSC curves recorded on a Shinku-
Riko DSC-1500 M5. About 10 mg of the sample were
placed in an aluminum crucible, and a-alumina was
used as a reference material. After the sample was set,

Y. Masuda et al. / Thermochimica Acta 318 (1998) 39±43
41
Table 1
Value of enthalpy and entropy changes for the phase transition and
fusion of rubidium formate
�
1
)
� 1
ÁS/ (J K )
T/ (8C)
ÁH/ (kJ mol
0.42Æ0.02
Phase transition
90
1.16Æ0.05
a
a
a
95
0.25
0.67
Melting
173
12.5Æ0.5
28.0Æ1.1
a
a
a
170
11.9
26.8
a
P. Ferloni et al., Z. Naturforsch. 30a (1995) 1447.
The structure of the phase before the transition was
solved using the heavy-atom Patterson method. The
space group and the lattice parameters are shown in
Table 2. The rubidium atoms were re®ned anisotro-
pically, while the rest were re®ned isotropically. The
position of hydrogen atom could not be determined by
the X-ray diffraction method, but hydrogen atom was
placed in the position expected from the form of the
formate ion. The ®nal positional parameters are listed
in Table 2 with the thermal parameters. The conven-
tional R factor, R ˆ ÆkFoj � jFcj=ÆjFoj, and the
Fig. 1. DTA curves of rubidium formate. The asterisk indicates
melting.
weighted R factor
w
q
2
2
Rw ˆ ꢀÆwꢀjF j � jF j† =ÆwjF j †
o
c
o
Table 2
Crystal data for the phase before the phase transition of rubidium
formate
Fig. 2. X-ray powder diffraction patterns measured before, and
after, the endothermic peak. (A), before transition; and (B), after
transition.
Space group
Pca2
1
(orthorhombic)
Ê
lattice constants
aˆ9.229(1) A
Ê
bˆ4.630(2) A
Ê
cˆ7.408(2) A
Z
4
referred to as a thermal hysteresis, and the phase
transition classi®ed as a reversible and ®rst-order
transition.
The enthalpy and entropy changes for phase transi-
tion and the fusion are listed in Table 1. The values
obtained in the present work are larger than those
reported by Ferloni et. al.; however, the values
obtained are the average of measurements repeated
Positional (x,y,z) and isotropic thermal (B) parameters)
2
Ê
B/(A
atom
Rb
x
y
z
a
0.2478
0.822(1)
1.005(1)
0.951(2)
1.0193
0.0608
0.447(2)
0.145(2)
0.385(3)
0.5372
0.0
1.44(3)
2.1(2)
1.8(3)
1.5(3)
5.0
O(1)
O(2)
C
0.264(8)
0.241(5)
0.269(5)
0.2942
H
ten times and have an accuracy of better 5%. The
tr
Reliability factor
values of enthalpy, ÁH , and entropy changes, ÁS ,
Rˆ8.6(%)
R
w
ˆ11.0(%)
tr
for the phase transition are much smaller than the ones
for fusion. This fact suggests that such a drastic
structural change as that of fusion is not to be expected
for the present phase transition.
a
8
2
Ã
2
Ã
2
Ã
2
Beq ˆ ꢅ ꢀB11ꢀaa † ‡ B22ꢀbb † ‡ B33ꢀcc †
3
Ã
Ã
Ã
Ã
‡ 2B12aa bb cos ꢀ ‡ 2B13aa bb cos ꢆ
‡ 2B23bb cc cos ꢄ†
Ã
Ã

4
2
Y. Masuda et al. / Thermochimica Acta 318 (1998) 39±43
Table 3
Crystal data for the phase after phase transition of rubidium
formate
Space group
lattice constants
P2
1
/m (monoclinic)
Ê
aˆ4.6522(4) A
Ê
bˆ4.6528(7) A
Ê
cˆ7.5170(6) A
ꢀ
2
ˆ97.610(6)8
Z
Positional (x,y,z) and isotropic thermal (B) parameters
2
Ê
B/A
atom
Rb
x
y
z
0.0
0.0
0.0
2.42
6.02
6.02
0.08
0.72
O(1)
O(2)
C
0.42(1)
0.35(1)
0.50(3)
0.70
0.26(1)
0.25
0.25
0.25
0.25
Fig. 3. Crystal structures of the phase before transition.
� 0.26(1)
0.0
H
0.0
were 0.086 and 0.110, respectively, where |F | is
o
Reliability factor
a
I
b
R
ˆ9.005(%)
R
F
ˆ5.79(%)
observed structural factor, |F | is the calculated one.
c
P
P
a
Fig. 3 shows the crystal structure before the transition.
In this phase, all the rubidium atoms were situated on
the x±y planes at zˆ0.0 and zˆ0.5, and the formate
ions are located on the planes situated between the
planes formed by rubidium atoms.
RI ˆ
jIkꢀo† � Ikꢀc†j=
Ikꢀo†.
k
k
P
P
b
1=2
1=2
1=2
RF ˆ
j‰Ikꢀo†Š � ‰Ikꢀc†Š j= k‰Ikꢀo†Š
k
observed is more compatible with that for the crystal
structural model on the basis of P2 /m than the one
The diffraction peaks observed for the phase after
transition were indexed on the monoclinic cell with
lattice constants of aˆ4.6522(4) , bˆ4.6528(7),
1
calculated for P2/m, as shown in Fig. 4. Table 3 and
Fig. 5 show the re®ned structural parameters and the
crystal structure, respectively. In the phase, all rubi-
dium atoms were situated on the x±y planes at zˆ0.0
and zˆ0.5, and the formate ions also form the layer
structures situated on the x±y planes at zˆ0.25 and
zˆ0.75.
Ê
cˆ7.5170(6) A and ꢀˆ97.610(6)8. Only one extinc-
tion rule was held for the special re¯ections, namely
0
sumed to be P2/m or P2 /m. The diffraction pro®le
0l : lˆ2n‡1. Therefore, the space group were pre-
1
Fig. 6 shows the three-dimensional frameworks
connecting the eight rubidium atoms for both the
Fig. 4. Observed (Á Á Á), calculated (ÐÐÐ), and difference (Áy,
lower ( Á Á Á) line) X-ray powder diffraction profiles of the phase
after transition. Reflection positions are indicated by vertical bars.
Fig. 5. Crystal structure of the phase after transition.

Y. Masuda et al. / Thermochimica Acta 318 (1998) 39±43
43
Fig. 7. The stacking sequences of layers along the c-axis of
rubidium formate. (A), before transition; and (B), after transition.
Fig. 6. The frameworks of rubidium atoms of rubidium formate.
A), before transition; and (B), after transition.
(
phases before, and after, transition. Both the dimen-
sions of the frameworks of the phases before, and
after, transition seem to agree fairly well with each
other. Therefore, the relative positon of rubidium
atoms for both the phases barely changed in the
transition. On the other hand, the orientation of
formate ions is clearly changed in both the phases
as shown in Fig. 7. A structural change through
the phase transition seems to be mostly attributa-
ble to the change of orientation of the formate ions.
The small values of ÁHtr and ÁStr are consistent
with the slight change of structure as the orienta-
tion of formate ions through the present phase
transition.
References
[1] S. Shishido, Y. Masuda, Nippon Kagaku Zasshi 92 (1971) 309.
[2] S. Shishido, Y. Masuda, Nippon Kagaku Kaishi, 185 (1973).
[3] S. Shishido, Y. Masuda, Nippon Kagaku Kaishi, 66 (1976).
[4] P. Feroni, M. Sansei, P. Franzosini, Z. Naturforsch. 30a (1975)
1
447; M. Braghetti, G. Bercheisie, Ann. Meeting, Chim.
Inorg., 101 (1969).
[5] Y. Masuda, K. Hashimoto, Y. Ito, Thermochim. Acta 163
1990) 171.
(
[
6] K. Hashimoto, Y. Ito, Y. Masuda, 59th Annual Meeting of the
Chemical Society of Japan, preprint 2F508, 1990.
7] A. Yahata, Y. Ito, Y. Masuda, 26th Annual Meeting of The
Japan Society of Calorimetry and Thermal Analysis, preprint,
1990, pp. 200±201.
[
[
8] A. Yahata, K. Hashimoto, Y. Ito, Y. Masuda, 61th Annual
Meeting of The Chemical Society of Japan, preprint 3A310,
1991.
[
9] A. Yahata, Y. Ito, Y. Masuda, 27th Annual Meeting of The
Japan Society of Calorimetry and Thermal Analysis, preprint,
Acknowledgements
1991, pp. 80±81.
[
[
[
10] Y. Masuda, A. Yahata, H. Ogawa, Inorg. Chem. 34 (1995) 3130.
11] H.M. Ritveld, J. Appl. Crystallogr., 65±71 (1969).
12] F. Izumi, H. Asano, H. Murata, N. Watanabe, J. Appl.
Crystallogr. 20 (1987) 411.
The authors would like to thank Dr. Shoichi Sato of
X-ray Research Laboratory of Rigaku Corporation,
for his kind assistance and advice in X-ray diffraction
measurements. We also gratefully acknowledge ®nan-
cial support provided from the Sasaki Environmental
Foundation.
[
13] The Chemical Society of Japan, Kagaku Binran Kisohen II
(Handbook of Chemistry), 3rd edn., Maruzen, Tokyo, 1984,
p. 267.