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Phase transitions of scandium triacetate
PHYSICA] ELSEVIER
Physica B 213&214 (1995) 402 404
Phase transitions of scandium triacetate Yousuke O h t a a, Takasuke Matsuo a'*, Hiroshi Suga a' 1, William. I.F. David b, Richard M. Ibberson b a Department of Chemistry and Microcalorimetry Research Center, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan b ISIS Division, Daresbury-RutherJbrd Appleton Laboratory, Chilton, Didcot, Oxon. OXIIOQX, UK
Abstract Lattice parameters of deuterated scandium triacetate have been determined by high-resolution neutron diffraction at temperatures between 5 and 200 K. Two phase transitions were located at 66 and 159 K in agreement with a calorimetric experiment. A large anisotropy of the thermal expansivity was found and it is discussed in terms of the different nature of the chemical bonds combining the molecules in the crystal.
1. Introduction Scandium triacetate, Sc(CH3CO/)3, crystallizes in the space group P63/mcm [1]. The structure may be described as a close packing of linear polymeric species {Sc(CH3CO2)3} ., in which adjacent scandium ions are connected by three bidentate acetate ions forming an equilateral triangle in a cross-section perpendicular to the polymeric axis, The polymeric unit is rather rigid, as each of the scandium ions is bound to its two neighbors by six strong ionic-coordination scandium-oxygen bonds. Methyl groups of the acetate ions point at right angles to the columnar structure. Considerations based on the van der Waals contact show that the skeletal parts (i.e., linear chain of scandium ions coordinated by six oxygen atoms) of the columns are effectively shielded from each Other by sheaths of methyl groups. This interaction between the polymeric columns occurs via methyl groups belonging to the neighboring columns. This crystal undergoes phase transitions at 59.0 and 167.0K, the former being first order and the latter a * Corresponding author. 1 Present address: Research Institute for Science and Technology, Kinki University, Kowakae, Higashi-Osaka 577, Japan.
higher order transition [2]. The site symmetry of the methyl groups in the structure is ram. This requires the methyl groups to be disordered. The crystal may be regarded as a collection of disordered methyl groups interacting with each other though van der Waals contact between hydrogen atoms. Thus it offers a simple system in which the collective dynamics of hindered rotors results in phase transitions. In the present study we employed neutron diffraction and calorimetry to investigate the structural and thermodynamic properties of deuterated scandium triacetate.
2. Experimental The sample was prepared by dissolving scandium hydroxide Sc(OH)3 in aqueous methyl-deuterated acetic acid CD3CO2H and subsequent evaporation of the solvent. The stated deuterium purity of the acetic acid was 99.5%. Elemental analysis agreed with a calculation based on the stoichiometry: C, 31.18% (calc. 31.10); D, 7.77% (7.80); Sc, 19.46% (19.45). The neutron-diffraction experiment was performed on the High-Resolution Powder Diffractometer (HRPD) at ISIS [3]. Variation of the lattice parameters as a function
0921-4526/95/$09.50 ~ 1995 Elsevier Science B.V. All rights reserved SSDI 0921 -4526(95)0t]1 70-0
Y. Ohta et al./Physica B 213&214 (1995) 402 404 of temperature was followed between 5 and 200 K at 5, 2 and 0.2 K intervals depending on the temperature. They were calculated from the diffraction data using the CAILS program (Cell and Intensity Least Squares). Two phase transitions occurred at 66 and 159 K. The higherorder transition occurring at 159 K was examined closely, using the high resolution of the instrument. The 66 K transition was discontinuous. The phase transitions were also studied thermodynamically by adiabatic calorimetry between 10 and 300 K [4].
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3. Results
and discussion
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0 The phase transitions are indicated by the discontinuous change in the a and b/x/"3 parameters at 66 K and their gradual merging at 159 K as shown in Fig. 1. Above the latter temperature, the two lattice parameters are identical, in agreement with the hexagonal symmetry of the high-temperature phase. The intermediate and lowtemperature phases are orthorhombic. Except at the transition temperatures, the temperature dependence of the a and b lattice parameters was smooth to +_ 0.0004 A. This was in striking contrast with the lattice parameter c. As shown in Fig. 2, the parameter c depended on the temperature only weakly. However, it has two local maxima within the intermediate temperature phase. Their magnitudes exceed the probable error bounds indicated by the error bars in the plot. The origin of this irregular temperature dependence is not understood. The transition temperatures are indicated by the arrows in the figure. The points shown by triangles represents the data collected at 0.2K intervals close to the upper transition temperature. The lattice constant was thus strongly temperature dependent as the transition temperature was approached from below, but it was still continuous. The heat capacity of deuterated scandium triacetate is plotted in Fig. 3. That of natural scandium triacetate is also plotted for comparison. In each of the curves, the sharp, first-order character of the lower-temperature transition on one hand and the gradual, higher order character of the higher-temperature transition on the other are clearly reproduced. The excess heat capacity associated with the phase transitions was determined by using a normal heat capacity representing the vibrational heat capacity, This was calculated by a lattice-dynamics method. The entropy of the transitions thus estimated was 20J/(Kmol). This compares favorably with 3(Rln2) = 17.3J/(Kmol) expected for a twofold rotational-disorder model of the methyl group. The difference between the lattice parameters a and b/x/3 may be regarded as the order parameter of the phase transition. This quantity decreases throughout
o°°°°°
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50
100
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T/K Fig. 1. Lattice parameters a and b/x/3 of deuterated scandium triacetate.
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T/K Fig. 2. Lattice parameter c of deuterated scandium triacetate. 300
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Fig. 3. Heat capacities of natural and deuterated scandium triacetate. The broken lines represent vibrational heat capacities of a lattice dynamic model.
404
Y. Ohta et al. / Pl~vsica B 213&214 (1995) 402 404
the intermediate phase. Thus the high-temperature transition has set in and the disorder already progressed considerably as the intermediate phase becomes stable at the lower transition point. The excess heat capacity was substantial at the same temperature in agreement with this interpretation. The thermal expansivity was remarkably anisotropic. The mean values of a and b//,j/3 increased by 6 x 10- 3 in the intermediate phase while c increased by only 5 x 10 4 in the same temperature interval. This is a consequence of the strong intracolumnar bonds and the weak van der Waals forces that bind the columns together. The rotational disorder of methyl groups is coupled with the intercolumnar packing and thus its effect is most evident in the a - b plane, even though the displacement of the
deuterium atoms involved in the rotational disorder has a major component along the c-axis. Determination of the actual positions of deuterium atoms is in progress and will be reported later.
References [l] R. Fuchs and J.Z. Strahle, Z. Naturforsch. B 39 (1984) 1662. [2] Y. Ohta, T. Matsuo and H. Suga, J. Chem. Thermodynamics 24 (1992) 1189. [3] W.I.F. David, in: Neutron Scattering at a Pulsed Source, eds. R.J. Newport, B.D. Rainford and R. Cywinski (Adam Hilger, Bristol, 1988) ch. 12. [4] T. Matsuo and H. Suga, Thermochim Acta 88 (1985) 149.
Physica B 213&214 (1995) 402 404
Phase transitions of scandium triacetate Yousuke O h t a a, Takasuke Matsuo a'*, Hiroshi Suga a' 1, William. I.F. David b, Richard M. Ibberson b a Department of Chemistry and Microcalorimetry Research Center, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan b ISIS Division, Daresbury-RutherJbrd Appleton Laboratory, Chilton, Didcot, Oxon. OXIIOQX, UK
Abstract Lattice parameters of deuterated scandium triacetate have been determined by high-resolution neutron diffraction at temperatures between 5 and 200 K. Two phase transitions were located at 66 and 159 K in agreement with a calorimetric experiment. A large anisotropy of the thermal expansivity was found and it is discussed in terms of the different nature of the chemical bonds combining the molecules in the crystal.
1. Introduction Scandium triacetate, Sc(CH3CO/)3, crystallizes in the space group P63/mcm [1]. The structure may be described as a close packing of linear polymeric species {Sc(CH3CO2)3} ., in which adjacent scandium ions are connected by three bidentate acetate ions forming an equilateral triangle in a cross-section perpendicular to the polymeric axis, The polymeric unit is rather rigid, as each of the scandium ions is bound to its two neighbors by six strong ionic-coordination scandium-oxygen bonds. Methyl groups of the acetate ions point at right angles to the columnar structure. Considerations based on the van der Waals contact show that the skeletal parts (i.e., linear chain of scandium ions coordinated by six oxygen atoms) of the columns are effectively shielded from each Other by sheaths of methyl groups. This interaction between the polymeric columns occurs via methyl groups belonging to the neighboring columns. This crystal undergoes phase transitions at 59.0 and 167.0K, the former being first order and the latter a * Corresponding author. 1 Present address: Research Institute for Science and Technology, Kinki University, Kowakae, Higashi-Osaka 577, Japan.
higher order transition [2]. The site symmetry of the methyl groups in the structure is ram. This requires the methyl groups to be disordered. The crystal may be regarded as a collection of disordered methyl groups interacting with each other though van der Waals contact between hydrogen atoms. Thus it offers a simple system in which the collective dynamics of hindered rotors results in phase transitions. In the present study we employed neutron diffraction and calorimetry to investigate the structural and thermodynamic properties of deuterated scandium triacetate.
2. Experimental The sample was prepared by dissolving scandium hydroxide Sc(OH)3 in aqueous methyl-deuterated acetic acid CD3CO2H and subsequent evaporation of the solvent. The stated deuterium purity of the acetic acid was 99.5%. Elemental analysis agreed with a calculation based on the stoichiometry: C, 31.18% (calc. 31.10); D, 7.77% (7.80); Sc, 19.46% (19.45). The neutron-diffraction experiment was performed on the High-Resolution Powder Diffractometer (HRPD) at ISIS [3]. Variation of the lattice parameters as a function
0921-4526/95/$09.50 ~ 1995 Elsevier Science B.V. All rights reserved SSDI 0921 -4526(95)0t]1 70-0
Y. Ohta et al./Physica B 213&214 (1995) 402 404 of temperature was followed between 5 and 200 K at 5, 2 and 0.2 K intervals depending on the temperature. They were calculated from the diffraction data using the CAILS program (Cell and Intensity Least Squares). Two phase transitions occurred at 66 and 159 K. The higherorder transition occurring at 159 K was examined closely, using the high resolution of the instrument. The 66 K transition was discontinuous. The phase transitions were also studied thermodynamically by adiabatic calorimetry between 10 and 300 K [4].
7.90
403 [
I
I
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oOO
*< -, 7.85
~oooOo
~ o ~ 2 oo
~°°°
d o
'Z3
o
7.80
o o %o o
o o
3. Results
and discussion
~oooo
7.75
o
o
o
I
0 The phase transitions are indicated by the discontinuous change in the a and b/x/"3 parameters at 66 K and their gradual merging at 159 K as shown in Fig. 1. Above the latter temperature, the two lattice parameters are identical, in agreement with the hexagonal symmetry of the high-temperature phase. The intermediate and lowtemperature phases are orthorhombic. Except at the transition temperatures, the temperature dependence of the a and b lattice parameters was smooth to +_ 0.0004 A. This was in striking contrast with the lattice parameter c. As shown in Fig. 2, the parameter c depended on the temperature only weakly. However, it has two local maxima within the intermediate temperature phase. Their magnitudes exceed the probable error bounds indicated by the error bars in the plot. The origin of this irregular temperature dependence is not understood. The transition temperatures are indicated by the arrows in the figure. The points shown by triangles represents the data collected at 0.2K intervals close to the upper transition temperature. The lattice constant was thus strongly temperature dependent as the transition temperature was approached from below, but it was still continuous. The heat capacity of deuterated scandium triacetate is plotted in Fig. 3. That of natural scandium triacetate is also plotted for comparison. In each of the curves, the sharp, first-order character of the lower-temperature transition on one hand and the gradual, higher order character of the higher-temperature transition on the other are clearly reproduced. The excess heat capacity associated with the phase transitions was determined by using a normal heat capacity representing the vibrational heat capacity, This was calculated by a lattice-dynamics method. The entropy of the transitions thus estimated was 20J/(Kmol). This compares favorably with 3(Rln2) = 17.3J/(Kmol) expected for a twofold rotational-disorder model of the methyl group. The difference between the lattice parameters a and b/x/3 may be regarded as the order parameter of the phase transition. This quantity decreases throughout
o°°°°°
o
I
I
50
100
150
T/K Fig. 1. Lattice parameters a and b/x/3 of deuterated scandium triacetate.
8.870 /h
°< \ . 8.865
°*°*°
8.860
® 1"
t~
~
5o
1
I
100
150
T/K Fig. 2. Lattice parameter c of deuterated scandium triacetate. 300
I
I
D 200 H
o~
loo
t
I
100
200
T/K
1
300
Fig. 3. Heat capacities of natural and deuterated scandium triacetate. The broken lines represent vibrational heat capacities of a lattice dynamic model.
404
Y. Ohta et al. / Pl~vsica B 213&214 (1995) 402 404
the intermediate phase. Thus the high-temperature transition has set in and the disorder already progressed considerably as the intermediate phase becomes stable at the lower transition point. The excess heat capacity was substantial at the same temperature in agreement with this interpretation. The thermal expansivity was remarkably anisotropic. The mean values of a and b//,j/3 increased by 6 x 10- 3 in the intermediate phase while c increased by only 5 x 10 4 in the same temperature interval. This is a consequence of the strong intracolumnar bonds and the weak van der Waals forces that bind the columns together. The rotational disorder of methyl groups is coupled with the intercolumnar packing and thus its effect is most evident in the a - b plane, even though the displacement of the
deuterium atoms involved in the rotational disorder has a major component along the c-axis. Determination of the actual positions of deuterium atoms is in progress and will be reported later.
References [l] R. Fuchs and J.Z. Strahle, Z. Naturforsch. B 39 (1984) 1662. [2] Y. Ohta, T. Matsuo and H. Suga, J. Chem. Thermodynamics 24 (1992) 1189. [3] W.I.F. David, in: Neutron Scattering at a Pulsed Source, eds. R.J. Newport, B.D. Rainford and R. Cywinski (Adam Hilger, Bristol, 1988) ch. 12. [4] T. Matsuo and H. Suga, Thermochim Acta 88 (1985) 149.