- Email: [email protected]
Raman spectroscopic and DTA studies of aqueous rare earth thiocyanate solutions
ELSEVIER
Journal of Alloys and Compounds 249 ( 1997) 119- 123
Raman Spectroscopic and DTA studies of aqueous rare earth thiocyanate solutions Y. Yoshimuraa’“, ‘Departrnetzt
Y. Taguchib, H. Kanno”, Y. Suzukib
“Department of Chemistry, National Defense Academy, of Industrial Chemistry, Faculty of Science and Technology,
Yokosukn, Kanagawa 239 Japarz. Meiji University, Tama-ku, Kawasaki
214 Japan.
Abstract
Ramanmeasurements werecarriedout for aqueous rare earththiocyanate[Ln(SCN),; Ln=La’+- Lu”] solutions(R=20; R is molesof waterper molesof salt) in the liquid stateat roomtemperatureandin the glassystateat liquid-nitrogentemperature.Ramanbandshifts for the C-S andC-N stretchingbandsof the SCN- ion indicatedthat the SCN- ion bindsto a rare earthion only at the N end.Thus, thesesolutionsare bettercalledasaqueousrareearthisothiocyanatesolutions.Glasstransitiontemperatures (T,‘s) of thesesolutionswere alsomeasured. The T, valueshave a distinct extendeds-shapeand arecomparedwith the previousonesfor aqueousrare earthchloride andperchloratesolutions.Seriesbehaviorof the T, resultsis discussed in conjunctionwith the coordinationnumberchangein the middle of the series. Keywords:
Aqueous rareearththiocyanate solution;Ramanspectrum; Glasstransitiontemperature; Thiocyanato-lanthanoid complex
1. Introduction The interaction of the thiocyanate ion with metal cations has been widely studied [l-lo]. Many such studies are motivated by the fact that the thiocyanate ion contains both the hard nitrogen and the soft sulphur atoms which can coordinate to metal ions. Among thiocyanato complexes of d ‘O metal (II) ions, N-bonding is found in the [Zn(NCS),]‘- complex that contains a relatively hard zinc (II) ion, while S-bonding is observed in the [Hg(SCN),]‘complex of the soft mercury (II) ion [l l-131. For a cadmium (II) ion, thiocyanate ions bind through two N and S ends to form [Cd(NCS),(SCN),]*in water [14] becausethe hardnessof a cadmium (II) ion is weaker than that of a zinc (II) ion and the softnessof a cadmium (II) ion is also less than that of a mercury (II) ion. The phenomenaof linkage isomerism (two complexes differing only in its bonding mode of an ambidentate ligand to metal ion) of the thiocyanate ion were reviewed [3]. For instance, the [Pd(As(C,H,),),(SCN),] complex undergoes partial isomerization to the N-bonded form when melted. In view of these observations, it is very interesting to see the
*Corresponding author.Tel.: 81-0468-41-3810 (ex. 3583);Fax: 810468-44-5901; e-mail:[email protected]. 09258388/97/$17.00 PII SO925-8388(96)02636-9
0
1997 Elsevier
ScienceS.A. A11rights
reserved
configuration of thiocyanate ions coordinating to rare earth ions. In previous X-ray diffraction [15] and Raman studies [16] of aqueousrare earth electrolyte solutions, it has been clearly shown that the inner-sphere hydration number of rare earth ions changes from nine for light rare earth members to eight for heavy ones. This coordination number change may be a major cause [17-191 of the irregularities which are observed for transport and thermodynamic properties versus atomic number or ionic radius plots [ 17,18,20]. According to the extensive work by Spedding et al. [20,21], the average hydration number gradually changes from nine to eight in the Nd3+-Tb3+ region at ordinary temperatures. On the other hand, a drastic change is observed in the T, versus atomic number plot for rare earth perchlorate solutions [17]. The importance of these observations is that solution properties are largely dependent on the inner-spherehydration number of rare earth ions in the solutions. In this study, as an extension of previous work [22-251 on aqueous rare earth electrolyte solutions, Raman measurements were carried out for aqueous rare earth thiocyanate [Ln(SCN),; Ln=La3 i -Lu3 ‘1 solutions in the liquid state at room temperature and in the glassy state at liquid-nitrogen temperature to elucidate the bonding types of SCN- ions to rare earth ions in the thiocyanate complexes. Glass transition temperatures (T,‘s) of these
120
Y. Yoshimura
et al. I Jourml
of Alloys
md Compounds
249 (1997)
119-123
solutions were also measured. The series behavior of thiocyanato rare earth complex ions in the solutions is clarified in conjunction with the coordination number change in the middle of the series. The results are compared with previous results [23] of aqueous rare earth chloride and perchlorate solutions.
2. Experimental
details
Rare earth thiocyanate solutions were prepared by dissolving rare earth chloride in distilled water and converting their chemical forms by passing through an anion exchange resin column of SCN- form. The resulting eluent solutions were heated on a hot plate at about 120 “C to evaporate excess water and the concentrations of all the sample solutions were adjusted to be R=20 (R is moles of water per moles of salt). The concentrations of the obtained solutions were checked by the EDTA titration of rare earth ions. Raman spectra were obtained with a JASCO NR- 1100 spectrophotometer using ca. 300 mW of the 514.5 nm line of an NEC argon-ion laser as an exciting source. The spectral resolution in this study was 1 cm-‘. To obtain the Raman spectrum of a glassy sample, we used a special quartz glass Dewar vessel designed specifically to maintain the glassy sample at liquid-nitrogen temperature during Raman measurements. Experimental details are described elsewhere [ 161. A simple differential thermal analysis (DTA) technique was used to measure T,. The sample solution (ca. 0.05 ml) in a 2 mm inner diameter Pyrex glass cell was vitrified in liquid-nitrogen. In this cell an alumel-chrome1 thermocouple junction was inserted. The overall cooling rate was about 600 K min-‘. DTA measurements were carried out at a heating rate of about 5 K min-’ in the glass transition temperature region. Benzene was used as the reference material. Glass formation was checked by visual inspection.
3. Results and discussion The infrared spectrum and structure of thiocyanate itself were studied by Jones [6]. In aqueous KSCN solution, the spectrum for a linear thiocyanate ion consists of a C-N stretching band at ~-2066 cm-‘, a C-S stretching band at -750 cm-‘, and a SCN bending band at -470 cm-‘. The SCN bending band is a very feeble band and is not always observed in a Raman spectrum. Therefore, we mainly examined the C-S and C-N stretching vibrational modes. Raman spectra of the C-S stretching vibration band for Ln(SCN),*20H,O solutions (Ln=La, Sm, Dy, Lu) in the liquid state are shown in Fig. 1. In the liquid state, this band appears as two peaks in all the solutions. The lower frequency peak remains unchanged in its frequency (755
,
850
800 Wavenumber
I
750 I cm”
I
I
800 750 Wavenumber I cm”
700
Fig. I. Raman C-S stretching vibrational spectra for the aqueous lanthanum, samarium, dysprosium and lutetium thiocyanate solutions (R=20) in the liquid and the glassy states.
cm-’ ) across the series, while the higher one increases on going from the aqueous lanthanum thiocyanate solution to the aqueous lutetium thiocyanate solutions. It is clear from this frequency shift that the higher frequency peak is ascribed to the SCN- ions coordinating to Ln3+ ions (the thiocyanate ions in the inner-sphere complexes) and the lower frequency peak is due to the solvated free SCNions. The increase of the higher frequency peak with decreasing ionic radius of rare earth ions shows that the interaction between SCN- ions and the rare earth ions is getting stronger on going from the aqueous lanthanum thiocyanate solution to the aqueous lutetium thiocyanate solution. Another important feature in Fig. 1 is the smooth change of relative intensities of the higher and lower frequency peaks from the La solution to the Lu solution. The higher peak increases in its intensity while the lower one becomes weaker. This corresponds to the increase of the number of the SCN- complexes from lanthanum to lutetium because the affinity for SCN- ions gets stronger with the decrease in the ionic radius of rare earth ions. Raman spectra for the C-S stretching band of the La, Sm, Dy and Lu solutions in the glassy state are also shown in Fig. 1. The most significant feature of the results is that the Raman spectra in the glassy state are different from those in the liquid state at room temperature. The lower frequency peak disappears and there remains a single envelope in the glassy state. However, the Raman spectra of the Lu solution and possibly all the other solutions in the glassy state show band asymmetry. So it seems likely that these bands consist of two components such as the coordinated and solvated free SCN- ions. The peak frequencies (the higher peaks in the liquid state) are plotted against the rare earth ionic radius in Fig. 2. The magnitude of the frequency for liquid state is larger than that for glassy state by about 7-10 cm-‘. There are several
Y. Yoshimum
La I,
Ce I
Pr Nd III,,,, Pm
800
7701,
Sm
Gd Eu
et al.
1 Jounml
of Alloys
and
Compounds
249
(1997)
121
119-123
Dy Er Yb Illllll Ho Tm Lu Tb
Liquid
Glassy
state I
' 1.05
' 1.00
'
' 0.95
Ionic
radius
' 0.90
' I 0.85
22Loo
/ A
Fig. 2. Frequency variation of the higher C-S stretching vibrational band for aqueous rare earth thiocyanate solutions (R=20) as a function of ionic radius.O:liquid state at room temperature,A:glassy state at liquid-nitrogen temperature.
experimental data [26-281 showing that the Lewis basicity of water increases at low temperatures. Therefore hydration is expected to be stronger at low temperaturesthan at ordinary temperatures. In this connection, Hage et al. [29] reported that the bound species(the SCN- ions coordinating to alkali metal ions) increases in an aqueous solutions of alkali metal thiocyanates when going from ambient temperature to the supercooled and glassy state. This conclusion was based on FT-IR spectra of the C-N stretching band for a dilute aqueoussolutions of lithium (lM), sodium (0.04, 0.2 and lM), and potassium (1M) thiocyanate in the glassy and supercooled states. They attributed the increase of the bound species with decreasing temperature to the anomaliesof supercooledwater and aqueoussolutions that are caused by structural changes of water toward a fully hydrogen-bonded tetrahedral network in the supercooled and glassy states. On the other hand, Raman spectra for the C-N stretching band of the SCN- ions show a single peak both in the liquid and glassy states for all the aqueous rare earth thiocyanate solutions examined. Raman spectra for the C-N stretching band of the La, Sm, Dy and Lu solutions are given in Fig. 3. Band asymmetries are evident in the Lu solution spectra in the C-N stretching region. These asymmetriescould also be due to the uncoordinated SCNions. As the frequency for the SCN- ions coordinated to rare earth ions is so similar to that for solvated free SCNions, it is considered that band asymmetries are not obvious in the other solution spectra. Peak frequencies of the C-N stretching bands are plotted as a function of the rare earth ionic radius in Fig. 4. Similar to the C-S
2150 2100 Wavenumber
2050
2000 I cm-’
state
h
2150 2100 Wavenumber
2050
2000 / cm“
1950
Fig. 3. Raman C-N stretching vibrational spectra for the aqueous lanthanum, samarium, dysprosium and lutetium thiocyanate solutions (R=20) in the liquid and glassy states,
stretching band, the frequency of the C-N stretching band increaseswith the decreasein the ionic radius of the rare earth ion. However, the peak position of the C-N stretching band in each solution for the glassy state is higher than that for the liquid state. This is a rather natural phenomenon in a vitrified solution becausethe interaction between ions should become stronger at low temperatures. As already noted in the introduction, there are several possible binding orientations in the configuration of a thiocyanate ion coordinating to a rare earth ion. From the correlations between the frequencies of the C-N and C-S stretching bands, the bonding types of an SCN- ion to a
La
Ce
/
1
2120
Pr Nd
!I
Sm
Gd
Pm
Eu
Er
Dy
/ ,111
Yb
/Illli Tb
Ho
Tm
Lu *
. $ 2100E z a, z 3 2080-
A
*
.
.AA
AA
A
*
l e*
8l e l
l 0*
l
'
t II
I
1.05
I
1.00
I
I
0.95
Ionic radius
1
0.90
I
I
0.85
/ h.
Fig. 4. Frequency variation of the C-N stretching band for aqueous rare earth thiocyanate solutions (R=20) as a function of ionic radius::liquid state at room temperature,A:glassy state at liquid-nitrogen temperature.
Y. Yoshimura
122
et al. I Jownni
of Alloys
rare earth ion in the thiocyanate complexes are suggested as follows [30]. For an isothiocyanate complex (M-NCS), C-N and C-S stretching bands appear in the approximate ranges below 2100 cm-’ and 780-860 cm-‘, respectively. For a thiocyanate complex (M-SCN), the corresponding ranges are approximately above 2100 cm-’ and 690-720 cm-‘. For a bridging thiocyanate complex (M-NCS-M), the C-N stretching band generally appears in a higher range than that for an M-NCS complex while the C-S stretching band usually occurs at intermediate frequency of the M-NCS and M-SCN complexes. In our results, all the frequencies of the C-S stretching bands appear between 780-800 cm-’ and those of the C-N stretching bands appear below 2100 cm-‘. From these facts, it is concluded that the coordination of a thiocyanato ligand to a rare earth ion is Ln-NCS form [30]. Thus, the solutions which we have investigated are better referred to as aqueous rare earth isothiocyanate solutions. In this connection, Takahashi et al. [31] investigated complexation between rare earth and thiocyanate ions in DMF by calorimetry, NMR and Raman spectroscopic measurements. They suggested that thiocyanate ions bind to rare earth ions through the N end and form inner-sphere complexes in the DMF solution. According to the HSAB principle proposed by Pearson [32], hard acids will prefer to coordinate to hard bases, and soft acids will prefer soft bases. Thus, the result obtained in this study indicates that rare earth ions in the Ln(SCN), solutions behave as a hard acid. The glass transition temperatures for Ln(SCN),*20H,O solutions are shown in Fig. 5 as a function of ionic radius of the rare earth ion. We immediately know that the T, values have a distinct extended s-shape though there is not a large break in the middle as observed for the perchlorate solutions [33]. Similar series behavior has been observed
La 1,
Ce
Sm
Pr Nd II,l,,l
Pm
-105
Gd
Er
/I,,,, Eu Tb Dy Ho
Yb
249 (1997)
119-123
for the rare earth chloride solutions [23]. As previously noted, the s-shape can be adequately explained by the coordination number change in the middle of the series. Taking account of the previous results [23], the coordination number of light rare earth ions is nine and that of heavy ones is eight with the middle range being the mixture of the two coordination numbers. Although the coordination number decreasesfrom nine for light rare earth ions to eight for heavy ones, the total hydration sphereincreasesacrossthe series with a rapid rise in the middle as evidenced by the sharp rise in T,. As a glass transition takes place at a temperature above which there are relatively rapid translational motions of moleculesin a liquid, a higher T, is expected for a solutions in which metal ions have a larger hydration sphere. Comparison of the T, results for the thiocyanate and perchlorate solutions indicates that the Ts change in the transition region (Sm3+- Tb3’) is more abrupt in the perchlorate solutions than in the thlocyanate solutions. This difference can be ascribed to the complexing abilities of anions. Although there have been reports that perchlorate ions can coordinate to metal ions [34], perchlorate ions are consideredto be non complexing in theserare earth perchlorate solutions [35]. Accordingly, the coordination spheresof rare earth ions consist exclusively of water molecules in the perchlorate solutions. On the other hand, those of rare earth ions in the thiocyanate solutions are partly occupied by thiocyanate ion(s). As the coordination (hydration) number change takes place due to the mismatch between the size of a rare earth ion and that of the coordination sphere of water molecules(and the thiocyanate ion), the participation of thiocyanate ion(s) into the inner-spherecoordination can mediate the size gap between the two sizesand lead to the gradual change of the coordination number, resulting in the gradual Tg variation for the thiocyanate solution.
Acknowledgments
Tm Lu
_
The presentwork was financially supportedby Grant-inAid for Scientific Research on Priority Area, “New Development of Rare Earth Complexes”, No. 07230105 and 08220106 from the Ministry of Education, Science and Culture.
l m
l
References
l
l -115
and Compounds
I, 1.05
D.P. Strommen and R.A. Plane, J. Chem. Phys., 60 (1974) 2643. [2] S. Fronaeus and R. Larrson, Acta Chern. Stand., 16 (1962) 1447. [3] J.L. Burmeister, Coord. Chem. Rev., I (1966) 20.5; 3 (1968) 225. [4] D.B. Sowerby, J. Am. Chem. SW., 84 (1962) 1831. [5] R.J.H. Clark and C.S. Williams, Spectrochim. Acra, 22 (1966) 1081. [6] L.H. Jones, J. Chem. Phys., 25 (1956) 1069. [7j K.A. Taylor, T.V. Long II and R.A. Plane, J. Chem. Phys., 47( 1967) [l]
l I 1.00
I
I
I
0.95
I, 0.90
I,
0.85
Ionic radius / A Fig.. 5. Glass transition temperatures of aqueous solutions (R=20) as a function of ionic radius.
rare earth thiocyanate
138. [S]
A. Tramer,
J. Chem. Phys., 59 (1962)
232.
Y. Yoshimurri
et al.
i
Journal
of Alloys
[9] R.E. Hester and K. Krishnan, J. Chern. Phys., 48 (1968) 1825. [lo] S. Ishiguro and K. Kato, Rare En&s, 27 (1995) 61. [l I] T. Yamaguchi, K. Yamamoto and H. Ohtaki, Bull. Chern. Sot., Jpn., 58 (1985) 3235. [12] S. Ahrland and L. Kullberg, Acta Chem. &and., 25 (1971) 3692. [13] I. Persson, A. Iverfeldt and S. Ahrland, Acra Chem. Scnnd., A35 (1981) 295. [14] S. Ishiguro, T. Tnkamuku and H. Ohtaki, Bull. Chem. Sot. Jpn., 61 (1988) 3901. [15] A. Habenschuss and F.H. Spedding, J. Chern. Phys., 70 (1979) 2797, 3785. [16] H. Kanno and J. Hiraishi, Chem. Phys. Lerr., 75 (1980) 553. [17] H. Kanno and Y. Akama, Chew. Phys. Letr., 72 (1980) 181. [18] F.H. Spedding, P.F. Cullen and A. Habenschuss, J. Chem. Phys., 78 (1974) 1106. [I91 G.R. Choppin and A.J. Graffeo, hzorg. Chem., 4 (1965) 1254. [20] F.H. Spedding, M.J. Pikal and B.O. Ayers, J. Plzys. Chem., 70 (1966) 2440. [21] F.H. Spedding and J.A. Rard, J. Phys. Chem., 78 (1974) 1435; F.H. Spedding, J.P. Walters and J.L. Baker, J. Chem. Eng. Dam, 20 (1975) 189, 438; F.H. Spedding, J.A. Rard and A. Habenschuss, J. Phys. Chem., 81 (1977) 1069.
and Compounds [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
[35]
249 (1997)
119-123
123
H. Kanno and J. Hiraishi, J. Phys. Chern., 86 (1982) 1488. H. Kanno and Y. Akama, J. Phys. Chem.: 91 (1987) 1263. H. Kanno and J. Hiraishi, J. Phys. Chem., 88 (1984) 2787. H. Kanno and Y. Yoshimura, 1. Alloys Camp., 225 (1995) 253. H. Kanno and J. Hiraishi, &l/i. Chem. Sot. Jpn., 58 (1985) 2701. H. Kanno, Chern. Phys. L&f., 170 (1990) 382. H. Kanno. Cvo-Lett., 15 (1994) 173. W. Hage. A. Hallbrucker and E. Mayer, J. Phys. Chim., 96, (1992) 6488. S.M. Nelson, T.M. Shepherd, J. Inorg. Chem., fl (1972) 1723. R. Takahashi and S. Ishiguro, J. Chern. Sot. Faraday Trans., 88 (1992) 3165. R.G. Pearson, J. Am. Chem. Sot., 85 (1963) 3533; 110 (1988) 7684. H. Kanno, J. Alloys Camp., 19.2 (1993) 271. A.J. Downs and C.J. Adams, in J.C. Bailar, H.J. Emeleus, R. Nyholm and A.F. Trotman-Dickenson (eds.): Comprehensive Inorganic Cheinisfry,Vol. 2, Chap. 26, Pergamon Press, Oxford, 1973, p. 1448. B.J. Hathaway, in G. Wilkinson, R.D. Gillard and J.A. McCleverty (eds.), Comprehensive Coorditmrion Chemisrry, Vol. 2, Chap. 15.5, Pergamon Press, Oxford, 1987, p. 414.
Journal of Alloys and Compounds 249 ( 1997) 119- 123
Raman Spectroscopic and DTA studies of aqueous rare earth thiocyanate solutions Y. Yoshimuraa’“, ‘Departrnetzt
Y. Taguchib, H. Kanno”, Y. Suzukib
“Department of Chemistry, National Defense Academy, of Industrial Chemistry, Faculty of Science and Technology,
Yokosukn, Kanagawa 239 Japarz. Meiji University, Tama-ku, Kawasaki
214 Japan.
Abstract
Ramanmeasurements werecarriedout for aqueous rare earththiocyanate[Ln(SCN),; Ln=La’+- Lu”] solutions(R=20; R is molesof waterper molesof salt) in the liquid stateat roomtemperatureandin the glassystateat liquid-nitrogentemperature.Ramanbandshifts for the C-S andC-N stretchingbandsof the SCN- ion indicatedthat the SCN- ion bindsto a rare earthion only at the N end.Thus, thesesolutionsare bettercalledasaqueousrareearthisothiocyanatesolutions.Glasstransitiontemperatures (T,‘s) of thesesolutionswere alsomeasured. The T, valueshave a distinct extendeds-shapeand arecomparedwith the previousonesfor aqueousrare earthchloride andperchloratesolutions.Seriesbehaviorof the T, resultsis discussed in conjunctionwith the coordinationnumberchangein the middle of the series. Keywords:
Aqueous rareearththiocyanate solution;Ramanspectrum; Glasstransitiontemperature; Thiocyanato-lanthanoid complex
1. Introduction The interaction of the thiocyanate ion with metal cations has been widely studied [l-lo]. Many such studies are motivated by the fact that the thiocyanate ion contains both the hard nitrogen and the soft sulphur atoms which can coordinate to metal ions. Among thiocyanato complexes of d ‘O metal (II) ions, N-bonding is found in the [Zn(NCS),]‘- complex that contains a relatively hard zinc (II) ion, while S-bonding is observed in the [Hg(SCN),]‘complex of the soft mercury (II) ion [l l-131. For a cadmium (II) ion, thiocyanate ions bind through two N and S ends to form [Cd(NCS),(SCN),]*in water [14] becausethe hardnessof a cadmium (II) ion is weaker than that of a zinc (II) ion and the softnessof a cadmium (II) ion is also less than that of a mercury (II) ion. The phenomenaof linkage isomerism (two complexes differing only in its bonding mode of an ambidentate ligand to metal ion) of the thiocyanate ion were reviewed [3]. For instance, the [Pd(As(C,H,),),(SCN),] complex undergoes partial isomerization to the N-bonded form when melted. In view of these observations, it is very interesting to see the
*Corresponding author.Tel.: 81-0468-41-3810 (ex. 3583);Fax: 810468-44-5901; e-mail:[email protected]. 09258388/97/$17.00 PII SO925-8388(96)02636-9
0
1997 Elsevier
ScienceS.A. A11rights
reserved
configuration of thiocyanate ions coordinating to rare earth ions. In previous X-ray diffraction [15] and Raman studies [16] of aqueousrare earth electrolyte solutions, it has been clearly shown that the inner-sphere hydration number of rare earth ions changes from nine for light rare earth members to eight for heavy ones. This coordination number change may be a major cause [17-191 of the irregularities which are observed for transport and thermodynamic properties versus atomic number or ionic radius plots [ 17,18,20]. According to the extensive work by Spedding et al. [20,21], the average hydration number gradually changes from nine to eight in the Nd3+-Tb3+ region at ordinary temperatures. On the other hand, a drastic change is observed in the T, versus atomic number plot for rare earth perchlorate solutions [17]. The importance of these observations is that solution properties are largely dependent on the inner-spherehydration number of rare earth ions in the solutions. In this study, as an extension of previous work [22-251 on aqueous rare earth electrolyte solutions, Raman measurements were carried out for aqueous rare earth thiocyanate [Ln(SCN),; Ln=La3 i -Lu3 ‘1 solutions in the liquid state at room temperature and in the glassy state at liquid-nitrogen temperature to elucidate the bonding types of SCN- ions to rare earth ions in the thiocyanate complexes. Glass transition temperatures (T,‘s) of these
120
Y. Yoshimura
et al. I Jourml
of Alloys
md Compounds
249 (1997)
119-123
solutions were also measured. The series behavior of thiocyanato rare earth complex ions in the solutions is clarified in conjunction with the coordination number change in the middle of the series. The results are compared with previous results [23] of aqueous rare earth chloride and perchlorate solutions.
2. Experimental
details
Rare earth thiocyanate solutions were prepared by dissolving rare earth chloride in distilled water and converting their chemical forms by passing through an anion exchange resin column of SCN- form. The resulting eluent solutions were heated on a hot plate at about 120 “C to evaporate excess water and the concentrations of all the sample solutions were adjusted to be R=20 (R is moles of water per moles of salt). The concentrations of the obtained solutions were checked by the EDTA titration of rare earth ions. Raman spectra were obtained with a JASCO NR- 1100 spectrophotometer using ca. 300 mW of the 514.5 nm line of an NEC argon-ion laser as an exciting source. The spectral resolution in this study was 1 cm-‘. To obtain the Raman spectrum of a glassy sample, we used a special quartz glass Dewar vessel designed specifically to maintain the glassy sample at liquid-nitrogen temperature during Raman measurements. Experimental details are described elsewhere [ 161. A simple differential thermal analysis (DTA) technique was used to measure T,. The sample solution (ca. 0.05 ml) in a 2 mm inner diameter Pyrex glass cell was vitrified in liquid-nitrogen. In this cell an alumel-chrome1 thermocouple junction was inserted. The overall cooling rate was about 600 K min-‘. DTA measurements were carried out at a heating rate of about 5 K min-’ in the glass transition temperature region. Benzene was used as the reference material. Glass formation was checked by visual inspection.
3. Results and discussion The infrared spectrum and structure of thiocyanate itself were studied by Jones [6]. In aqueous KSCN solution, the spectrum for a linear thiocyanate ion consists of a C-N stretching band at ~-2066 cm-‘, a C-S stretching band at -750 cm-‘, and a SCN bending band at -470 cm-‘. The SCN bending band is a very feeble band and is not always observed in a Raman spectrum. Therefore, we mainly examined the C-S and C-N stretching vibrational modes. Raman spectra of the C-S stretching vibration band for Ln(SCN),*20H,O solutions (Ln=La, Sm, Dy, Lu) in the liquid state are shown in Fig. 1. In the liquid state, this band appears as two peaks in all the solutions. The lower frequency peak remains unchanged in its frequency (755
,
850
800 Wavenumber
I
750 I cm”
I
I
800 750 Wavenumber I cm”
700
Fig. I. Raman C-S stretching vibrational spectra for the aqueous lanthanum, samarium, dysprosium and lutetium thiocyanate solutions (R=20) in the liquid and the glassy states.
cm-’ ) across the series, while the higher one increases on going from the aqueous lanthanum thiocyanate solution to the aqueous lutetium thiocyanate solutions. It is clear from this frequency shift that the higher frequency peak is ascribed to the SCN- ions coordinating to Ln3+ ions (the thiocyanate ions in the inner-sphere complexes) and the lower frequency peak is due to the solvated free SCNions. The increase of the higher frequency peak with decreasing ionic radius of rare earth ions shows that the interaction between SCN- ions and the rare earth ions is getting stronger on going from the aqueous lanthanum thiocyanate solution to the aqueous lutetium thiocyanate solution. Another important feature in Fig. 1 is the smooth change of relative intensities of the higher and lower frequency peaks from the La solution to the Lu solution. The higher peak increases in its intensity while the lower one becomes weaker. This corresponds to the increase of the number of the SCN- complexes from lanthanum to lutetium because the affinity for SCN- ions gets stronger with the decrease in the ionic radius of rare earth ions. Raman spectra for the C-S stretching band of the La, Sm, Dy and Lu solutions in the glassy state are also shown in Fig. 1. The most significant feature of the results is that the Raman spectra in the glassy state are different from those in the liquid state at room temperature. The lower frequency peak disappears and there remains a single envelope in the glassy state. However, the Raman spectra of the Lu solution and possibly all the other solutions in the glassy state show band asymmetry. So it seems likely that these bands consist of two components such as the coordinated and solvated free SCN- ions. The peak frequencies (the higher peaks in the liquid state) are plotted against the rare earth ionic radius in Fig. 2. The magnitude of the frequency for liquid state is larger than that for glassy state by about 7-10 cm-‘. There are several
Y. Yoshimum
La I,
Ce I
Pr Nd III,,,, Pm
800
7701,
Sm
Gd Eu
et al.
1 Jounml
of Alloys
and
Compounds
249
(1997)
121
119-123
Dy Er Yb Illllll Ho Tm Lu Tb
Liquid
Glassy
state I
' 1.05
' 1.00
'
' 0.95
Ionic
radius
' 0.90
' I 0.85
22Loo
/ A
Fig. 2. Frequency variation of the higher C-S stretching vibrational band for aqueous rare earth thiocyanate solutions (R=20) as a function of ionic radius.O:liquid state at room temperature,A:glassy state at liquid-nitrogen temperature.
experimental data [26-281 showing that the Lewis basicity of water increases at low temperatures. Therefore hydration is expected to be stronger at low temperaturesthan at ordinary temperatures. In this connection, Hage et al. [29] reported that the bound species(the SCN- ions coordinating to alkali metal ions) increases in an aqueous solutions of alkali metal thiocyanates when going from ambient temperature to the supercooled and glassy state. This conclusion was based on FT-IR spectra of the C-N stretching band for a dilute aqueoussolutions of lithium (lM), sodium (0.04, 0.2 and lM), and potassium (1M) thiocyanate in the glassy and supercooled states. They attributed the increase of the bound species with decreasing temperature to the anomaliesof supercooledwater and aqueoussolutions that are caused by structural changes of water toward a fully hydrogen-bonded tetrahedral network in the supercooled and glassy states. On the other hand, Raman spectra for the C-N stretching band of the SCN- ions show a single peak both in the liquid and glassy states for all the aqueous rare earth thiocyanate solutions examined. Raman spectra for the C-N stretching band of the La, Sm, Dy and Lu solutions are given in Fig. 3. Band asymmetries are evident in the Lu solution spectra in the C-N stretching region. These asymmetriescould also be due to the uncoordinated SCNions. As the frequency for the SCN- ions coordinated to rare earth ions is so similar to that for solvated free SCNions, it is considered that band asymmetries are not obvious in the other solution spectra. Peak frequencies of the C-N stretching bands are plotted as a function of the rare earth ionic radius in Fig. 4. Similar to the C-S
2150 2100 Wavenumber
2050
2000 I cm-’
state
h
2150 2100 Wavenumber
2050
2000 / cm“
1950
Fig. 3. Raman C-N stretching vibrational spectra for the aqueous lanthanum, samarium, dysprosium and lutetium thiocyanate solutions (R=20) in the liquid and glassy states,
stretching band, the frequency of the C-N stretching band increaseswith the decreasein the ionic radius of the rare earth ion. However, the peak position of the C-N stretching band in each solution for the glassy state is higher than that for the liquid state. This is a rather natural phenomenon in a vitrified solution becausethe interaction between ions should become stronger at low temperatures. As already noted in the introduction, there are several possible binding orientations in the configuration of a thiocyanate ion coordinating to a rare earth ion. From the correlations between the frequencies of the C-N and C-S stretching bands, the bonding types of an SCN- ion to a
La
Ce
/
1
2120
Pr Nd
!I
Sm
Gd
Pm
Eu
Er
Dy
/ ,111
Yb
/Illli Tb
Ho
Tm
Lu *
. $ 2100E z a, z 3 2080-
A
*
.
.AA
AA
A
*
l e*
8l e l
l 0*
l
'
t II
I
1.05
I
1.00
I
I
0.95
Ionic radius
1
0.90
I
I
0.85
/ h.
Fig. 4. Frequency variation of the C-N stretching band for aqueous rare earth thiocyanate solutions (R=20) as a function of ionic radius::liquid state at room temperature,A:glassy state at liquid-nitrogen temperature.
Y. Yoshimura
122
et al. I Jownni
of Alloys
rare earth ion in the thiocyanate complexes are suggested as follows [30]. For an isothiocyanate complex (M-NCS), C-N and C-S stretching bands appear in the approximate ranges below 2100 cm-’ and 780-860 cm-‘, respectively. For a thiocyanate complex (M-SCN), the corresponding ranges are approximately above 2100 cm-’ and 690-720 cm-‘. For a bridging thiocyanate complex (M-NCS-M), the C-N stretching band generally appears in a higher range than that for an M-NCS complex while the C-S stretching band usually occurs at intermediate frequency of the M-NCS and M-SCN complexes. In our results, all the frequencies of the C-S stretching bands appear between 780-800 cm-’ and those of the C-N stretching bands appear below 2100 cm-‘. From these facts, it is concluded that the coordination of a thiocyanato ligand to a rare earth ion is Ln-NCS form [30]. Thus, the solutions which we have investigated are better referred to as aqueous rare earth isothiocyanate solutions. In this connection, Takahashi et al. [31] investigated complexation between rare earth and thiocyanate ions in DMF by calorimetry, NMR and Raman spectroscopic measurements. They suggested that thiocyanate ions bind to rare earth ions through the N end and form inner-sphere complexes in the DMF solution. According to the HSAB principle proposed by Pearson [32], hard acids will prefer to coordinate to hard bases, and soft acids will prefer soft bases. Thus, the result obtained in this study indicates that rare earth ions in the Ln(SCN), solutions behave as a hard acid. The glass transition temperatures for Ln(SCN),*20H,O solutions are shown in Fig. 5 as a function of ionic radius of the rare earth ion. We immediately know that the T, values have a distinct extended s-shape though there is not a large break in the middle as observed for the perchlorate solutions [33]. Similar series behavior has been observed
La 1,
Ce
Sm
Pr Nd II,l,,l
Pm
-105
Gd
Er
/I,,,, Eu Tb Dy Ho
Yb
249 (1997)
119-123
for the rare earth chloride solutions [23]. As previously noted, the s-shape can be adequately explained by the coordination number change in the middle of the series. Taking account of the previous results [23], the coordination number of light rare earth ions is nine and that of heavy ones is eight with the middle range being the mixture of the two coordination numbers. Although the coordination number decreasesfrom nine for light rare earth ions to eight for heavy ones, the total hydration sphereincreasesacrossthe series with a rapid rise in the middle as evidenced by the sharp rise in T,. As a glass transition takes place at a temperature above which there are relatively rapid translational motions of moleculesin a liquid, a higher T, is expected for a solutions in which metal ions have a larger hydration sphere. Comparison of the T, results for the thiocyanate and perchlorate solutions indicates that the Ts change in the transition region (Sm3+- Tb3’) is more abrupt in the perchlorate solutions than in the thlocyanate solutions. This difference can be ascribed to the complexing abilities of anions. Although there have been reports that perchlorate ions can coordinate to metal ions [34], perchlorate ions are consideredto be non complexing in theserare earth perchlorate solutions [35]. Accordingly, the coordination spheresof rare earth ions consist exclusively of water molecules in the perchlorate solutions. On the other hand, those of rare earth ions in the thiocyanate solutions are partly occupied by thiocyanate ion(s). As the coordination (hydration) number change takes place due to the mismatch between the size of a rare earth ion and that of the coordination sphere of water molecules(and the thiocyanate ion), the participation of thiocyanate ion(s) into the inner-spherecoordination can mediate the size gap between the two sizesand lead to the gradual change of the coordination number, resulting in the gradual Tg variation for the thiocyanate solution.
Acknowledgments
Tm Lu
_
The presentwork was financially supportedby Grant-inAid for Scientific Research on Priority Area, “New Development of Rare Earth Complexes”, No. 07230105 and 08220106 from the Ministry of Education, Science and Culture.
l m
l
References
l
l -115
and Compounds
I, 1.05
D.P. Strommen and R.A. Plane, J. Chem. Phys., 60 (1974) 2643. [2] S. Fronaeus and R. Larrson, Acta Chern. Stand., 16 (1962) 1447. [3] J.L. Burmeister, Coord. Chem. Rev., I (1966) 20.5; 3 (1968) 225. [4] D.B. Sowerby, J. Am. Chem. SW., 84 (1962) 1831. [5] R.J.H. Clark and C.S. Williams, Spectrochim. Acra, 22 (1966) 1081. [6] L.H. Jones, J. Chem. Phys., 25 (1956) 1069. [7j K.A. Taylor, T.V. Long II and R.A. Plane, J. Chem. Phys., 47( 1967) [l]
l I 1.00
I
I
I
0.95
I, 0.90
I,
0.85
Ionic radius / A Fig.. 5. Glass transition temperatures of aqueous solutions (R=20) as a function of ionic radius.
rare earth thiocyanate
138. [S]
A. Tramer,
J. Chem. Phys., 59 (1962)
232.
Y. Yoshimurri
et al.
i
Journal
of Alloys
[9] R.E. Hester and K. Krishnan, J. Chern. Phys., 48 (1968) 1825. [lo] S. Ishiguro and K. Kato, Rare En&s, 27 (1995) 61. [l I] T. Yamaguchi, K. Yamamoto and H. Ohtaki, Bull. Chern. Sot., Jpn., 58 (1985) 3235. [12] S. Ahrland and L. Kullberg, Acta Chem. &and., 25 (1971) 3692. [13] I. Persson, A. Iverfeldt and S. Ahrland, Acra Chem. Scnnd., A35 (1981) 295. [14] S. Ishiguro, T. Tnkamuku and H. Ohtaki, Bull. Chem. Sot. Jpn., 61 (1988) 3901. [15] A. Habenschuss and F.H. Spedding, J. Chern. Phys., 70 (1979) 2797, 3785. [16] H. Kanno and J. Hiraishi, Chem. Phys. Lerr., 75 (1980) 553. [17] H. Kanno and Y. Akama, Chew. Phys. Letr., 72 (1980) 181. [18] F.H. Spedding, P.F. Cullen and A. Habenschuss, J. Chem. Phys., 78 (1974) 1106. [I91 G.R. Choppin and A.J. Graffeo, hzorg. Chem., 4 (1965) 1254. [20] F.H. Spedding, M.J. Pikal and B.O. Ayers, J. Plzys. Chem., 70 (1966) 2440. [21] F.H. Spedding and J.A. Rard, J. Phys. Chem., 78 (1974) 1435; F.H. Spedding, J.P. Walters and J.L. Baker, J. Chem. Eng. Dam, 20 (1975) 189, 438; F.H. Spedding, J.A. Rard and A. Habenschuss, J. Phys. Chem., 81 (1977) 1069.
and Compounds [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
[35]
249 (1997)
119-123
123
H. Kanno and J. Hiraishi, J. Phys. Chern., 86 (1982) 1488. H. Kanno and Y. Akama, J. Phys. Chem.: 91 (1987) 1263. H. Kanno and J. Hiraishi, J. Phys. Chem., 88 (1984) 2787. H. Kanno and Y. Yoshimura, 1. Alloys Camp., 225 (1995) 253. H. Kanno and J. Hiraishi, &l/i. Chem. Sot. Jpn., 58 (1985) 2701. H. Kanno, Chern. Phys. L&f., 170 (1990) 382. H. Kanno. Cvo-Lett., 15 (1994) 173. W. Hage. A. Hallbrucker and E. Mayer, J. Phys. Chim., 96, (1992) 6488. S.M. Nelson, T.M. Shepherd, J. Inorg. Chem., fl (1972) 1723. R. Takahashi and S. Ishiguro, J. Chern. Sot. Faraday Trans., 88 (1992) 3165. R.G. Pearson, J. Am. Chem. Sot., 85 (1963) 3533; 110 (1988) 7684. H. Kanno, J. Alloys Camp., 19.2 (1993) 271. A.J. Downs and C.J. Adams, in J.C. Bailar, H.J. Emeleus, R. Nyholm and A.F. Trotman-Dickenson (eds.): Comprehensive Inorganic Cheinisfry,Vol. 2, Chap. 26, Pergamon Press, Oxford, 1973, p. 1448. B.J. Hathaway, in G. Wilkinson, R.D. Gillard and J.A. McCleverty (eds.), Comprehensive Coorditmrion Chemisrry, Vol. 2, Chap. 15.5, Pergamon Press, Oxford, 1987, p. 414.