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Studies on lanthanide complexes with pyronic ligands—III. Synthesis and characterization of novel complexes with 4-methylesculetin
0277-5387/W $3.00+.00 0 1990 Pergamon Press plc
Vol. 9, No. I, pp. 939442, 1990 Printed in Great Britain Polyhedron
STUDIES ON LANTHANIDE COMPLEXES WITH PYRONIC LIGANDS-111.” SYNTHESIS AND CHARACTERIZATION OF NOVEL COMPLEXES WITH 4-METHYLESCULETIN C. BISI CASTELLANIT Dipartimento
and 0.
CARUGO
di Chimica Generale, Universita di Pavia, via Taramelli 12, 27100 Pavia, Italy (Received 13 March 1989 ; accepted 2 November 1989)
novel complexes Ln(Hmes)Cl,* 3H20 (Ln = La, Sm, Eu, Gd, Tb, Dy ; Hmes = monodeprotonated 4-methylesculetin) have been synthesized and characterized by elemental and thermogravimetric analysis, and by vibrational and electronic absorption spectra. The ligand, potentially diprotic, is only monodeprotonated. The complexation constants have been determined potentiometrically. Contrary to many other complexes of lanthanides with benzopyrone ligands, the compounds described in this paper do not show metal luminescence promoted by the excited states of the ligand. This behaviour is discussed. Abstract-The
The luminescence of lanthanide cation complexes and its applications are the focus of attention of many researchers. ’ In previous works, we showed that ligands containing the a- or the y-pyronic rings are able to stimulate the lanthanide ion luminescence.2 Among the a-pyrone derivatives, the 7hydroxocoumarins are of relevant interest because of their peculiar electronic structure. 3 The presence of a hydroxy group in the 7-position causes a shift to lower energy of the bands in the electronic absorption spectra, with respect to the parent coumarin. This shift results from mesomeric effects, as shown in Scheme 1. The decrease in electronic transition energy may influence the ability of these molecules to transfer their electronic energy to the lanthanide cations. In order to verify if the 7-hydroxocoumarins behave as good chromophores toward lanthanide ions, like other a-pyrone ligands, we used the 4-methylesculetin as ligand and obtained the novel complexes Ln(Hmes)Cl, - 3H20 [Ln = La, Sm, Eu, Gd, Tb, Dy ; Hmes = deprotonated form of 4-methylesculetin (referred to as H,mes in Scheme l)]. The compounds were characterized by means of elemental and thermogravimetrical analysis, and electronic
* For the preceding parts see ref. 2. Author to whom correspondence should be addressed.
t
I
II
Scheme 1. H *mes(4-methylesculetin): R = OH, R’ = Me.
and vibrational spectroscopy. The formation constants of all the complexes were determined. No increase in lanthanide cation luminescence was observed in any case. EXPERIMENTAL
Synthesis 6,7-Dihydroxo-4-methyl-2H1- benzopyran-Z one (6methylesculetin) was obtained from AldrichChemie. The lanthanide chlorides were prepared as previously described.4 An ethanolic solution of H 2mes (1 mmol in 40 cm3) was added to an aqueous solution of the appropriate lanthanide salt (1 mmol in 15 cm’). On adjusting the pH to 6.2-6.5 with aqueous NaOH, the product separated from the solution as a brown-orange powder. It was centrifugated, washed with ethanol and water, and dried at 1 mm Hg and room temperature for 3 h.
940
C. BISI CASTELLANI
and 0.
CARUGO
If kept in contact with the solution, it decomposes in a few hours to a dark pitch.
group in the 6-position ;3 thus it seems probable that in our complexes the 7-hydroxy group is deprotonated. The proton of the 6-hydroxy group might be stabilized through hydrogen bonding to one or Physicochemical measurements more of the several oxygen atoms available in the IR and electronic spectra were recorded, and complexes. TGA measurements performed, as previously The water content of the complexes has been described.’ Emission and excitation spectra were verified by TGA, as reported in Table 1; in each obtained with an Aminco-Baumen MPF spec- compound all the water molecules are lost in a onetrophotofluorimeter. Proton-ligand and metalstep process at relatively low temperature, indiligand stability constants were determined using the cating that they are not coordinatively bonded to Calvin-Wilson pH-titration technique,6 in 1 : 1 the metal. This fact, together with the well known requirement of the lanthanide cations to reach a ethanol-water. An E366 Metrohm pH-meter, equipped with a Metrohm glass electrode, was used high coordination number, might indicate a polyfor pH measurements ; the concentrations of the meric structure, with a ligand behaving as a bridge. various species were in the range 1 x 10p4-4 x lo- 4 This hypothesis is supported by the insolubility of M, temperature 28.O+O.l”C, and Z = 0.1 M the complexes in most common solvents (including DMSO). A polymeric structure has often been NaClO,. The accuracy in the stability constants determination was not very high, because of the observed in lanthanide complexes.’ Attempts to precipitation of the complexes, after addition of a synthesize complexes with a ligand-metal ratio small volume of NaOH 0.05 M. The elemental and higher than 1: 1 have been unsuccessful, even when thermogravimetric analysis and the stability con- operating with a large excess of H,mes. The electronic spectra of all the complexes in the stants are reported in Table 1. solid state are very similar, and quite different from that of H,mes (Fig. 1). In DMSO they are someRESULTS AND DISCUSSION what different from those in the solid state, indiSome complexes of 4-methylesculetin have cating partial decomposition of the complexes in solution ; therefore it seems more appropriate to already been described ; in all cases it was found that the metal cation is bonded by H,mes in its compare the spectra in the solid state. The two bands at higher energy are shifted from 198 and 236 dianionic form. 7 On the contrary, in the complexes nm in the ligand, to 220 and 244 nm in the synthesized by us the ligand behaves as a monoanion (Hmes). It is known that the OH group in the complexes. The main band at lower energy under7-position is significantly more acidic than the OH goes a larger shift passing from 340 nm in H2mes
Table 1. Elemental
and thermogravimetric
analytical
data, and stability constants
% C Found Calc.
% H Found Calc.
% Ln Found Calc.
TGA” Found Calc. (for 3Hz0)
La(Hmes)C1,*3H,O
26.4 26.4
3.0 2.9
30.4 30.5
12.04 11.88
4.19+0.04
Sm(Hmes)Cl,-
3H,O
25.4 25.7
2.9 2.8
32.0 32.2
11.50 11.59
4.92 f0.07
- 3H,O
25.6 25.7
2.9 2.8
32.3 32.5
11.77 11.55
4.99 &-0.07
3H20
25.4 25.4
2.8 2.8
33.3 33.2
11.50 11.42
4.64 +0.06
* 3H20
25.3 25.3
2.7 2.8
33.6 33.5
11.30 11.38
4.99kO.06
25.2 25.1
2.7 2.7
33.8 33.9
10.66 10.76
5.04kO.05
Compound
Eu(Hmes)Cl, Gd(Hmes)Cl,Tb(Hmes)Cl,
Dy(Hmes)Cl,-3H,O
“Percent weight loss in the range 54105°C. b Referred to the reaction : LnCl 3+ H,mes = Ln(Hmes)Cl,
+ Cl- + H+.
log (B I)b
Lanthanide complexes with pyronic ligands-III.
OJ-
Y 20.5ii? 8 ! 0.3-
0.1-
2bo
3bo
460 A
560
"Ill
Fig. 1. Solid state absorption spectra of (a) 4-methylesculetin, and (b) Eu(Hmes)Cl, - 3Hz0.
390 nm in the complexes ; also its pattern changes noticeably. The decrease in the electronic energy transitions of the complexes with respect to the free ligand could result from an increase of the mesomeric effect (see Scheme 1) after deprotonation and complexation of H,mes. This hypothesis is supported by IR spectral data. The H2mes vibrational spectrum is characterized, in addition to a large v(OH) band at 3350 cm- ‘, by a strong signal at 1660 cm-’ due to the pyrone v(M) and by two other bands at 1610 and 1560 In the complexes of 4cn-’ assigned to v(W). methylesculetin which have already been described in the literature, the v(C=O) bond remains unaltered, but the v(C=C) bonds are very slightly shifted to lower energy (less than 10 cm-‘).’ In the lanthanide complexes described in this work, the pyrone v(O) is located at 1645 cm- ’ and the v(C=C) bonds are found at 1575 and 1525 cm-‘. These shifts to lower energy with respect to the free H,mes could indicate that in the complexes the importance of form II of the free ligand (see Scheme 1) is increased. However, the IR data might also be indicative of the formation of a coordinative bond between the lanthanide cations and the coumarine carbonyl oxygen atom. Packing effects may also be present. The formation constants of all the described complexes have been determined, and are reported in to
* The experimental described in ref. 9.
conditions
are not exhaustively
941
Table 1. In order to calculate them, the pK, of H,mes was previously determined, and a value of 8.12 f 0.3 was found. The slight difference from the value (7.90) reported by other authors may be due to different experimental conditions.*,’ The variation of metal-ligand stability constants with increasing atomic number of the cation follows an expected trend. ‘O In fact at the Gd level there appears a break, which has often been noted and theoretically studied. ’ ’ As far as the luminescence is concerned, we recorded the emission spectra of the complexes of 4methylesculetin with Sm”‘, Eu”‘, Tb”’ and Dy”’ which are known to be lanthanide cations which can luminesce in the visible region. ‘* However, when exciting in the range 300-430 nm, no lanthanide ion luminescence was observed, either in DMSO or in water-methanol (pH 5.5). The ligand fluorescence is reduced to about 70% with respect to the free H,mes fluorescence. The fact that the metal ions do not luminesce could result from different reasons. For example, we could suppose a very efficient quenching via multiphonon-like processes due to the OH stretching vibrations of water molecules or of the non-deprotonated OH group of the ligand. However, in our opinion, it is more reasonable to suppose that 4-methylesculetin is not effective in transferring energy to luminescent lanthanide cations. In fact, if the metal ion luminescence was quenched by OH stretching vibrations, we should have observed some difference on passing from DMSO to water-methanol as solvent; moreover, some differences had to arise by changing the energy gap between the lowest emitting level and the highest accepting level of the metal ; the gaps are 7500, 7800, 12,300 and 14,700 cm- ’ for Sm”‘, Dy”‘, Eu”’ and Tb”‘, respectively. ’ 3 Moreover, the brownorange colour of all the complexes indicates that the first singlet excited state of the ligands is quite low in energy ; as a consequence, the first triplet excited state of the ligands, generally involved in the ligand to lanthanide energy transfer,” is probably at too low energy to allow an effective energy transfer.
REFERENCES 1. J.-C. Btinzli, Znorg. Chim. Acta 1987, 139, 219; W. dew. Horrocks and A. Albin, Prog. Znorg. Chem. 1984,31, 1; F. S. Richardson, Chem. Rev. 1982,82, 541. 2. C. Bisi Castellani, 0. Carugo, C. Tomba and A. Gamba Invernizzi, Znorg. Chem. 1988, 27, 3965 ; C. Bisi Castellani and 0. Carugo, Znorg. Chim. Acta. 1989,159, 157. 3, M. Katyal and H. B. Singh, Tulunta 1968, 15, 1043.
942
C. BISI CASTELLANI
4. C. Bisi Castellani, J. Znorg. Nucl. Chem. 1970, 32, 2899. 5. C. Bisi Castellani, 0. Carugo, C. Tomba, V. Berbenni and S. Cinquetti, Znorg. Chim. Acta 1988, 145, 157. 6. F. J. C. Rossotti and H. Rossotti, The Determination of the Stability Constants. McGraw-Hill, New York (1961). 7. D. Singh and H. B. Singh, 2. Naturfor. 1977, 32b, 438. 8. C. Bisi Castellani and V. Tazzoli, Acta Cryst. 1984, C40, 1834; C, Bisi Castellani and A. Coda, Acta Cryst. 1985, C41, 186. 9. B. D. Jain and H. B. Singh, Znd. J. Chem. 1963, 1, 369.
and 0. CARUGO 10. L. C. Thompson, in Handbook of the Physics and Chemistry of Rare Earths (Edited by K. A. Gschneider and L. Eyring), Vol. 3, p. 45. NorthHolland, Amsterdam (1979). 11. V. G. Vokhmin and G. V. Ionova, Russ. J. Znorg. Chem. 1987, 32, 1552; B. F. Dzhurinskii, Russ. J. Znorg. Chem. 1980,25,41. 12. A. P. B. Sinha, in Spectroscopy in Inorganic Chemistry (Edited by C. N. R. Rao and J. R. Ferraro), p. 255. Academic Press, New York (1971). 13. W. T. Carnall, in Handbook on the Physics and Chemistry of Rare Earths (Edited by K. A. Gschneider and L. Eyring), Vol. 3, p. 171. NorthHolland, Amsterdam (1979).
Vol. 9, No. I, pp. 939442, 1990 Printed in Great Britain Polyhedron
STUDIES ON LANTHANIDE COMPLEXES WITH PYRONIC LIGANDS-111.” SYNTHESIS AND CHARACTERIZATION OF NOVEL COMPLEXES WITH 4-METHYLESCULETIN C. BISI CASTELLANIT Dipartimento
and 0.
CARUGO
di Chimica Generale, Universita di Pavia, via Taramelli 12, 27100 Pavia, Italy (Received 13 March 1989 ; accepted 2 November 1989)
novel complexes Ln(Hmes)Cl,* 3H20 (Ln = La, Sm, Eu, Gd, Tb, Dy ; Hmes = monodeprotonated 4-methylesculetin) have been synthesized and characterized by elemental and thermogravimetric analysis, and by vibrational and electronic absorption spectra. The ligand, potentially diprotic, is only monodeprotonated. The complexation constants have been determined potentiometrically. Contrary to many other complexes of lanthanides with benzopyrone ligands, the compounds described in this paper do not show metal luminescence promoted by the excited states of the ligand. This behaviour is discussed. Abstract-The
The luminescence of lanthanide cation complexes and its applications are the focus of attention of many researchers. ’ In previous works, we showed that ligands containing the a- or the y-pyronic rings are able to stimulate the lanthanide ion luminescence.2 Among the a-pyrone derivatives, the 7hydroxocoumarins are of relevant interest because of their peculiar electronic structure. 3 The presence of a hydroxy group in the 7-position causes a shift to lower energy of the bands in the electronic absorption spectra, with respect to the parent coumarin. This shift results from mesomeric effects, as shown in Scheme 1. The decrease in electronic transition energy may influence the ability of these molecules to transfer their electronic energy to the lanthanide cations. In order to verify if the 7-hydroxocoumarins behave as good chromophores toward lanthanide ions, like other a-pyrone ligands, we used the 4-methylesculetin as ligand and obtained the novel complexes Ln(Hmes)Cl, - 3H20 [Ln = La, Sm, Eu, Gd, Tb, Dy ; Hmes = deprotonated form of 4-methylesculetin (referred to as H,mes in Scheme l)]. The compounds were characterized by means of elemental and thermogravimetrical analysis, and electronic
* For the preceding parts see ref. 2. Author to whom correspondence should be addressed.
t
I
II
Scheme 1. H *mes(4-methylesculetin): R = OH, R’ = Me.
and vibrational spectroscopy. The formation constants of all the complexes were determined. No increase in lanthanide cation luminescence was observed in any case. EXPERIMENTAL
Synthesis 6,7-Dihydroxo-4-methyl-2H1- benzopyran-Z one (6methylesculetin) was obtained from AldrichChemie. The lanthanide chlorides were prepared as previously described.4 An ethanolic solution of H 2mes (1 mmol in 40 cm3) was added to an aqueous solution of the appropriate lanthanide salt (1 mmol in 15 cm’). On adjusting the pH to 6.2-6.5 with aqueous NaOH, the product separated from the solution as a brown-orange powder. It was centrifugated, washed with ethanol and water, and dried at 1 mm Hg and room temperature for 3 h.
940
C. BISI CASTELLANI
and 0.
CARUGO
If kept in contact with the solution, it decomposes in a few hours to a dark pitch.
group in the 6-position ;3 thus it seems probable that in our complexes the 7-hydroxy group is deprotonated. The proton of the 6-hydroxy group might be stabilized through hydrogen bonding to one or Physicochemical measurements more of the several oxygen atoms available in the IR and electronic spectra were recorded, and complexes. TGA measurements performed, as previously The water content of the complexes has been described.’ Emission and excitation spectra were verified by TGA, as reported in Table 1; in each obtained with an Aminco-Baumen MPF spec- compound all the water molecules are lost in a onetrophotofluorimeter. Proton-ligand and metalstep process at relatively low temperature, indiligand stability constants were determined using the cating that they are not coordinatively bonded to Calvin-Wilson pH-titration technique,6 in 1 : 1 the metal. This fact, together with the well known requirement of the lanthanide cations to reach a ethanol-water. An E366 Metrohm pH-meter, equipped with a Metrohm glass electrode, was used high coordination number, might indicate a polyfor pH measurements ; the concentrations of the meric structure, with a ligand behaving as a bridge. various species were in the range 1 x 10p4-4 x lo- 4 This hypothesis is supported by the insolubility of M, temperature 28.O+O.l”C, and Z = 0.1 M the complexes in most common solvents (including DMSO). A polymeric structure has often been NaClO,. The accuracy in the stability constants determination was not very high, because of the observed in lanthanide complexes.’ Attempts to precipitation of the complexes, after addition of a synthesize complexes with a ligand-metal ratio small volume of NaOH 0.05 M. The elemental and higher than 1: 1 have been unsuccessful, even when thermogravimetric analysis and the stability con- operating with a large excess of H,mes. The electronic spectra of all the complexes in the stants are reported in Table 1. solid state are very similar, and quite different from that of H,mes (Fig. 1). In DMSO they are someRESULTS AND DISCUSSION what different from those in the solid state, indiSome complexes of 4-methylesculetin have cating partial decomposition of the complexes in solution ; therefore it seems more appropriate to already been described ; in all cases it was found that the metal cation is bonded by H,mes in its compare the spectra in the solid state. The two bands at higher energy are shifted from 198 and 236 dianionic form. 7 On the contrary, in the complexes nm in the ligand, to 220 and 244 nm in the synthesized by us the ligand behaves as a monoanion (Hmes). It is known that the OH group in the complexes. The main band at lower energy under7-position is significantly more acidic than the OH goes a larger shift passing from 340 nm in H2mes
Table 1. Elemental
and thermogravimetric
analytical
data, and stability constants
% C Found Calc.
% H Found Calc.
% Ln Found Calc.
TGA” Found Calc. (for 3Hz0)
La(Hmes)C1,*3H,O
26.4 26.4
3.0 2.9
30.4 30.5
12.04 11.88
4.19+0.04
Sm(Hmes)Cl,-
3H,O
25.4 25.7
2.9 2.8
32.0 32.2
11.50 11.59
4.92 f0.07
- 3H,O
25.6 25.7
2.9 2.8
32.3 32.5
11.77 11.55
4.99 &-0.07
3H20
25.4 25.4
2.8 2.8
33.3 33.2
11.50 11.42
4.64 +0.06
* 3H20
25.3 25.3
2.7 2.8
33.6 33.5
11.30 11.38
4.99kO.06
25.2 25.1
2.7 2.7
33.8 33.9
10.66 10.76
5.04kO.05
Compound
Eu(Hmes)Cl, Gd(Hmes)Cl,Tb(Hmes)Cl,
Dy(Hmes)Cl,-3H,O
“Percent weight loss in the range 54105°C. b Referred to the reaction : LnCl 3+ H,mes = Ln(Hmes)Cl,
+ Cl- + H+.
log (B I)b
Lanthanide complexes with pyronic ligands-III.
OJ-
Y 20.5ii? 8 ! 0.3-
0.1-
2bo
3bo
460 A
560
"Ill
Fig. 1. Solid state absorption spectra of (a) 4-methylesculetin, and (b) Eu(Hmes)Cl, - 3Hz0.
390 nm in the complexes ; also its pattern changes noticeably. The decrease in the electronic energy transitions of the complexes with respect to the free ligand could result from an increase of the mesomeric effect (see Scheme 1) after deprotonation and complexation of H,mes. This hypothesis is supported by IR spectral data. The H2mes vibrational spectrum is characterized, in addition to a large v(OH) band at 3350 cm- ‘, by a strong signal at 1660 cm-’ due to the pyrone v(M) and by two other bands at 1610 and 1560 In the complexes of 4cn-’ assigned to v(W). methylesculetin which have already been described in the literature, the v(C=O) bond remains unaltered, but the v(C=C) bonds are very slightly shifted to lower energy (less than 10 cm-‘).’ In the lanthanide complexes described in this work, the pyrone v(O) is located at 1645 cm- ’ and the v(C=C) bonds are found at 1575 and 1525 cm-‘. These shifts to lower energy with respect to the free H,mes could indicate that in the complexes the importance of form II of the free ligand (see Scheme 1) is increased. However, the IR data might also be indicative of the formation of a coordinative bond between the lanthanide cations and the coumarine carbonyl oxygen atom. Packing effects may also be present. The formation constants of all the described complexes have been determined, and are reported in to
* The experimental described in ref. 9.
conditions
are not exhaustively
941
Table 1. In order to calculate them, the pK, of H,mes was previously determined, and a value of 8.12 f 0.3 was found. The slight difference from the value (7.90) reported by other authors may be due to different experimental conditions.*,’ The variation of metal-ligand stability constants with increasing atomic number of the cation follows an expected trend. ‘O In fact at the Gd level there appears a break, which has often been noted and theoretically studied. ’ ’ As far as the luminescence is concerned, we recorded the emission spectra of the complexes of 4methylesculetin with Sm”‘, Eu”‘, Tb”’ and Dy”’ which are known to be lanthanide cations which can luminesce in the visible region. ‘* However, when exciting in the range 300-430 nm, no lanthanide ion luminescence was observed, either in DMSO or in water-methanol (pH 5.5). The ligand fluorescence is reduced to about 70% with respect to the free H,mes fluorescence. The fact that the metal ions do not luminesce could result from different reasons. For example, we could suppose a very efficient quenching via multiphonon-like processes due to the OH stretching vibrations of water molecules or of the non-deprotonated OH group of the ligand. However, in our opinion, it is more reasonable to suppose that 4-methylesculetin is not effective in transferring energy to luminescent lanthanide cations. In fact, if the metal ion luminescence was quenched by OH stretching vibrations, we should have observed some difference on passing from DMSO to water-methanol as solvent; moreover, some differences had to arise by changing the energy gap between the lowest emitting level and the highest accepting level of the metal ; the gaps are 7500, 7800, 12,300 and 14,700 cm- ’ for Sm”‘, Dy”‘, Eu”’ and Tb”‘, respectively. ’ 3 Moreover, the brownorange colour of all the complexes indicates that the first singlet excited state of the ligands is quite low in energy ; as a consequence, the first triplet excited state of the ligands, generally involved in the ligand to lanthanide energy transfer,” is probably at too low energy to allow an effective energy transfer.
REFERENCES 1. J.-C. Btinzli, Znorg. Chim. Acta 1987, 139, 219; W. dew. Horrocks and A. Albin, Prog. Znorg. Chem. 1984,31, 1; F. S. Richardson, Chem. Rev. 1982,82, 541. 2. C. Bisi Castellani, 0. Carugo, C. Tomba and A. Gamba Invernizzi, Znorg. Chem. 1988, 27, 3965 ; C. Bisi Castellani and 0. Carugo, Znorg. Chim. Acta. 1989,159, 157. 3, M. Katyal and H. B. Singh, Tulunta 1968, 15, 1043.
942
C. BISI CASTELLANI
4. C. Bisi Castellani, J. Znorg. Nucl. Chem. 1970, 32, 2899. 5. C. Bisi Castellani, 0. Carugo, C. Tomba, V. Berbenni and S. Cinquetti, Znorg. Chim. Acta 1988, 145, 157. 6. F. J. C. Rossotti and H. Rossotti, The Determination of the Stability Constants. McGraw-Hill, New York (1961). 7. D. Singh and H. B. Singh, 2. Naturfor. 1977, 32b, 438. 8. C. Bisi Castellani and V. Tazzoli, Acta Cryst. 1984, C40, 1834; C, Bisi Castellani and A. Coda, Acta Cryst. 1985, C41, 186. 9. B. D. Jain and H. B. Singh, Znd. J. Chem. 1963, 1, 369.
and 0. CARUGO 10. L. C. Thompson, in Handbook of the Physics and Chemistry of Rare Earths (Edited by K. A. Gschneider and L. Eyring), Vol. 3, p. 45. NorthHolland, Amsterdam (1979). 11. V. G. Vokhmin and G. V. Ionova, Russ. J. Znorg. Chem. 1987, 32, 1552; B. F. Dzhurinskii, Russ. J. Znorg. Chem. 1980,25,41. 12. A. P. B. Sinha, in Spectroscopy in Inorganic Chemistry (Edited by C. N. R. Rao and J. R. Ferraro), p. 255. Academic Press, New York (1971). 13. W. T. Carnall, in Handbook on the Physics and Chemistry of Rare Earths (Edited by K. A. Gschneider and L. Eyring), Vol. 3, p. 171. NorthHolland, Amsterdam (1979).