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Synthesis, crystallographic data, magnetic properties and vibrational study of the new series KLn(CrO4)4 (Ln = Eu, Gd, Tb)
J. Phys. Ckm. Solids Vol. 51. No. 9. pp. I 117-l 121. 1990 Printed in Great Britain.
0022-3697/w s3.w + 0.00 Pergamon Pms plc
SYNTHESIS, CRYSTALLOGRAPHIC DATA, MAGNETIC PROPERTIES AND VIBRATIONAL STUDY OF THE NEW SERIES KLn(CrO& (Ln = Eu, Gd, Tb) I. BUENO,~C. PARADA,~~ R. SAEZ PUCHE,~ I. L. BOTTO$ and E. J. BARAN$ t Departamento 4 Departamento
de Quimica Inorganica, Facultad de Ciencias Quimicas, U.C.M. 28040, Madrid de Quimica, Facultad de Ciencias Exactas, Universidad National de la Plats. 1900 La Plats, Argentina (Received 2 January 1990; accepted 14 March 1990)
Abstract-Single crystals of the new series KLn(CrO,)r were grown by the hydrothermal method. Lattice dimensions decrease from Eu to Tb due to the lanthanide contraction. Magnetic susceptibility measurements were made in the 4.2-300 K temperature range. A vibrational study (IR and Raman) was made using a correlation between the point group of the “free” ions (Td), its site symmetry (C,) and its factor group (G). Keywork
Lanthanides, chromate, crystal data, magnetic properties, spectra.
1. INTRODUCTION In previous work we have found that in KLn(CrO,), there is a first isomorphous series when Ln = La-Eu, whose structure has been determined and whose thermal, magnetic and spectroscopic properties have been studied in detail [l-4]. In this paper we show the existence of a second series in the same system when Ln = Eu-Tb, the europium compound being dimorphic. The structure of this new series has been determined for the terbium compound KTb(Cr04)t [S] and showed important structural differences from the other one. This fact can be explained by the smaller size of these cations due to the lanthanide contraction which determines a lower coordination for the Ln’+ ions.
compounds
2. EXPERIMENTAL The synthesis of these new compounds was carried out by the following procedures: (i) KGd(CrO& and KTb(CrG,), were prepared by a hydrothermal procedure in sealed glass tubes heated at 150°C for 3 days for the Gd, and at 130°C for 15 days for the Tb compound. K,Cr,O, and Ln,03 (Ln = Gd, Tb) were used as reactants in molar ratios 21:1 and l&l, respectively. Single crystals of the two phases were grown by this method. (ii) KLn(CtQ)~ (Ln = Eu-Tb) were synthesized as microcrystalline powders by solid-state reaction from Ln,O,, KNO,, and CrG3 in molar ratios l/2 : 1: 2, with successive thermal treatments at 450, 500 and 550°C for 8 h each.
$ Author to whom correspondence should be addressed. PCS 5lP-o~
The synthesis of the europium compound using the first method generates the modification belonging to the other monoclinic structural type. The X-ray powder patterns were recorded using a Siemens Kristalloflex 810 diffractometer and a D-500 goniometer with nickel-filtered copper radiation (1 = 1.540598 A), selected with a graphite monochromator. The d-spacing measurements were made at a scanning rate of 0.1’ 20 cm-i, using silicon (a = 5.430881 A) as an internal standard. Magnetic susceptibility x measurements were made in a DMS 5 pendule susceptometer using the Faraday method. The maximum magnetic field was 1.5 kG with B dB/dZ = 30 kGz cm-i between 4.2 and 300 K. The set-up was calibrated with Hg [Co(SCN),] and Gdz(SOI)3.8H20 and 1 was independent of the field in the temperature range of measurements. Values of the effective moments 01) and Weiss constants (0) were obtained by least-squares fits of the linear part of the reciprocal susceptibility curves at the higher temperatures. The IR spectra were recorded on a Perkin Elmer 580B spectrophotometer using the KBr pellet technique. The Raman spectra of the crystalline powders sealed in glass capillaries were obtained on a spectrometer JARREL ASH mod.25-300 using the 514.5 nm line of an Ar+ laser for excitation. The spectra1 resolution for both spectra was better than + 1 cm-i.
3. RESULTS AND DISCUSSION
X-Ray spectra This new phase crystallizes in the orthorhombic system S.G. P2,2,2, and Z = 4 [5]. Table 1 shows the
1117
I. BWNO er al.
1118
Table 1. X-Ray diffraction data For KLn(CrO,)s (Ln = Eu, Gd, Tb) with e.s.d. in parentheses Ln
u (A)
b (A)
c (A)
FiYl
13.844(8) 13.804(5) 13.779(5)
5.479(4) 5.746(3) 5.735( 1)
P&48(4) 9.037(2) 9.029(2)
Tb
D, (g cm?
24.4
-
16.3
-
E’ d & 12.2
-
-
3.90 3.98 4.04
I
! T
unit cell parameters of the three compounds. A decrease of these values can be observed with increasing atomic number of the lanthanide cation. Table 2 shows the observed and calculated interplanar dspacings and the observed intensities for this series.
X 6.1
-
&9
I
I 100
0
J 300
2uJ TtKf
Fig. 1. Temperature dependence of the reciprocal magnetic susceptibitity per mole of Tb’+ in KTb(CrO,),.
Magnetic properties
The magnetic susceptibility of KTb(CrO,), shows a Curie-Weiss behavior over a wide temperature range from 300 down to 20 K (Fig. 1). At lower temperatures x-‘(T) deviates from the Curie-Weiss law and the curve bends upward, analogous to that observed in some other terbium and praseodymium compounds [6,7]. The susceptibility of KGd(CrG,)t, as can be observed in Fig. 2, obeys a quasi-Curie law over the entire range of temperatures from 4.2 to 300 K and yields a value of the effective magnetic moment of 8.10 B.M. which agrees fairly well with that expected for the free ion without taking into account the
crystal field (CF) contributions (81, as shown in Table 3. The ground state of Gd’+ is ‘S, and the CF splitting reported for different compounds of gadoiinium is very small (< I cm-‘). Consequently the crystal field effects should be operative only at very low temperatures [9]. For Tb3+ and Gd3+ cations, the smalf values of the @ constants (4.5 and -0.5 K, respectively) indicate that the CF effect on the magnetic interaction should be very small [lo]. The anomalous variation of the magnetic susceptibility with temperature in the case of KEu(C~G,)~
Table 2. Observed (d,) and calculated (d,) relative intensities of KLn(CrO,), hkf
4
101
7.589
200 201 011 002 400 311 302 212 020 312 “411 > 203 013 501 221 022 511 104 420 512 322 “421 > 610 413 611 123 602 304 214 223
6.943 5.508 4.872 4.531 3.457 3.340 3.233 3.166 2.877 2.817 2.766 2.672 2.650 2.549 2.422 2.404 2.235 2.214 2.183 2.149 2.144 2.115 2.084 2.056 2.052 2.030 2.013 1.990
EU 4 7.574 6.922 5.498 4.852 4.524 3.461 3.344 3.231 3.163 2.875 2.816 2.818 > 2.765 2.671 2.648 2.547 2.426 2.405 2.232 2.211 2.185 2.148 2.149 > 2.141 2.115 2.084 2.058 2.053 2.031 2.014 I.992
III*
: 40
16 12 100 :: so 20 39
4 7.519 6.873 5.464 4.845 4.502 3.440 3.334 3.217 3.151 2.874 2.809 2.754 2.660 2.637 2.546 2.423 2.395 2.225 2.206 2.176 2.143
6 : 10 6 6
2.134 2.107 2.075 2.054 2.046 2.022 1.987 1.987
Gd 4 7.541 6.889 5.473 4.846 4.505 3.445 3.333 3.216 3.153 2.874 2.807 2.808 > 2.753 2.662 2.635 2.545 2.423 2.395 2.223 2.207 2.176 2.143 2.143 > 2.133 2.106 2.075 2.053 2.046 2.022 1.988 1.988
II4 2 3; 10 10 100 8 16 30 8 1s 5 : : 3 14 : 4
compounds 4 7.519 6.878 5.458 4.824 4.495 3.435 3.332 3.212 3.149 2.862 2.801 2.752 2.659 2.630 2.537 2.419 2.302 2.222 2.201 2.136 2.136 2.129 2.102 2.069 2.052 2.044 2.003 1.984 1.924
Tb 4 7.534 6.874 5.464 4.831 4.503 3.437 3.325 3.212 3.147 2.863 2.801 2.801 > 2.751 2.659 2.630 2.536 2.416 2.390 2.222 2.200 2.137 2.137 2.137> 2.127 2.103 2.070 0.049 2.042 2.004 1.984 1.924
III* : 3s 12 11 LOO 12 23 3s 13 18 : 8 4 4 3 16 4 7 7 7 5 4 : 4 5 10
Study of the new series KLn(CrO,),
0
100
200
300
0
1119
I loo
2eo
300
T(K)
T(K)
Fig. 2. Temperature dependence of the reciprocal magnetic susceptibility per mole of Gd’+ in KGd(CrO&.
Fig. 3. Temperature variation of the magnetic susceptibility per mole of Eul+ in KEu(c10,)~. Inset: energy levels estimated for EuJ+.
may be attributed to the existence of closely spaced multiplets and for this reason x(T) does not follow a Curie-Weiss law and the magnetic moment changes with temperature. Below 100 K the susceptibility remains almost constant which is indicative that the only non-magnetic ‘F, ground state is populated. As Fig. 3 shows, the experimental susceptibility data are in good agreement with the theoretical values calculated by using van Vleck’s equation [ll], which does not consider either the ‘F, CF splitting or the J-mixing, that strongly modify the wave functions. The best fit of the experimental data to Van Vleck’s equation is obtained for an orbital coupling constant I equal to 330 cm-‘. Assuming Russel-Saunders coupling the energy levels for the different ‘F, multiplets of Eu’+ can be calculated. Estimated energy levels for EL? are plotted inset in Fig. 3, and represent the center-of-gravity of the different CF levels. We are currently undertaking optical spectra studies by fluorescence and absorption techniques in order to calculate the CF parameters [12, 131. This will allow us to compute more precisely the temperature dependence of the magnetic susceptibility.
and OCrO angles [S]. They are located on the general C, sites of the orthorhombic unit cell. In order to facilitate the assignment for the 72 internal vibrational modes corresponding to the eight chromate ions a correlation between the point group of the “free” ions (Td), its site symmetry (C,) and its factor group (D2) was undertaken. The results are presented in Table 4. The result of this analysis implies that the following components should be expected for the different vibrational modes:
Vibrational spectra
The vibrational spectra of the compounds belonging to this orthorhombic series are somewhat simpler than those previously reported for the monoclinic series [4]. Nevertheless, the assignment is not easy, as we will show later. The two crystallographically inequivalent CrO:are not very different, showing similar bond lengths Table 3. Calculated (c) and measured (exp.) magnetic moments (B.M.) and. Weiss temperature (K) for the Ln’+ Ions m KLn(CrO,)*
Compound KEu(CQ, KGd(CrO,), KTb(CrO, )z
r..,.(B.M.)
r,(B.M.)
3.47t 8.10 9.31
t Calculated at room temperature.
3.50 7.94 9.12
B(K) -0.5 4.5
symmetric stretching v, : symmetric bending v2: antisymmetric
stretching v,:
antisymmetric
bending vq:
8 6 16 12 24 18 24 18
Raman-active and IR-active bands Raman-active and IR-active bands Raman-active and IR-active bands Raman-active and IR-active bands.
The spectra of these three compounds are very similar in the different spectral regions. As a typical example Fig. 4 shows the IR and Raman spectra of KGd(CrOd)*. Although the number of observed bands is clearly smaller than predicted above, the general spectral features point to important correlation field effects, showing strong coupling between the anions in the unit cell. In the case of the previously investigated monoclinic series of KLn(CrO& compounds the intercalation of the v, components between the vj
Table 4. Correlation between the point group, site group and factor group of the CrO:- ion in the orthorhombic KLn(CrO,), series Point group
7-d v,(A,) VICE 1
v,(Fd
Site group
c,
A
b 2(A, 4 9%. 4)
2(2A, 2B,, 24.24) 3(3A, 38,. 3&r 38,) 2(3A, 3B,. 3&, 3B,)
2A 3A 3A
v,(6) Activity of the factor A,B,,B2,B,; IR: B,,&rB,.
Factor group x2
group
components:
Raman:
I. BUENOet uf.
the typica aspect of the vt components of tetrahedral oxo-anions they do not coincide with the stronger Raman lines, which are usually assigned to the vt modes 14, 141. The fact that these strong lines show similar frequency values to those observed in the middle part of the IR spectra suggests important coupling between the vl and vs modes. Similar couplings probably also occur in the deformation region. Therefore, in Table 5 in which we present the set of measured vibrational modes, no attempt has been made to differentiate between the different symmetric and antis~met~c modes in the two spectral ranges. It is also interesting to mention that in this series most of the corresponding IR and Raman bands are slightly displaced to higher frequencies with increasing mass of the lanl;hanide cation. This behavior is more evident in the stretching region of the spectra
and it is directly related to the reinforcement of the Cr-0 bonds with the diminution of the unit cell dimensions in the Eu to Tb direction [4, IS, 16) Finaliy, the present results show that it is very easy to differentiate between the two groups of KLn(Ct+CI& materials using IR and Raman spectroscopy. ~~k~~~fedge~e~ts_T~is
Fig. 4. Infrared (above) and Raman (below) spectra of KGd(CrO,), .
work is part of a joint research program of the Inorganic Chemistry Groups of Madrid and La Plats which is supported by the Ministry of Education and Science of Spain. I. L. Botto and E. J. Baran acknowledge the support of CQNICET, Argentina.
components could be very clearly demonstrated. The situation is not so straightforward in the present case and it is difficult to differentiate unambiguously between the components of the two stretching modes. Although the three lower-frequency IR bands present Table 5. Vibrationa fR 980
spectra of the investigated KLn(CrO&
KGd(CrW, Ramad
m
979 w 939 m 914vw 898 vs
941 s 917vw 901 vs 878 VW 870 vs
fR
892 vs 886 vs 870 sb 865 vs
854 VW 846 m
883 vw 871 s
-
KEu(CrW Raman 982 w 922 w 915w 895 w 890 vs 88I YS 872 sh 860 vs
m
compounds (values in cm-‘) fR 944s
929 VW 906 vs
930 w
886 vs 873 vs
885 vs
898 vs
833 m
834 m 833 m
446s 415s 403 s 384 m 376 m 356 m 344w
MS
448s 4to s
383 w 375 w 355 s 338 w
330 vw
325 VW
“3 + VI
862 vs
829 m
4OOW
Assignment
982 m
%31m
413s 397 s 385 w 369 m 354m 339 w
KTb(CrU& Raman
858 wr 843 w
84%m 839
REFERENCES 1. Bueno I., Garcia O., Parada C.. Gutierrez E., Mange A. and Ruiz C., J. c&em. SW. Dufton Truns. 1911 (1988). 2. Bueno f., Garcia O., Parada C. and Saez Puche R., f. iess-common Me& 139, 261 (1988).
402w 385 w 372 w 355 s 338 w 329 w
377 s 357 m 342 w 326 VW
vs: very strong; s: strong; m: medium; w: weak; VW:very weak; sh: shoulder.
375 w 355 m 339 w
“4 + “2
Study of the new series KLn(CrG,), 3. Bueno I., Parada C. and Saez Puche R., Solid St. Ionics 32/B, 488( 1989). 4. Baran E. J., Ferrer E. G., Bueno I. and Parada C., J. Raman Specwosc. (in press). 5. Bueno I.. Parada C., Herrnoso J. A., Vegas A. and Martinez-Ripoll M., J. Solid St. Chem. (in press). 6. Saez Puche R., Norton M., White T. R. and Glaunsinger G. S., J. Solid Sr. C/rem. SO, 281 (1983). 7. Laureiro Y., Veiga M. L.. Fernandez F., Saez Puche R.. Jerez A. and Pica C.. J. less-common Metals (in press). 8. Vlcck J. H. van, The Theory of the Electric and Magnetic Suscepribilities. Oxford University Press, Oxford (1965).
1121
9. Gwo M. D.. Aldred A. T. and Chan S. K., J. Phys. Chem. Solids 48, 229 (1987). 10. Saez Puche R., Femandez F., Rodriguez Carvajal J. and Martinez J. L., Solid St. Commun. 72, 273 (1989). 11. Percher P. and Caro P., J. them. Phys. 63.4176 (1978). 12. VIeck J. H. van, J. appl. Phys. 39, 365 (1968). 13. Jayasankar C. K., Antic-Fidance E., Lemaitre-Blaise M. and Percher P., Phys. Starus Solidi (b) 133, 345 (1986). 14. Milller A., Weinstock N. and Baran E. J.. An. Asoc. Quim. Argent. 64, 239 (1976). 15. Baran E. J., Escobar M. E., Foumier L. L. and Filgueira R. R., Z. anorg. allg. Chem. 472, 193 (1981). 16. Lavat A. E., Trezza M., Botto I. L., Roncaglia D. I. and Baran E. J., Specrrosc. Lefr. 31, 355 (1988).
0022-3697/w s3.w + 0.00 Pergamon Pms plc
SYNTHESIS, CRYSTALLOGRAPHIC DATA, MAGNETIC PROPERTIES AND VIBRATIONAL STUDY OF THE NEW SERIES KLn(CrO& (Ln = Eu, Gd, Tb) I. BUENO,~C. PARADA,~~ R. SAEZ PUCHE,~ I. L. BOTTO$ and E. J. BARAN$ t Departamento 4 Departamento
de Quimica Inorganica, Facultad de Ciencias Quimicas, U.C.M. 28040, Madrid de Quimica, Facultad de Ciencias Exactas, Universidad National de la Plats. 1900 La Plats, Argentina (Received 2 January 1990; accepted 14 March 1990)
Abstract-Single crystals of the new series KLn(CrO,)r were grown by the hydrothermal method. Lattice dimensions decrease from Eu to Tb due to the lanthanide contraction. Magnetic susceptibility measurements were made in the 4.2-300 K temperature range. A vibrational study (IR and Raman) was made using a correlation between the point group of the “free” ions (Td), its site symmetry (C,) and its factor group (G). Keywork
Lanthanides, chromate, crystal data, magnetic properties, spectra.
1. INTRODUCTION In previous work we have found that in KLn(CrO,), there is a first isomorphous series when Ln = La-Eu, whose structure has been determined and whose thermal, magnetic and spectroscopic properties have been studied in detail [l-4]. In this paper we show the existence of a second series in the same system when Ln = Eu-Tb, the europium compound being dimorphic. The structure of this new series has been determined for the terbium compound KTb(Cr04)t [S] and showed important structural differences from the other one. This fact can be explained by the smaller size of these cations due to the lanthanide contraction which determines a lower coordination for the Ln’+ ions.
compounds
2. EXPERIMENTAL The synthesis of these new compounds was carried out by the following procedures: (i) KGd(CrO& and KTb(CrG,), were prepared by a hydrothermal procedure in sealed glass tubes heated at 150°C for 3 days for the Gd, and at 130°C for 15 days for the Tb compound. K,Cr,O, and Ln,03 (Ln = Gd, Tb) were used as reactants in molar ratios 21:1 and l&l, respectively. Single crystals of the two phases were grown by this method. (ii) KLn(CtQ)~ (Ln = Eu-Tb) were synthesized as microcrystalline powders by solid-state reaction from Ln,O,, KNO,, and CrG3 in molar ratios l/2 : 1: 2, with successive thermal treatments at 450, 500 and 550°C for 8 h each.
$ Author to whom correspondence should be addressed. PCS 5lP-o~
The synthesis of the europium compound using the first method generates the modification belonging to the other monoclinic structural type. The X-ray powder patterns were recorded using a Siemens Kristalloflex 810 diffractometer and a D-500 goniometer with nickel-filtered copper radiation (1 = 1.540598 A), selected with a graphite monochromator. The d-spacing measurements were made at a scanning rate of 0.1’ 20 cm-i, using silicon (a = 5.430881 A) as an internal standard. Magnetic susceptibility x measurements were made in a DMS 5 pendule susceptometer using the Faraday method. The maximum magnetic field was 1.5 kG with B dB/dZ = 30 kGz cm-i between 4.2 and 300 K. The set-up was calibrated with Hg [Co(SCN),] and Gdz(SOI)3.8H20 and 1 was independent of the field in the temperature range of measurements. Values of the effective moments 01) and Weiss constants (0) were obtained by least-squares fits of the linear part of the reciprocal susceptibility curves at the higher temperatures. The IR spectra were recorded on a Perkin Elmer 580B spectrophotometer using the KBr pellet technique. The Raman spectra of the crystalline powders sealed in glass capillaries were obtained on a spectrometer JARREL ASH mod.25-300 using the 514.5 nm line of an Ar+ laser for excitation. The spectra1 resolution for both spectra was better than + 1 cm-i.
3. RESULTS AND DISCUSSION
X-Ray spectra This new phase crystallizes in the orthorhombic system S.G. P2,2,2, and Z = 4 [5]. Table 1 shows the
1117
I. BWNO er al.
1118
Table 1. X-Ray diffraction data For KLn(CrO,)s (Ln = Eu, Gd, Tb) with e.s.d. in parentheses Ln
u (A)
b (A)
c (A)
FiYl
13.844(8) 13.804(5) 13.779(5)
5.479(4) 5.746(3) 5.735( 1)
P&48(4) 9.037(2) 9.029(2)
Tb
D, (g cm?
24.4
-
16.3
-
E’ d & 12.2
-
-
3.90 3.98 4.04
I
! T
unit cell parameters of the three compounds. A decrease of these values can be observed with increasing atomic number of the lanthanide cation. Table 2 shows the observed and calculated interplanar dspacings and the observed intensities for this series.
X 6.1
-
&9
I
I 100
0
J 300
2uJ TtKf
Fig. 1. Temperature dependence of the reciprocal magnetic susceptibitity per mole of Tb’+ in KTb(CrO,),.
Magnetic properties
The magnetic susceptibility of KTb(CrO,), shows a Curie-Weiss behavior over a wide temperature range from 300 down to 20 K (Fig. 1). At lower temperatures x-‘(T) deviates from the Curie-Weiss law and the curve bends upward, analogous to that observed in some other terbium and praseodymium compounds [6,7]. The susceptibility of KGd(CrG,)t, as can be observed in Fig. 2, obeys a quasi-Curie law over the entire range of temperatures from 4.2 to 300 K and yields a value of the effective magnetic moment of 8.10 B.M. which agrees fairly well with that expected for the free ion without taking into account the
crystal field (CF) contributions (81, as shown in Table 3. The ground state of Gd’+ is ‘S, and the CF splitting reported for different compounds of gadoiinium is very small (< I cm-‘). Consequently the crystal field effects should be operative only at very low temperatures [9]. For Tb3+ and Gd3+ cations, the smalf values of the @ constants (4.5 and -0.5 K, respectively) indicate that the CF effect on the magnetic interaction should be very small [lo]. The anomalous variation of the magnetic susceptibility with temperature in the case of KEu(C~G,)~
Table 2. Observed (d,) and calculated (d,) relative intensities of KLn(CrO,), hkf
4
101
7.589
200 201 011 002 400 311 302 212 020 312 “411 > 203 013 501 221 022 511 104 420 512 322 “421 > 610 413 611 123 602 304 214 223
6.943 5.508 4.872 4.531 3.457 3.340 3.233 3.166 2.877 2.817 2.766 2.672 2.650 2.549 2.422 2.404 2.235 2.214 2.183 2.149 2.144 2.115 2.084 2.056 2.052 2.030 2.013 1.990
EU 4 7.574 6.922 5.498 4.852 4.524 3.461 3.344 3.231 3.163 2.875 2.816 2.818 > 2.765 2.671 2.648 2.547 2.426 2.405 2.232 2.211 2.185 2.148 2.149 > 2.141 2.115 2.084 2.058 2.053 2.031 2.014 I.992
III*
: 40
16 12 100 :: so 20 39
4 7.519 6.873 5.464 4.845 4.502 3.440 3.334 3.217 3.151 2.874 2.809 2.754 2.660 2.637 2.546 2.423 2.395 2.225 2.206 2.176 2.143
6 : 10 6 6
2.134 2.107 2.075 2.054 2.046 2.022 1.987 1.987
Gd 4 7.541 6.889 5.473 4.846 4.505 3.445 3.333 3.216 3.153 2.874 2.807 2.808 > 2.753 2.662 2.635 2.545 2.423 2.395 2.223 2.207 2.176 2.143 2.143 > 2.133 2.106 2.075 2.053 2.046 2.022 1.988 1.988
II4 2 3; 10 10 100 8 16 30 8 1s 5 : : 3 14 : 4
compounds 4 7.519 6.878 5.458 4.824 4.495 3.435 3.332 3.212 3.149 2.862 2.801 2.752 2.659 2.630 2.537 2.419 2.302 2.222 2.201 2.136 2.136 2.129 2.102 2.069 2.052 2.044 2.003 1.984 1.924
Tb 4 7.534 6.874 5.464 4.831 4.503 3.437 3.325 3.212 3.147 2.863 2.801 2.801 > 2.751 2.659 2.630 2.536 2.416 2.390 2.222 2.200 2.137 2.137 2.137> 2.127 2.103 2.070 0.049 2.042 2.004 1.984 1.924
III* : 3s 12 11 LOO 12 23 3s 13 18 : 8 4 4 3 16 4 7 7 7 5 4 : 4 5 10
Study of the new series KLn(CrO,),
0
100
200
300
0
1119
I loo
2eo
300
T(K)
T(K)
Fig. 2. Temperature dependence of the reciprocal magnetic susceptibility per mole of Gd’+ in KGd(CrO&.
Fig. 3. Temperature variation of the magnetic susceptibility per mole of Eul+ in KEu(c10,)~. Inset: energy levels estimated for EuJ+.
may be attributed to the existence of closely spaced multiplets and for this reason x(T) does not follow a Curie-Weiss law and the magnetic moment changes with temperature. Below 100 K the susceptibility remains almost constant which is indicative that the only non-magnetic ‘F, ground state is populated. As Fig. 3 shows, the experimental susceptibility data are in good agreement with the theoretical values calculated by using van Vleck’s equation [ll], which does not consider either the ‘F, CF splitting or the J-mixing, that strongly modify the wave functions. The best fit of the experimental data to Van Vleck’s equation is obtained for an orbital coupling constant I equal to 330 cm-‘. Assuming Russel-Saunders coupling the energy levels for the different ‘F, multiplets of Eu’+ can be calculated. Estimated energy levels for EL? are plotted inset in Fig. 3, and represent the center-of-gravity of the different CF levels. We are currently undertaking optical spectra studies by fluorescence and absorption techniques in order to calculate the CF parameters [12, 131. This will allow us to compute more precisely the temperature dependence of the magnetic susceptibility.
and OCrO angles [S]. They are located on the general C, sites of the orthorhombic unit cell. In order to facilitate the assignment for the 72 internal vibrational modes corresponding to the eight chromate ions a correlation between the point group of the “free” ions (Td), its site symmetry (C,) and its factor group (D2) was undertaken. The results are presented in Table 4. The result of this analysis implies that the following components should be expected for the different vibrational modes:
Vibrational spectra
The vibrational spectra of the compounds belonging to this orthorhombic series are somewhat simpler than those previously reported for the monoclinic series [4]. Nevertheless, the assignment is not easy, as we will show later. The two crystallographically inequivalent CrO:are not very different, showing similar bond lengths Table 3. Calculated (c) and measured (exp.) magnetic moments (B.M.) and. Weiss temperature (K) for the Ln’+ Ions m KLn(CrO,)*
Compound KEu(CQ, KGd(CrO,), KTb(CrO, )z
r..,.(B.M.)
r,(B.M.)
3.47t 8.10 9.31
t Calculated at room temperature.
3.50 7.94 9.12
B(K) -0.5 4.5
symmetric stretching v, : symmetric bending v2: antisymmetric
stretching v,:
antisymmetric
bending vq:
8 6 16 12 24 18 24 18
Raman-active and IR-active bands Raman-active and IR-active bands Raman-active and IR-active bands Raman-active and IR-active bands.
The spectra of these three compounds are very similar in the different spectral regions. As a typical example Fig. 4 shows the IR and Raman spectra of KGd(CrOd)*. Although the number of observed bands is clearly smaller than predicted above, the general spectral features point to important correlation field effects, showing strong coupling between the anions in the unit cell. In the case of the previously investigated monoclinic series of KLn(CrO& compounds the intercalation of the v, components between the vj
Table 4. Correlation between the point group, site group and factor group of the CrO:- ion in the orthorhombic KLn(CrO,), series Point group
7-d v,(A,) VICE 1
v,(Fd
Site group
c,
A
b 2(A, 4 9%. 4)
2(2A, 2B,, 24.24) 3(3A, 38,. 3&r 38,) 2(3A, 3B,. 3&, 3B,)
2A 3A 3A
v,(6) Activity of the factor A,B,,B2,B,; IR: B,,&rB,.
Factor group x2
group
components:
Raman:
I. BUENOet uf.
the typica aspect of the vt components of tetrahedral oxo-anions they do not coincide with the stronger Raman lines, which are usually assigned to the vt modes 14, 141. The fact that these strong lines show similar frequency values to those observed in the middle part of the IR spectra suggests important coupling between the vl and vs modes. Similar couplings probably also occur in the deformation region. Therefore, in Table 5 in which we present the set of measured vibrational modes, no attempt has been made to differentiate between the different symmetric and antis~met~c modes in the two spectral ranges. It is also interesting to mention that in this series most of the corresponding IR and Raman bands are slightly displaced to higher frequencies with increasing mass of the lanl;hanide cation. This behavior is more evident in the stretching region of the spectra
and it is directly related to the reinforcement of the Cr-0 bonds with the diminution of the unit cell dimensions in the Eu to Tb direction [4, IS, 16) Finaliy, the present results show that it is very easy to differentiate between the two groups of KLn(Ct+CI& materials using IR and Raman spectroscopy. ~~k~~~fedge~e~ts_T~is
Fig. 4. Infrared (above) and Raman (below) spectra of KGd(CrO,), .
work is part of a joint research program of the Inorganic Chemistry Groups of Madrid and La Plats which is supported by the Ministry of Education and Science of Spain. I. L. Botto and E. J. Baran acknowledge the support of CQNICET, Argentina.
components could be very clearly demonstrated. The situation is not so straightforward in the present case and it is difficult to differentiate unambiguously between the components of the two stretching modes. Although the three lower-frequency IR bands present Table 5. Vibrationa fR 980
spectra of the investigated KLn(CrO&
KGd(CrW, Ramad
m
979 w 939 m 914vw 898 vs
941 s 917vw 901 vs 878 VW 870 vs
fR
892 vs 886 vs 870 sb 865 vs
854 VW 846 m
883 vw 871 s
-
KEu(CrW Raman 982 w 922 w 915w 895 w 890 vs 88I YS 872 sh 860 vs
m
compounds (values in cm-‘) fR 944s
929 VW 906 vs
930 w
886 vs 873 vs
885 vs
898 vs
833 m
834 m 833 m
446s 415s 403 s 384 m 376 m 356 m 344w
MS
448s 4to s
383 w 375 w 355 s 338 w
330 vw
325 VW
“3 + VI
862 vs
829 m
4OOW
Assignment
982 m
%31m
413s 397 s 385 w 369 m 354m 339 w
KTb(CrU& Raman
858 wr 843 w
84%m 839
REFERENCES 1. Bueno I., Garcia O., Parada C.. Gutierrez E., Mange A. and Ruiz C., J. c&em. SW. Dufton Truns. 1911 (1988). 2. Bueno f., Garcia O., Parada C. and Saez Puche R., f. iess-common Me& 139, 261 (1988).
402w 385 w 372 w 355 s 338 w 329 w
377 s 357 m 342 w 326 VW
vs: very strong; s: strong; m: medium; w: weak; VW:very weak; sh: shoulder.
375 w 355 m 339 w
“4 + “2
Study of the new series KLn(CrG,), 3. Bueno I., Parada C. and Saez Puche R., Solid St. Ionics 32/B, 488( 1989). 4. Baran E. J., Ferrer E. G., Bueno I. and Parada C., J. Raman Specwosc. (in press). 5. Bueno I.. Parada C., Herrnoso J. A., Vegas A. and Martinez-Ripoll M., J. Solid St. Chem. (in press). 6. Saez Puche R., Norton M., White T. R. and Glaunsinger G. S., J. Solid Sr. C/rem. SO, 281 (1983). 7. Laureiro Y., Veiga M. L.. Fernandez F., Saez Puche R.. Jerez A. and Pica C.. J. less-common Metals (in press). 8. Vlcck J. H. van, The Theory of the Electric and Magnetic Suscepribilities. Oxford University Press, Oxford (1965).
1121
9. Gwo M. D.. Aldred A. T. and Chan S. K., J. Phys. Chem. Solids 48, 229 (1987). 10. Saez Puche R., Femandez F., Rodriguez Carvajal J. and Martinez J. L., Solid St. Commun. 72, 273 (1989). 11. Percher P. and Caro P., J. them. Phys. 63.4176 (1978). 12. VIeck J. H. van, J. appl. Phys. 39, 365 (1968). 13. Jayasankar C. K., Antic-Fidance E., Lemaitre-Blaise M. and Percher P., Phys. Starus Solidi (b) 133, 345 (1986). 14. Milller A., Weinstock N. and Baran E. J.. An. Asoc. Quim. Argent. 64, 239 (1976). 15. Baran E. J., Escobar M. E., Foumier L. L. and Filgueira R. R., Z. anorg. allg. Chem. 472, 193 (1981). 16. Lavat A. E., Trezza M., Botto I. L., Roncaglia D. I. and Baran E. J., Specrrosc. Lefr. 31, 355 (1988).