the paramagnetic susceptibility of c-type europium sesquioxide

Journal of Magnetism and Magnetic Materials 58 (1986) 61-66 61 North-Holland, Amsterdam

T H E P A R A M A G N E T I C S U S C E P T I B I L I T Y O F C-TYPE EUROPIUM SESQUIOXIDE

Paul CARO and Pierre PORCHER

ER 210 du CNRS, 1 place A. Briand, 92195 Meudon Cedex, France

Received 11 March 1985; in revised form 4 September 1985

The experimental variation with temperature of the paramagnetic susceptibility of C-type europium sesquioxide has been simulated taking into account different crystal field for the two cationic sites in the unit cell. Because of the very large value for the second rank crystal field parameter at the S 6 site (one fourth of the cationic sites) the susceptibility is much higher than free atom expectation values. The calculation using crystal field parameters derived from optical data is in perfect agreement with experimental magnetic measurements. The "anomalous" paramagnetic susceptibility of C-Eu203 is consequently the result of a crystal field effect and is not due to some sort of "exchange" between the magnetic ions.

1. Introduction

Huang and Van Vleck [1] attributed to "aniso- tropic exchange" the "anomalous" magnetic susceptibility of C-type Eu 203, which exhibits a 30% increase over that of the free Eu 3+ ion (and most europium compounds) below 50 K. It is well known that there are two crystallographic sites for the E u 3 + ion in the C-type structure which has the T 7 space group. Three quarter of the cations are in a low symmetry C 2 site, and one fourth in a centro symmetrical S 6 site. The energy levels of the Stark components of the 7Fj multiplets are easily ob- tained from spectroscopy in the case of the C 2 site [2,3], but the fluorescence signals from the S 6 site are very difficult to detect, because electric dipole transitions, which feed the excited levels of the f 6 configuration, are forbidded by the centrosymme- try.

Huang and Van Vleck used the C 2 site optical data of Chang and Gruber [2] to compute the paramagnetic susceptibility, which mostly depends on the position of the 7F 1 Stark components. Ex- plicitly they made " the crude approximation that the field for all the sites is the same as that determined optically for one of them" (ref. [1] p. 1145). A recent analysis of the fluorescence spectrum of C-Gd203 [4] revealed that the 4f 7, 6p7/2

8S7/2 transition which is a dipolar magnetic one,

exhibits Stark components for both C 2 and S 6 sites. In the case of 7F 1 (Eu3+), as well as in the case of 6P7/2 (Gd3+), the Stark splitting depends essentially on the second rank crystal field parameters, and from the experiment on C-Gd203 it is clear that the S 6 axial B 2 parameter is very large, close to - 1 7 0 0 cm-1; whereas for Eu 3+ in Y203 the C 2 site parameters are B 2 = - 1 9 5 cm 1, B22 = 695 c m - t [3] and for Gd203 B 2 = - 2 6 6 cm-1, B 2 = - 7 2 0 cm-1 [4]. Then the "strength" of the crystal field, measured for instance by the parame-

ter 1 /3 ~ ( B 2)2 + 2(B 2)2, is different for the two sites: 351 cm -1 for C 2, 566 cm -1 for S 6 in Gd203.

2. The C 2 site

The energy levels of E u 3+ in the C 2 site are experimentally well known [2,3,5-7]. It is not difficult to fit the 7Fj Stark level splittings to a set of crystal field parameters which reproduce them correctly. It is however more difficult to reproduce the absolute values of the position of the Stark levels in a calculation where the crystal field operators are applied together with free atom operators on an extended basis. The experimental barycenter of the 7F t level for the C 2 site is 370 cm - t (table 1) whereas the 7F 2 level barycenter is estimated to be at 1061 cm -~, values in agreement with usual

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62 P. Caro, P. Porcher / Paramagnetic susceptibility of C- EueO 3

Table 1 Experimental and computed (with values in table 3) energy levels of the C a site of Eu 3+ in C-Y203

Level Computed Experimental Experimental leadingket energy(cm 1) UV/77 K[5] [6,7]

7F 0 0 0 0 7F 1 1 200.7 202 7F 1 0 368.2 361 7F 1-1 551.9 546 7F 2 2 837.0 859 7F 2-1 879.1 907 7F 2 1 905.0 949 7F 2-2 1179.7 7F 2 0 1332.6 1378

5 D 0 0 17205.7 17224 5D1 1 18905 18937 5D1 0 18944.2 18961 5D 1-1 19015.5

0 201 363 550

17215 18931 18953 19007

results for the E u 3+ compounds. To reproduce correctly these barycenters, in order to obtain wave functions associated with the experimental energies of the Stark levels, it is necessary to use a large I SLJMj) states basis. We used 387 of them covering the multiplets 7Fj, 5D(1)(2)(3)j, 5G(1)(2) (3)s, 3P(1)(2)(3)(4)(5)(6)s and 1S 0. The result is nevertheless very sensitive to the truncation, and values of the free atom parameters. We used ex- trapolated Carnall values [8] (table 3), in order to adjust the position of the barycenter of 7F 1, the important level for the paramagnetic susceptibility. The spin-orbit coupling, constant was taken as 1270 cm -1. In these conditions the barycenter of 7F 2 is computed a little bit low (1027 cm-1), whereas the 7F 1 one, can be deemed acceptable at 374 cm-1. If the barycenter of 7F 2 is well adjusted, the 7F 1 barycenter is calculated too hip~la by 20 cm-1, a difference which affects the value of the paramagnetic susceptibility.

The program calculates automatically the magnetic susceptibility for the principal axes at the site. The diagonal and nondiagonal matrix elements of the ( L + 2 S ) operator are computed between all the states involved in the wave vectors up to eigenvalues of 6000 cm-1 and fed into the well known Van Vleck formula which also uses the Hamiltonian eigenvalues for the energies of the levels [9]. Because of the extended basis the wave

vectors are rather complete, including for example all J-mixing effects, and spin-orbit mixing effects (mostly 7F-SD). The 5D 0 and 5D 1 positions and splittings are also reasonably well reproduced (they are of course unimportant for the magnetic calculations) (table 1). We have used, for the C 2 site, crystal field parameters slightly modified with respect to those determined for Eu 3+ in C-Y203 in ref. [3] (table 3).

3. The S 6 site

Heber and co-workers [6,7] extracted from fluorescence data on Eu 3 ÷ in C-Y203 informations on the S 6 site 7F 1 splittings and assigned a doubly degenerate E level at 132 cm -1 above the ground 7F 0 state and a A level at 429 cm-1 with the 5D 0 level at 17302 cm -1 (table 2). One should remark that t h e 7 F 1 barycenter computed with these data seems to be very low (231 cm -1) with respect to usual values (typified by the C 2 site above). The data on the 6P7/2 level for the S 6 site in C-Gd203. (4) however unambiguously assign a negative sign to the large B 2 axial parameter and consequently the A level should be lowest. But even in reversing the assignment the barycenter is still low (330 cm-1). Recently Tola and co-workers [10] used the powerful XEOL (X-ray excited optical luminescence) technique with a synchrotron radiation beam to produce the emission spectrum of Eu 3 + in C-Y203. Three lines at 5822, 5930 and 5955 were strongly enhanced in this experiment corresponding to lines tentatively assigned by previous workers [6,7,13] to the magnetic dipole lines 5D 0 ~ T F 1 of the S 6 site. There is no determination however of the position of the 5D 0 level. Those data can be interpreted in two ways:

a) the line at 5955 A which is broad and complex can be a vibronic satellite (seen also by the authors of refs. [6,7]) and the 7F1, $6, site splitting is 293 cm-1.

b) the two lines at 5923 ~, (16883 cm -1) and 5955 ~, (16778 cm -1) are the result of a degener- acy lifting of the E level, due, not to a lower site symmetry (the dipolar electric lines will appear), but to a zero-phonon line splitting due to a reso- nance with peaks in the phonon density of states,

P. Caro, P. Porcher / Paramagnetic susceptibility of C-Eu203

Table 2 Experimental and computed (with values in table 3) energy levels of the S 6 site of Eu 3+ in C-Y203

63

Level Computed Experimental Experimental (with symmetry energy [6,7] [10] label from ( c m - 1) calculation)

Experimental [131

7F o A 0 0 0 7F1A 128.3 132 (E) 126 7F 1E 500.0 429 (A) 424

504 7F2E 881 7F2A 991.8 7F 2 E 1318.5

5D0 A 17315.8 17302 (17302) * 5D 1 A 18958.5 18992 (E) 5D 1 E 19115.4 19083 (A)

0

830 (E) 948 (E)

1184(A)

* Computed with the 5 Do value of ref. [6].

of the type encountered for the 7F~ E level of the C3v site of Eu 3+ in A-La203 [12]. Taking into account the three levels, the barycenter of 7 F 1 is then 351 cm -1, much more on line with usual observations [12].

There is however no doubt, from the 5822 ,~ line, that there is for the 7F~ of the S 6 site of Eu 3+ in C-Y203 a very low lying Stark level (132 cm-1 according to the 5 Do position determined by Heber and coworkers [6,7] to be compared with the 202 cm-1 of the low-lying 7F 1 level at the C 2 site).

The splitting of the 7F 2 level of the S 6 site was

Table 3 Crystal field parameters for C-Eu203 (adapted to Eu203 from the results in ref. [3], the imaginary parameters for the C 2 site were omitted)

B~ (cm -1 C 2 site S 6 site

B 2 -188 .2 - 1 7 3 5 B22 - 692.2 B04 - 1266 - 907 B~ - 1530 B 4 - 2452 B44 1093 B 6 230.7 725 B 6 234.2 B36 536 B 6 915 B 6 165.4 744

Free atom parameters ( c m - l ) : E 0 = 3025.95, E 1 = 5500, E 2 = 24.45, E 3 = 582, or= 20, fl = - 6 4 0 , "/=1750, ~ =1270.

tentatively determined from Raman 7 F0 _._, 7 i=2 electronic transitions in Y203 (Eu 3+) by Schaack and Koningstein [13]. They found a level at 830 cm -1 (E), another one at 948 cm-1 (E) and the third at 1184 cm -1 (A). The computed barycenter (948 cm -1) seems low, the ordinary value being usually 100 cm -~ above. Surprisingly, no electronic Ra- man scattering was found corresponding to the C 2 site.

We have computed the Stark energy levels and their wave vectors for the S 6 site using the Bo z value determined for C-Gd203, the other crystal field parameters being derived from the electro- static calculation in ref. [3]. The free atom parameters were the same as in the identical calculation for the C 2 site (table 3). From the C-Gd~Oa spec-

Table 4 Computed magnetic susceptibility values for one Eu 3+ atom in C-Eu203

T X for one Eu 3 + X for Eu 3 + Bulk X (K) in C 2 site in S 6 site (emu X 103)

(emu × 103) (emu x 103)

10 7.955 11.76 8.906 50 7.933 11.35 8.787

100 7.535 9.434 7.987 200 6.072 6.700 6.229 300 4.947 5.275 5.029 500 3.712 3.871 3.752

1000 2.555 2.619 2.571

64 P. Caro, P. Porcher / Paramagnetic susceptibility of C-Eu20 J

t rum one knows that the free atom parameters are nearly the same for the two sites.

The computed energies together with the availa- ble experimental data are collected in table 2. The paramagnetic susceptibilities were computed in the same conditions as for the C 2 site and selected values are given in table 4.

4. Results

reported on fig. 1 with selected numerical values for reference given in table 4.

Due to the very low energy of the lower 7F 1 Stark component the contribution of the S 6 site to the X value is very large, and although it enters for only a fourth in the overall bulk susceptibility, it induces a noticeable deviation from usual values close to "f ree-a tom" expectations. The computed mean value is very close to the experimental re- suits.

The experimental values of the paramagnetic susceptibility of C-Eu203 were determined by several groups [14-19], and they are basically in agreement.

The results of our computation, using crystal field parameters derived from optical experiments on Eu 3+ in C-Y203 and Gd 3+ in C-Gd203, are

10"3 [ emu

I0"

• Trapnell, Selwood

o Borovik-Rornanov, Kreine5

• Arajs, Colvin • Henry la Blanchetais

- 2 0 0 4 0 0 6 0 0 T K

Fig. 1. Computed susceptibility for the C 2 and the S 6 site in C-Eu203. The average is ¼(3x(C2)+Ix(S6) ). Triangles or points are experimental data from literature.

5. Discussion

The apparent "anomalous" magnetic behaviour of C-Eu203 is easy to explain on the basis of very different crystal field parameters at the two europium sites in the structure. This explanation was sometime ago (see ref. [18]) supposed to be "unreasonable". However, the fitting of the optical data to the experimental results as well as the theoretical calculations support the evidence of a large difference in the values of the second rank crystal field parameters between the two europium sites in C-Eu203. The S 6 site data are consistent with a rather large value of the axial B02 parameter. Now, the second rank parameters affect, almost exclusively, the Stark splitting of the first excited state of the 4f 6 configuration: 7F 1. The larger the second-rank parameters, the smaller the energy difference between the 7F 0 ground state and the lowest Stark level of 7F 1, then the in- fluence on the paramagnetic susceptibility of the off-diagonal matrix elements of the magnetic operator increases, and the X value of the low temperature plateau also increases as shown by the calculation of the susceptibility curve.

Values of the second-rank crystal field parameters for lanthanides in the 1500-2000 cm-1 range are rare, but not so uncommon (values like that were observed in our laboratory for complex rare earth apatites, and for exotic niobates or tanta- lates).

The measurement of the paramagnetic susceptibility and the discovery of "anomalously" high X values at the low temperature plateau, is in fact an excellent method to recognize such crystal field situations which may even produce difficulties in

P. Caro, P. Porcher / Paramagnetic susceptibility of C-Eu203 65

the identification of the 5D 0 ---,7F 1 lines in the optical spectrum due to the very large and unusual splitting of the 7F 1 level. In inorganic (or organic)

2 values extend from chemistry the range of Bq almost zero (for example in numerous europium chelates, or the oxysulfides) to the large values close to 2000 cm -a reported here. The paramagnetic susceptibility will reflect that trend with more and more sensitivity as the values increase. The second rank parameters are basically of elec- trostatic origin and they are a consequence of the combination of distances and particular geometri- cal arrangement of the ligands at the site, with an additional large sensitivity to atoms outside of the first coordination sphere. It reflects the electro- static potential imposed by the structure at the site. It can only be predicted through careful elec- trostatic calculations including not only point charges but also dipole and multipole contribu- tions [20].

In situations where large values of the second- rank crystal field parameters are at work it is necessary in order to simulate the experimental paramagnetic susceptibility curve to solve first the crystal field problem, because one needs wave vectors derived from an extended ISLJMj) basis calculation. This is necessary because in those conditions J-mixing effects cannot be neglected. For instance in C-EuzO 3 the ground "7F00" wave vector for the S 6 site ("large" Bo 2 value) has the following leading components:

17F00):78.62%; 17F20): 12.22%; 15D(1)00):

2.98%; 15D(3)00): 2.20%; I 7F4-3): 1.25%; etc.

Whereas the ground state wave vector for the 2 values) has: C 2 site ( "normal" Bq

17F00) :85.56%; [ 5D(1)00) :3.16%; I 7F22) :

2.85%; [ 7F2-2) :2.85%; 15D(3) 00) :2.42%, etc.

One can note the very large increase in the 7F 2 components which dominates for the S 6 site the usual minor 5D 0 components induced by spin- orbit coupling.

Because the crystallographic situations which generate large values of the second rank crystal field parameters are rare, the deviations with re-

spect to the "normal" value of europium paramagnetic susceptibility have not been previously recognized as simple crystal field effects and have been interpreted instead as the consequence of "magnet ic interactions". The proof of those inter- action if they occur at all, is possible only when crystal field effects have been ruled out as a possible cause of the deviations. As the chemists, syn- thetize more and more "exotic" structures for europium compounds as part of the hunt for new phosphors or laser materials, new cases will appear of large second-rank crystal field parameters, with the automatic consequence of large deviations for the europium paramagnetic susceptibility with respect to "f ree-a tom" values. The simple origin of these deviations is in fact of great help to the spectroscopist for the determination of the Bq k crystal-field parameters. Those parameters are in- trinsic properties of the materials, roughly the same for all of the lanthanides in an isomorphous crystallographic series. They command the optical properties and decide of the usefulness of the material for practical applications such as suitabil- ity as phosphor host or solid state lasers. The particular energy level sequence of the 4f 6, Eu 3+, configuration is well suited to the determination of crystal field parameters from optical data, the present paper shows that the paramagnetic data can also be very useful to help in such determination.

References

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[2] N.C. Chang and J.S. Gruber, J. Chem. Phys. 41 (1964) 3227.

[3] J. Dexpert-Ghys and M. Faucher, Phys. Rev. B24 (1981) 3138.

[4] E. Antic-Fidancev, M. Lemaitre-Blaise and P. Caro, J. Chem. Phys. 76 (1982) 2906.

[5] J. Dexpert-Ghys and M. Faucher, Phys. Rev. B20 (1979) 10.

[6] J. Heber, K.H. Hellwege, W. Kobler and M. Murmann, Z. Phys. 237 (1970) 189.

[7] J. Heber, and V. Kobler, in: Luminescence of Crystals, Molecules and Solutions, ed. F. Williams (Plenum Press, New York, 1973) p. 379.

[8] W.T. Carnall, H. Crosswhite and H.M. Crosswhite, Un- published Report, Argonne National Laboratory (1977).

66 P. Caro, P. Porcher / Paramagnetic susceptibility of C-Eu203

[9] P. Caro, D. Derouet, L. Beaury and E. Souli6, J. Chem. Phys. 70 (1979) 2542.

[10] P. Tola, J. Dexpert-Ghys, M. Lemonnier, A. Retournard, M. Pagel and J. Goulon, Chem. Phys. 78 (1983) 339.

[11] H. Forest and G. Ban, J. Electrochem. Soc. 116 (1969) 474.

[12] O.K. Moune-Minn, P. Porcher and P. Caro, J. Solid State Chem. 50 (1983) 41.

[13] G. Shaack and J.A. Koningstein, J. Opt. Soc. Am. 60 (1970) 1110.

[14] B.M.W. Trapnell and P.W. Selwood, Nature 169 (1952) 840.

[15] A.S. Borovik-Romanov and N.M. Kreines, Soviet Phys. JETP 2 (1956) 657.

[16] S. Arajs and R.V. Colvin, J. Appl. Phys. 35 (1964) 1181. [17] A. Grill and M. Schierber, Phys. Rev. B1 (1970) 2241. [18] S. Kern and R. Kostelecky, J. Appl. Phys. 42 (1971) 1773. [19] C. Henry la Blanchetais, Private communication. [20] M. Faucher and D. Garcia, Phys. Rev. B26 (1982) 5451.

the paramagnetic susceptibility of c-type europium sesquioxide

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