Volume 105A, number 6 PHYSICS LETTERS 22 October 1984

HIGH-ORDER EPR TRANSITIONS OF Gd 3+ IN Ce2Zn3(NO3)12 • 24H20

V.K. JAIN Department of Physics, M.D. University, Rohtak-124001, India

Received 4 June 1984

The angular variation of the EPR spectrum of Gd 3÷ in Ce 2 Zn3(NO3)12 ' 2 4 H 2 0 has been studied. In addition to the allowed fine structure lines some weak low-field lines, identified as forbidden transitions (AM= ± 2, ± 3), have been ob- served.

1. Introductiog Electron paramagnetic resonance (EPR) absorption by Gd 3+ present as a dilute guest in single crystals of Ce2Zn3(NO3)12 • 24H20 (CZN) has been studied by Misumi et al. [1 ] at room tem- perature. However, the weak lines which occur at the low-maguetic field side of the allowed (AM = +1) fine Structure lines in the EPR spectra of Gd 3+ in CZN have not been investigated previously. As will be shown in this paper these lines have been identi- fied as arising from the higher-order EPR transitions (IAMI > 1).

2. Crystal structure. The crystal structure of Ce2Mg3(NO3)I2 • 24H20 has been determined by Zalkin et al. [2]. CZN can be expected to have a sim- ilar structure. The primitive cell containing one for- mula unit is rhombohedral with lattice constant a = 1.3165 nm and a = 49.37 °. The space group is R3. The unit cell contains three divalent ions situated at two different lattice sites. One of them occupies a lattice site with the point symmetry C3. i and the other two divalent ions occupy lattice sites with the point symmetry C 3. The trivalent ion is found at a site of C 3 point symmetry and the rest of the atoms are at positions of general types of the space group. The di- valent ions are surrounded by six water molecules forming together with the central ion a nearly regular octahedral complex. Each trivalent ion is coordinated with 12 oxygens belonging to six nitrate ions, located at the corners of a somewhat irregular icosahedron.

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3. Experimental. Single crystals of CZN were grown at room temperature (~290 K) by slow evap- oration of an aqueous solution. The Gd 3+ was in- troduced into the host lattice by adding (0.2 wt%) gadolinium nitrate. The crystal grows in flat hexago- nal plates, the plane of which is perpendicular to the trigonal axis.

The EPR experiments were performed on a JEOL FE-3X homodyne spectrometer operating at "9.45 GHz equipped with a TEoll-cylindrical cavity and 100 kHz field modulation. A speck of powdered DPPH used as a field marker (taking gD1VH = 2.0036) was inserted simultaneously into the sample cavity. The crystals were mounted on quartz rods. The angular-variation studies were done using a JES- UCR-2X sample angular rotating device.

4. Results and discussiott For an arbitrary orienta- tion of the crystal the EPR spectrum consists of a single set of seven lines between "0.268 and "0.410 T which arise from the allowed fine structure transi- tions (AM= +1) of the Gd 3+ centre (formed by the substitution of Ce 3. by Gd3+). In addition to these lines there are many weak fines below "-0.23 T (fig. 1). The intensity of these weak lines is 1/10 to 1/500 of the AM = +1 transitions. Angular variation studies of the spectrum reveal the following:

(1). The maximum spread of the spectrum (z axis) corresponding to AM = ±I transitions occurs when the magnetic field is parallel to the c axis of the crys- tal.

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Volume 105A, number 6 PHYSICS LETIERS 22 October 1984

f f f f ff

50"0 mT B t 4 •

i

t131

I: I

Fig. 1. EPR spectrum ofGd 3+ in CeaZn3(NO3)12 -24H20 single crystals at 290 K; B makes an angle of 30 ° to the z-axis in the zx plane. Forbidden f'me structure lines marked ' f ' belongs to AM = ±2. Forbidden transitions are amplified by a factor 5.

(2). The subsidiary maxima (x-axis) occur when the magnetic field is at 90 ° to the c-axis. However, at this orientation, none of the observed transitions at- tains an extremum.

(3). The spectrum shows a periodicity of 2n/3 when the crystal is rotated in a plane perpendicular to the c-axis. However, in this plane the angular varia- tion of the line position is very small ("0 .6 mT).

The spin-hamiltonian for Gd 3+ corresponding to a trigonal symmetry and with the z-axis parallel to the c-axis can be written as [3]

. I z.Or~O • t ~ 0 r ~ 0 + _ _ L . I , 0 0 0 ~=~S'g'B'r~o2v2"r6-oO4V4 t26°~6 6

+1,_3~3 + t , 3~3 __k_t,606 "J°4v4 3"0°6v6 + 1260u6 6 ' (1)

where the symbols have their usual meaning and S = 7/2 for Gd 3÷. The crystal field parameters b 3 and b63 are identically zero for C3h and D3d symmetry and ~ 0 for C3v symmetry [4].

The parameters obtained for the spin-hamiltonian (1) at 290,, K are (in units,,of 10 --4 cm- l ) : b 0 = 107.7 -+ 1.0, b~ = 0.2 ± 0.05, b~ _ --- 0.42 ± 0.05,gn = 1.991 -+ 0.002. The signs of the parameters are only relative and have been determined assuming b 0 to be positive. The parameters b66 and g± are not quoted because of the anomalous angular variation of fine structure transitions in a plane perpendicular to the z-axis.

To identify the observed low-field lines we have in- vestigated higher-order EPR transitions (AM > ±1) for the Gd 3+ centre. For this purpose we have used third- order perturbation to evaluate the position of all the fine structure transitions.

The field position BM_., M_R at which a line due to the EPR transition (M -+ M - R) occurs was ob- tained by using the eigenvalue of the spin-hamiltonian (retaining only the predominant b°O 0 term) to third order of perturbation [5]. The line position is given by

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Volume 105A, number 6 PHYSICS LETTERS 22 October 1984

T 0D

- - -~ AM=_+I . . ,

0"342

0 .270

0.198

0-126, . • •

0.054 0 I 0 20 30 40 50 60 70 80 g0 z - a x i s Ar~3te O in d e g r e e s ~ x-axis

Fig. 2. Angular dependence of Gd 3+ EPR lines in the zx plane for Ce2Zn3(NO3)t2.24H20 at 290 K. ~ theoretical curves. • ex perJmental positions.

BM._,M_R = Bo/R ~ 0 - ~ b 2 ( 3 cos20 - I X 2 M - R )

- ( b O ) 2 s i n 4 0 T/aB 0 - (bO)2sin20 COS20 U/2B 0

-- (bO)3sin20 cos2O(3 cos20 - 1)V/aB20

- (b0)3s in40(3 cos20 _ 1)W/32B 2

_ 3(b0)3sin40 cos20 X/4B 2 ,

where

T = - 6 M 2 + 6 M R - 2 R 2 - 1 + 2S(S + 1) ,

U = 2 4 M 2 - 24MR + 8R 2 + 1 - 4S(S + 1 ) ,

V = (2M - R)[-80M 2 - 40R 2 + 80M__R - 14

+ 2 4 5 ( S + 1)] ,

W= ( 2 M - R ) [ 2 0 M 2 - 2 0 M R + 10R 2 + 14

- 1 2 S ( S + 1 ) ] ,

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Volume 105A, number 6 PHYSICS LETTERS 22 October 1984

X = ( 2 M - R ) [ 2 0 M 2 + 10R 2 - 20MR + 5

- 8s(s + 1)1, a n d M = ~ , - + ~ , 3 1 +~, ---[;R = 1 ,2 , 3, 4, 5, 6.

Fig. 2 shows the angular variation of the spectrum in the zx plane. In the figure 0 is the angle between the external magnetic field and the z axis o f the Gd 3+ centre. Because o f the low intensity, AM = +3 transi- tions are observed only for certain orientations as is shown in fig. 2. The reasonably good agreement of the calculated angular variation with the experimental one suggests that the weak lines at the low magnetic field side o f the free structure lines are due to AM 1> 2 transitions.

This work was supported by U.G.C. New Delhi.

References

[ 1 ] S. Misumi, T. Isobe and T. Higa, Nippon Kagaku Kaishi 10(1974) 1829.

[2] A. Zalkin, J.D. Forrester and D.H. Templeton, J. Chem. Phys. 39 (1963) 2881.

[3] H.A. Buckmaster and Y.H. Shing, Phys. Stat. Sol. 12a (1972) 325.

[4] H.A. Buckmaster, J.C. Dering and J.1. Fry, J. Phys. CI (1968) 599.

[5] R.M. Golding and W.C. Tennant, Mol. Phys. 25 (1973) 1163; 28 (1974) 167.

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