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Mössbauer spectroscopy of Dy2O2X (X = S, Se, Te)
102 MOSSBAUER SPECTROSCOPY OF Dy202X (X = S, Se, Te) J. C H A P P E R T * , Y. A B B A S t and J. R O S S A T - M I G N O D t Centre d'Etudes Nucl~aires, 85 X - 3 8 0 4 1 Grenoble, France
Magnetic and quadrupole hyperfine interactions have been determined by M 6 s s b a u e r spectroscopy of '6'Dy in antiferromagnetic Dy202X (X = S, Se and Te). L o w temperature results are compared with neutron diffraction and optical data, and point charge calculations. Above the N6el temperature quadrupole split spectra are observed.
1. Introduction A systematic study of the magnetic properties of the rare-earth(R) oxichalcogenides RzO2X with X = S, Se and Te has shown that they are antiferromagnets [1]. S and Se compounds have the same hexagonal structure (P3m) while for X = T e a quadratic form (I4/mmm) is observed. In this work we have examined the 161Dy MSssbauer resonance of powder samples at various temperatures. The source was J61Tb obtained by neutron irradiation of 90% enriched 16°GdF3. A neutron diffraction determination of the magnetic structures was conducted simultaneously [2].
13
15
Dy202Se
m
13 14 1516
qF qF
2. Experimental results Because of the low ordering temperatures of these compounds the MOssbauer experiments were performed at 1.6 K. Hyperfine patterns (fig. 1) indicate the superposition of magnetic (gotz,He~) and quadrupole (e2qQ) interactions which are listed in table I. Due to different values of the ratio of these interactions, lines 13 and 14, and 15 and 16 overlap for X = Se, are slightly spaced for S and are completely split for Te (fig. 1). MOssbauer spectra have also been recorded at higher temperatures up to 300 K. A typical spectrum of DyzO2Se at 102 K (inset of fig. 2) consists of five lines due to a pure quadrupole interaction. This interaction decreases rapidly with an increase of temperature (fig. 2). At 300 K its value is only 370 MHz. Quadrupole split spectra in the paramagnetic region are very seldom observed, because of thermal averaging over crystal field levels, particularly in metallic systems. Here we are * D R F / G r o u p e Interactions Hyperfines. t DRF/Laboratoire de Diffraction Neutronique.
Physica 86--88B (1977) 102-104 (~) North-Holland
T=I.6K -30-20
-10 () 10 2C1 VELOCITY (cm/sec)
30
Fig. 1. M f s s b a u e r spectra of '61Dy in Dy202X (X = S, Se and Te) at 1.6 K. A m o n g the 16 lines only the positions of lines 13-16 are indicated. Table I Hyperfine parameters and magnetic m o m e n t s at 1.6K and N6el temperatures for Dy in Dy202X. Errors in the last figure are given in brackets
eZqQ
gol~.H~n
X
(cm/sec)
(MHz)
//-Moss (~e)
,U'neut (~B)
TN (K)
s Se Te
10.0 (2) 12.8 (2) 10.7 (2)
704 (5) 807 (5) 845 (5)
8.4 (5) 9.6 (2) 10.1 (2)
7.2 (5) 9.0 (5) 10.0 (5)
5.13 8.5 10.2
dealing with insulators where we expect the crystal field splitting to be more important. Below 55 K thermal averaging is no longer established, as shown by the onset of paramagnetic hyperfine structure [2]. The only other reported
103 3x10 a
agrees well with the M6ssbauer value/z = 8.4/z a. The quadrupole interaction e2qQ comes from a non-zero electric field gradient at the Dy nucleus. Its principal component is written
\
o [ -r
~:
::
X
S
I I
k "/t,, ~
"-"
0 1 ~¢o"
-4
I
-2
I
I
o
2
4
v(cm/sec)
Dy202 Se
~ ~ ....
I I 10 O 200 TEMPERATURE
0
102 K
I
where j/_ _ I 300
(K)
Fig. 2. Temperature dependence of e2qQ in Dy20~Se. The horizontal line at 210 MHz represents the calculated lattice contribution to e2qQ. The inset shows a typical '+lDy Mfssbauer spectrum at 102 K. The stick diagram indicates the positions and the intensities of the five lines of the pure quadrupole pattern. The overall splitting is 3~e2qQ.
example of quadrupole split spectra is Dy2Ti207 [3].
3. Discussion
Since the overall splitting of the low temperature hyperfine pattern is proportional to the effective field at the Dy nucleus, analysis of the spectra of fig. 1 allows a determination of the Dy 3+ magnetic moment /z. Assuming 10/x 8 for Dy metal, the values o f / z at 1.6 K are listed in table I. T h e y are in good agreement with those derived from neutron diffraction data [2]. The magnitude of /z indicates that the electronic ground state of Dy 3+ is essentially a pure 11512) state for X = Se and Te while crystal field effects reduce strongly /z in Dy202S [4]. In that compound the crystal field hamiltonian is =
v°o ° + / 3 ( v ° o ° + 0 q,- Y ( V 6 0 6
0
eVz~ = (1 -- R ) e V ~+e: + ( l - y ~ ) e V ~ zLat,
3 3 q._ V 6 0 6 + V 6 0 6 ) ,66
where the O~" are the Stevens operator equivalents, V~' are the crystal field parameters, and a,/3 and 3' are coefficients tabulated by Elliott and Stevens [5]. Optical experiments provide the V~' parameters [6]. Diagonalization of ~ shows that the ground state wave function in DY202S is IA) = 0.815 115/2) - 0.478 19/2) + 0.28013/2) + . . . . therefore (A [Jz[A ) = 6.12 a n d / x = 8.2/XB, which
ev~rz = - e 2 a (r 3)(3J2~- J(J + 1)) is the 4f contribution and eVzz" ,Eat = --4 V°/(1 - tr2)(r 2) is the lattice term. The parameters R =0.15 and y = = 8 0 are the Sternheimer coefficients and cr2 = 0.4 is a screening coefficient. Taking the quantization axis along c and setting the charges of Dy, O and X equal to +3, - 2 and - 2 , respectively, the values of eVz~ t a r e calculated with a point charge model (table II). Table II Comparison of eV. in Dy2OzX from M6ssbauer data at 1.6 K with calculated values (in 10-6 erg/cm2). 0 is the angle between the Dy ~÷ spin and the crystal axis (in degree) [2]
V° X S
(cm-')
62 (opt.) Se 72 (calc.) Te 185 (calc.)
eV~z
eV.
(1 - y®)eV.
(1 - R)eVzz
(calc.) (Moss.)
0
-0.46
4.73
4.27
5.7
0
-0.57
7.50
6.93
7.1
90
-1.38
8.16
6.78
6.2
0
For X = S , we used the optical value V ° = 6 2 c m -I (6). In eV~fz, a = - 0 . 0 0 6 3 and (r -3)-9.2 a.u. for Dy 3÷. One observes that the resultant values eVzz agree well with the M6ssbauer values, especially for X = Se and Te. The temperature dependence of eZqQ (fig. 2) is due solely to the 4f contribution. It reflects the spacing of the eight crystal field doublets of Dy 3÷. In Dy2OESe the 4f term is close to zero at high temperature ( > 3 0 0 K ) because of thermal averaging. Then e2qQ is roughly the lattice contribution whose calculated value is 210 MHz. In order to interpret quantitatively the temperature dependence of eEqQ, it is necessary to know accurately the energy and the wave function of each electronic doublet. We do not yet have such information. However, since at - 3 0 0 K eV4/z approaches zero, we may state that the overall
104
c r y s t a l field s p l i t t i n g is o f t h e o r d e r o f 2 5 0 300 c m -~. A m o r e d e t a i l e d a n a l y s i s o f t h e s e d a t a is u n d e r w a y .
References [1] Y. Abbas, J. Rossat-Mignod, G. Quezel and C. Vettier, Solid State Commun. 14 (1974) lll5. Y. Abbas, Thesis, Universit6 de Grenoble, France (1976).
[2] Y. Abbas, J. Chappert, F. Tch6ou and J. Rossat-Mignod, to be published. [3] A. AImog, E.R. Bauminger, A. Levy, I. Nowik and S. Ofer, Solid State Commun. 12 (1973) 693. [4] M. Belakhovsky, J. Phys. 32 0971) CI-915. [5] R.J. Elliott and K.W.H. Stevens, Proc. Roy. Soc. A218 (1953) 553. [6] J. Rossat-Mignod, J.C. Souillat and G. Quezel, Phys. Stat. Sol. (b) 62 (1974) 519.
Magnetic and quadrupole hyperfine interactions have been determined by M 6 s s b a u e r spectroscopy of '6'Dy in antiferromagnetic Dy202X (X = S, Se and Te). L o w temperature results are compared with neutron diffraction and optical data, and point charge calculations. Above the N6el temperature quadrupole split spectra are observed.
1. Introduction A systematic study of the magnetic properties of the rare-earth(R) oxichalcogenides RzO2X with X = S, Se and Te has shown that they are antiferromagnets [1]. S and Se compounds have the same hexagonal structure (P3m) while for X = T e a quadratic form (I4/mmm) is observed. In this work we have examined the 161Dy MSssbauer resonance of powder samples at various temperatures. The source was J61Tb obtained by neutron irradiation of 90% enriched 16°GdF3. A neutron diffraction determination of the magnetic structures was conducted simultaneously [2].
13
15
Dy202Se
m
13 14 1516
qF qF
2. Experimental results Because of the low ordering temperatures of these compounds the MOssbauer experiments were performed at 1.6 K. Hyperfine patterns (fig. 1) indicate the superposition of magnetic (gotz,He~) and quadrupole (e2qQ) interactions which are listed in table I. Due to different values of the ratio of these interactions, lines 13 and 14, and 15 and 16 overlap for X = Se, are slightly spaced for S and are completely split for Te (fig. 1). MOssbauer spectra have also been recorded at higher temperatures up to 300 K. A typical spectrum of DyzO2Se at 102 K (inset of fig. 2) consists of five lines due to a pure quadrupole interaction. This interaction decreases rapidly with an increase of temperature (fig. 2). At 300 K its value is only 370 MHz. Quadrupole split spectra in the paramagnetic region are very seldom observed, because of thermal averaging over crystal field levels, particularly in metallic systems. Here we are * D R F / G r o u p e Interactions Hyperfines. t DRF/Laboratoire de Diffraction Neutronique.
Physica 86--88B (1977) 102-104 (~) North-Holland
T=I.6K -30-20
-10 () 10 2C1 VELOCITY (cm/sec)
30
Fig. 1. M f s s b a u e r spectra of '61Dy in Dy202X (X = S, Se and Te) at 1.6 K. A m o n g the 16 lines only the positions of lines 13-16 are indicated. Table I Hyperfine parameters and magnetic m o m e n t s at 1.6K and N6el temperatures for Dy in Dy202X. Errors in the last figure are given in brackets
eZqQ
gol~.H~n
X
(cm/sec)
(MHz)
//-Moss (~e)
,U'neut (~B)
TN (K)
s Se Te
10.0 (2) 12.8 (2) 10.7 (2)
704 (5) 807 (5) 845 (5)
8.4 (5) 9.6 (2) 10.1 (2)
7.2 (5) 9.0 (5) 10.0 (5)
5.13 8.5 10.2
dealing with insulators where we expect the crystal field splitting to be more important. Below 55 K thermal averaging is no longer established, as shown by the onset of paramagnetic hyperfine structure [2]. The only other reported
103 3x10 a
agrees well with the M6ssbauer value/z = 8.4/z a. The quadrupole interaction e2qQ comes from a non-zero electric field gradient at the Dy nucleus. Its principal component is written
\
o [ -r
~:
::
X
S
I I
k "/t,, ~
"-"
0 1 ~¢o"
-4
I
-2
I
I
o
2
4
v(cm/sec)
Dy202 Se
~ ~ ....
I I 10 O 200 TEMPERATURE
0
102 K
I
where j/_ _ I 300
(K)
Fig. 2. Temperature dependence of e2qQ in Dy20~Se. The horizontal line at 210 MHz represents the calculated lattice contribution to e2qQ. The inset shows a typical '+lDy Mfssbauer spectrum at 102 K. The stick diagram indicates the positions and the intensities of the five lines of the pure quadrupole pattern. The overall splitting is 3~e2qQ.
example of quadrupole split spectra is Dy2Ti207 [3].
3. Discussion
Since the overall splitting of the low temperature hyperfine pattern is proportional to the effective field at the Dy nucleus, analysis of the spectra of fig. 1 allows a determination of the Dy 3+ magnetic moment /z. Assuming 10/x 8 for Dy metal, the values o f / z at 1.6 K are listed in table I. T h e y are in good agreement with those derived from neutron diffraction data [2]. The magnitude of /z indicates that the electronic ground state of Dy 3+ is essentially a pure 11512) state for X = Se and Te while crystal field effects reduce strongly /z in Dy202S [4]. In that compound the crystal field hamiltonian is =
v°o ° + / 3 ( v ° o ° + 0 q,- Y ( V 6 0 6
0
eVz~ = (1 -- R ) e V ~+e: + ( l - y ~ ) e V ~ zLat,
3 3 q._ V 6 0 6 + V 6 0 6 ) ,66
where the O~" are the Stevens operator equivalents, V~' are the crystal field parameters, and a,/3 and 3' are coefficients tabulated by Elliott and Stevens [5]. Optical experiments provide the V~' parameters [6]. Diagonalization of ~ shows that the ground state wave function in DY202S is IA) = 0.815 115/2) - 0.478 19/2) + 0.28013/2) + . . . . therefore (A [Jz[A ) = 6.12 a n d / x = 8.2/XB, which
ev~rz = - e 2 a (r 3)(3J2~- J(J + 1)) is the 4f contribution and eVzz" ,Eat = --4 V°/(1 - tr2)(r 2) is the lattice term. The parameters R =0.15 and y = = 8 0 are the Sternheimer coefficients and cr2 = 0.4 is a screening coefficient. Taking the quantization axis along c and setting the charges of Dy, O and X equal to +3, - 2 and - 2 , respectively, the values of eVz~ t a r e calculated with a point charge model (table II). Table II Comparison of eV. in Dy2OzX from M6ssbauer data at 1.6 K with calculated values (in 10-6 erg/cm2). 0 is the angle between the Dy ~÷ spin and the crystal axis (in degree) [2]
V° X S
(cm-')
62 (opt.) Se 72 (calc.) Te 185 (calc.)
eV~z
eV.
(1 - y®)eV.
(1 - R)eVzz
(calc.) (Moss.)
0
-0.46
4.73
4.27
5.7
0
-0.57
7.50
6.93
7.1
90
-1.38
8.16
6.78
6.2
0
For X = S , we used the optical value V ° = 6 2 c m -I (6). In eV~fz, a = - 0 . 0 0 6 3 and (r -3)-9.2 a.u. for Dy 3÷. One observes that the resultant values eVzz agree well with the M6ssbauer values, especially for X = Se and Te. The temperature dependence of eZqQ (fig. 2) is due solely to the 4f contribution. It reflects the spacing of the eight crystal field doublets of Dy 3÷. In Dy2OESe the 4f term is close to zero at high temperature ( > 3 0 0 K ) because of thermal averaging. Then e2qQ is roughly the lattice contribution whose calculated value is 210 MHz. In order to interpret quantitatively the temperature dependence of eEqQ, it is necessary to know accurately the energy and the wave function of each electronic doublet. We do not yet have such information. However, since at - 3 0 0 K eV4/z approaches zero, we may state that the overall
104
c r y s t a l field s p l i t t i n g is o f t h e o r d e r o f 2 5 0 300 c m -~. A m o r e d e t a i l e d a n a l y s i s o f t h e s e d a t a is u n d e r w a y .
References [1] Y. Abbas, J. Rossat-Mignod, G. Quezel and C. Vettier, Solid State Commun. 14 (1974) lll5. Y. Abbas, Thesis, Universit6 de Grenoble, France (1976).
[2] Y. Abbas, J. Chappert, F. Tch6ou and J. Rossat-Mignod, to be published. [3] A. AImog, E.R. Bauminger, A. Levy, I. Nowik and S. Ofer, Solid State Commun. 12 (1973) 693. [4] M. Belakhovsky, J. Phys. 32 0971) CI-915. [5] R.J. Elliott and K.W.H. Stevens, Proc. Roy. Soc. A218 (1953) 553. [6] J. Rossat-Mignod, J.C. Souillat and G. Quezel, Phys. Stat. Sol. (b) 62 (1974) 519.