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Mössbauer and magnetization studies of the U1−xNpxO2 fluorites
Solid State Communications, Vol. 50, No. 4, pp. 357-361, 1984. Printed in Great Britain.
0038-1098/84 $3.00 + .00 Pergamon Press Ltd.
MOSSBAUER AND MAGNETIZATION STUDIES OF THE UI-xNp~O2 FLUORITES A. Tabuteau, J. Jovd and M. Pag6s Institut Curie, Section de Physique et Chimie, Equipe Physicochimie des Transuraniens (ERA CNRS 1006), 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France C.H. de Novion SESI, Bat. 31, CEN, BP 6, 92260 Fontenay-aux-Roses, France and J. Gal
NRCN, BP 9001, Beer-Sheva, Israel
(Received 29 November 1983 by E.F. Bertaut)
2aTNp Mossbauer effect and magnetic susceptibility measurements of the U~-xNpxO2 fluorite solid solution have been performed in the composition range 0.15 ~
1. INTRODUCTION ACTINIDE DIOXIDES AnO2 have drawn the attention of chemists and physicists for their technological interest as well as for their special physical properties. Of particular interest are UO2 and NpO2, the magnetic properties of which have been the subject of recent extensive experimental and theoretical studies [1-6]. UO2 orders antiferromagnetically below TN = 30.8 K, with an uranium ordered moment of 1.8/a n [7]; a slight distortion of the oxygen sublattice was found in association with the magnetic order [ 1]. The transition, which is of first order, could be well explained in terms of a collectwe Jahn-Teller mechanism [2]. But Solt and Erdos [5] have pointed out recently that this process would not lead alone to the observed structure, and that the lattice deformation is favoured by direct electric quadrupole-quadrupole interactions. Early specific heat [8] and magnetic susceptibility measurements [9] (and more recently [3])had shown in NpO2 the existence of an anomaly at 25 K which was first interpreted-as a transition to an antiferromagnetic state. However, neutron diffraction studies [4, 9, 10] led to the conclusion that the Np ordered magnetic moment, if it exists, is smaller than 0.2 tzB (when the paramagnetic moment is 2.95/an [ 11 ] ). Mossbauer experiments on polycrystalline NpO2 samples showed a moderate broadening of the absorption line below
25 K, which, if interpreted as a magnetic ordering, would correspond to an ordered moment of 0.01/l n [12]. Gal et al. [6] claimed that this broadening, which is strongly dependent on the stoichiometry of the compound (NpO2+x)is non-magnetic and could result from the Orbach-Blume spin-lattice relaxation process within the Vs cubic ground state quartet of Np rv split into two Kramer's doublets by the crystalline field due to deformations of the cubic unit cell. It was then concluded that the NpO2÷x compounds (-- 0.08 ~
357
358
STUDIES OF THE UI_~Np~O2 FLUORITES
Vol. 50, No. 4
Table 1. Magnetic properties of Ul-xNpx02 deduced from Mossbauer and magnetic susceptibility measurements (the hyper~ne fieM Hen(0) were deduced from the fit with the Wegenermodel; the values obtained from the Nowik- Wickman model are quite similar) X
0
0.15
Hen(0) (kOe) To (K) /-tord*/NP (#B)
TN (K) /aen QzB) 0o (K)
30.8 3.6(3) --244(16)
0.25
0.50
1000(40) 18.7(9) 0.5(1)
1120(40) 15.5(7) 0.6(1)
1200(20) 10.0(6) 0.65(5)
27(2) 2.95(8) -- 180(12)
23(5) 3.43(6) -- 180(12)
11.5 t
0.75 1210(50) 9.6(6) 0.65(9) 8(2) 3.0(1) -- 132(20)
1 25? 0.017
2.95 --22
* P-ord derived from Hen(0) by the relationship of Dunlap and Lander [21]. t TN (-+ 1 K) for x = 0.50 deduced from neutron diffraction measurements [14].
' X-'cxlo'
,X-l(xto- )
)
701 60 50
m
S "V
t
O
10
26
.:"o..""" X -1 .... o, a ..s. o
39
tOO
150 T K)
Fig. 2. X-1 = f ( T ) f o r Uo.2sNp~TsO2 (applied field =
14 kG).
2.1. Samples The Ul-xNpxO2 samples were prepared by coprecipitation of the hydroxides in H2SO4 by NH4OH. The precipitate was then heated to 1000°C under vacuum. X-ray diffraction measurements revealed a single phase of CaF2-type structure, with linear x dependent lattice parameter (5,470 h for UO2 and 5.324 A for NpO2) and without broadening of the diffraction lines [15].
the sample. The uncertainty on the sample temperature is -+ 0.5 K after calibration. At temperatures above 50 K, all the Ul_xNpxO 2 follow the Curie-Weiss law, with effective moments #en of about 3/~B and large negative Curie paramagnetic temperatures 0p (see Table 1). The susceptibility curve for U~asNpo.lsO2 is shown in Fig. 1 : it is the only composition which presents, similarly to UO2, a sharp maximum of magnetic susceptibility suggesting an antiferromagnetic transition with TN = 27(2) K. The other two samples (x = 0.25 and 0.75) show more shallow transitions (see Fig. 2): TN was then defined, somewhat arbitrarily, as the temperature of change of slope of X-~ (T). The values of TN are reported in Table 1.
2.2. Magnetic susceptibility The magnetic susceptibility measurements were performed with a Faraday electrobalance in the temperature range 4-300 K. The temperature was measured by an Au (0.07 Fe)-Chromel thermocouple situated near
2.3. Mossbauer spectroscopy The Mossbauer absorption spectra of the 2aTNp 59.6 keV wray were recorded by a conventional constant acceleration transducer attached to a multiscaler device. The temperature of the absorber was controlled within
Fig. 1. X-1 = f ( T ) for Uo.ssNpo.lsO2. (a) Applied field = 14 kG. (b) Low temperature range (applied field = 3.34 kG). 2. EXPERIMENTAL RESULTS
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STUDIES OF THE Ui_xNpxO 2 FLUORITES
spectra are displayed in Fig. 3; these spectra are similar to those observed for all the other solid solutions. Their main characteristics are the following:
,9°f,.. :
t ,,oL_
Above a "transition" temperature To a single narrow absorption line is always observed. The absorption line width Fo (full width at half maximum FWHM) is practically temperature independent from To to 77 K and equal to 3.2(2)mm sec -1. Below To, the spectrum broadens significantly and at 5 K a relatively resolved hyperfine splitting is observed (see Fig. 3); this suggests the presence of spin relaxation phenomena in a magnetically ordered system. In Uo.ssNpo.lsO 2 and UoosNpoasO2, To was found somewhat lower than the transition temperature TN deduced from magnetic susceptibility (see Table 1); on the other hand for x -- 0.50 (sample where TN was deduced from a recent power neutron diffraction experiment [14]) a n d x -- 0.75, To and TN are quite close. -- The MOssbauer isomer shifts, for all the Ul-xNpxO2 studied are not affected by the transition. Above and below To, the value found (-- 5.6(1)mm sec -1 relative to NpA12, + 8.3(1)mm s e c - 1 relative to the Am metal source) coincides, within the experimental error, with that of pure NpO2 [12]. This shows that the Np ion in all the solid solution investigated has a IV formal charge state (5/3, 419/2) independent of the uranium neighbours. -
- "\
oo.o .
-\ /
-
~..
j'5.7K
99.0
°
-40-20
0
+20 +40
1(30.0
99.8 99.6 99.4
The analysis of the Mossbauer absorption spectra was based on the following assumptions:
99.2 I00.0;
359
:" '" "." '." "" ~......'- .".
"."""~ ""
"
99.8 99.6 99.4 '
99,2~
-40
-20
0
+20 +40
V e l o c i t y (mm/sec) Fig. 3. M6ssbauer absorption spectra o f some Ul-xNpxO2 compounds. The solid line represents the least square fit using: (a) the Wegener model (x = 0.25); (b) the Nowik-Wickman model assuming relaxation effects within an isolated Kramers doublet with A = 4K, r = 4.5 x 10 -zI sec and/-/e~ = 1213 kOe as fit parameters for the 4.5 K, x = 0.15 case (see text). -+0.5 K in a furnace embedded in liquid helium as described in [16]. Recoilless absorption spectra were obtained for Ul-xNpxO2 (x = 0.15, 0.25, 0.50 and 0.75) at various temperatures from 4.2 to 77 K. Typical 2aTNp Mossbauer
(a) The Np ions are in a IV formal charge state (see above) with a Cubic Ps quartet ground state [17]. (b) Below TN or To, the ground Ps quartet of the Np ion will split as a result of the axial crystalline field originated by the oxygen displacements and of the internal magnetic field. If one of the splittlngs A is of the order of a few degrees Kelvin, a relaxation channel is opened and a relaxation process is expected. (c) The random distribution of the Np ions within the fluorite U sublattlce will certainly introduce a spread in the hyperfine fields in addition to the relaxation phenomena. We assumed that below To the FWHM l~ of each individual absorption line is given by F = Po + FM, where Fo is the FWHM at 77 K (see Section 2.3) and FM the broadening due to the spread In the hyperfine fields. In order to get a rough estimation of the values of the magnetic hyperflne field constants, a first fitting procedure was carried out using the phenomenological Wegener model [18, 19]. Later, the molecular field Nowik-Wickman model [20] was used to obtain the values of the doublet splitting A and of the relaxation times z; this model expresses the present physical
360
STUDIES OF THE Ul-xNpxO2 FLUORITES
[
.....
:
100(
50C
o Fig. 4. Hyperfine field evolution as function of the temperature. situation if we make the approximation that only one dominant relaxation frequency (within the fundamental Kramers doublet) should be considered.* It should be stressed that we were able to fit our data only by modifying the Wegener and the Nowlk-Wickman model's by introducing P = Fo + FM into the corresponding formulas [19, 20]. Reasonably good fits of the experimental spectra were obtained using these models (see Fig. 3). The hyperfine parameters determined in this way may be summarized as follows: The average magnetic hyperfine fields acting on the Np nuclei are shown on Fig. 4; they decrease as the temperature is raised and vanish at the transition temperature To; the extrapolated values of Hen at 0 K are given in Table 1. FM is found to be nearly composition and temperature independent below To (3 +- 1 mm sec-1), and drops sharply to zero for T >~ To. - The derived correlation times 7- at 4.5 K, using the Nowik-Wickman model [20], are of the order of 10 -11 sec. - Quadrupolar interactions were found to be negligible below and above To. - The value derived for the exchange splitting of the fundamental Kramers doublet at 4.5 K, using the Nowik-Wickman model, is A ~ 4 K. -
-
3. DISCUSSION Our interpretation of the Mossbauer and susceptibility results may be summarized as follows: (a) The 237Np isomer shift is independent of temperature and composition: the Np ion remains Np rv
(5I "3, %,~).
Vol. 50, No. 4
(b) All the Ul-xNpxO2 sohd solutions mvestigated in the present work were found to order magnetically at low temperatures; this experimental fact contradicts the Mossbauer source experiment on UO2 doped by 237Np [22], which stated the absence of an induced Np moment m the ordered antlferromagnetic state. The transition observed at To corresponds to a magnetic ordering of the Np ions, and not only to magnetic relaxation phenomena, as confirmed by the recent observation by neutron diffraction of a magnetic reflexxon in U0.sNpo.sO2 below TN ~ 12 K [14], temperature which is very near To 10K. (c) The dependence of TN and To with x (see Table 1) indicates a constant decrease of ordering temperatures as x increases: the Interchange of Np in UO2 lowers the U - U exchange interactions stmllarly to thorium m the Ul_xThxO 2 solid solution [23]. One also remarks that at low neptunium concentration (x = 0.15 and 0.25), the Np sites order at lower temperature than the U sites (To < TN). (d) The non-magnetic behaviour of Np between TN and To for x = 0.15 and x = 0.25 is certainly the most striking result of the present investigation. The sharp maximum of magnettc susceptibility observed at TN = 27 K in Uo.ssNpo.lsO2 argues, by analogy with UO2, for antiferromagnetlc ordering probably coupled with a Jahn-Teller deformation of the oxygen sublattice. If the Np ion carried a non-zero ordered moment below TN, a sharp onset of hyperfine field should occur at TN, as a result of transferred field, leading to a broadening of the 237Np Mossbauer absorption line. The fact that this is not observed strongly supports the Solt and Erdos model [13] for NpO2. The Np ion ground state (P8 quartet) remains then unperturbed by the reduced magnetic field between TN and To The behaviour of the hyperf'me field at the 237Np nucleus below TN is typical of temperature dependent core polarization, and similar to the behavlour of diamagnetic ions m a magnetic environment (for example Sn substituted m iron garnet systems [24]), thus supporting the idea that (Jz} equals zero. The crystal field parameter A4[A6 is therefore insignificantly affected by composition and by the Jahn-Teller deformation down to To. (e) The derived zero value of the quadrupolar moment (q) below TN is in apparent discrepancy with the existence of a distortion of the oxygen sublattice as predicted by Solt and Erdos [13] for pure NpO2. But no calculation of the value of (q) has been made using this model m order to compare with the experimental results. 4. CONCLUSION
* It was found from preliminary fits with the Wegener model that transverse relaxation times can be neglected in the studied samples.
Our magnetic susceptxbility and Mossbauer effect studies of the solid solution Ux-xNpxO2 have shown
Vol. 50, No. 4
STUDIES OF THE Ul-xNpxO2 FLUORITES
that long-range magnetic order occurs at low temperature (T<~ TN) via ordering of the magnetic uranium ions. The neptunium ions could carry a zero effective moment (Jz) as predicted by the Solt and Erdos model and be ordered by the transferred field from the uranium. From the value of the Np induced moment, 0.5/l n (Table 1) and polarized neutron diffraction experiment on NpO2 at 4.2 K (a 0.07/a B moment is induced by an external field of 4 6 T [4]), one may estimate the transferred field on the Np sites m the sohd solution as about 400 kOe. The ordering temperature for the Np ions, To, is definitely smaller than TN at low neptunium content (x = 0.15 and 0.25), but very near to it f o r x = 0.50 and 0.75. This difference is not understood: it might be due to a change of ordered magnetic structure with composition, as suggested by neutron diffraction measurements [14]. Studies for extreme x values, like 0.1 and 0.9 and neutron diffraction experiments are needed to improve our understanding of the magnetic behaviour of this system.
4. 5. 6. 7. 8.
9. 10. 11. 12. 13. 14. 15. 16. 17.
Acknowledgement - The authors wish to thank Dr R. Pascard who initiated this work. REFERENCES 1. 2. 3.
J. Faber, G.H. Lander & B.R. Cooper, Phys. Rev. Lett. 35, 1770 (1975). R. Siemann & B.R. Cooper, Phys. Rev. B20, 2869 (1979). P. Erdos, G. Solt, Z. Zolnierek, A. Blaise & J.M. Fournier, Physica BI02, 164 (1980).
18. 19. 20. 21. 22. 23. 24.
361
A. Delapalme, M. Forte, J.M. Fournier, J. Rebizant & J.C. Spirlet, Physica B102, t71 (1980). G. Solt & P. Erdos, Physica B102, 313 (1980). J. Gal, C. Musikas, J. Joy6, M. Pages & I. Nowik, Phys. Rev. B23, 4524 (1981). B.C. Frazer, G. Shlrane, D.E. Cox & C.E. Olson, Phys. Rev. 140, 1448 (1965). D.W. Osborne & E.F. Westrum Jr., J. Chem. Phys. 21, 1884 (1953). J.W. Ross&D.J. Lam, J. Appl. Phys. 38,1451 (1967). D. Cox & B.C. Frazer, J. Phys. Chem. Solids 28, 1649 (1967). L. Heaton, M.H. Mueller & J.M. Williams, J. Phys. Chem. Solids 28, 1651 (t967). B.D. Dunlap, G.M. Kalvlus, D.J. I_am & M.B. Brodsky, J. Phys. Chem. Solids 29, 1365 (1968). G. Solt & P. Erdos, J. Magn. Magn. Mat. 15-18, 57 (1980). A. Tabuteau, M. Pages, A. Boeuf, R. Cacluffo, F. Rustichelli, J. Rebizant & L. Man~s (to be published). A. Tabuteau, Thesis, Universit~ Pierre et Mane Curie, Paris (1981 ). J. Gal & J. Hess, Rev. Sci. Instrum. 42, 543 (1971). R.P. 1Tlchardson & J.B. Gruber, J. Chem. Phys. 16, 256 (1972). H. Wegener, Z. Phys. 186, 498 (1965). J. Gal, Z. Hadari, U. Atzmony, E.R. Bauminger, I. Nowik & S. Ofer, Phys. Rev. B8, 1901 (1973). I. Nowik & H.H. Wickman, Phys. Rev. Lett. 17, 949 (1966). B.D. Dunlap & G.H. Lander, Phys. Rev. Lett. 33, 1046 (1974). R.D. Meeker, B.D. Dunlap & D. Cohen, J. Phys. Chem. Solids 37, 551 (1976). J.B. Comly, J. Appl. Phys. 39, 716 (1968). D. Lebenbaum, I. Nowik, E.R. Bauminger & S. Ofer, SolidState Commun. 9, 1885 (1971).
0038-1098/84 $3.00 + .00 Pergamon Press Ltd.
MOSSBAUER AND MAGNETIZATION STUDIES OF THE UI-xNp~O2 FLUORITES A. Tabuteau, J. Jovd and M. Pag6s Institut Curie, Section de Physique et Chimie, Equipe Physicochimie des Transuraniens (ERA CNRS 1006), 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France C.H. de Novion SESI, Bat. 31, CEN, BP 6, 92260 Fontenay-aux-Roses, France and J. Gal
NRCN, BP 9001, Beer-Sheva, Israel
(Received 29 November 1983 by E.F. Bertaut)
2aTNp Mossbauer effect and magnetic susceptibility measurements of the U~-xNpxO2 fluorite solid solution have been performed in the composition range 0.15 ~
1. INTRODUCTION ACTINIDE DIOXIDES AnO2 have drawn the attention of chemists and physicists for their technological interest as well as for their special physical properties. Of particular interest are UO2 and NpO2, the magnetic properties of which have been the subject of recent extensive experimental and theoretical studies [1-6]. UO2 orders antiferromagnetically below TN = 30.8 K, with an uranium ordered moment of 1.8/a n [7]; a slight distortion of the oxygen sublattice was found in association with the magnetic order [ 1]. The transition, which is of first order, could be well explained in terms of a collectwe Jahn-Teller mechanism [2]. But Solt and Erdos [5] have pointed out recently that this process would not lead alone to the observed structure, and that the lattice deformation is favoured by direct electric quadrupole-quadrupole interactions. Early specific heat [8] and magnetic susceptibility measurements [9] (and more recently [3])had shown in NpO2 the existence of an anomaly at 25 K which was first interpreted-as a transition to an antiferromagnetic state. However, neutron diffraction studies [4, 9, 10] led to the conclusion that the Np ordered magnetic moment, if it exists, is smaller than 0.2 tzB (when the paramagnetic moment is 2.95/an [ 11 ] ). Mossbauer experiments on polycrystalline NpO2 samples showed a moderate broadening of the absorption line below
25 K, which, if interpreted as a magnetic ordering, would correspond to an ordered moment of 0.01/l n [12]. Gal et al. [6] claimed that this broadening, which is strongly dependent on the stoichiometry of the compound (NpO2+x)is non-magnetic and could result from the Orbach-Blume spin-lattice relaxation process within the Vs cubic ground state quartet of Np rv split into two Kramer's doublets by the crystalline field due to deformations of the cubic unit cell. It was then concluded that the NpO2÷x compounds (-- 0.08 ~
357
358
STUDIES OF THE UI_~Np~O2 FLUORITES
Vol. 50, No. 4
Table 1. Magnetic properties of Ul-xNpx02 deduced from Mossbauer and magnetic susceptibility measurements (the hyper~ne fieM Hen(0) were deduced from the fit with the Wegenermodel; the values obtained from the Nowik- Wickman model are quite similar) X
0
0.15
Hen(0) (kOe) To (K) /-tord*/NP (#B)
TN (K) /aen QzB) 0o (K)
30.8 3.6(3) --244(16)
0.25
0.50
1000(40) 18.7(9) 0.5(1)
1120(40) 15.5(7) 0.6(1)
1200(20) 10.0(6) 0.65(5)
27(2) 2.95(8) -- 180(12)
23(5) 3.43(6) -- 180(12)
11.5 t
0.75 1210(50) 9.6(6) 0.65(9) 8(2) 3.0(1) -- 132(20)
1 25? 0.017
2.95 --22
* P-ord derived from Hen(0) by the relationship of Dunlap and Lander [21]. t TN (-+ 1 K) for x = 0.50 deduced from neutron diffraction measurements [14].
' X-'cxlo'
,X-l(xto- )
)
701 60 50
m
S "V
t
O
10
26
.:"o..""" X -1 .... o, a ..s. o
39
tOO
150 T K)
Fig. 2. X-1 = f ( T ) f o r Uo.2sNp~TsO2 (applied field =
14 kG).
2.1. Samples The Ul-xNpxO2 samples were prepared by coprecipitation of the hydroxides in H2SO4 by NH4OH. The precipitate was then heated to 1000°C under vacuum. X-ray diffraction measurements revealed a single phase of CaF2-type structure, with linear x dependent lattice parameter (5,470 h for UO2 and 5.324 A for NpO2) and without broadening of the diffraction lines [15].
the sample. The uncertainty on the sample temperature is -+ 0.5 K after calibration. At temperatures above 50 K, all the Ul_xNpxO 2 follow the Curie-Weiss law, with effective moments #en of about 3/~B and large negative Curie paramagnetic temperatures 0p (see Table 1). The susceptibility curve for U~asNpo.lsO2 is shown in Fig. 1 : it is the only composition which presents, similarly to UO2, a sharp maximum of magnetic susceptibility suggesting an antiferromagnetic transition with TN = 27(2) K. The other two samples (x = 0.25 and 0.75) show more shallow transitions (see Fig. 2): TN was then defined, somewhat arbitrarily, as the temperature of change of slope of X-~ (T). The values of TN are reported in Table 1.
2.2. Magnetic susceptibility The magnetic susceptibility measurements were performed with a Faraday electrobalance in the temperature range 4-300 K. The temperature was measured by an Au (0.07 Fe)-Chromel thermocouple situated near
2.3. Mossbauer spectroscopy The Mossbauer absorption spectra of the 2aTNp 59.6 keV wray were recorded by a conventional constant acceleration transducer attached to a multiscaler device. The temperature of the absorber was controlled within
Fig. 1. X-1 = f ( T ) for Uo.ssNpo.lsO2. (a) Applied field = 14 kG. (b) Low temperature range (applied field = 3.34 kG). 2. EXPERIMENTAL RESULTS
Vol. 50, No. 4
STUDIES OF THE Ui_xNpxO 2 FLUORITES
spectra are displayed in Fig. 3; these spectra are similar to those observed for all the other solid solutions. Their main characteristics are the following:
,9°f,.. :
t ,,oL_
Above a "transition" temperature To a single narrow absorption line is always observed. The absorption line width Fo (full width at half maximum FWHM) is practically temperature independent from To to 77 K and equal to 3.2(2)mm sec -1. Below To, the spectrum broadens significantly and at 5 K a relatively resolved hyperfine splitting is observed (see Fig. 3); this suggests the presence of spin relaxation phenomena in a magnetically ordered system. In Uo.ssNpo.lsO 2 and UoosNpoasO2, To was found somewhat lower than the transition temperature TN deduced from magnetic susceptibility (see Table 1); on the other hand for x -- 0.50 (sample where TN was deduced from a recent power neutron diffraction experiment [14]) a n d x -- 0.75, To and TN are quite close. -- The MOssbauer isomer shifts, for all the Ul-xNpxO2 studied are not affected by the transition. Above and below To, the value found (-- 5.6(1)mm sec -1 relative to NpA12, + 8.3(1)mm s e c - 1 relative to the Am metal source) coincides, within the experimental error, with that of pure NpO2 [12]. This shows that the Np ion in all the solid solution investigated has a IV formal charge state (5/3, 419/2) independent of the uranium neighbours. -
- "\
oo.o .
-\ /
-
~..
j'5.7K
99.0
°
-40-20
0
+20 +40
1(30.0
99.8 99.6 99.4
The analysis of the Mossbauer absorption spectra was based on the following assumptions:
99.2 I00.0;
359
:" '" "." '." "" ~......'- .".
"."""~ ""
"
99.8 99.6 99.4 '
99,2~
-40
-20
0
+20 +40
V e l o c i t y (mm/sec) Fig. 3. M6ssbauer absorption spectra o f some Ul-xNpxO2 compounds. The solid line represents the least square fit using: (a) the Wegener model (x = 0.25); (b) the Nowik-Wickman model assuming relaxation effects within an isolated Kramers doublet with A = 4K, r = 4.5 x 10 -zI sec and/-/e~ = 1213 kOe as fit parameters for the 4.5 K, x = 0.15 case (see text). -+0.5 K in a furnace embedded in liquid helium as described in [16]. Recoilless absorption spectra were obtained for Ul-xNpxO2 (x = 0.15, 0.25, 0.50 and 0.75) at various temperatures from 4.2 to 77 K. Typical 2aTNp Mossbauer
(a) The Np ions are in a IV formal charge state (see above) with a Cubic Ps quartet ground state [17]. (b) Below TN or To, the ground Ps quartet of the Np ion will split as a result of the axial crystalline field originated by the oxygen displacements and of the internal magnetic field. If one of the splittlngs A is of the order of a few degrees Kelvin, a relaxation channel is opened and a relaxation process is expected. (c) The random distribution of the Np ions within the fluorite U sublattlce will certainly introduce a spread in the hyperfine fields in addition to the relaxation phenomena. We assumed that below To the FWHM l~ of each individual absorption line is given by F = Po + FM, where Fo is the FWHM at 77 K (see Section 2.3) and FM the broadening due to the spread In the hyperfine fields. In order to get a rough estimation of the values of the magnetic hyperflne field constants, a first fitting procedure was carried out using the phenomenological Wegener model [18, 19]. Later, the molecular field Nowik-Wickman model [20] was used to obtain the values of the doublet splitting A and of the relaxation times z; this model expresses the present physical
360
STUDIES OF THE Ul-xNpxO2 FLUORITES
[
.....
:
100(
50C
o Fig. 4. Hyperfine field evolution as function of the temperature. situation if we make the approximation that only one dominant relaxation frequency (within the fundamental Kramers doublet) should be considered.* It should be stressed that we were able to fit our data only by modifying the Wegener and the Nowlk-Wickman model's by introducing P = Fo + FM into the corresponding formulas [19, 20]. Reasonably good fits of the experimental spectra were obtained using these models (see Fig. 3). The hyperfine parameters determined in this way may be summarized as follows: The average magnetic hyperfine fields acting on the Np nuclei are shown on Fig. 4; they decrease as the temperature is raised and vanish at the transition temperature To; the extrapolated values of Hen at 0 K are given in Table 1. FM is found to be nearly composition and temperature independent below To (3 +- 1 mm sec-1), and drops sharply to zero for T >~ To. - The derived correlation times 7- at 4.5 K, using the Nowik-Wickman model [20], are of the order of 10 -11 sec. - Quadrupolar interactions were found to be negligible below and above To. - The value derived for the exchange splitting of the fundamental Kramers doublet at 4.5 K, using the Nowik-Wickman model, is A ~ 4 K. -
-
3. DISCUSSION Our interpretation of the Mossbauer and susceptibility results may be summarized as follows: (a) The 237Np isomer shift is independent of temperature and composition: the Np ion remains Np rv
(5I "3, %,~).
Vol. 50, No. 4
(b) All the Ul-xNpxO2 sohd solutions mvestigated in the present work were found to order magnetically at low temperatures; this experimental fact contradicts the Mossbauer source experiment on UO2 doped by 237Np [22], which stated the absence of an induced Np moment m the ordered antlferromagnetic state. The transition observed at To corresponds to a magnetic ordering of the Np ions, and not only to magnetic relaxation phenomena, as confirmed by the recent observation by neutron diffraction of a magnetic reflexxon in U0.sNpo.sO2 below TN ~ 12 K [14], temperature which is very near To 10K. (c) The dependence of TN and To with x (see Table 1) indicates a constant decrease of ordering temperatures as x increases: the Interchange of Np in UO2 lowers the U - U exchange interactions stmllarly to thorium m the Ul_xThxO 2 solid solution [23]. One also remarks that at low neptunium concentration (x = 0.15 and 0.25), the Np sites order at lower temperature than the U sites (To < TN). (d) The non-magnetic behaviour of Np between TN and To for x = 0.15 and x = 0.25 is certainly the most striking result of the present investigation. The sharp maximum of magnettc susceptibility observed at TN = 27 K in Uo.ssNpo.lsO2 argues, by analogy with UO2, for antiferromagnetlc ordering probably coupled with a Jahn-Teller deformation of the oxygen sublattice. If the Np ion carried a non-zero ordered moment below TN, a sharp onset of hyperfine field should occur at TN, as a result of transferred field, leading to a broadening of the 237Np Mossbauer absorption line. The fact that this is not observed strongly supports the Solt and Erdos model [13] for NpO2. The Np ion ground state (P8 quartet) remains then unperturbed by the reduced magnetic field between TN and To The behaviour of the hyperf'me field at the 237Np nucleus below TN is typical of temperature dependent core polarization, and similar to the behavlour of diamagnetic ions m a magnetic environment (for example Sn substituted m iron garnet systems [24]), thus supporting the idea that (Jz} equals zero. The crystal field parameter A4[A6 is therefore insignificantly affected by composition and by the Jahn-Teller deformation down to To. (e) The derived zero value of the quadrupolar moment (q) below TN is in apparent discrepancy with the existence of a distortion of the oxygen sublattice as predicted by Solt and Erdos [13] for pure NpO2. But no calculation of the value of (q) has been made using this model m order to compare with the experimental results. 4. CONCLUSION
* It was found from preliminary fits with the Wegener model that transverse relaxation times can be neglected in the studied samples.
Our magnetic susceptxbility and Mossbauer effect studies of the solid solution Ux-xNpxO2 have shown
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STUDIES OF THE Ul-xNpxO2 FLUORITES
that long-range magnetic order occurs at low temperature (T<~ TN) via ordering of the magnetic uranium ions. The neptunium ions could carry a zero effective moment (Jz) as predicted by the Solt and Erdos model and be ordered by the transferred field from the uranium. From the value of the Np induced moment, 0.5/l n (Table 1) and polarized neutron diffraction experiment on NpO2 at 4.2 K (a 0.07/a B moment is induced by an external field of 4 6 T [4]), one may estimate the transferred field on the Np sites m the sohd solution as about 400 kOe. The ordering temperature for the Np ions, To, is definitely smaller than TN at low neptunium content (x = 0.15 and 0.25), but very near to it f o r x = 0.50 and 0.75. This difference is not understood: it might be due to a change of ordered magnetic structure with composition, as suggested by neutron diffraction measurements [14]. Studies for extreme x values, like 0.1 and 0.9 and neutron diffraction experiments are needed to improve our understanding of the magnetic behaviour of this system.
4. 5. 6. 7. 8.
9. 10. 11. 12. 13. 14. 15. 16. 17.
Acknowledgement - The authors wish to thank Dr R. Pascard who initiated this work. REFERENCES 1. 2. 3.
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