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Induced magnetic form factor of NpO2
Physiea 102B (1980) 171-173 © North-Holland Publishing Company
INDUCED MAGNETIC FORM FACTOR OF NpO2"~ A. D E L A P A L M E a, M. F O R T E b, J.M. F O U R N I E R c, J. R E B I Z A N T ~ and J.C. S P I R L E T d aLaboratoire L. Brillouin, l'Orme des Merisiers, BP no. 2, Crif sur Yvette, France bCommission des Communaut~s Europdennes, J.R.C. ISPRA, 21020 Varese, Italy cCEN/Grenoble, DRF/PHS, 85 X, 38041 Grenoble Cddex, France °Commission des Communaut~s Europ#ennes, J.R.C. Karlsruhe, Postfach 2266, Leopoldsherfen, R.F.A. tWork carried out at ILL, 156X, 38042 Grenoble, Cedex. A polarized neutron diffraction experiment has been performed at ILL on a single crystal of NpO2 weighting only 5 mg. A moment of 0.07/~a at 4.2K was induced by an applied field of 4.6T, in good agreement with recent magnetic susceptibility measurements. Corrections were made for the strong extinction. A new value of bNp has been determined (bNp = 1.00(3)× 10-~2cm) and, in agreement with published data, reanalysed in a consistent fashion regarding stoichiometry. The form factor obtained is satisfactory fitted with the theoretical one for Np 4+ ions calculated from relativistic atomic wave functions.
1. Introduction
NpO2 was a cube (0.75 mm edge) grown in a vapor atmosphere and then perfected [6]. For the perfect CaF2 structure, there are three kinds of nuclear structure factors F N :
Neptunium dioxide, NpO2, has the same fluorite structure as the other actinide dioxides (fig. 1). It is interesting to study it by means of the polarized neutron diffraction techniques for three reasons. (1) It is generally a c c e p t e d - b u t not always [ 1 ] - t h a t the actinide dioxides AnO2 are ionic with An 4÷ ions. It is thus possible to compare experimental and theoretical form factors in order to try to obtain the groundstate of Np in NpO2. (2) There exists, at 25 K, a transition which was first interpreted as an antiferromagnetic ordering [2] but no moment was detected either directly by powder neutron diffraction [3] or indirectly by M6ssbauer effect [4]; polarized neutron diffraction results may help to elucidate the nature of this transition [5]. (3) Last, but not least, no single crystals of any Np compounds have been grown up to now, and the NpO2 single crystals used are the first sizeable ones reported [6].
cbol, 4lbNpl,
class a (h + k + l = 4n): IFNa (0)1 = 41bNp + class b (h + k + l = 2n + 1): IFNb (0)[ = class c (h + k + l = 4n + 2): [FNc (0)1 = 4[bsp -- cbol,
where b is the coherent scattering length and c is the oxygen to neptunium ratio. It is important to know the F N values well because the polarized neutron experiments yield only the ratio between magnetic and nuclear structure factors. Class a and b reflections are strong and mainly sensitive to extinction, while class c reflexions are very weak (bNp~ 10-12 cm) and thus very sensitive to bNp and c (b0 = 0.58 x 10-12 cm being assumed perfectly known). Class a and b reflexions have been corrected for the strong extinction, thanks to an independent study [7].* The study states that class c has a too low FN, which can be corrected either by increasing c or decreasing b~p from the published 1.055 × 10-~2 cm value. Uranium and neptunium dioxides can easily be
2. Experimental results The polarized neutron diffraction was carried out on the D3 diffractometer at ILL (A = 0.9 and 1.0,~, a = 5.431 ,~, [001] parallel to a vertical magnetic field of 4.6 T, T = 4.2 K). The crystal of
* Refinement on unpolarized data yields B ( N p ) ~ 0 . A 2, B ( O X ) = 0 . 1 ~ , mosaic parameter 0.15 z, particle size ~2/~m. 171
172
A. Delapalme et al./Induced magnetic form factor of Np02
"': :
hyperstoichiometric (c > 2). Recent investigation by Willis [8] on UO2+x showed that extra oxygens (x) cannot be in vacancies of the CaF2 structure, but form a (2.2.2) complex where two oxygens O" move toward vacancies (along the (111) direction) and two new oxygens O' occupy the two-vacancy hole created, parallel to the (110) direction). The consequences are changes in the crystal color, a hump in IP/v~Ifor increasing sin 0/A, and important decrease of I~1. None of these effects has been observed, indicating good stoichiometry and thus bNp has to be decreased. Fig. 2 shows the deconvolution of polarized data with bNp = 1.0 × 10 -12 cm. Empty circles correspond to bNp = 1.055 × 10-12 cm and empty triangles to no extinction corrections (diamagnetic corrections are negligible). To confirm these results a powder neutron diffraction was performed on the
o_'
Fig. 1. Fluorite structure: • U atoms; × oxygen atoms; [] vacancy. The extra-oxygen positions are indicated by a large dark cross. T h e figure below shows the perpendicular location of O' and O" (2.2.2) complex.
'p,
..... ::::..--~.,~.............
O
""
~'| I ""'::::'":~"~":.:i Np-4'÷ ,
,
,
,
,
.1
.2
.3
.4
.5
'..r
"°_"
_
.6sine/7 ~
Fig. 2. Magnetic form factor of NpO2 for bNp = 10-12 cm and showing theoretical Np 3+, Np 4+, and Np 5+ form factors [9].
A. Delapalme et al./ lnduced magnetic form factor of Np02
multidetector D1B at ILL, and the experiment carried out by Heaton et al. [3] has been reanalysed with Willis's conclusions, because we noticed a hump for FNb on their data, and there is careful chemical analysis indicating hyperstoichiometry. It gives bNp = 1.025 (21) × 10 ~2cm. However, our polarized data are best deconvoluted with bNp = 1.00 (3) x 10-12 cm.
3. Interpretation From the few reflexions collected in eight days at ILL we tried only to look for a spherical form factor. Assuming a pure Russel-Saunders configuration and within the dipole approximation, the form factor is given by (]0)+ C2(j2), where (Ji) are radial integrals and C2 = 2.333, 1.75, and 1.5, respectively, for Np 3÷, Np 4÷, and Np 5÷. After normalization to 0.28/*a (magnetization on the unit cell), the large value of C2 for Np 3÷ allows a real difference from the Np 4÷ and Np 5÷ form factors and leads us to believe that Np 4÷ is the realistic ionization for Np in NpO2 (see fig. 2) where theoretical form factors has been calculated from relativistic atomic wave functions given by Desclaux et al. [9].
4. Conclusion Two types of conclusions can be drawn from this study. (1) Technical conclusions. To obtain the form factor from the experimental flipping ratios a number of corrections had to be made in a self-consistent way which concern the extinction, the Fermi length, and the stoichiometry. It has been found that the stoichiometry was very good and apparent deviation from the structure factor had to be attributed to an incorrect Fermi length
173
of Np which is redetermined as bNp = (1.00 +- 0.03)1012 cm. (2) Physical conclusions. The form factor is well fitted by the atomic one for Np 4÷ (possibly Np 5÷) and not Np 3÷. Three types of models have been proposed for the transition at 25 K, two of which involve Np 3÷ ions [5]; our form factor results are thus in favor of the model using Np 4+ ions. There is no measurable ordered moment down to a limit of 0.2/~B and the agreement with magnetic susceptibility data at 4.2 K is good [5]. There seems to be an anisotropy of the form factor at high sin 0/A but data are not numerous enough due to a lack of experimental time and the precision is not high because the sample was very small and paramagnetic.
Acknowledgements We wish to thank ILL for the use of its facilities and F. Tasset for his friendly assistance. References [1] V.A. Gubanov, A. Rosen and D.J. Ellis, J. Phys. Chem. Solids 40 (1979) 17. [2] D.W. Osborne and E.F. Westrum Jr., J. Chem. Phys. 21 (1953) 1884. [3] L. Heaton, M.H. Mueller and J.M. Williams, J. Phys. Chem. Solids 28 (1967) 1651; D.E. Cox and B.C. Frazer, J. Phys. Chem. Solids 28 (1967) 1649. [4] J.A. Stone and W.L. Pillinger, Phys. Rev. Lett. 13 (1964) 200. [5] A. Blaise, P. Erdrs, J.M. Fournier, G. Solt and Z. Zolnierek, this Conference. [6] J.C. Spirlet, J. de Phys. C4 (1979) 87. [7] A. Boeuf, J.M. Fournier, G. Heger, L. Manes, J. Rebizant, R. Rusticelli and J.C. Spirlet, private communication. [8] B.T.M. Willis, Acta Cryst. A34 (1978) 88. [9] J.P. Desclaux and A.J. Freeman, J. Magn. Mag. Mater. 8 (1978) 119.
INDUCED MAGNETIC FORM FACTOR OF NpO2"~ A. D E L A P A L M E a, M. F O R T E b, J.M. F O U R N I E R c, J. R E B I Z A N T ~ and J.C. S P I R L E T d aLaboratoire L. Brillouin, l'Orme des Merisiers, BP no. 2, Crif sur Yvette, France bCommission des Communaut~s Europdennes, J.R.C. ISPRA, 21020 Varese, Italy cCEN/Grenoble, DRF/PHS, 85 X, 38041 Grenoble Cddex, France °Commission des Communaut~s Europ#ennes, J.R.C. Karlsruhe, Postfach 2266, Leopoldsherfen, R.F.A. tWork carried out at ILL, 156X, 38042 Grenoble, Cedex. A polarized neutron diffraction experiment has been performed at ILL on a single crystal of NpO2 weighting only 5 mg. A moment of 0.07/~a at 4.2K was induced by an applied field of 4.6T, in good agreement with recent magnetic susceptibility measurements. Corrections were made for the strong extinction. A new value of bNp has been determined (bNp = 1.00(3)× 10-~2cm) and, in agreement with published data, reanalysed in a consistent fashion regarding stoichiometry. The form factor obtained is satisfactory fitted with the theoretical one for Np 4+ ions calculated from relativistic atomic wave functions.
1. Introduction
NpO2 was a cube (0.75 mm edge) grown in a vapor atmosphere and then perfected [6]. For the perfect CaF2 structure, there are three kinds of nuclear structure factors F N :
Neptunium dioxide, NpO2, has the same fluorite structure as the other actinide dioxides (fig. 1). It is interesting to study it by means of the polarized neutron diffraction techniques for three reasons. (1) It is generally a c c e p t e d - b u t not always [ 1 ] - t h a t the actinide dioxides AnO2 are ionic with An 4÷ ions. It is thus possible to compare experimental and theoretical form factors in order to try to obtain the groundstate of Np in NpO2. (2) There exists, at 25 K, a transition which was first interpreted as an antiferromagnetic ordering [2] but no moment was detected either directly by powder neutron diffraction [3] or indirectly by M6ssbauer effect [4]; polarized neutron diffraction results may help to elucidate the nature of this transition [5]. (3) Last, but not least, no single crystals of any Np compounds have been grown up to now, and the NpO2 single crystals used are the first sizeable ones reported [6].
cbol, 4lbNpl,
class a (h + k + l = 4n): IFNa (0)1 = 41bNp + class b (h + k + l = 2n + 1): IFNb (0)[ = class c (h + k + l = 4n + 2): [FNc (0)1 = 4[bsp -- cbol,
where b is the coherent scattering length and c is the oxygen to neptunium ratio. It is important to know the F N values well because the polarized neutron experiments yield only the ratio between magnetic and nuclear structure factors. Class a and b reflections are strong and mainly sensitive to extinction, while class c reflexions are very weak (bNp~ 10-12 cm) and thus very sensitive to bNp and c (b0 = 0.58 x 10-12 cm being assumed perfectly known). Class a and b reflexions have been corrected for the strong extinction, thanks to an independent study [7].* The study states that class c has a too low FN, which can be corrected either by increasing c or decreasing b~p from the published 1.055 × 10-~2 cm value. Uranium and neptunium dioxides can easily be
2. Experimental results The polarized neutron diffraction was carried out on the D3 diffractometer at ILL (A = 0.9 and 1.0,~, a = 5.431 ,~, [001] parallel to a vertical magnetic field of 4.6 T, T = 4.2 K). The crystal of
* Refinement on unpolarized data yields B ( N p ) ~ 0 . A 2, B ( O X ) = 0 . 1 ~ , mosaic parameter 0.15 z, particle size ~2/~m. 171
172
A. Delapalme et al./Induced magnetic form factor of Np02
"': :
hyperstoichiometric (c > 2). Recent investigation by Willis [8] on UO2+x showed that extra oxygens (x) cannot be in vacancies of the CaF2 structure, but form a (2.2.2) complex where two oxygens O" move toward vacancies (along the (111) direction) and two new oxygens O' occupy the two-vacancy hole created, parallel to the (110) direction). The consequences are changes in the crystal color, a hump in IP/v~Ifor increasing sin 0/A, and important decrease of I~1. None of these effects has been observed, indicating good stoichiometry and thus bNp has to be decreased. Fig. 2 shows the deconvolution of polarized data with bNp = 1.0 × 10 -12 cm. Empty circles correspond to bNp = 1.055 × 10-12 cm and empty triangles to no extinction corrections (diamagnetic corrections are negligible). To confirm these results a powder neutron diffraction was performed on the
o_'
Fig. 1. Fluorite structure: • U atoms; × oxygen atoms; [] vacancy. The extra-oxygen positions are indicated by a large dark cross. T h e figure below shows the perpendicular location of O' and O" (2.2.2) complex.
'p,
..... ::::..--~.,~.............
O
""
~'| I ""'::::'":~"~":.:i Np-4'÷ ,
,
,
,
,
.1
.2
.3
.4
.5
'..r
"°_"
_
.6sine/7 ~
Fig. 2. Magnetic form factor of NpO2 for bNp = 10-12 cm and showing theoretical Np 3+, Np 4+, and Np 5+ form factors [9].
A. Delapalme et al./ lnduced magnetic form factor of Np02
multidetector D1B at ILL, and the experiment carried out by Heaton et al. [3] has been reanalysed with Willis's conclusions, because we noticed a hump for FNb on their data, and there is careful chemical analysis indicating hyperstoichiometry. It gives bNp = 1.025 (21) × 10 ~2cm. However, our polarized data are best deconvoluted with bNp = 1.00 (3) x 10-12 cm.
3. Interpretation From the few reflexions collected in eight days at ILL we tried only to look for a spherical form factor. Assuming a pure Russel-Saunders configuration and within the dipole approximation, the form factor is given by (]0)+ C2(j2), where (Ji) are radial integrals and C2 = 2.333, 1.75, and 1.5, respectively, for Np 3÷, Np 4÷, and Np 5÷. After normalization to 0.28/*a (magnetization on the unit cell), the large value of C2 for Np 3÷ allows a real difference from the Np 4÷ and Np 5÷ form factors and leads us to believe that Np 4÷ is the realistic ionization for Np in NpO2 (see fig. 2) where theoretical form factors has been calculated from relativistic atomic wave functions given by Desclaux et al. [9].
4. Conclusion Two types of conclusions can be drawn from this study. (1) Technical conclusions. To obtain the form factor from the experimental flipping ratios a number of corrections had to be made in a self-consistent way which concern the extinction, the Fermi length, and the stoichiometry. It has been found that the stoichiometry was very good and apparent deviation from the structure factor had to be attributed to an incorrect Fermi length
173
of Np which is redetermined as bNp = (1.00 +- 0.03)1012 cm. (2) Physical conclusions. The form factor is well fitted by the atomic one for Np 4÷ (possibly Np 5÷) and not Np 3÷. Three types of models have been proposed for the transition at 25 K, two of which involve Np 3÷ ions [5]; our form factor results are thus in favor of the model using Np 4+ ions. There is no measurable ordered moment down to a limit of 0.2/~B and the agreement with magnetic susceptibility data at 4.2 K is good [5]. There seems to be an anisotropy of the form factor at high sin 0/A but data are not numerous enough due to a lack of experimental time and the precision is not high because the sample was very small and paramagnetic.
Acknowledgements We wish to thank ILL for the use of its facilities and F. Tasset for his friendly assistance. References [1] V.A. Gubanov, A. Rosen and D.J. Ellis, J. Phys. Chem. Solids 40 (1979) 17. [2] D.W. Osborne and E.F. Westrum Jr., J. Chem. Phys. 21 (1953) 1884. [3] L. Heaton, M.H. Mueller and J.M. Williams, J. Phys. Chem. Solids 28 (1967) 1651; D.E. Cox and B.C. Frazer, J. Phys. Chem. Solids 28 (1967) 1649. [4] J.A. Stone and W.L. Pillinger, Phys. Rev. Lett. 13 (1964) 200. [5] A. Blaise, P. Erdrs, J.M. Fournier, G. Solt and Z. Zolnierek, this Conference. [6] J.C. Spirlet, J. de Phys. C4 (1979) 87. [7] A. Boeuf, J.M. Fournier, G. Heger, L. Manes, J. Rebizant, R. Rusticelli and J.C. Spirlet, private communication. [8] B.T.M. Willis, Acta Cryst. A34 (1978) 88. [9] J.P. Desclaux and A.J. Freeman, J. Magn. Mag. Mater. 8 (1978) 119.