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Some structural and magnetic properties of 239PuD2.25 by neutron diffraction
0038-1098/84 $3.00 + .OO Pergamon Press Ltd.
Solid State Communications, Vol. 52, No. 6, pp. 619-621, 1984. Printed in Great Britain.
SOME STRUCTURAL AND MAGNETIC PROPERTIES OF 239pUD2.2s BY NEUTRON DIFFRACTION W. Bartscher,’ A. Boeuf: R. Caciuffo: J.M. Fournier,“ J.M. Haschke,’ L. Manes,’ J. Rebizant,’ F. Rustichelli3 and J.W. Ward’ ‘Commission of European Communities, J.R.C. Karlsruhe, Federal Republic of Germany ZCommissionof European Communities, J.R.C. Ispra, Italy and Institut Laue-Langevin, Grenoble, France aUniversita de Ancona, Italy yentre d’Etudes Nucleaires de Grenoble and Universite de Grenoble, France (Received 10 July 1984 by E.F. Bertaut)
A neutron powder diffraction study of 23tiD2.25 compound was performed at different temperatures, in order to determine the deuterium atoms positions and to study the occurrence of structural and magnetic phase transitions. Vacancies of tetrahedral sites were found together with partial occupancy of octahedral special positions. No order disorder transitions were observed at low temperature. Below T = 60 K PL~D,.~, becomes ferromagnetically ordered with an ordered magnetic moment p,d = 0.8 pg per Pu atom. 1. INTRODUCTION THE PHYSICO-CHEMICAL properties of the actinide elements are fascinating because of the large differences exhibited from element across the row. Whereas other metallic hydride systems in the periodic table (e.g. early transition metals, trivalent rare-earths) can be rather simply classified and intercompared, homologs are not easily found for the actinide hydrides. For example, ThH2 already shows interesting anomalies as compared to the supposedly similar Ti-, Zr- and HfI-I* series, and PaH3 and UH3 are unique. PuH2 crystallizes in the face-centered cubic system and remains in the same crystallographic system when the stoichiometry is increased till it reached PuH3. It is then at Np, Pu and beyond that some comparisons can be done with various rare-earth systems, since the chemistry begins to look very similar. However, a closer look at some of the electronic properties shows immediately some major differences: (a) Progressive filling of octahedral sites causes a steady decrease in lattice parameter for all the rareearths and Pu, but not for Np, whose hydride actually grows larger. (b) As the composition MH2 is approached, the electrical conductivity of the rare-earth hydrides increases to as much as 50% better than the parent metal. In contrast, the parent actinide metals are poor conductors to begin with, and the conductivities of the hydrides [ 1,2] are even poorer. This is an indication of large involvement of 5felectrons in bonding, which is an area of theoretical and experimental interest. (c) The magnetic properties of the rare-earth
hydrides are rather uncomplicated and the magnetic moments remain about the same as for the parent metals, indicating highly-localized 4felectrons. However the Np-hydrides are non-magnetic (UH3 is magnetic), and the Pu-hydrides exhibit a complex series of magnetic and electrical transitions [2] between 4-300 K. The present paper reports the results of the first, at our knowledge, neutron diffraction study performed on a 23?u deuteride, namely 23’?~D2.2sin order to get information on the deuterium atoms positions and to study the magnetic ordering at low temperature. 2. EXPERIMENTAL DETAILS The bulk plutonium deuteride sample was prepared at the Institute for Transuranium Elements, Joint Research Centre, Karlsruhe, Germany, by direct reaction of high purity o-plutonium with deuterium in the plastic region at high pressure. About 2.5 g of the obtained compound was sealed in a double-wall aluminium container by electron beam in order to overcome the contamination hazard. Since 23?u has a very high thermal neutron cross section, the PLID~.~~ powder was shaped as a ring of 0.5 mm thickness to avoid excessive absorption and kept under helium atmosphere to insure good temperature exchange (?Ju has a self heating power of 2 mW g-‘). Powder diffraction patterns were recorded on the D2 diffractometer at the high flux reactor of the Institut Laue-Langevin in Grenoble. The neutron wavelength was 1.219 f 0.0014 A. Measurements were done at 20,65, 150 and 290 K in order to determine the structure of the solid solution at room 619
SOME STRUCTURAL
620
AND MAGNETIC PROPERTIES
30
T-20K
0”
20"
IO" BRAGG
40"
30° ANGLE
Fig. 1. Typical neutron powder spectrum obtained with 23gP~D2.2s at a temperature T = 20 K. f(8c)
SITE OCCUPATION
100
0.95 I
FACTOR 0.85 0.90 I I Theorehcal curves
-
0 T=290K 0 T:150K AT65K
n”
I
0.45
0.35
0.25 f(4b)
I
I
SITE OCCUPATION
I 0.55
I
FACTOR
Fig. 2. Occupation factor of tetrahedral (8~) and octahedral sites (4b) as a function of the relative intensity I111/~220,
~311/~22~,~2~~x
points and theoretical
10/1220.Ex~erh=tal
curves.
temperature and to detect the eventual occurrence at low temperature of structural order-disorder transitions or of magnetic ordering. 3. STRUCTURAL
RESULTS
Figure 1 shows a typical diffraction pattern obtained at T = 20 K. In spite of the high neutron capture crosssection of 239Pu, giving a transmission coefficient of the order of 7%, six Bragg reflections of the PuD,.2, sample were correctly observed. For Bragg angles 6’> 36” data
OF 23gPuD2.2,
Vol. 52, No. 6
are hardly workable because of the aluminium container’s reflections. X-ray data [3] have shown that in PtlH 2+r hydrides Pu atoms are in a f.c.c. lattice and by analogy with rare earth hydrides, one expects that crystallize in the CaFa structure (Fm3m symPuD2.m metry) with the deuterium atoms occupying the tetrahedral interstitials of the metal f.c.c. lattice. It is also expected that excess deuterium atoms progressively occupy the octahedral interstitials. The neutron data of this experiment confirm the general validity of this assumption, the lattice parameter being a = 5.344 + 0.007 A at T = 290 K. However, it is found that no agreement between calculated and measured intensities can be obtained unless one allows the presence of vacancies in the tetrahedral sites. By normalizing the different reflections with the strongest one i.e. (2 2 0) only the (1 1 l), (3 1 1) and (2 0 0) reflections have an intensity which depends upon the occupation factor f(8c) of the 8c tetrahedral sites. Figure 2 shows the theoretical variation of the (1 1 1) (3 1 1) and (2 0 0) intensities relative to the (2 2 0) one as a function of the occupation factor I. The calculations were performed using a value [4] b,, = (0.81 * 0.05) x lo-l2 cm. In particular it can be seen that the intensity of the (2 0 0) reflection shows a strong variation with a minimum for j(Sc) z 0.90. It is possible to compute the occupation factor giving the best overall agreement between the experimental and the theoretical intensities for each temperature of the measurement. The f(8c) values obtained by this procedure are: f(8c)
= 0.96 f 0.02
for both T = 150K and T = 65 K
f(8c)
= 0.95 + 0.02
for T= 290K.
By convenience these values were chosen as common abscissa for the experimental relative intensities reported in Fig. 2. As a consequence in the PuD,.,, about 5% of the tetrahedral sites appear to be vacant. No evidence of structural phase transition, related to a possible ordering of the deuterium in the octahedral interstitials was found. The presence of vacancies in the tetrahedral sites of the PuD2.as fluorite structure is not unlike the one found [5] and also in some fluorite type compound for UOZ.~~ with mixed valency [6]. This leads us to propose the following structural model. The excess of deuterium atoms occupies octahedral sites and by trapping each one conduction electron becomes D- ions. In order to reduce the lattice electrostatic energy, some D- ions move from tetrahedral to octahedral sites giving rise to cluster formation. A possible cluster in a fluorite lattice is shown in Fig. 3. The positions of metallic atoms are indicated by small circles, octahedral sites by big circles
Vol. 52, No. 6
SOME STRUCTURAL
AND MAGNETIC PROPERTIES
OF 23vPuD2~2,
621
ferromagnetic at low temperature [7, 81 with low ordered moments Cu, - 0.4~~If.u.) Effectively extra intensities of magnetic origin has been found for the diffraction pattern at T = 20 K. The obtained ordered moment value is p = (0.8 + 0.3)llg/f.u. pointing toward Pu3+ ions. The discrepancy with the saturation moment value is probably due to a strong anisotropy of the magnetization.
REFERENCES Fig. 3. Schematic representation of a cluster with four deuteride ions (large circles) on octahedral sites around a vacant tetrahedral site (square). Small circles are plutonium atoms. and tetrahedral vacancies by squares. The D- ions are probably not in the ideal octahedral position but displaced toward tetrahedral faces. In this approach deuterium atoms are assumed to be hydric, becoming D-. In this perspective, it should be interesting to study the PuD,,, solid solutions which x > 0.7 to see if such clustering is efficient enough to drive a metal to semiconductor transition.
4. MAGNETIC RESULTS PuH,+,
solid solutions have been reported to be
1. 2.
3. 4.
5. 6. 7.
8.
J.W. Ward, L.E. Cox, J.L. Smith, G.R. Stewart & J.M. Wood, J. Phys. C40, 15 (1979). J.W. Ward, J.L. Smith, J.O. Willis, S.T. Kosiewicz, J.M. Haschke & A.E. Hodges, Los Alamos Report: LA-UR81-1275 (1981). T. Moromura, T. Yahata, K. Ouchi & M. Iseki, J. Znorg. Chem. 34,171 (1972). A. Boeuf, R. Caciuffo, J.M. Fournier, L. Manes, J. Rebizant, E. Roudaut & F. Rustichelli, (to be published). B.T. Willis, J. de Phys. 25,43 1 (1964). D.J.M. Bevan, J. Strlhle, 0. Greis, J. Solid State Chem. 44,75 (1982). J.W. Ward, J.L. Smith, J.U. Willis, S.T. Kosiewicz, J.M, Haschke & A.E. Hodges, US DOE-Rep. LBL-12441,86 (1981). G. Cinader, D. Zamir & Z. Hadari, PhJx. Rev. B14, 912 (1976).
Solid State Communications, Vol. 52, No. 6, pp. 619-621, 1984. Printed in Great Britain.
SOME STRUCTURAL AND MAGNETIC PROPERTIES OF 239pUD2.2s BY NEUTRON DIFFRACTION W. Bartscher,’ A. Boeuf: R. Caciuffo: J.M. Fournier,“ J.M. Haschke,’ L. Manes,’ J. Rebizant,’ F. Rustichelli3 and J.W. Ward’ ‘Commission of European Communities, J.R.C. Karlsruhe, Federal Republic of Germany ZCommissionof European Communities, J.R.C. Ispra, Italy and Institut Laue-Langevin, Grenoble, France aUniversita de Ancona, Italy yentre d’Etudes Nucleaires de Grenoble and Universite de Grenoble, France (Received 10 July 1984 by E.F. Bertaut)
A neutron powder diffraction study of 23tiD2.25 compound was performed at different temperatures, in order to determine the deuterium atoms positions and to study the occurrence of structural and magnetic phase transitions. Vacancies of tetrahedral sites were found together with partial occupancy of octahedral special positions. No order disorder transitions were observed at low temperature. Below T = 60 K PL~D,.~, becomes ferromagnetically ordered with an ordered magnetic moment p,d = 0.8 pg per Pu atom. 1. INTRODUCTION THE PHYSICO-CHEMICAL properties of the actinide elements are fascinating because of the large differences exhibited from element across the row. Whereas other metallic hydride systems in the periodic table (e.g. early transition metals, trivalent rare-earths) can be rather simply classified and intercompared, homologs are not easily found for the actinide hydrides. For example, ThH2 already shows interesting anomalies as compared to the supposedly similar Ti-, Zr- and HfI-I* series, and PaH3 and UH3 are unique. PuH2 crystallizes in the face-centered cubic system and remains in the same crystallographic system when the stoichiometry is increased till it reached PuH3. It is then at Np, Pu and beyond that some comparisons can be done with various rare-earth systems, since the chemistry begins to look very similar. However, a closer look at some of the electronic properties shows immediately some major differences: (a) Progressive filling of octahedral sites causes a steady decrease in lattice parameter for all the rareearths and Pu, but not for Np, whose hydride actually grows larger. (b) As the composition MH2 is approached, the electrical conductivity of the rare-earth hydrides increases to as much as 50% better than the parent metal. In contrast, the parent actinide metals are poor conductors to begin with, and the conductivities of the hydrides [ 1,2] are even poorer. This is an indication of large involvement of 5felectrons in bonding, which is an area of theoretical and experimental interest. (c) The magnetic properties of the rare-earth
hydrides are rather uncomplicated and the magnetic moments remain about the same as for the parent metals, indicating highly-localized 4felectrons. However the Np-hydrides are non-magnetic (UH3 is magnetic), and the Pu-hydrides exhibit a complex series of magnetic and electrical transitions [2] between 4-300 K. The present paper reports the results of the first, at our knowledge, neutron diffraction study performed on a 23?u deuteride, namely 23’?~D2.2sin order to get information on the deuterium atoms positions and to study the magnetic ordering at low temperature. 2. EXPERIMENTAL DETAILS The bulk plutonium deuteride sample was prepared at the Institute for Transuranium Elements, Joint Research Centre, Karlsruhe, Germany, by direct reaction of high purity o-plutonium with deuterium in the plastic region at high pressure. About 2.5 g of the obtained compound was sealed in a double-wall aluminium container by electron beam in order to overcome the contamination hazard. Since 23?u has a very high thermal neutron cross section, the PLID~.~~ powder was shaped as a ring of 0.5 mm thickness to avoid excessive absorption and kept under helium atmosphere to insure good temperature exchange (?Ju has a self heating power of 2 mW g-‘). Powder diffraction patterns were recorded on the D2 diffractometer at the high flux reactor of the Institut Laue-Langevin in Grenoble. The neutron wavelength was 1.219 f 0.0014 A. Measurements were done at 20,65, 150 and 290 K in order to determine the structure of the solid solution at room 619
SOME STRUCTURAL
620
AND MAGNETIC PROPERTIES
30
T-20K
0”
20"
IO" BRAGG
40"
30° ANGLE
Fig. 1. Typical neutron powder spectrum obtained with 23gP~D2.2s at a temperature T = 20 K. f(8c)
SITE OCCUPATION
100
0.95 I
FACTOR 0.85 0.90 I I Theorehcal curves
-
0 T=290K 0 T:150K AT65K
n”
I
0.45
0.35
0.25 f(4b)
I
I
SITE OCCUPATION
I 0.55
I
FACTOR
Fig. 2. Occupation factor of tetrahedral (8~) and octahedral sites (4b) as a function of the relative intensity I111/~220,
~311/~22~,~2~~x
points and theoretical
10/1220.Ex~erh=tal
curves.
temperature and to detect the eventual occurrence at low temperature of structural order-disorder transitions or of magnetic ordering. 3. STRUCTURAL
RESULTS
Figure 1 shows a typical diffraction pattern obtained at T = 20 K. In spite of the high neutron capture crosssection of 239Pu, giving a transmission coefficient of the order of 7%, six Bragg reflections of the PuD,.2, sample were correctly observed. For Bragg angles 6’> 36” data
OF 23gPuD2.2,
Vol. 52, No. 6
are hardly workable because of the aluminium container’s reflections. X-ray data [3] have shown that in PtlH 2+r hydrides Pu atoms are in a f.c.c. lattice and by analogy with rare earth hydrides, one expects that crystallize in the CaFa structure (Fm3m symPuD2.m metry) with the deuterium atoms occupying the tetrahedral interstitials of the metal f.c.c. lattice. It is also expected that excess deuterium atoms progressively occupy the octahedral interstitials. The neutron data of this experiment confirm the general validity of this assumption, the lattice parameter being a = 5.344 + 0.007 A at T = 290 K. However, it is found that no agreement between calculated and measured intensities can be obtained unless one allows the presence of vacancies in the tetrahedral sites. By normalizing the different reflections with the strongest one i.e. (2 2 0) only the (1 1 l), (3 1 1) and (2 0 0) reflections have an intensity which depends upon the occupation factor f(8c) of the 8c tetrahedral sites. Figure 2 shows the theoretical variation of the (1 1 1) (3 1 1) and (2 0 0) intensities relative to the (2 2 0) one as a function of the occupation factor I. The calculations were performed using a value [4] b,, = (0.81 * 0.05) x lo-l2 cm. In particular it can be seen that the intensity of the (2 0 0) reflection shows a strong variation with a minimum for j(Sc) z 0.90. It is possible to compute the occupation factor giving the best overall agreement between the experimental and the theoretical intensities for each temperature of the measurement. The f(8c) values obtained by this procedure are: f(8c)
= 0.96 f 0.02
for both T = 150K and T = 65 K
f(8c)
= 0.95 + 0.02
for T= 290K.
By convenience these values were chosen as common abscissa for the experimental relative intensities reported in Fig. 2. As a consequence in the PuD,.,, about 5% of the tetrahedral sites appear to be vacant. No evidence of structural phase transition, related to a possible ordering of the deuterium in the octahedral interstitials was found. The presence of vacancies in the tetrahedral sites of the PuD2.as fluorite structure is not unlike the one found [5] and also in some fluorite type compound for UOZ.~~ with mixed valency [6]. This leads us to propose the following structural model. The excess of deuterium atoms occupies octahedral sites and by trapping each one conduction electron becomes D- ions. In order to reduce the lattice electrostatic energy, some D- ions move from tetrahedral to octahedral sites giving rise to cluster formation. A possible cluster in a fluorite lattice is shown in Fig. 3. The positions of metallic atoms are indicated by small circles, octahedral sites by big circles
Vol. 52, No. 6
SOME STRUCTURAL
AND MAGNETIC PROPERTIES
OF 23vPuD2~2,
621
ferromagnetic at low temperature [7, 81 with low ordered moments Cu, - 0.4~~If.u.) Effectively extra intensities of magnetic origin has been found for the diffraction pattern at T = 20 K. The obtained ordered moment value is p = (0.8 + 0.3)llg/f.u. pointing toward Pu3+ ions. The discrepancy with the saturation moment value is probably due to a strong anisotropy of the magnetization.
REFERENCES Fig. 3. Schematic representation of a cluster with four deuteride ions (large circles) on octahedral sites around a vacant tetrahedral site (square). Small circles are plutonium atoms. and tetrahedral vacancies by squares. The D- ions are probably not in the ideal octahedral position but displaced toward tetrahedral faces. In this approach deuterium atoms are assumed to be hydric, becoming D-. In this perspective, it should be interesting to study the PuD,,, solid solutions which x > 0.7 to see if such clustering is efficient enough to drive a metal to semiconductor transition.
4. MAGNETIC RESULTS PuH,+,
solid solutions have been reported to be
1. 2.
3. 4.
5. 6. 7.
8.
J.W. Ward, L.E. Cox, J.L. Smith, G.R. Stewart & J.M. Wood, J. Phys. C40, 15 (1979). J.W. Ward, J.L. Smith, J.O. Willis, S.T. Kosiewicz, J.M. Haschke & A.E. Hodges, Los Alamos Report: LA-UR81-1275 (1981). T. Moromura, T. Yahata, K. Ouchi & M. Iseki, J. Znorg. Chem. 34,171 (1972). A. Boeuf, R. Caciuffo, J.M. Fournier, L. Manes, J. Rebizant, E. Roudaut & F. Rustichelli, (to be published). B.T. Willis, J. de Phys. 25,43 1 (1964). D.J.M. Bevan, J. Strlhle, 0. Greis, J. Solid State Chem. 44,75 (1982). J.W. Ward, J.L. Smith, J.U. Willis, S.T. Kosiewicz, J.M, Haschke & A.E. Hodges, US DOE-Rep. LBL-12441,86 (1981). G. Cinader, D. Zamir & Z. Hadari, PhJx. Rev. B14, 912 (1976).