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Orthorhombic structure of UMn2 at low temperatures
Journal of the Less-Common
ORTHORHOMBIC
Metals,
STRUCTURE
107 (1985) 243 - 248
OF UMn,
243
AT LOW TEMPERATURES
A. C. LAWSON, J. L. SMITH, J. 0. WILLIS and J. A. O’ROURKE Materials Science and Technology Alamos, NM 87545 (U.S.A.)
Division,
Los
Alamos
National
Laboratory,
Los
J. FABER and R. L. HITTERMAN Materials IL 60439
Science (U.S.A.)
and
Technology
Division,
Argonne
National
Laboratory,
Argonne,
(Received July 12, 1984)
Summary We refined the crystal structure of the cubic Laves phase UMn, at 295 K and 12 K using the method of Rietveld profile analysis. Structural data and interatomic distances are given.
1. Introduction A low temperature crystallographic transformation was discovered in the cubic Laves phase (Cl5 structure) UMn2 by Marpoe and Lander [l] in 1978. They discussed the structure, which is highly distorted from cubic below 220 K, in terms of a monoclinic unit cell. Lawson [2] pointed out that the low temperature structure could be described using a metrically equivalent orthorhombic structure for which the atomic positions are severely constrained by symmetry and made an approximate determination of these positions using- the published intensity data of Marpoe and Lander. The present study was undertaken in order to provide a more precise description of the low temperature structure of UMn,. In particular, some of the conclusions drawn in Lawson’s earlier study are faulty because of a sign error, and these are now corrected. Apart from the intrinsic crystallographic interest in the transformation, we were motivated to study UMn, by the positions of its constituents in the periodic system. According to the nearly periodic system of Smith and Kmetko [3], uranium and manganese both originate in that region of the periodic system which is characterized by a crossover in behavior between localized magnetic moment formation and metallic bonding of the 5f and 3d electrons respectively. UMn, is neither magnetically ordered nor superconducting down to 20 mK but appears instead to exhibit spin fluctuations at low temperatures [4]. While this behavior of UMn, can readily be correlated with the positions of its constituents in the periodic system, the crys0022-5088/85/$3.30
0 Elsevier Sequoia/Printed in The Netherlands
244
tallographic behavior cannot be, and we can speculate that the occurrence of the cubic-orthorhombic transformation allows this compound to avoid more conspicuous magnetic behavior. The present study does not directly address this point, but some of our conclusions may bear on it.
2. Experimental
method
Approximately 40 g of polycryst~line UMn, was prepared by arc melting in a zirconium-gettered argon atmosphere. In order to ensure homogeneity, the sample was melted 12 times. It was turned over each time, and while molten it was made to flow. Similar UMn, samples would fracture on reheating after the fourth melting if permitted to cool below red heat; good homogeneity is therefore indicated. The calculated sample composition is UMn,.02s based on the assumption that the observed 73 mg weight loss was entirely manganese. Attempts to produce a sample with 1% or less excess manganese always resulted in such large weight losses that the samples were deficient in manganese. The sample actually measured was weighed out to have a 2.3% manganese excess and had a very small weight loss, suggesting that the equilib~um composition contains at least 1% manganese excess. The sample was weighed and immediately transferred to a helium-filled glove-bag where it was ground to a powder of approximately 100 mesh and epoxy-sealed into a 3/8 in vanadium tube containing the helium atmosphere. A small fragment was retained for superconductivity measurement in a 3He-4He dilution refrigerator. Throughout these procedures care was taken to prevent contact of the powdered sample with air, as spontaneous ignition of this extraordinarily pyrophoric material would almost certainly result. For the same reason, it is fortunate that no annealing procedures are necessary for the preparation of this congruently melting compound. Even with these precautions, traces of UOp (perhaps several per cent) were found in the diffraction pattern of the powdered sample. Time-of-flight neutron diffraction data were obtained using the General Purpose Powder Diffractometer (GPPD) at the Intense Pulsed Neutron Source (IPNS), Argonne National Laboratory. The sample temperature was measured with an accuracy to about 0.5 K and maintained with a precision of about 0.1 K using a closed-cycle helium refrigerator. Runs of about 24 h duration were made at room temperature and 12 K; the data were analyzed using the Rietveld profile analysis technique with a standard package of computer codes maintained at IPNS and slightly modified for use at the Los Alamos National Laboratory.
3. Results and discussion The refinement profiles for UMn? are shown in Figs. 1 and 2, where the calculated and observed intensities and their difference are given at both
245 TABLE
1
Structural
parameters
of UMnz
Temperature Space group U (b = 8.53 fm) Atomic positions Temperature factor (AZ) Occupation Mn(1) (b = -3.73 fm) Atomic positions Temperature factor (AZ) Occupation Mn(2) Atomic positions Temperature factor (AZ) Occupation Lattice constants (II) a b c Residuals Profile intensity R (%) Weighted profile R (%)
TABLE
295 K
12 K
Fd3m
Imma
l/8 l/8 0.85(3) 1.00
l/8
0 l/4 0.1345(l) 0.28( 2) 1.00
l/2 l/2 0.73(3) 1.05(l)
l/2
l/4 l/4 0.35(3) 1.04(l)
314
0 0 l/2 0.28(3) 1.03(l) 7.1595(9)
4.8163(l) 5.2478(l) 7.1820(l)
4.72 7.37
4.19
7.60
2
Bond lengths
in UMn2 (in %ngstrBms)
Bond
295 K
12 K
u-u
3.102
2.924 3.256
Mn-Mn
2.533
U-Mn
2.968
Mn( 1 )-Mn( 1) Mn(l)-Mn(2) Mn( 2)-Mn( 2) U-Mn( 1) U-Mn(
2)
2.408 2.588 2.639 3.013 3.004 2.935 2.908
room temperature and 12 K; some relevant quantities extracted from the refinement are given in Table 1. The analysis reveals that the sample used in this investigation is about 4% hyperstoichiometric with respect to manganese. This conclusion remains unaffected by recent slight revisions of the scattering length of uranium [5, 61 and is consistent with the calculated sample composition considered along with the observation of UO? in the sample. The table shows that the atomic positions are in good agreement with the earlier analysis [2], except that the sign of the deviation of the z coordinate of the uranium atoms in the low temperature structure is now
246
i,,,,l,.,,l.~..l~~.~l~.,~1~1,~l~~..l~~.~l.~~~l~~~~l~~.~
0.500
0.672
0.844
1.016
1.188
d -SPACING
Rietveld profile for UMn2 at 295 K (d = 0.5 - 1.5 8): calculated pattern; the lower curve is the residual.
~mIAlIIIIIIIIIIOHIa111111111111
. ..Lr.
0.500
0.672
0.844
lllll 1111111111 lnllll I
1.016
I.188
d-SPACING& Fig. 2(a). Rietveld
profile
1.532
(h,
Fig. l(a). -,
1.360
for UMnz at 12 K (d = 0.5 - 1.5 8).
X,
measured
I II Ill I I
1.360
scattering;
II
1.532
241
6.0 cn = 2 0
3.0 0.0
I.513
1.685
1.857
2.029 d-SPACING
2.201
2.373
2.545
2.373
2.545
(8,
Fig. l(b). Rietveld profile for UMnz at 295 K (d = 1.5 - 2.5 8).
*lo3
111111,,111,1,111,11,,,1,,,,,,,,,,,,,,1,,,,,,,,,,,,,,,,,,,,
I.513
1.685
1.857
2.029 d-SPACING&
2.201
Fig. 2(b). Rietveld profile for UMnz at 12 K (d = 1.5 - 2.5 A).
243
changed. The atomic distances are now substantially different from those found earlier; these are given in Table 2. The distinct U-U and Mn-Mn distances differ from each other by considerably more than the variation of the U-Mn distances, indicating that the latter are approximately preserved during the transformation. The U-U distances are comparable with those found for P-U [ 71. A diffraction pattern obtained at 222 K could not be fitted with either the cubic or the orthorhombic structure. This finding is consistent with the results of Marpoe and Lander [l] who found considerable broadening in the vicinity of the transition temperature. The behavior of UMn, in this temperature region will be the subject of a future investigation.
Acknowledgments We are pleased to thank M. H. Mueller, G. H. Lander, F. J. Rotella, R. B. Vondreele and A. C. Larson for helpful discussions during the course of this work. This work was supported under the auspices of the U.S. Department of Energy.
References 1 2 3 4 5
G. R. Marpoe and G. H. Lander, Solid State Commun., 26 (1978) 559. A. C. Lawson, J. Less-Common Met., 90 (1983) L13. J. L. Smith and E. A. Kmetko, J. Less-Common Met., 90 (1983) 83. J. M. Fournier, Solid State Commun., 36 (1980) 245. A. Roeuf, R. Caciuffo, R. Rebonato, F. Rustichelli, J. M. Fournier, U. Kischko and L. Manes,Phys. Rev. Lett., 49 (1982) 1086. 6 K. Clausen, W. Hayes, J. E. Macdonald, P. Schnabel, M. T. Hutchings and J. K. Kjems, High Temp. High Pressures, 15 (1983) 383. 7 J. Donohue, The Structures of the Elements, John Wiley, New York, 1974, p. 150.
ORTHORHOMBIC
Metals,
STRUCTURE
107 (1985) 243 - 248
OF UMn,
243
AT LOW TEMPERATURES
A. C. LAWSON, J. L. SMITH, J. 0. WILLIS and J. A. O’ROURKE Materials Science and Technology Alamos, NM 87545 (U.S.A.)
Division,
Los
Alamos
National
Laboratory,
Los
J. FABER and R. L. HITTERMAN Materials IL 60439
Science (U.S.A.)
and
Technology
Division,
Argonne
National
Laboratory,
Argonne,
(Received July 12, 1984)
Summary We refined the crystal structure of the cubic Laves phase UMn, at 295 K and 12 K using the method of Rietveld profile analysis. Structural data and interatomic distances are given.
1. Introduction A low temperature crystallographic transformation was discovered in the cubic Laves phase (Cl5 structure) UMn2 by Marpoe and Lander [l] in 1978. They discussed the structure, which is highly distorted from cubic below 220 K, in terms of a monoclinic unit cell. Lawson [2] pointed out that the low temperature structure could be described using a metrically equivalent orthorhombic structure for which the atomic positions are severely constrained by symmetry and made an approximate determination of these positions using- the published intensity data of Marpoe and Lander. The present study was undertaken in order to provide a more precise description of the low temperature structure of UMn,. In particular, some of the conclusions drawn in Lawson’s earlier study are faulty because of a sign error, and these are now corrected. Apart from the intrinsic crystallographic interest in the transformation, we were motivated to study UMn, by the positions of its constituents in the periodic system. According to the nearly periodic system of Smith and Kmetko [3], uranium and manganese both originate in that region of the periodic system which is characterized by a crossover in behavior between localized magnetic moment formation and metallic bonding of the 5f and 3d electrons respectively. UMn, is neither magnetically ordered nor superconducting down to 20 mK but appears instead to exhibit spin fluctuations at low temperatures [4]. While this behavior of UMn, can readily be correlated with the positions of its constituents in the periodic system, the crys0022-5088/85/$3.30
0 Elsevier Sequoia/Printed in The Netherlands
244
tallographic behavior cannot be, and we can speculate that the occurrence of the cubic-orthorhombic transformation allows this compound to avoid more conspicuous magnetic behavior. The present study does not directly address this point, but some of our conclusions may bear on it.
2. Experimental
method
Approximately 40 g of polycryst~line UMn, was prepared by arc melting in a zirconium-gettered argon atmosphere. In order to ensure homogeneity, the sample was melted 12 times. It was turned over each time, and while molten it was made to flow. Similar UMn, samples would fracture on reheating after the fourth melting if permitted to cool below red heat; good homogeneity is therefore indicated. The calculated sample composition is UMn,.02s based on the assumption that the observed 73 mg weight loss was entirely manganese. Attempts to produce a sample with 1% or less excess manganese always resulted in such large weight losses that the samples were deficient in manganese. The sample actually measured was weighed out to have a 2.3% manganese excess and had a very small weight loss, suggesting that the equilib~um composition contains at least 1% manganese excess. The sample was weighed and immediately transferred to a helium-filled glove-bag where it was ground to a powder of approximately 100 mesh and epoxy-sealed into a 3/8 in vanadium tube containing the helium atmosphere. A small fragment was retained for superconductivity measurement in a 3He-4He dilution refrigerator. Throughout these procedures care was taken to prevent contact of the powdered sample with air, as spontaneous ignition of this extraordinarily pyrophoric material would almost certainly result. For the same reason, it is fortunate that no annealing procedures are necessary for the preparation of this congruently melting compound. Even with these precautions, traces of UOp (perhaps several per cent) were found in the diffraction pattern of the powdered sample. Time-of-flight neutron diffraction data were obtained using the General Purpose Powder Diffractometer (GPPD) at the Intense Pulsed Neutron Source (IPNS), Argonne National Laboratory. The sample temperature was measured with an accuracy to about 0.5 K and maintained with a precision of about 0.1 K using a closed-cycle helium refrigerator. Runs of about 24 h duration were made at room temperature and 12 K; the data were analyzed using the Rietveld profile analysis technique with a standard package of computer codes maintained at IPNS and slightly modified for use at the Los Alamos National Laboratory.
3. Results and discussion The refinement profiles for UMn? are shown in Figs. 1 and 2, where the calculated and observed intensities and their difference are given at both
245 TABLE
1
Structural
parameters
of UMnz
Temperature Space group U (b = 8.53 fm) Atomic positions Temperature factor (AZ) Occupation Mn(1) (b = -3.73 fm) Atomic positions Temperature factor (AZ) Occupation Mn(2) Atomic positions Temperature factor (AZ) Occupation Lattice constants (II) a b c Residuals Profile intensity R (%) Weighted profile R (%)
TABLE
295 K
12 K
Fd3m
Imma
l/8 l/8 0.85(3) 1.00
l/8
0 l/4 0.1345(l) 0.28( 2) 1.00
l/2 l/2 0.73(3) 1.05(l)
l/2
l/4 l/4 0.35(3) 1.04(l)
314
0 0 l/2 0.28(3) 1.03(l) 7.1595(9)
4.8163(l) 5.2478(l) 7.1820(l)
4.72 7.37
4.19
7.60
2
Bond lengths
in UMn2 (in %ngstrBms)
Bond
295 K
12 K
u-u
3.102
2.924 3.256
Mn-Mn
2.533
U-Mn
2.968
Mn( 1 )-Mn( 1) Mn(l)-Mn(2) Mn( 2)-Mn( 2) U-Mn( 1) U-Mn(
2)
2.408 2.588 2.639 3.013 3.004 2.935 2.908
room temperature and 12 K; some relevant quantities extracted from the refinement are given in Table 1. The analysis reveals that the sample used in this investigation is about 4% hyperstoichiometric with respect to manganese. This conclusion remains unaffected by recent slight revisions of the scattering length of uranium [5, 61 and is consistent with the calculated sample composition considered along with the observation of UO? in the sample. The table shows that the atomic positions are in good agreement with the earlier analysis [2], except that the sign of the deviation of the z coordinate of the uranium atoms in the low temperature structure is now
246
i,,,,l,.,,l.~..l~~.~l~.,~1~1,~l~~..l~~.~l.~~~l~~~~l~~.~
0.500
0.672
0.844
1.016
1.188
d -SPACING
Rietveld profile for UMn2 at 295 K (d = 0.5 - 1.5 8): calculated pattern; the lower curve is the residual.
~mIAlIIIIIIIIIIOHIa111111111111
. ..Lr.
0.500
0.672
0.844
lllll 1111111111 lnllll I
1.016
I.188
d-SPACING& Fig. 2(a). Rietveld
profile
1.532
(h,
Fig. l(a). -,
1.360
for UMnz at 12 K (d = 0.5 - 1.5 8).
X,
measured
I II Ill I I
1.360
scattering;
II
1.532
241
6.0 cn = 2 0
3.0 0.0
I.513
1.685
1.857
2.029 d-SPACING
2.201
2.373
2.545
2.373
2.545
(8,
Fig. l(b). Rietveld profile for UMnz at 295 K (d = 1.5 - 2.5 8).
*lo3
111111,,111,1,111,11,,,1,,,,,,,,,,,,,,1,,,,,,,,,,,,,,,,,,,,
I.513
1.685
1.857
2.029 d-SPACING&
2.201
Fig. 2(b). Rietveld profile for UMnz at 12 K (d = 1.5 - 2.5 A).
243
changed. The atomic distances are now substantially different from those found earlier; these are given in Table 2. The distinct U-U and Mn-Mn distances differ from each other by considerably more than the variation of the U-Mn distances, indicating that the latter are approximately preserved during the transformation. The U-U distances are comparable with those found for P-U [ 71. A diffraction pattern obtained at 222 K could not be fitted with either the cubic or the orthorhombic structure. This finding is consistent with the results of Marpoe and Lander [l] who found considerable broadening in the vicinity of the transition temperature. The behavior of UMn, in this temperature region will be the subject of a future investigation.
Acknowledgments We are pleased to thank M. H. Mueller, G. H. Lander, F. J. Rotella, R. B. Vondreele and A. C. Larson for helpful discussions during the course of this work. This work was supported under the auspices of the U.S. Department of Energy.
References 1 2 3 4 5
G. R. Marpoe and G. H. Lander, Solid State Commun., 26 (1978) 559. A. C. Lawson, J. Less-Common Met., 90 (1983) L13. J. L. Smith and E. A. Kmetko, J. Less-Common Met., 90 (1983) 83. J. M. Fournier, Solid State Commun., 36 (1980) 245. A. Roeuf, R. Caciuffo, R. Rebonato, F. Rustichelli, J. M. Fournier, U. Kischko and L. Manes,Phys. Rev. Lett., 49 (1982) 1086. 6 K. Clausen, W. Hayes, J. E. Macdonald, P. Schnabel, M. T. Hutchings and J. K. Kjems, High Temp. High Pressures, 15 (1983) 383. 7 J. Donohue, The Structures of the Elements, John Wiley, New York, 1974, p. 150.