- Email: [email protected]
Magnetic properties of the ternary phosphide U3Ni3.34P6
0022-36!97(%)003044
Pergamon
MAGNETIC
J PhJs. Chem Solr& Vol57, No. 5, pp. 521-525, 1996 CopyrIght c: 1996 Elsevier Science Ltd Printed in Great Bntain. All rights reserved 0022-3697i96 $15.04 + 0.00
PROPERTIES
M. REEHUIS,?
T. EBEL,$
OF THE TERNARY W. JEITSCHKO,$
PHOSPHIDE
R. SONNTAG?
U3Ni3,34P6
and N. STijSSERf.
tHahn-Meitner-Institut, Glienicker Strasse 100, D-14109 Berlin, Germany fAnorganisch-Chemisches Institut, Universitiit Miinster, Wilhelm-Klemm-Strasse 8. D-48149 Miinster. Germany (Received 9 March 1995; accepted in revisedform 28 October 1995)
Abstract-The magnetic order of the uranium sublattice of U3Ni3.34P6was investigated for polycrystalline samples with a SQUID magnetometer and by neutron diffraction. Below the Curie temperature 7’~ = 139 f 2 K the magnetic moments on the two different uranium sites (with the Wyckoff notations 2h and la of the space group P4/mmm) were found to order parallel to the tetragonal c axis and antiparallel to each other, resulting in ferrimagnetism. The nickel atoms do not carry magnetic moments. Rietveld refinements resulted in the magnetic moments of peexp (“)(Ul) = 1.4 f 0. I pBand pcXp(“I(U2) = I .6 & 0.2 pa at 2 K. Accordingly, a magnetic moment along the c axis of AP,,~~~) = 2. ~l~~~(~)(Ul)- ,L,,~(~,(U2) = 1.2f 0.3 /~a is obtained. This ferrimagnetism was confirmed by the magnetization measurements, where a saturation moment pexp(sm)= 0.41 f 0.01 pa was observed. This value is reduced, because it is obtained from a randomly orientated powder sample. A minimum in the magnetization curve was observed at 130 K. which is rationalized as the compensation point of that ferrimagnet. Keywords:
A. inorganic compounds, C. neutron scattering, D. magnetic properties, D. magnetic structure.
1. INTRODUCTION
structure
The magnetic properties of various ternary phosphides were investigated during recent years. Antiferromagnetic and ferromagnetic behaviour of the uranium sublattices were found for UN&P, [ 1, 21, UCuzPz [1], UCuP2 [I] and U2Cu2P30 [3, 41. In these compounds the nickel and copper atoms are essentially nonmagnetic, while in UMhP2 [5] and UCozPz [6] the transition metal atoms carry magnetic moments, which order at higher temperatures than the uranium moments. Recently the tetragonal crystal structure (Fig. 1) of the new ternary phosphide U3Ni3.34P6 was determined from X-ray data. It may be considered as being composed by ThCr2Siz-type building blocks of the composition U3Ni4P4, which are separated from each other by an additional phosphorus layer resulting in the ideal composition U3NidP6 [7]. As is known for the simple ThCrZSiztype compound UNi2_xP2 [1, 2, 81 considerable defects were observed for the nickel sites resulting in the composition U3Ni3,34P6. Magnetic susceptibility measurements (Fig. 2) indicated Curie-Weiss behaviour above the ordering temperature Tc = 139 f 2 K with an average paramagnetic moment of pexp(pj = 2.1 f 0.1 PB per uranium atom. The maximum at 130 f 2 K in the reciprocal susceptibility curve was interpreted as the compensation point assuming U3Ni3,34P6 to be a ferrimagnet. This is confirmed in the present paper by magnetization measurements. In addition we report the magnetic
diffraction
of U3Ni334P6
as obtained
from
neutron
data.
2. EXPERIMENTAL
A sample of U3Ni3.34P6 was prepared by reaction of the elemental components in a tin flux with an atomic ratio U : Ni : P : Sn = 3 : 4 : 15: 32 essentially as described earlier [7]. Ten samples with the total weight of about 45g were prepared to yield 9.5g of U3Ni3.34P6after the tin-rich matrix had been dissolved in slightly diluted (1 : 1) hydrochloric acid. The Guinier powder patterns of the samples showed small amounts of NiP2, U02 and UP2, as the only impurities. The temperature and field dependence of the magnetic moments were measured in the temperature range between 5 and 300K using a SQUID magnetometer (Quantum Design, Inc.). The compacted polycrystalline samples were cooled down in zero magnetic field. They were measured with increasing temperature or increasing magnetic flux density, respectively. Very similar results were obtained from several samples, prepared with slightly different starting compositions. The neutron powder diffraction measurements were carried out on the instruments E2 and E6 at the BERII reactor of the HMI at Berlin. The E2 instrument uses a germanium monochromator with the 3 11 reflection (X = 121.8pm) and the E6 instrument has a double
521
M. REEHUIS et al.
Fig. 1. Crystal and magnetic structure of U3Ni3.34P6. The magnetic moments of the two different uranium sites are found to order parallel to the tetragonal c axis and antiparallel to each other.
focussing graphite monochromator using the 002 reflection (X = 238pm). The powder data of U3Ni3.s4P6 were recorded on the E2 diffractometer to the 20 range from 4 to 84” with the 80” multidetector. With the E6 instrument the data were recorded between the 28 values of 8 and 68” at 2 K. The temperature dependence of the magnetic intensities of the peak 100 at 20 = 37.0” was measured with the E6 diffractometer because of the higher neutron flux using the 20” multidetector. Further details of these instruments are described elsewhere [9]. The Rietveld refinements of the neutron powder diffraction data were carried out with the programme FULLPROF [lo]. The neutron scattering lengths used were b(U) = 8.417fm, b(Ni) = 10.3Ofm and b(P) = 5.13 fm [l 11. The magnetic form factors of uranium were taken from Ref. [ 121assuming U4+, although the results do not change much when form factors for U3’ are used.
3. RESULTS AND DISCUSSION
I I
"3"i3.34P6
T [KlFig. 2. The temperature dependence of the reciprocal susceptibility of U3Ni3.s4Ps at different flux densities. The
straight line represents the least-squares fit according to the Curie-Weiss law.
The structure of U3Ni3,s4Ps is tetragonal (Fig. 1) with two different uranium sites at the positions 2h and la of the space group P4/mmm (Table 1). The nickel atoms were found [7] with partial occupancy at the site 4i, and of the three different phosphorus positions one is the split position 4m (0.451, 0, l/2), slightly off the position 2e (l/2, 0, l/2). At first the nuclear structure was refined from the E2 data measured at room temperature. It was possible to refine both the thermal and the occupancy parameters, and only the parameter of the nickel position showed a significant deviation from the full occupancy. Nevertheless, in the final least-squares refinements the thermal parameters were held at the values previously obtained from the single-crystal
Table 1. Refinement results of the tetragonal (P4/mmm) U3Ni3.s4P6structure? Instrument Temperature [K]
CAD4 300
E2 300
E2 2
a bml c bml c/a V [nm3]
381.82(3) 1350.1(2) 3.536 0.1968(l)
3815(l) 1350.2(3) 3.539 0.1965(l)
380.6( 1) 1348.3(3) 3.543 0.1953(l)
Y
z
z
.r
B
112
0.34794(4) 0 0.1733(l) 0.2702(4) 0.0857(4) l/2 0.018(200)
0.3471(S) 0 0.1727(9) 0.268(2) 0.086(l) l/2 0.053(117)
0.3466(7) 0 0.1741(8) 0.270(l) 0.085(l) l/2 0.061(117)
0.317(7) 0.31(l) 0.53(3) 0.44(8) 0.50(S) 0.42(9)
Atom
Site
x
Ul 2h 112 u2 ill 0 Ni 4i 0 PI 0 2g P2 2h 112 P3 4m 0.451(2) Residual (no. of F values)
0 112
0 112
0
CAD4 300
tThe lattice constants and positional parameters as obtained from the four-circle CAD4 X-ray data and the neutron powder diffraction data (E2 instrument) are compared. During the Rietveld refinement of the neutron data the positional parameter x of the P3 position and the thermal parameters were fixed at the values as obtained from the X-ray data. Standard deviations in the place values of the last listed digits are given in parentheses.
523
Magnetic properties of the ternary phosphide U,N& MP, X-ray data, to reduce the standard deviations of the positional (and later also the magnetic) parameters. It can be seen (Table 1) that the agreement between the results obtained from the present neutron diffraction data and from the four-circle X-ray diffractometer data is rather good, albeit the latter are more accurate. Nevertheless, the partial occupancy of the nickel site was confirmed. The occupancy of 75(2)% is within four standard deviations equal to the value of 83.6(5)% obtained from the X-ray data. The conventional residuals for structure factors obtained for the refinements of the nuclear structure from the data collected at 2K and at room temperature are also listed in Table 1. The neutron diffraction powder patterns recorded at 2 K did not contain any additional reflections and therefore the magnetic structure did not require a larger cell. The 001 reflection did not contain any additional intensity, while the 100 reflection showed a strong increase of its intensity as compared to the room temperature data. Therefore a structure, where the magnetic moments are aligned parallel to the tetragonal c axis was suggested. A reasonable fit between the observed and calculated intensities was obtained for the ferrimagnetic structure as shown in Fig. 1 already with the data recorded with the E2 instrument. Nevertheless, another d;ra set was collected at the E6 instrument, which gives a better statistics and a better resolution due to a higher flux and a larger wavelength. The refinements of these data resulted in magnetic moments of P,,~(~) (Ul) = 1.4 & 0.1 pg and pexpcn)(U2) = 1.6 f 0.2~~. A residual of R = 0.091 was obtained for 11 intensity values of the magnetic structure. A comparison of the intensities for the magnetic structure is given in Table 2. The ferrimagnetic structure obtained from the neutron diffraction experiments was confirmed by the magnetization measurements on the SQUID magnetometer. Figure 3 shows the hysteresis loop as obtained at 5K. It can be seen that the hysteresis at this temperature is rather small with a remanence
I 7
9 s
0.4
0.2
0
‘Z
m
A ‘G
cm-0.2
P I -0.4
"S"3.34h L---l
I
-6
-4
-2
L 0
I
4
2 B
6
VI -
Fig. 3. Isothermal variation of the magnetization as a function of the applied flux density B measured at 5 K. The inset shows the magnetization at low flux densities.
of 0.19 f 0.01 pa (0.97 f 0.05Am2/kg) and a coercitivity of about 0.005 fO.OO1 T. The saturation magnetization is also very small with a value of ~exp(smj= 0.41 * 0.01 pa. This value should be compared with the difference of the magnetic moments as obtained by the neutron diffractionstudy. Since there are two Ul atoms with their moments parallel to the c axis and only one U2 atom per cell with its moment antiparallel to the c axis the resulting moment corresponds to A/lexr+) = ~.cL~~~(,,)(U~)- CL,,~(~)(U~)= 1.2 f 0.3 pa. This value is much higher (however, with a large standard deviation) than the value obtained from the saturation magnetization, because in a magnetization experiment of a compacted powder sample with random orientation of the crystallites the full magnetization cannot be reached. The resulting moments are aligned parallel to the c axis in the magnetic field, but the crystallites are oriented randomly. The reciprocal susceptibility curve of U3Ni3,34P6 (Fig. 2) shows a sharp maximum at 130 f 2 K. This unusual behaviour is also found for single crystals and therefore it must be an intrinsic property of that compound. The maximum in the reciprocal susceptibility curve is reflected in the temperature dependence
Table 2. Observed and calculated magnetic intensities for U3Ni3.s4P6 hkl
I,
I,
A
003 100 101 004 102 103 005 110 111 104 112
233 14 99 58 22
0 226 0 0 5 102 0 5 65 1 29
0 I 0 0 9 -3 0 -5 -1 -1 -1
T [Kl Fig. 4. Temperature dependence of the spontaneous magnetization of U3Ni3 34P6.
524
M. REEHUIS er al.
of the spontaneous magnetization (Fig. 4). This curve was drawn by extracting the spontaneous magnetization (obtained by extrapolation to zero magnetic field) from a series of magnetization curves, recorded at different temperatures, of the type shown in Fig. 3 for 5 K. These magnetization curves were practically linear for the range of the magnetic field between 0.2 and 3.0 T for all temperatures up to the curve obtained at 138K (i.e., up to the Curie temperature) and therefore no Arrott plots were needed to obtain the spontaneous magnetization as shown in Fig. 4. Nevertheless, we have also used Arrott plots, evaluating the magnetization data recorded with magnetic flux densities of less than OST. The results look very similar to those shown in Fig. 4, except that the minimum at 130K almost reaches a spontaneous magnetization of zero pa/f.u. This curve of Fig. 4 looks very similar to the curves calculated for a ferrimagnet with a compensation point [13-151. As an example for such a calculation we have assumed that the magnetic moments of the two ferrimagnetically orientated species (in our case the moments of the Ul and U2 atoms) follow Brillouin functions, which in the vicinity of the Curie temperature TC can be approximated by ,u N (1 - T/Tc)‘, where p is a critical exponent. In Fig. 5 we show the results of this calculation (which is not a fit), where the magnetic moments for the two Ul atoms are orientated antiparallel to the moment of the U2 atom. For T = 0 K the moments were assumed to correspond to those obtained from the neutron diffraction experiTo simulate their orientation behaviour with
I 0
I 50
100
150
T [Kl Fig. 5. Result of a model calculation for the spontaneous magnetization of U3Ni3.34P6.The magnetic moments of the two Ul atoms per cell are oriented antiparallel to that of the U2 atom. It is assumed, that their orientations have temperature dependences following two Brillouin functions, which can be approximated by p N (1 - T/Tc)O. In the model calculation the critical exponents p were assumed to have the values of 0.5 and 0.3 for the Ul and U2 atoms, respectively. The absolute values of the difference curve have a minimum at 130 K, which is the compensation point of this ferrimagnet. The magnitudes of the moments at T = OK were assumed to be equal to the moments as obtained from the neutron diffraction data.
“3’43.34% _I 0
20
40
60
all
100
120 T
140
160
[Kl -
Fig. 6. Temperature dependence of the magnetic intensity of the reflection 100.The solid line in the diagram is only a guide for the eye. temperature we have chosen a pair of /3 values (0.5 for Ul and 0.3 for U2), which result in a minimum at the experimentally observed compensation point of 130 K. It can be seen, that such a compensation point will occur in all cases, whenever the smaller magnetic moments of a ferrimagnet maintain their orientation better with increasing temperature than the larger moments of antiparallel orientation. In principle such a compensation point could also be seen by the temperature behaviour of those magnetic reflections with structure factors, where the ferrimagnetically oriented moments are subtracted from each other, e.g., the 200 reflection in U3Ni3.34P6. Unfortunately (but of course by definition) such magnetic reflections are always weak and in our case they are superimposed by strong nuclear reflections, and thus we could not observe this compensation point in the neutron diffraction data. We have, however, followed the temperature dependence of the magnetic contribution for the reflection 100 (Fig. 6). For this reflection the structure factor contains the contribution of the U 1 and U2 moments in an additive way. It can be seen, that this Brillouin function approaches zero at the Curie temperature of about TC = 139K in good agreement with the Curie temperature of 139 f 2 K observed from the SQUID data. Acknowledgements--We
thank the Hoechst AG, Werk Knapsack and Dr. G. Hiifer (Heraeus Quarzschmelze, Hanau) for generous gifts of ultrapure red phosphorus and silica tubes. This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. REFERENCES 1. Zolnierek Z., Kaczorowski D. and Tr& R., J. LessCommon Mer. 121, 193 (1986). 2. Fischer P., Murasik A., Kaczorowski D. and TroC R., Physica B 156 & 157,829 (1989). 3. Burlet P., Trod R., Kaczorowski D., No&l H. and Rossat-Mignod J., J. Magn. Mugn. Ma&r. 130, 237 (1994).
Magnetic properties of the ternary phosphide U3Ni3,s4P6 4. Kaczorowski D., Potel M. 8c No&l H., J. Solid Stare Chem. 112,228 (1994). 5. Jeitschko W., Terbiichte L. J., Reinbold E. J., Pollmeier P. G. and Vomhof T., .I. Less-Common Me!. 161, 125 (1990). 6. Reehuis M., Vomhof T. and Jeitschko W., J. Phys. Chem. Solids 55,625 (1994). 7. Ebel T. and Jeitschko W., J. Solid State Chem. 116,307 (1995). 8. Albering J. H. and Jeitschko W., Z. Kristallogr. Suppl. 7, 5 (1993). 9. Robertson T., Neutron Scattering-Instrumentation at the Upgraded Research Reactor BERII. Berlin Neutron Scattering Center, BENSC, Berlin (1992).
525
10. Rodriguez-Carvajal J., FULLPROF: A Program for Rietveld Refinement and Pattern Matching Analysis, Abstracts of the Satellite Meeting on Powder-Diffraction of the XV Coneress of the IUCr. D. 127. Toulouse (1990). 11. Sears V. F., ALmic Energy of Canada Limited, AECL8490 (1984). 12. Brown, P. J., Magnetic Form Factors, in International Tables for Crystallography (Edited by A. J. C. Wilson) Vol. C, p. 391. Kluwer Academic Press, Dordrecht (1992). 13. Neel L., Ann. Phys. (Paris), Ser. 12,3, 137 (1948). 14. Goodenough, J. B., Magnetism and the Chemical Bond. Wiley, New York (1963). 15. Crangle J., Solid Siate Magnetism. Edward Arnold, London (1991).
Pergamon
MAGNETIC
J PhJs. Chem Solr& Vol57, No. 5, pp. 521-525, 1996 CopyrIght c: 1996 Elsevier Science Ltd Printed in Great Bntain. All rights reserved 0022-3697i96 $15.04 + 0.00
PROPERTIES
M. REEHUIS,?
T. EBEL,$
OF THE TERNARY W. JEITSCHKO,$
PHOSPHIDE
R. SONNTAG?
U3Ni3,34P6
and N. STijSSERf.
tHahn-Meitner-Institut, Glienicker Strasse 100, D-14109 Berlin, Germany fAnorganisch-Chemisches Institut, Universitiit Miinster, Wilhelm-Klemm-Strasse 8. D-48149 Miinster. Germany (Received 9 March 1995; accepted in revisedform 28 October 1995)
Abstract-The magnetic order of the uranium sublattice of U3Ni3.34P6was investigated for polycrystalline samples with a SQUID magnetometer and by neutron diffraction. Below the Curie temperature 7’~ = 139 f 2 K the magnetic moments on the two different uranium sites (with the Wyckoff notations 2h and la of the space group P4/mmm) were found to order parallel to the tetragonal c axis and antiparallel to each other, resulting in ferrimagnetism. The nickel atoms do not carry magnetic moments. Rietveld refinements resulted in the magnetic moments of peexp (“)(Ul) = 1.4 f 0. I pBand pcXp(“I(U2) = I .6 & 0.2 pa at 2 K. Accordingly, a magnetic moment along the c axis of AP,,~~~) = 2. ~l~~~(~)(Ul)- ,L,,~(~,(U2) = 1.2f 0.3 /~a is obtained. This ferrimagnetism was confirmed by the magnetization measurements, where a saturation moment pexp(sm)= 0.41 f 0.01 pa was observed. This value is reduced, because it is obtained from a randomly orientated powder sample. A minimum in the magnetization curve was observed at 130 K. which is rationalized as the compensation point of that ferrimagnet. Keywords:
A. inorganic compounds, C. neutron scattering, D. magnetic properties, D. magnetic structure.
1. INTRODUCTION
structure
The magnetic properties of various ternary phosphides were investigated during recent years. Antiferromagnetic and ferromagnetic behaviour of the uranium sublattices were found for UN&P, [ 1, 21, UCuzPz [1], UCuP2 [I] and U2Cu2P30 [3, 41. In these compounds the nickel and copper atoms are essentially nonmagnetic, while in UMhP2 [5] and UCozPz [6] the transition metal atoms carry magnetic moments, which order at higher temperatures than the uranium moments. Recently the tetragonal crystal structure (Fig. 1) of the new ternary phosphide U3Ni3.34P6 was determined from X-ray data. It may be considered as being composed by ThCr2Siz-type building blocks of the composition U3Ni4P4, which are separated from each other by an additional phosphorus layer resulting in the ideal composition U3NidP6 [7]. As is known for the simple ThCrZSiztype compound UNi2_xP2 [1, 2, 81 considerable defects were observed for the nickel sites resulting in the composition U3Ni3,34P6. Magnetic susceptibility measurements (Fig. 2) indicated Curie-Weiss behaviour above the ordering temperature Tc = 139 f 2 K with an average paramagnetic moment of pexp(pj = 2.1 f 0.1 PB per uranium atom. The maximum at 130 f 2 K in the reciprocal susceptibility curve was interpreted as the compensation point assuming U3Ni3,34P6 to be a ferrimagnet. This is confirmed in the present paper by magnetization measurements. In addition we report the magnetic
diffraction
of U3Ni334P6
as obtained
from
neutron
data.
2. EXPERIMENTAL
A sample of U3Ni3.34P6 was prepared by reaction of the elemental components in a tin flux with an atomic ratio U : Ni : P : Sn = 3 : 4 : 15: 32 essentially as described earlier [7]. Ten samples with the total weight of about 45g were prepared to yield 9.5g of U3Ni3.34P6after the tin-rich matrix had been dissolved in slightly diluted (1 : 1) hydrochloric acid. The Guinier powder patterns of the samples showed small amounts of NiP2, U02 and UP2, as the only impurities. The temperature and field dependence of the magnetic moments were measured in the temperature range between 5 and 300K using a SQUID magnetometer (Quantum Design, Inc.). The compacted polycrystalline samples were cooled down in zero magnetic field. They were measured with increasing temperature or increasing magnetic flux density, respectively. Very similar results were obtained from several samples, prepared with slightly different starting compositions. The neutron powder diffraction measurements were carried out on the instruments E2 and E6 at the BERII reactor of the HMI at Berlin. The E2 instrument uses a germanium monochromator with the 3 11 reflection (X = 121.8pm) and the E6 instrument has a double
521
M. REEHUIS et al.
Fig. 1. Crystal and magnetic structure of U3Ni3.34P6. The magnetic moments of the two different uranium sites are found to order parallel to the tetragonal c axis and antiparallel to each other.
focussing graphite monochromator using the 002 reflection (X = 238pm). The powder data of U3Ni3.s4P6 were recorded on the E2 diffractometer to the 20 range from 4 to 84” with the 80” multidetector. With the E6 instrument the data were recorded between the 28 values of 8 and 68” at 2 K. The temperature dependence of the magnetic intensities of the peak 100 at 20 = 37.0” was measured with the E6 diffractometer because of the higher neutron flux using the 20” multidetector. Further details of these instruments are described elsewhere [9]. The Rietveld refinements of the neutron powder diffraction data were carried out with the programme FULLPROF [lo]. The neutron scattering lengths used were b(U) = 8.417fm, b(Ni) = 10.3Ofm and b(P) = 5.13 fm [l 11. The magnetic form factors of uranium were taken from Ref. [ 121assuming U4+, although the results do not change much when form factors for U3’ are used.
3. RESULTS AND DISCUSSION
I I
"3"i3.34P6
T [KlFig. 2. The temperature dependence of the reciprocal susceptibility of U3Ni3.s4Ps at different flux densities. The
straight line represents the least-squares fit according to the Curie-Weiss law.
The structure of U3Ni3,s4Ps is tetragonal (Fig. 1) with two different uranium sites at the positions 2h and la of the space group P4/mmm (Table 1). The nickel atoms were found [7] with partial occupancy at the site 4i, and of the three different phosphorus positions one is the split position 4m (0.451, 0, l/2), slightly off the position 2e (l/2, 0, l/2). At first the nuclear structure was refined from the E2 data measured at room temperature. It was possible to refine both the thermal and the occupancy parameters, and only the parameter of the nickel position showed a significant deviation from the full occupancy. Nevertheless, in the final least-squares refinements the thermal parameters were held at the values previously obtained from the single-crystal
Table 1. Refinement results of the tetragonal (P4/mmm) U3Ni3.s4P6structure? Instrument Temperature [K]
CAD4 300
E2 300
E2 2
a bml c bml c/a V [nm3]
381.82(3) 1350.1(2) 3.536 0.1968(l)
3815(l) 1350.2(3) 3.539 0.1965(l)
380.6( 1) 1348.3(3) 3.543 0.1953(l)
Y
z
z
.r
B
112
0.34794(4) 0 0.1733(l) 0.2702(4) 0.0857(4) l/2 0.018(200)
0.3471(S) 0 0.1727(9) 0.268(2) 0.086(l) l/2 0.053(117)
0.3466(7) 0 0.1741(8) 0.270(l) 0.085(l) l/2 0.061(117)
0.317(7) 0.31(l) 0.53(3) 0.44(8) 0.50(S) 0.42(9)
Atom
Site
x
Ul 2h 112 u2 ill 0 Ni 4i 0 PI 0 2g P2 2h 112 P3 4m 0.451(2) Residual (no. of F values)
0 112
0 112
0
CAD4 300
tThe lattice constants and positional parameters as obtained from the four-circle CAD4 X-ray data and the neutron powder diffraction data (E2 instrument) are compared. During the Rietveld refinement of the neutron data the positional parameter x of the P3 position and the thermal parameters were fixed at the values as obtained from the X-ray data. Standard deviations in the place values of the last listed digits are given in parentheses.
523
Magnetic properties of the ternary phosphide U,N& MP, X-ray data, to reduce the standard deviations of the positional (and later also the magnetic) parameters. It can be seen (Table 1) that the agreement between the results obtained from the present neutron diffraction data and from the four-circle X-ray diffractometer data is rather good, albeit the latter are more accurate. Nevertheless, the partial occupancy of the nickel site was confirmed. The occupancy of 75(2)% is within four standard deviations equal to the value of 83.6(5)% obtained from the X-ray data. The conventional residuals for structure factors obtained for the refinements of the nuclear structure from the data collected at 2K and at room temperature are also listed in Table 1. The neutron diffraction powder patterns recorded at 2 K did not contain any additional reflections and therefore the magnetic structure did not require a larger cell. The 001 reflection did not contain any additional intensity, while the 100 reflection showed a strong increase of its intensity as compared to the room temperature data. Therefore a structure, where the magnetic moments are aligned parallel to the tetragonal c axis was suggested. A reasonable fit between the observed and calculated intensities was obtained for the ferrimagnetic structure as shown in Fig. 1 already with the data recorded with the E2 instrument. Nevertheless, another d;ra set was collected at the E6 instrument, which gives a better statistics and a better resolution due to a higher flux and a larger wavelength. The refinements of these data resulted in magnetic moments of P,,~(~) (Ul) = 1.4 & 0.1 pg and pexpcn)(U2) = 1.6 f 0.2~~. A residual of R = 0.091 was obtained for 11 intensity values of the magnetic structure. A comparison of the intensities for the magnetic structure is given in Table 2. The ferrimagnetic structure obtained from the neutron diffraction experiments was confirmed by the magnetization measurements on the SQUID magnetometer. Figure 3 shows the hysteresis loop as obtained at 5K. It can be seen that the hysteresis at this temperature is rather small with a remanence
I 7
9 s
0.4
0.2
0
‘Z
m
A ‘G
cm-0.2
P I -0.4
"S"3.34h L---l
I
-6
-4
-2
L 0
I
4
2 B
6
VI -
Fig. 3. Isothermal variation of the magnetization as a function of the applied flux density B measured at 5 K. The inset shows the magnetization at low flux densities.
of 0.19 f 0.01 pa (0.97 f 0.05Am2/kg) and a coercitivity of about 0.005 fO.OO1 T. The saturation magnetization is also very small with a value of ~exp(smj= 0.41 * 0.01 pa. This value should be compared with the difference of the magnetic moments as obtained by the neutron diffractionstudy. Since there are two Ul atoms with their moments parallel to the c axis and only one U2 atom per cell with its moment antiparallel to the c axis the resulting moment corresponds to A/lexr+) = ~.cL~~~(,,)(U~)- CL,,~(~)(U~)= 1.2 f 0.3 pa. This value is much higher (however, with a large standard deviation) than the value obtained from the saturation magnetization, because in a magnetization experiment of a compacted powder sample with random orientation of the crystallites the full magnetization cannot be reached. The resulting moments are aligned parallel to the c axis in the magnetic field, but the crystallites are oriented randomly. The reciprocal susceptibility curve of U3Ni3,34P6 (Fig. 2) shows a sharp maximum at 130 f 2 K. This unusual behaviour is also found for single crystals and therefore it must be an intrinsic property of that compound. The maximum in the reciprocal susceptibility curve is reflected in the temperature dependence
Table 2. Observed and calculated magnetic intensities for U3Ni3.s4P6 hkl
I,
I,
A
003 100 101 004 102 103 005 110 111 104 112
233 14 99 58 22
0 226 0 0 5 102 0 5 65 1 29
0 I 0 0 9 -3 0 -5 -1 -1 -1
T [Kl Fig. 4. Temperature dependence of the spontaneous magnetization of U3Ni3 34P6.
524
M. REEHUIS er al.
of the spontaneous magnetization (Fig. 4). This curve was drawn by extracting the spontaneous magnetization (obtained by extrapolation to zero magnetic field) from a series of magnetization curves, recorded at different temperatures, of the type shown in Fig. 3 for 5 K. These magnetization curves were practically linear for the range of the magnetic field between 0.2 and 3.0 T for all temperatures up to the curve obtained at 138K (i.e., up to the Curie temperature) and therefore no Arrott plots were needed to obtain the spontaneous magnetization as shown in Fig. 4. Nevertheless, we have also used Arrott plots, evaluating the magnetization data recorded with magnetic flux densities of less than OST. The results look very similar to those shown in Fig. 4, except that the minimum at 130K almost reaches a spontaneous magnetization of zero pa/f.u. This curve of Fig. 4 looks very similar to the curves calculated for a ferrimagnet with a compensation point [13-151. As an example for such a calculation we have assumed that the magnetic moments of the two ferrimagnetically orientated species (in our case the moments of the Ul and U2 atoms) follow Brillouin functions, which in the vicinity of the Curie temperature TC can be approximated by ,u N (1 - T/Tc)‘, where p is a critical exponent. In Fig. 5 we show the results of this calculation (which is not a fit), where the magnetic moments for the two Ul atoms are orientated antiparallel to the moment of the U2 atom. For T = 0 K the moments were assumed to correspond to those obtained from the neutron diffraction experiTo simulate their orientation behaviour with
I 0
I 50
100
150
T [Kl Fig. 5. Result of a model calculation for the spontaneous magnetization of U3Ni3.34P6.The magnetic moments of the two Ul atoms per cell are oriented antiparallel to that of the U2 atom. It is assumed, that their orientations have temperature dependences following two Brillouin functions, which can be approximated by p N (1 - T/Tc)O. In the model calculation the critical exponents p were assumed to have the values of 0.5 and 0.3 for the Ul and U2 atoms, respectively. The absolute values of the difference curve have a minimum at 130 K, which is the compensation point of this ferrimagnet. The magnitudes of the moments at T = OK were assumed to be equal to the moments as obtained from the neutron diffraction data.
“3’43.34% _I 0
20
40
60
all
100
120 T
140
160
[Kl -
Fig. 6. Temperature dependence of the magnetic intensity of the reflection 100.The solid line in the diagram is only a guide for the eye. temperature we have chosen a pair of /3 values (0.5 for Ul and 0.3 for U2), which result in a minimum at the experimentally observed compensation point of 130 K. It can be seen, that such a compensation point will occur in all cases, whenever the smaller magnetic moments of a ferrimagnet maintain their orientation better with increasing temperature than the larger moments of antiparallel orientation. In principle such a compensation point could also be seen by the temperature behaviour of those magnetic reflections with structure factors, where the ferrimagnetically oriented moments are subtracted from each other, e.g., the 200 reflection in U3Ni3.34P6. Unfortunately (but of course by definition) such magnetic reflections are always weak and in our case they are superimposed by strong nuclear reflections, and thus we could not observe this compensation point in the neutron diffraction data. We have, however, followed the temperature dependence of the magnetic contribution for the reflection 100 (Fig. 6). For this reflection the structure factor contains the contribution of the U 1 and U2 moments in an additive way. It can be seen, that this Brillouin function approaches zero at the Curie temperature of about TC = 139K in good agreement with the Curie temperature of 139 f 2 K observed from the SQUID data. Acknowledgements--We
thank the Hoechst AG, Werk Knapsack and Dr. G. Hiifer (Heraeus Quarzschmelze, Hanau) for generous gifts of ultrapure red phosphorus and silica tubes. This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. REFERENCES 1. Zolnierek Z., Kaczorowski D. and Tr& R., J. LessCommon Mer. 121, 193 (1986). 2. Fischer P., Murasik A., Kaczorowski D. and TroC R., Physica B 156 & 157,829 (1989). 3. Burlet P., Trod R., Kaczorowski D., No&l H. and Rossat-Mignod J., J. Magn. Mugn. Ma&r. 130, 237 (1994).
Magnetic properties of the ternary phosphide U3Ni3,s4P6 4. Kaczorowski D., Potel M. 8c No&l H., J. Solid Stare Chem. 112,228 (1994). 5. Jeitschko W., Terbiichte L. J., Reinbold E. J., Pollmeier P. G. and Vomhof T., .I. Less-Common Me!. 161, 125 (1990). 6. Reehuis M., Vomhof T. and Jeitschko W., J. Phys. Chem. Solids 55,625 (1994). 7. Ebel T. and Jeitschko W., J. Solid State Chem. 116,307 (1995). 8. Albering J. H. and Jeitschko W., Z. Kristallogr. Suppl. 7, 5 (1993). 9. Robertson T., Neutron Scattering-Instrumentation at the Upgraded Research Reactor BERII. Berlin Neutron Scattering Center, BENSC, Berlin (1992).
525
10. Rodriguez-Carvajal J., FULLPROF: A Program for Rietveld Refinement and Pattern Matching Analysis, Abstracts of the Satellite Meeting on Powder-Diffraction of the XV Coneress of the IUCr. D. 127. Toulouse (1990). 11. Sears V. F., ALmic Energy of Canada Limited, AECL8490 (1984). 12. Brown, P. J., Magnetic Form Factors, in International Tables for Crystallography (Edited by A. J. C. Wilson) Vol. C, p. 391. Kluwer Academic Press, Dordrecht (1992). 13. Neel L., Ann. Phys. (Paris), Ser. 12,3, 137 (1948). 14. Goodenough, J. B., Magnetism and the Chemical Bond. Wiley, New York (1963). 15. Crangle J., Solid Siate Magnetism. Edward Arnold, London (1991).