- Email: [email protected]
Structural, magnetic and transport properties of NpIrSn
Journal of Alloys and Compounds 335 (2002) 77–80
L
www.elsevier.com / locate / jallcom
Structural, magnetic and transport properties of NpIrSn P. Javorsky´ a , *, L. Havela a , F. Wastin b , J.C. Griveau b , J. Rebizant b , G.H. Lander b , G. Rousse c a
b
Department of Electronic Structures, Charles University, Ke Karlovu 5, 12116 Prague 2, Czech Republic European Commission, Joint Research Centre, Institute for Transuranium Elements ( ITU), Postfach 2340, 76125 Karlsruhe, Germany c Institut Max von Laue Paul Langevin, rue Jules Horowitz, 38042 Grenoble Cedex 9, France Received 17 August 2001; accepted 3 September 2001
Abstract Structural, magnetic and electrical transport properties of a new compound, NpIrSn, crystallizing in the ZrNiAl-type hexagonal structure are reported. NpIrSn orders antiferromagnetically below T N 540 K and undergoes a second magnetic phase transition around 22 K. The antiferromagnetic order is not affected by magnetic fields up to 9 T. Powder neutron diffraction revealed an incommensurate magnetic structure in both magnetic phases. 2002 Elsevier Science B.V. All rights reserved. Keywords: Actinide compounds; Magnetically ordered materials; Neutron diffraction; Electrical transport; Magnetic measurements
1. Introduction The equiatomic actinide intermetallic compounds AnTX (An5actinide, T5transition metal, X5p-metal) form a large group of materials with various electronic properties [1]. In comparison to uranium-based compounds, the Npbased ones represent a relatively new subject for experimental studies [2–4]. In this paper, we present a study of NpIrSn, crystallizing in the ZrNiAl-type hexagonal structure. We present results of X-ray characterization, magnetic susceptibility, electrical resistivity and neutron diffraction measurements, all (except the X-ray characterization) on polycrystalline material.
2. Experimental A sample was prepared by arc-melting the pure metals in the stoichiometric ratio under an argon atmosphere. The ingot was melted four times to maximize homogeneity. Weight loss was checked after melting and found to be negligible. The characterization of the sample was performed by X-ray diffraction with a Debye–Scherrer camera, using a Philips type PW1120 / 90 generator with Cu Kb radiation filtered by Ni. Small single crystals isolated from the fragmented button were examined with an EnrafNonius CAD4 diffractometer using Mo Ka radiation. *Corresponding author. Fax: 1420-2-2491-5050. ´ E-mail address: [email protected] (P. Javorsky).
The magnetic susceptibility was studied on a sample with grains randomly oriented and fixed with an acetonebased glue. Measurements were performed using a SQUID-magnetometer (Quantum Design) in magnetic fields up to 7 T and in the temperature range 2–300 K. Electrical resistivity was measured on encapsulated bulk piece using a conventional four-probe AC method in the temperature range 4.2–300 K. Magnetic fields up to 9 T were applied perpendicular to the direction of electric current. The neutron diffraction experiment was performed with the D20 diffractometer at the Institute Laue-Langevin ˚ at several (ILL), Grenoble, at a wavelength l52.40 A selected temperatures between 4 and 80 K. The sample (mass¯0.6 g) was placed in a special container used for transuranium samples. Program MXD [5] was used for the refinement of the nuclear structure. The scattering factors and absorption cross-sections tabulated in Ref. [6] have been used in the analysis.
3. Results and discussion The X-ray analysis performed both on a powdered sample and small single crystals showed that NpIrSn crystallizes in the ZrNiAl-type hexagonal structure (space group P-62 m) with the lattice parameters a5735.0 pm and c5397.5 pm at room temperature (Fig. 1). These parameters as well as the c /a ratio are close to the values reported
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01846-1
78
P. Javorsky´ et al. / Journal of Alloys and Compounds 335 (2002) 77 – 80
Fig. 2. The H /M versus T dependence of NpNiSn measured in m0 H54 T and corrected for the ferromagnetic impurity; full line represents the fit to Eq. (1) with meff 52.48mB and Qp 5 219.6 K.
Fig. 1. Schematic picture of the two layers of the ZrNiAl-type crystal structure.
for isostructural UIrSn [7] and RIrSn (R5rare earth) [8]. Neutron diffraction patterns measured down to 4 K confirm this crystal structure in the whole temperature range. Table 1 summarizes the refined structural parameters. Small discrepancies between the results of the refinement of X-ray and neutron-diffraction data can be found for the Np and Sn atomic-position parameters. Although these parameters might change with temperature, we do not expect such a change as almost temperature-independent
values are found in all former studies on isostructural compounds [1]. Here we note that the neutron-data treatment was extremely difficult due to a large background scattering from the aluminum shielding. The aluminum shielding was a part of the safety procedure for this transuranium sample, and resulted in three times more Al atoms in the neutron beam than the sample atoms. Only five nuclear reflections, sufficiently separated from the aluminum peaks, can be used in the refinement. This is insufficient for a precise structure analysis. In the X-ray case, the absorption is difficult to account for, so both methods have their limitations. We find, in fact, that the neutron parameters are closer to those of isostructural uranium compounds. The temperature dependence of magnetic susceptibility in the paramagnetic region follows modified Curie–Weiss law (Fig. 2) 2 2 M NA m0 m B m eff x 5 ] 5 ]]]] 1 x0 H 3k B (T 2 up )
(1)
The fit to experimental data above 70 K gives the effective moment meff 52.48mB / Np, paramagnetic Curie tempera-
Table 1 Refined structural parameters for NpIrSn
Space group Structure type Lattice parameters
P-62 m (No. 189) ZrNiAl
Atomic positions
Np Ir Sn
Number of reflections refined R factors
3(g): x 0 1 / 2 2(c): 1 / 3 2 / 3 0 1(b): 0 0 1 / 2 3(f): x 0 0
X-ray diffraction (T5295 K)
Neutron diffraction (T580 K)
a5735.0(2) pm c5397.5(1) pm x50.5890(4)
a5732(1) pm c5396(1) pm x50.581(2)
x50.2537(6) 101 R p 53.53%
x50.225(2) 5 R wABS 51.78%
P. Javorsky´ et al. / Journal of Alloys and Compounds 335 (2002) 77 – 80
ture up 5 219.6 K and the temperature-independent term x0 54310 29 m 3 / mol. The other symbols in Eq. (1) have their usual meaning. The fitted effective moment is slightly reduced with respect to the theoretical value of 2.68mB for the Np 31 (5f 4 ) configuration obtained in the L–S coupling scheme. The negative paramagnetic Curie temperature up indicates a dominant antiferromagnetic exchange interaction in NpIrSn. The M /H versus T dependence, represented in Fig. 3, shows a maximum at T N 540 K marking the transition to the antiferromagnetic state. An additional magnetic phase transition can be seen at T 1 524 K. When applying the field of 7 T, the phase transitions shift to lower temperatures (39 and 22 K, respectively) which is also an indication of antiferromagnetic order. It is further corroborated by the magnetization curves (Fig. 4) which are linear up to the highest applied field of 7 T. The temperature dependence of electrical resistivity r (T ) of NpIrSn is represented in Fig. 5. The high absolute value (¯3 mV cm at room temperature) could be affected by shape uncertainties and possible cracks in the sample. Nevertheless, such a high value points to the narrow 5f-band intersected by the Fermi energy. The data measured in zero magnetic field show a transition related to magnetic ordering at 40 K, in agreement with the magnetization data. The overall behaviour does not change in external magnetic fields up to 9 T. As already mentioned, the powder neutron diffraction data taken at 80 K, i.e. well above T N , are in agreement with the ZrNiAl-type hexagonal crystal structure. We have not observed any change in the intensities of the nuclear peaks when cooling down to T54 K. This confirms that: (i) no structural changes occur and (ii) there is no ferromagnetic component. To study the magnetic structures, diffraction patterns with high statistics (¯10 h, the same as at 80 K) have been taken at T54 and 35 K, i.e. in both magnetic phases. Additional patterns with lower statistics (¯1 h) were taken at 9, 14, 21, 25, 30, 39, 44 and 60 K to follow the temperature development. Several purely magnetic reflections are seen in the differential patterns (IT 2I80K ), represented in Fig. 6. Three magnetic peaks appear at 4 K. A single propagation vector that is commensurate with the crystal lattice or a vector incommensurate along one of the axes cannot index all three. The search in the whole reciprocal space gives many vectors that describe these three reflections, all of them incommensurate, both along the c-axis and within the basal plane. It is not possible to choose one of them unambiguously and refine the details of the magnetic structure. Two new magnetic reflections appear at 35 K, whereas those observed at 4 K vanish, except for that at 438, which persists with considerably reduced intensity. The results of our search in the reciprocal space are similar to that at 4 K — the propagation vector is incommensurate, and cannot be determined unambiguously. The additional low-statistic patterns taken at different temperatures between 4 and 80
79
Fig. 3. The M /H versus T dependence of NpIrSn measured at 0.1 T (open circles) and 7 T (filled triangles).
Fig. 4. Magnetization curves measured on NpIrSn at different temperatures.
Fig. 5. Temperature dependence of electrical resistivity of NpIrSn.
P. Javorsky´ et al. / Journal of Alloys and Compounds 335 (2002) 77 – 80
80
below T N 540 K and undergoes additional magnetic phase transition at 24 K. The magnetic structure in both phases is incommensurate and probably rather complex. The antiferromagnetic order is not affected by magnetic fields at least up to 9 T.
Acknowledgements
Fig. 6. Differential diffraction patterns of NpIrSn.
K do not reveal any new reflections and confirm the presence of only two magnetic phases. No magnetic reflections appear in the 60 K pattern. ¨ We may also mention that Mossbauer experiments were performed on this compound, leading to rather complicated spectra with multiple Np sites. These results also point to a rather complex magnetic structure for both magnetic phases observed [9]. Comparison to isostructural UIrSn reveals substantially different behaviour. UIrSn orders ferromagnetically below T C 521–23 K [10,11] with the uranium magnetic moments along the c-axis. NpIrSn shows an antiferromagnetic order with complex magnetic structure that cannot be solved completely from our present data. The magnetic properties of the RIrSn compounds crystallizing in the same structure [8] have not yet been reported. Their study, technically certainly easier than for Np-based materials, and possible similarities to NpIrSn might be helpful to solve the details of magnetic order in NpIrSn.
4. Conclusion We can conclude that NpIrSn crystallizes in the hexagonal ZrNiAl-type structure, orders antiferromagnetically
The work was supported by the Grant Agency of the Czech Republic (grant No. 202 / 98 / P245). The high-purity Np metal required for the fabrication of this compound was made available through a loan agreement between Lawrence Livermore National Laboratory and ITU Karlsruhe, in the framework of a collaboration involving LLNL, Los Alamos National Laboratory, and the U.S. Department of Energy.
References ´ L. Havela, in: K.H.J. Buschow (Ed.), Handbook of [1] V. Sechovsky, Magnetic Materials, Vol. 11, North Holland, Amsterdam, 1998, pp. 1–289, and references cited therein. [2] F. Wastin, J.C. Spirlet, J. Rebizant, J. Alloys Comp. 219 (1995) 232. [3] Y. Kergadallan, Ph.D. Thesis, Universite´ de Paris XI Orsay, 1993. ´ L. Havela, F. Wastin, J.C. Griveau, E. Bednarczyk, J. [4] P. Javorsky, Rebizant, J.P. Sanchez, P. Vulliet, J. Alloys Comp. 283 (1999) 16. [5] P. Wolfers, J. Appl. Crystallogr. 23 (1990) 554. [6] V.F. Sears, Neutron News 3 (1992) 26. [7] A.V. Andreev, M.I. Bartashevich, Phys. Met. Metallogr. 62 (2) (1986) 50. [8] P. Salamakha, O. Sologub, J.K. Yakinthos, C.D. Routsi, J. Alloys Comp. 265 (1998) L1. [9] J.P. Sanchez, P. Vulliet, private communication. [10] R. Kruk, R. Kmiec, K. Latka, K. Tomala, R. Troc, V.H. Tran, Phys. Rev. B 55 (1997) 5851. [11] V.H. Tran, R. Troc, D. Badurski, J. Alloys Comp. 219 (1995) 285.
L
www.elsevier.com / locate / jallcom
Structural, magnetic and transport properties of NpIrSn P. Javorsky´ a , *, L. Havela a , F. Wastin b , J.C. Griveau b , J. Rebizant b , G.H. Lander b , G. Rousse c a
b
Department of Electronic Structures, Charles University, Ke Karlovu 5, 12116 Prague 2, Czech Republic European Commission, Joint Research Centre, Institute for Transuranium Elements ( ITU), Postfach 2340, 76125 Karlsruhe, Germany c Institut Max von Laue Paul Langevin, rue Jules Horowitz, 38042 Grenoble Cedex 9, France Received 17 August 2001; accepted 3 September 2001
Abstract Structural, magnetic and electrical transport properties of a new compound, NpIrSn, crystallizing in the ZrNiAl-type hexagonal structure are reported. NpIrSn orders antiferromagnetically below T N 540 K and undergoes a second magnetic phase transition around 22 K. The antiferromagnetic order is not affected by magnetic fields up to 9 T. Powder neutron diffraction revealed an incommensurate magnetic structure in both magnetic phases. 2002 Elsevier Science B.V. All rights reserved. Keywords: Actinide compounds; Magnetically ordered materials; Neutron diffraction; Electrical transport; Magnetic measurements
1. Introduction The equiatomic actinide intermetallic compounds AnTX (An5actinide, T5transition metal, X5p-metal) form a large group of materials with various electronic properties [1]. In comparison to uranium-based compounds, the Npbased ones represent a relatively new subject for experimental studies [2–4]. In this paper, we present a study of NpIrSn, crystallizing in the ZrNiAl-type hexagonal structure. We present results of X-ray characterization, magnetic susceptibility, electrical resistivity and neutron diffraction measurements, all (except the X-ray characterization) on polycrystalline material.
2. Experimental A sample was prepared by arc-melting the pure metals in the stoichiometric ratio under an argon atmosphere. The ingot was melted four times to maximize homogeneity. Weight loss was checked after melting and found to be negligible. The characterization of the sample was performed by X-ray diffraction with a Debye–Scherrer camera, using a Philips type PW1120 / 90 generator with Cu Kb radiation filtered by Ni. Small single crystals isolated from the fragmented button were examined with an EnrafNonius CAD4 diffractometer using Mo Ka radiation. *Corresponding author. Fax: 1420-2-2491-5050. ´ E-mail address: [email protected] (P. Javorsky).
The magnetic susceptibility was studied on a sample with grains randomly oriented and fixed with an acetonebased glue. Measurements were performed using a SQUID-magnetometer (Quantum Design) in magnetic fields up to 7 T and in the temperature range 2–300 K. Electrical resistivity was measured on encapsulated bulk piece using a conventional four-probe AC method in the temperature range 4.2–300 K. Magnetic fields up to 9 T were applied perpendicular to the direction of electric current. The neutron diffraction experiment was performed with the D20 diffractometer at the Institute Laue-Langevin ˚ at several (ILL), Grenoble, at a wavelength l52.40 A selected temperatures between 4 and 80 K. The sample (mass¯0.6 g) was placed in a special container used for transuranium samples. Program MXD [5] was used for the refinement of the nuclear structure. The scattering factors and absorption cross-sections tabulated in Ref. [6] have been used in the analysis.
3. Results and discussion The X-ray analysis performed both on a powdered sample and small single crystals showed that NpIrSn crystallizes in the ZrNiAl-type hexagonal structure (space group P-62 m) with the lattice parameters a5735.0 pm and c5397.5 pm at room temperature (Fig. 1). These parameters as well as the c /a ratio are close to the values reported
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01846-1
78
P. Javorsky´ et al. / Journal of Alloys and Compounds 335 (2002) 77 – 80
Fig. 2. The H /M versus T dependence of NpNiSn measured in m0 H54 T and corrected for the ferromagnetic impurity; full line represents the fit to Eq. (1) with meff 52.48mB and Qp 5 219.6 K.
Fig. 1. Schematic picture of the two layers of the ZrNiAl-type crystal structure.
for isostructural UIrSn [7] and RIrSn (R5rare earth) [8]. Neutron diffraction patterns measured down to 4 K confirm this crystal structure in the whole temperature range. Table 1 summarizes the refined structural parameters. Small discrepancies between the results of the refinement of X-ray and neutron-diffraction data can be found for the Np and Sn atomic-position parameters. Although these parameters might change with temperature, we do not expect such a change as almost temperature-independent
values are found in all former studies on isostructural compounds [1]. Here we note that the neutron-data treatment was extremely difficult due to a large background scattering from the aluminum shielding. The aluminum shielding was a part of the safety procedure for this transuranium sample, and resulted in three times more Al atoms in the neutron beam than the sample atoms. Only five nuclear reflections, sufficiently separated from the aluminum peaks, can be used in the refinement. This is insufficient for a precise structure analysis. In the X-ray case, the absorption is difficult to account for, so both methods have their limitations. We find, in fact, that the neutron parameters are closer to those of isostructural uranium compounds. The temperature dependence of magnetic susceptibility in the paramagnetic region follows modified Curie–Weiss law (Fig. 2) 2 2 M NA m0 m B m eff x 5 ] 5 ]]]] 1 x0 H 3k B (T 2 up )
(1)
The fit to experimental data above 70 K gives the effective moment meff 52.48mB / Np, paramagnetic Curie tempera-
Table 1 Refined structural parameters for NpIrSn
Space group Structure type Lattice parameters
P-62 m (No. 189) ZrNiAl
Atomic positions
Np Ir Sn
Number of reflections refined R factors
3(g): x 0 1 / 2 2(c): 1 / 3 2 / 3 0 1(b): 0 0 1 / 2 3(f): x 0 0
X-ray diffraction (T5295 K)
Neutron diffraction (T580 K)
a5735.0(2) pm c5397.5(1) pm x50.5890(4)
a5732(1) pm c5396(1) pm x50.581(2)
x50.2537(6) 101 R p 53.53%
x50.225(2) 5 R wABS 51.78%
P. Javorsky´ et al. / Journal of Alloys and Compounds 335 (2002) 77 – 80
ture up 5 219.6 K and the temperature-independent term x0 54310 29 m 3 / mol. The other symbols in Eq. (1) have their usual meaning. The fitted effective moment is slightly reduced with respect to the theoretical value of 2.68mB for the Np 31 (5f 4 ) configuration obtained in the L–S coupling scheme. The negative paramagnetic Curie temperature up indicates a dominant antiferromagnetic exchange interaction in NpIrSn. The M /H versus T dependence, represented in Fig. 3, shows a maximum at T N 540 K marking the transition to the antiferromagnetic state. An additional magnetic phase transition can be seen at T 1 524 K. When applying the field of 7 T, the phase transitions shift to lower temperatures (39 and 22 K, respectively) which is also an indication of antiferromagnetic order. It is further corroborated by the magnetization curves (Fig. 4) which are linear up to the highest applied field of 7 T. The temperature dependence of electrical resistivity r (T ) of NpIrSn is represented in Fig. 5. The high absolute value (¯3 mV cm at room temperature) could be affected by shape uncertainties and possible cracks in the sample. Nevertheless, such a high value points to the narrow 5f-band intersected by the Fermi energy. The data measured in zero magnetic field show a transition related to magnetic ordering at 40 K, in agreement with the magnetization data. The overall behaviour does not change in external magnetic fields up to 9 T. As already mentioned, the powder neutron diffraction data taken at 80 K, i.e. well above T N , are in agreement with the ZrNiAl-type hexagonal crystal structure. We have not observed any change in the intensities of the nuclear peaks when cooling down to T54 K. This confirms that: (i) no structural changes occur and (ii) there is no ferromagnetic component. To study the magnetic structures, diffraction patterns with high statistics (¯10 h, the same as at 80 K) have been taken at T54 and 35 K, i.e. in both magnetic phases. Additional patterns with lower statistics (¯1 h) were taken at 9, 14, 21, 25, 30, 39, 44 and 60 K to follow the temperature development. Several purely magnetic reflections are seen in the differential patterns (IT 2I80K ), represented in Fig. 6. Three magnetic peaks appear at 4 K. A single propagation vector that is commensurate with the crystal lattice or a vector incommensurate along one of the axes cannot index all three. The search in the whole reciprocal space gives many vectors that describe these three reflections, all of them incommensurate, both along the c-axis and within the basal plane. It is not possible to choose one of them unambiguously and refine the details of the magnetic structure. Two new magnetic reflections appear at 35 K, whereas those observed at 4 K vanish, except for that at 438, which persists with considerably reduced intensity. The results of our search in the reciprocal space are similar to that at 4 K — the propagation vector is incommensurate, and cannot be determined unambiguously. The additional low-statistic patterns taken at different temperatures between 4 and 80
79
Fig. 3. The M /H versus T dependence of NpIrSn measured at 0.1 T (open circles) and 7 T (filled triangles).
Fig. 4. Magnetization curves measured on NpIrSn at different temperatures.
Fig. 5. Temperature dependence of electrical resistivity of NpIrSn.
P. Javorsky´ et al. / Journal of Alloys and Compounds 335 (2002) 77 – 80
80
below T N 540 K and undergoes additional magnetic phase transition at 24 K. The magnetic structure in both phases is incommensurate and probably rather complex. The antiferromagnetic order is not affected by magnetic fields at least up to 9 T.
Acknowledgements
Fig. 6. Differential diffraction patterns of NpIrSn.
K do not reveal any new reflections and confirm the presence of only two magnetic phases. No magnetic reflections appear in the 60 K pattern. ¨ We may also mention that Mossbauer experiments were performed on this compound, leading to rather complicated spectra with multiple Np sites. These results also point to a rather complex magnetic structure for both magnetic phases observed [9]. Comparison to isostructural UIrSn reveals substantially different behaviour. UIrSn orders ferromagnetically below T C 521–23 K [10,11] with the uranium magnetic moments along the c-axis. NpIrSn shows an antiferromagnetic order with complex magnetic structure that cannot be solved completely from our present data. The magnetic properties of the RIrSn compounds crystallizing in the same structure [8] have not yet been reported. Their study, technically certainly easier than for Np-based materials, and possible similarities to NpIrSn might be helpful to solve the details of magnetic order in NpIrSn.
4. Conclusion We can conclude that NpIrSn crystallizes in the hexagonal ZrNiAl-type structure, orders antiferromagnetically
The work was supported by the Grant Agency of the Czech Republic (grant No. 202 / 98 / P245). The high-purity Np metal required for the fabrication of this compound was made available through a loan agreement between Lawrence Livermore National Laboratory and ITU Karlsruhe, in the framework of a collaboration involving LLNL, Los Alamos National Laboratory, and the U.S. Department of Energy.
References ´ L. Havela, in: K.H.J. Buschow (Ed.), Handbook of [1] V. Sechovsky, Magnetic Materials, Vol. 11, North Holland, Amsterdam, 1998, pp. 1–289, and references cited therein. [2] F. Wastin, J.C. Spirlet, J. Rebizant, J. Alloys Comp. 219 (1995) 232. [3] Y. Kergadallan, Ph.D. Thesis, Universite´ de Paris XI Orsay, 1993. ´ L. Havela, F. Wastin, J.C. Griveau, E. Bednarczyk, J. [4] P. Javorsky, Rebizant, J.P. Sanchez, P. Vulliet, J. Alloys Comp. 283 (1999) 16. [5] P. Wolfers, J. Appl. Crystallogr. 23 (1990) 554. [6] V.F. Sears, Neutron News 3 (1992) 26. [7] A.V. Andreev, M.I. Bartashevich, Phys. Met. Metallogr. 62 (2) (1986) 50. [8] P. Salamakha, O. Sologub, J.K. Yakinthos, C.D. Routsi, J. Alloys Comp. 265 (1998) L1. [9] J.P. Sanchez, P. Vulliet, private communication. [10] R. Kruk, R. Kmiec, K. Latka, K. Tomala, R. Troc, V.H. Tran, Phys. Rev. B 55 (1997) 5851. [11] V.H. Tran, R. Troc, D. Badurski, J. Alloys Comp. 219 (1995) 285.