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The magnetic structure of U2N2Te
Solid State Communications, Vol. 22, pp. 697—699, 1977.
Pergamon Press.
Printed in Great Britain
ThE MAGNETIC STRUCTURE OF U2N2Te J. Leciejewicz Institute of Nuclear Research, ~wierk Research Establishment, 05-400 Otwock, Poland and Z. Zolnierek and R. Troi Institute for Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wroclaw, POB 937, Poland (Received 21 February 1977 by E.F. Bertaut) The magnetic structure of tetragonal U2N2Te has been studied by means of neutron diffraction on polyciystailine sample. A ferromagnetic alignment of the magnetic moments below 68 K has been confirmed. The best agreement between the calculated and observed intensities of the magnetic reflections has been obtained for the moment direction forming an angle 70 ±50 to the tetragonal axis. The mapitude of the uranium ordered moment was found to be 2.50 ±0.05 ~
AMONG ternary uranium nitrides of general formula U2N2(M, Z) with the third element either a pnictide or a chalcogenide, those containing lighter elements, i.e. M = P, As, S or Se were found to be hexagonal (Ce202S-type of crystal structure) [1] and exhibit antiferromagnetic properties [2—4].The U2N2Z ternaries with heavier elements, where Z = Sb, Bi or Te, are tetragonal (space group 14/mmm) with c/a ratio amounting to about 3 [5]. They are ferromagnetic at low temperatures [2]. The magnetic properties of the latter have been recently studied in detail [6]. Contrary to the antimonide and bismuthide ternaries, U2N2Te with a Curie point of 71 K turned out to be a ferromagnet with an unusual magnetic behaviour. The magnetization of this compound at 4.2 K increases slowly with applied field up to H = S kOe. At this value, the magnetization begins to increase more rapidly but even at H = 140 kOe it is still far from saturation. The ordered magnetic moment2•78PB• of uranium, p~,at the thisthermal field achieves a value as Similarly, variation of the large as magnetization of U 2N2Te is quite different from that of U2N2Sb and U2N2Bi. For the former the magnetization increases with temperature and goes at first through a flat niveau at about 43 K and then through a fairly sharp maximum in the vicinity of the Curie temperature. At higher field strengths it still manifests itself as a deflection point persisting in applied fields close to 50 kOe. Thetoabove niveau remainsgradually well pronounced up fieldsmentioned of S kOe and disappears at higher field strengths. The magnetization curves at fields higher than 25 kOe attains an almost usual ferromagnetic behaviour.
In order to explain such an unusual properties of U2N2Te we have undertaken a neutron diffraction study to check wheather the observed effect is bound to any change in the direction of the magnetic moment. The powder sample of U2N2Te prepared after reference [2] was found by an X-ray to be single phase with the lattice constants: a = 3.967 ±0.002 A and c = 12.58 ±0.010 A. The powder neutron diffraction experiment was made on the DN 500-spectrometer using a wavelength of 1.32 ±0.01 A. Diffraction pattems were taken at room temperature, 100 and 4.2 K. The observed nuclear peaks at room temperature could be indexed in tetragonal unit cell of space group 14/mmm. There are two molecules in the unit cell, with the atoms at the following positions: 4U 4’ ‘OOu’ ‘00 at ~e u 4 N at 4(d) (0k): (~0*) ~
. ~
2 Te at 2(a) + (~~) (000) The u parameter was refmed by a least square method using the intensities of 8 peaks (11 reflections) observed on the neutron diffraction pattern taken at 100K. This parameter was found to be 0.3399 ±0.0012 yielding R = 3.99%. The neutron scattering amplitudes for U, N2cm, and Te were used[7]. to be 0.85, 0.94 and 0.57 x respectively 10’ Five first peaks observed on neutron diffraction pattern at 4.2 K contained fairly large magnetic contributions. These contributions were isolated by subtracting the peak intensities measured at about 100K 697
698
THE MAGNETIC STRUCTURE OF U2N2Te
Vol. 22, No. 11
2.80
I
101
~
U2N2Te
092,, 004
2.60 68±3K 8
21+0
(103~101)
101 112
7 0
220 50
100 —~-T[K]
Fig. 1. Temperature dependence of the (101) magnetic for the sample at the same geometrical conditions as at 4.2K. However, no purely magnetic diffraction peaks or satellites could be observed on the pattern at 4.2 K, which can indicate the ferromagnetic behaviour of this compound. The temperature dependence of a fairly large magnetic contribution to a small (101) nuclear peak shows only monotonous decrease with temperature yielding the Curie temperature of 68 ±3K (Fig. 1). Such a behaviour of this curve means that the niveau previouslyobservedaheadyat l600edoesnotappear at zero field strength within experimental errors. Among the magnetic peaks there are also the [001] reflections, as (002) or (004), indicating thus that magnetic aligned either the basalthe plane [001] moments or form anare ~-anglewith the in c-axis. The magnitude of the uranium ordered moment was calculated using the scaling factor, the u-parameter and the magnetic form factor of uranium atom having the 5f2-configuration [8]. The values of ji,. calculated from six reflections under an assumption that the moments lie exactly in the basal plane show relatively large discrepancy. Therefore we have performed a refmement of ~ as a function of 0 taking these six reflections into consideration. This function is presented in Fig. 2. The observed and calculated intensities of the magnetic reflections considered above for = 90 and 70°,respectively, are compared in Table 1. A proximity in the R values found for these two cases would not allow one to judge as to the 0 angle as it could be done on the basis of Fig. 2. However, due to known limitation of neutron powder method we were not able to determine uniquely the direction ofp~. vectors but only their angle with the tetragonal axis [9].
I
90
80
70
~[ I
60
Fig. 2. The magnitude of the calculated j.z~for given magnetic reflection hid in dependence on the 0 angle. Table 1. The calculated and observed integrated intensities of the low-angle resolved magnetic peaks h kI
1cpJc =
002 10 1 004 103 11 0 11 2
t* RR ==
6,246 8,135 2,800 19738 2,000 1.9%, 1.5%, ~ ~2, = 2.46 2.50
90~’
= 7()0~
6,565 7,402 3,224 19,112
5,949 8,188 2,921 19,214
1,890
2,059
0•101~~B• ± ±°•°5~B~
The magnitude of the ordered moment derived from neutron diffraction experiment without an external magnetic field is smaller by almost O.31.LB than that obtained from magnetization measurements of polycrystalline samples. In order to correlate this difference, if it does not originate from the polarization of the conduction electrons, and to explain the unusual temperature dependence of the magnetization it is necessary to undertake a neutron diffraction study of U 2N2Te in an external magnetic field. The obtained results indicate for the first time in the case of the ferromagnetic actinide compounds that the magnetic moment alignment is not usually along the c-axis but slightly tilted from the basal plane. This rather unexpected fact can be understood in terms of a peculiar scheme of low-lying crystal-field levels detailed discussed elsewhere [6].
Vol. 22, No. 11 Acknowledgements
THE MAGNETIC STRUCTURE OF U2N2Te —
699
The authors wish to thank Prof. W. Trzebiatowski for kind interest and Dr. J. Przystawa for
helpful discussions. The authors are indebted Mrs H. Ptasiewicz-B~kfor performing a least square refinement. REFERENCES 1.
BENZ R. & ZACHARIASEN W.H., Acta Cryst. B26, 823 (1970).
2.
TROC R. & ZOLNIEREK Z.,F~-oc.mt. Conf Magnetism Moscow 1973 (Edited by Nauka), Vol. 6, p. 59. Moscow (1974). ZOLNIEREK Z. & TROd R., Proc. 5th Tnt. Conf Plutonium and otherActinides, Baden-Baden, West Germany 1975 (Edited by BLANK H. & LINDNER R.), p. 589. North-Holland, Amsterdam (1976). LECIEJEWICZ J., ~OLNIEREK Z., LIGENZA S., TROd R. & PTASIEWICZ H., J. Phys. C: Solid State Phys. 8, 1697 (1975).
3. 4. 5.
BENZ R. & ZACHARIASEN W.H.,Acta C,yst. B25, 295 (1969).
6.
ZOLNIEREK Z. & TROd R. (to be published).
7.
Neutron Diffraction Commission International Union of Ciystallogr., Acta Cryst. A28, 357 (1972).
8.
WEDGWOOD F.A.,J. Phys. C: Solid State Phys. 5,2427 (l972),
9.
SHIRANE G.,Acta Cryst. 12, 282 (1959).
Pergamon Press.
Printed in Great Britain
ThE MAGNETIC STRUCTURE OF U2N2Te J. Leciejewicz Institute of Nuclear Research, ~wierk Research Establishment, 05-400 Otwock, Poland and Z. Zolnierek and R. Troi Institute for Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wroclaw, POB 937, Poland (Received 21 February 1977 by E.F. Bertaut) The magnetic structure of tetragonal U2N2Te has been studied by means of neutron diffraction on polyciystailine sample. A ferromagnetic alignment of the magnetic moments below 68 K has been confirmed. The best agreement between the calculated and observed intensities of the magnetic reflections has been obtained for the moment direction forming an angle 70 ±50 to the tetragonal axis. The mapitude of the uranium ordered moment was found to be 2.50 ±0.05 ~
AMONG ternary uranium nitrides of general formula U2N2(M, Z) with the third element either a pnictide or a chalcogenide, those containing lighter elements, i.e. M = P, As, S or Se were found to be hexagonal (Ce202S-type of crystal structure) [1] and exhibit antiferromagnetic properties [2—4].The U2N2Z ternaries with heavier elements, where Z = Sb, Bi or Te, are tetragonal (space group 14/mmm) with c/a ratio amounting to about 3 [5]. They are ferromagnetic at low temperatures [2]. The magnetic properties of the latter have been recently studied in detail [6]. Contrary to the antimonide and bismuthide ternaries, U2N2Te with a Curie point of 71 K turned out to be a ferromagnet with an unusual magnetic behaviour. The magnetization of this compound at 4.2 K increases slowly with applied field up to H = S kOe. At this value, the magnetization begins to increase more rapidly but even at H = 140 kOe it is still far from saturation. The ordered magnetic moment2•78PB• of uranium, p~,at the thisthermal field achieves a value as Similarly, variation of the large as magnetization of U 2N2Te is quite different from that of U2N2Sb and U2N2Bi. For the former the magnetization increases with temperature and goes at first through a flat niveau at about 43 K and then through a fairly sharp maximum in the vicinity of the Curie temperature. At higher field strengths it still manifests itself as a deflection point persisting in applied fields close to 50 kOe. Thetoabove niveau remainsgradually well pronounced up fieldsmentioned of S kOe and disappears at higher field strengths. The magnetization curves at fields higher than 25 kOe attains an almost usual ferromagnetic behaviour.
In order to explain such an unusual properties of U2N2Te we have undertaken a neutron diffraction study to check wheather the observed effect is bound to any change in the direction of the magnetic moment. The powder sample of U2N2Te prepared after reference [2] was found by an X-ray to be single phase with the lattice constants: a = 3.967 ±0.002 A and c = 12.58 ±0.010 A. The powder neutron diffraction experiment was made on the DN 500-spectrometer using a wavelength of 1.32 ±0.01 A. Diffraction pattems were taken at room temperature, 100 and 4.2 K. The observed nuclear peaks at room temperature could be indexed in tetragonal unit cell of space group 14/mmm. There are two molecules in the unit cell, with the atoms at the following positions: 4U 4’ ‘OOu’ ‘00 at ~e u 4 N at 4(d) (0k): (~0*) ~
. ~
2 Te at 2(a) + (~~) (000) The u parameter was refmed by a least square method using the intensities of 8 peaks (11 reflections) observed on the neutron diffraction pattern taken at 100K. This parameter was found to be 0.3399 ±0.0012 yielding R = 3.99%. The neutron scattering amplitudes for U, N2cm, and Te were used[7]. to be 0.85, 0.94 and 0.57 x respectively 10’ Five first peaks observed on neutron diffraction pattern at 4.2 K contained fairly large magnetic contributions. These contributions were isolated by subtracting the peak intensities measured at about 100K 697
698
THE MAGNETIC STRUCTURE OF U2N2Te
Vol. 22, No. 11
2.80
I
101
~
U2N2Te
092,, 004
2.60 68±3K 8
21+0
(103~101)
101 112
7 0
220 50
100 —~-T[K]
Fig. 1. Temperature dependence of the (101) magnetic for the sample at the same geometrical conditions as at 4.2K. However, no purely magnetic diffraction peaks or satellites could be observed on the pattern at 4.2 K, which can indicate the ferromagnetic behaviour of this compound. The temperature dependence of a fairly large magnetic contribution to a small (101) nuclear peak shows only monotonous decrease with temperature yielding the Curie temperature of 68 ±3K (Fig. 1). Such a behaviour of this curve means that the niveau previouslyobservedaheadyat l600edoesnotappear at zero field strength within experimental errors. Among the magnetic peaks there are also the [001] reflections, as (002) or (004), indicating thus that magnetic aligned either the basalthe plane [001] moments or form anare ~-anglewith the in c-axis. The magnitude of the uranium ordered moment was calculated using the scaling factor, the u-parameter and the magnetic form factor of uranium atom having the 5f2-configuration [8]. The values of ji,. calculated from six reflections under an assumption that the moments lie exactly in the basal plane show relatively large discrepancy. Therefore we have performed a refmement of ~ as a function of 0 taking these six reflections into consideration. This function is presented in Fig. 2. The observed and calculated intensities of the magnetic reflections considered above for = 90 and 70°,respectively, are compared in Table 1. A proximity in the R values found for these two cases would not allow one to judge as to the 0 angle as it could be done on the basis of Fig. 2. However, due to known limitation of neutron powder method we were not able to determine uniquely the direction ofp~. vectors but only their angle with the tetragonal axis [9].
I
90
80
70
~[ I
60
Fig. 2. The magnitude of the calculated j.z~for given magnetic reflection hid in dependence on the 0 angle. Table 1. The calculated and observed integrated intensities of the low-angle resolved magnetic peaks h kI
1cpJc =
002 10 1 004 103 11 0 11 2
t* RR ==
6,246 8,135 2,800 19738 2,000 1.9%, 1.5%, ~ ~2, = 2.46 2.50
90~’
= 7()0~
6,565 7,402 3,224 19,112
5,949 8,188 2,921 19,214
1,890
2,059
0•101~~B• ± ±°•°5~B~
The magnitude of the ordered moment derived from neutron diffraction experiment without an external magnetic field is smaller by almost O.31.LB than that obtained from magnetization measurements of polycrystalline samples. In order to correlate this difference, if it does not originate from the polarization of the conduction electrons, and to explain the unusual temperature dependence of the magnetization it is necessary to undertake a neutron diffraction study of U 2N2Te in an external magnetic field. The obtained results indicate for the first time in the case of the ferromagnetic actinide compounds that the magnetic moment alignment is not usually along the c-axis but slightly tilted from the basal plane. This rather unexpected fact can be understood in terms of a peculiar scheme of low-lying crystal-field levels detailed discussed elsewhere [6].
Vol. 22, No. 11 Acknowledgements
THE MAGNETIC STRUCTURE OF U2N2Te —
699
The authors wish to thank Prof. W. Trzebiatowski for kind interest and Dr. J. Przystawa for
helpful discussions. The authors are indebted Mrs H. Ptasiewicz-B~kfor performing a least square refinement. REFERENCES 1.
BENZ R. & ZACHARIASEN W.H., Acta Cryst. B26, 823 (1970).
2.
TROC R. & ZOLNIEREK Z.,F~-oc.mt. Conf Magnetism Moscow 1973 (Edited by Nauka), Vol. 6, p. 59. Moscow (1974). ZOLNIEREK Z. & TROd R., Proc. 5th Tnt. Conf Plutonium and otherActinides, Baden-Baden, West Germany 1975 (Edited by BLANK H. & LINDNER R.), p. 589. North-Holland, Amsterdam (1976). LECIEJEWICZ J., ~OLNIEREK Z., LIGENZA S., TROd R. & PTASIEWICZ H., J. Phys. C: Solid State Phys. 8, 1697 (1975).
3. 4. 5.
BENZ R. & ZACHARIASEN W.H.,Acta C,yst. B25, 295 (1969).
6.
ZOLNIEREK Z. & TROd R. (to be published).
7.
Neutron Diffraction Commission International Union of Ciystallogr., Acta Cryst. A28, 357 (1972).
8.
WEDGWOOD F.A.,J. Phys. C: Solid State Phys. 5,2427 (l972),
9.
SHIRANE G.,Acta Cryst. 12, 282 (1959).