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Crystal structure, magnetic and electrical transport properties of UPS single crystals
Pergamon
00223697EWOO132-4 . I
J. Phys. Chem. Soiids Vol. 55. No. 1 I. pp. 1363-1367. I994 Copyright ‘is 1994 Elwvier Science Lid Printed in tireal Britain. All rights nscrwd 0022-3697/94 -S7.00 + 0.00
CRYSTAL STRUCTURE, MAGNETIC AND ELECTRICAL TRANSPORT PROPERTIES OF UPS SINGLE CRYSTALS D. KACZOROWSKI,~~ H. NOEL,? M. POTELT and A. ZY~MUNT§ tLaboratoire
de Chimie du Solide et Inorganique Mofeculaire, Universite de Rennes, URA CNRS I495, 35042 Rennes, France §W. Trzebiatowski Institute for Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wrodaw, Poland (Received 12 November 1993; accepted 26 July 1994)
Abstract-The crystal structure of UPS was refined from single-crystal X-ray diffraction data to a residual of 0.032 for 10 variables and 336 structure factors. It is of the tetragonal ZrSiS type (space group P4/nmm) with the lattice parameters a = 3.8064(5) A and c = 7.981(2) A. The magnetic and electrical properties of UPS were measured on single crystals, within the temperature range 5-300K. The compound orders ferromagnetically below 118 K, with the magnetic moments aligned along the c-axis. At 5 K, the ordered magnetic moment amounts to 1.07 ps per U atom. Both in the ordered and paramagnetic regions, UPS exhibits a huge magnetic anisotropy. Above T,, the electrical resistivity of UPS decreases logarithmically with increasing tem~ratu~. Beiow 20 K., a p * Tz dependence is observed. Keywork:
A. chalcogenides, A. magnetic materials, D. crystal structure, D. electrical conductivity, D. magnetic properties.
up to form of thin plates with dimensions 5x2x0.2mm. The polycrystalline sample and powdered single crystals were examined by X-ray diffraction and found to be single phase. The X-ray patterns were easily indexed in the tetragonal system with the lattice parameters a = 3.810 A and c = 7.980 A. They agree well with the literature data [2]. Magnetic measurements on single-crystalline specimens were performed over the temperature range 5-300 K and in magnetic fields up to 5 T using a SHE SQUID magnetometer. The electrical resistivity was measured between 4.2 and 300 K using a conventional four-point direct current method.
1. INTRODUCTION
UPS belongs to a large family of tetragonal uranium compounds crystallizing in the PbFClor related crystal structures. Many of these materials have already been characterized with respect to their magnetic and electrical transport behaviour [l]; however, the properties of UPS are still hardly known. This ternary
compound
has been reported
to order ferro-
magnetically below about 120 K [2] but no results of any magnetic measurements were shown. In this paper, we present the magnetic and electrical transport properties of UPS, studied on single-crystalline specimens. refinement
Moreover, we report here on the X-ray of its crystal structure.
3. CRYSTAL STRUCTURE ~NEMENTS 2. EXPER~ENTAL
Single-crystal intensity data were collected on an automated four-circle diffractometer (Nonius CAD4) with graphite-monochromated MO Ku radiation. Least-squares refinement of the diffraction angles of 25 reflections yielded the lattice parameters a = 3.8064(5) A and c = 7.981(2) A. The observed systematic extinction rule of h + k # 2n for h/c0
The powder sample of UPS was prepared by heating stoichiometric quantities of the components in an evacuated quartz tube at 800°C for 7 days. The reaction product was used for the single-crystal growth by the chemical vapour transport method. Iodine served as a transporting agent, in the applied temperature gradient of 900-950°C. The crystals grew in a cooler part of the ampoule. They had the
reflections P4/nmm
2 On leave from: W. Trzebiatowski Institute for Low Temperature and Structure Research, Polish Academy of Sciences, B-950 WrocIaw, Poland. PCS SW I-hi
was compatible
with
the
space
group
reported previously for UPS [2]. The main crystallographic and experimental data are summarized in Table 1. A total of 744 integrated intensities was measured in the reciprocal sphere up
1363
D. KACZOROWSKI ef al.
1364
Table I. Crystallographic and experimental data for UPS Lattice constants u (A) c (A) v (A’) Formula units per cell Space group Formula weight Calculated density (g cm-‘) Absorption coefficient (cm-‘) Crystal dimensions (pm’) 0120 scans up to Range in hkl Absorption correction Total number of reflections with I z o(I) Merging R Unique reflections Reflections with I > 20 (I) Number of variables Conventional residual Weighted residual Goodness of fit
to 0 = 50”. An absorption correction was made from psi-scan data and later by DIFABS [3]. After averaging of equivalent reflections and omitting those with I < 2a (I), 336 independent intensities remained which were used for the structure refinements. The calculations were performed by full-matrix leastsquares cycles using the SPD program system [4]. The starting positional parameters for U, P and S were taken from the previous powder study [2]. To check the deviations from the ideal composition the occupancy parameters of the P and S atoms were allowed to vary. No significant deviations from the full occupancies were observed, therefore in the last cycles the idea1 occupancies were assumed with anisotropic thermal parameters for all the atoms. The final conventional and weighted residuals are R = 0.035 and R, = 0.032. The positional and thermal parameters are listed in Table 2, and the interatomic distances in Table 3. The structure factor tables are available from the authors. 4. MAGNETIC PROPERTIES
The temperature dependence of the magnetization of UPS single crystal, measured with magnetic field applied along the crystallographic c-axis (H IIc),is
3.8064 (5) 7.981 (2) 115.6 z=2 P4/nmm (No.129) 301.07 8.65 677 120x60~30 0 = 50” o-8,(r8,Cl7 PSISCAN + DIFABS 567 0.042 341 336 10 0.035 0.032 1.036
displayed in Fig. 1. In rough agreement with a previous powder study [2], UPS orders ferromagnetically at T, = 118 K. The B,,(T) variation, taken on the sample cooled without an applied magnetic field (zero field cooled ZFC), exhibits a characteristic broad maximum which is typical for highly anisotropic ferromagnets. It is well known that a strong magnetocrystalline anisotropy usually results in the formation of very narrow Bloch walls and thus in the occurrence of a pronounced domain effect. Indeed, the maximum in a,,(T)for UPS originates from the competition between the domain movement induced by the temperature (leading to an increase in the magnetization) and the thermal disorder in the magnetic moment system. Such a behaviour does not appear in the magnetization measured on the sample cooled in an applied magnetic field (field cooled FC) which may correspond to the results obtained on a single-domain crystal. A strong domain effect is also clearly visible on the low-field dependence of the u,, magnetization of UPS (see Fig. 2). Here, a high value of the nucleation field H. (H. = 3 kOe at 5 K) probably originates from the occurrence of a compensated narrow-wall domain structure of the 180” type which is rapidly reconstructed at a field H,. It is worthwhile noting that a
Table 2. Positional and thermal parameters in UPS
Atom
Site
X
U
2c 2a 2c
0.25 0.25 0.25
P S
Y 0.25 0.25 0.25
z 0.27324(6) 0 0.6343(4)
8, I = 822 0.308(5) 1.96(8) 0.42(4)
The form of the anisotropic thermal parameters is: exp(-fZuh,hia:a:/?,,), lattice parameter.
BJ, 0.36(l) 0.05(8) 0.08(7)
0.326(3) 1.32(3) 0.31(2)
where at is a reciprocal
1365
Properties of UPS single crystals
Table 3. Interatomic distances (in angstroms) in UPS u-4s 1s 4P 4u
2.791(1) 2.882(I) 2.894(2) 3.806(I)
P-4P 4U 4S
very similar behaviour of a,,(T) and c,,(H) was observed previously [5] for the UAsY (Y = S, Se, Te) ternaries for which huge anisotropy constants of the order of 10’ ergg-’ (anisotropy fields of about 800-1300 kOe) were estimated [6]. Our preliminary results for UPS point to anisotropy constants of the same order of magnitude as those for UAsY but more detailed investigations are needed to determine them accurately. Above H,, the magnetization of UPS increases rapidly but saturates in a field of about 5 kOe. The saturation value of c,, amounts to 19.6e.m.u. g-’ which corresponds to the ordered magnetic moment of 1.06 pLgper U atom. As is clear from the first magnetization curve (Fig. 2), UPS appears to be a magnetically hard material with the remanent magnetization as high at 98% and a nearly rectangular hysteresis loop (not shown). In contrast to q, the magnetization ul, taken within the basal plane (H J-c), is extremely low. It raises almost linearly with increasing magnetic field and does not exhibit any hysteresis effect (not shown). Thus, we can conclude that there is no transverse component of the spontaneous magnetic moment, i.e. the magnetic moments localized on the uranium atom sites are aligned along the c-axis. This statement is in line with the neutron diffraction data for other UXY ternaries [7l which are all uniaxial ferromagnets with the easy magnetization c-axis.
Tempcr~turc
2.692(I) 2.894(2) 3.484(2)
S-4U 1U 4S 4P
2.791(1) 2.882(1) 3.441(l) 3.484(2)
The temperature dependences of the transverse (H Ic) and longitudinal (H 11~)components of the reciprocal magnetic susceptibility of UPS are displayed in Fig. 1. As can be seen, in the whole paramagnetic region studied, x,, is at least three times larger than xl. This feature indicates that a large magnetocrystalline anisotropy in UPS is present also above T,. Both the x;‘(T) and x;‘(T) variations can be approximated by the Curie-Weiss law with the following parameters: 8 i = 123 K, & = 2.57 ps and 8 ,’ = -2703 K, r,‘R= 5.74 pe. Although these values have pure formal meaning and should be considered as the material characteristics only, they reflect a strongly uniaxial character of the magnetic susceptibility of UPS and clearly indicate the importance of crystal-field interactions in this compound (see discussion below). 5. ELECTRICAL TRANSPORT PROPERTIES The temperature variation of the electrical resistivity of a UPS single crystal, measured with the current flowing within the basal plane of the tetragonal unit cell, is shown in Fig. 3. At room temperature, the resistivity is equal to 270 @cm. Upon cooling the sample, it increases gradually to 324 ~Qcrn just above Tc and then falls down to p. = 202 pf2 cm at 4.2 K. Below 20 K, a T* dependence of the resistivity is observed (see Fig. 3). It most probably reflects the electron-magnon scattering processes with a gapless spin-wave spectrum. The ferromagnetic phase transition at 118 K manifests itself as a kink in p(T) and a sharp maximum in the temperature derivative of the resistivity (not shown).
(K)
Fig. 1. Temperature dependence of the magnetization (lefthand scale) and the reciprocal magnetic susceptibility (righthand scale) for UP!3 single crystal. The magnetization was measured along the c-axis in the field-cooled (FC) and the zero-field cooled (ZFC) regimes in a field of 2 kOe. The susceptibility in the paramagnetic region was measured along (0) and perpendicular (0) to the c-axis. The solid lines represent fits of x i:(T) to the Cur&Weiss law,
Fig. 2. Magnetization vs magnetic field (H II c-axis) for UPS single crystals measured at 5 K with increasing (closed symbols) and decreasing (open symbols) magnetic field.
1366
D. KACZOROWSKI
et al. 6. DISCUSSION
6.1.
Crystal structure
The uranium itially reported PbFCl-type
pnictochalcogenides UXY were in[2] to crystallize in the tetragonal
structure.
Then,
adoption
related anti-Fe,As type of structure [13]. However, as shown by Klein
of a closely was proposed Haneveld and
Jellinek [14], the above compounds are isostructural to ZrSiS. Although all three structure types belong to the same space group P4/nmm and have identically occupied atomic positions, they differ between themFig. 3. Temperature variation of the electrical resistivity in the basal plane of UPS single crystal. The solid lines are fits with functions and parameters specified in the figure.
selves
with
bondings
respect
to the character
and some additional
particular,
of chemical
structural
for the UXY compounds
features.
In
with the ZrSiS-
type structure c/a > 2, zu > 0.25 and zu + zy < 1 [14]. As is apparent from Table 2, UPS fulfills all these criteria. The high value of the residual supposed
to be owing
measured
UPS crystal,
resistivity
is not
quality
of the
to a poor
Moreover,
strongly
since p0 is well reproducible
as deduced
from a rather
short
P-P distance (2.692 A) within the basal plane, the bonding of the phosphorus atoms in UPS exhibits a covalent
separation
character.
In contrast,
of 3.4418, can be regarded
the S-S
as a non-bond-
for other specimens. It originates rather from the domain structure (see Section 4) and/or from localiz-
ing one. This observation
ation effects of the conduction f-d hybridization [S].
ence between UPS and PbFCl-type compounds (e.g. the UOY oxychalcogenides) which show all the bondings to be strongly ionic. The aforementioned bond-
electrons
due to the
The most intriguing feature of the p (7’) dependence for UPS is its negative temperature coefficient above Tc. As shown in Fig. 3, the resistivity decreases logarithmically up to the room temperature suggesting
the
presence
in this
compound
of a
ing of the phosphorus
makes an important
atoms
in UPS results
differ-
in the
reduction of the normal oxidation states of P with respect to uranium [ 151. Therefore, the formal valence formula of this compound may be represented U4+[P]*-S2where the brackets indicate the
as in-
Kondo-like scattering of the conduction electrons with localized magnetic moments. It is worthwhile
tralayer
noting
ization should be treated as a first approximation only, because the semimetallic character of UPS (see
that a very similar
behaviour
of p (T)
was
observed before [9] for the arsenides UAsY. The negative curvature of the resistivity above rc was interpreted
for these
ternaries
either
as a Kondo
effect [lO,l l] or a result of scattering of the conduction electrons by collective excitations in the system with a singlet crystal-field
ground
state [lo].
On the other hand ThAsSe [12], which is believed to be a diamagnetic reference material to UAsSe, exhibits a strong negative curvature of p (7’). Thus, the authors
of Ref.
[12] have
concluded
that
in
UAsSe structural effects dominate by far the magnetic scattering mechanisms and the characteristic shape of the p (T) curve above rc for this compound is due only to a strongly anomalous phonon contribution to the total resistivity. Therefore, in view of the above contradictory interpretations of the results obtained for UAsSe, we think that any further discussion of the electrical properties observed for UPS requires first an extension of resistivity measurements to higher temperatures as well as an examination of the p (T) behaviour of its non-magnetic counterpart ThPS.
Section
P-P bonding.
5) indicates
Of course, the above rational-
unambiguously
that the charge
balance in this compound goes also through conduction electron and valence hole bands.
the
UPS reveals a typically layered character with the following sequence of atomic layers along the c-axis: -P-U-S-S-U-P-. All the coordination polyhedra in UPS are essentially the same as in the UOY (PbFCl type) and UX2 (ZrSiS type) compounds and they are discussed in detail in Ref. [2]. In particular, the uranium atom is surrounded by four phosphorus and four sulphur atoms forming together a deformed square antiprism and additionally capped on the S plane side by an additional sulphur atom. 6.2.
Magnetic
properties
The crystallochemical characteristics of UPS, discussed above, have a great influence on its magnetic behaviour. As pointed out by Zygmunt [16], the dominant mechanism leading to the occurrence of magnetic ordering in the UXY compounds is a superexchange interaction via the metalloid atoms
1367
Properties of UPS single crystals being either in the ionic or strongly covalent states. In the case of UPS, two possible ways of superexchange should be considered, namely U4+-S*--U4+ and U4+-[PI*--U4+, both having a positive, i.e. ferromagnetic, character. Moreover, owing to the semimetallic behaviour of this compound, the action of indirect RKKY-type interactions mediated by the conduction electrons must also be taken into account. Certainly the ferromagnetic ordering of the uranium atoms in UPS arises as a result of an interplay between these both exchange mechanisms. Furthermore, these interactions are probably strongly influenced by the hybridization effect between the uranium 5felectrons and the valence p electrons of the surrounding metalloids (p-f mixing [ 171). This latter mechanism, together with crystal-field interactions, seems to be the origin of the huge magnetocrystalline anisotropy characteristic of the UXY ternaries. The crystal-field effect is certainly the main mechanism responsible for the strong reduction of the experimental values of the ordered and effective magnetic moments in UPS (see Section 4) with respect to those expected for the 5f*configuration of the free U4+ ion. On the other hand, from our electrical resistivity results, it seems likely that a Kondo-like screening effect also plays an important role in the overall reduction of the magnetic moment of uranium in this compound. In an environment with the C,,. point symmetry the ground multiplet 3H, of the U4+ ion in UPS splits into seven crystal-field levels: two doublets Q:2r, built up of the I+ 3) and jr 1) states, three singlets r\v”’ and rZI, comprising the /4), 10) and / -4) states, and two singlets TJr and r,,, containing the 12) and / -2) states. Unfortunately, because of an insufficient number of experimental data and the rather small temperature range of our single-crystal susceptibility measurements, we cannot propose as yet any crystal-field mode1 which would fit the experimental x,,(T) and ~~(7’) dependences of UPS. However, a clear tendency to follow the Curie-Weiss law with a relatively large effective magnetic moment, observed for both components of the magnetic susceptibility of UPS, may suggest that a few closely lying crystal field levels, forming at tem~ratures above T, a pseudodegenerated “ground state”, are energetically quite well separated from the remaining excited states. Moreover, because of the
strong uniaxial anisotropy found in the ordered region, it appears likely that the ground state properties of UPS are determined predominantly by a non-Kramers’ T5, doublet. We hope that a quantitative interpretation of the magnetic behaviour of UPS in terms of the crystal-field effect will be possible in the near future after completing the high temperature susceptibility measurements which are under way.
Acknowledgements-We thank Prof. R. Tr& for his kind interest in this work. One of us (D. K.) is indebted to the French Ministry for Research and Technology for a stipend. Part of this study was supported by the Committee for Scientific Research (KBN) under Grant No. 202969101 (Poland).
REFERENCES I. Fournier J. M. and Trod R., in Hundbook on the Physics and Chemistry of the Act&ides (Edited by A. J. Freeman and G. H. Lander), Vol. 2, pp. 29-173. North-Holland, Amsterdam (1985). 2. Hulliger F., J. Less-Common Met. 16, 113 (1968). 3. Walker N. and Stuart D., Acta Cr~sta~~o~r. _ _ A39. 158 (1983).
Frenz B. A., in Computing in Crystallography (Edited by H. Schenk, R. Olthof-Hazekamp, H. Van Koninnsveld and G. C. Bassi), p. 64. Delft‘ University Press, The Netherlands (1978). 5. Bazan C. and Zygmunt Z., Phys. Status Solidi (a) 12,
4.
640 (1972). 6.
Bielov K. P., Dimit~evskii A. S., Zygmunt A., Levitin R. Z. and Trzebiatowski W., Zh. Eksp. Teor. Fi:. 64, 582 (1973).
7.
Rossat-Mignod Handbook
J., Lander G. H. and Burlet P., in
on the Physics and Chemistry
of the Actinides
(Edited by A. J. F&man and G. H.-Lander). Vol. 1. DD. 415-515. North-Holland. Amsterdam (19841. 8. Ghoenes J., J. ~ss-Common Met. 121, Si (1986). 9. Wojakowski A., Henkie Z. and Kletowski Z., Phys. Status
Solidi
(a) 14, 517 (1972).
Wojakowski A., Markowski P. J., Henkie Z. and Laurent Ch., Phys. Status Solidi (a) 100, K47 (1987). 11. Henkie Z.. Fabrowski R. and Woiakowski A.. Acta 10.
Phys. Pal. 85, 249 (1994). 12.
-
Schoenes J., Bacsa W. and Hulliger F..
Solid St. C’ommun. 68, 287 (1988). 13. Flahaut J., J. So/id St. Chem. 9, 124 (1974). 14. Klein Haneveld A. J. and Jellinek F., J. Less-Common Met. 18, 123 (1969). 15. Johnson V. and Jeitschko W., J. Solid St. Chem. 6, 306 (1973). 16. Zygmunt A., in OnziPme Journ~es des Aciinides (Edited
by G. Bombieri, G. de Paoli and P. Zanella) p. 122. Stampa Anastatica, Padua (1981). 17. Takegahara K., Takahashi A., Yanase A. and Kasuya T., So/id St. Commun. 39, 857 (1981).
00223697EWOO132-4 . I
J. Phys. Chem. Soiids Vol. 55. No. 1 I. pp. 1363-1367. I994 Copyright ‘is 1994 Elwvier Science Lid Printed in tireal Britain. All rights nscrwd 0022-3697/94 -S7.00 + 0.00
CRYSTAL STRUCTURE, MAGNETIC AND ELECTRICAL TRANSPORT PROPERTIES OF UPS SINGLE CRYSTALS D. KACZOROWSKI,~~ H. NOEL,? M. POTELT and A. ZY~MUNT§ tLaboratoire
de Chimie du Solide et Inorganique Mofeculaire, Universite de Rennes, URA CNRS I495, 35042 Rennes, France §W. Trzebiatowski Institute for Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wrodaw, Poland (Received 12 November 1993; accepted 26 July 1994)
Abstract-The crystal structure of UPS was refined from single-crystal X-ray diffraction data to a residual of 0.032 for 10 variables and 336 structure factors. It is of the tetragonal ZrSiS type (space group P4/nmm) with the lattice parameters a = 3.8064(5) A and c = 7.981(2) A. The magnetic and electrical properties of UPS were measured on single crystals, within the temperature range 5-300K. The compound orders ferromagnetically below 118 K, with the magnetic moments aligned along the c-axis. At 5 K, the ordered magnetic moment amounts to 1.07 ps per U atom. Both in the ordered and paramagnetic regions, UPS exhibits a huge magnetic anisotropy. Above T,, the electrical resistivity of UPS decreases logarithmically with increasing tem~ratu~. Beiow 20 K., a p * Tz dependence is observed. Keywork:
A. chalcogenides, A. magnetic materials, D. crystal structure, D. electrical conductivity, D. magnetic properties.
up to form of thin plates with dimensions 5x2x0.2mm. The polycrystalline sample and powdered single crystals were examined by X-ray diffraction and found to be single phase. The X-ray patterns were easily indexed in the tetragonal system with the lattice parameters a = 3.810 A and c = 7.980 A. They agree well with the literature data [2]. Magnetic measurements on single-crystalline specimens were performed over the temperature range 5-300 K and in magnetic fields up to 5 T using a SHE SQUID magnetometer. The electrical resistivity was measured between 4.2 and 300 K using a conventional four-point direct current method.
1. INTRODUCTION
UPS belongs to a large family of tetragonal uranium compounds crystallizing in the PbFClor related crystal structures. Many of these materials have already been characterized with respect to their magnetic and electrical transport behaviour [l]; however, the properties of UPS are still hardly known. This ternary
compound
has been reported
to order ferro-
magnetically below about 120 K [2] but no results of any magnetic measurements were shown. In this paper, we present the magnetic and electrical transport properties of UPS, studied on single-crystalline specimens. refinement
Moreover, we report here on the X-ray of its crystal structure.
3. CRYSTAL STRUCTURE ~NEMENTS 2. EXPER~ENTAL
Single-crystal intensity data were collected on an automated four-circle diffractometer (Nonius CAD4) with graphite-monochromated MO Ku radiation. Least-squares refinement of the diffraction angles of 25 reflections yielded the lattice parameters a = 3.8064(5) A and c = 7.981(2) A. The observed systematic extinction rule of h + k # 2n for h/c0
The powder sample of UPS was prepared by heating stoichiometric quantities of the components in an evacuated quartz tube at 800°C for 7 days. The reaction product was used for the single-crystal growth by the chemical vapour transport method. Iodine served as a transporting agent, in the applied temperature gradient of 900-950°C. The crystals grew in a cooler part of the ampoule. They had the
reflections P4/nmm
2 On leave from: W. Trzebiatowski Institute for Low Temperature and Structure Research, Polish Academy of Sciences, B-950 WrocIaw, Poland. PCS SW I-hi
was compatible
with
the
space
group
reported previously for UPS [2]. The main crystallographic and experimental data are summarized in Table 1. A total of 744 integrated intensities was measured in the reciprocal sphere up
1363
D. KACZOROWSKI ef al.
1364
Table I. Crystallographic and experimental data for UPS Lattice constants u (A) c (A) v (A’) Formula units per cell Space group Formula weight Calculated density (g cm-‘) Absorption coefficient (cm-‘) Crystal dimensions (pm’) 0120 scans up to Range in hkl Absorption correction Total number of reflections with I z o(I) Merging R Unique reflections Reflections with I > 20 (I) Number of variables Conventional residual Weighted residual Goodness of fit
to 0 = 50”. An absorption correction was made from psi-scan data and later by DIFABS [3]. After averaging of equivalent reflections and omitting those with I < 2a (I), 336 independent intensities remained which were used for the structure refinements. The calculations were performed by full-matrix leastsquares cycles using the SPD program system [4]. The starting positional parameters for U, P and S were taken from the previous powder study [2]. To check the deviations from the ideal composition the occupancy parameters of the P and S atoms were allowed to vary. No significant deviations from the full occupancies were observed, therefore in the last cycles the idea1 occupancies were assumed with anisotropic thermal parameters for all the atoms. The final conventional and weighted residuals are R = 0.035 and R, = 0.032. The positional and thermal parameters are listed in Table 2, and the interatomic distances in Table 3. The structure factor tables are available from the authors. 4. MAGNETIC PROPERTIES
The temperature dependence of the magnetization of UPS single crystal, measured with magnetic field applied along the crystallographic c-axis (H IIc),is
3.8064 (5) 7.981 (2) 115.6 z=2 P4/nmm (No.129) 301.07 8.65 677 120x60~30 0 = 50” o-8,(r8,Cl7 PSISCAN + DIFABS 567 0.042 341 336 10 0.035 0.032 1.036
displayed in Fig. 1. In rough agreement with a previous powder study [2], UPS orders ferromagnetically at T, = 118 K. The B,,(T) variation, taken on the sample cooled without an applied magnetic field (zero field cooled ZFC), exhibits a characteristic broad maximum which is typical for highly anisotropic ferromagnets. It is well known that a strong magnetocrystalline anisotropy usually results in the formation of very narrow Bloch walls and thus in the occurrence of a pronounced domain effect. Indeed, the maximum in a,,(T)for UPS originates from the competition between the domain movement induced by the temperature (leading to an increase in the magnetization) and the thermal disorder in the magnetic moment system. Such a behaviour does not appear in the magnetization measured on the sample cooled in an applied magnetic field (field cooled FC) which may correspond to the results obtained on a single-domain crystal. A strong domain effect is also clearly visible on the low-field dependence of the u,, magnetization of UPS (see Fig. 2). Here, a high value of the nucleation field H. (H. = 3 kOe at 5 K) probably originates from the occurrence of a compensated narrow-wall domain structure of the 180” type which is rapidly reconstructed at a field H,. It is worthwhile noting that a
Table 2. Positional and thermal parameters in UPS
Atom
Site
X
U
2c 2a 2c
0.25 0.25 0.25
P S
Y 0.25 0.25 0.25
z 0.27324(6) 0 0.6343(4)
8, I = 822 0.308(5) 1.96(8) 0.42(4)
The form of the anisotropic thermal parameters is: exp(-fZuh,hia:a:/?,,), lattice parameter.
BJ, 0.36(l) 0.05(8) 0.08(7)
0.326(3) 1.32(3) 0.31(2)
where at is a reciprocal
1365
Properties of UPS single crystals
Table 3. Interatomic distances (in angstroms) in UPS u-4s 1s 4P 4u
2.791(1) 2.882(I) 2.894(2) 3.806(I)
P-4P 4U 4S
very similar behaviour of a,,(T) and c,,(H) was observed previously [5] for the UAsY (Y = S, Se, Te) ternaries for which huge anisotropy constants of the order of 10’ ergg-’ (anisotropy fields of about 800-1300 kOe) were estimated [6]. Our preliminary results for UPS point to anisotropy constants of the same order of magnitude as those for UAsY but more detailed investigations are needed to determine them accurately. Above H,, the magnetization of UPS increases rapidly but saturates in a field of about 5 kOe. The saturation value of c,, amounts to 19.6e.m.u. g-’ which corresponds to the ordered magnetic moment of 1.06 pLgper U atom. As is clear from the first magnetization curve (Fig. 2), UPS appears to be a magnetically hard material with the remanent magnetization as high at 98% and a nearly rectangular hysteresis loop (not shown). In contrast to q, the magnetization ul, taken within the basal plane (H J-c), is extremely low. It raises almost linearly with increasing magnetic field and does not exhibit any hysteresis effect (not shown). Thus, we can conclude that there is no transverse component of the spontaneous magnetic moment, i.e. the magnetic moments localized on the uranium atom sites are aligned along the c-axis. This statement is in line with the neutron diffraction data for other UXY ternaries [7l which are all uniaxial ferromagnets with the easy magnetization c-axis.
Tempcr~turc
2.692(I) 2.894(2) 3.484(2)
S-4U 1U 4S 4P
2.791(1) 2.882(1) 3.441(l) 3.484(2)
The temperature dependences of the transverse (H Ic) and longitudinal (H 11~)components of the reciprocal magnetic susceptibility of UPS are displayed in Fig. 1. As can be seen, in the whole paramagnetic region studied, x,, is at least three times larger than xl. This feature indicates that a large magnetocrystalline anisotropy in UPS is present also above T,. Both the x;‘(T) and x;‘(T) variations can be approximated by the Curie-Weiss law with the following parameters: 8 i = 123 K, & = 2.57 ps and 8 ,’ = -2703 K, r,‘R= 5.74 pe. Although these values have pure formal meaning and should be considered as the material characteristics only, they reflect a strongly uniaxial character of the magnetic susceptibility of UPS and clearly indicate the importance of crystal-field interactions in this compound (see discussion below). 5. ELECTRICAL TRANSPORT PROPERTIES The temperature variation of the electrical resistivity of a UPS single crystal, measured with the current flowing within the basal plane of the tetragonal unit cell, is shown in Fig. 3. At room temperature, the resistivity is equal to 270 @cm. Upon cooling the sample, it increases gradually to 324 ~Qcrn just above Tc and then falls down to p. = 202 pf2 cm at 4.2 K. Below 20 K, a T* dependence of the resistivity is observed (see Fig. 3). It most probably reflects the electron-magnon scattering processes with a gapless spin-wave spectrum. The ferromagnetic phase transition at 118 K manifests itself as a kink in p(T) and a sharp maximum in the temperature derivative of the resistivity (not shown).
(K)
Fig. 1. Temperature dependence of the magnetization (lefthand scale) and the reciprocal magnetic susceptibility (righthand scale) for UP!3 single crystal. The magnetization was measured along the c-axis in the field-cooled (FC) and the zero-field cooled (ZFC) regimes in a field of 2 kOe. The susceptibility in the paramagnetic region was measured along (0) and perpendicular (0) to the c-axis. The solid lines represent fits of x i:(T) to the Cur&Weiss law,
Fig. 2. Magnetization vs magnetic field (H II c-axis) for UPS single crystals measured at 5 K with increasing (closed symbols) and decreasing (open symbols) magnetic field.
1366
D. KACZOROWSKI
et al. 6. DISCUSSION
6.1.
Crystal structure
The uranium itially reported PbFCl-type
pnictochalcogenides UXY were in[2] to crystallize in the tetragonal
structure.
Then,
adoption
related anti-Fe,As type of structure [13]. However, as shown by Klein
of a closely was proposed Haneveld and
Jellinek [14], the above compounds are isostructural to ZrSiS. Although all three structure types belong to the same space group P4/nmm and have identically occupied atomic positions, they differ between themFig. 3. Temperature variation of the electrical resistivity in the basal plane of UPS single crystal. The solid lines are fits with functions and parameters specified in the figure.
selves
with
bondings
respect
to the character
and some additional
particular,
of chemical
structural
for the UXY compounds
features.
In
with the ZrSiS-
type structure c/a > 2, zu > 0.25 and zu + zy < 1 [14]. As is apparent from Table 2, UPS fulfills all these criteria. The high value of the residual supposed
to be owing
measured
UPS crystal,
resistivity
is not
quality
of the
to a poor
Moreover,
strongly
since p0 is well reproducible
as deduced
from a rather
short
P-P distance (2.692 A) within the basal plane, the bonding of the phosphorus atoms in UPS exhibits a covalent
separation
character.
In contrast,
of 3.4418, can be regarded
the S-S
as a non-bond-
for other specimens. It originates rather from the domain structure (see Section 4) and/or from localiz-
ing one. This observation
ation effects of the conduction f-d hybridization [S].
ence between UPS and PbFCl-type compounds (e.g. the UOY oxychalcogenides) which show all the bondings to be strongly ionic. The aforementioned bond-
electrons
due to the
The most intriguing feature of the p (7’) dependence for UPS is its negative temperature coefficient above Tc. As shown in Fig. 3, the resistivity decreases logarithmically up to the room temperature suggesting
the
presence
in this
compound
of a
ing of the phosphorus
makes an important
atoms
in UPS results
differ-
in the
reduction of the normal oxidation states of P with respect to uranium [ 151. Therefore, the formal valence formula of this compound may be represented U4+[P]*-S2where the brackets indicate the
as in-
Kondo-like scattering of the conduction electrons with localized magnetic moments. It is worthwhile
tralayer
noting
ization should be treated as a first approximation only, because the semimetallic character of UPS (see
that a very similar
behaviour
of p (T)
was
observed before [9] for the arsenides UAsY. The negative curvature of the resistivity above rc was interpreted
for these
ternaries
either
as a Kondo
effect [lO,l l] or a result of scattering of the conduction electrons by collective excitations in the system with a singlet crystal-field
ground
state [lo].
On the other hand ThAsSe [12], which is believed to be a diamagnetic reference material to UAsSe, exhibits a strong negative curvature of p (7’). Thus, the authors
of Ref.
[12] have
concluded
that
in
UAsSe structural effects dominate by far the magnetic scattering mechanisms and the characteristic shape of the p (T) curve above rc for this compound is due only to a strongly anomalous phonon contribution to the total resistivity. Therefore, in view of the above contradictory interpretations of the results obtained for UAsSe, we think that any further discussion of the electrical properties observed for UPS requires first an extension of resistivity measurements to higher temperatures as well as an examination of the p (T) behaviour of its non-magnetic counterpart ThPS.
Section
P-P bonding.
5) indicates
Of course, the above rational-
unambiguously
that the charge
balance in this compound goes also through conduction electron and valence hole bands.
the
UPS reveals a typically layered character with the following sequence of atomic layers along the c-axis: -P-U-S-S-U-P-. All the coordination polyhedra in UPS are essentially the same as in the UOY (PbFCl type) and UX2 (ZrSiS type) compounds and they are discussed in detail in Ref. [2]. In particular, the uranium atom is surrounded by four phosphorus and four sulphur atoms forming together a deformed square antiprism and additionally capped on the S plane side by an additional sulphur atom. 6.2.
Magnetic
properties
The crystallochemical characteristics of UPS, discussed above, have a great influence on its magnetic behaviour. As pointed out by Zygmunt [16], the dominant mechanism leading to the occurrence of magnetic ordering in the UXY compounds is a superexchange interaction via the metalloid atoms
1367
Properties of UPS single crystals being either in the ionic or strongly covalent states. In the case of UPS, two possible ways of superexchange should be considered, namely U4+-S*--U4+ and U4+-[PI*--U4+, both having a positive, i.e. ferromagnetic, character. Moreover, owing to the semimetallic behaviour of this compound, the action of indirect RKKY-type interactions mediated by the conduction electrons must also be taken into account. Certainly the ferromagnetic ordering of the uranium atoms in UPS arises as a result of an interplay between these both exchange mechanisms. Furthermore, these interactions are probably strongly influenced by the hybridization effect between the uranium 5felectrons and the valence p electrons of the surrounding metalloids (p-f mixing [ 171). This latter mechanism, together with crystal-field interactions, seems to be the origin of the huge magnetocrystalline anisotropy characteristic of the UXY ternaries. The crystal-field effect is certainly the main mechanism responsible for the strong reduction of the experimental values of the ordered and effective magnetic moments in UPS (see Section 4) with respect to those expected for the 5f*configuration of the free U4+ ion. On the other hand, from our electrical resistivity results, it seems likely that a Kondo-like screening effect also plays an important role in the overall reduction of the magnetic moment of uranium in this compound. In an environment with the C,,. point symmetry the ground multiplet 3H, of the U4+ ion in UPS splits into seven crystal-field levels: two doublets Q:2r, built up of the I+ 3) and jr 1) states, three singlets r\v”’ and rZI, comprising the /4), 10) and / -4) states, and two singlets TJr and r,,, containing the 12) and / -2) states. Unfortunately, because of an insufficient number of experimental data and the rather small temperature range of our single-crystal susceptibility measurements, we cannot propose as yet any crystal-field mode1 which would fit the experimental x,,(T) and ~~(7’) dependences of UPS. However, a clear tendency to follow the Curie-Weiss law with a relatively large effective magnetic moment, observed for both components of the magnetic susceptibility of UPS, may suggest that a few closely lying crystal field levels, forming at tem~ratures above T, a pseudodegenerated “ground state”, are energetically quite well separated from the remaining excited states. Moreover, because of the
strong uniaxial anisotropy found in the ordered region, it appears likely that the ground state properties of UPS are determined predominantly by a non-Kramers’ T5, doublet. We hope that a quantitative interpretation of the magnetic behaviour of UPS in terms of the crystal-field effect will be possible in the near future after completing the high temperature susceptibility measurements which are under way.
Acknowledgements-We thank Prof. R. Tr& for his kind interest in this work. One of us (D. K.) is indebted to the French Ministry for Research and Technology for a stipend. Part of this study was supported by the Committee for Scientific Research (KBN) under Grant No. 202969101 (Poland).
REFERENCES I. Fournier J. M. and Trod R., in Hundbook on the Physics and Chemistry of the Act&ides (Edited by A. J. Freeman and G. H. Lander), Vol. 2, pp. 29-173. North-Holland, Amsterdam (1985). 2. Hulliger F., J. Less-Common Met. 16, 113 (1968). 3. Walker N. and Stuart D., Acta Cr~sta~~o~r. _ _ A39. 158 (1983).
Frenz B. A., in Computing in Crystallography (Edited by H. Schenk, R. Olthof-Hazekamp, H. Van Koninnsveld and G. C. Bassi), p. 64. Delft‘ University Press, The Netherlands (1978). 5. Bazan C. and Zygmunt Z., Phys. Status Solidi (a) 12,
4.
640 (1972). 6.
Bielov K. P., Dimit~evskii A. S., Zygmunt A., Levitin R. Z. and Trzebiatowski W., Zh. Eksp. Teor. Fi:. 64, 582 (1973).
7.
Rossat-Mignod Handbook
J., Lander G. H. and Burlet P., in
on the Physics and Chemistry
of the Actinides
(Edited by A. J. F&man and G. H.-Lander). Vol. 1. DD. 415-515. North-Holland. Amsterdam (19841. 8. Ghoenes J., J. ~ss-Common Met. 121, Si (1986). 9. Wojakowski A., Henkie Z. and Kletowski Z., Phys. Status
Solidi
(a) 14, 517 (1972).
Wojakowski A., Markowski P. J., Henkie Z. and Laurent Ch., Phys. Status Solidi (a) 100, K47 (1987). 11. Henkie Z.. Fabrowski R. and Woiakowski A.. Acta 10.
Phys. Pal. 85, 249 (1994). 12.
-
Schoenes J., Bacsa W. and Hulliger F..
Solid St. C’ommun. 68, 287 (1988). 13. Flahaut J., J. So/id St. Chem. 9, 124 (1974). 14. Klein Haneveld A. J. and Jellinek F., J. Less-Common Met. 18, 123 (1969). 15. Johnson V. and Jeitschko W., J. Solid St. Chem. 6, 306 (1973). 16. Zygmunt A., in OnziPme Journ~es des Aciinides (Edited
by G. Bombieri, G. de Paoli and P. Zanella) p. 122. Stampa Anastatica, Padua (1981). 17. Takegahara K., Takahashi A., Yanase A. and Kasuya T., So/id St. Commun. 39, 857 (1981).