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31P NMR Knight shifts in the UPY(Y = S, Se, Te) compounds
293
Journal of Magnetism and Magnetic Materials 27 (1982) 293-297 North-Holland Publishing Company
31P NMR KNIGHT SHIFTS IN THE UPY(Y = S, Se, Te) COMPOUNDS
0.~. i0GAL
and A. ZYGMUNT
Institute for Law Temperature
and Structure Research, Polish Academy
of Sciences, 50-950 Wroctaw, pl. Katedralny
1, Poland
Received 26 January 1982
Nuclear magnetic resonance of 3’P measurements have been made for UPY (where Y = S, Se and Te) compounds. The resonane lines show the effect of anisotropic Knight shifts. In the UPS, the NMR data indicate an axial symmetry of the phosphorus environment whereas for UPSe and UPTe the symmetry is lower than the axial one. The result for UPSe disagrees with earlier structural studies. An analysis of NMR spectra indicates that both the asymmetry parameter e and the isotropic component of the Knight shift increase with the increase of atomic weight of the Y element.
1. Introduction
Recent magnetic [ 11,neutron and X-ray diffraction [2,3] studies in UPSe and UPTe have revealed many interesting properties of these materials. Here, we report on the continuation of the investigation on these dompounds using NMR technique. Moreover, we have extended these studies to UPS. The three uranium temaries UPY (Y = S, Se, Te) order magnetically at low temperatures becoming uniaxial ferromagnets. Their Curie temperatures are 118, 102 and 85 K for UPS, UPSe and UPTe, respectively. The electrical resistivity of UPY compounds is relatively low, about 200 /.LO cm at room temperature [4], so that they can be treated as uranium intermetallics. X-ray and neutron diffraction studies suggest that the UPY compounds crystallize in two competitive structures: a PbFCl-type and a UGeTe one. UPS and UPSe have been described as having the PbFCl-type of structure, space group P4/nmm with 2 formula units in tetragonal unit cell. The P atoms located in 2c sites form a square net with comparatively short (2.7-2.8A) P-P distances in the 110 plane. The two P layers are isolated from each other (7.9%8.17A) by four layers coming from the U and Y atoms. The UPTe, in turn crystallizes in the UGeTe-type of structure with 14/mmm space group, 4 formula units in the tetragonal unit cell. Similarly, as in the UPS and 0304-8853/82/oooO-0000/$02.75
0 1982 North-Holland
UPSe compounds, the P atoms form a square net with small distances P-P and phosphorus layers separated by the distance c/2 = 8.513 A. In spite of the fact that both (PbFCl and UGeTe) types of struct&e can be built frond the same uranium polyhedra [UP,Y,], the coordination polyhedra formed around the P atoms by uranium atoms are different. The coordination polyhedron [PU,] for UPS and UPSe is a distorted tetrahedron and rectangle for UPTe. The UP, compound also has the PbFCl-type of structure. However, Pietraszko and tukaszewicz [5] report that the crystal lattice undergoes slight transformation on cooling below 83’C and four different sites of P atoms can be distinguished. There have been no such X-ray experiments on UPS and UPSe as yet.
2. Experimental
The preparation procedure for UPSe and UPTe was described previously [2,3]. The UPS was obtained in similar manner as UPSe. The plateshaped single crystals of UPSe were grown by chemical transport method using iodine (5 mg/cm3). The Varian cross-coil spectrometer was used to collect NMR data on 3’P nuclei. In most cases the
t
294
O.J. iogd
A. Zygmunr / “P NMR knight shifts in UPY
spectra were taken on powder specimens at room temperature. ‘The spectrometer used a swept field (typical sweep length between 60 to 100 Oe) and field modulation (typically 0.9 to 3.4 Oe). The time constant following phase-sensitive detection was 0.3 s. The sweep time was 90s. To achieve the necessary signal-to-noise ratio, the records of each sweep were summed using a Northern Tracer NS 575A digital signal analyzer. The resonance line shifts were measured with respect to the 3’P reference line in H,PO,. The spectrometer frequency (usually equal to 5.52 MHz) was determined by means of a PFL-20 digital frequency meter. The magnetic field calibration was performed using MJ-110 type proton digital magnetometer (made by the Institute of Physics, Polish Academy of Sciences).
UPS 3’P NMR 5.5244 MHz
I
3. The results and discussion
I
,V,
1
I
3140
3160 MAGNETIC
In figs. 1 and 2 we show a 3’P NMR powder spectra for UPS and UPTe, respectively. They represent the first derivatives of the absorption mode signal. The characteristic feature of these spectra is the presence of the strong anisotropic broadening. This is due to an angular-dependent Knight shift in noncubic environment of resonating nuclei. In UPS (fig. 1) the local symmetry at the phosphorus is Dzd and therefore the Knight shift may have two different components, K, = KY = Kl and K, = K ,,. The K, and K ,, are defined as
H,
,
K,,=
H,,, - HI, HII
I
3200
FIELD
(Oel
Fig. 1. Powder 3’P NMR spectrum (absorption derivative) for UPS, measured at a nominal resonance frequency of 5.5244 MHz, showing the asymmetric lineshape characteristic of axial Knight shift anisotropy. The inset shows the positions of the H, and HI, fields. They are shifted on the flanks if the dipole-dipole linewidth is not small enough compared to the anisotropy. Since the anisotropy in the UPS is much larger than the dipole broadening, only a small error results from taking the H, and H,, as shown in the inset.
I
Hre,- H, K,=
I
3160
I
UPTe 3’P NMR 5.9040 MHz
n
1
9
where H,, is the resonance magnetic field for reference material, H, and H,, are resonance fields when crystal symmetry axis (tetragonal) is perpendicular and parallel oriented in respect to the external magnetic field, respectively. Deduction of H, and H,, from the powder spectra [6] are shown in the inset in fig. 1. The estimated values of K, and K,, for UPS are given in table 1. The powder spectrum taken for UPTe (fig. 2) is typical when anisotropic Knight shift effects are important and the site symmetry of P is lower than axial [7]. This agrees with the X-ray analysis which
I
I
3410
1
!
3430 MAGNETIC
1
I
I
3450 FIELD
(Oel
Fig. 2. Powder “P NMR spectrum for UPTe taken at 5.9848 MHz. The positions of Hz, H,, and H, fields are shown in the inset.
O.J. &d
A. ZVgmunt
/ ‘tP NMR
295
knight shifts in UPY
Table 1 Measured 3’P NMR Knight shift parameters in the UPY compounds. All K’s are positive and in S. Typical uncertainty is *0.03% Cornpound
K,
KY
K,
Kiso
K,
e
UPS UPSe UPTe
1.45 1.26 1.56
0.45 0.71 1.09
0.45 0.48 0.73
0.78 0.82 1.13
0.66 0.44 0.43
0 0.52 0.837
indicated the point symmetry at P sites as D,,. In that case the Knight shift has three components: Ki=
HreLHi, (i=x,y,z),
(2)
I
where the Hi’s are the magnetic fields, whose positions can be identified in the spectrum as shown in the inset in fig. 2. The estimated values of Ki are given in table 1. The 31P NMR powder spectrum for UPSe is shown in fig. 3. Although it is similar to that of UPS, pronounced differences suggest that site symmetry in WPSe is lower than the axial one. In order to find a more solid base for this conclusion and to get better access into Knight shift parameters, measurements on the single crystal of UPSe were carried out. The crystals in a form of slabs, each with a thickness of the order of or less than the rf skin depth and each insulated from the
z-d
MAGNETIC
FIELD
IOe)
Fig. 4. “P NMR spectrum is a stack of single crystals of UPSe with the magnetic field along the crystal c axis. The trace is the average of 120 sweeps. One resonance line is seen.
other, were used to build up a stack of parallel slabs. Although the c symmetry axis of all crystals were oriented along the same direction, there were random positions of each crystal in the plane perpendicular to the c-axis. The NMR spectra obtained for two orientations of the crystallographic c-axis with respect to external magnetic field are shown in figs. 4 and 5. When the external magnetic field is parallel to the c-axis we observe the single line with K,, = + 1.26% whilst there are two well separated lines for HI c, whose K values are +0.48 and +0.71%, respectively. Only one line should be observed for HI c, if the P site would have axial symmetry. The K,, can be identi-
UPSe ” P NMR 5.985 MHz
L$ 3160
, 3400
1
I
3170
3180 MAGNETIC
I
3190 FIELD (Oel
3200
I
3420 3440 3480 MAGNETIC FIELD (Oe)
Fig. 3. Powder “P NMR spectrum for UPSc, measured nominal resonance frequency of 5.985 MHz.
at a
Fig. 5. 3’ P NMR spectrum in a stack of single crystals of UPSe with the magnetic field perpendicular to the crystal c axis. The two resonance lines are seen. This spectrum was obtained by averaging 128 sweeps.
2%
O.J. ifogat
A. Zygmunt / “P NMR
fied as K, whereas two latter K’s as K, and K,,, respectively. The Knight shift data for all three studied compounds are summarized in table 1. Besides the parameters mentioned above, the others related to them have also been shown. They are, for the case of lower than axial symmetry: Ki,=f(Kx+Ky+Kz), K, = K, - Kim,
(34
E= (KY-K,) K,
and Kiso=i(KII
+2K,),
K,=2K,,=$(K,,-K,),
(3b)
for axial symmetry. These parameters are frequently used for description of angular dependence of the Knight shift for single crystals [6]. Results presented in table 1 show the effect of the change of Y element on the local properties of phosphorus in UPY compounds. When sulphur is replaced by selenium, the axial symmetry is lowered and nonzero asymmetry parameter E appears. Moreover, the K, decreases while Ki, does not change much. When tellurium, in turn, replaces selenium, further increase of E is observed whereas K, remains almost unchanged. Simultaneously, isotropic Knight shift Ki, increases by ca 38% of the UPSe value. Before discussing the Knight shift in terms of electronic structure, some comments are needed on effects which may contribute to the observed Knight shift but not directly connected to the object of interest. One such effect is due to the relatively large macroscopic susceptibility and demagnetization fields in powdered samples not having an all-over spherical geometry. This contribution is hard to calculate since the shape of the powder particles is not regular and demagnetization factors are only known for a particular geometry. Nevertheless, the order of magnitude of the demagnetization fields can be estimated. Since the crystals of the UPY compounds grow as thin slabs, we assume that the powder sample consists of particles of similar shape. The particles
knight shifts in UPY
are assumed to be randomly oriented with respect to the external field. Then, for equatorial directions (HI c) we have H, (dem) = (47r/3)x vH, and H,,(dem) = -(8s/3)x,H, for axial direction (H IIc). Here, xv is the volume magnetic susceptibility. Evaluation of H,,(dem) and iY,(dem) at room temperature and 3200 Oe shows them to be, at worst, only m 9% of the observed shifts so that the effect is of secondary importance. Another contribution to the observed shifts which should be considered is a magnetic dipolar interaction between the average magnetic moment (P) localized on the uranium atoms and the 31P nuclei. This interaction was discussed by Kroon [8], Ibers et al. [9] and Grunzweig-Genossar et al. [lo]. The local fields produced by paramagnetic ions in the position of phosphorus depend on ,ii and the sums over the dipole sites:
where XL, xi, XL are the coordinates of the k th dipole and r, the radius vector between the origin and the k th atoms. The average magnetic moment (p) in the paramagnetic state is given by: F= +,&/3k(T-
@) = x&o/NA,
(5)
where np is the paramagnetic moment per uranium ion in Bohr magnetons (pa), k is the Boltzman constant, t9 is the Curie temperature, X~ is the molar magnetic susceptibility and NA is the Avogardo number. The p’s for the UPY were estimated from magnetic susceptibility measurements [1,4] and appropriate lattice sums have been calculated numerically knowing the crystal structure [2,3] of those compounds. Hence, the components of the local field, Hij = jiSij, were calculated. The results show again that contribution of these fields to the observed Knight shifts are small (= 9%). Moreover, they have the opposite sign to the demagnetization fields, and therefore one can cancel the other to some extent. Thus we conclude that a considerable part of the measured shifts is to be intrinsic in the sense that it is related to the electronic structure of the investigated compounds. We shall assume, therefore, that the interaction
O.J. gogal. A. Zygmunt / “P NMR knight shifts in UPY
between the 5f electrons on the uranium ions and phosphorus nucleus has a substantial contribution to the Knight shift of 31P in UPY. This can be carried either by the conduction electrons (RKKY mechanism) or through an exchange interaction between localized (bonding) electrons. Since both mechanisms are simultaneously present, it is difficult to analyze their contributions. In the previous NMR studies on 3’P in rareearth phosphides [ 1 l] the results were interpreted in terms of the RKKY model assuming uniform conduction-electron spin polarization. However, the s-f exchange energy constant can be also anisotropic [12]. On the other hand, “P NMR studies [ 131 in UP and in UP, _,S, provided arguments that 31P Knight shifts are primarily due to superhyperfine interactions, through exchange by the covalent bonds between neighboring cations and anions. The authors [ 131 came to this conclusion through the observation of constancy of aK/axu in the UP-US system. In the “P NMR studies presented in this paper, we do not have such arguments and we are not aware if either one or other mechanism is dominating or both are of the same strength in UPY compounds. Although recent photoemission studies [14] in UPS revealed a presence of Sf electrons in the conduction band, further investigation of the electronic structure of UPY compounds are needed to make the interpretation of the Knight shifts reliable.
Acknowledgements We wish to express our gratitude to Prof. Dr. B. Stalinski for his stimulating interest in this work.
297
It is also a pleasure to acknowledge the discussion of Dr. S. Sagnowki in the subject of this paper.
References [l] A. Zygmunt and A. Cxopnik, Phys. Stat. Sol. (a) 18 (1973) 731. C. Bazan and Z. Zygmunt, Preprint ILHFLT, WrocIaw, Poland, No. 17 (1973). [2] A. Zygmunt, A. Murasik, S. Ligenxa and J. Leciejewicx, Phys. Stat. Sol. (a) 22 (1974) 75. [3] A. Zygmunt, S. Ligenxa, H. Ptasiewicz and J. Leciejewicz, Phys. Stat. Sol. (a), 25 (1974) K77. F. Hulliger, J. Less-Common Metals 16 (1968) 113. [4] A. Zygmunt, unpublished data. [5] D. Pietraszko and K. Lukaszewicx, Bull. Acad. Polon. Sci. Ser. Sci. Chim. XIX, No. 4 (1971) 237. [6] G.C. Carter, L.H. Bennett and D.J. Kahan, in: Progress in Materials Science, vol. 20, eds. B. Chalmers, J.W. Christian and T.B. Massalski (Pergamon Press, Oxford-New York, 1977). [7] N. Bloembergen and T.J. Rowland, Acta Met. 1 (1953) 731. [S] D.J. Kroon, Philips Res. Repts. 15 (1960) 501. (91 J.A. Ibers, C.H. Holm and C.R. Adams, Phys. Rev. 121 (1961) 1620. [lo] J. Grunzweig-Genossar, M. Kuxnietc and B. Meerovici, Phys. Rev. Bl (1970) 1958. [1 1] E.D. Jones, Phys. Rev. 180 (1969) 455. [12] B.D. Rainford and J.G. Houmann, Phys. Rev. L&t. 26 (1971) 1254. [13] F. Friedman and J. Grunxweig-Genossar, Phys. Rev. B4 (1971) 180. [14] J. Brunner, M. Erbudak and F. Hulliger, in: Abstr. 1 l&me ,JoumQs des Actinides, Venice, Italy (25-27 May 1981) p. 37.
Journal of Magnetism and Magnetic Materials 27 (1982) 293-297 North-Holland Publishing Company
31P NMR KNIGHT SHIFTS IN THE UPY(Y = S, Se, Te) COMPOUNDS
0.~. i0GAL
and A. ZYGMUNT
Institute for Law Temperature
and Structure Research, Polish Academy
of Sciences, 50-950 Wroctaw, pl. Katedralny
1, Poland
Received 26 January 1982
Nuclear magnetic resonance of 3’P measurements have been made for UPY (where Y = S, Se and Te) compounds. The resonane lines show the effect of anisotropic Knight shifts. In the UPS, the NMR data indicate an axial symmetry of the phosphorus environment whereas for UPSe and UPTe the symmetry is lower than the axial one. The result for UPSe disagrees with earlier structural studies. An analysis of NMR spectra indicates that both the asymmetry parameter e and the isotropic component of the Knight shift increase with the increase of atomic weight of the Y element.
1. Introduction
Recent magnetic [ 11,neutron and X-ray diffraction [2,3] studies in UPSe and UPTe have revealed many interesting properties of these materials. Here, we report on the continuation of the investigation on these dompounds using NMR technique. Moreover, we have extended these studies to UPS. The three uranium temaries UPY (Y = S, Se, Te) order magnetically at low temperatures becoming uniaxial ferromagnets. Their Curie temperatures are 118, 102 and 85 K for UPS, UPSe and UPTe, respectively. The electrical resistivity of UPY compounds is relatively low, about 200 /.LO cm at room temperature [4], so that they can be treated as uranium intermetallics. X-ray and neutron diffraction studies suggest that the UPY compounds crystallize in two competitive structures: a PbFCl-type and a UGeTe one. UPS and UPSe have been described as having the PbFCl-type of structure, space group P4/nmm with 2 formula units in tetragonal unit cell. The P atoms located in 2c sites form a square net with comparatively short (2.7-2.8A) P-P distances in the 110 plane. The two P layers are isolated from each other (7.9%8.17A) by four layers coming from the U and Y atoms. The UPTe, in turn crystallizes in the UGeTe-type of structure with 14/mmm space group, 4 formula units in the tetragonal unit cell. Similarly, as in the UPS and 0304-8853/82/oooO-0000/$02.75
0 1982 North-Holland
UPSe compounds, the P atoms form a square net with small distances P-P and phosphorus layers separated by the distance c/2 = 8.513 A. In spite of the fact that both (PbFCl and UGeTe) types of struct&e can be built frond the same uranium polyhedra [UP,Y,], the coordination polyhedra formed around the P atoms by uranium atoms are different. The coordination polyhedron [PU,] for UPS and UPSe is a distorted tetrahedron and rectangle for UPTe. The UP, compound also has the PbFCl-type of structure. However, Pietraszko and tukaszewicz [5] report that the crystal lattice undergoes slight transformation on cooling below 83’C and four different sites of P atoms can be distinguished. There have been no such X-ray experiments on UPS and UPSe as yet.
2. Experimental
The preparation procedure for UPSe and UPTe was described previously [2,3]. The UPS was obtained in similar manner as UPSe. The plateshaped single crystals of UPSe were grown by chemical transport method using iodine (5 mg/cm3). The Varian cross-coil spectrometer was used to collect NMR data on 3’P nuclei. In most cases the
t
294
O.J. iogd
A. Zygmunr / “P NMR knight shifts in UPY
spectra were taken on powder specimens at room temperature. ‘The spectrometer used a swept field (typical sweep length between 60 to 100 Oe) and field modulation (typically 0.9 to 3.4 Oe). The time constant following phase-sensitive detection was 0.3 s. The sweep time was 90s. To achieve the necessary signal-to-noise ratio, the records of each sweep were summed using a Northern Tracer NS 575A digital signal analyzer. The resonance line shifts were measured with respect to the 3’P reference line in H,PO,. The spectrometer frequency (usually equal to 5.52 MHz) was determined by means of a PFL-20 digital frequency meter. The magnetic field calibration was performed using MJ-110 type proton digital magnetometer (made by the Institute of Physics, Polish Academy of Sciences).
UPS 3’P NMR 5.5244 MHz
I
3. The results and discussion
I
,V,
1
I
3140
3160 MAGNETIC
In figs. 1 and 2 we show a 3’P NMR powder spectra for UPS and UPTe, respectively. They represent the first derivatives of the absorption mode signal. The characteristic feature of these spectra is the presence of the strong anisotropic broadening. This is due to an angular-dependent Knight shift in noncubic environment of resonating nuclei. In UPS (fig. 1) the local symmetry at the phosphorus is Dzd and therefore the Knight shift may have two different components, K, = KY = Kl and K, = K ,,. The K, and K ,, are defined as
H,
,
K,,=
H,,, - HI, HII
I
3200
FIELD
(Oel
Fig. 1. Powder 3’P NMR spectrum (absorption derivative) for UPS, measured at a nominal resonance frequency of 5.5244 MHz, showing the asymmetric lineshape characteristic of axial Knight shift anisotropy. The inset shows the positions of the H, and HI, fields. They are shifted on the flanks if the dipole-dipole linewidth is not small enough compared to the anisotropy. Since the anisotropy in the UPS is much larger than the dipole broadening, only a small error results from taking the H, and H,, as shown in the inset.
I
Hre,- H, K,=
I
3160
I
UPTe 3’P NMR 5.9040 MHz
n
1
9
where H,, is the resonance magnetic field for reference material, H, and H,, are resonance fields when crystal symmetry axis (tetragonal) is perpendicular and parallel oriented in respect to the external magnetic field, respectively. Deduction of H, and H,, from the powder spectra [6] are shown in the inset in fig. 1. The estimated values of K, and K,, for UPS are given in table 1. The powder spectrum taken for UPTe (fig. 2) is typical when anisotropic Knight shift effects are important and the site symmetry of P is lower than axial [7]. This agrees with the X-ray analysis which
I
I
3410
1
!
3430 MAGNETIC
1
I
I
3450 FIELD
(Oel
Fig. 2. Powder “P NMR spectrum for UPTe taken at 5.9848 MHz. The positions of Hz, H,, and H, fields are shown in the inset.
O.J. &d
A. ZVgmunt
/ ‘tP NMR
295
knight shifts in UPY
Table 1 Measured 3’P NMR Knight shift parameters in the UPY compounds. All K’s are positive and in S. Typical uncertainty is *0.03% Cornpound
K,
KY
K,
Kiso
K,
e
UPS UPSe UPTe
1.45 1.26 1.56
0.45 0.71 1.09
0.45 0.48 0.73
0.78 0.82 1.13
0.66 0.44 0.43
0 0.52 0.837
indicated the point symmetry at P sites as D,,. In that case the Knight shift has three components: Ki=
HreLHi, (i=x,y,z),
(2)
I
where the Hi’s are the magnetic fields, whose positions can be identified in the spectrum as shown in the inset in fig. 2. The estimated values of Ki are given in table 1. The 31P NMR powder spectrum for UPSe is shown in fig. 3. Although it is similar to that of UPS, pronounced differences suggest that site symmetry in WPSe is lower than the axial one. In order to find a more solid base for this conclusion and to get better access into Knight shift parameters, measurements on the single crystal of UPSe were carried out. The crystals in a form of slabs, each with a thickness of the order of or less than the rf skin depth and each insulated from the
z-d
MAGNETIC
FIELD
IOe)
Fig. 4. “P NMR spectrum is a stack of single crystals of UPSe with the magnetic field along the crystal c axis. The trace is the average of 120 sweeps. One resonance line is seen.
other, were used to build up a stack of parallel slabs. Although the c symmetry axis of all crystals were oriented along the same direction, there were random positions of each crystal in the plane perpendicular to the c-axis. The NMR spectra obtained for two orientations of the crystallographic c-axis with respect to external magnetic field are shown in figs. 4 and 5. When the external magnetic field is parallel to the c-axis we observe the single line with K,, = + 1.26% whilst there are two well separated lines for HI c, whose K values are +0.48 and +0.71%, respectively. Only one line should be observed for HI c, if the P site would have axial symmetry. The K,, can be identi-
UPSe ” P NMR 5.985 MHz
L$ 3160
, 3400
1
I
3170
3180 MAGNETIC
I
3190 FIELD (Oel
3200
I
3420 3440 3480 MAGNETIC FIELD (Oe)
Fig. 3. Powder “P NMR spectrum for UPSc, measured nominal resonance frequency of 5.985 MHz.
at a
Fig. 5. 3’ P NMR spectrum in a stack of single crystals of UPSe with the magnetic field perpendicular to the crystal c axis. The two resonance lines are seen. This spectrum was obtained by averaging 128 sweeps.
2%
O.J. ifogat
A. Zygmunt / “P NMR
fied as K, whereas two latter K’s as K, and K,,, respectively. The Knight shift data for all three studied compounds are summarized in table 1. Besides the parameters mentioned above, the others related to them have also been shown. They are, for the case of lower than axial symmetry: Ki,=f(Kx+Ky+Kz), K, = K, - Kim,
(34
E= (KY-K,) K,
and Kiso=i(KII
+2K,),
K,=2K,,=$(K,,-K,),
(3b)
for axial symmetry. These parameters are frequently used for description of angular dependence of the Knight shift for single crystals [6]. Results presented in table 1 show the effect of the change of Y element on the local properties of phosphorus in UPY compounds. When sulphur is replaced by selenium, the axial symmetry is lowered and nonzero asymmetry parameter E appears. Moreover, the K, decreases while Ki, does not change much. When tellurium, in turn, replaces selenium, further increase of E is observed whereas K, remains almost unchanged. Simultaneously, isotropic Knight shift Ki, increases by ca 38% of the UPSe value. Before discussing the Knight shift in terms of electronic structure, some comments are needed on effects which may contribute to the observed Knight shift but not directly connected to the object of interest. One such effect is due to the relatively large macroscopic susceptibility and demagnetization fields in powdered samples not having an all-over spherical geometry. This contribution is hard to calculate since the shape of the powder particles is not regular and demagnetization factors are only known for a particular geometry. Nevertheless, the order of magnitude of the demagnetization fields can be estimated. Since the crystals of the UPY compounds grow as thin slabs, we assume that the powder sample consists of particles of similar shape. The particles
knight shifts in UPY
are assumed to be randomly oriented with respect to the external field. Then, for equatorial directions (HI c) we have H, (dem) = (47r/3)x vH, and H,,(dem) = -(8s/3)x,H, for axial direction (H IIc). Here, xv is the volume magnetic susceptibility. Evaluation of H,,(dem) and iY,(dem) at room temperature and 3200 Oe shows them to be, at worst, only m 9% of the observed shifts so that the effect is of secondary importance. Another contribution to the observed shifts which should be considered is a magnetic dipolar interaction between the average magnetic moment (P) localized on the uranium atoms and the 31P nuclei. This interaction was discussed by Kroon [8], Ibers et al. [9] and Grunzweig-Genossar et al. [lo]. The local fields produced by paramagnetic ions in the position of phosphorus depend on ,ii and the sums over the dipole sites:
where XL, xi, XL are the coordinates of the k th dipole and r, the radius vector between the origin and the k th atoms. The average magnetic moment (p) in the paramagnetic state is given by: F= +,&/3k(T-
@) = x&o/NA,
(5)
where np is the paramagnetic moment per uranium ion in Bohr magnetons (pa), k is the Boltzman constant, t9 is the Curie temperature, X~ is the molar magnetic susceptibility and NA is the Avogardo number. The p’s for the UPY were estimated from magnetic susceptibility measurements [1,4] and appropriate lattice sums have been calculated numerically knowing the crystal structure [2,3] of those compounds. Hence, the components of the local field, Hij = jiSij, were calculated. The results show again that contribution of these fields to the observed Knight shifts are small (= 9%). Moreover, they have the opposite sign to the demagnetization fields, and therefore one can cancel the other to some extent. Thus we conclude that a considerable part of the measured shifts is to be intrinsic in the sense that it is related to the electronic structure of the investigated compounds. We shall assume, therefore, that the interaction
O.J. gogal. A. Zygmunt / “P NMR knight shifts in UPY
between the 5f electrons on the uranium ions and phosphorus nucleus has a substantial contribution to the Knight shift of 31P in UPY. This can be carried either by the conduction electrons (RKKY mechanism) or through an exchange interaction between localized (bonding) electrons. Since both mechanisms are simultaneously present, it is difficult to analyze their contributions. In the previous NMR studies on 3’P in rareearth phosphides [ 1 l] the results were interpreted in terms of the RKKY model assuming uniform conduction-electron spin polarization. However, the s-f exchange energy constant can be also anisotropic [12]. On the other hand, “P NMR studies [ 131 in UP and in UP, _,S, provided arguments that 31P Knight shifts are primarily due to superhyperfine interactions, through exchange by the covalent bonds between neighboring cations and anions. The authors [ 131 came to this conclusion through the observation of constancy of aK/axu in the UP-US system. In the “P NMR studies presented in this paper, we do not have such arguments and we are not aware if either one or other mechanism is dominating or both are of the same strength in UPY compounds. Although recent photoemission studies [14] in UPS revealed a presence of Sf electrons in the conduction band, further investigation of the electronic structure of UPY compounds are needed to make the interpretation of the Knight shifts reliable.
Acknowledgements We wish to express our gratitude to Prof. Dr. B. Stalinski for his stimulating interest in this work.
297
It is also a pleasure to acknowledge the discussion of Dr. S. Sagnowki in the subject of this paper.
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