Brun, T.O.

Lam, D.J.

Lander, G.H.

Veal, B.W.

UNT Digital Library

INELASTIC NEUTRON AND X-RAY PHOTOEMISSION EXPERIMENTS ON USnnbyG. H. Lander, T. 0. Brun, B. W. Veal, and D. J. Lam-NOTICC-Tllts report wus prepared as an account of worksponsored by thu United States Government. Neitherthe United States nor the United States Atomic EnergyConirnissiL-n, nor any o f their employees, nor any o flhe.tr contractors, subcontractors, or their employees,makes any warranty, express or implied, or assumes any\egul l iability or ri'spunsibtjity for the accuracy, com-pleteness or usefulness of any Information, apparatus,product or process disclosed, or represents that its usewould nol infringe privately owned rights.Presented At:American Physical Society Conference on CrystallineElectric Field Effects in Metals and Alloys,Montreal, Canada, June 26-29, 1974 MASTERUolC-AUAUSAECBINELASTIC NEUTRON AND X-RAY PHOTOEMISSION EXPERIMENTS ON USn3*G. H. Lander, T. 0. Brun, B. W. Veal, and D. J. LamArgonne National LaboratoryArgonne, Illinois 60439August 1974*Work performed under the auspices of the U.S. Atomic Energy Commission.To be published in the Proceedings of the Montreal Conference on"Crystalline Electric Field Effects in Metals and Alloys"INELASTIC NEUTRON AND X-RAY PHOTOEMISSION EXPERIMENfS ON USn3G. H. Land&r, T. 0. Bvun, B. W. Veal, and D. J. LamArgonne National LaboratoryArgonne, Illinois 60439ABSTRACTLow-temperature susceptibility measurements on USn3 indicatethat the material does not order magnetically, and that the lowestcrystal-field state is a nonmagnetic doublet or singlet with anexcited state some 20 - 100 K above in energy. Neutron Inelastictime-of-flight experiments have been performed to search for theanticipated transitions between the crystal-field states. Spectrataken at 297 and 90 K show no sign of any discrete magnetic excitations.X-ray photemission spectra from USn, show that the 5f_ electrons arelocated near the Fermi level so that strong hybridization between the5t_ and conduction electrons probably occurs.Work performed under the auspices of the U.S. Atomic Energy Commission.A..- 2 -I. INTRODUCTIONThe observation by inelastic neutron scattering of transitionsbetween crystal-field states in a variety of lanthanide systems hasaptly demonstrated the importance of the electrostatic interactionsthat give rise to the crystal field. The consequence of the crystalfield potential acting on a localized 4f_ electron state provides afirm phenomenological base from which to explore the magnetic behaviorof lanthanide compounds. If we turn our attention to the actinide (5f)series and compounds, no such convenient "base" appears to exist. Theextended spatial distribution of the 5f_ electrons, as compared to thatof the 4f electrons, suggests that crystal-field effects should be evenmore important in the actinide series, and photon spectroscopy on ionicsystems confirms this assumption. Strong crystal-field interactionsrequire a consideration of whether J mixing is important, although theconcept of discrete crystal-field levels still remains valid. Inattempting to observe the crystal-field transitions in the actinides wemust also recognize that if the splittings are very large the energydifferences might well be outside the range of thermal neutron spectro-scopy. In this case the neutron experiments would not observe the transi-tions between the states, but a null experiment should not be interpretedas evidence that no crystal-field states exist.- 3 -II. NEUTRON MEASUREMENTSIn an attempt tc measure the crystal-field transitions in anactlnide system we aave chosen USn.. This compound has the cubiceAuCu, structure with a lattice parameter of 4.63 A. MagnetizationA 5measurements ' show that the susceptibility x is independent oftemperature below *> 20 K. The experimental results are shown in2 3Fig. 1. Also included is a calculation based on the 5f_ : H, statewith F. as the ground state. A similar calculation can be performedwith the F, singlet as the ground state. No antiferromagnetic ex-change is included in this calculation. The inclusion of this termsimply shifts the theoretical curve rigidly upwards. Provided USn.,is a well localized system, the x vs. T curve in Fig. 1 indicatesthat the splitting A between the ground state and the first excitedstate is approximately 20-100 K. The transitions between these statesshould be readily observable by means of neutron spectroscopy.The results of tiine-of-flight inelastic neutron experiments on a18 g sample of polycrystalline USn- are shown in Fig. 2. This figure°-lrepresents the summation of all scattering such that 0.9 < Q < 3.3 A .At 297 K the "hump" on the energy gain side resembles crystal-fieldtransitions, but the spectrum at 90Kshows no significant peak other thanat E • 0. The difference between the 297 and 90 K data immediatelysuggests that phonon scattering may be present. Fhonon population factorsobey Bose statistics and the phonon scattering will be negligible at 90 K.On the other hand, the population of the crystal-field states obeys Boltz-mann statistics and these transitions should be observable, at least onthe energy loss side, even at low temperature.- 4 -QThe spectra of Ftp,. 2 may also be processed to yield the symmetrized>\»scattering function S (Q, u) as a function of energy hu for a series ofo-lmomentum transfers Q (- 4 rr sin 6/A). The results for Q - 2.2 and 4.5 Aare presented in Fig. 3 as a function of temperature for USn, and for theisostructural compound ThSn_. The latter compound has no 5f_ magneticelectrons but the lattice properties should be similar to those of USn 3«Figure 3 demonstrates (a) that no significant differences exist betweenthe scattering from USn, and ThSn_, (b) that the scattering is reducedsignificantly at lower temperatures, and (c) that the scattering increaseswith higher Q values. All these observations are compatible with thesuggestion that the scattering from USn- arises entirely from phonons.The solid lines drawn in the lower sections (90 K) are based on theassumption that the scattering at 297 K arises only from phonons and isreduced at 90 K by the Bose factor. This solid line is in reasonableagreement with the data. In addition, the scattering function at•-1 8-l 2Q - 4.5 A is approximately 4 times greater than at 2.2 A (i.e., a Qdependence), again compatible with phonon scattering.What, then, would be expected in the neutron spectra if crystal-fieldtransitions were really present? In Fig. 4 experimental spectra foro-lQ - 2.2 A and T » 90 K (taken from Fig. 3) are compared with the scatter-ing calculated for a model in which E (r 3) - E ( i y - A » 8 meV - 93 K(the overall splitting of the H^ manifold is then <v> 4 5 0 K ) . The solidline corresponds to spectra from the r~ - r, (8 m e V ) , r, - r, (8 m e V ) ,and r. - r, (16 meV) transitions. In simulating these spectra we haveassumed the crystal-field levels to be Gaussian in energy with a variable- 5 -full width at half maximum (FWHM). For the sclid line FWHM - 3 meV,for the dashed curve the FWHM « 10 meV. The physical origin of crystallevel broadening is at present not understood; however, widths in the9literature vary between 1 and 5 meV at 90 K for lanthanide materials.Of course, a further increase in the FWHM would make it impossible forneutrons to observe the crystal-field transitions, although the variouscomparisons in Fig. 3 suggest that all the inelastic scattering originatesfrom phonons.We emphasize that possible scattering from a number of other crystal-field schemes has also been examined (in particular those with I", singletground states), but in all cases fot which A £ 20 meV the models predictthe observation of crystal-field transitions. If A > 20 meV the neutronswould probably see no transitions, but, in this case, the x~ vs T curve(Fig. 1) would be difficult to interpret on the basis of a localizedelectron model.III. X-RAY PHOTOEMISSION EXPERIMENTSThe X-ray photoemission spectra (XPS) for VO2, a-V, and USn3 areshown in Fig. 5. The data were taken with a Hewlett-Packard 5950Aspectrometer using monochromatized AlKa radiation. The samples were—8prepared and measured in a pressure of less than 10 torr. However,uranium has a high affinity for oxygen, and, since the XPS measurementprobes only a few atomic layers of the surface, some residual oxidecontaminant always remains. Thus, the shoulders near -3eV in the o-Uand USn. spectra are probably caused by oxide.- 6 -For U0- a well-defined localized state exists. For uranium metal,however, the 5f electrons are known to be delocalized. The band containingthe hybridized 5_f electrons is seen cutting E_, in Fig. 5. The spectra forUSn, are remarkably similar to those obtained for uranium metal. In viewof the negative neutron experiment, the question then arises whether USn.,,like ct-U, might be more appropriately treated within the framework of bandtheory. The XFS of Fig. 5 show that the 5f electron states overlap theconduction band and may, therefore, significantly hybridize with it. How-ever, because of the limits imposed by the experimental resolution, theXPS results do not conclusively establish that the 5f_ electrons exist ina partially filled band that intersects E_.IV. CONCLUSIONSMagnetization experiments on cubic USn. strongly suggest that thiscompound would be a good candidate in which to observe crystal-fieldtransitions with inelastic neutron scattering. However, tlroe-of-flightexperiments indicate that the level of magnetic scattering from suchtransitions is at least an order of magnitude lower than the predictionsof any simple model. X-ray photoemission spectra from USn. show that the5f_ electrons lie near Ep and that hybridization between the 5f 6d, and7§_ electrons might occur.11 12Recently Brodsky end van Maaren e£ al., have examined the re-sistivity and specific heat of USn, at low temperature and independentlyconcluded that the electronic properties can best be understood by a spin-fluctuation model. In this model the 5f states are broadened due to direct- 7 -overlap of 5f_ wavefunctions, to 5jC—6d̂  hybridization, or to 5f_-conductionelectron interactions. However, the spin-fluctuation model is usuallyassociated with systems in which the actinide-actinide separation ise osmall (̂  3 A as in a-U), whereas this separation in USn. is 4.6 A. Whilethe interpretation of our experimental results appears to favor the spin-fluctuation model, we cannot exclude the possibility that broadened crystal-field states exist in USn_.ACKNOWLEDGMENTSWe are grateful to J. F. Reddy for sample preparation, to A. T. Aldredfor measuring the susceptibility of our sample, and to F. Faulikas, D. L.Price, and G. Ostrowskl for experimental assistance.4 * ' i- 8 -REFERENCES1. K. C. Turberfield, L. Passell, R. J. Birgeneau, and E. Bucher, Fhys.Rev. Lett. 25, 752 (1970); J. Appl. Phys. 4£, 1746 (1971); Phys. Rev.B 4^ 718 (1971); A. Furrer, J. J. Kjems, and 0. Vogt, J. Phys. C 5_,2246 (1972).2. B. G. Wybourne, Spectroscopic Properties of Rare Earths. (InterscienceMew York, 1965).3. D. J. Lam and S. K. Chan, Phys. Rev. B £, 808 (1974).4. J. Mulafe, Bull. Acad. Pol. Sci. 20, 883 (1972); A.Misiuk, J. Mulak,and A. Czopnik, Bull. Acad. Pol. Sci. 20, 891 (1972).5. K. H. J. Buschow and H. J. van Daal, AIP Conf. Proc. £, 1464 (1971).6. B. R. Cooper and 0. Vogt. Phys. Rev. B _1, 1218 (1970).7. R. Kleb, G. E. Ostrowskl, D. L. Price, and J. M. Rowe, Nucl. Inst.and Methods 106, 221 (1973).8. J. R. D. Copley, D. L. Price, and J. H. Rowe, Nucl. Inst. and Methods,107, 501 (1973).9. T. 0. Brun, G. H. Lander, D. L. Price, G. P. Felcher, and J. F. Reddy,Phys. Rev. B 9.. 248 (1974).10. B. W. Veal and D. J. Lara, Phys. Rev. (to be published).11. M. B. Brodsky, Phys. Rev. B 9_, 1381 (1974).12. M. H. van Maaren, H. J. van Daal, and K. H. J. Buschow, Solid StateComm. 14, 145 (1974).- 9 -FIGURE CAPTIONSFig. 1. Inverse susceptibility as a function of temperature for USn- fromRef. 4. The solid line Is f*rom a calculation discussed In the text.2The free-Ion f line (dashed) assumes no crystal-field or exchangeparameters.Fig. 2. Accumulated tirae-of-flight spectra for USn- at 297 and 90 K.Fig. 3. Scattering function versus energy for USn. and ThSn- at 297 and 90 Kfor two momentum transfers. The solid line corresponds to the phononscattering anticipated at 90 K (see text).Fig. 4. Scattering from USn- (points). The solid curve represents the antic-ipated crystal-field transitions If the levels have a FWHM of 3 meV.For the dashed curve FWHM - 10 meV.Fig. 5. XPS of U02, ot-U, and USn3>20 40 60 80 100TEMPERATURE i°K)120Fig. 1. Inverse susceptibility as a function of temperature forUSn3 from Ref. 4. The solid line is from a calculationdiscussed in the text. The free-ion f line (dashed)assumes no crystal-field or exchange parameters.Neg. No. MSD-59889.1000-wO3111MM—I—rUSn, 0.9 <O< 3.3 A~'297 K9»M''<*•«!•*«»»».1000-I I I 1 I I I I90 KiU2015 10 5 0 -5 -10 -15 -20 m«VI I I 1 I I I300 400 500 600 /xsec/mFig. 2. Accumulated time-o£-flight spectra for USru at297 and 90 K. Neg. No. MSD-59891.-20.3x100.1>wE32T T= 2.2 A"1 O USr>3 A ThSn 3297 K0-2 - f * 1 UOjO— o <s o10 15 ao-20.3x10 —0.20.190 Ko o __ oO-A101iw(«,eV)IS 20ZxlO2I.SI 'o«» 0.5OSUSn3 * T h S n 3297 Kooo o o V10 15ti (i> (ire V)20Fig. 3. Scattering frunction versus energy for USru and ThSn, at 297 and 90 K for twomomentum transfers. The solid line corresponds to the phonon scatteringanticipated at 90 K (see text). Neg. No. MSD-59892, 59890.Fig. 4. Scattering from USn, (points). The solid curve representsthe anticipated crystal-field transitions if the levels havea FWHM of 3 meV. For the dashed curve FWHM - 10 meV.Neg. No. MSD-S9887.USD,"S'\»v*i'''*!la-u•vuo2A/•• ^-14 -12 -10 -8 -6 -4 -2ENERGY (ev)2 4Fig. 5. XPS of U02, o-U, and USn3.Neg. No. MSD-60041

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