Physica B 312–313 (2002) 899–901 Crystal field excitations in U 3 Pd 20 Si 6 K. Kuwahara a, *, M. Kohgi a , N. Tateiwa b , R.I. Bewley c , J. Allen d , K.A. McEwen d , N. Kimura e , H. Aoki e , T. Komatsubara e a Department of Physics, Faculty of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan b Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan c ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UK d Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK e Center for Low Temperature Science, Tohoku University, Sendai 980-8578, Japan Abstract Crystal field (CF) excitations in the ternary intermetallic uranium compound U 3 Pd 20 Si 6 have been measured using inelastic neutron scattering. Three inelastic peaks were observed at energy transfers B12, B23 and B32 meV. This is direct evidence that the 5f electrons in this material are well localized. From this result and the bulk properties of U 3 Pd 20 Si 6 ; possible CF level schemes are discussed. The strong magnetic excitation below the antiferromagnetic phase transition temperature of 19 K is also reported. r 2002 Elsevier Science B.V. All rights reserved. Keywords: U 3 Pd 20 Si 6 ; Crystal field excitations; Inelastic neutron scattering Recently, a new ternary intermetallic compound U 3 Pd 20 Si 6 has been found [1]. The crystal structure of U 3 Pd 20 Si 6 belongs to a cubic space group of Fm % 3m with a lattice parameter 12.205 ( A, where U ions occupy two different crystallographic sites (8c and 4a). Macroscopic measurements on U 3 Pd 20 Si 6 revealed two successive magnetic phase transitions at T N ¼ 19 K and T C ¼ 2 K. Neutron diffraction experiments identified the magnetic phase below T N as an antiferromagnetic (AF) ordering of magnetic moments at 8c site with a propagation vector Q ¼½1; 1; 1: On the other hand, the phase transition below T C is considered to be a ferromagnetic transition at 4a site, because magnetic susceptibility diverges at T C : The large magnetic entropy (R ln3 per U ion at T N ) and the effective Curie–Weiss paramagnetic moment (m eff ¼ 3:30m B /U ) indicate that the 5f electrons in this system are likely to be localized. Furthermore, a strong softening of elastic constant c 44 in the para- magnetic phase and in the AF phase suggests that the localized orbital (quadrupolar) degrees of freedom remain even in the AF phase (T pT N ). To confirm this localized nature of 5f electrons in U 3 Pd 20 Si 6 ; we carried out inelastic neutron experiment. The experiment was done on the chopper spectro- meter HET at the ISIS spallation neutron source. A large amount (B30 g) of fine U 3 Pd 20 Si 6 powder was filled under helium gas atmosphere into an aluminum container and mounted in a closed cycle refrigerator. Fig. 1 shows the HET energy spectra with an incident energy of E i ¼ 60 meV for different temperatures. The symbols represent the spectra at the low angle detector bank (an average scattering angle f ¼ 51), while the lines represent the phonon spectra estimated by multi- plying the data of U 3 Pd 20 Si 6 at the high angle detector bank (f ¼ 1331) by the ratio of the low angle data to the high angle data obtained from the nonmagnetic reference material LaRhSb at the HET spectrometer. We observed three inelastic peaks, A at B12 meV, B at B23 meV and C at B32 meV in both the paramagnetic phase and the AF phase. Clearly, these three peaks are magnetic in origin. Among these three peaks, peaks B and C decrease as temperature is increased in the paramagnetic phase, as expected for excitations from the ground state. This direct observation of CF excitations evidences the localized character of the 5f electrons in *Corresponding author. Tel.: +81-426-77-1111; fax: +81- 426-77-2483. E-mail address: kuwahara@phys.metro-u.ac.jp (K. Kuwahara). 0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0921-4526(01)01267-4