- Email: [email protected]
Crystal-field measurements and structural investigations by neutron scattering of quasi crystalline Pd58.8U20.6Si20.6
Physica B 156 & 157 (1989) 31-32 North-Holland, Amsterdam
CRYSTAL-FIELD MEASUREMENTS AND STRUCTURAL INVESTIGATIONS BY NEUTRON SCATTERING OF QUASI CRYSTALLINE Pd,8~,U,,~,Si,,~, R. FUCHS’, A. FURRER*, P. FISCHER’,
H. HEER2 and H. RUDIN’
‘lnstitut fiir Physik, Klingelbergstr. 82, 4056 Basel, Switzerland ‘Laboratorium fiir Neutronenstreuung, Eidgendsische Technische Hochschule
Ziirich, 5303 Wiirenlingen, Switzerland
Quasi crystalline Pd,,,,U,, &,, 6 has been investigated by elastic and inelastic neutron scattering (INS). In the energy loss range of O-50 meV in the INS spectra we found a crystal-field transition at about 3 meV of considerable broadness which is in agreement with a model calculation with point charges located at the vertices of a slightly disturbed icosahedron.
1. Introduction
3. Experimental
Since the discovery of the fivefold symmetry by electron diffraction in rapidly quenched AlMn alloys [l] crystallographers and physicists try to figure out the arrangement of the atoms in materials with crystallographic forbidden symmetries. Various theories explaining the structure of these new materials have been published was the alloy we investi[2? 31.Ph3.8U20.6Si20.6 gated [4,5]. Although ternary, this material has two striking advantages: - The icosahedral phase can be reached by annealing the metallic glass without disturbing crystalline contributions (less than 1 percent crystalline phase percent). - Uranium offers the possibility of measuring crystal-field splitting.
The elastic neutron scattering experiment was performed with thermal neutrons of 1.093 A wavelength from the research reactor Saphir at Wiirenlingen. The intensities were measured with a position sensitive detector. The cylindrical sample was cooled to 16 K. The raw data were corrected for absorption, multiple scattering and incoherent contributions and then normalized at high angles. The inelastic neutron scattering experiments (INS) have been carried out on the triple-axis spectrometer R5 operated in the neutron energy loss configuration with an energy of 13.7 meV for the incident beam. 4. Results and discussion factor structure S(Q) = was calculated (fig. 1). All peaks can be indexed. Following Elser [6] and identifying the strongest peaks with (211111) and (221001), we get a quasi crystalline lattice constant corresponding to a vertex distance of the rhombohedron of 5.15 A. In the energy-loss configuration we measured at various temperatures and scattering vectors in the range of 1.1 G Q G 4.0 A-’ and 10 G T 6 300 K respectively. In the energy loss range of O-50 meV we observed several excitations. With one exception The
2. Sample preparation The metallic glass Pd58,8U2,,,6Si20.6was produced by melt spinning. By annealing the glass at 790-805 K for 120-200 s in a helium atmosphere, the material is transformed into the quasi crystalline phase. A differential scanning calorimetry (DSC) analysis determined the proportions of the amorphous and crystalline phases in the quasi crystalline material to within 7% and less than 1% respectively.
total
Zcoh( Q)lm
0921-4526/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
32
R. Fuchs et al.
I Crystal field in quasi crystalline 1
oL”--“i/
I
2
0
I
4 Q
Fig. 1. Structure
factor
I
I
E
10
ri?11
of quasi
crystalline
Pd,, RUZ,, hSiZ(,h.
they can be interpreted as phonon interactions getting stronger with increasing temperature and higher Q-values. At about 3 meV we see a crystal-field transition with a FWHM of about 3-4 meV, which is considerably larger than the instrumental resolution of 1 meV (fig. 2). The spectra in the low energy-loss range with momentum transfer Q < 2 A-’ were corrected for phonon scattering by extrapolating the data with Q = 2.9 A-’ using the Q2-dependence of the differential neutron cross section for phonons. In various theories of the quasi crystal the icosahedral symmetry plays a dominant role
Pd-U-5
[3]. We therefore made a model calculation for the crystal-field splitting, locating point charges at the vertices and the uranium atom in the center of a icosahedron. In the Stevens operator equivalent’s representation of
only Bi and Bi are different from zero in this calculation [7]. This leads to an energy splitting of five- and fourfold degeneracy in the J = 4 state and of six- and fourfold degeneracy in the J = 9/2 state. In both cases it exists just one transition from the ground level which is consistent with our measurements. It has been noticed in recent papers [8,9] that the quasi crystalline order in Pd,,~,U,,,,Si,, 6 is mostly due to the U-U correlations and that the Pd and Si atoms which are the preferred neighbours of the uranium atom seem to be rather disordered, similar to the glassy state. So we calculated the change in the splitting of the energy levels due to a slight deviation from the perfect icosahedral symmetry. This calculation leads to a shift in the energy difference between ground state and excited state by an amount which explains the width of the measured crystalfield peak. Acknowledgements
1600
We would like to thank H. Breitenstein and P. Reimann for the preparation of the metallic glasses and the Swiss National Science foundation for financial support. References [I] D. Shechtman, [2] [3]
600 -10
[4] -8
-6 &ne~l
O
Fig. 2. Energy spectra of neutrons scattered from quasi and corrected for phonon scattercrystalline Pd,s,U,,,,Si,,, ing. Squares and circles indicate measurements at Q = 1.1 A-’ and Q = 2.0 A-’ respectively, (open symbols for T= 10 K, and full symbols for T = 120 K). The lines represent a Lorentz fit.
[5] [6] [7] [8] [9]
I. Blech, D. Gratias and J.W. Chan, Phys. Rev. Lett. 53 (1984) 1951. D. Levine and P.J. Steinhardt, Phys. Rev. Lett. 53 (1984) 2477. P.W. Stephens and A.I. Goldmann, Phys. Rev. Lett. 56 (1986) 1168. S.J. Poon, A.J. Drehman and K.R. Lawless, Phys. Rev. Lett. 55 (1985) 2324. K.M. Wong and S.J. Poon, Phys. Rev. B 34 (1986) 7371. V. Elser, Phys. Rev. B 32 (1985) 4892. K.W.H. Stevens, Proc. Phys. Sot. A 65 (1952) 209. D.D. Kofalt, S. Nanao, T. Egami, K.M. Wong and S.J. Poon, Phys. Rev. Lett. 57 (1986) 114. R. Fuchs, S. Jost, H. Rudin, H.-J. Giintherodt and P. Fischer, Mat. Sci. and Eng. 89 (1989), in press.
CRYSTAL-FIELD MEASUREMENTS AND STRUCTURAL INVESTIGATIONS BY NEUTRON SCATTERING OF QUASI CRYSTALLINE Pd,8~,U,,~,Si,,~, R. FUCHS’, A. FURRER*, P. FISCHER’,
H. HEER2 and H. RUDIN’
‘lnstitut fiir Physik, Klingelbergstr. 82, 4056 Basel, Switzerland ‘Laboratorium fiir Neutronenstreuung, Eidgendsische Technische Hochschule
Ziirich, 5303 Wiirenlingen, Switzerland
Quasi crystalline Pd,,,,U,, &,, 6 has been investigated by elastic and inelastic neutron scattering (INS). In the energy loss range of O-50 meV in the INS spectra we found a crystal-field transition at about 3 meV of considerable broadness which is in agreement with a model calculation with point charges located at the vertices of a slightly disturbed icosahedron.
1. Introduction
3. Experimental
Since the discovery of the fivefold symmetry by electron diffraction in rapidly quenched AlMn alloys [l] crystallographers and physicists try to figure out the arrangement of the atoms in materials with crystallographic forbidden symmetries. Various theories explaining the structure of these new materials have been published was the alloy we investi[2? 31.Ph3.8U20.6Si20.6 gated [4,5]. Although ternary, this material has two striking advantages: - The icosahedral phase can be reached by annealing the metallic glass without disturbing crystalline contributions (less than 1 percent crystalline phase percent). - Uranium offers the possibility of measuring crystal-field splitting.
The elastic neutron scattering experiment was performed with thermal neutrons of 1.093 A wavelength from the research reactor Saphir at Wiirenlingen. The intensities were measured with a position sensitive detector. The cylindrical sample was cooled to 16 K. The raw data were corrected for absorption, multiple scattering and incoherent contributions and then normalized at high angles. The inelastic neutron scattering experiments (INS) have been carried out on the triple-axis spectrometer R5 operated in the neutron energy loss configuration with an energy of 13.7 meV for the incident beam. 4. Results and discussion factor structure S(Q) = was calculated (fig. 1). All peaks can be indexed. Following Elser [6] and identifying the strongest peaks with (211111) and (221001), we get a quasi crystalline lattice constant corresponding to a vertex distance of the rhombohedron of 5.15 A. In the energy-loss configuration we measured at various temperatures and scattering vectors in the range of 1.1 G Q G 4.0 A-’ and 10 G T 6 300 K respectively. In the energy loss range of O-50 meV we observed several excitations. With one exception The
2. Sample preparation The metallic glass Pd58,8U2,,,6Si20.6was produced by melt spinning. By annealing the glass at 790-805 K for 120-200 s in a helium atmosphere, the material is transformed into the quasi crystalline phase. A differential scanning calorimetry (DSC) analysis determined the proportions of the amorphous and crystalline phases in the quasi crystalline material to within 7% and less than 1% respectively.
total
Zcoh( Q)lm
0921-4526/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
32
R. Fuchs et al.
I Crystal field in quasi crystalline 1
oL”--“i/
I
2
0
I
4 Q
Fig. 1. Structure
factor
I
I
E
10
ri?11
of quasi
crystalline
Pd,, RUZ,, hSiZ(,h.
they can be interpreted as phonon interactions getting stronger with increasing temperature and higher Q-values. At about 3 meV we see a crystal-field transition with a FWHM of about 3-4 meV, which is considerably larger than the instrumental resolution of 1 meV (fig. 2). The spectra in the low energy-loss range with momentum transfer Q < 2 A-’ were corrected for phonon scattering by extrapolating the data with Q = 2.9 A-’ using the Q2-dependence of the differential neutron cross section for phonons. In various theories of the quasi crystal the icosahedral symmetry plays a dominant role
Pd-U-5
[3]. We therefore made a model calculation for the crystal-field splitting, locating point charges at the vertices and the uranium atom in the center of a icosahedron. In the Stevens operator equivalent’s representation of
only Bi and Bi are different from zero in this calculation [7]. This leads to an energy splitting of five- and fourfold degeneracy in the J = 4 state and of six- and fourfold degeneracy in the J = 9/2 state. In both cases it exists just one transition from the ground level which is consistent with our measurements. It has been noticed in recent papers [8,9] that the quasi crystalline order in Pd,,~,U,,,,Si,, 6 is mostly due to the U-U correlations and that the Pd and Si atoms which are the preferred neighbours of the uranium atom seem to be rather disordered, similar to the glassy state. So we calculated the change in the splitting of the energy levels due to a slight deviation from the perfect icosahedral symmetry. This calculation leads to a shift in the energy difference between ground state and excited state by an amount which explains the width of the measured crystalfield peak. Acknowledgements
1600
We would like to thank H. Breitenstein and P. Reimann for the preparation of the metallic glasses and the Swiss National Science foundation for financial support. References [I] D. Shechtman, [2] [3]
600 -10
[4] -8
-6 &ne~l
O
Fig. 2. Energy spectra of neutrons scattered from quasi and corrected for phonon scattercrystalline Pd,s,U,,,,Si,,, ing. Squares and circles indicate measurements at Q = 1.1 A-’ and Q = 2.0 A-’ respectively, (open symbols for T= 10 K, and full symbols for T = 120 K). The lines represent a Lorentz fit.
[5] [6] [7] [8] [9]
I. Blech, D. Gratias and J.W. Chan, Phys. Rev. Lett. 53 (1984) 1951. D. Levine and P.J. Steinhardt, Phys. Rev. Lett. 53 (1984) 2477. P.W. Stephens and A.I. Goldmann, Phys. Rev. Lett. 56 (1986) 1168. S.J. Poon, A.J. Drehman and K.R. Lawless, Phys. Rev. Lett. 55 (1985) 2324. K.M. Wong and S.J. Poon, Phys. Rev. B 34 (1986) 7371. V. Elser, Phys. Rev. B 32 (1985) 4892. K.W.H. Stevens, Proc. Phys. Sot. A 65 (1952) 209. D.D. Kofalt, S. Nanao, T. Egami, K.M. Wong and S.J. Poon, Phys. Rev. Lett. 57 (1986) 114. R. Fuchs, S. Jost, H. Rudin, H.-J. Giintherodt and P. Fischer, Mat. Sci. and Eng. 89 (1989), in press.