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Yb valence in YbMn 2 (Si,Ge) 2 J.M. Cadogan and D.H. Ryan Department of Physics and Astronomy, University of Manitoba Winnipeg, MB, R3T 2N2, Canada E-mail: [email protected] Department of Physics, McGill University Montreal, QC, H3A 2T8, Canada E-mail: [email protected]. - PowerPoint PPT Presentation
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“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Yb valence in YbMn2(Si,Ge)2
J.M. Cadogan and D.H. Ryan
Department of Physics and Astronomy, University of ManitobaWinnipeg, MB, R3T 2N2, Canada
E-mail: [email protected]
Department of Physics, McGill UniversityMontreal, QC, H3A 2T8, Canada E-mail: [email protected]
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Previous work
Nowik et al. [1] used magnetometry and 57Fe Mössbauer (on doped samples) to show that the Mn sublattice in YbMn2Si2 is antiferromagnetic (AF) below 520 K. A further transition at 35 K was attributed to a possible magnetic ordering of the Yb3+ sublattice. YbMn2Ge2 was shown to order at 495 K and to exhibit multiple magnetic transitions at lower temperatures. Yb was suggested to be divalent in the germanide, on the basis of cell-volume considerations.
Subsequent neutron diffraction work by Hofmann et al. [2-4] showed that YbMn2Ge2 is a planar AF below 510 K and exhibits spin-canting below 185 K. No ordering of the Yb sublattice was detected. Analysis of the T-dependence of the lattice parameters led to the suggestion that the Yb ion has a valence of 2.35 in the germanide.
Neutron diffraction indicated that the silicide orders in an axial AF structure below 526 K. The ‘event’ at 35 K was shown to be due to a rearrangement of the Mn moments into a cell-doubled AF state. The Yb3+ moments ordered below 10 K.
Electronic structure determinations by XPS were interpreted by Szytula et al. [5] as showing Yb to be trivalent in the silicide and divalent in the germanide.
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
YbMn2(Si,Ge)2
• Mn is the only transition metal to carry a magnetic moment in the RT2X2 series.
• Ytterbium is a Lanthanide (“Rare-Earth” R) element with an atomic number of 70.
• The most common ionization state for R ions is 3+, leaving Yb3+ with an outer electron configuration of 4f13, one electron short of a full 4f shell.
• Thus, we have the possibility of valence fluctuations or a mixed valence state since Yb2+ would have a 4f14 configuration i.e. a full 4f shell.
• Mössbauer spectroscopy can easily distinguish between Yb3+ and Yb2+
• Yb3+ has both a magnetic moment and a 4f contribution to the electric field gradient at the 170Yb nucleus; the full-4f-shell of Yb2+ has neither.
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
I=2
Mössbauer Spectroscopy of 170Yb
The 84.2 keV Mössbauer gamma-ray arises from the transition between the I=2 excited nuclear state and the I=0 ground state of the 170Yb nucleus.
170Tm 130 d
170Yb
84.2 keV, 1.6 ns
0 keV
–
I=0
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Experimental details• YbMn2(Si,Ge)2 samples were prepared by
arc-melting
• The crystal structure of YbMn2(Si,Ge)2 is
body-centred tetragonal ThCr2Si2-type with
the I4/mmm space group (#139)
• The Yb ions occupy the 2a sites with the point-group 4/mmm. Mn occupies the 4d sites and Si/Ge occupies the 4e sites.
• The 10 mCi 170Tm Mössbauer source was prepared by neutron activation of 25 mg of Tm as a 10 wt-% alloy in Aluminium.
• The source and sample were mounted vertically in a helium cryostat and the Mössbauer drive was operated in sine mode.
• The 84.2 keV Mössbauer -rays were detected with a HPGe detector.
• The drive was calibrated with a laser interferometer.
Yb
Mn
Si,Ge
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Yb2+ (small EFG)
170Yb Mössbauer spectra
All spectra were fitted using a non-linear, least-squares minimization routine with line positions and intensities derived from an exact solution to the full Hamiltonian [6].
Yb3+ (larger EFG due to 4f contribution)
YbMn2Si2-xGex
x
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Relative fractions of Yb2+ and Yb3+ in YbMn2(Si,Ge)2
x
Determined from the relative areas of the magnetic (3+) and non-magnetic (2+) spectral components
2+
3+
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
References[1] I. Nowik et al. J. Magn. Magn. Mater. 185 91-3 (1998)[2] M. Hofmann et al. J. Alloys Comp. 311 137-42 (2000)[3] M. Hofmann et al. J. Phys.: Condens. Matter 13 9773-80
(2001)[4] M. Hofmann et al. Appl. Phys A74 S713-5 (2002)[5] A. Szytula et al. J. Alloys Comp. 366 313-8 (2004)[6] D.H. Ryan et al. J. Phys.: Condens. Matter 16 6129-38 (2004)
Conclusion170Yb Mössbauer spectroscopy provides a direct and unambiguous determination of the valence of the Yb ion in the YbMn2(Si,Ge)2 family of intermetallics.
