1. 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: cadogan@physics.umanitoba.ca Department of Physics, McGill University Montreal, QC, H3A 2T8, Canada E-mail: dhryan@physics.mcgill.ca

2. Previous work Nowik et al. [1] used magnetometry and 57 Fe Mössbauer (on doped samples) to show that the Mn sublattice in YbMn 2 Si 2 is antiferromagnetic (AF) below 520 K. A further transition at 35 K was attributed to a possible magnetic ordering of the Yb 3+ sublattice. YbMn 2 Ge 2 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 YbMn 2 Ge 2 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 Yb 3+ 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.

3. YbMn 2 (Si,Ge) 2 Mn is the only transition metal to carry a magnetic moment in the RT 2 X 2 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 Yb 3+ with an outer electron configuration of 4f 13, one electron short of a full 4f shell. Thus, we have the possibility of valence fluctuations or a mixed valence state since Yb 2+ would have a 4f 14 configuration i.e. a full 4f shell. Mössbauer spectroscopy can easily distinguish between Yb 3+ and Yb 2+ Yb 3+ has both a magnetic moment and a 4f contribution to the electric field gradient at the 170 Yb nucleus; the full-4f-shell of Yb 2+ has neither.

4. I=2 Mössbauer Spectroscopy of 170 Yb 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 170 Yb nucleus. 170 Tm 130 d 170 Yb 84.2 keV, 1.6 ns 0 keV –– I=0

5. Experimental details YbMn 2 (Si,Ge) 2 samples were prepared by arc-melting The crystal structure of YbMn 2 (Si,Ge) 2 is body-centred tetragonal ThCr 2 Si 2 -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 170 Tm 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

6. Yb 2+ (small EFG) 170 Yb 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]. Yb 3+ (larger EFG due to 4f contribution) YbMn 2 Si 2-x Ge x x

7. Relative fractions of Yb 2+ and Yb 3+ in YbMn 2 (Si,Ge) 2 x Determined from the relative areas of the magnetic (3+) and non-magnetic (2+) spectral components 2+ 3+

8. 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) Conclusion 170 Yb Mössbauer spectroscopy provides a direct and unambiguous determination of the valence of the Yb ion in the YbMn 2 (Si,Ge) 2 family of intermetallics.