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 Department of Physics, McGill University Montreal, QC, H3A 2T8, Canada

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.

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.

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

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

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

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+

References [1]I. Nowik et al. J. Magn. Magn. Mater (1998) [2]M. Hofmann et al. J. Alloys Comp (2000) [3]M. Hofmann et al. J. Phys.: Condens. Matter (2001) [4]M. Hofmann et al. Appl. Phys A74 S713-5 (2002) [5]A. Szytula et al. J. Alloys Comp (2004) [6]D.H. Ryan et al. J. Phys.: Condens. Matter (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.