Yb valence in YbMn2(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
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.
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.
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
b–
I=2
84.2 keV, 1.6 ns
I=0
0 keV
170Yb
130 d
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.
Yb
•
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 g-rays
were detected with a HPGe
detector.
•
The drive was calibrated with a
laser interferometer.
Mn
Si,Ge
170Yb
Mössbauer spectra
YbMn2Si2-xGex
All spectra were fitted using a non-linear, leastsquares minimization routine with line positions
and intensities derived from an exact solution to the
full Hamiltonian [6].
Yb2+
(small EFG)
Yb3+
(larger EFG due to
4f contribution)
x
Relative fractions of Yb2+ and Yb3+ in YbMn2(Si,Ge)2
Determined from the relative areas of the magnetic (3+)
and non-magnetic (2+) spectral components
2+
3+
x
Conclusion
170Yb
Mössbauer spectroscopy provides a direct and unambiguous
determination of the valence of the Yb ion in the YbMn2(Si,Ge)2
family of intermetallics.
References
[1]
[2]
[3]
[4]
[5]
[6]
I. Nowik et al. J. Magn. Magn. Mater. 185 91-3 (1998)
M. Hofmann et al. J. Alloys Comp. 311 137-42 (2000)
M. Hofmann et al. J. Phys.: Condens. Matter 13 9773-80 (2001)
M. Hofmann et al. Appl. Phys A74 S713-5 (2002)
A. Szytula et al. J. Alloys Comp. 366 313-8 (2004)
D.H. Ryan et al. J. Phys.: Condens. Matter 16 6129-38 (2004)