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Abstract:
A novel intermetallic compound with two crystallographic modifications,
alpha- and beta-YbAgGa2, was prepared by argon high frequency melting
from ingots of the elements in tantalum crucibles with homogenization
heat treatments at 600 degrees C (beta-YbAgGa2) and 400 degrees C
(alpha-YbAgGa2).
The crystal structure of beta-YbAgGa2 has been refined from powder
diffraction data obtained using a Ge(111) primary monochromator and a
linear position-sensitive detector. The sample was measured in
transmission mode with Cu K alpha(1) radiation in the range 10 degrees<2
theta<130 degrees, Delta theta=0.02 degrees: a=6.9653(3) Angstrom,
b=4.3362(2) Angstrom, c=10.3386(4) Angstrom, V=311.85(4) Angstrom(3).
Full profile refinement led to the residual values R(I)=0.061 and
R(P)=0.081 (for 303 reflections), space group Pnma, oP16; 4 Yb in 4(c) x
1/4 z, x=0.2241(2), z=0.82774(9), 4 Ga1=0.926(6)Ga + 0.074(6)Ag in 4(c),
x=0.2950(3), z=0.5369(2), 4 Ga2 in 4(c), x=0.0751(3), z=0.1497(2), 4
Ag=0.951(6)Ag + 0.049(6) Ca in 4(c), x=0.4595(2), z=0.0883(1).
Coordination numbers are 16 for Yb, 10 for Ga and 12 for Ag. Shortest
interatomic distances were revealed for the following: Yb-Ga, 3.014
Angstrom; Yb-Ag, 3.183 Angstrom; Yb-Yb, 3.832 Angstrom; Ga-Ga, 2.622
Angstrom; Ga-Ag, 2.720 Angstrom; Ag-Ag, 3.024 Angstrom. The structure of
beta-YbAgGa2 is a distorted variant of the ScRhSi2 type.
Magnetic susceptibility measurements (4.8-900 K) revealed a rather low
effective paramagnetic moment for beta-YbAgGa2 (0.5 mu(B) (Yb atom)(-1),
Theta(p)=6.5 K) whereas a tripositive ground state is inferred for
alpha-YbAgGa2 (about 4.5 mu(B) (Yb atom)(-1), Theta(p)=-71 K).
