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Abstract:
The compound Yb3Au5.5Ga5.5 was found during a systematic research of
ternary ytterbium gallides with transition metals. The crystal structure
was investigated by means of powder and single-crystal methods, with the
following results: space group Pmm2, a = 4.4746(8) angstrom, b =
13.294(2) angstrom, c = 18.067(3) angstrom, V = 1074.7(5) angstrom3, Z =
4, D(x) = 12.17 mg mm-3. The intensities of 1500 reflections were
measured (four-circle diffractometer, Mo Kalpha radiation, (sin
theta/lambda)max = 0.65 angstrom-1, T = 293 K, F(000) = 3260); 1065
reflections were used for structure solution and refinement. The final R
value is 0.0427 (anisotropic thermal coefficients) for 36 atomic sites
(186 variable parameters). There are 12 atom sites occupied fully by Au,
12 by Ga and four sites show the statistical Au/Ga occupation. The
shortest interatomic distances are: Yb-Au, 3.046 A; Yb-(Au,Ga), 3.086
angstrom; Yb-Ga, 3.075 angstrom; Yb-Yb, 3.901 angstrom; Au-Au, 2.880
angstrom; Au-(Au,Ga), 2.648 angstrom; Au-Ga, 2.52 angstrom;
(Au,Ga)-(Au,Ga), 2.721 A; (Au,Ga)-Ga, 2.63 angstrom; Ga-Ga, 2.77
angstrom. The coordination numbers of the atoms are as follows: for Yb,
17 or 18; for Au, (Au,Ga) and Ga, 10 or 12.
The crystal structure of Yb3Au5.5Ga5.5 is the first representative of a
new structure type (Pearson symbol oP56) and can be described as an
occupation and deformation variant of the La3Al11 structure as well as a
deformation and defect variant of the BaAl4 type of structure:
V(Yb3Au5.5Ga5.5) almost-equal-to 6V(La2Al11) almost-equal-to 6V(BaAl4),
where V is the unit cell volume; (a,b,c)Yb3Au5.5Ga5.5 almost-equal-to
(a,b,2c)La2Al11 almost-equal-to (a,3b,2c)BaAl4. The representatives of
all structure types mentioned exist in the ternary systems
Yb-{Cu,Ag,Au)-Ga at 600-degrees-C.
