YbAs compounds

Q Calculate the average copper oxidation state in the compounds YBa,Cu30g and YBa2Cu30y and hence predict whether they could be hole-based superconductors. 

The structurally simplest cuprate superconductors are doped La CuO (the T or T/0 structure), doped Nd CuO (the T stmcture) and oxygen-deficient YBa GUjO,. At room temperature, La CuO adopts a distorted version of the K NiF structure, in which the CuO octahedra are Jahn-Teller lengthened along the c-axis (Figure 4.7). At higher temperatures the Jahn-Teller distortion is lost and the material adopts the undistorted K NiF stmcture. The room temperature insulating stoichiometric compound is transformed into a superconductor by replacement of some of the La " cations with the alkaline earth cations Ba, Sr + or Ca. The stmcture of Nd CuO ... 

With the exception of Ce systems, lanthanide compounds and alloys made with Yb most frequently exhibit Kondo anomalies. Two of the most thoroughly studied Yb systems are YbA and YbAls. The phase diagram for the Yb-Al system has been reported by Palenzona (1972). YbA crystallizes in the cubic MgCu2 Laves phase while YbAL forms in the cubic CujAu structure. landelli and Palenzona (1972) measured the lattice constants of both compounds from 80-800 K. 

Another significant development was the superconducting system of a mixed oxide of yttrium reported by Wu, Chu and co-workers. The compound has a formula YBa CujO, where x is between 6 and 7 and a T, 93 K which means that the expensive liquid helium can be replaced by cheap liquid nitrogen (77 K). For simplicity, the above system is called the 1-2-3 system based on the ratio of the metals present. This also has a perovskite structure. Similar compounds substituting yttrium by other lanthanides like europium, samarium etc., are known. These are called warm superconductors. A lot of research to obtain high T are going on and superconductors with Tj, as high as 164 K have been reported. 

Figure 28. Comparison of tbe Cu 2py2 and satellite XPS spectra from several copper compounds with the spectrum from the superconducting oxide YBa CuiOr (67) a) CuO, r=3.25 eV b) La2Cu04. r=3.30eV c) YBajCujO,. r =. 3.20eV d) NaCuO . T= 1.60 eV e) Cu satellites...

The 4f electron character of Yb compounds is supposed to be similar to that of Ce compoimds. The dHvA measurements for Yb compounds are small in number. The Fermi surface of YbAs with magnetic ordering is similar to that of LaSb. 

By 1987, superconductivity was observed in another ceramic, yttrium barium copper oxide (YBCO) (YBa CuiOy). Commonly called a 1-2-3 compound because of the 1 2 3 ratio of yttrium to barium to copper. YBCO has a 7 of 93 K (above the temperature of hquid nitrogen). Although 93 K is still cold, it qualifies as quite warm in the world of superconductors, so YBCO is considered to be the first high-temperature superconductor. YBCO was first prepared by combining the three metal carbonates at high temperature (1000 to 1300 K). The copper ions are present in a mixture of -1-2 and 3-3 oxidation states ... 

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