Figure 1: Shows the crystal structure of LnBa2Cu3O7, where Ln is the element Y or a rare earth element.
Outcome: Synthesis and diffraction studies of Ln2-xBa2+xCu2+yTi2-yO11, abbreviated LnBCTO here, with Ln=Tb and Nd; compounds with structural similarities to cuprate superconductors
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Cuprate high-temperature superconductors were first synthesized 1986. A common feature of the cuprate high-temperature superconductors is the presense of CuO2 sheets in the crystal structure as illustrated in Fig. 1 for the LnBa2Cu3O7 family of compounds.
Another cuprate family is Ln2-xBa2+xCu2+yTi2-yO11 (abbreviated LnBCTO), where Ln is a rare earth element. However, so far no high-temperature compound of this family has been synthesized. The first member of this family to be prepared was GdBCTO [3] and the LnBCTO crystal structure takes the form:
This family of compounds is seen to contain CuO2 sheets - the familiar constituent of cuprate high-temperature superconductors. Superconductivity is in many cases induced in cuprates by aliovalent doping. However, doping this family of compounds has been shown to be difficult and often resulting in multi phase samples [4].
In the work [1-2] we presented the first successful substitution of trivalent Ln with divalent ion. Neodynium in Nd2-xBa2+xCu2Ti2O11 was substituted with barium for vaues of x up to 0.8, and terbium in Tb2-xBa2+xCu2Ti2O11 was substituted with barium for the value x=0.2. In addition a cupper rich Tb2-xBa2+xCu2+yTi2-yO11 compound with x=0.2 and y=0.1 was synthesized. SQUID measurements found these compounds not to superconduct at high temperatures.
A closer analysis of the crystal structure of these compounds reveals a number of potential reasons for the lack of superconductivity. One of these reasons is Ti/Cu disorder, i.e. the Cu sites in the crystal are partially occupied by Ti ions and vice versa. Ti 'impurities' in the CuO2 sheets could explain the lack of superconductivity in these materials. However compounds with excess cupper, as reported in [2] and [5], have been found from neutron powder diffraction studies to contain fully occupied CuO2 sheets, still no high-temperature superconductivity was detected.