T-O transition

The step appears also in the apical bond (Cu2-01) (fig. 14). Cava et al. (1990) concluded from this abrupt structural change that the superconductivity is triggered by the T —> O transition and, therefore, it appears at the onset of the orthorhombic phase. Their paper presents the first systematic interdisciplinary investigation of the structural parameters and magnetic properties as a function of the nonstoichiometry, supporting the charge-transfer model from the chains to the planes which is now generally accepted. 

Fig. 13. Site occupancies for oxygen in the 123-0, structure as a function of nonstoichiometry from RT neutron diffiaction. 6 1-x is based on the measured weight loss. Note the use of 01 for the chain sites and 04 for the apical sites. A smoodi T-O transition takes place, indicating a second-order transition. This is in contrast to later experiments (Radaelli et al. (1992) cf. lig. 34, sect. 3.3). After Jorgensen et al. (1990a).

Fig. 14. Dependence of the apical bond Cu2-01 on the oxygen nonstoichiometry. Note the abrupt change of the Zr-getteied samples (solid squares) at the T — O transition. NPD data, after Cava et al. (1990) and Hewat (1994) at r=5K, and Jorgensen et al. (1990a) (open circles) at RT.

Fig. 21. Bond valence sums of Cul and Cu2 as a ftmction of oxygen nonstoichiometry. Note that the BVS of Cu2 follows very nearly the shape of the curve (fig. 2) and reflects the T —> O transition and the optimally doped phase. The BVS of the chain Cul increases lineaily due to the increase of the carriers concentration with doping. After Cava et al. (1990).

Fig. 25. Lattice parameters of equilibrium CAR samples as a function of oxygen nonstoichiometry, after data of Kruger et al. (1997) and Conder et al. (1999). (a) a-parameter (b) 6-parameter (c) orthorhombic strain (d) c-parameter (e) unit cell volume. Note the steep change at the T — O transition, and the deviations from linearity.

The stepwise change of the apical bond (fig. 14) and of the BVS (fig. 21) at the transition supported the idea (Miceli et al. 1988, Cava et al. 1988a,b, 1990) that the I-M transition is coupled with the T-O transition enabling the onset of the charge transfer. 

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