The dependence of the lattice parameters on oxygen content in orthorhombic YBa2Cu3O6+x:: A high precision reinvestigation of near equilibrium samples

A new high precision investigation of the unit cell geometry as a function of the oxygen content is presented for YBa2Cu3O6+x, The differences from earlier work by other groups are (a) the use of "near-equilibrium," i.e., not quenched but very slowly cooled samples; (b) the use of high accuracy volumetric oxygen determination; and (c) the large number of compositions investigated (36 in the range of 0.5 < x < 1.0). Special attention has been paid to very accurate control of the oxygen content during synthesis and to homogenization of the samples. A similarity to most of the work in the literature is that, differing from our previous work, the samples used here were synthesized from a carbonate Ba source. The results show that the parameters of the synthesis can influence the unit cell geometry. Thus, in samples synthesized with a Ba-carbonate precursor the c-parameter remains linear with the oxygen content. This is in agreement,vith the literature and opposite to our samples synthesized in the absence of carbonate. However, the a- and b-parameters change nonlinearly with the oxygen content, in agreement with our previous work with samples without carbonate precursors. For this reason the orthorhombicity shows a maximum corresponding to the T-c maximum. These deviations from linearity of the lattice parameters mirror lattice distortions and phase transitions in 123, briefly discussed here. Analytical functions are given which allow the estimation of the oxygen content from the lattice parameter. (C) 1997 Academic Press.