VACANCY BEHAVIOR AND HOLE NUMBERS IN YBA2CU3O7-X

We have performed a systematic theoretical study of the effects of de-oxygenation on the plane hole numbers in YBa2Cu3O7 - x compounds, by calculating the total and partial densities of electronic states for several different orthorhombic structures corresponding to various forms of vacancy orderings. We show that as x increases the Fermi energy Ef moves upwards and the density of states at Ef decreases. The movement of the Fermi energy depends upon the type of structure under consideration and for structures having fragmented chains Ef remains constant over a large interval of compositions. This behaviour leads to plateaus in the hole number versus x curve which can be related to the behaviour of the critical temperature. © 1990.