Structural Order Disorder Research Articles

Superconductivity and order disorder transformations in YBa 2(Cu 1−yCr y) 3O x

Homogeneous materials with perovskite related structures can be prepared over some range in YBa 2(Cu 1−yCr y) 3O x. Materials are studied which have been quenched from various annealing temperatures T a at various O 2 pressures (pO 2). Structural transformations as a function of increasing T a involve in all cases changes from an orthorhombic modification with b = c 3 ( O a,bc ) to an orthorhombic modification with a ≠ b ≠ c 3 ( O a,b,c ) to a tetragonal modification (T). Substituted and unsubstituted materials have ranges of T a resulting in relatively constant superconducting transition temperatures, T s, separated by critical annealing temperatures, T cr, beyond which T s strongly decrease. The first T cr T 1 cr = 777K) for YBa 2Cu 3O x is shown to correspond roughly to the extent of the O a,bc phase. A dependence of structural order disorder transformations on the mode of preparation (T, pO 2) is observed. As an example materials with orthorhombic O a,b,c and O a,bc structures can be prepared at low T and pO 2 values of x lower than the limit under air. T cr and T s decrease slightly with increasing y but follow the general trends of the parent compound. Results are discussed in the light of order-disorder transformations, superstructures and their temperature and composition dependence.

Structural order and partial disorder in biological systems

Structural order and partial disorder in biological systems