Mixed Layers in Copper Based Superconducting Materials

Abstract

Abstract. 2014 Recently discovered series of high Tc superconductors, characterized by the existence of two types of cations within the same layer, are presented. The first family concerns the mercury based cuprates, Hg1-xMxA2Cam-1CumO2m+2+03B4, with A = Ba and/or Sr, which exhibit structures closely related to that of the thallium cuprates TlBa2Cam-1CumO2m+3. They differ from the thallium cuprates by a high oxygen deficiency at the level of the mercury layer. It is shown that cations such as M = Cu, Pb, Tl, Bi, Ce, Pr, Cr, V, Mo, W, Ti, Sr, Ca,... can partially substitute for mercury ions, stabilizing the structures. The cationic composition of the layer depends indeed on the nature of the M cation but also on that of the alkaline earth A. For given A and M cations, the x value remains unchanged even when the number of copper layers varies. M and Hg cations are either statistically distributed over the same site or ordered. Different types of ordering have been detected. Another way of generating mixed layers is to shear periodically the structure, leading to the formation of the so called "collapsed phase". In the collapsed bismuth cuprates, bismuth and copper segments, a few octahedra long, alternate in strongly waving layers. In the collapsed oxycarbonates, carbonate groups and M cations are ordered within the intermediate layer so that they can be simply described from a partial and ordered substitution of carbon for Hg,Tl, Bi and other M cations building the intermediate layer. The oxycarbonitrates (Y1-xCax)nBa2nCu3n-1(C,N)O3O7n-3 can also be described as an ordered substitution of carbon for copper in the 123 matrix. The different families of superconducting materials which are generated by such mechanisms are described as well as the way the different species are distributed within the mixed layers. Their influence on the physical properties are discussed.

Cation Ordering in [(Tl,M)O] Layers of “1201”-Based Cuprates: Similarity to Ordering in fcc-Based Alloys

“…These results suggest that the transition elements are located on both the Tl and the Cu positions, particularly for Nb-and Ta-based samples. This behavior is different from the results observed for M"Mo (11) (Mo is only substituting for Tl), as well as from the results in Cr-based compounds which form chromate groups exhibiting long-range ordering with the thallium polyhedra, similar to those observed in the carbonate groups in the 1201 phases (see (21) for a review). It should be noted that there is always a small cation deficiency since M#Tl+0.9, but this is not unusual for thallium-based layered cuprates.…”

Nanostructural Transitions Related to Flexible Shearing Phenomena in the Superconductors Hg1−xVxSr4−yBayCu2O7−δCO3

“…The electron diffraction investigation showed that the three structural types of oxycarbonates are stabilized: -For 0 Յ y Ͻ 1.8, a [S 2 CC] 1 [1201] structure is obtained. The subcell is tetragonal with a ϭ a p and c Ȃ 16.5 Å and P-type symmetry.…”

“…This oxycarbonate exhibits an orthorhombic supercell with a Ȃ a p , b Ȃ 8a p , c Ȃ 17 Å and A-type symmetry. The superstructure along b results from the periodic formation, in the [S 2 CC] 1 [1201] structure, of (100) crystallographic shear planes (CSP), every four octahedra ( Fig. 1b and c Varying the Ba/Sr ratio, a third family has been observed, which only differs from the second family by the orientation and the periodicity of the CSPs, which are (110) planes ( Fig.…”

“…3a), the 0k 0 0 reflections with k 0 ϭ2n ϩ 1 appear as a result of the twinning phenomenon and double diffraction. The superstructure and the twinning phenomena are clearly observed along the oblique zone [31 ළ0] of the tetragonal [S 2 CC] 1 [1201] subcell. The corresponding images show that the structural phenomenon which is responsible for the superstructure takes place at the level of the carbonate layers.…”

“…The first takes into consideration the coexistence of two different superstructures of the tetragonal subcell: one centered tetragonal supercell with a I Ȃ a ͙2 Ȃ a p ͙2 and c I ϭ 2c and one P-type tetragonal supercell with a II Ȃ a ͙2 Ȃ a p ͙2 and c II ϭ c (a and c being the parameters of the tetragonal [[S 2 CC] 1 [1201] subcell). The two variants would be systematically present in every crystal.…”