New aspects in the crystal chemistry of (M2CU2O3)m(CuO2)n-type (M = Ca, Sr, Y, Bi) cuprates
The structural regularities of (M2Cu2O3)m(CuO2)n-type (M = Ca, Sr, Y, Bi) cuprates with ladder-type planes have been considered.  On the basis of the structural studies of the samples grown by floating-zone and flux methods, it was concluded that the phase (M2Cu2O3)(CU1+deltaO2+gamma), described as the m/n = 1/1 member, with Cu-deficient Cu-O ribbons in the series (M2Cu2O3)m(CuO2)n can be treated as an individual structural type intermediate between SrCu2O3 and incommensurate-type phases (M2Cu2O3)m(CuO2)n with m not equal n. The deficiency of the Cu sites in CuO ribbons is a particular crystal-chemistry feature of this structural type, in addition to the geometrical feature (m = n).  The structural type of the 'Cu-deficient ladder' can transform into the incommensurate-type phases (M2Cu2O3)m(CUO2)n (m/n = 5/7, 7/10, 9/13 etc.)  in the presence of free [CuO4]6- radicals in the melt.  A realization of the (M2Cu2O3)m(CuO2)n phase with a particular m/n value is caused by the type of M cation.  On the basis of recent data, the role of trivalent ions in the chemical composition of the (M2Cu2O3)m(CuO2)n-type compounds was also analyzed.  The occurrence of the superconductivity in (Sr,Ca)CuO2 samples is discussed by analogy with cuprates of the infinite-layer structure.