Cation disorder in Ga1212

Substitution of calcium for strontium in LnSr(2-x)Ca(x)Cu(2)GaO(7) (Ln = La , Pr, Nd, Gd, Ho, Er, Tm, and Yb) materials at ambient pressure and 975 deg rees C results in complete substitution of calcium for strontium in the lan thanum and praseodymium systems and partial substitution in the other lanth anide systems. The calcium saturation level depends on the size of the Ln c ation, and in all cases, a decrease in the lattice parameters with calcium concentration was observed until a common, lower bound, average A-cation si ze is reached. Site occupancies from X-ray and neutron diffraction experime nts for LnSr(2-x)Ca(x)Cu(2)GaO(7) (x = 0 and x = 2) confirm that the A-cati ons distribute between the two blocking-layer sites and the active-layer si te based on size. A quantitative link between cation distribution and relat ive site-specific cation enthalpy for calcium, strontium, and lanthanum wit hin the gallate structure is derived. The cation distribution in other simi lar materials can potentially be modeled.