LOW-TEMPERATURE BEHAVIOR OF OXYGEN ORDERING IN YBA2CU3O6+2C - CALCULATION OF LENGTH OF O(1)-CU(1)-O(1) CHAINS AT T-LESS-THAN-100-K

Oxygen ordering in the basal plane of YBa2Cu3O6+2c is investigated in terms of the two-dimensional Ising model with the repulsive nearest-neighbor O-O bond (V1) and the second-neighbor O-O bond (V3), while O-O interaction through Cu (V2) is attractive. The phase diagram is calculated using the modified cluster field model (MCFM) for values of interactions obtained by the linear muffin-tin orbital (LMTO) first-principle calculations. The analytic closed-form expression, based on the cluster variation method (CVM), for the length of O(1)-Cu(1)-O(1) chains l at arbitrary low temperatures, is derived and compared with MCFM calculations for T>100 K. The agreement with the MCFM calculations is excellent. We found that the slope of ln(l) versus inverse temperature plot practically equals 2\V2\ for all values of oxygen content c and temperatures below room temperature.