STRUCTURAL PHASE-TRANSITIONS IN BULK YBA2CU3O6+X WITH X=0.35 AND X=0.36

The structural behavior of mm(3)-sized single crystals of YBa2Cu3O6+x with oxygen concentrations close to the metal-insulator transition is studied as a function of temperature, using 95-keV synchrotron x-ray d iffraction. At x=0.36, no evidence is found of a room-temperature phas e separation into tetragonal and orthorhombic phases, nor of a phase b oundary between Ortho-II and tetragonal. Instead, we observe two disti nct phase transitions: tetragonal to Ortho-I with a critical temperatu re T-OI=246(2) degrees C and Ortho-I to Ortho-II with T-OII 85(10) deg rees C. Measurements of the spontaneous strain show the O/T transition to be nearly continuous with a critical exponent beta=0.34(2), consis tent with a 3D Ising model driven weakly first order, presumably by th e strain. A memory effect is observed, where relies of the twin domain s-possibly related to tweed formations-continue to exist in the tetrag onal phase when the temperature is increased above T-OI. Corresponding measurements for x=0.35 gave similar results with T-OI=181(2) degrees C, T-OII=95(10) degrees C, and beta=0.35(2)-but with the appearance o f a small tetragonal component at room temperature. This component is interpreted as a nonequilibrium feature. In both cases the Ortho-I to Ortho-II transformations are very broad with a characteristic temperat ure dependence of the widths of the superstructure peaks that are simi lar to results obtained in a previous study for x=0.50. By comparison of the Ortho-II correlation lengths along a, b, and c with the corresp onding data for x=0.50 we find evidence for a strong x dependency of A SYNNNI-type effective interaction parameters. The present results cann ot be explained in terms of prevalent lattice gas models of the oxygen ordering and emphasizes the need for a theoretical basis that incorpo rates the strain and charge degrees of freedom.