Tetragonal-orthorhombic phase transition in YBaCuO thin films observed by perturbed angular correlation spectroscopy

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Tetragonal-orthorhombic phase transition in YBaCuO thin films observed by perturbed angular correlation spectroscopy R. Platzer, I. D. Dumkow, D. W. Tom, J. A. Gardner, and J. Tate Department of Physics and Center for Advanced Materials Research, Oregon State University, Corvallis, Oregon 97331-6507 (Received 6 June 1997; accepted 8 September 1997)

Oxygen-deficient, tetragonal thin films of YBa2 Cu3 O61x with x ø 0.25, quenched from the deposition temperature, change to the oxygenated, orthorhombic phase with x ø 1, between 200 ±C and 400 ±C in flowing oxygen. The transition is not reversible in flowing oxygen, and cannot be completely reversed by cooling in flowing argon. We do not observe a transition of the orthorhombic films to the tetragonal phase up to 800 ±C in flowing oxygen. We observe that the major impurity phases to appear under nonoptimal annealing conditions are oriented phases of YCuO2 and BaCu2 O2 , with Y2 BaCuO5 and Y2 Cu2 O5 conspicuously absent. These conclusions have been drawn from a study that uses perturbed angular correlation spectroscopy to probe the local microstructure of the films.

I. INTRODUCTION

A great deal of effort has been expended in establishing the phase diagram of YBa2 Cu3 O61x .1–3 One of the important features of this material is that it is possible to tune the oxygen content through the range 0 , x , 1 by establishing a particular temperature and oxygen pressure. Quenching the resulting material to room temperature allows investigation of these metastable states at ambient pressure and temperature, and determination of the conditions under which oxygen becomes mobile as a function of temperature and different oxidizing environments. Much of the knowledge of the phase diagram of YBaCuO is contained in a widely used plot generated by Bormann and N¨olting and modified by others.1,2 This pressure-temperature phase plot is based on observations of the equilibrium reactions of bulk material, but has nevertheless become useful for film growers. However, it is clear that such a plot cannot be completely accurate for films since kinetic effects at the growth surface and substrate interactions are important. Phase changes in the film/substrate system may differ from the bulk, and should be documented. The growth of thin films of the high temperature superconductors is an important aspect of the commercial application of superconductors, and it is critical that processing issues are understood. High quality films of YBa2 Cu3 O7 are not grown directly in the superconducting form, but must be grown as an oxygendeficient insulating form and annealed in oxygen. For many purposes, in situ annealing by cooling from deposition conditions in a moderate oxygen pressure is sufficient, and sometimes that cooldown can be rather rapid. Nonetheless, it is important to understand the exact J. Mater. Res., Vol. 13, No. 4, Apr 1998

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conditions of the transition to optimize the growth and processing.

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