164 K resistive transition in yttrium barium copper oxide-a cautionary tale?

A halide-flux grown large grain size sample of YBa2Cu3O7-x has been found to develop a sharp drop in resistance at 164 K after thermal cycling. Susceptibility measurements show no evidence of a Meissner effect at this temperature. Ultrasonic measurements are presented that indicate a structural phase transition occurring at about 170 K in low oxygen content samples. The sharp drop in resistance is attributed to the structural phase transition and not to superconductivity. Attention is drawn to the similarity of these effects and those advanced as evidence of superconductivity in some new materials.