Elastic properties of polycrystalline YBa_2Cu_3O_7: Evidence for   granularity induced martensitic behavior
  In this work we present the study of the elastic properties of polycrystalline samples of superconducting YBa_2Cu_3O_7 prepared by the sol-gel method. The quality of all samples was checked by x-ray diffraction and scanning electronic microscopy while their physical properties were verified by transport and magnetic measurements. The elastic study was performed using the standard pulse-echo technique through measuring the phase velocity and the attenuation of ultrasonic waves (in the range of a few MHz) as a function of temperature. We have focused this study on the low temperatures interval (T < 200K). The obtained results show a strong hysteretic behavior in the ultrasonic attenuation (in addition to usually observed hysteretic behavior for the velocity) which strongly supports the existence of a martensitic-like phase above the superconducting critical temperature T_C. We argue that this peculiar behavior can be attributed to the granularity present in the samples. 
C. Stari, A. Moreno-Gobbi, A.W. Mombru, S. Sergeenkov, A.J.C.   Lanfredi, C.A. Cardoso and F.M. Araujo-Moreira