The role of the oxygen deficiency in the change of sound velocity in YBa2Cu3O7-d below 20 K - GAIDUK, A. L.; GASAN, V. M.; DEMIRSKII, V. V.; ZHERLITSYN, S. V.; KOLOBOV, I. G.; TABACHNIKOVA, E. D.; D, FIV V. (ФІЗИКА НИЗЬКИХ ТЕМПЕРАТУР, 1990)