[en] A new ceramic perovskite, GdBa₂HfO_5.5, has been synthesized and developed for use as substrate for the YBa₂Cu₃O_7-δ superconductor. The dielectric constant and loss factor of this material are in a range suitable for its use as substrate for microwave applications. No detectable chemical reaction between YBa₂Cu₃O_7-δ and GdBa₂HfO_5.5 was observed even under severe heat treatment. YBa₂Cu₂O_7-δ and YBa₂Cu₃O_7-δ-Ag thick films dip coated on GdBa₂HfO_5.5 substrate gave a zero-resistivity superconducting transition of 92 K. The critical current density (J_c) of YBa₂Cu₃O_7-δ thick film was 3*10³ A cm^-2 whereas a current density of 2*10⁴ A cm^-2 was obtained for YBa²Cu³O^7-δ-Ag thick film. The YBa₂Cu₃O_7-δ-Ag thick film showed preferred (001) orientation on polycrystalline GdBa₂HfO_5.5 substrate. (author)