What is the crystal structure for the formula yba 2 cu 3 o 7?
CREATE TABLE "critical_temperature_t_c_crystal_structu" (
  "formula" text,
  "notation" text,
  "t_c_k" real,
  "no_of_cu_o_planes_in_unit_cell" real,
  "crystal_structure" text
);
SELECT "crystal_structure" FROM "critical_temperature_t_c_crystal_structu" WHERE "formula"='YBa 2 Cu 3 O 7';