What is the notion when the crystal structure is tetragonal and the formula is bi 2 sr 2 cacu 2 o 8
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 "notation" FROM "critical_temperature_t_c_crystal_structu" WHERE "crystal_structure"='Tetragonal' AND "formula"='Bi 2 Sr 2 CaCu 2 O 8';