1. A method for the treatment of a disease that is responsive to the inhibition of platelet aggregation, the method comprising administering to a subject in need thereof a pharmaceutical composition comprising as active ingredient a P2Y₁₂ receptor antagonist which is 4-((R)-2-{[6-((S)-3-methoxy-pyrrolidin-1-yl)-2-phenyl-pyrimidine-4-carbonyl]-amino}-3-phosphono-propionyl)-piperazine-1-carboxylic acid butyl ester, or a pharmaceutically acceptable salt thereof, and at least one pharmaceutically acceptable carrier material, wherein 4-((R)-2-{[6-((S)-3-methoxy-pyrrolidin-1-yl)-2-phenyl-pyrimidine-4-carbonyl]-amino}-3-phosphono-propionyl)-piperazine-1-carboxylic acid butyl ester, or the pharmaceutically acceptable salt thereof, is obtained from a crystalline form of 4-((R)-2-{[6-((S)-3-methoxy-pyrrolidin-1-yl)-2-phenyl-pyrimidine-4-carbonyl]-amino}-3-phosphono-propionyl)-piperazine-1-carboxylic acid butyl ester hydrochloride, wherein the crystalline form of 4-((R)-2-{[6-((S)-3-methoxy-pyrrolidin-1-yl)-2-phenyl-pyrimidine-4-carbonyl]-amino}-3-phosphono-propionyl)-piperazine-1-carboxylic acid butyl ester hydrochloride is characterized by: