Dumbledore said that Voldemort didnt tell Lucius that the diary was a horcrux, but he might have said that it would cause the Chamber to open again:
“No doubt he thought that Lucius would not dare do anything with the Horcrux other than guard it carefully, but he was counting too much upon Lucius’s fear of a master who had been gone for years and whom Lucius believed dead. Of course, Lucius did not know what the diary really was. I understand that Voldemort had told him the diary would cause the Chamber of Secrets to reopen because it was cleverly enchanted.”
HBP, pg. 508