Is the questioner's statement to her colleagues, "It is assumed that they should put up a large barrier, or that women should wear a modest, non-revealing abaya," considered a defamation of the women present in the market, and what is the expiation if that occurs?

What has been mentioned is not considered slander (qadhf), for slander is accusing a legally responsible, free, Muslim, sane, chaste person of denying paternity or of fornication. It is the accusation of indecency against one who is innocent of it. The punishment for slandering chaste women is eighty lashes, the rejection of one's testimony, and deeming the accuser wicked.