What is the expiation for someone who, in a moment of anger, said to his wife: "You are forbidden to me for a week," then had intercourse with her during that period, knowing that by his oath he only intended to refrain from intercourse with her?

So long as you did not intend dhahir (a form of divorce in which the husband declares his wife unlawful to him by comparing her to his mother's back) or divorce by this prohibition, but rather intended abstinence from intercourse, then its ruling is that of an oath, and you must offer an expiation for an oath: feeding ten poor persons or clothing them. If you cannot find them, then fasting three days.