why are melting and boiling points of ammonia are higher than expected ?

Ammonia has higher boiling and melting point than phosphine and then boiling and melting point increases down the group because of increase in size.
The abnormally high boiling and melting point of ammonia is due to its tendency to form hydrogen bonds.


In PH₃ and other hydrides, the intermolecular forces are van der Waals forces. These vander Waal's forces increases withe increase in molecular size and therefore, boiling points increases on moving from PH₃ to BiH₃