[phenixbb] how to generate an isomorphous difference map

Hi, I am trying to generate an isomorphous difference map with phenix GUI. I input reflection 1 and 2 with mtz format and a PDB model, I got an MTZ format map file. Because I want to load it to coot or pymol with negative and positive density, I need to transform this mtz map to ccp4 map. However, FFT map cannot work. Does any know how can I get a ccp4 format maps with positive or negative density? Thanks!