SYSTEMATICS IN THE COMMENSURATE-INCOMMENSURATE PHASE-TRANSITION IN [N(CH3)(4)](2)MCL(4) (M=ZN, FE, AND MN) UNDER PRESSURE

We have carried out x-ray diffraction studies on the modulated phases of [N(CH3)(4)]2ZnCl4 and [N(CH3)(4)]2FeCl4 under hydrostatic pressure. The geometrical similarities among the pressure-temperature phase dia grams of these compounds and that of [N(CH3)(4)]2MnCl4 reported previo usly lead to proposing a universal pressure-temperature phase diagram. The universal diagram contains several commensurate phases with long periods and the incommensurate phase with contour lines of equal modul ation wave vectors. Thus obtained universal diagram and the sequences of the commensurate wave vector are in good agreement with the predict ions of theoretical models with competing interactions such as the axi al-next-nearest-neighbor Ising model.