HYDROSTATIC-PRESSURE EFFECT ON THE FERROELASTIC MONOCLINIC PHASE OF [N(CH3)(4)](2)MCL(4) (M=FE,ZN)

The deviation of the monoclinic angle from 90 degrees, Delta beta, in the ferroelastic phase (space group P12(1)/c1) of [N(CH3)(4)](2) M Cl- 4 (M = Fe, Zn) has been measured as a function of hydrostatic pressure by X-ray diffraction method. The contour lines of the equal Delta bet a in the pressure-temperature phase diagram converge on the boundary b etween the normal (Pmcn) and ferroelastic phases, drawing arcs. System atic feature among the isostructural [N(CH3)(4)](2)MX(4) (X = Cl and B r) compounds is discussed on the universality of the pressure-temperat ure phase diagrams.