MASS-SPECTROMETRIC OBSERVATION OF A CONDENSED-PHASE TRANSITION DURINGEFFUSION OF GALLIUM SESQUISULFIDE

A theory of condensed-phase transitions during vaporization processes, i.e. a theory of triple points, was tested by mass-spectrometric obse rvation of the effusing vapor of gallium sesquisulfide, Ga2S3(c), as t he condensed phase underwent a phase transition both in the increasing -temperature direction and in the decreasing-temperature direction thr ough 25 cycles. Theoretical prediction of the trajectory of partial-pr essure ratios in pressure, temperature, composition space through a te mperature-driven phase transition was tested by measuring ionic intens ity ratios. The vapor species, principally Ga2S(g) and S-2(g), had a r atio of partial pressures independent of temperature while either the low-temperature or the high-temperature condensed phase was effusing c ongruently, but the ratio showed strong predictable dependence on temp erature and composition during a condensed-phase transition, in line w ith the theory. This work is the first direct test of these theoretica l predictions. (C) 1997 Elsevier Science B.V.