By a combination of x-ray diffraction, Raman, and neutron-scattering measurements, the (x,T) phase diagram of the dipolar glass ${\mathrm{K}}_{1\mathrm{-}\mathit{x}}$(${\mathrm{NH}}_4$${)}_{\mathit{x}}$I has been derived. Three compositional ranges are observed at low temperatures. Above the critical ammonium concentration ${\mathit{x}}_{\mathit{c}}$, estimated between x=0.72 and 0.82, x-dependent phase transitions occur from NaCl to CsCl cubic structures, as in pure ammonium iodide. For 0.3≤x<${\mathit{x}}_{\mathit{c}}$, an orientational glassy phase is characterized by dipolar short-range order of an antiferroelectric type. Below x∼0.3, the transition is no longer collective but replaced by a single-ion freezing.

• Received 1 July 1992

DOI:https://doi.org/10.1103/PhysRevB.46.13747