The phase diagram of the deuteron glass Rbq (ND4), DsPO4 has been determined experimentally in the entire range of concentration z. A recently introduced temperature-frequency plot has been used to analyze the shape of the dielectric relaxation spectrum, indicating that in the glassy regime the longest relaxation time diverges according to the Vogel-Fulcher law. The corresponding Vogel-Fulcher temperature has been identified as the static limit of the freezing temperature Ty. The phase boundaries of the ferroelectric and antiferroelectric phases have been obtained in a standard manner by observing the peaks and breaks, respectively, in the temperature dependence of the quasistatic dielectric constant. It is shown that in a broad concentration range the observed phase diagram can be quantitatively described by a mean-field theory based on the static random-bond random-field model of dipolar glasses. The absence of macroscopic polarization in the ferroelectric region close to the glassy phase is probably due to the formation of microdomains in the presence of local random fields, however, recent NMR data suggest that a phase segregation between ferroelectric and glassy regions may occur.

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The perovskite‐type layer structure compound (CH3NH3)2CdCl4 exhibits an unusual phase sequence: The phase transitions are governed by changes both in the motion of CH3NH3 groups and the hydrogen bonding scheme. Structural data are given for the tetragonal high‐temperature and the monoclinic low‐temperature phase. The whole phase sequence is discussed in detail.

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