On heating, the compounds P4S3, P4Se3 and AS4S3, and with rapid heating rates AS4Se3, are transformed into the plastic crystalline beta-modification, the structure of which is related to the beta-Mn structure. This transition occurs in As4S3 and As4Se3 only under nonequilibrium conditions. P4S3 and P4Se3 show a phase transition within-the plastic crystalline state, possibly in a cubic structure. The transformation temperature of P4S3 is very close to the melting point and can be observed only on cooling. The alpha-beta transition shows a large hysteresis which is nearly independent of the cooling rate. In annealing experiments at elevated temperatures, As4S3 and As4Se3 decompose into As4S4 and AS4Se4, respectively, and unidentified amorphous products. The system P4S3-P4Se3 contains three regions of solid solubility, one from P4S3 to 45 mol% of PSe3 with the alpha-P4S3 structure, the second from almost-equal-to 48 to 60 mol% of P4Se3 with a new structural type (alpha*-P4S3), and the third from almost-equal-to 65 mol% Of P4Se3 with the alpha-P4Se3 Structure. On heating, all these phases transform into a plastic crystalline modification (beta) with complete solid solubility between P4S3 and P4Se3. At higher temperatures this beta-phase changes into the plastic gamma-phase. The system P4S3-As4S3 reveals a broad metastable region of solid solubility, based on the structure of alpha-AS4S3. All solid solutions undergo transformation into the plastic crystalline modifications at elevated temperatures. At almost-equal-to 75 mol% of AsS3 a new structure is observed after equilibration at 390 K. Crystalline samples of P4Se3-As4Se3 at ambient temperature show regions of solid solubility with alpha-P4Se3, alpha*-P4S3 and alpha-P4S3 structure. Solid solutions in the system P4S3-P4Se3 consist of molecules of the type PSnSe3-n (n = 0-3) in statistical distribution, the formation being entropy controlled. In the mixtures of P4S3-As4S3, molecules of the type P4-nAsnS3 (n = 1-3), in which the occupation of the apical position for P atoms is favoured, were found. PapicalAs3S3 is the most stable of these isomers. The high abundance of PapicalAs3S3 reveals that the formation is enthalpy controlled. In the system P4Se3-As4Se3 a statistical distribution of substituted molecules was found. The system P4S3-P4Se3-As4S3-As4Se3 shows five regions of solid solubility at room temperature. At all compositions these phases transform into the plastic crystalline beta- and gamma-modifications at higher temperature, forming a broad range of complete solid solubility. A decomposition reaction of the As4B3 molecules into As4B4 and amorphous products was observed in the As4Se3-As4S3 part of the system. The solid solutions in this system are formed from all possible isomers of the type PnAs4-nSmSe3-m (n = 0-4, m = 0-3).
