The compounds P 4S 3, P 4Se 3, As 4S 3 and As 4Se 3 and the quaternary system P 4S 3-P 4Se 3-As 4S 3-As 4Se 3

Abstract

On heating, the compounds P 4S 3, P 4Se 3 and As 4S 3, and with rapid heating rates As 4Se 3, are transformed into the plastic crystalline β-modification, the structure of which is related to the β-Mn structure. This transition occurs in As 4S 3 and As 4Se 3 only under non-equilibrium conditions. P 4S 3 and P 4Se 3 show a phase transition within the plastic crystalline state, possibly in a cubic structure. The transformation temperature of P 4S 3 is very close to the melting point and can be observed only on cooling. The α-β transition shows a large hysteresis which is nearly independent of the cooling rate. In annealing experiments at elevated temperatures, As 4S 3 and As 4Se 3 decompose into As 4S 4 and As 4Se 4, respectively, and unidentified amorphous products. The system P 4S 3-P 4Se 3 contains three regions of solid solubility, one from P 4S 3 to 45 mol% of P 4Se 3 with the α-P 4S 3 structure, the second from ≈ 48 to 60 mol% of P 4Se 3 with a new structural type (α ∗-P 4S 3), and the third from ≈ 65 mol% of P 4Se 3 with the α-P 4Se 3 structure. On heating, all these phases transform into a plastic crystalline modification (β) with complete solid solubility between P 4S 3 and P 4Se 3. At higher temperatures this β-phase changes into the plastic γ-phase. The system P 4S 3-As 4S 3 reveals a broad metastable region of solid solubility, based on the structure of α-As 4S 3. All solid solutions undergo transformation into the plastic crystalline modifications at elevated temperatures. At ≈ 75 mol% of As 4S 3 a new structure is observed after equilibration at 390 K. Crystalline samples of P 4Se 3-As 4Se 3 at ambient temperature show regions of solid solubility with α-P 4Se 3, α ∗-P 4S 3 and α-P 4S 3 structure. Solid solutions in the system P 4S 3-P 4Se 3 consist of molecules of the type P 4S n Se 3- n ( n = 0–3) in statistical distribution, the formation being entropy controlled. In the mixtures of P 4S 3-As 4S 3, molecules of the type P 4- n As n S 3 ( n = 1–3), in which the occupation of the apical position for P atoms is favoured, were found. P apicalAs 3S 3 is the most stable of these isomers. The high abundance of P apicalAs 3S 3 reveals that the formation is enthalpy controlled. In the system P 4Se 3-As 4Se 3 a statistical distribution of substituted molecules was found. The system P 4S 3-P 4Se 3-As 4S 3-As 4Se 3 shows five regions of solid solubility at room temperature. At all compositions these phases transform into the plastic crystalline β- and γ-modifications at higher temperature, forming a broad range of complete solid solubility. A decomposition reaction of the AS 4B 3 molecules into As 4B 4 and amorphous products was observed in the As 4Se 3-As 4S 3 part of the system. The solid solutions in this system are formed from all possible isomers of the type P n As 4- n S m Se 3- m ( n = 0–4, m = 0–3).