THE PHASE EQUILIBRIA OF THE PBS – PR2S3 – ER2S3 SYSTEM
DOI:
https://doi.org/10.32782/pcsd-2021-1-6Keywords:
crystalline structure, coordinate setting, isothermal section, quasi-binary sectionAbstract
In this work the theoretical analysis of structural features of initial phases of quasi-ternary system PbS – Pr2S3 – Er2S3 is carried out. It is important to note that they are characterized by a congruent type of melting and can use as components of the above pointed quasi-binary system. The crystalline structure of PbS and Er2S3 is described by octahedral polyhedra of cations, and in the structure of Pr2S3, due to the increase in the radius of the Pr atom compared to the radius of the Er atom, the coordination number increases to 8. Thus, a prism with two additional atoms is obtained. During the transition to ternary phases, we observe some changes in the coordination numbers of Pb and Pr atoms. In the structure of Er2PbS4, the coordination number of Er atoms remains octahedral, but in Pb atoms it becomes prismatic with one additional atom. In the structure of Pr2PbS4 a dense packing of atoms is formed and as a result a mixture of atoms {0.667 La + 0.333 Pb} is formed, which occupies the site 12a. This motif of coordination transitions ends with the formation of only ternary phases in the quasi-ternary system PbS - Pr2S3 - Er2S3. In addition, more than 40 samples were synthesized and their X-ray phase analysis was performed. According to its results, the existence of ternary phases Er2PbS4 and Pr2PbS4 was confirmed. The maximum synthesis temperature was 1323 K. The synthesis of the initial samples of the system was carried out using solid-phase reactions in vacuum quartz ampoules at a residual pressure of 10-2. The presence of new ternary phases has not been established. Experimental studies point to the existence of quasi-binary equilibria Er2PbS4 - Pr2PbS4 and a sufficiently bulky two-phase region Er2S3 + Pr2+2/3xPb1-xS4 (x = 0 ÷ 0.54). According to the results of the study, an isothermal section of the quasi-ternary system PbS - Pr2S3 - Er2S3 at a temperature of 770 K was constructed.
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