PHYSICO-CHEMICAL INTERACTION IN THE Ag7PSe6 – AgSbP2Se6 SYSTEM
Authors: Sabov V.I. , Barchiy I.Ye. , Piasecki M. , Filep M.J. , Pogodin A.I. , Sabov M.Yu. 
Published: April 2022

The study of physicochemical interaction was carried out by investigation of 12 samples within the Ag7PSe6 – AgSbP2Se6 system by DTA and XRD methods. The starting materials: Ag7PSe6 and AgSbP2Se6 were pre-synthesized from the elementary components of high purity. The thermal effects temperatures on the thermograms and X-ray diffraction data of compounds are in good agreement with the literature data. It was found that a cubic (SG P213) low-temperature modification of Ag7PSe6 and a trigonal (SG R3) modification of AgSbP2Se6, was obtained.
A large number of effects appeared on the heating and cooling curves of the system samples, indicated that the physicochemical interaction in the Ag7PSe6 – AgSbP2Se6 system is complex. The thermograms show that the system consists of three distinct parts. X-ray diffraction results are in good agreement with DTA data. According to the results it is established:

In the concentration range 100 – 44mol.% Ag7PSe6, up to the Ag4P2Se6–AgSbSe2–Se ternary system intersection the following reaction occurs:

хAg7PSe6+(1-х)AgSbP2Se6=(7-7х)/5Ag4P2Se6+(1-х)AgSbSe2+(9х-4)/5Ag7PSe6+(2-2х)/5Se,
where 0.44 ˂ x ˂ 1.00. 2.

In the concentrations range 44 – 37.5 mol.% Ag7PSe6, from Ag4P2Se6–AgSbSe2–Se ternary system to Ag4P2Se6 –Sb2Se3 – Se ternary system intersections, the following reaction occurs:

хAg7PSe6+(1-х) AgSbP2Se6=(2-х)/2Ag4P2Se6+(8х-3)AgSbSe2+(2-3.5х)Sb2Se3+2.5Se,
where 0.375 ˂ x ˂ 0.44.

In the concentrations range 37.5 – 0 mol.% Ag7PSe6, from the Ag4P2Se6 –Sb2Se3 – Se ternary system intersection with to AgSbP2Se6 the interaction is described by the equation:

хAg7PSe6+(1-х)AgSbP2Se6=(3-8х)/3AgSbP2Se6+7х/4Ag4P2Se6+(1-х)/2Sb2Se3+(9-18х)/2Se,
where 0˂ x ˂ 0.375.
At the intersection points with the ternary systems the following chemical reactions take:
4Ag7PSe6 + 5AgSbP2Se6 = 7Ag4P2Se6 + 5AgSbSe2 + 2Se
6Ag7PSe6 + 10AgSbP2Se6 = 13Ag4P2Se6 + 5Sb2Se3 + 3Se.
To clarify the nature of the interaction, 3 samples were additionally synthesized, with 30, 37.5, and 44.4 mol.% Ag7PSe6. The obtained samples were investigated by DTA, MSA and XRD. In the thermograms the absence of Ag7PSe6 polymorphic transformation effect and the presence of the selenium melting thermal effect were established. The study of the microstructure proved that all samples are multiphase. In the powder patterns of the sample with 30 mol.% Ag7PSe6 reflexes of Ag4P2Se6, AgSbP2Se6 and Sb2Se3 also. Diffractograms of samples with 37.5 and 44.4 mol% Ag7PSe6, which correspond to the intersection points of Ag7PSe6–AgSbP2Se6 with ternary systems Ag4P2Se6–Sb2Se3–Se and Ag4P2Se6–AgSbSe2–Se, were compared with the calculated for three-component mixtures in appropriate ratios. The calculated diffractograms are completely consistent with the experimental ones, which proves that the cross section Ag7PSe6–AgSbP2Se6 intersects Ag4P2Se6–Sb2Se3–Se and Ag4P2Se6–AgSbSe2–Se ternary, and Ag7PSe6–Ag4P2Se6–AgSbSe2–Se, Ag4P2Se6–AgSbSe2–Sb2Se3–Se, AgSbP2Se6–Ag4P2Se6–Sb2Se3–Se quaternary systems.
Keywords: physic-chemical interaction; DTA; XRD; phase composition.