PHASE EQUILIBRIA IN THE Cu2Se – Ga2Se3 – In2Se3 QUASI-TERNARY SYSTEM

Isothermal section of the quasi-ternary system Cu2Se – Ga2Se3 – In2Se3 at 820 K has been built according to the X-ray analysis results of 100 samples. The system is characterized by the formation of the region of α-solid solutions with the chalcopyrite structure, S.G. I-42d. δ'-Solid solutions based on CuIn3Se5 and δ-solid solutions based on CuGa3Se5 are formed at 820 K. The first one have tetragonal structure and crystallize in S.G. I-42m with changing of the cell parameters from a = 0.57540(1) nm, c = 1.1520(2) nm for CuIn3Se5 to a = 0.56207(9) nm, c = 1.1286(6) nm for the composition 48 mol.% CuIn3Se5 – 52 mol.% CuGa3Se5. δ-Solid solutions crystallize in tetragonal system, S.G. I-42m. The cell parameters change from a = 0.55092(3) nm, c = 1.0973(2) nm for CuGa3Se5 to a = 0.56040(7) nm, c = 1.1179(9) nm for the composition 28 mol.% CuIn3Se5 – 72 mol.% CuGa3Se5. The mechanism of the formation of solid solutions, namely the substitution In3+ « Ga3+, was established. In addition to these solid solutions, the system is characterized by the formation of ε-solid solutions based on HTM Ga2Se3 with cubic structure, S.G. F-43m. The unit cell parameters vary from a = 0.5430(2) nm for Ga2Se3 to a = 0.5512(3) nm for the sample 4 mol.% Cu2Se – 83 mol.% Ga2Se3 – 13 mol.% In2Se3. There are also γ2 and γ1 phases, which areas of existence extend into the quasiternary system till 1-2 mol.% and streach along the Ga2Se3 – In2Se3 system (γ1 – up to 10 mol.% In2Se3, γ2 – up to 30 mol.% In2Se3). There are small regions of homogeneity based on HTM Cu2Se, CuIn5Se8, CuIn7Se11, CuIn11Se17, 1-HTM In2Se3. These one-phase regions are separated by two- and three-phase equilibria.The liquidus surface projection has been built based on the results of investigations of the quasi-binary CuInSe2 – Ga2In8Se15 system and 6 polythermal sections. The liquidus surface projection of the Cu2Se – Ga2Se3 – In2Se3 system consists of 11 fields of the primary crystallization: μ-solid solutions based on HTM-Cu2Se, Cu3InSe3 compound, α-solid solutions with the chalcopyrite structure, β-solid solutions based on HTM-CuInSe2, quaternary high-temperature γ-phase formed in the Cu2Se – Ga2Se3 system, δ-solid solutions based on CuGa3Se5, ε-solid solutions based on HТМ-Ga2Se3, γ1-phase based on the Ga0.4In1.6Se3 composition, compounds CuIn5Se8 and CuIn11Se17, and 3-HТМ In2Se3. These fields are separated by 21 monovariant curves and 22 nonvariant points which lie on the corresponding planes of nonvariant four-phase processes. There is one quasibinary section CuInSe2 – Ga2In8Se15 (γ1) in the system. The compounds CuGaSe2, CuGa3Se5, CuIn5Se8, CuIn11Se17 have a peritectic type of formation. The CuIn3Se5, CuIn7Se11, Ga6In4Se15 compounds have a solid-phase type of formation, therefore, they do not have areas of primary crystallization on the liquidus surface projection.