Quaternary rare-earth selenides Ba2REGaSe5 and Ba2REInSe5

Twelve quaternary rare-earth selenides Ba₂REGaSe₅ and Ba₂REInSe₅ were prepared in the form of single crystals from reactions of BaSe, RE, M₂Se₃ (M = Ga, In), and Se at 1373K. The compounds Ba₂REGaSe₅ fall into two separate series adopting an orthorhombic (RE = La, Ce; space group Pnma, Z=4, a = 12.49-12.50Å, b = 9.60-9.63Å, c = 8.74Å) or a triclinic structure (RE = Tb, Ho, Tm, Yb, Lu; space group P 1 bar , Z = 2, a = 7.28-7.31 Å, b = 8.61-8.72 Å, c = 9.37-9.43 Å, α = 103.4-103.5°, β = 103.0-103.1°, γ = 107.3-107.5°). The compounds Ba₂REInSe₅ adopt an orthorhombic structure (RE = Pr, Tb, Ho, Tm, Lu; space group Cmc2₁, Z = 4, a = 4.23-4.33 Å, b = 18.75-18.96 Å, c = 13.29-13.31 Å). All structures contain anionic M-centred tetrahedra, isolated Se^2- anions, Ba²⁺ cations in eight-fold coordination, and RE³⁺ cations in either six- or seven-fold coordination. A structure map developed based on radius ratios involving the RE³⁺, M³⁺, and Ch^2- ions is effective in segregating four structures (in space groups Cmc2₁, P 1 bar , Pnma, and I4/mcm) found for Ba₂REMCh₅ (M = Ga, In; Ch = S, Se, Te). Bond valence sum calculations reveal that the upper limit for the largest RE components that can be substituted within a given structural series is set by the need to avoid overbonding of the RE atoms. An optical band gap of 1.34(2) eV was measured for the compound Ba₂LaGaSe₅, slightly less than a value of 1.76 eV obtained from electronic band structure calculations, which suggest an indirect transition.