Quaternary rare-earth selenides Ba2REGaSe5 and Ba2REInSe5

Abstract

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 1373 K. The compounds Ba2REGaSe5 fall into two separate series adopting an orthorhombic (RE = La, Ce; space group Pnma, Z = 4, a = 12.49–12.50 Å, b = 9.60–9.63 Å, c = 8.74 Å) or a triclinic structure (RE = Tb, Ho, Tm, Yb, Lu; space group P1¯, 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 Se2– 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 Ch2– ions is effective in segregating four structures (in space groups Cmc21, P1¯, 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.