Synthesis, characterisation and properties of rare earth oxyselenides A4O4Se3 (A = Eu, Gd, Tb, Dy, Ho, Er, Yb and Y)

Abstract

Rare earth oxyselenides A₄O₄Se₃ (A = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb and Y) were synthesised using solid state reactions and three new structure types (β, γ, and δ) were observed. A₄O₄Se₃ materials adopt either the α (A = Nd, Sm), β (A = Eu), γ (A = Gd, Tb) or δ (A = Dy, Ho, Er, Yb, Y) structure depending on the rare earth radius. Each structure type contains alternating [A₂O₂]²⁺ and Se²⁻/Se₂²⁻ layers. Different ordered and disordered arrangements of Se²⁻ and [Se–Se]²⁻ give the Se layer flexibility and lead to the four different structure types observed. The volume coefficients of expansion for A₄O₄Se₃ ranged from +1.746(9) × 10⁻⁵ to +2.237(3) × 10⁻⁵ K⁻¹ from 12 to 300 K; no structural phase transitions were observed in this temperature range. Diffuse reflection spectra show A₄O₄Se₃ are semiconductors with band gap E_g 1.02–1.46 eV. Gd₄O₄Se₃, Dy₄O₄Se₃, and Tb₄O₄Se₃ samples show antiferromagnetic ordering with Néel temperature, T_N, of 7–9 K. DFT calculations confirm the two different valence states of Se²⁻ and Se₂²⁻ in Eu₄O₄Se₃.