The oxygen-poor ternary lanthanide oxide selenides M₁₀OSe₁₄ crystallize with the Pr₁₀OS₁₄-type structure [12,13] in the tetragonal system with the space group I4₁/acd (no. 142) and eight formula units per unit cell (M = La: a = 1592.04(9) pm, c = 2106.48(14) pm; M = Ce: a = 1578.96(9) pm, c = 2086.59(14) pm; M = Pr: a = 1568.74(8) pm, c = 2073.42(13) pm; M = Nd: a = 1559.83(8) pm, c = 2062.91(12) pm). Compared to the lattice constants of La₁₀OSe₁₄ at 153 K (a = 1588.8(2), c = 2101.4(3) pm) [3] and within all expectations, the lattice parameters presented here show slightly larger values than those subjected to the X-ray experiments at room temperature (298 K).

Eight crystallographically independent atoms (see Table 1) reside at four different Wyckoff positions: O at 8a (site symmetry: ..), Se4 at 16e (.2.), M3 at 16f (..2) along with M1, M2, Se1, Se2 and Se3 all at the general 32g site with symmetry 1. The coordination spheres of the trivalent lanthanide cations M³⁺ exhibit a trigonal prismatic shape with two caps each (see Figure 1). From these, just (M2)³⁺ binds the light O^2– anion apart from seven contacts to Se^2–, while (M1)³⁺ and (M3)³⁺ show eight bonds to only Se^2– anions.

All important interatomic distances (Table 2) are very similar to those found in other well-investigated compounds (Table 3). According to the oxygen implementation, the Se^2––M³⁺ distance ranges expands largely from the C-type M₂Se₃ to the M₁₀OSe₁₄compounds (Table 2 and Table 3). Regarding the significantly different distances between O^2–/Se^2– and (M2)³⁺ (243–248 pm vs. 288–348 pm) the refinement of a mixed site occupation of O^2– and Se^2– at the Wyckoff position 8a just like in La₁₀O_0.945Se_14.055 [14] is certainly not appropriate.

Isolated oxide-anion centered metal tetrahedra [O(M2)₄]¹⁰⁺ (Figure 2b) embedded in a complex anionic lanthanide selenide matrix {[(M1)₃(M3)₃Se₁₄]^10–} dominate the crystal structure of the title compounds (Figure 2a). The distances between O^2– and (M2)³⁺ in these oxide selenides M₁₀OSe₁₄ decrease from 248 pm for M = La to 243 pm for M = Nd caused by the lanthanide contraction, but they also amount to values slightly higher than in the corresponding lanthanide oxide sulfides M₁₀OS₁₄ (Table 3, M = La–Nd). Although most trends remain the same in the oxide chalcogenides M₁₀OCh₁₄ from M = La to M = Nd, the angles M2-O-M2 exhibit lower values in the selenide compounds, while the angles M2-O-M2' show higher values as compared to the sulfide representatives. These effects certainly originate from the different sizes of the chalcogenide anions within the complex anionic lanthanide chalcogenide matrix {[(M1)₃(M3)₃Ch₁₄]^10–} (S^2–vs. Se^2–). Similar to the M₁₀OS₁₄-type compounds (M = La-Nd, Sm and Gd) [8,9], most of the oxygen-free part in this crystal structure of the M₁₀OSe₁₄ series (M₁₀Se₁₄ ≡ M₂Se_2.8, M = La–Nd) can be interpreted as closely related to the cation-defective Th₃P₄-type structure [20] of the corresponding lanthanide sesquiselenides M₂Se₃ known as their C-type modification [15,16,17]. Hence the internuclear distances between Se^2– and M³⁺ do not differ significantly, with the exception of the contacts M1-Se2'' and M2-Se1' in the title compounds (compare Table 2 and Table 3).

The optical band gaps of the oxide selenides M₁₀OSe₁₄ amount to values between 1.89 eV and 2.04 eV (see Table 4 and Figure 3), so they should represent typical semiconducting materials. In comparison with the oxygen-richer oxide selenide diselenides M₄O₄Se[Se₂] the band gaps are exhibiting lower values than those of the title compounds. Similar to the M₄O₄Se[Se₂] series the transitions of electrons from the 4f to the 5d orbitals or within the 4f shell of the lanthanides in the related M₁₀OSe₁₄ representatives can be detected below the actual band gap for the corresponding cerium (mainly 4f–5d), praseodymium and neodymium compounds (mainly intra 4f).

Strobel et al. [7] interpret the conduction band (CB) as predominately an attribute of the 4p states of Se and the valence band (VB) as the main attribute of the empty 5d orbitals of the respective lanthanides. This should be also true for the oxide selenides M₁₀OSe₁₄. But the differences in the crystal structures (tetragonal for M₁₀OSe₁₄ vs. orthorhombic for M₄O₄Se₃) and the different kinds of selenide units (Se^2– in M₁₀OSe₁₄ vs. Se^2– and [Se₂]^2– in M₄O₄Se₃) cause some variations in the optical band gaps. While for the title compounds just the eightfold coordination is realized for the trivalent lanthanide cations, the M₄O₄Se₃ representatives exhibit eight- as well as sixfold coordination spheres for these cations. More importantly, the diselenide units [Se₂]^2– in the M₄O₄Se₃ cases probably shift the band gaps to significantly smaller values as the bonding situation and therefore the energy levels of VB should be quite different from those examples with just Se^2– anions (M₂Se₃ and M₁₀OSe₁₄). All these band gaps are somewhat lower than those of the C-type lanthanide sesquiselenides M₂Se₃ with M = La–Nd investigated by Prokofiev et al. [21] (1.85–2.30 eV), but they clearly support a strong similarity between all these selenide representatives (M₂Se₃, M₁₀OSe₁₄ and M₄O₄Se[Se₂]). Furthermore, it opens not only for the oxide selenides with the general formula M₁₀OSe₁₄ (M = La–Nd) but a new scientific field for their applications as red pigments without toxic metals like cadmium [22].