Rare-Earth Metal(III) Oxide Selenides M₄O₄Se[Se₂] (M = La, Ce, Pr, Nd, Sm) with Discrete Diselenide Units: Crystal Structures, Magnetic Frustration, and Other Properties

Abstract

The rare-earth metal(III) oxide selenides of the formula La4O4Se[Se2], Ce4O4Se[Se2], Pr4O4Se[Se2], Nd4O4Se[Se2], and Sm4O4Se[Se2] were synthesized from a mixture of the elements with selenium dioxide as the oxygen source at 750 degrees C. Single crystal X-ray diffraction was used to determine their crystal structures. The isostructural compounds M4O4Se[Se2] (M=La, Ce, Pr, Nd, Sm) crystallize in the orthorhombic space group Amm2 with cell dimensions a=857.94(7), b=409.44(4), c=1316.49(8) pm for M=La; a=851.37(6), b=404.82(3), c=1296.83(9) pm for M=Ce; a=849.92(6), b=402.78(3), c=1292.57(9) pm for M=Pr; a=845.68(4), b=398.83(2), c=1282.45(7) pm for M=Nd; and a=840.08(5), b=394.04(3), c=1263.83(6) pm for M=Sm (Z=2). In their crystal structures, Se2- anions as well as [Se-Se]2- dumbbells interconnect {[M4O4]4+} infinity 2 layers. These layers are composed of three crystallographically different, distorted [OM4]10+ tetrahedra, which are linked via four common edges. The compounds exhibit strong Raman active modes at around 215 cm(-1), which can be assigned to the Se-Se stretching vibration. Optical band gaps for La4O4Se[Se2], Ce4O4Se[Se2], Pr4O4Se[Se2], Nd4O4Se[Se2], and Sm4O4Se[Se2] were derived from diffuse reflectance spectra. The energy values at which absorption takes place are typical for semiconducting materials. For the compounds M4O4Se[Se2] (M=La, Pr, Nd, Sm) the fundamental band gaps, caused by transitions from the valence band to the conduction band (VB-CB), lie around 1.9 eV, while for M=Ce an absorption edge occurs at around 1.7 eV, which can be assigned to f-d transitions of Ce3+. Magnetic susceptibility measurements of Ce4O4Se[Se2] and Nd4O4Se[Se2] show Curie-Weiss behavior above 150 K with derived experimental magnetic moments of 2.5 micro B/Ce and 3.7 micro B/Nd and Weiss constants of theta p=-64.9 K and theta p=-27.8 K for the cerium and neodymium compounds, respectively. Down to 1.8 K no long-range magnetic ordering could be detected. Thus, the large negative values for theta p indicate the presence of strong magnetic frustration within the compounds, which is due to the geometric arrangement of the magnetic sublattice in form of [OM4]10+ tetrahedra.

Optimizing accuracy and efficacy in data-driven materials discovery for the solar production of hydrogen

“…This list includes compounds with d 0 and d 10 transition-metal cations and main-group metal oxides which are not expected to induce mid-gap states and magnetic structures. The predicted band gaps are compared to experimental data, where available[62][63][64][65][66][67][68][69][70][71][72]…”

The Short Series of the Oxygen-Poor Lanthanide Oxide Selenides M10OSe14 with M = La–Nd

“…The optical band gaps of the oxide selenides M 10 OSe 14 amount to values between 1.89 eV and 2.04 eV (see Table 4 and Figure 3 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 M 10 OSe 14 .…”

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

“…The change from sharing 3 to 4 tetrahedral edges increases the area per 2+ charge of the [M 2 5 ) materials have been prepared as bulk phases and structurally characterised for A = La, Ce, Pr, Nd and Sm and have the same layered structures. 12 These systems contain a mixture of Se 2-and Se 2 2-anions, and the consequent increased number of Se required for charge balance relative to A 2 O 2 Se leads to these structures having pseudo tetragonal layers based on sharing four tetrahedral edges as found for A 2 O 2 Te materials. The structures of previously reported A = La-Sm materials were determined from single crystals flux-grown from the rare-earth metal, selenium and selenium dioxide.…”

“…Ternary rare-earth oxyselenides are known with a variety of A:O:Se ratios. Four compositions have been reported from conventional solid state routes: A 10 OSe 14 (A = La-Nd), [7][8][9] A 2 OSe 2 (A = Pr & Gd), 7,10 A 4 O 4 Se 3 (A = La-Nd, Sm) 7,11,12 and A 2 O 2 Se (A = La, Pr, Nd, Sm, Gd, Er, Ho, Yb). 7,13 A liquid phase synthesis of nanoplates of A 4 O 4 Se 3 (A = Nd, Sm, Gd-Ho) has also been reported.…”

“…The structures of previously reported A = La-Sm materials were determined from single crystals flux-grown from the rare-earth metal, selenium and selenium dioxide. 12 Each crystallises in the non-centrosymmetric space group Amm2 with cell parameters of ~2a, a, c relative to the tetragonal A 2 O 2 Te structure. The structure will be discussed in more detail below, but contains infinite chains of alternating Se 2-/Se 2 2-running parallel to the a-axis.…”