C-type Nd₂Se₃

The title compound, neodymium sesquiselenide, is isotypic with the other known rare-earth metal(III) selenides M₂Se₃ (M = La–Pr and Sm–Lu) with the cubic C-type structure. It adopts a cation-defective Th₃P₄-type arrangement with close to 8/9 of the unique neodymium-cation site occupied, leading to the composition Nd_2.667Se₄ (Z = 4) or Nd₂Se₃ (Z = 5.333), respectively. The Nd³⁺ cations are thus surrounded by eight selenide anions, forming trigonal [NdSe₈]¹³⁻ dodeca­hedra, whereas the Se²⁻ anions exhibit a sixfold coordination, but due to the under-occupation of neodymium, each one is statistically surrounded by only 5.333 cations. The crystal studied was a merohedral twin with a 0.31 (6):0.69 (6) domain ratio.

Related literature

For the structural family with the cation-defective Th₃P₄-type arrangement, see: Pardo et al. (1963); Flahaut et al. (1965); Lashkarev & Paderno (1965). For the rare-earth sesquiselen­ides M₂Se₃ with M = La–Pr and Sm–Lu, see: Grundmeier & Urland (1995); Folchnandt (1997); Folchnandt & Schleid (2001); Folchnandt et al. (2004).

Experimental

Crystal data

• Nd_2.667Se₄

• M_r = 700.48

• Cubic,

• a = 8.8632 (6) Å

• V = 696.26 (8) ų

• Z = 4

• Mo Kα radiation

• μ = 40.39 mm⁻¹

• T = 293 K

• 0.03 × 0.03 × 0.02 mm

Data collection

• Stoe IPDS-I diffractometer

• Absorption correction: numerical (X-SHAPE; Stoe & Cie, 1999) T_min = 0.305, T_max = 0.401

• 8964 measured reflections

• 220 independent reflections

• 214 reflections with I > 2σ(I)

• R_int = 0.065

Refinement

• R[F² > 2σ(F²)] = 0.026

• wR(F²) = 0.060

• S = 1.22

• 220 reflections

• 7 parameters

• Δρ_max = 1.01 e Å⁻³

• Δρ_min = −2.11 e Å⁻³

• Absolute structure: Flack (1983), 92 Friedel pairs

• Flack parameter: 0.31 (6)

Table 1
Selected bond lengths (Å)

Nd—Se^i (4×)	2.9675 (5)
Nd—Se (4×)	3.1732 (6)
Symmetry codes: (i) .

Data collection: DIF4 (Stoe & Cie, 1992); cell refinement: DIF4; data reduction: REDU4 (Stoe & Cie, 1992); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: SHELXL97.

Supporting information

S1. Comment

C-type Nd₂Se₃ (Fig. 1) belongs to a structural family with the cation-defect Th₃P₄-type arrangement (Pardo et al., 1963; Flahaut et al., 1965; Lashkarev & Paderno, 1965) adopted by rare-earth sesquiselenides M₂Se₃ with M = La – Pr and Sm – Lu (Grundmeier & Urland, 1995; Folchnandt, 1997; Folchnandt & Schleid, 2001; Folchnandt et al., 2004) following the general formula M_2.667□_0.333Se₄. The Nd³⁺ cations occupy the 12a position, whereas selenium resides at the 16c position. Despite the fact that out of the 12 possible cationic sites (per 16 Se^2- and unit cell), only 10.667 are allowed to be occupied to realise the composition Nd₂Se₃ (with Z = 5.333, i.e. M_2.667□_0.333Se₄ with Z = 4); these exhibit the coordination number 8 with respect to the selenide anions. The [NdSe₈]^13- coordination polyhedra can be described as trigonal dodecahedra with 4-symmetry (Fig. 2). On average, the Se^2- anions are surrounded by 5.333 Nd³⁺ cations in a trigonal hemiprism of symmetry .3. with faces rotated 38.2° with respect to each other (Fig. 3).

Related literature

For the structural family with the cation-defective Th₃P₄-type arrangement, see: Pardo et al. (1963); Flahaut et al. (1965); Lashkarev & Paderno (1965). For the rare-earth sesquiselenides M₂Se₃ with M = La–Pr and Sm–Lu, see: Grundmeier & Urland (1995); Folchnandt (1997); Folchnandt & Schleid (2001); Folchnandt et al. (2004).

S2. Experimental

Ruby-red, multifaceted, transparent crystals of Nd₂Se₃ were obtained from stoichiometric reaction of the elements in the presence of CsCl as a flux, placed within a torch-sealed evacuated fused-silica vessel. The mixture was heated at 1123 K for seven days, followed by cooling to ambient temperature with 10 K/h.

Computing details

Data collection: DIF4 (Stoe & Cie, 1992); cell refinement: DIF4 (Stoe & Cie, 1992); data reduction: REDU4 (Stoe & Cie, 1992); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. Crystal structure of C-Type Nd2Se3.
	Fig. 2. Coordination sphere of the Nd^3+ cation in the shape of a trigonal dodecahedron. Displacement ellipsoids are drawn at 95% probability level. [Symmetry codes: (i) -x + 1/2, -y, z + 1/2; (ii) y + 1/4, x + 1/4, z + 1/4; (iii) y + 1/4, -x - 1/4, -z + 1/4; (iv) -x + 1/2, y, -z; (v) -y + 3/4, -x + 1/4, z + 1/4; (vi) -y + 3/4, x - 1/4, -z + 1/4; (vii) x, -y, -z + 1/2.]
	Fig. 3. Coordination sphere of the Se^2- anion. Due to the under-occupation of the neodymium site, each selenium is surrounded by 5.333 Nd^3+ cations. Displacement ellipsoids are drawn at 95% probability level. [Symmetry codes: (viii) y, -z, -x + 1/2; (ix) -x + 1/2, -y, z - 1/2; (x) -y - 1/4, x - 1/4, -z + 1/4; (xi) y, z, x; (xii) y + 1/4, -x + 3/4, -z + 1/4.]

dineodymium(III) triselenide

Crystal data

Nd2.667Se4	Dx = 6.682 Mg m^−3
Mr = 700.48	Mo Kα radiation, λ = 0.71069 Å
Cubic, I43d	Cell parameters from 5000 reflections
Hall symbol: I -4bd 2c 3	θ = 1.0–32.7°
a = 8.8632 (6) Å	µ = 40.39 mm^−1
V = 696.26 (8) Å^3	T = 293 K
Z = 4	Block, red
F(000) = 1184	0.03 × 0.03 × 0.03 mm

Data collection

Stoe IPDS-I diffractometer	220 independent reflections
Radiation source: fine-focus sealed tube	214 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.065
imaging plate detector system scans	θmax = 32.7°, θmin = 5.6°
Absorption correction: numerical (X-SHAPE; Stoe & Cie, 1999)	h = −13→13
Tmin = 0.305, Tmax = 0.401	k = −13→13
8964 measured reflections	l = −13→13

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0359P)^2] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.026	(Δ/σ)max = 0.007
wR(F^2) = 0.060	Δρmax = 1.01 e Å^−3
S = 1.22	Δρmin = −2.11 e Å^−3
220 reflections	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
7 parameters	Extinction coefficient: 0.0086 (7)
0 restraints	Absolute structure: Flack (1983), 92 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.31 (6)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Nd	0.3750	0.0000	0.2500	0.0053 (2)	0.89
Se	0.07261 (5)	0.07261 (5)	0.07261 (5)	0.0028 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Nd	0.0066 (3)	0.0046 (3)	0.0046 (3)	0.000	0.000	0.000
Se	0.0028 (3)	0.0028 (3)	0.0028 (3)	0.00063 (16)	0.00063 (16)	0.00063 (16)

Geometric parameters (Å, º)

Nd—Se^i	2.9675 (5)	Nd—Se	3.1732 (6)
Nd—Se^ii	2.9675 (5)	Se—Nd^viii	2.9675 (5)
Nd—Se^iii	2.9675 (5)	Se—Nd^ix	2.9675 (5)
Nd—Se^iv	2.9675 (5)	Se—Nd^x	2.9675 (5)
Nd—Se^v	3.1732 (6)	Se—Nd^xi	3.1732 (6)
Nd—Se^vi	3.1732 (6)	Se—Nd^xii	3.1732 (6)
Nd—Se^vii	3.1732 (6)
Se^i—Nd—Se^ii	91.403 (3)	Se^ii—Nd—Se	77.283 (2)
Se^i—Nd—Se^iii	91.403 (3)	Se^iii—Nd—Se	87.467 (16)
Se^ii—Nd—Se^iii	162.00 (2)	Se^iv—Nd—Se	67.092 (10)
Se^i—Nd—Se^iv	162.00 (2)	Se^v—Nd—Se	135.510 (1)
Se^ii—Nd—Se^iv	91.403 (3)	Se^vi—Nd—Se	135.510 (1)
Se^iii—Nd—Se^iv	91.403 (3)	Se^vii—Nd—Se	64.738 (1)
Se^i—Nd—Se^v	77.284 (2)	Nd^viii—Se—Nd^ix	88.609 (17)
Se^ii—Nd—Se^v	67.092 (10)	Nd^viii—Se—Nd^x	88.609 (17)
Se^iii—Nd—Se^v	130.811 (11)	Nd^ix—Se—Nd^x	88.609 (17)
Se^iv—Nd—Se^v	87.468 (16)	Nd^viii—Se—Nd^xi	107.535 (2)
Se^i—Nd—Se^vi	87.468 (16)	Nd^ix—Se—Nd^xi	162.372 (6)
Se^ii—Nd—Se^vi	130.811 (11)	Nd^x—Se—Nd^xi	84.849 (2)
Se^iii—Nd—Se^vi	67.092 (10)	Nd^viii—Se—Nd^xii	162.372 (6)
Se^iv—Nd—Se^vi	77.283 (2)	Nd^ix—Se—Nd^xii	84.849 (2)
Se^v—Nd—Se^vi	64.739 (1)	Nd^x—Se—Nd^xii	107.534 (2)
Se^i—Nd—Se^vii	67.092 (10)	Nd^xi—Se—Nd^xii	81.565 (16)
Se^ii—Nd—Se^vii	87.468 (16)	Nd^viii—Se—Nd	84.849 (2)
Se^iii—Nd—Se^vii	77.283 (2)	Nd^ix—Se—Nd	107.535 (2)
Se^iv—Nd—Se^vii	130.810 (11)	Nd^x—Se—Nd	162.372 (6)
Se^v—Nd—Se^vii	135.510 (1)	Nd^xi—Se—Nd	81.565 (16)
Se^vi—Nd—Se^vii	135.510 (1)	Nd^xii—Se—Nd	81.565 (16)
Se^i—Nd—Se	130.811 (11)

Symmetry codes: (i) −x+1/2, −y, z+1/2; (ii) y+1/4, x+1/4, z+1/4; (iii) y+1/4, −x−1/4, −z+1/4; (iv) −x+1/2, y, −z; (v) −y+3/4, −x+1/4, z+1/4; (vi) −y+3/4, x−1/4, −z+1/4; (vii) x, −y, −z+1/2; (viii) y, −z, −x+1/2; (ix) −x+1/2, −y, z−1/2; (x) −y−1/4, x−1/4, −z+1/4; (xi) y, z, x; (xii) y+1/4, −x+3/4, −z+1/4.