Calcium nitride (Ca₂N), a redetermination

Ca₂N adopts the anti-CdCl₂ structure in which two sheets of calcium ions enclose a sheet of N^3- ions, with each calcium bonded to three N^3- at 2.4500 (3) Å and each nitro­gen octahedrally coordinated by six calciums. Within one calcium sheet, each metal has three others at 3.2944 (8) Å and a further six at 3.6271 (3) Å. The distance between two calcium layers with no intervening nitro­gen layer is 4.3221 (6) Å, and this intervening region contains no observable electron density. Neutron and X-ray powder diffraction studies have shown that the c-axis dimension changes with the synthetic method employed. In the single-crystal, c is significantly lower, reflecting the higher temperature employed in its formation.

Key indicators

• Single-crystal X-ray study
• T = 298 K
• Mean [sigma](Ca-N) = 0.0003 Å
• R factor = 0.017
• wR factor = 0.038
• Data-to-parameter ratio = 14.9

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

General Notes



ABSTM_02  When printed, the submitted absorption T values will be replaced
          by the scaled T values. Since the ratio of scaled T's is
          identical to the ratio of reported T values, the scaling does
          not imply a change to the absorption corrections used in the
          study.
          Ratio of Tmax expected/reported      0.861
          Tmax scaled      0.561 Tmin scaled      0.494

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S1. Comment

There is considerable interest in the A₂N nitrides of the Group 2 elements due to their unusual formal oxidation state and electrical properties. A previous crystal structure determination (Keve & Skapski, 1968) showed the major features of the Ca₂N structure but could not be completed as the crystals were lost. We have repeated the determination on high quality crystals obtained from an unusual preparative route. Single crystals of Ca₂N were prepared by firstly reacting calcium metal dissolved in an excess of liquid sodium with a positive pressure of nitrogen gas at 723 K for 3 d. The excess sodium was removed by vacuum distillation at 973 K and 10 ^-5 Torr leaving a polycrystalline product. The product was then heated at 1393 K for 5 d in a stainless steel crucible lined with cobalt foil and welded shut in an argon atmosphere. Single crystals of the air-sensitive Ca₂N were removed from the crucible in a high-integrity, nitrogen-filled glove-box (O₂ content <2 p.p.m., H₂O content <5 p.p.m.) and placed in RS3000 perfluoropolyether for mounting on the diffractometer.

Ca₂N has the anti-CdCl₂ structure in which two sheets of calcium ions enclose a sheet of N^3- ions and each calcium is bonded to three N^3- ions at 2.4500 (3) Å. Each nitrogen is octahedrally coordinated by calciums. Within the layer each calcium has three calcium ions at 3.2944 (8) Å and a further six at 3.6271 (3) Å. The distance between two calcium layers with no intervening nitrogen layers is 4.3221 (6) Å, and this intervening region contains no observable electron density. Neutron and X-ray powder diffraction studies (Gregory et al., 2000) have shown that the c axis dimension changes according to the synthetic method used: the c dimension of the single-crystal was significantly lower than those of the powders, reflecting the higher temperature employed in the formation of the single crystals.

S2. Experimental

Synthetic details are given in the Comment section.

S3. Refinement

There were no unusual aspects to the structure analysis.

Computing details

Data collection: STADI4 (Stoe & Cie, 1997); cell refinement: STADI4; data reduction: X-RED (Stoe & Cie, 1997); program(s) used to solve structure: SIR97 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); software used to prepare material for publication: SHELXL97 and PLATON (Spek, 2000).

Figures

	Fig. 1. A view of the crystal structure of Ca2N showing the layer lattice (Ca blue spheres; N pink spheres). The figure was produced using ATOMS (Dowty, 1998).
	Fig. 2. A view showing the atom labelling and the immediate coordination around Ca and N. Displacement ellipsoids are drawn at 50% the probability level.

(I)

Crystal data

Ca2N	Dx = 2.170 Mg m^−3
Mr = 94.17	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3m	Cell parameters from 49 reflections
a = 3.6271 (3) Å	θ = 12.5–16.5°
c = 18.972 (2) Å	µ = 3.61 mm^−1
V = 216.15 (3) Å^3	T = 298 K
Z = 3	Sphenoid, dark red
F(000) = 141	0.24 × 0.16 × 0.16 mm

Data collection

Stoe Stadi-4 four-circle diffractometer	99 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.055
Graphite monochromator	θmax = 29.9°, θmin = 3.2°
ω/θ scans	h = −5→5
Absorption correction: ψ scan (X-RED; Stoe & Cie, 1997)	k = −5→5
Tmin = 0.574, Tmax = 0.652	l = −25→25
864 measured reflections	3 standard reflections every 60 min
104 independent reflections	intensity decay: random variation +−1.6%

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.017	w = 1/[σ^2(Fo^2) + 0.28P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.038	(Δ/σ)max = 0.001
S = 1.28	Δρmax = 0.23 e Å^−3
104 reflections	Δρmin = −0.30 e Å^−3
7 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.063 (8)

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
Ca	1.0000	1.0000	0.26631 (3)	0.0136 (2)
N	0.6667	0.3333	0.3333	0.0134 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ca	0.0125 (3)	0.0125 (3)	0.0157 (3)	0.00624 (13)	0.000	0.000
N	0.0114 (8)	0.0114 (8)	0.0175 (13)	0.0057 (4)	0.000	0.000

Geometric parameters (Å, º)

Ca—N	2.4500 (3)	Ca—Ca^ii	3.6271 (3)
Ca—Ca^i	3.2944 (8)	Ca—Ca^iii	4.3221 (6)
Ca^ii—N—Ca^iv	95.503 (14)	Ca^v—Ca—Ca^ix	90.0
Ca^ii—N—Ca	95.503 (14)	Ca^iv—Ca—Ca^ix	180
Ca^iv—N—Ca	95.503 (14)	N^viii—Ca—Ca^x	137.752 (7)
Ca^ii—N—Ca^v	84.496 (14)	N^ix—Ca—Ca^x	42.248 (7)
Ca^iv—N—Ca^v	180	N—Ca—Ca^x	90.0
Ca—N—Ca^v	84.496 (14)	Ca^i—Ca—Ca^x	90.0
Ca^ii—N—Ca^vi	84.496 (14)	Ca^vii—Ca—Ca^x	123.401 (9)
Ca^iv—N—Ca^vi	84.496 (14)	Ca^v—Ca—Ca^x	56.599 (9)
Ca—N—Ca^vi	180	Ca^iv—Ca—Ca^x	120
Ca^v—N—Ca^vi	95.504 (14)	Ca^ix—Ca—Ca^x	60
Ca^ii—N—Ca^vii	180	N^viii—Ca—Ca^viii	42.248 (7)
Ca^iv—N—Ca^vii	84.496 (14)	N^ix—Ca—Ca^viii	90.0
Ca—N—Ca^vii	84.496 (14)	N—Ca—Ca^viii	137.752 (7)
Ca^v—N—Ca^vii	95.505 (14)	Ca^i—Ca—Ca^viii	56.599 (9)
Ca^vi—N—Ca^vii	95.505 (14)	Ca^vii—Ca—Ca^viii	90.0
N^viii—Ca—N^ix	95.503 (14)	Ca^v—Ca—Ca^viii	123.401 (9)
N^viii—Ca—N	95.503 (14)	Ca^iv—Ca—Ca^viii	120
N^ix—Ca—N	95.503 (14)	Ca^ix—Ca—Ca^viii	60
N^viii—Ca—Ca^i	47.752 (7)	Ca^x—Ca—Ca^viii	120
N^ix—Ca—Ca^i	47.751 (7)	N^viii—Ca—Ca^xi	42.248 (7)
N—Ca—Ca^i	98.20 (2)	N^ix—Ca—Ca^xi	137.752 (7)
N^viii—Ca—Ca^vii	47.752 (7)	N—Ca—Ca^xi	90.0
N^ix—Ca—Ca^vii	98.20 (2)	Ca^i—Ca—Ca^xi	90.0
N—Ca—Ca^vii	47.751 (7)	Ca^vii—Ca—Ca^xi	56.599 (9)
Ca^i—Ca—Ca^vii	66.802 (18)	Ca^v—Ca—Ca^xi	123.401 (9)
N^viii—Ca—Ca^v	98.20 (2)	Ca^iv—Ca—Ca^xi	60
N^ix—Ca—Ca^v	47.752 (7)	Ca^ix—Ca—Ca^xi	120
N—Ca—Ca^v	47.752 (7)	Ca^x—Ca—Ca^xi	180
Ca^i—Ca—Ca^v	66.802 (18)	Ca^viii—Ca—Ca^xi	60
Ca^vii—Ca—Ca^v	66.802 (18)	N^viii—Ca—Ca^ii	137.752 (7)
N^viii—Ca—Ca^iv	90.0	N^ix—Ca—Ca^ii	90.0
N^ix—Ca—Ca^iv	137.752 (7)	N—Ca—Ca^ii	42.248 (7)
N—Ca—Ca^iv	42.248 (7)	Ca^i—Ca—Ca^ii	123.401 (9)
Ca^i—Ca—Ca^iv	123.401 (9)	Ca^vii—Ca—Ca^ii	90.0
Ca^vii—Ca—Ca^iv	56.599 (9)	Ca^v—Ca—Ca^ii	56.599 (9)
Ca^v—Ca—Ca^iv	90.0	Ca^iv—Ca—Ca^ii	60
N^viii—Ca—Ca^ix	90.0	Ca^ix—Ca—Ca^ii	120
N^ix—Ca—Ca^ix	42.248 (7)	Ca^x—Ca—Ca^ii	60
N—Ca—Ca^ix	137.752 (7)	Ca^viii—Ca—Ca^ii	180
Ca^i—Ca—Ca^ix	56.599 (9)	Ca^xi—Ca—Ca^ii	120
Ca^vii—Ca—Ca^ix	123.401 (9)

Symmetry codes: (i) −x+7/3, −y+8/3, −z+2/3; (ii) x, y−1, z; (iii) −x+5/3, −y+4/3, −z+1/3; (iv) x−1, y−1, z; (v) −x+7/3, −y+5/3, −z+2/3; (vi) −x+4/3, −y+2/3, −z+2/3; (vii) −x+4/3, −y+5/3, −z+2/3; (viii) x, y+1, z; (ix) x+1, y+1, z; (x) x+1, y, z; (xi) x−1, y, z.