Synthesis and crystal structure of γ-SrNCN at 38 GPa

This article is part of the collection Early Career Scientists in Structural Science.

γ-Strontium carbodi­imide, γ-SrNCN, was synthesized from a mixture of strontium subnitride (Sr₂N) and tetra­cyano­ethyl­ene (C₆N₄) at 38 (3) GPa in a laser-heated diamond anvil cell. Its crystal structure was solved and refined using synchrotron single-crystal X-ray diffraction. The new polymorph crystallizes in space group I4/mcm (No. 140), where the Sr²⁺ and NCN²⁻ packing can be derived from the CsCl (B2) structure type. γ-SrNCN (tI16-SrNCN) is isostructural to tI16-BaNCN and represents the first high-pressure polymorph of SrNCN.

1. Chemical context

Inorganic carbodi­imide salts are an inter­esting and well-established class of materials that can exhibit exciting optical, magnetic, and catalytic properties (Corkett et al., 2024). Two polymorphs of SrNCN have been reported thus far. The first characterized polymorph of strontium carbodi­imide, α-SrNCN (oP16-SrNCN, NaSCN structure type), was synthesized through a reaction of melamine (C₃N₆H₆) with strontium subnitride (Sr₂N) at 1123 K (Berger & Schnick, 1994). Polycrystalline β-SrNCN (hR12-SrNCN, β-NaN₃ structure type) was synthesized from SrCO₃ in liquid NH₃ (Wissmann, 2001), while crystals suitable for single-crystal X-ray diffraction were obtained by heating reactive fluxes of SrI₂, NaCN and NaN₃ (2:1:1) at 1073 K, followed by slow cooling (Liao & Dronskowski, 2004). Krings et al. (2010) further expanded the range of synthetic routes to both α- and β-SrNCN and showed that β-SrNCN is the ground-state polymorph. α-SrNCN is used as a host lattice for Eu²⁺ doping, yielding an efficient orange-emitting phosphor (Krings et al., 2011). Here, we report the synthesis of a high-pressure polymorph, γ-SrNCN (tI16-SrNCN), from a mixture of strontium subnitride (Sr₂N) and tetra­cyano­ethyl­ene (C₆N₄) at 38 (3) GPa. γ-SrNCN is isostructural to tI16-BaNCN, which is produced in a reaction of BaCO₃ in liquid NH₃ at ambient pressure and an elevated temperature of 1173 K (Masubuchi et al., 2018). tI16-BaNCN remains stable upon compression up to 23 GPa. At higher pressures, it undergoes a symmetry-lowering phase transition to mC16-BaNCN, driven predominantly by tilting of the NCN²⁻ anions (Masubuchi et al., 2022; Yamamoto et al., 2026). There are a few other examples of high-pressure studies of carbodi­imides (Solozhenko et al., 2004; Glaser et al., 2008; Möller et al., 2018; Meinerzhagen et al., 2024; Yang et al., 2024). Furthermore, high-pressure and high-temperature conditions have proven the feasibility of synthesizing ternary nitridocarbonates with increased coordination numbers of three and four for carbon (Brüning et al., 2023; Aslandukov et al., 2024).

2. Structural commentary

γ-SrNCN crystallizes in the space group I4/mcm (No. 140, KN₃ structure type), where Sr, C, and N occupy the Wyckoff positions 4a (site symmetry 422), 4d (site symmetry m.mm) and 8h (site symmetry m.2m), respectively. The bond lengths and geometry of the NCN²⁻ anion are only weakly affected by pressure, with d(C—N) = 1.222 (13) Å in the reported structure compared to d(C—N) = 1.232 (5) Å in β-SrNCN at ambient pressure. In contrast to α-SrNCN and β-SrNCN, where Sr is sixfold coordinated in an octa­hedral environment with d(Sr—N) = 2.600 (8) − 2.657 (8) Å, γ-SrNCN features Sr in an eightfold tetra­gonal anti­prismatic coordination with d(Sr—N) = 2.460 (4) Å. Similar to β-SrNCN, γ-SrNCN is built from stacked layers of Sr²⁺ cations and linear NCN²⁻ anions (Fig. 1). The main difference between the polymorphs arises from the rearrangement of NCN²⁻ units: in γ-SrNCN, they are oriented parallel to the Sr²⁺ layers rather than perpendicular to them like in β-SrNCN.

Figure 1 Crystal structure representations of (a) γ-SrNCN and (b) β-SrNCN along different crystallographic axes. Displacement ellipsoids are drawn at the 70% probability level. Sr, C and N atoms are colored green, brown and blue, respectively. Semitransparent atoms imply the next layer.

Within the NCN²⁻ layers, each linear unit is rotated by 90° in the ab plane with respect to the corresponding unit within the adjacent layer. Treating the NCN²⁻ unit as a single pseudo-atom would result in an octa­hedral coordination for both Sr and NCN²⁻ in β-SrNCN, while the coordination would be cubic for γ-SrNCN. Therefore, β-SrNCN can be derived from the NaCl (B1) structure type and γ-SrNCN packing follows the CsCl (B2) structure type, consistent with the pressure-coordination rule. A parallel can be drawn to the B1 to B2 phase transition in NaCl at 30 GPa (Bassett et al., 1968) and to the polymorphism of NaN₃, for which the isostructural high-pressure polymorph tI16-NaN₃ was described (Pulham et al., 2014). The results also align well with the pressure homologue rule, since CsCl (B2) packing can be achieved in tI16-BaNCN at ambient pressure (Masubuchi et al., 2018).

3. Synthesis and crystallization

Strontium subnitride (Sr₂N) was synthesized via direct reaction of strontium metal with N₂ gas at 1273 K, as described in the literature (Reckeweg & DiSalvo, 2002). A 20 µm piece of Sr₂N was embedded in tetra­cyano­ethyl­ene (C₆N₄, Thermo Fischer, purity > 98%), compressed to 38 (3) GPa and laser-heated in a diamond anvil cell (BX90 body design; Boehler–Almax type diamonds with a conical aperture of 70° and 200 µm culet size) with a Nd:YAG laser (λ = 1064 nm, T > 1500 K, 4 s heating time). The same synthesis approach and detailed description of data evaluation can be found in previous works on ternary nitridocarbonates (Brüning et al., 2023, 2025; Ranieri et al., 2025; Jurzick et al., 2026). Pressure was determined using the pressure–frequency relationship of the stressed diamond Raman band (Akahama & Kawamura, 2006). The reaction product, γ-SrNCN, was polycrystalline with submicron grain sizes, as indicated by XRD mapping of the sample chamber.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. The reaction product was studied by means of synchrotron single-crystal X-ray diffraction at the extreme conditions beamline P02.2 at Deutsches Elektronen Synchrotron (PetraIII, DESY). The X-ray beam had a full width at half maximum of ∼2 µm and a wavelength of 0.2908 Å, and the X-ray diffraction data were measured using a PerkinElmer XRD1621 2D flat panel detector. To obtain single-crystal datasets, the diamond anvil cell was rotated around the vertical ω axis within a range of ±32°. Diffraction data were acquired in 0.5° ω steps with an exposure time of 4s per °. Data reduction was performed using CrysAlis^PRO software package. Since the dataset contains diffraction data from multiple crystallites with different orientations, the algorithm DaFi (Aslandukov et al., 2022) was used to group and extract the orientation matrices from individual crystalline domains. The most prominent domain was used for integration and the resulting hkl file was then used for structure solution with SHELXT (Sheldrick 2015a) and refinement with SHELXL (Sheldrick 2015b) within the OLEX2-1.5 inter­face (Dolomanov et al., 2009). The limited opening of the diamond anvil cell results in reduced completeness of the datasets. As a standard procedure, we carefully examine reconstructed precession images of each dataset to check for missing superlattice reflections and to cross-check the choice of the space-group symmetry. In the case of γ-SrNCN, the systematic absences are consistent with the I4/mcm space group. It can be shown that (0kl): k,l = 2n [≡(h0l): h,l = 2n] for a c-glide plane perpendicular to [100] (≡[010]) in a tetra­gonal body-centered Bravais lattice holds (Fig. 2).

Table 1
Experimental details

Crystal data
Chemical formula	SrNCN
Mr	127.65
Crystal system, space group	Tetragonal, I4/mcm
Temperature (K)	293
a, c (Å)	5.311 (2), 5.790 (6)
V (Å^3)	163.3 (2)
Z	4
Radiation type	Synchrotron, λ = 0.2908 Å
μ (mm^−1)	3.01
Crystal size (mm)	0.001 × 0.001 × 0.001
Data collection
Diffractometer	Customized ω-circle diffractometer
Absorption correction	Multi-scan (CrysAlis PRO; Rigaku OD, 2025)
Tmin, Tmax	0.150, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	366, 109, 70
Rint	0.072
(sin θ/λ)max (Å^−1)	1.006
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.060, 0.156, 1.15
No. of reflections	109
No. of parameters	10
Δρmax, Δρmin (e Å^−3)	1.97, −1.69
Computer programs: CrysAlis PRO (Rigaku OD, 2025), SHELXT2014/5 (Sheldrick, 2015a), SHELXL2014/7 (Sheldrick, 2015b) and OLEX2 (Dolomanov et al., 2009).

Figure 2 (h0l)-reciprocal lattice plane of γ-SrNCN (No. 140 I4/mcm) at 38 (3) GPa with indexed reflections fulfilling (hkl): h + k + l = 2n for I-centering and (0kl): k,l = 2n [≡ (h0l): h,l = 2n] for a c-glide plane perpendicular to [100] (≡ [010]).

Supporting information

Computing details

γ-Strontium carbodiimide

Crystal data

SrNCN	Dx = 5.191 Mg m^−3
Mr = 127.65	Synchrotron radiation, λ = 0.2908 Å
Tetragonal, I4/mcm	Cell parameters from 94 reflections
a = 5.311 (2) Å	θ = 2.2–14.7°
c = 5.790 (6) Å	µ = 3.01 mm^−1
V = 163.3 (2) Å^3	T = 293 K
Z = 4	Irregular, dull dark gray
F(000) = 232	0.001 × 0.001 × 0.001 mm

Data collection

Customized ω-circle diffractometer	109 independent reflections
Radiation source: synchrotron, PETRAIII, Beamline P02.2	70 reflections with I > 2σ(I)
Synchrotron monochromator	Rint = 0.072
Detector resolution: 5.0 pixels mm^-1	θmax = 17.0°, θmin = 3.6°
ω scans	h = −7→7
Absorption correction: multi-scan (CrysAlisPro; Rigaku OD, 2025)	k = −8→7
Tmin = 0.150, Tmax = 1.000	l = −7→8
366 measured reflections

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: dual
R[F^2 > 2σ(F^2)] = 0.060	w = 1/[σ^2(Fo^2) + (0.0651P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.156	(Δ/σ)max < 0.001
S = 1.15	Δρmax = 1.97 e Å^−3
109 reflections	Δρmin = −1.69 e Å^−3
10 parameters

Special details

Experimental. X-ray diffraction was measured at 38 (3) GPa on synthesis products, produced by laser-heating in a diamond anvil cell with an opening angle of 70°. Tetracyanoethylene was used as the pressure-transmitting medium. Pressure was determined using the pressure-frequency relationship of the stressed diamond Raman band.

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

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

	x	y	z	Uiso*/Ueq
Sr01	0.5000	0.5000	0.2500	0.0118 (5)
C1	0.5000	0.0000	0.0000	0.012 (3)
N1	0.3373 (18)	0.1627 (18)	0.0000	0.013 (2)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sr01	0.0132 (6)	0.0132 (6)	0.0092 (11)	0.000	0.000	0.000
C1	0.012 (5)	0.012 (5)	0.013 (11)	−0.008 (6)	0.000	0.000
N1	0.009 (3)	0.009 (3)	0.019 (8)	0.001 (4)	0.000	0.000

Geometric parameters (Å, º)

Sr01—Sr01^i	2.895 (3)	C1—Sr01^ix	3.0245 (11)
Sr01—Sr01^ii	2.895 (3)	C1—Sr01^x	3.0245 (11)
Sr01—C1^iii	3.0245 (11)	C1—Sr01^xi	3.0245 (11)
Sr01—C1	3.0245 (11)	C1—Sr01^vii	3.0245 (11)
Sr01—N1^iv	2.460 (4)	C1—Sr01^xii	3.0245 (11)
Sr01—N1^v	2.460 (4)	C1—Sr01^i	3.0245 (11)
Sr01—N1	2.460 (4)	C1—Sr01^xiii	3.0245 (11)
Sr01—N1^vi	2.460 (4)	C1—N1	1.222 (13)
Sr01—N1^i	2.460 (4)	C1—N1^xii	1.222 (13)
Sr01—N1^vii	2.460 (4)	N1—Sr01^xi	2.460 (4)
Sr01—N1^iii	2.460 (4)	N1—Sr01^vii	2.460 (4)
Sr01—N1^viii	2.460 (4)	N1—Sr01^i	2.460 (4)
Sr01^ii—Sr01—Sr01^i	180.0	N1^viii—Sr01—N1^vi	86.5 (5)
Sr01^ii—Sr01—C1	118.59 (3)	N1^iv—Sr01—N1^v	149.1 (6)
Sr01^i—Sr01—C1^iii	118.59 (3)	N1^iii—Sr01—N1^vii	69.74 (7)
Sr01^i—Sr01—C1	61.41 (3)	N1—Sr01—N1^vi	69.74 (7)
Sr01^ii—Sr01—C1^iii	61.41 (3)	N1^iv—Sr01—N1^iii	80.5 (2)
C1—Sr01—C1^iii	180.0	Sr01—C1—Sr01^xi	103.24 (2)
N1^viii—Sr01—Sr01^i	126.04 (7)	Sr01^xii—C1—Sr01^vii	103.24 (2)
N1^iv—Sr01—Sr01^ii	126.04 (7)	Sr01^vii—C1—Sr01^x	180.0
N1^iii—Sr01—Sr01^ii	53.96 (7)	Sr01^xi—C1—Sr01^vii	57.19 (5)
N1—Sr01—Sr01^i	53.96 (7)	Sr01^ix—C1—Sr01^x	57.19 (5)
N1—Sr01—Sr01^ii	126.04 (7)	Sr01^xiii—C1—Sr01^x	103.24 (2)
N1^iv—Sr01—Sr01^i	53.96 (7)	Sr01^ix—C1—Sr01^vii	122.81 (5)
N1^vi—Sr01—Sr01^i	53.96 (7)	Sr01^ix—C1—Sr01^xi	180.0
N1^i—Sr01—Sr01^i	53.96 (7)	Sr01^i—C1—Sr01^vii	103.24 (2)
N1^viii—Sr01—Sr01^ii	53.96 (7)	Sr01—C1—Sr01^x	103.24 (2)
N1^i—Sr01—Sr01^ii	126.04 (7)	Sr01^xii—C1—Sr01^x	76.76 (2)
N1^vii—Sr01—Sr01^ii	53.96 (7)	Sr01—C1—Sr01^xii	180.0
N1^vi—Sr01—Sr01^ii	126.04 (7)	Sr01—C1—Sr01^ix	76.76 (2)
N1^vii—Sr01—Sr01^i	126.04 (7)	Sr01—C1—Sr01^vii	76.76 (2)
N1^v—Sr01—Sr01^i	126.04 (7)	Sr01^xi—C1—Sr01^xiii	103.24 (2)
N1^v—Sr01—Sr01^ii	53.96 (7)	Sr01^i—C1—Sr01^x	76.76 (2)
N1^iii—Sr01—Sr01^i	126.04 (7)	Sr01^xii—C1—Sr01^xiii	57.19 (5)
N1^i—Sr01—C1	110.97 (18)	Sr01^xiii—C1—Sr01^vii	76.76 (2)
N1^vii—Sr01—C1^iii	91.5 (2)	Sr01—C1—Sr01^i	57.19 (5)
N1^iii—Sr01—C1	157.1 (3)	Sr01^xi—C1—Sr01^xii	76.76 (2)
N1^iii—Sr01—C1^iii	22.9 (3)	Sr01^ix—C1—Sr01^i	103.24 (2)
N1^viii—Sr01—C1^iii	53.8 (3)	Sr01^xi—C1—Sr01^x	122.81 (5)
N1^vii—Sr01—C1	88.5 (2)	Sr01^xi—C1—Sr01^i	76.76 (2)
N1^i—Sr01—C1^iii	69.03 (18)	Sr01—C1—Sr01^xiii	122.81 (5)
N1^vi—Sr01—C1	53.8 (3)	Sr01^xii—C1—Sr01^i	122.81 (5)
N1^viii—Sr01—C1	126.2 (3)	Sr01^xiii—C1—Sr01^i	180.0
N1^vi—Sr01—C1^iii	126.2 (3)	Sr01^ix—C1—Sr01^xiii	76.76 (2)
N1—Sr01—C1^iii	157.1 (3)	Sr01^ix—C1—Sr01^xii	103.24 (2)
N1^v—Sr01—C1	69.03 (18)	N1^xii—C1—Sr01^vii	128.380 (11)
N1—Sr01—C1	22.9 (3)	N1^xii—C1—Sr01^i	128.380 (11)
N1^iv—Sr01—C1^iii	88.5 (2)	N1—C1—Sr01^xii	128.380 (11)
N1^iv—Sr01—C1	91.5 (2)	N1—C1—Sr01^x	128.380 (11)
N1^v—Sr01—C1^iii	110.97 (18)	N1^xii—C1—Sr01^ix	51.620 (11)
N1^viii—Sr01—N1^i	80.5 (2)	N1^xii—C1—Sr01^xii	51.620 (11)
N1^vi—Sr01—N1^v	80.5 (2)	N1^xii—C1—Sr01	128.380 (11)
N1—Sr01—N1^v	86.5 (5)	N1—C1—Sr01^vii	51.620 (11)
N1^i—Sr01—N1^v	138.9 (5)	N1—C1—Sr01^xiii	128.380 (11)
N1^viii—Sr01—N1^iii	69.74 (7)	N1—C1—Sr01^i	51.620 (11)
N1^viii—Sr01—N1^vii	107.91 (13)	N1^xii—C1—Sr01^xi	128.380 (11)
N1^vii—Sr01—N1^v	69.74 (7)	N1—C1—Sr01^ix	128.380 (11)
N1—Sr01—N1^i	107.91 (13)	N1^xii—C1—Sr01^x	51.620 (11)
N1^viii—Sr01—N1^iv	138.9 (5)	N1—C1—Sr01	51.620 (11)
N1—Sr01—N1^iv	69.74 (7)	N1^xii—C1—Sr01^xiii	51.620 (11)
N1^viii—Sr01—N1^v	69.74 (7)	N1—C1—Sr01^xi	51.620 (11)
N1^iv—Sr01—N1^i	69.74 (7)	N1^xii—C1—N1	180.0 (13)
N1^iv—Sr01—N1^vii	86.5 (5)	Sr01—N1—Sr01^i	72.09 (13)
N1^iii—Sr01—N1^v	107.91 (13)	Sr01^xi—N1—Sr01	149.1 (6)
N1^iv—Sr01—N1^vi	107.91 (13)	Sr01^xi—N1—Sr01^i	99.5 (2)
N1^iii—Sr01—N1^i	86.5 (5)	Sr01^xi—N1—Sr01^vii	72.09 (13)
N1^iii—Sr01—N1^vi	149.1 (6)	Sr01—N1—Sr01^vii	99.5 (2)
N1^viii—Sr01—N1	149.1 (6)	Sr01^vii—N1—Sr01^i	149.1 (6)
N1^vii—Sr01—N1^vi	138.9 (5)	C1—N1—Sr01^i	105.5 (3)
N1^vii—Sr01—N1^i	149.1 (6)	C1—N1—Sr01^xi	105.5 (3)
N1^i—Sr01—N1^vi	69.74 (7)	C1—N1—Sr01^vii	105.5 (3)
N1—Sr01—N1^iii	138.9 (5)	C1—N1—Sr01	105.5 (3)
N1—Sr01—N1^vii	80.5 (2)
Sr01—C1—N1—Sr01^xi	180.000 (1)	Sr01^ix—C1—N1—Sr01	0.000 (1)
Sr01^xii—C1—N1—Sr01^vii	75.25 (6)	Sr01^xiii—C1—N1—Sr01^xi	−75.25 (6)
Sr01^xiii—C1—N1—Sr01^i	180.0	Sr01^xi—C1—N1—Sr01	180.0
Sr01^xi—C1—N1—Sr01^vii	75.25 (6)	Sr01^xiii—C1—N1—Sr01^vii	0.0
Sr01—C1—N1—Sr01^i	75.25 (6)	Sr01^xii—C1—N1—Sr01	180.0
Sr01^xii—C1—N1—Sr01^i	−104.75 (6)	Sr01^i—C1—N1—Sr01^xi	104.75 (6)
Sr01^ix—C1—N1—Sr01^vii	−104.75 (6)	Sr01^xiii—C1—N1—Sr01	104.75 (6)
Sr01^ix—C1—N1—Sr01^xi	180.0	Sr01^ix—C1—N1—Sr01^i	75.25 (6)
Sr01^i—C1—N1—Sr01^vii	180.0	Sr01^i—C1—N1—Sr01	−75.25 (6)
Sr01^x—C1—N1—Sr01^vii	180.0	Sr01^vii—C1—N1—Sr01^xi	−75.25 (6)
Sr01^xi—C1—N1—Sr01^i	−104.75 (6)	Sr01^vii—C1—N1—Sr01	104.75 (6)
Sr01^xii—C1—N1—Sr01^xi	0.000 (1)	Sr01^vii—C1—N1—Sr01^i	180.0
Sr01^x—C1—N1—Sr01^i	0.000 (1)	Sr01^x—C1—N1—Sr01	−75.25 (6)
Sr01—C1—N1—Sr01^vii	−104.75 (6)	Sr01^x—C1—N1—Sr01^xi	104.75 (6)

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