Synthesis and Structure Determination of the Quaternary Zinc Nitride Halides Zn 2 NX 1 − y X ′ y ( X , X ′ = Cl , Br , I ; 0 < y < 1 )

The quaternary series Zn2NCl1−yBry and Zn2NBr1−yIy were synthesized from solid-liquid reactions between zinc nitride and the respective zinc halides in closed ampoules, and the evolution of their crystal structures was investigated by single-crystal and powder X-ray diffraction. Zn2NX1−yXy (X, X′ = Cl, Br, I) adopts the anti-β-NaFeO2 motif in which each nitride ion is tetrahedrally coordinated by four zinc cations, and the halide anions are located in the voids of the skeleton formed by corner-sharing [NZn4] tetrahedra. While Zn2NCl1−yBry crystallizes in the acentric orthorhombic space group Pna21 (No. 33), isotypic to Zn2NX (X = Cl, Br), the structure of Zn2NBr1−yIy is a function of the iodide concentration, namely, Zn2NBr (Pna21) for low iodine content and Zn2NI (Pnma) for higher (y ≥ 0.38).

Likewise, the N 3− ion is coordinated by four zinc atoms (2 × Zn1 and 2 × Zn2), with an average Zn-N distance of 1.902 Å and Zn-N-Zn angles between 103° and 116°, in good accordance with what is known from Zn2NX (X = Cl, Br) [30].For further comparison, the Zn-N distance varies between 2.13 and 2.16 Å in Zn3N2 [33], so it is significantly larger in the binary phase than in the ternary which most probably goes back to the more ionic bonding character in the latter.This, however, is an admittedly crude guess.Also, the Zn-Cl distance is between 2.28 and 2.33 Å in ZnCl2 while the mean Zn-Br distance is 2.42 Å in ZnBr2 [34,35].The compounds Zn2NBr0.62I0.38,see Figure 1b, and Zn2NI [30] are isostructural.Zn2NBr0.62I0.38 adopts the orthorhombic space group Pnma in which the N 3− ion is coordinated by four zinc atoms, whereas the halide anions occupy the tetrahedral voids in the framework.
There are two crystallographically independent zinc atoms.Zn1 forms a distorted tetrahedron with its nitrogen, bromide, and iodide neighbors, with bond lengths of Zn1-N = 1.914(5)Å and 1.920 Likewise, the N 3− ion is coordinated by four zinc atoms (2 × Zn1 and 2 × Zn2), with an average Zn-N distance of 1.902 Å and Zn-N-Zn angles between 103 • and 116 • , in good accordance with what is known from Zn 2 NX (X = Cl, Br) [30].For further comparison, the Zn-N distance varies between 2.13 and 2.16 Å in Zn 3 N 2 [33], so it is significantly larger in the binary phase than in the ternary which most probably goes back to the more ionic bonding character in the latter.This, however, is an admittedly crude guess.Also, the Zn-Cl distance is between 2.28 and 2.33 Å in ZnCl 2 while the mean Zn-Br distance is 2.42 Å in ZnBr 2 [34,35].
2.1.2.Crystal Structure of Zn 2 NBr 0.62 I 0.38 The compounds Zn 2 NBr 0.62 I 0.38 , see Figure 1b, and Zn 2 NI [30] are isostructural.Zn 2 NBr 0.62 I 0.38 adopts the orthorhombic space group Pnma in which the N 3− ion is coordinated by four zinc atoms, whereas the halide anions occupy the tetrahedral voids in the framework.
Owing to the increasing halide radius, the Zn-X distances enlarge in going from Cl to I. The halide anions are located in the voids resulting from the corner-sharing [NZn 4 ] tetrahedral framework.The voids occupied by Cl/Br (N-Zn2-N is not linear) are smaller than those occupied by Br/I (N-Zn2-N strictly linear).Hence, the crystal packing of Zn 2 NBr 1−y I y exhibits a higher symmetry than the one of Zn 2 NCl 1−y Br y for y ≥ 0.38 or even slightly lower.PXRD patterns of the quaternary zinc nitride halides are presented in Figure 2.There is an obvious shift of peaks to lower 2θ with increasing halide radii, as expected.One also witnesses tiny amounts of ZnO either resulting from the starting material Zn 3 N 2 or from the quartz tube.Within the Zn 2 NBr 1−y I y system, see Figure 2b, space group Pna2 1 (the one of Zn 2 NBr) and Pnma (the one of Zn 2 NI) cannot be distinguished for trivial crystallographic reasons.
Owing to the increasing halide radius, the Zn-X distances enlarge in going from Cl to I. The halide anions are located in the voids resulting from the corner-sharing [NZn4] tetrahedral framework.The voids occupied by Cl/Br (N-Zn2-N is not linear) are smaller than those occupied by Br/I (N-Zn2-N strictly linear).Hence, the crystal packing of Zn2NBr1−yIy exhibits a higher symmetry than the one of Zn2NCl1−yBry for y ≥ 0.38 or even slightly lower.

Structure Discussion of Zn2NX1−yX′y (X, X′ = Cl, Br, I; 0 < y < 1)
PXRD patterns of the quaternary zinc nitride halides are presented in Figure 2.There is an obvious shift of peaks to lower 2θ with increasing halide radii, as expected.One also witnesses tiny amounts of ZnO either resulting from the starting material Zn3N2 or from the quartz tube.Within the Zn2NBr1−yIy system, see Figure 2b, space group Pna21 (the one of Zn2NBr) and Pnma (the one of Zn2NI) cannot be distinguished for trivial crystallographic reasons.Figure 3 displays the course of the lattice parameters and the unit cell volume (all taken from the powder, not the single-crystal data) against the bromide and iodide content in the Zn 2 NX 1−y X y system.For Zn 2 NCl 1−y Br y , a, b, c, and V increase linearly with the bromide content, see Figure 3a.
For Zn 2 NBr 1−y I y , the behavior is different, as shown in Figure 3b: b and c increase linearly with the iodide content but a first increases slightly for small iodide contents, followed by a sharper increase for larger iodine contents.This effect mirrors the structural change of the Zn 2 NBr 1−y I y system in going from the acentric Pna2 1 to the centric Pnma space group which, according to the single-crystal data, sets in at about y = 0.38, possibly even slightly earlier than that.
Figure 3 displays the course of the lattice parameters and the unit cell volume (all taken from the powder, not the single-crystal data) against the bromide and iodide content in the Zn2NX1−yX′y system.For Zn2NCl1−yBry, a, b, c, and V increase linearly with the bromide content, see Figure 3a.For Zn2NBr1−yIy, the behavior is different, as shown in Figure 3b: b and c increase linearly with the iodide content but a first increases slightly for small iodide contents, followed by a sharper increase for larger iodine contents.This effect mirrors the structural change of the Zn2NBr1−yIy system in going from the acentric Pna21 to the centric Pnma space group which, according to the single-crystal data, sets in at about y = 0.38, possibly even slightly earlier than that.

Synthesis of Zn2NX1−yX′y (X, X′ = Cl, Br, I; 0 < y < 1)
Because the starting materials are air and moisture sensitive, all manipulations were carried out under a continuously purified and monitored argon atmosphere in a glove-box (H2O and O2 below 1 ppm) or under vacuum.The quaternary zinc nitride halides were prepared from solid-liquid reactions.The starting materials, dark gray Zn3N2 (Alfa, Karlsruhe, Germany, 99%) and white ZnCl2 (Alfa, 99.99%; m.p.: 283 °C), ZnBr2 (Alfa, 99.999%; m.p.: 394 °C), or ZnI2 (Alfa, 99.995%; m.p.: 446 °C) were thoroughly mixed using a 1:(1 − y):y molar ratio.For X = Cl, X′ = Br or X = Br, X′ = I, the ratio of y was varied from 0 to 1 in increments of 0.25.The mixture was loaded in a quartz tube which was then sealed under vacuum.The ampoule was heated and kept at a temperature of 550 °C for 20 h.The reaction follows the simple equation:  Because the starting materials are air and moisture sensitive, all manipulations were carried out under a continuously purified and monitored argon atmosphere in a glove-box (H 2 O and O 2 below 1 ppm) or under vacuum.The quaternary zinc nitride halides were prepared from solid-liquid reactions.The starting materials, dark gray Zn 3 N 2 (Alfa, Karlsruhe, Germany, 99%) and white ZnCl 2 (Alfa, 99.99%; m.p.: 283 • C), ZnBr 2 (Alfa, 99.999%; m.p.: 394 • C), or ZnI 2 (Alfa, 99.995%; m.p.: 446 • C) were thoroughly mixed using a 1:(1 − y):y molar ratio.For X = Cl, X = Br or X = Br, X = I, the ratio of y was varied from 0 to 1 in increments of 0.25.The mixture was loaded in a quartz tube which was then sealed under vacuum.The ampoule was heated and kept at a temperature of 550 • C for 20 h.The reaction follows the simple equation: Pale white powders of Zn 2 NCl 1−y Br y and Zn 2 NBr 1−y I y were obtained and checked by X-ray powder diffraction (XRPD).Colorless single crystals of Zn 2 NCl 0.47 Br 0.53 and Zn 2 NBr 0.62 I 0.38 were also obtained by the reaction of Zn 3 N 2 with the respective ZnX 2 at temperatures from 550 to 600 • C for about three days.However, any attempts to synthesize Zn 2 NCl 1−y I y were unsuccessful.
Quaternary zinc nitride halides are stable in dry air for several hours, thereby resembling the ternary zinc nitride halides.

X-ray Crystallography
Single crystals of Zn 2 NCl 0.47 Br 0.53 and Zn 2 NBr 0.62 I 0.38 were fixed on a glass fiber in air.The single-crystal data were collected at 293(2) K with a Bruker SMART APEX CCD diffractometer (Bruker AXS Inc., Madison, WI, USA) using monochromatic Mo-Kα radiation.The collection and reduction of the data were implemented with the Bruker Suite software package [37,38].An empirical absorption correction was carried out with SADABS.
The structures of Zn 2 NCl 0.47 Br 0.53 and Zn 2 NBr 0.62 I 0.38 were solved by analogy with the ternary phases and refined by full-matrix least-squares techniques on the basis of intensities with SHELXL [37,38].Undoubtedly, Zn 2 NCl 0.47 Br 0.53 crystallizes in the acentric space group Pna2 1 (No.33) and is isotypic with Zn 2 NX (X = Cl, Br).Zn 2 NBr 0.62 I 0.38 , however, crystallizes in the centrosymmetric space group Pnma (No. 62) and is isotypic with Zn 2 NI.The halide contents result from the single-crystal refinements which are more reliable in terms of stoichiometry.
The powder X-ray diffraction data of Zn 2 NX 1−y X y (X, X = Cl, Br, I) were recorded at room temperature by means of a calibrated Huber Image Plate (G 670) powder diffractometer (Rimsting, Germany) (Cu-Kα 1 radiation, 6 • -100 • in 2θ) with a flat-sample holder.The background was manually subtracted by linear interpolation, and the FULLPROF program package [39] was used for Rietveld refinements using a pseudo-Voigt profile function.The final structural models of Zn 2 NCl 0.47 Br 0.53 and Zn 2 NBr 0.62 I 0.38 derived from single-crystal XRD were fully confirmed from the Rietveld data, as depicted in Figure 4.  Details of the crystallographic data collection and structure refinement are given in Table 1.Lattice parameters, refined atomic coordinates, equivalent isotropic displacement parameters, and anisotropic displacement parameters are listed in Tables 2-4.Selected bond distances and angles are presented in Table 5.Further information in the form of CIF data has been deposited at Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany, and may be obtained from there using the depository CSD numbers 431875 (Zn2NCl0.47Br0.53)and 431876 Details of the crystallographic data collection and structure refinement are given in Table 1.Lattice parameters, refined atomic coordinates, equivalent isotropic displacement parameters, and anisotropic displacement parameters are listed in Tables 2-4.Selected bond distances and angles are presented in Table 5.Further information in the form of CIF data has been deposited at Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany, and may be obtained from there using the depository CSD numbers 431875 (Zn 2 NCl 0.47 Br 0.53 ) and 431876 (Zn 2 NBr 0.62 I 0.38 ), respectively.

Figure 1 .
Figure 1.Polyhedral representation of [NZn 4 ] tetrahedra and the local nitrogen coordination in Zn 2 NCl 0.47 Br 0.53 (a) and Zn 2 NBr 0.62 I 0.38 (b).The Cl/Br and the Br/I anions statistically occupy the tetrahedral voids in the framework.

Figure 3 .
Figure 3. Course of the lattice parameters a, b, and c (left and blue) and volume V (right and red) based on XRPD data as a function of the bromide content for Zn2NCl1−yBry (a) and the iodide content for Zn2NBr1−yIy (b).

Figure 3 .
Figure 3. Course of the lattice parameters a, b, and c (left and blue) and volume V (right and red) based on XRPD data as a function of the bromide content for Zn 2 NCl 1−y Br y (a) and the iodide content for Zn 2 NBr 1−y I y (b).

Figure 4 .
Figure 4. Rietveld refinement of the X-ray powder pattern of Zn2NCl0.47Br0.53(a) and Zn2NBr0.62I0.38(b) showing measured and fitted intensities (red/black), the position of the Bragg peaks (green) and the difference curve (blue).

Figure 4 .
Figure 4. Rietveld refinement of the X-ray powder pattern of Zn 2 NCl 0.47 Br 0.53 (a) and Zn 2 NBr 0.62 I 0.38 (b) showing measured and fitted intensities (red/black), the position of the Bragg peaks (green) and the difference curve (blue).

Table 1 .
Crystal data and details of the structural refinements of Zn 2 NCl 0.47 Br 0.53 and Zn 2 NBr 0.62 I 0.38 .

Table 3 .
Refined atomic coordinates and equivalent isotropic displacement parameters for Zn 2 NCl 0.47 Br 0.53 and Zn 2 NBr 0.62 I 0.38 .