The A-Type Ln 4 N 2 S 3 Series : New Nitride Sulfides of the Light Lanthanoids ( Ln = Ce – Nd )

The reaction of lanthanoid metal powders (Ln) with sulfur and cesium azide (CsN3) as a nitrogen source in the presence of lanthanoid tribromides (LnBr3) yields lanthanoid nitride sulfides with the composition Ln4N2S3 (Ln = Ce–Nd) when appropriate molar ratios of the starting material are used. Additional cesium bromide (CsBr) as a flux secures quantitative conversion (7 days) at 900 ◦C in evacuated silica tubes as well as the formation of black single crystals. All compounds crystallize isotypically with the orthorhombic crystal structure of La4N2S3 (Pnnm, Z = 2) and their structures were determined from single-crystal X-ray diffraction data (Ce4N2S3: a = 644.31(4), b = 1554.13(9), c = 404.20(3) pm; Pr4N2S3: a = 641.23(4), b = 1542.37(9), c = 400.18(3) pm; Nd4N2S3: a = 635.19(4), b = 1536.98(9), c = 397.85(3) pm). Compared to La4N2S3 the a-axes do not fulfill the expectation of the lanthanide contraction. The main feature of the crystal structure comprises N3−-centered (Ln)4 tetrahedra arranging as pairs [N2Ln6] of edge-shared [NLn4] units, which are further connected via four vertices to form double chains 1 ∞{([NLn4/2]2)}. Bundled along [001] like a hexagonal rod packing, they are held together by two crystallographically different S2− anions. Two compounds of a second modification (B-type La4N2S3 and Pr4N2S3) will also be presented and discussed for comparison.


Results and Discussion
The members of the short Ln 4 N 2 S 3 series (Ln = Ce-Nd) crystallize orthorhombically in the space group Pnnm with two formula units (Z = 2) per unit cell (Tables 1-3) and are therefore isotypical with the A-type structure of La 4 N 2 S 3 [7].Each of the two crystallographically independent Ln 3+ cations is firstly surrounded by two N 3− anions.For (Ln1) 3+ another four, for (Ln2) 3+ even four plus one S 2− anions appear in their coordination spheres, thus resulting in overall coordination numbers (C.N.) of 6 and 6+1.The polyhedron around (Ln1) 3+ having the site symmetry (..m) can be described as a trigonal prism (Figure 1, left), in which both a prism edge (N•••N ) as well as the center (Ln1) reside on a mirror plane.(Ln2) 3+ , likewise with the site symmetry (..m), shows a trigonal prism or octahedron as a coordination polyhedron, which again proves to be very distorted, since it exhibits, in addition, another extra sulfur ligand (S2") as a cap (Figure 1, right).The distances d(Ln 3+ -S 2− ) for Ln = Ce-Nd start at 283 pm and increase continuously up to a value of 308 pm.For A-type La 4 N 2 S 3 (a = 641.98(4), b = 1581.42(9), c = 409.87(3)pm) [7], the following ligand provides an abrupt increase of distance (d(La2-S2") = 341 pm), but shows an ECoN = 0.26 (effective coordination number [13]); nevertheless, it is a sound contribution to be considered for the whole coordination sphere of (La2) 3+ .In spite of the lanthanide contraction as anticipated, the compounds Ln 4 N 2 S 3 (Ln = Ce-Nd) investigated in this work show a remarkable devolution of this mentioned distance d(Ln2-S2").First an increase happens from 341 to 351 pm during the transition from the lanthanum to the cerium compound, accompanied by a decreasing ECoN value of 0.13.With the subsequent compounds Pr 4 N 2 S 3 and Nd 4 N 2 S 3 , this distance stagnates and finally decreases again to values of 350 and 343 pm (Table 4 and Figure 2, yellow graph), so one can at most speak of a 6+1-fold but never of a real seven-fold coordination for (Ln2) 3+ (Ln = Ce-Nd).This behavior is also repeated in the lattice constants (Table 1 and Figure 2), where in the extreme case an unusual increase of the a-axis from the lanthanum to the cerium compound can be observed.The molar volumes V m monotonically decrease with the increasing atomic number of Ln, which finally reflects the lanthanide contraction again.

Results and Discussion
The members of the short Ln4N2S3 series (Ln = Ce-Nd) crystallize orthorhombically in the space group Pnnm with two formula units (Z = 2) per unit cell (Tables 1-3) and are therefore isotypical with the A-type structure of La4N2S3 [7].Each of the two crystallographically independent Ln 3+ cations is firstly surrounded by two N 3− anions.For (Ln1) 3+ another four, for (Ln2) 3+ even four plus one S 2− anions appear in their coordination spheres, thus resulting in overall coordination numbers (C.N.) of 6 and 6+1.The polyhedron around (Ln1) 3+ having the site symmetry (.m) can be described as a trigonal prism (Figure 1, left), in which both a prism edge (N•••N′) as well as the center (Ln1) reside on a mirror plane.(Ln2) 3+ , likewise with the site symmetry (.m), shows a trigonal prism or octahedron as a coordination polyhedron, which again proves to be very distorted, since it exhibits, in addition, another extra sulfur ligand (S2″) as a cap (Figure 1, right).The distances d(Ln 3+ -S 2− ) for Ln = Ce-Nd start at 283 pm and increase continuously up to a value of 308 pm.For A-type La4N2S3 (a = 641.98(4), b = 1581.42(9), c = 409.87(3)pm) [7], the following ligand provides an abrupt increase of distance (d(La2-S2″) = 341 pm), but shows an ECoN = 0.26 (effective coordination number [13]); nevertheless, it is a sound contribution to be considered for the whole coordination sphere of (La2) 3+ .In spite of the lanthanide contraction as anticipated, the compounds Ln4N2S3 (Ln = Ce-Nd) investigated in this work show a remarkable devolution of this mentioned distance d(Ln2-S2″).First an increase happens from 341 to 351 pm during the transition from the lanthanum to the cerium compound, accompanied by a decreasing ECoN value of 0.13.With the subsequent compounds Pr4N2S3 and Nd4N2S3, this distance stagnates and finally decreases again to values of 350 and 343 pm (Table 4 and Figure 2, yellow graph), so one can at most speak of a 6+1-fold but never of a real seven-fold coordination for (Ln2) 3+ (Ln = Ce-Nd).This behavior is also repeated in the lattice constants (Table 1 and Figure 2), where in the extreme case an unusual increase of the a-axis from the lanthanum to the cerium compound can be observed.The molar volumes Vm monotonically decrease with the increasing atomic number of Ln, which finally reflects the lanthanide contraction again.12) 50( 9) 86( 10    In analogy to all the rare-earth metal(III) nitride chalcogenides and their halide derivatives known to date [1][2][3], the N 3− anions are again surrounded by a more or less distorted tetrahedron of Ln 3+ cations, in which the four N 3− -Ln 3+ distances (228-241 pm) differ by a maximum of 13 pm and the angles range between 97 • and 114 • (Table 4).In fact, the typical characteristic of the structural construction is actually created by the individual linkage of these [NLn 4 ] 9+ tetrahedra.As shown in Figure 3, the [NLn 4 ] 9+ units initially occur as dimers [N 2 Ln 6 ] 12+ by sharing a common edge (Ln1•••Ln1), and they are then condensed to one-dimensional infinite strands along [001] by corner-linkage (via Ln2) with two similar neighboring units corresponding to 1  ∞ {[N(Ln1) e 2/2 (Ln2) v 2/2 ] 3+ } (e = edge-linking, v = vertex-linking).This type of [NLn 4 ] 9+ -tetrahedral linkage is also found in the crystal structures of the nitride chlorides β-Y 2 NCl 3 and β-Gd 2 NCl 3 [18] and in those of nitride sulfide halides Ln 6 N 3 S 4 X (Ln = La-Nd; X = Cl, Br) [19,20].In the latter, however, the crystal structure is made up of two kinds of strands that are commensurable with each other along their propagation axis.Figure 4 shows a projection of the crystal structure of the new Ln 4 N 2 S 3 representatives with an A-type La 4 N 2 S 3 structure with a view along the c-axis.The 1 ∞ {([NLn 2 ] 2 ) 6+ } double strands are separated by two crystallographically different S 2− anions with almost octahedral Ln 3+ -coordination spheres (Table 5).The neighboring cationic chain units in the a-direction are similarly oriented per se, but compared to their adjacent chains in the b-direction, they get mirrored by a diagonal glide plane n that runs vertical to the b-axis at heights of one-fourth and three-fourths and are shifted by one-half in the aand c-directions, respectively.Thus, a single strand is surrounded by a total of six more in the manner of a hexagonal rod packing.

N
In analogy to all the rare-earth metal(III) nitride chalcogenides and their halide derivatives known to date [1][2][3], the N 3− anions are again surrounded by a more or less distorted tetrahedron of Ln 3+ cations, in which the four N 3− -Ln 3+ distances (228-241 pm) differ by a maximum of 13 pm and the angles range between 97° and 114° (Table 4).In fact, the typical characteristic of the structural construction is actually created by the individual linkage of these [NLn4] 9+ tetrahedra.As shown in Figure 3, the [NLn4] 9+ units initially occur as dimers [N2Ln6] 12+ by sharing a common edge (Ln1•••Ln1), and they are then condensed to one-dimensional infinite strands along [001] by corner-linkage (via Ln2) with two similar neighboring units corresponding to ] 3+ } (e = edge-linking, v = vertex-linking).This type of [NLn4] 9+ -tetrahedral linkage is also found in the crystal structures of the nitride chlorides β-Y2NCl3 and β-Gd2NCl3 [18] and in those of nitride sulfide halides Ln6N3S4X (Ln = La-Nd; X = Cl, Br) [19,20].In the latter, however, the crystal structure is made up of two kinds of strands that are commensurable with each other along their propagation axis.Figure 4 shows a projection of the crystal structure of the new Ln4N2S3 representatives with an A-type La4N2S3 structure with a view along the c-axis.The 1 ∞ {([NLn2]2) 6+ } double strands are separated by two crystallographically different S 2− anions with almost octahedral Ln 3+ -coordination spheres (Table 5).The neighboring cationic chain units in the a-direction are similarly oriented per se, but compared to their adjacent chains in the b-direction, they get mirrored by a diagonal glide plane n that runs vertical to the b-axis at heights of one-fourth and three-fourths and are shifted by one-half in the aand c-directions, respectively.Thus, a single strand is surrounded by a total of six more in the manner of a hexagonal rod packing.

Ln1
Ln2 Apart from the nitride sulfides Ln4N2S3 (Ln = Ce-Nd) and La4N2S3 [7] of the orthorhombic A-type modification presented here, a monoclinic form (B-type) for La4N2S3 [12] and Pr4N2S3 [9] has been reported for each with a crystal structure quite different from the orthorhombic one.Unlike the crystal structure of the A-type Ln4N2S3 members (Ln = La, Pr), in which linear chains are built by linkage of [N2Ln6] 12+ bitetrahedra, in the B-type structure layers are produced by their cross-linkage via common vertices according to ] 3+ } with four-and eight-fold pores (Figure 5).Accompanied by a quadruplication of the cell volume for the B-type (Z = 8) as compared to the A-type (Z = 2), the unit cell of the B-Ln4N2S3 representatives contains four times the total number of cations and anions, but only twice the number of crystallographically different unkind particles owing to the doubling of the respective Wyckoff positions (8f and 4c or 4e as compared to 4g and 2a).With the exception of the already-mentioned distance Ln2-S2″ which loses its coordinative influence upon the transition from A-La4N2S3 to A-Ce4N2S3, both kinds of anions (N 3− and S 2− ) as well as the cations can analogously be assigned to each other in their respective modifications as shown in Table 5.  Apart from the nitride sulfides Ln 4 N 2 S 3 (Ln = Ce-Nd) and La 4 N 2 S 3 [7] of the orthorhombic A-type modification presented here, a monoclinic form (B-type) for La 4 N 2 S 3 [12] and Pr 4 N 2 S 3 [9] has been reported for each with a crystal structure quite different from the orthorhombic one.Unlike the crystal structure of the A-type Ln 4 N 2 S 3 members (Ln = La, Pr), in which linear chains are built by linkage of [N 2 Ln 6 ] 12+ bitetrahedra, in the B-type structure layers are produced by their cross-linkage via common vertices according to 2  ∞ {[N(Ln3/4) e 2/2 (Ln1/2) v 2/2 ] 3+ } with four-and eight-fold pores (Figure 5).Accompanied by a quadruplication of the cell volume for the B-type (Z = 8) as compared to the A-type (Z = 2), the unit cell of the B-Ln 4 N 2 S 3 representatives contains four times the total number of cations and anions, but only twice the number of crystallographically different unkind particles owing to the doubling of the respective Wyckoff positions (8f and 4c or 4e as compared to 4g and 2a).With the exception of the already-mentioned distance Ln2-S2" which loses its coordinative influence upon the transition from A-La 4 N 2 S 3 to A-Ce 4 N 2 S 3 , both kinds of anions (N 3− and S 2− ) as well as the cations can analogously be assigned to each other in their respective modifications as shown in Table 5.In addition to La4N2S3 [7,12] crystallizing dimorphously in the A-La4N2S3-and in the B-Pr4N2S3-type structures and Ce4N2Se3, which is observed either with the Nd4N2Se3-or with the Ce4N2Te3-type arrangement, now the next nitride sulfide of the lanthanoids with the composition Pr4N2S3 can represent both the A-and B-type structures.In order to determine the respective high-pressure and/or high-temperature phases, the theoretically calculated densities using X-ray diffraction (Dx) give at least uniform indications, even though they are not strong.With values of 5.426 [7] and 5.772 g/cm 3 (Table 1) the A-type Ln4N2S3 members (Ln = La, Pr) show somewhat larger densities as compared to 5.363 [12] and 5.740 g/cm 3 [9], respectively, which are available for the possible low-pressure and/or high-temperature phases of the B-type representatives.To what extent these differences of 1.2% and 0.6% could be significant is left to the reader to determine.As the physical parts of the preparation methods for members of both modifications are identical (seven days at 900 °C in evacuated fused silica ampoules, see Experimental), only the chemical conditions can provide an explanation.If for the synthesis of the A-type Ln4N2S3 representatives (Ln = La-Nd), in addition to the lanthanoid metal and sulfur, cesium azide (CsN3) and the corresponding lanthanide bromide (LnBr3, Ln = La-Nd) with CsBr as a fluxing agent were used (see Experimental), the alkali metal and the halides in the form of the triiodides LnI3 (Ln = La, Pr), sodium azide (NaN3) and fluxing NaI varied for the preparation of the B-type Ln4N2S3 ones.Whether, in this case, the intermediates formed, such as elemental iodine (causing changes in pressure or chemical transport) or ternary halides (such as Cs3LnBr6 [21] in the first or Na3LnI6 [22] in the second case) play a role can only be speculated.

Experimental
As adapted from the standard methodology reported in [1], the new lanthanoid nitride sulfides Ln4N2S3 (Ln = Ce-Nd) are obtained by the reaction of lanthanoid metal (Ln; ChemPur: 99.9%) with sulfur (S; ChemPur: 99.9999%) and lanthanoid tribromide (LnBr3; prepared from CeO2, Pr6O11 and Nd2O3 (all: Johnson-Matthey: 99.999%) by the ammonium-bromide method [23]) and cesium azide (CsN3; Ferak: 99.9%).On adding cesium bromide (CsBr; ChemPur: 99.9%) as flux almost black, rod-shaped single crystals of the target compounds Ln4N2S3 (Ln = Ce-Nd) that reflect strongly in the In addition to La 4 N 2 S 3 [7,12] crystallizing dimorphously in the A-La 4 N 2 S 3 -and in the B-Pr 4 N 2 S 3 -type structures and Ce 4 N 2 Se 3 , which is observed either with the Nd 4 N 2 Se 3 -or with the Ce 4 N 2 Te 3 -type arrangement, now the next nitride sulfide of the lanthanoids with the composition Pr 4 N 2 S 3 can represent both the A-and B-type structures.In order to determine the respective high-pressure and/or high-temperature phases, the theoretically calculated densities using X-ray diffraction (D x ) give at least uniform indications, even though they are not strong.With values of 5.426 [7] and 5.772 g/cm 3 (Table 1) the A-type Ln 4 N 2 S 3 members (Ln = La, Pr) show somewhat larger densities as compared to 5.363 [12] and 5.740 g/cm 3 [9], respectively, which are available for the possible low-pressure and/or high-temperature phases of the B-type representatives.To what extent these differences of 1.2% and 0.6% could be significant is left to the reader to determine.As the physical parts of the preparation methods for members of both modifications are identical (seven days at 900 • C in evacuated fused silica ampoules, see Experimental), only the chemical conditions can provide an explanation.If for the synthesis of the A-type Ln 4 N 2 S 3 representatives (Ln = La-Nd), in addition to the lanthanoid metal and sulfur, cesium azide (CsN 3 ) and the corresponding lanthanide bromide (LnBr 3 , Ln = La-Nd) with CsBr as a fluxing agent were used (see Experimental), the alkali metal and the halides in the form of the triiodides LnI 3 (Ln = La, Pr), sodium azide (NaN 3 ) and fluxing NaI varied for the preparation of the B-type Ln 4 N 2 S 3 ones.Whether, in this case, the intermediates formed, such as elemental iodine (causing changes in pressure or chemical transport) or ternary halides (such as Cs 3 LnBr 6 [21] in the first or Na 3 LnI 6 [22] in the second case) play a role can only be speculated.

Figure 2 .
Figure 2. Lattice parameters (a, b, and c), selected distances (d(Ln2-S")), and molar volumes (V m ) of the complete A-type Ln 4 N 2 S 3 series (Ln = La-Nd, error bars with a percentage of 0.5%) versus the ionic radii (r i ) of the trivalent lanthanide cations [17].

Figure 4 .
Figure 4. Projection of the crystal structure of the A-type Ln4N2S3 series (Ln = La-Nd) on the [001] plane.

Figure 4 .
Figure 4. Projection of the crystal structure of the A-type Ln 4 N 2 S 3 series (Ln = La-Nd) on the [001] plane.

Table 1 .
Crystallographic data for the three members of the Ln 4 N 2 S 3 series (Ln = Ce-Nd).

Table 2 .
Fractional atomic coordinates for the three members of the Ln 4 N 2 S 3 series (Ln = Ce-Nd).

Table 3 .
Anisotropic displacement parameters (U ij 1 /pm 2 ) for the three members of the Ln 4 N 2 S 3 series (Ln = Ce-Nd).

Table 4 .
Selected interatomic distances (d/pm) and angles ( / • ) for the three members of the Ln 4 N 2 S 3 series (Ln = Ce-Nd) compared to A-type La 4 N 2 S 3 .

Table 4 .
Selected interatomic distances (d/pm) and angles (∡/°) for the three members of the Ln4N2S3 series (Ln = Ce-Nd) compared to A-type La4N2S3.

Table 5 .
Motifs of mutual adjunction for the A-and B-type Ln 4 N 2 S 3 structures (Ln = La-Nd).
A-type Ln 4 N 2 S 3