Topochemical anion metathesis routes to the Zr2N2S phases and the Na2S and ACL derivatives (A = Na, K, Rb)

Article abstract of DOI:10.1021/ja0210650

Anion metathesis reactions between ZrNCl and A₂S (A = Na, K, Rb) in the solid state follow three different pathways depending on reaction temperature and reactant stoichiometry: (1) the reaction of ZrNCl with A₂S in the 2:1 stoichiometry at 800 °C/72 h/in vacuo yields α-Zr₂N₂S with the expected layered structure of La₂O₂S. Above 850 °C, α-Zr₂N₂S (P3m1; a = 3.605(1) A, c = 6.421(3) A) neatly transforms to β-Zr₂N₂S (P6₃/mmc: a = 3.602(1) A, c = 12.817(1) A). The structures of the α- and β-forms are related by an a/2 shift of successive Zr₂N₂ layers. (2) The same reaction at low temperatures (300-400 °C) yields ACl intercalated phases of the formula A_xZr₂N₂SCl_x (0 < x < ～0.15), where alkali ions are inserted between the S/Cl...S/Cl van der Waals gap of a ZrNCl-type structure. The S and Cl ions are disordered and the c lattice parameters are alkali dependent (R3m, a ～ 3.6 A, c ～ 28.4 (Na), 28.9 (K), and 30.5 A (Rb). A_xZr₂N₂SCl_x phases are hygroscopic and reversibly absorb water to give monohydrates. (3) Reaction of ZrNCl with excess A₂S at 400-1000 °C gives A₂S intercalated phases of the formula A_2xZr₂N₂S_1+x (0 < x < 0.5), where the alkali ions reside between the S...S van der Waals gap of a ZrNCl type structure (R3m, a ～ 3.64 A, c ～ 29.48 A). Structural characterization of the new phases and implications of the results are described.

Full text of DOI:10.1021/ja0210650

Published on Web 03/18/2003
Topochemical Anion Metathesis Routes to the Zr2N2S Phases
and the Na2S and ACl Derivatives (A ) Na, K, Rb)
C. Stoltz,† K. Ramesha,† S. A. Sirchio,† Z. S. Go¨nen,† B. W. Eichhorn,*,†
L. Salamanca-Riba,‡ and J. Gopalakrishnan§
Contribution from the Departments of Chemistry and Biochemistry and Materials Engineering,
Center for SuperconductiVity Research, UniVersity of Maryland, College Park, Maryland 20742,
and Solid State and Structural Chemistry Unit, Indian Institute of Science,
Bangalore 560012, India
Received August 8, 2002; E-mail: eichhorn@umd.edu
Abstract: Anion metathesis reactions between ZrNCl and A2S (A ) Na, K, Rb) in the solid state follow
three different pathways depending on reaction temperature and reactant stoichiometry: (1) the reaction
of ZrNCl with A2S in the 2:1 stoichiometry at 800 °C/72 h/in vacuo yields R-Zr2N2S with the expected layered
structure of La2O2S. Above 850 °C, R-Zr2N2S (P3hm1; a ) 3.605(1) Å, c ) 6.421(3) Å) neatly transforms to
â-Zr2N2S (P63/mmc: a ) 3.602(1) Å, c ) 12.817(1) Å). The structures of the R- and â-forms are related by
an a/2 shift of successive Zr2N2 layers. (2) The same reaction at low temperatures (300-400 °C) yields
ACl intercalated phases of the formula AxZr2N2SClx (0 < x < ∼0.15), where alkali ions are inserted between
the S/Cl‚‚‚S/Cl van der Waals gap of a ZrNCl-type structure. The S and Cl ions are disordered and the c
lattice parameters are alkali dependent (R3hm, a ∼ 3.6 Å, c ∼ 28.4 (Na), 28.9 (K), and 30.5 Å (Rb).
AxZr2N2SClx phases are hygroscopic and reversibly absorb water to give monohydrates. (3) Reaction of
ZrNCl with excess A2S at 400-1000 °C gives A2S intercalated phases of the formula A2xZr2N2S1+x (0 < x
< 0.5), where the alkali ions reside between the S‚‚‚S van der Waals gap of a ZrNCl type structure (R3hm,
a ∼ 3.64 Å, c ∼ 29.48 Å). Structural characterization of the new phases and implications of the results are
described.
The group 4 nitride-halides have received considerable
attention of late due to their interesting structures and physical
properties. Recently, it was shown that alkali-doped derivatives
of ZrNCl and HfNCl are superconductors with Tc’s as high as
25 K.1-4 The ZrNCl structure is lamellar with a van der Waals-
type stacking of Cl-Zr2N2-Cl sheets (see I in Chart 1).5
Various alkali ions and organometallics can be inserted into
the van der Waals gap and effectively inject charge into the
ZrN layers.6-10 The superconducting transition temperatures of
the intercalated (doped) phases are relatively insensitive to the
nature of the intercalant, which is reminiscent of the behavior
in the doped AxMS2 superconductors (A ) alkali, M ) early
transition metal).11-13 Unlike the MS2 derivatives, however, the
halide anions of the HfNCl and ZrNCl phases can be changed
(Cl f Br f I), which effects a shift to lower Tc in the doped
materials.4,8 For these reasons, we and others have been
interested in preparing different derivatives of this class of
compounds in order to study the anion effect on Tc and achieve
higher transition temperatures.
One intriguing but elusive derivative of this class of materials
is the sulfide compound Zr2N2S, which is expected14,15 to have
an La2O2S structure16 (see II in Chart 1). The transformation
from ZrNCl to Zr2N2S involves replacement of two chloride
ions by one sulfide ion, which would eliminate the van der
Waals gap and covalently link the ZrN layers. Although the
synthesis of Zr2N2S has not been reported to date, Lissner and
Schleid recently described15 a novel synthetic route to a NaCl-
stabilized Zr2N2S derivative of formula NaZr2N2SCl (see III in
Chart 1 and eq 1). The NaZr2N2SCl structure is closely related
† Department of Chemistry and Biochemistry, University of Maryland.
‡ Department of Materials Engineering, University of Maryland.
§ Indian Institute of Science.
(1) Tou, H.; Maniwa, Y.; Koiwasaki, T.; Yamanaka, S. Phys. ReV. B 2001,
63, 20508.
2NaN3 + 7Zr + 6S + xs NaCl f
(2) Yamanaka, S.; Hotehama, K.; Kawaji, H. Nature 1998, 392, 580.
(3) Yamanaka, S.; Hotehama, K.; Koiwasaki, T.; Kawaji, H.; Fukuoka, H.;
Shamoto, S.; Kajitani, T. Physica C 2000, 341, 699.
Na2ZrS3 + 3NaZr2N2SCl (850 °C) (1)
(4) Yamanaka, S.; Kawaji, H.; Hotehama, K.; Ohashi, M. AdV. Mater. 1996,
8, 771.
to that of ZrNCl. The Na+ ions of NaZr2N2SCl fill the octahedral
(5) Fogg, A. M.; Evans, J. S. O.; O’Hare, D. Chem. Commun. 1998, 2269.
(6) Ohashi, M.; Uyeoka, K.; Yamanaka, S.; Hattori, M. Chem. Lett. 1990, 93.
(7) Ohashi, M.; Uyeoka, K.; Yamanaka, S.; Hattori, M. Bull. Chem. Soc. Jpn.
1991, 64, 2814.
(12) Intercalation Chemistry; Whittingham, M. S., Jacobson, A. J., Eds.;
Academic Press: New York, 1982.
(8) Kawaji, H.; Hotehama, K.; Yamanaka, S. Chem. Mater. 1997, 9, 2127.
(9) Fogg, A. M.; Green, V. M.; O’Hare, D. Chem. Mater. 1999, 11, 216.
(10) Fogg, A. M.; Green, V. M.; O’Hare, D. J. Mater. Chem. 1999, 9, 1547.
(11) Scho¨llhorn, R. in Physics of Intercalation Compounds; Pietronero, l., Tosatti,
E., Eds.; Springer-Verlag: Berlin, New York, 1983; p 33.
(13) Jacobson, A. J. In Solid State Chemistry Compounds; Cheetham, A., Day,
P., Eds.; Oxford University Press: New York, 1992; p 182.
(14) Lissner, F.; Schleid, T. Z. Anorg. Allg. Chem. 1999, 625, 195.
(15) Lissner, F.; Schleid, T. Z. Anorg. Allg. Chem. 2001, 627, 2307.
(16) Morosin, B. Acta Crystallogr. 1973, B29, 2647.
9
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J. AM. CHEM. SOC. 2003, 125, 4285-4292
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A R T I C L E S
Stoltz et al.
Chart 1
Scheme 1
the ZrO2 impurity content. The compound has the expected
La2O2S structure type with P3hm1 crystal symmetry and will be
discussed in the next section.
Above 850 °C, R-Zr2N2S neatly converts to a different
polymorph, â-Zr2N2S, that possesses P63/mmc crystal symmetry
and, to our knowledge, represents a new structure type (see next
section). The â-Zr2N2S phase can be directly synthesized from
the ZrNCl and A2S precursors when reaction 2 is carried out at
900 °C. The â-phase is dark brownish-green in color and is
stable to 1050 °C in the absence of air. This compound has
been characterized by EDX, microanalysis, and powder XRD
(Rietveld refinement). Both the R- and â-Zr2N2S phases are
stable in air at room temperature and are not hygroscopic.
Reactions of ZrNCl with excess A2S (A ) Na, K) at 400-
1000 °C result in the formation of A2S excess phases of formula
A2xZr2N2S1+x (0 < x < 0.5) according to eq 3. The A2S phases
holes in the van der Waals gap of the ZrNCl structure whereas
a disordered 1:1 mixture of S and Cl resides on the chloride
sites.15 We report here the synthesis of nonstabilized Zr2N2S
by way of topotactic anion metathesis chemistry. The low-
temperature form of the compound has the expected La2O2S
structure type (P3hm1 crystal symmetry)16 but undergoes a phase
transition to a more thermodynamically stable â-structure (P63/
mmc crystal symmetry). In addition, we describe the synthesis
and characterization of the Na2S excess phases, Na2xZr2N2S1+x
(0 < x < 0.5) and defect members of the Lissner and Schleid-
type compounds15 of formula AxZr2N2SClx where A ) Na, K,
Rb and 0 < x < ∼0.15.
2ZrNCl + 1+xA2S f A2xZr2N2S1+x + 2ACl
(A ) Na, K; 400-1000 °C, 0 < x < 0.5) (3)
appear to exist throughout the range 0 < x e 0.5; however, we
have only characterized two members of the series; namely,
Na0.5Zr2N2S1.25 (x ) 0.25) and Na0.95Zr2N2S1.84 (x ≈ 0.5). The
Na compounds can be prepared as single-phase materials
whereas the potassium compounds K2xZr2N2S1+x are formed
competitively with R-Zr2N2S and have not been prepared in pure
form. Reactions with Rb2S give exclusively Zr2N2S. High
synthesis temperatures (∼1000 °C) are required to obtain
Na2xZr2N2S1+x members with x > 0.25. Reactions conducted
at 800 °C or below result in a maximum of 25% Na2S
incorporation regardless of reaction stoichiometry. WDS analy-
ses and bulk sulfur microanalyses conducted on the x ) 0.25
and x ) 0.50 Na2xZr2N2S1+x compounds gave the formulations
Results
Synthesis. ZrNCl reacts with A2S (A ) Na, K, Rb) reagents
to give a series of sulfide/chloride materials that depend on
reaction conditions and temperature. The reactions are sum-
marized in Scheme 1 and are described individually below.
1
ZrNCl reacts with /2 equiv of A2S (A ) Na, K, Rb) at 800
°C (in vacuo) to give R-Zr2N2S and ACl after 72 h according
to eq 2. The brown R-Zr2N2S product is obtained by removing
2ZrNCl + A2S f R-Zr2N2S + 2ACl
(A ) Na, K, Rb; 800 °C) (2)
the ACl byproduct with water. The compound has been
characterized by powder XRD, EDX, TEM, and ED studies.
The XRD analysis (Figure 1) shows a ZrO2 impurity that
presumably originates from the ZrNCl starting material. EDX
analyses show only Zr and S in a 2:1 ratio after accounting for
(normalized to Zr) of Na0.49(3)Zr2.00(7)N2S1.28(4) and Na0.90(3)-
Zr2.0(2)N2S1.85(1), respectively. The formula are subsequently
referred to as Na0.5Zr2N2S1.25 and Na0.95Zr2N2S1.84, respectively.
The apparent excess sulfur in the Na0.95Zr2N2S1.84 phase may
be due to the presence of disulfide units (S22-) within the sulfide
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4286 J. AM. CHEM. SOC. VOL. 125, NO. 14, 2003

Anion Metathesis Routes to Zr2N S Phases
A R T I C L E S
2
Figure 1. Calculated and observed XRD profiles for R-Zr2N2S. The asterisks denote reflections due to the ZrO2 impurity. The broad background is due to
the emerging â-Zr2N2S high-temperature phase.
Table 1. Crystallographic Data
b
c
b
empirical formula
Na Zr2N SClxa
K Zr2N SClxa
Rb Zr2N SClxa
Na0.95Zr2N S
R-Zr2N S
â-Zr2N S
x
2
x
2
x
2
2
1.84
2
2
space group
a/Å
c/Å
c/Å hydrated
reaction temp/°C
R3hm
3.63
28.4
32.5
R3hm
3.61
28.9
33.9
R3hm
3.57
30.5
33.3
R3hm
P3hm1
P63/mmc
3.611(1)
12.818(1)
3.637(1)
3.605(2)
6.412(3)
29.490(7)
300-350
300-350
300-350
400-1000
700-800
850-1050
a Unit cell estimates from diffraction data. Not refined due to broadness of diffraction peaks. b Refined unit cell parameters from Rietveld analysis (Topas,
Bruker). c Refined unit cell from diffracction data (MDI; Jade).
layer. The A2xZr2N2S1+x compounds adopt a highly defective
NaZr2N2SCl type structure (see III in Chart 1). The compounds
are yellow-brown in color, free of chloride (WDS and EDX
analyses), and stable in air at room temperature.
The sulfide-for-chloride exchange also occurs at much lower
temperatures (300-400 °C), but the resulting products contain
residual alkali halide (5-15 mol %) and adopt a highly defective
NaZr2N2SCl-type structure (see III in Chart 1 and eq 4). The
section) are relatively insensitive to the size of the alkali ions
and are presumably dictated by the size of the H2O intercalant
(see IV in Chart 1). TGA analyses indicate that approximately
one water molecule of hydration is incorporated for each Zr2N2S
subunit. The exact hydration level is difficult to determine due
to the heterogeneity of the sample (see below). The alkali
chloride-stabilized compounds have been characterized by
powder XRD, EDX, WDS, TEM, and ED analysis. The WDS/
EDX analyses from stoichiometric reactions are highly variable
from sample to sample showing 5-15% ACl contents, but Cl
is often in significantly higher concentration than the alkali
content. The data indicate that the products are most likely
heterogeneous with various members of the AxZr2N2SClx solid
solution, the corresponding hydrates, and potentially some
products from incomplete metathesis.
2ZrNCl + A2S f AxZr2N2SClx + 2-xACl
(A ) Na, K; 0 < x e 0.15; 300-400 °C) (4)
use of excess Na2S in the reactions or regrinding and refiring
with additional Na2S does not reduce the amount of residual
chloride in the 300-350 °C temperature regime. Although the
product compositions vary with reaction temperature, conditions,
and time, the general formula for these compounds is given by
NaxZr2N2SClx, where 0 < x e ∼0.15 (see below).
Structural Studies. A summary of the structural data is given
in Table 1. Each compound is described individually below.
r-Zr2N2S. The low-temperature R-Zr2N2S compound adopts
the expected14,15 La2O2S structure16 with P3hm1 crystal symmetry
where a ) 3.605(1) Å and c ) 6.412(3) Å. The crystal
symmetry has been assigned on the basis of the XRD patterns,
the TEM data, and the electron diffraction studies. The simulated
and observed XRD profiles are shown in Figure 1, and an
idealized ball-and-stick drawing of the structure is given in
Figure 2. The broad background in the diffraction profile arises
from the emerging â-Zr2N2S phase (see next section) that
crystallizes above 850 °C. The ZrO2 impurity presumably
originates from the ZrNCl starting material. Regardless, the
agreement between calculated and observed XRD profiles is
excellent.
The x ) 1 end member is the NaZr2N2SCl phase described
by Lissner and Schleid15 but cannot be prepared by this
chemistry. Due to the low reaction temperatures, the compounds
are poorly crystalline (see next section) but the prominent 0 0
3 reflections show that the c lattice parameter increases with
the size of the alkali ion (see Table 1).
Unlike the Zr2N2S and A2xZr2N2S1+x phases, the AxZr2N2-
SClx compounds are hygroscopic and reversibly form mono-
hydrates of formula Ax(H2O)Zr2N2SClx (see eq 5). The c lattice
AxZr2N2SClx + H2O a Ax(H2O)Zr2N2SClx
(5)
parameters of the hydrated materials (∼33-34 Å; see next
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A R T I C L E S
Stoltz et al.
Figure 4. Ball-and-stick drawing of (a) the ZrN4S3 coordination sphere of
Zr2N2S compounds, (b) the octahedral SZr6 site in R-Zr2N2S, and (c) the
trigonal prismatic SZr6 site in â-Zr2N2S. The coloring scheme is as
follows: zirconium, blue; sulfur, yellow; nitrogen, light gray.
Figure 2. Idealized ball-and-stick drawing of the R-Zr2N2S structure (a)
perpendicular to the c-axis and (b) down the c-axis. The coloring scheme
is as follows: zirconium, blue; sulfur, yellow; nitrogen, light gray.
and zirconium coordination numbers are lower in the later
compounds. The sulfur atoms of R-Zr2N2S symmetrically bridge
the ZrN layers and reside in Zr6 octahedral holes (see Figure
4b).
â-Zr2N2S. The high-temperature form of Zr2N2S, denoted
â-Zr2N2S, adopts a new structure type with P63/mmc crystal
symmetry with a ) 3.611(1) Å and c ) 12.818(1) Å. The
calculated, observed, and difference XRD profiles for â-Zr2N2S
are shown in Figure 5 and a ball-and-stick drawing of this
structure is given in Figure 6. A summary of the refinement
data is given in Table 2 and selected bond distances and angles
are given in Table 3. The ZrO2 impurity in the sample was
accommodated by way of a two-phase refinement. The â-Zr2N2S
structure differs from the R-form by shifting the interleaving
Zr2N2 layers by a/2, which leaves the interlayer sulfide ions in
trigonal prismatic Zr6 holes instead of octahedral holes (see
Figure 4c). The shift changes the stacking of the ZrN layers
along the c axis to give an AB‚‚‚BA‚‚‚AB‚‚‚ repeat sequence
(Figure 6). This alternate stacking sequence doubles the c lattice
parameter relative to the AB‚‚‚AB‚‚‚AB arrangement in the
R-structure and generates the trigonal prismatic Zr6 interlayer
holes. The shift also aligns the Zr atoms from adjacent layers
along the c axis. However, the zirconium coordination sphere
and individual ZrN layers are the same in both forms. The
refined Zr-S distances (2.814(1) Å) are again long relative to
other zirconium sulfides14,17,18 and presumably reflect the high
coordination numbers (CNs) of both sulfur (CN ) 6) and
zirconium (CN ) 7). The refined Zr-N distances (2.10, 2.35
Å) are quite reasonable15 but are less reliable due to the low
scattering power of nitrogen.
Figure 3. TEM image of the R-Zr2N2S showing the [1 0 0] zone axis and
the 6.4 Å unit cell. The inset shows the electron diffraction pattern, which
is consistent with the La2O2S structure type.
The TEM image of R-Zr2N2S (Figure 3) shows the layered
nature of the structure and the well-defined 6.4 Å repeat unit
that defines the c lattice parameter. The selected area electron
diffraction (SAED) studies of the [0 0 1] zone axis (see inset,
Figure 3) are also consistent with the refined unit cell parameters
and crystal symmetry and do not reveal any superstructure
reflections.
The R-Zr2N2S structure contains Zr2N2 layers similar to that
of â-ZrNCl and related zirconium nitrides. The individual ZrN
layers stack in an AB‚‚‚AB‚‚‚AB repeat pattern that gives rise
to interlayer Zr6 octahedral holes and the 6.4-Å c lattice
parameter (Figure 3). Due to the broadness of the diffraction
profiles and the emergence of the â-Zr2N2S phase, we could
not obtain reasonable refinements of the structure. However,
the idealized model constructed from the refined unit cell
constants imposed on the La2O2S structure type gives very
reasonable metric parameters. In the idealized model, each Zr
is 7 coordinate with a ZrN4S3 coordination sphere (see Figure
4a). When the Zr-N distances are adjusted to equal those of
â-ZrNCl, the optimized Zr-S distances are 2.74 Å. For
Na2xZr2N2S1+x. The structure of the Na2xZr2N2S1+x phases
is closely related to that of â-ZrNCl and NaZr2N2SCl. The
compounds are rhombohedral, space group R3hm, with a ≈ 3.6
Å and c ≈ 29.5 Å. The results of a Rietveld refinement for
Na0.95Zr2N2S1.84 are given in Table 2, and selected bond
distances and angles are given in Table 3. The calculated,
observed, and difference XRD profiles are shown in Figure 7.
The analytical data gave a formula of Na0.90(3)Zr2.0(2)N2S1.85(1)
,
which is in excellent agreement with the refined occupancies
of Na0.95(2)Zr2N2S1.84(3) for the same sample. The structure is
essentially that of NaZr2N2SCl (see III in Chart 1) with the
following exceptions. The excess sulfide in the Na0.95Zr2N2S1.84
formula (i.e. for x ) 0.47 in Na2xZr2N2S1+x; Na ) 0.95 and S
) 1.47) presumably arises from the formation of disulfide units
in the sulfide layer. The formation of disulfides is quite common
comparison, the Zr-S distances in NaZr2S3 and the Ban+1
-
ZrnS3n+1 perovskites are 2.5-2.6 Å (av)14,17,18 but the sulfur
(18) Hung, Y.-C.; Fettinger, J. C.; Eichhorn, B. W. Acta Crystallogr. 1997, C53,
(17) Chen, B.-H.; Eichhorn, B.; Fanwick, P. Inorg. Chem. 1992, 31, 1788.
827.
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4288 J. AM. CHEM. SOC. VOL. 125, NO. 14, 2003

Anion Metathesis Routes to Zr2N S Phases
A R T I C L E S
2
Figure 5. Calculated, observed, and difference XRD profiles (Rietveld analysis, Topas) for â-Zr2N2S. The asterisks denote the ZrO2 impurities (two polymorphs)
that were accommodated by way of a three-phase refinement. The tick marks indicate the Bragg reflections for â-Zr2N2S (top), ZrO2 JCPDF 83-0939
(middle), and ZrO2 JCPDF 81-1550 (bottom).
Table 2. XRD Rietveld Refinement Data
â-Zr2N S
Na0.95Zr2N S
2
2
1.84
crystal system
space group
a/Å
c/Å
Pref Or
Rwp
hexagonal
P63/mmc
3.611(1)
12.818(1)
0 0 1
7.8
trigonal
R3hm
3.637(1)
29.490(7)
0 0 1
9.5
Rp
5.9
6.11
RBragg
Zr
4.1
1/3
4.1
0
x
y
2/3
0
z
0.8975(1)
0.21(8)
1.0
0.2135(3)
0.46(6)
1.0
0
Beq
occ
x
N
1/3
y
z
Beq
occ
x
y
z
Beq
occ
x
2/3
0.0809(5)
0.10
1.0
0
0
1/4
0.10(1)
1.0
0
Figure 6. Ball-and-stick drawing of the â-Zr2N2S structure showing (a)
an approximate 1 0 0 projection and (b) a 0 0 1 projection. The coloring
scheme is as follows: zirconium, blue; sulfur, yellow; nitrogen, light gray.
0.1430(3)
0.20
1.0
0
S
and would alleviate instabilities associated with large vacancy
concentrations. However, in Na0.95Zr2N2S1.84, the sulfide layer
still contains ∼15% vacancies that are randomly distributed.
For lower x members in the Na2xZr2N2S1+x series, the vacancies
approach 50%. Although we do not detect disulfides in these
compounds through composition analysis, it is quite possible
that disulfide units could exist there as well.
The octahedral sites that host the Na+ ions in the van der
Waals gaps can be fully occupied (x ) 0.5) or virtually empty
as x approaches zero. This behavior is quite similar to that of
the AxMS2 compounds (A ) alkali, M ) early transition
metal),11-13 where x can vary from 0 to 1. The unit cell
parameters for Na0.5Zr2N2S1.25 (a ) 3.634(1) Å, c ) 29.469(7)
Å) are very similar to those of Na0.95Zr2N2S1.84 (a ) 3.637(1)
Å, c ) 29.490(7) Å).
The Zr-S bond distances for NaZr2N2S1.5 are 2.671(2) Å,
which are shorter than those of the Zr2N2S phases and are more
akin to other zirconium sulfides.14,17,18 The shorter distances
presumably reflect the lower coordination numbers of both Zr
and S due to vacancies in the NaZr2N2S1.5 structure. The refined
Zr-N and Na-S distances are typical.5,15
0
0.3973(1)
0.08(1)
0.918(5)
0
0
0
Na
y
z
Beq
occ
2.0(2)
0.95(2)
NaZr2N2SCl (see III in Chart 1)15 but have vacancies on both
the Na and S/Cl sites. The compounds are rhombohedral, space
group R3hm, with alkali-dependent lattice parameters of a ≈ 3.6
Å and c ) 28.4 (Na), 28.9 (K), and 30.5 Å (Rb). The crystal
symmetries and structures are based upon the similarities with
the well-characterized15 NaZr2N2SCl end member, the XRD
profiles, and the TEM and ED data. Because layer shifts and
alternate crystal symmetries are well known for layered oxides
with large alkali ions,19 the actual crystal symmetry for the Rb
phase may be different. The XRD profiles are characteristically
broad in appearance (see Figure 8) and are quite similar to those
of ZrNCl as expected. The c lattice constants were estimated
AxZr2N2SClx and Its Hydrates. The AxZr2N2SClx phases are
also isostructural with the Lissner and Schleid compound
(19) Schaak, R. E.; Mallouk, T. E. Chem. Mater. 2002, 14, 1455.
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Figure 7. Calculated, observed, and difference XRD profiles (Rietveld analysis, Topas) for Na0.95Zr2N2S1.84. The asterisks denote the ZrO2 impurities (not
refined). The tick marks indicate the Na0.95Zr2N2S1.84 Bragg reflections.
Table 3. Selected Bond Distances (Å) and Angles (deg)
â-Zr2N S
Na0.95Zr2N S
2
2
1.84
Zr-N
2.103(1)
2.35(1)
2.814(1)
2.078(12)
2.208(4)
2.671(2)
2.822(2)
110.9(3)
72.0(3)
Zr-N
Zr-S
Na-S
N-Zr-N
N-Zr-N
Zr-N-Zr
Zr-S-Zr
S-Zr-S
S-Na-S
S-Na-S
118.3(2)
82.4(2)
97.6(2)
82.4(2)
79.81(1)
108.0(3)
85.8(1)
99.7(1)
80.3(1)
from the 0 0 3 reflections and the TEM images whereas the a
lattice constants were estimated from the electron diffraction
data and the higher angle diffraction data. Cell constants and
structures could not be refined due to the low quality of the
diffraction data.
Figure 8. XRD profile for KxZr2N2SClx(H2O), where x ∼ 0.15. The inset
shows the 0 0 3 peak shift upon hydration.
hydrophilic interactions of organic and inorganic layers, one
can obtain exfoliated monolayers and nanotubes by inserting
organic cations between the layers of inorganic oxides.22,23
However, the intergallery exchangeable cations in layered oxides
are highly ionic by design and have little electronic or chemical
influence on the host oxide matrix. In contrast, since anions
are an integral part of the host, exchange of anions in layered
materials can directly alter the band structure of the host lattice.
Moreover, if organic anions can be incorporated into the host,
they can directly affect the electronic properties of the materials
through covalent interactions and potentially introduce additional
functionality. Of course, topotactic anion exchange is much more
difficult than cation exchange due to the anion’s large size and
high polarizability. However, the sulfide-for-chloride exchanges
described here illustrate that such transformations are indeed
possible and also give rise to new solid-state compounds that
are not accessible by traditional means. Importantly, these
reactions illustrate that generic coordination chemistry available
The hydrated samples contain water molecules in the inter-
gallery sites, and all of the AxZr2N2SClx(H2O) phases (where
A ) Na, K, Rb) have c lattice parameters of ∼33-34 Å (see
IV in Chart 1 and Table 1). The TEM images of the dehydrated
(Figure 9a) and hydrated (Figure 9b) potassium phase shows
the 29- and 34-Å lattice spacings, respectively, in accord with
the observed XRD profiles (see Figure 8). Importantly, the TEM
images of the hydrated phase show both hydrated and dehy-
drated regions in the same crystallite (see Figure 9b), which is
consistent with the reversible nature of the hydration process
(eq 5). The insensitivity of the c parameters to the size of the
alkali ion in the hydrated samples indicates that the intergallery
spacing is dominated by water molecules that are presumably
H-bonded to the Cl/S anions (see IV in Chart 1). The exact
crystal symmetries of the hydrated phases are unkown.
Discussion
Cation methathesis in layered oxides has received significant
recent attention as a method of preparing low-temperature and
metastable-layered oxides.19-21 By exploiting the hydrophobic/
(21) Gopalakrishnan, J.; Sivakumar, T.; Ramesha, K.; Thangadurai, V.; Sub-
banna, G. N. J. Am. Chem. Soc. 2000, 122, 6237.
(22) Urban, J. J.; Yun, W. S.; Gu, Q.; Park, H., J. Am. Chem. Soc. 2002, 124,
1186.
(20) Sivakumar, T.; Seshadri, R.; Gopalakrishnan, J. J. Am. Chem. Soc. 2001,
123, 11496.
(23) Schaak, R. E.; Mallouk, T. E. Chem. Mater. 2000, 12, 3427.
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Anion Metathesis Routes to Zr2N S Phases
A R T I C L E S
2
Figure 9. TEM image of KxZr2N2SClx, where x ∼ 0.15. (a) A crystallite showing a dehydrated KxZr2N2SClx region; (b) a crystallite showing dehydrated
and hydrated KxZr2N2SClx regions.
Conlclusion
to the molecular chemist may be viable in these and related
layered halides.
We have shown that anion exchange reactions between ZrNCl
and A2S (A ) Na, K, Rb) in the solid state follow three different
pathways depending on reaction temperature and reactant
stoichiometry. The reactions of ZrNCl with A2S in the 2:1
stoichiometry gives R-Zr2N2S with the expected layered struc-
ture of La2O2S at 800 °C but gives â-Zr2N2S above 850 °C.
The structures of the R- and â-forms are related by an a/2 shift
of successive Zr2N2 layers. The same reaction at low temper-
atures (300-400 °C) yields ACl intercalated phases of the
formula AxZr2N2SClx (0 < x < ∼0.2) that are defect derivatives
of the Lissner and Schlied compound NaZr2N2SCl.15 Reactions
of ZrNCl with excess A2S at 400-1000 °C give A2S intercalated
phases of the formula A2xZr2N2S1+x (0< x < 0.5), where the
alkali ions reside between the S‚‚‚S van der Waals gap of a
ZrNCl-type structure. Importantly, these anion methathesis
synthetic procedures appear to be the only routes to the new
materials described here and may evolve into a general method
for the preparation of a wide variety of new, fuctionalized
compounds.
Ironically, the poor crystallinity and small particle size of
the ZrNCl precursor, which makes characterization difficult, is
presumably a key feature in the success of the metathesis
chemistry described here. Highly crystalline ZrNCl starting
materials show much slower reaction kinetics for metathesis,
which is consistent with a topochemical metathesis exchange
process.
The sulfide-for-chloride exchange reactions described here
show that kinetic control can be obtained in a topchemical anion
exchange process involving a refractory inorganic host matrix.
To our knowledge, there are no alternate synthetic routes to
either form of Zr2N2S, and in addition, the reduced temperatures
associated with the metathesis routes allow for the isolation of
the low-temperature R-Zr2N2S form. Conversion to the â-form
is irreversible. It is also interesting to note that the Zr2N2S phases
are formed competitively with the Na2xZr2N2S1+x compound and
cannot be interconverted. For example, neither phase of Zr2N2S
reacts with Na2S or NaCl to form the respective Na2xZr2N2S1+x
or NaxZr2N2SClx compounds. Likewise, the Na2xZr2N2S1+x
phases do not undergo deintercalation reactions to form Zr2N2S
at any temperature. In contrast, the anhydrous NaxZr2N2SClx
compounds are intermediates in the formation of the Zr2N2S
phases. The synthesis of the Zr2N2S phases are favored at high
temperatures (>650 °C), and even though NaCl is a byproduct
of the reaction (see eq 2), the NaCl intercalation products are
not obtained under the high-temperature conditions described
herein. This observation suggests that these topochemical
reactions are quite different from that involving the direct
synthesis of NaZr2N2SCl described by Lissner and Schleid (see
eq 1). The latter was prepared at a similar reaction temperature
(800 °C) but does not afford the desired Zr2N2S phases.
Moreover, our low-temperature chemistry gives low x members
of the NaxZr2N2SClx phases but does not give the Lissner and
Schleid end member NaZr2N2SCl.15 These data show that
topochemical exchange chemistry described here is mechanisti-
cally distinct and gives rise to different products.
Experimental Section
General Information. Air- and moisture-sensitive materials were
stored and handled in a nitrogen-filled Vacuum Atmospheres drybox.
Standard Schlenk techniques were used for all air-sensitive solution
processes. Microanalysis was performed by Atlantic Microlab, Inc.
(Norcross, GA).
Synthesis. â-ZrNCl was prepared using slight modifications of
previously reported methods.24 In a typical reaction, 1.5 g of Zr metal
(99.7% Cerac, 325 mesh) was placed in a quartz boat in the center of
an argon flow-through furnace. Approximately 12 g of dry, sublimed
NH4Cl was placed in a second quartz boat ∼12 cm from the center of
the furnace in the direction of the argon inlet. The furnace was purged
with argon for 10 h and then heated at 30 °C/min to 650 °C. The
ammonium chloride was heated to ∼350 °C, while the boat containing
the Zr metal at the center of the furnace was heated to 650 °C, as
determined by external thermocouple readings. After cooling to room
temperature under argon, the center boat containing amorphous â-ZrNCl
(24) Ohashi, M.; Yamanaka, S.; Sumihara, M.; Hattori, M. J. Solid State Chem.
1988, 75, 99.
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A R T I C L E S
Stoltz et al.
was removed. Occasionally, a thin layer of white ZrO2 forms on the
surface of the product that is manually scraped away to give the pale
green product.
A2S (A ) Na, K, Rb) reagents were prepared by direct, stoichio-
metric reactions of the alkali metals with sulfur in refluxing liquid
ammonia. The resulting products were stored in the drybox. Caution:
reactions using liquid ammonia should be properly cooled and vented
to avoid dangerous pressure buildup and explosion.
software packages, respectively. Parameters in the refinements were
introduced in a stepwise fashion. With the exception of the thermal
parameters and occupancies, all parameters were refined together in
the latter stages. In the final stages, the thermal parameters or
occupancies were refined in alternate cycles, with all other parameters
fixed, until convergence was reached. For both refinements, the nitrogen
thermal parameters and occupancies were fixed at idealized values.
Because of the layered nature of the structures, preferred orientations
were included. The simulation of R-Zr2N2S was performed using Topas
software with the La2O2S fractional coordinates16 and the refined cell
constants of R-Zr2N2S. The background peak profiles were refined to
better simulate the observed diffractograms. The coordinates and
thermal parameters were fixed at ideal values and not refined.
R-Zr2N2S and â-Zr2N2S were prepared by the following methods.
In a typical reaction, 91 mg (1.0 mmol) of ZrNCl and 39 mg (0.5 mmol)
of Na2S or 55 mg (0.5 mmol) of K2S were ground and intimately mixed.
The mixtures were loaded into silica tubes in the drybox and
subsequently evacuated and sealed. During evacuation, a heat gun was
used to warm the reaction mixtures to ∼200 °C in order to drive off
any residual moisture. R-Zr2N2S was prepared by heating the samples
at 1 °C/min to 800 °C for 3 days. â-Zr2N2S was prepared by heating
1 °C/min to 900 °C for 3 days. R-Zr2N2S is brown whereas â-Zr2N2S
is dark brownish-green in color.
A2xZr2N2S1+x was prepared according to the above procedure, using
appropriate stoichiometric amounts of ZrNCl and Na2S, and heating
to 800-1000 °C for 3 days. The title compounds are yellow-brown in
color. A less crystalline product is obtained between 400 and 800 °C.
AxZr2N2SClx was also prepared by a similar procedure. In a typical
reaction, 91 mg (1.0 mmol) of ZrNCl and 39 mg (0.5 mmol) of Na2S
were heated to 350 °C for 7 days yielding a tan product. In all reactions,
excess alkali halide salt byproducts were washed away by sonicating
with 10 mL of distilled water 4 times, followed by a final acetone
wash. For the AxZr2N2SClx compounds, washing, exposure to air, or
both resulted in the formation of the hydrated AxZr2N2SClx(H2O) phases.
The hydrates are quantitatively converted back to the anhydrous form
by heating the samples under dynamic vacuum at 200 °C. XRD data
for the anhydrous material were recorded using sealed, anaerobic sample
holders.
Energy dispersive X-ray analysis (EDX) was performed with an
AMRAY 1820K scanning electron microscope with an acceleration
potential of 20 kV. Wavelength dispersive spectroscopy (WDS) was
performed with the JXA-8900 Superprobe, using ZrNCl as a Zr and
Cl standard, molybdenite as a S standard, albite as a Na standard, and
K2La2Ti3O10 as a K standard. Samples and standards were mounted
into casting resin epoxy plugs. EDX and WDS measurements were
carried out by analyzing 10 spots/sample.
Compound formulas were calculated from WDS data and mi-
croanalysis data. The nitrogen contents were assumed to be 2.0. The
reported errors were generated from propagated counting statistics
associated with the WDS analysis.
Hydration-TGA analysis was carried out in a series of three runs by
heating the sample in a small Pt boat at 200 °C, starting with initial
product masses of ∼300 mg.
High-resolution transmission electron micrographs were recorded
using a JEOL 4000 FX transmission electron microscope operated at
300 kV. A 200-mesh copper grid with a Formvar/carbon film was
introduced in the vial containing the powder of the sample to get some
of the nanoparticles attached to the film. Electron diffraction patterns
and high-resolution lattice images were obtained from several nano-
particles to get statistical information on the structure of the powder.
Characterization. X-ray diffraction (XRD) patterns were recorded
using a Bruker C2 Discover X-ray powder diffractometer with an HiStar
area detector and Cu KR radiation. Typically, five frames were collected
and merged to give 2θ scans from 4° to 93°. Unit cell indexing and
Rietveld refinements were performed using MDI Jade25 and Topas26
Acknowledgment. We are grateful to the NSF-DMR for
support of this work and the LINK foundation for the fellowship
for Z.S.G.
(25) Jade 5.0, general profile analysis software for powder diffraction data.
Materials Data, Inc., Livermore, CA, 1999.
(26) Topas v.2.0, general profile and structure analysis software for powder
diffraction data. Bruker AXS, Karlsruhe, Germany, 2000.
JA0210650
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