Crystalline Nitridophosphates by Ammonothermal Synthesis

Article abstract of DOI:10.1002/chem.201905227

Nitridophosphates are a well-studied class of compounds with high structural diversity. However, their synthesis is quite challenging, particularly due to the limited thermal stability of starting materials like P₃N₅. Typically, it requires even high-pressure techniques (e.g. multianvil) in most cases. Herein, we establish the ammonothermal method as a versatile synthetic tool to access nitridophosphates with different degrees of condensation. α-Li₁₀P₄N₁₀, β-Li₁₀P₄N₁₀, Li₁₈P₆N₁₆, Ca₂PN₃, SrP₈N₁₄, and LiPN₂ were synthesized in supercritical NH₃ at temperatures and pressures up to 1070 K and 200 MPa employing ammonobasic conditions. The products were analyzed by powder X-ray diffraction, energy dispersive X-ray spectroscopy, and FTIR spectroscopy. Moreover, we established red phosphorus as a starting material for nitridophosphate synthesis instead of commonly used and not readily available precursors, such as P₃N₅. This opens a promising preparative access to the emerging compound class of nitridophosphates.

Full text of DOI:10.1002/chem.201905227

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Title: Crystalline Nitridophosphates by Ammonothermal Synthesis
Authors: Mathias Mallmann, Sebastian Wendl, and Wolfgang Schnick
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Crystalline Nitridophosphates by Ammonothermal Synthesis
Mathias Mallmann,[a] Sebastian Wendl,[a] and Wolfgang Schnick*[a]
Abstract: Nitridophosphates are a well-studied class of compounds
with high structural diversity. However, their synthesis is quite
challenging, particularly due to the limited thermal stability of starting
materials like P3N5. Typically, it requires even high-pressure
techniques (e. g. multianvil) in most cases. Herein, we establish the
ammonothermal method as a versatile synthetic tool to access
nitridophosphates with different degrees of condensation. α-Li10P4N10,
β-Li10P4N10, Li18P6N16, Ca2PN3, SrP8N14 and LiPN2 were synthesized
in supercritical NH3 at temperatures and pressures up to 1070 K and
200 MPa employing ammonobasic conditions. The products were
analyzed by powder X-ray diffraction, energy dispersive X-ray
Accordingly, compounds that are built up from non-condensed
PN4 tetrahedra (e.g. Li7PN4)[23] possess a degree of condensation
of k = 1/4, while highly condensed frameworks feature k ≥ 1/2
(e.g. LiPN2).[24] For 1/4 < k < 1/2, partially condensed PN4
tetrahedra may form complex anions, such as adamantane-like
26]
groups (α-Li10P4N10, β-Li10P4N10),[25,
chain structures (e.g.
Ca2PN3),[27] or layers (e.g. Ho2P3N7).[28] The degree of
condensation may further be correlated with materials properties,
such as chemical inertness and rigidity of the network as well as
physical properties like ion conductivity.[18] Nitridophosphate
synthesis, however, is a challenging issue, as these compounds
spectroscopy, and Fourier-transform infrared spectroscopy. Moreover, are prone to thermal decomposition and the elimination of N2 at
we established red phosphorus as starting material for
nitridophosphate synthesis instead of commonly used and not readily
elevated temperatures (Eq. 1).
ꢀꢁꢂꢁ ꢃ
ꢆ ꢀꢁꢂꢁ ꢇ
available precursors such as P3N5. This opens
a promising
P3N5 ꢄ⎯⎯⎯ꢅ 3 PN + N2 ꢄ⎯⎯⎯⎯⎯ꢅ 3 P + 5/2 N2
(Eq. 1)
preparative access to the emerging compound class of
nitridophosphates.
Consequently, the number of nitridophosphates synthesized at
ambient pressure so far is limited (e.g. Ca2PN3, α-Li10P4N10,
β-Li10P4N10, LiPN2 or Mg2PN3).[24-27, 29] Following Le Chatelier's
principle, thermal decomposition, however, can be suppressed by
applying pressure. In this context, especially the multianvil
technique (p ≤ 25 GPa) turned out as a powerful synthetic tool.[18]
This high-pressure high-temperature method revealed numerous
nitridophosphates with different types of anionic tetrahedra-based
networks (e.g. SrP8N14, Li18P6N16 or LiNdP4N8).[30-32] Nevertheless,
utilizing high-pressure techniques implicates small sample
volumes, which limits detailed characterization of materials
properties as well as practical applications. Furthermore,
precursors like P3N5 are typically used,[18] requiring a multi-step
synthesis procedure. Thus, the ammonothermal method is a
promising and innovative alternative, as it enables the preparation
of large-volume samples, while suppressing thermal
decomposition by medium pressures (p ≤ 300 MPa). However,
there has been no systematic investigation on the
ammonothermal synthesis of nitridophosphates that covers their
broad structural diversity.
In this contribution, we exemplarily present the ammonothermal
synthesis (T ≤ 1070 K, p ≤ 200 MPa) of six nitridophosphates that
feature non-condensed PN4 tetrahedra groups, infinite PN4
tetrahedra chains, layered substructures, and highly condensed
frameworks, namely α-Li10P4N10, β-Li10P4N10, Li18P6N16, Ca2PN3,
SrP8N14, and LiPN2. This is a major extension of the structural
diversity of ammonothermally accessible ternary and multinary
compounds, which have hitherto been limited mainly to wurtzite
type derivatives and oxide nitride perovskites. In addition, red
phosphorus (Pred), which was up to now only used for synthesis
of HPN2 in ammonia,[33] is employed as a starting material for
nitridophosphates. This makes highly reactive and chlorine-
containing precursors (e.g. PCl5, (PNCl2)3) dispensable, which
Introduction
By analogy with well-known hydrothermal syntheses, the
ammonothermal method was developed by Jacobs and co-
workers and was established as an innovative synthetic approach
for amides, imides and nitrides.[1-5] The ammonothermal
technique gained fundamental interest in materials science as it
facilitates growth of high-quality GaN single crystals up to 50 mm
in diameter with growth rates of several hundred µm per day.[6-9]
Recent explorative syntheses under ammonothermal conditions
made crystalline wurtzite-type Grimm-Sommerfeld analogous
nitrides available, such as InN, II-IV-N2 compounds (II = Mg, Mn,
Zn; IV = Si, Ge) and CaGaSiN3, as well as oxide nitride
perovskites such as LnTaON2 (Ln = La, Ce, Pr, Nd, Sm, Gd).[10-15]
Applying the ammonothermal technique, even the challenging
preparation of a few nitridophosphates has been accomplished
successfully as reported for K3P6N11 and the double nitrides
Mg2PN3 and Zn2PN3.[16-18] Furthermore, various phosphorus
containing imidonitrides were synthesized in supercritical
ammonia and thus, the ammonothermal method appears as a
promising general synthetic approach for nitridophosphate
synthesis.[19-22]
Nitridophosphates are built up from PN4 tetrahedra and their
tetrahedra-based networks can be characterized by the degree of
condensation k = n(T)/n(X), which represents the atomic ratio of
tetrahedral centers (T = P) and coordinating atoms (X = N).
[a]
M. Mallmann, S. Wendl, Prof. Dr. W. Schnick
Department of Chemistry
University of Munich (LMU)
Butenandtstraße 5-13 (D), 81377 Munich (Germany)
E-mail: wolfgang.schnick@uni-muenchen.de
Supporting information for this article is given via a link at the end of
the document.
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can produce toxic and corrosive by-products, emphasizing the
innovative character of the ammonothermal approach.
Results and Discussion
Ammonothermal Synthesis
Nitridophosphates α-Li10P4N10, β-Li10P4N10, Li18P6N16, Ca2PN3,
SrP8N14, and LiPN2 were synthesized ammonothermally using
custom-built high-pressure, high-temperature autoclaves. P3N5 or
Pred were used as phosphorus source during syntheses. The other
starting materials as well as the corresponding reaction conditions
Scheme 1. Simplified condensation sequence of nitridophosphates during
ammonothermal synthesis, starting from Pred
.
method.[20,36-38]
A
possible condensation mechanism of
phosphorus containing intermediates is illustrated in Scheme 1.
When starting from Pred, the element has to be oxidized in a first
step to an oxidation state of +V forming an intermediate species
like hypothetical “P(NH2)5”, where two possible mechanisms are
conceivable. On the one hand, N2, which originates from the
decomposition of NH3, could act as oxidizing agent, on the other
hand, NH3 could directly react with Pred under elimination of H2.
“P(NH2)5” could immediately form “NP(NH2)2” by elimination of
ammonia, which can react to above mentioned reactive P/N
compounds such as (PN(NH2)2)3.[37] However, for a precise
statement on possible reaction mechanisms or phosphorus
containing intermediates, in situ measurements like Raman or
NMR spectroscopy could be helpful.
(maximum reaction temperature Tmax, maximum pressure pmax
,
reaction time at maximum temperature t) are summarized in
Table 1. Ammonobasic mineralizers, such as alkali metals, alkali
metal nitrides, alkali metal azides or alkaline earth metal azides,
which react in situ to the corresponding metal amides, were added
to increase the solubility of the starting materials by forming
soluble intermediate species. Since such intermediates are
preferentially formed at lower temperatures, an additional
temperature step at 670 K (holding time: 16 h) was conducted for
[5]
all reactions before heating to Tmax
.
The addition of these
mineralizers can also dissolve compounds such as Pred, which are
actually insoluble in NH3 even at temperatures above the critical
point.[34] Therefore, in the case of the synthesized lithium
nitridophosphates (α-Li10P4N10, β-Li10P4N10, Li18P6N16 and LiPN2),
Li3N or Li, respectively, was added in access to increase the
solubility of Pred. While NaN3 was added for the synthesis of
Ca2PN3, to increase both, the solubility of Ca and Pred, no
additional mineralizer was added for the synthesis of SrP8N14.
Instead, Sr(N3)2 acts as ammonobasic mineralizer itself by
forming Sr(NH2)2, as the heavier alkaline earth metals have been
discussed as ammonobasic mineralizers as well.[35] Possible
intermediates are mixed metal amides such as NaCa(NH2)3 and
reactive P/N compounds, e.g. hexaaminocyclotriphosphazene
(PN(NH2)2)3, the corresponding ammoniate (PN(NH2)2)3∙0.5NH3
Subsequent heating from 670 K to the maximum temperature
Tmax (see Table 1) leads to decomposition of the discussed
intermediates
and
formation
of
the
corresponding
nitridophosphates under elimination of NH3 (see Scheme 1). After
reaction, residual mineralizers and intermediate species were
removed by washing of the products with dry ethanol (α-Li10P4N10,
β-Li10P4N10, Li18P6N16 and Ca2PN3) or 1 M HCl (SrP8N14 and
LiPN2). SEM images of octahedrally shaped β-Li10P4N10 and
needle shaped SrP8N14 crystallites are illustrated in Figure 1,
while the other compounds were obtained as microcrystalline
powders.
or imidonitrides in analogy to Na10[P4(NH)6N4](NH2)6(NH3)0.5
,
which have already been synthesized using the ammonothermal
Table 1. Starting materials, mineralizers and reaction conditions of the
ammonothermal synthesis of α-Li10P4N10, β-Li10P4N10, Li18P6N16, Ca2PN3,
SrP8N14, and LiPN2.
Compounds
Starting
materials
Mineralizer
Tmax
[K]
pmax
[MPa]
t
[h]
Figure 1. SEM images of octahedrally shaped crystals of β-Li10P4N10 (a) and
crystalline needles of SrP8N14 (b).
α-Li10P4N10
β-Li10P4N10
Li18P6N16
Ca2PN3
Li3N + Pred
Li3N + Pred
Li3N + P3N5
CaH2 + Pred
Sr(N3)2 + P3N5
Li + Pred
Li3N
Li3N
Li3N
NaN3
Sr(N3)2
Li
920
100
135
165
200
170
135
72
72
50
96
96
96
As mentioned above, both P3N5 and Pred were used as starting
materials. While α-Li10P4N10, β-Li10P4N10, Ca2PN3 and LiPN2 were
synthesized starting from Pred, Li18P6N16 and SrP8N14 could only
be obtained starting from P3N5. A possible explanation could be
the higher reactivity of P3N5 compared to Pred, which is needed for
the synthesis of Li18P6N16 and SrP8N14.[24-27, 30, 31] Probably, higher
synthesis temperatures and pressures would lead to successful
synthesis of these two compounds starting from Pred as well.
The introduction of Pred as starting material for nitridophosphate
synthesis as well as the use of simple starting materials like pure
elements, lower reaction temperatures, pressures and larger
sample volumes compared to other synthesis methods, indicate
1070
970
870
SrP8N14
1070
1070
LiPN2
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the high potential of the ammonothermal approach as an
Crystal structures
alternative synthetic tool for
nitridophosphates.
Crystallographic investigation
The purified products were analyzed by powder X-ray diffraction
(PXRD). Rietveld refinements of α-Li10P4N10, β-Li10P4N10,
Li18P6N16, Ca2PN3, and LiPN2 were conducted starting from
a
systematic access to
α-Li10P4N10, β-Li10P4N10 and Li18P6N16 are built up from corner
sharing PN4 tetrahedra. While α-Li10P4N10 and β-Li10P4N10 contain
adamantane-like T2 supertetrahedra ([P4N10]10−) with a degree of
condensation of k = 2/5, Li18P6N16 is built up from [P6N16]18− anions
corresponding to a degree of condensation of k = 3/8 (see
Figure 3). These [P6N16]18 units consist of four PN4 tetrahedra
−
atomic coordinates and Wyckoff positions known from
forming a vierer-ring, which is connected to two further PN4
tetrahedra forming two dreier-rings.[39, 40] In contrast to these non-
condensed tetrahedra groups, the anionic P/N-structure of
Ca2PN3 is composed of zweier single chains running along [100]
made up of corner sharing PN4 tetrahedra (see Figure 3).[39, 40]
31]
literature.[24-27,
An exemplary Rietveld plot of Ca2PN3 is
illustrated in Figure 2. The Rietveld plots of α-Li10P4N10, β-
Li10P4N10, Li18P6N16, and LiPN2 can be found in the Supporting
Information (Figures S1 and S6). The crystallographic data as
well as atomic coordinates are summarized in Tables S1-S4, S6-
7 and S10-11 in the Supporting Information. In the case of
Li18P6N16 additional reflections could be observed, which can be
attributed to α-Li10P4N10 and LiPN2. Due to the fact that Li18P6N16
is so far only reported using high-pressure conditions (1270 K,
5.5 GPa), a possible explanation for these side phases could be
that higher reaction pressures would be necessary to achieve
phase purity.[31] In analogy, higher pressures as well as
temperatures would be necessary for the synthesis of SrP8N14, as
the synthesis at 1070 K and 170 MPa resulted in broad reflections
in the measured PXRD pattern (see Figure S5 in the Supporting
Information), suggesting a nanocrystalline sample morphology.
However, further increase of temperature or pressure is
challenging and not possible with the current high-pressure
equipment. Therefore, the measured PXRD was only compared
with a simulated pattern from literature data (see Figure S5)[30]
and may most likely be characterized as SrP8N14.
The chains exhibit a stretching factor of ꢈ = 1.0 and a degree of
ꢉ
condensation of k = 1/3. The crystal structure of SrP8N14 is
composed of PN4 tetrahedra forming a layered structure (see
EDX measurements of all compounds are summarized in
Tables S5, S8-9 and S12 in the Supporting Information.
Deviations from the theoretical values can be explained by
surface hydrolysis during sample preparation, washing treatment
or by crystalline and amorphous side phases. The absence of any
NHx functionality in the Li containing nitridophosphates was
confirmed by FTIR spectroscopy (Figures S2-4 and S7 in the
Supporting Information).
Figure 2. Rietveld refinement of PXRD pattern (λ = 1.5406 Å) of Ca2PN3 with
experimental data (black line), calculated data (red line), difference profile (blue
line) and reflection positions (black bars). Start values for Rietveld refinement
were taken from literature.[27] Unknown reflections between 6 and 10° only occur
after washing treatment and were therefore not taken into account during the
refinement.
Figure 3. Crystal structures and/or constituting PN4 tetrahedra units (green)
occurring in α-Li10P4N10, β-Li10P4N10, Li18P6N16, Ca2PN3, SrP8N14 and LiPN2.
Ca2+ and Sr2+ cations as well as LiN4 tetrahedra are depicted in red.
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Figure 3) and can be described as
nitridophosphate with a degree of condensation of k = 4/7. This is
the highest degree of condensation observed in
a
highly condensed
dissolution/crystallization processes, however, might be
necessary. Therefore, in situ measurements such as X-ray, NMR
or Raman techniques may provide important insights into these
processes.[41, 42]
nitridophosphates so far. LiPN2 is composed of all-side vertex-
sharing PN4 tetrahedra, which are connected via common corners
forming a 3D anionic network with a degree of condensation of
k = 1/2 (see Figure 3) isoelectronic and homeotypic to β-
cristobalite (SiO2). Detailed crystal structure descriptions of all six
compounds are given in literature.[24-27, 30, 31] As shown in Figure 3,
the above described nitridophosphates can be divided into
different groups regarding their anionic P/N-substructures (non-
condensed tetrahedra groups, tetrahedra chains, tetrahedra
layers and tetrahedra networks). This is a major extension of the
structural diversity of ammonothermally accessible ternary and
multinary nitrides, which have hitherto been limited mainly to
wurtzite type derivatives and oxide nitride perovskites.
Furthermore, the degree of condensation of ammonothermally
accessible nitridophosphates is widely extended and ranges now
from k = 1/3 to 4/7 (see Figure 4), covering almost the entirely
accessible range. These results show the great potential of the
ammonothermal method and can pave the way for synthesis of
hitherto unknown nitridophosphates using the ammonothermal
approach.
Experimental Section
Loading of the autoclaves with solid starting materials (see below) were
conducted under exclusion of oxygen and moisture in an argon-filled
glovebox (Unilab, MBraun, Garching, O2 < 1 ppm, H2O < 1 ppm). The
condensation of ammonia into the autoclaves was performed using a
vacuum line (≤ 0.1 Pa) with argon and ammonia (both: Air Liquide,
99.999%) supply. The gases were further purified by gas cartridges (Micro
Torr FT400-902 (for Ar) and MC400-702FV (for NH3), SAES Pure Gas Inc.,
San Luis Obispo, CA, USA), providing a purity level of < 1 ppbV H2O, O2
and CO2. The amount of condensed ammonia was detected using a mass
flow meter (D-6320-DR, Bronkhorst, Ruurlo, Netherlands).
Synthesis of P3N5
P3N5 was synthesized by reaction of P4S10 (Sigma Aldrich, 99%) in a
continuous flow of NH3 (Air Liquid, 99.999%).[43] After saturation with NH3
(4 h), the silica tube was heated with a rate of 5 K/min to 1125 K and held
for 4 h. After cooling to room temperature (5 K/min), the orange product
was washed with ethanol, water and acetone and dried under vacuum.
Powder X-ray diffraction was used to confirm phase purity.
Synthesis of Sr(N3)2
Based on the work of Suhrmann and Karau,[44, 45] Sr(N3)2 was synthesized
by reaction of in situ formed diluted HN3 (by passing aqueous NaN3 (Acros
Organics, 99%) through a cation exchanger (Amberlyst 15)) with SrCO3
(Sigma Aldrich, 99.995%). The HN3-solution was dropped slowly into an
aqueous suspension of SrCO3 and stirred until the liquid phase turned
completely clear. Residual SrCO3 was removed by filtration and the clear
filtrate was evaporated under reduced pressure (50 mbar, 40 °C). After
evaporation, the product was recrystallized from acetone and dried in
vacuo. PXRD and FTIR measurements were used to confirm phase purity.
Caution: Since HN3 solutions are potentially explosive and the vapor is
highly poisonous, special care issues are necessary.
Figure 4. Ammonothermally synthesized nitridophosphates, arranged in order
of their degree of condensation.
Ammonothermal synthesis of α- and β-Li10P4N10
For ammonothermal synthesis of α- and β-Li10P4N10
, 3 mmol Li3N
Conclusions
(104.5 mg, Sigma-Aldrich, 99.99%) and 3 mmol red P (92.9 mg, Merck,
99%) were ground and transferred to Ta-liners, for protection of the
samples against autoclave impurities. The liners were then placed in high-
temperature autoclaves (Haynes® 282®, max. 1100 K, 170 MPa, 10 mL)
and sealed with a lid via flange joints using a sealing gasket (silver coated
Inconel® 718 ring, GFD seals). The autoclave body and the upper part,
consisting of a hand valve (SITEC) with integrated bursting disc (SITEC)
and pressure transmitter (HBM P2VA1/5000 bar), are connected by an
Inconel® 718 high-pressure tube.[12] The closed autoclave was evacuated
and cooled to 198 K using an ethanol/liquid nitrogen mixture. Afterwards,
NH3 (≈ 4mL) was directly condensed into the autoclaves via a pressure
regulating valve. For both reactions, the autoclaves were primarily heated
to 670 K within 2 h, kept at this temperature for 16 h and subsequently
heated to 920 K (α-Li10P4N10) or 1070 K (β-Li10P4N10) within 3 h and held
at this temperature for 72 h, reaching maximum pressures of 100 MPa (α-
Li10P4N10) and 135 MPa (β-Li10P4N10), respectively. After cooling and
removal of NH3, the colorless products were separated under argon,
washed with EtOH and dried in vacuo.
Recently, we reported on synthesis and crystal growth of wurtzite
type Mg2PN3 and Zn2PN3 under ammonothermal conditions,
raising the question of a systematic access to nitridophosphates
using supercritical NH3.[17] In this contribution we report on the
ammonothermal syntheses of α-Li10P4N10, β-Li10P4N10, Li18P6N16,
Ca2PN3, SrP8N14 and LiPN2. Those compounds feature a degree
of condensation in the range 1/3 ≤ k ≤ 4/7 (Figure 4),
corresponding to different types of anionic tetrahedra-based
substructures such as non-condensed tetrahedra groups, chains,
layers and 3D-networks. In contrast to established high-pressure
techniques, the ammonothermal method requires only moderate
pressures and temperatures, exemplifying the high potential of
this preparative approach. Furthermore, readily available red
phosphorus was introduced as starting material in
nitridophosphate syntheses, avoiding the usage of halide or sulfur
containing precursors (e.g. (PNCl2)3, P4S10). Using simple starting
materials and yielding large sample volumes, the ammonothermal
method enables more detailed characterization of material
properties of nitridophosphates. Supporting fundamental
research on the reaction mechanisms, intermediate species and
Ammonothermal synthesis of Li18P6N16
Li18P6N16 was synthesized ammonothermally starting from 3.75 mmol Li3N
(130.6 mg, Sigma-Aldrich, 99.99%), 1.5 mmol P3N5 (244.4 mg) and
NH3 (≈ 5mL) in
a Ta-liner. Following the autoclave preparation (as
described for Li10P4N10), the vessel was heated to 670 K within 2 h, kept
at this temperature for 16 h, heated to 970 K within 3 h and held at this
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temperature for 50 h reaching a maximum pressure of 165 MPa. After
cooling and removing of NH3, the colorless product was separated under
argon, washed with EtOH and dried in vacuo.
Netzwerken”. Furthermore, we want to thank the group of Prof. Dr.
E. Schlücker for fabrication of the autoclaves (FAU Erlangen-
Nürnberg), Marion Sokoll for IR measurements and Christian
Minke for EDX measurements (both at Department of Chemistry,
LMU Munich).
Ammonothermal synthesis of Ca2PN3
Ca2PN3 was synthesized under ammonothermal conditions using an
Inconel® 718 autoclave (max. 870 K, 300 MPa, 10 mL). The setup and
preparation of the autoclave is analogous to the autoclaves described
above. 3 mmol CaH2 (126.3 mg, Sigma-Aldrich, 99.99%), 1.5 mmol red P
(46.5 mg, Merck, 99%), 7.5 mmol NaN3 (487.5 mg, Sigma-Aldrich, 99.5%)
as mineralizer and NH3 (≈ 6.5 mL) were used as starting materials in a
Ta-liner. After autoclave preparation (as described above), the reaction
mixture was heated to 670 K within 2 h, held for 16 h, heated to 870 K
within 2 h and kept at this temperature for 96 h, resulting in a maximum
pressure of 200 MPa. The beige product was separated after cooling and
removing of ammonia under argon, washed with EtOH and dried in vacuo.
Ammonothermal synthesis of SrP8N14
0.375 mmol Sr(N3)2 (64.4 mg), 1 mmol P3N5 (163 mg) were ground,
transferred to a Ta-liner, which was placed in a Haynes® 282® autoclave.
After preparation of the autoclave as described above, NH3 (≈ 5 mL) was
condensed into the autoclave. Subsequently, the autoclave was heated to
670 K within 2 h, held at this temperature for 16 h, heated to 1070 K within
3 h, and kept at this temperature for 96 h, reaching a maximum pressure
of 170 MPa. After cooling and removing of NH3, the colorless product was
isolated in air, washed with 1 M HCl and dried at 370 K.
Keywords: ammonothermal synthesis • supercritical fluids •
nitridophosphates • nitrides • phosphorus
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Ammonothermal synthesis of LiPN2
For synthesis of LiPN2 in supercritical ammonia, 10 mmol Li (69.4 mg, Alfa
Aesar, 99%) and 7.5 mmol red phosphorus (232.3 mg, Merck, 99%) were
transferred in a Ta-liner and placed in a Haynes® 282® autoclave. After
preparation of the autoclave as described above, approximately 4 mL NH3
were added. The reaction mixture was heated to 670 K within 2 h, held for
16 h, heated to 1070 K within 3 h and kept at this temperature for 96 h,
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Johrendt, J. Minár, W. Schnick, Chem. Eur. J.,
DOI:10.1002/chem.201903897.
resulting in
a maximum pressures of 135 MPa. After cooling and
elimination of NH3, the isolated colorless product was washed with 1 M HCl
and dried at 370 K.
Powder X-ray diffraction
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The purified products were filled and sealed in glass capillaries (0.3-
0.5 mm diameter, 0.01 mm wall thickness, Hilgenberg GmbH). A Stoe
STADI P diffractometer with Cu-Kα1 radiation (λ = 1.5406 Å), Ge(111)
monochromator and Mythen 1K detector in Debye-Scherrer geometry was
used for data collection. TOPAS was used for Rietveld refinement.[46]
Scanning electron microscopy
A Dualbeam Helios Nanolab G3 UC (FEI) scanning electron microscope,
equipped with an EDX detector (X-Max 80 SDD, Oxford instruments) was
used for EDX measurements. For this purpose, the samples were placed
on adhesive carbon pads and coated with a conductive carbon film using
a high-vacuum sputter coater (BAL-TEC MED 020, Bal Tec A).
Fourier Transform Infrared (FTIR) spectroscopy
A FTIR-IFS 66 v/S spectrometer (Bruker) was used for recording of IR
spectra of air-sensitive samples. The samples were mixed with KBr (Acros
Organics, 99%) under argon and pressed into a pellet. The spectra were
measured in the range of 400 – 4000 cm−1 and evaluated by OPUS.[47]
FTIR spectrum of LiPN2 was recorded on a Perkin Elmer BX II FTIR
spectrometer equipped with a DuraSampler Diamond ATR (attenuated
total reflection) unit under exposure to air.
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Acknowledgements
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The authors gratefully acknowledge financial support by the
Deutsche Forschungsgemeinschaft (DFG) within the research
group “Chemistry and Technology of the Ammonothermal
Synthesis of Nitrides” (FOR 1600), project SCHN377/16-2 and
the project SCHN377/18-1 “Neue Wege zu nitridischen Phosphat-
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This article is protected by copyright. All rights reserved.

10.1002/chem.201905227
Chemistry - A European Journal
FULL PAPER
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The terms dreier rings, vierer rings and zweier single chain were
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coined by Liebau and are derived from the German words “dreier,
vierer and zweier”; a dreier ring comprises three tetrahedra
centers, a vierer ring four tetrahedra centers, a zweier chain can
be described as two polyhedra within one repeating unit of the
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This article is protected by copyright. All rights reserved.

10.1002/chem.201905227
Chemistry - A European Journal
FULL PAPER
FULL PAPER
Nitridophosphates:
Various
M. Mallmann, S. Wendl, W. Schnick*
nitridophosphates with different types
of anionic PN4 tetrahedra-based
substructures were synthesized in
supercritical ammonia using custom-
built high-temperature, high-pressure
autoclaves. Red phosphorus was
introduced as starting material,
emphasizing the innovative character
of the ammonothermal approach.
Page No. – Page No.
Crystalline Nitridophosphates by
Ammonothermal Synthesis
This article is protected by copyright. All rights reserved.