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NH3 (Air Liquide, 5.0), followed by vacuum heat treatment of the
mixture at 6008C (2 h) to obtain P3N5 or at 4508C (2 h) to obtain
amorphous HPN2. In the case of P3N5, the mixture was additionally
heated at 9508C (2 h). This step was essential for complete con-
densation of the product. Detailed information is available in the
literature.[38–41] The phase purities of the respective products were
confirmed by powder X-ray diffraction analysis and FTIR spectros-
copy.
Powder X-ray diffraction analysis
Powder X-ray diffraction data for both compounds were collected
on a STOE StadiP powder diffractometer (CuKa1 radiation, Ge(111)
monochromator, MYTHEN 1 K Si strip detector) in parafocusing
Debye–Scherer geometry. Rietveld refinements were carried out
using the TOPAS Academic 4.1 package.[44] The preferred orienta-
tion of the crystallites was described using a fourth-order spherical
harmonic. Peak shapes were modeled by the fundamental parame-
ters approach (direct convolution of source emission profiles, axial
instrument contributions, and crystallite size and microstrain ef-
fects).
Synthesis
Solid-state NMR spectroscopy
MH4P6N12 (M=Mg, Ca) were synthesized from stoichiometric
amounts of the respective alkaline-earth metal nitrides and amor-
phous HPN2 and P3N5 using a Walker-type multi-anvil apparatus.[6]
A catalytic amount of NH4Cl was used as a mineralizer. Because of
the high air-sensitivity of Mg3N2 and Ca3N2, all manipulations were
carried out under exclusion of oxygen and moisture in an argon-
filled glove box (Unilab, MBraun, Garching, O2 <1 ppm, H2O<
0.1 ppm). The respective starting mixture was thoroughly ground
and tightly packed into a cylindrical capsule of hexagonal boron
nitride (Henze, Kempten). The filled capsule was sealed with a hex-
agonal boron nitride cap and placed in the center of a Cr2O3-
doped MgO octahedron (edge length 18 mm, Ceramic Substrates
& Components Ltd., Isle of Wight, U.K.). The MgO octahedron was
equipped with a ZrO2 tube (Cesima Ceramics, Wust-Fischbach, Ger-
many), which served as a thermal insulator. Furthermore, two
graphite tubes (one long tube and one short tube) were used as
electrical resistance furnaces. In order to ensure that the short
graphite tube was positioned in the center of the long tube, two
MgO spacers (one on each side) were used. Finally, a Mo plate was
placed on each side of the ZrO2 tube in order to achieve electrical
contact between the graphite tubes and the anvils of the multi-
anvil press. The MgO octahedron was then placed in the center of
an assembly of eight truncated tungsten carbide cubes (truncation
edge lengths 11 mm, Hawedia, Marklkofen, Germany), which were
separated with pyrophyllite gaskets. Detailed information on the
construction of the described multi-anvil assembly can be found in
the literature.[19] The sample was compressed to 8 GPa at room
temperature. It was then heated to 10008C over a period of
60 min, and the temperature was held at this level for 120 min.
Subsequently, the sample was cooled to room temperature over
a period of 60 min. After slow decompression (10 h), both products
were recovered as colorless crystalline solids, which were not sensi-
tive to air or moisture. NH4Cl was removed from the products by
washing with water and ethanol. However, for the solid-state NMR
experiments, the washing step was omitted.
For all measurements, the 1H resonance of 1% Si(CH3)4 in CDCl3
served as an external secondary reference, using the X value for
31P relative to 85% H3PO4 as reported by the IUPAC.[45] Solid-state
NMR spectra were measured on a Bruker Avance III spectrometer
1
with an 11.7 T magnet, operating at a H frequency of 500.25 MHz,
equipped with commercial 1.3 mm and 2.5 mm double-resonance
MAS probes. 31P–31P 2D double-quantum (DQ) single-quantum (SQ)
correlation MAS NMR spectra were obtained at a sample spinning
frequency of 20 kHz with
a transient-adapted POSTC7 se-
quence.[46,47] The conversion period was set at 1.2 ms. Rotor-
synchronized data sampling of the indirect dimension accumulated
16 transients per FID. Proton decoupling was implemented by CW
decoupling with a nutation frequency of 110 kHz. Repetition delays
were set at 60 s and 42 s for MgH4P6N12 and CaH4P6N12, respective-
ly. 31P{1H} heteronuclear correlation MAS NMR spectra were ob-
tained through a 2D correlation experiment based on the PRES-
TO II pulse sequence[48] as described in the literature.[49] Here,
proton decoupling was implemented by TPPM decoupling[50] with
1
a nutation frequency of 115 kHz. The H nutation frequency for the
R1852 recoupling sequence was 90 kHz for the R-elements, which
consisted of simple p-pulses. All other hard pulses applied in both
channels were implemented with a nutation frequency of 100 kHz.
Both experiments were performed at a sample spinning frequency
of 20 kHz with a repetition delay of 1.5 s. The numbers of accumu-
lated transients per FID were 256 and 512 for MgH4P6N12 and
CaH4P6N12, respectively.
FTIR spectroscopy
The FTIR spectra of MH4P6N12 (M=Mg, Ca) were measured using
the KBr pellet method on a Spectrum BX II spectrometer (Perkin-
Elmer, Waltham MA, USA).
Scanning electron microscopy and energy-dispersive X-ray
spectroscopy
SEM imaging and EDX analysis were performed using a JEOL JSM-
6500F field-emission scanning electron microscope (SEM),
equipped with a Si/Li EDX detector 7418 (Oxford Instruments). In
order to impart electrical conductivity to the sample surfaces, they
were coated with carbon using an electron beam evaporator (BAL-
TEC MED 020, Bal Tec AG).
Single-crystal X-ray diffraction analysis
Single-crystal X-ray diffraction data were collected on a Nonius
Kappa CCD diffractometer (MoKa radiation, graphite monochroma-
tor, Bruker, Karlsruhe). A semi-empirical absorption correction was
applied using the program XPREP.[42] The crystal structures were
solved by direct methods using SHELXS,[42] and refined by full-
matrix least-squares methods using SHELXL.[43] Further details of
the crystal structure determinations can be obtained from the Fa-
chinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldsha-
fen, Germany (Fax: +49–7247–808–666; e-mail: crysdata@fiz-karls-
ruhe.de) on quoting the depository numbers CSD-427952
(MgH4P6N12) and CSD-427953 (CaH4P6N12).
Acknowledgements
We thank Dr. Peter Mayer for collecting single-crystal X-ray
data and Christian Minke for EDX measurements. Furthermore,
we gratefully acknowledge financial support from the Fonds
&
&
Chem. Eur. J. 2015, 21, 1 – 8
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