Synthesis and characterization of a new ternary imide-Li 2Ca(NH)2

Article abstract of DOI:10.1021/ic060769y

The ternary imide Li₂Ca(NH)₂ was successfully synthesized by dehydrogenating a mixture of LiNH₂ and CaH₂ at a molar ratio of 2:1 in a stream of purified argon at 300°C. A powder X-ray diffraction measurement revealed that Li₂Ca(NH)₂ was of the trigonal anti-La₂O₃ structure (space group P3m1) with lattice constants of a = 3.5664(3)A and c = 5.9540(8) A. Ca occupied the 1b site (0, 0, 1/2), Li occupied the 2d site (1/3, 2/3, 0.8841(22)), and N occupied the 2d site (1/3, 2/3, 0.2565(15)). Nuclear magnetic resonance and X-ray absorption fine structure analyses demonstrated that each Li ion was coordinated with four imide ions and each Ca ion was coordinated with six imide ions.

Full text of DOI:10.1021/ic060769y

Inorg. Chem. 2007, 46, 517−521
Synthesis and Characterization of a New Ternary ImidesLi2Ca(NH)2
Guotao Wu,† Zhitao Xiong,† Tao Liu,† Yongfeng Liu,† Jianjiang Hu,† Ping Chen,*,†,‡
Yuanping Feng,† and Andrew T. S. Wee†
Department of Physics and Department of Chemistry, National UniVersity of Singapore, 10 Kent
Ridge Crescent, Singapore 117542
Received May 6, 2006
The ternary imide Li2Ca(NH)2 was successfully synthesized by dehydrogenating a mixture of LiNH2 and CaH2 at
a molar ratio of 2:1 in a stream of purified argon at 300 C. A powder X-ray diffraction measurement revealed that
Li2Ca(NH)2 was of the trigonal anti-La2O3 structure (space group P3m1) with lattice constants of a 3.5664(3)Å
and c 5.9540(8) Å. Ca occupied the 1b site (0, 0, 1/2), Li occupied the 2d site (1/3, 2/3, 0.8841(22)), and N
°
h
)
)
occupied the 2d site (1/3, 2/3, 0.2565(15)). Nuclear magnetic resonance and X-ray absorption fine structure analyses
demonstrated that each Li ion was coordinated with four imide ions and each Ca ion was coordinated with six
imide ions.
Introduction
diverse crystal structures and interesting physical properties.1-4
Recent investigations on synthesis methods, structures, and
properties of ternary and higher nitrides were summarized
by Gregory and Niewa et al., respectively.2-4 However,
research in ternary and higher inorganic imides is rare. It
was only in recent years that ternary imides containing both
an alkaline metal and an alkaline earth metal were synthe-
sized and investigated.8,9 As an example, Li2Mg(NH)2 can
be synthesized by reacting LiNH2 with MgH2 at elevated
temperatures.8,9 Synchrotron X-ray diffraction (XRD) and
neutron diffraction measurements reveal that Li2Mg(NH)2
possesses three different crystal structures depending on the
synthesis temperature.10 A similar synthetic route was applied
to the Li-Ca-N-H system, but little attention was paid to
its structural identification and chemical properties.8 If the
structure of Li2Ca(NH)2 is determined, the physical and
chemical properties of this new ternary imide can be well
predicted and understood by theoretical means.11,12
Binary nitrides, amides, and imides of various metals and
nonmetals have been well studied by Juza and co-workers
since the 1920s.1 These compounds have many interesting
properties that make them attractive for technological ap-
plications. Binary nitrides (e.g., GaN, InN, AlN, BN, and
TiN) have a number of applications in the microelectronic,
optoelectronic, abrasion, and corrosion protection industries.2-4
Binary amides, such as lithium and sodium amides, are
widely used as deprotonating agents in organic synthesis.5
Recently, lithium nitride and lithium imide were found to
exhibit strong affinity to H2, which enable them to be
potential candidates for hydrogen storage.6,7 Rapid growth
in nitride chemistry occurred in the last decades owing to
the improved classification of nitride crystal chemistry and
the development of new synthetic techniques, leading to the
discovery of a wide range of new ternary compounds with
Systematic studies of amides and imides were performed
by Juza et al., Jacobs and Schmidt, and Rouxel et al.
previously.13-17 Normally, imides (e.g., Li2NH, CaNH, and
BaNH) are prepared by heating amides under vacuum.13-15
* To whom correspondence should be addressed. Fax: +65-67776126.
Tel.: +65-65165100. E-mail: phychenp@nus.edu.sg.
† Department of Physics.
‡ Department of Chemistry.
(1) Brese, N. E.; O’Keeffe, M. Struct. Bonding (Berlin) 1992, 79, 307-
378.
(2) Gregory, D. H. J. Chem. Soc., Dalton Trans. 1999, 3, 259-270.
(3) Niewa, R.; Jacobs, H. Chem. ReV. 1996, 96, 2053.
(4) Niewa, R.; DiSalvo, F. J. Chem. Mater. 1998, 10, 2733-2752.
(5) Erden, I. In Handbook of Reagents for Organic Synthesis: Acidic and
Basic Reagents; Reich, H. J., Rigby, J. H., Eds.; Wiley: Chichester,
U.K., 1999; p 204.
(6) Chen, P.; Xiong, Z. T.; Luo, J. Z.; Lin, J. Y.; Tan, K. L. Nature 2002,
420, 302-304.
(7) Chen, P.; Xiong, Z. T.; Luo, J. Z.; Lin, J. Y.; Tan, K. L. J. Phys.
Chem. B 2003, 107, 10967-10970.
(8) Xiong, Z. T.; Wu, G. T.; Hu, J. J.; Chen, P. AdV. Mater. 2004, 16,
1522-1525.
(9) Luo, W. F. J. Alloys Compd. 2004, 381, 284-287.
(10) Zhao, J. C. Phase formation and reaction pathway of Mg(NH2)2+2LiH
mixture for reversible hydrogen storage. Presented at the International
Partnership for the Hydrogen Economy, Jucca, Italy, June 19-22,
(11) Z20h0an5g. , C. J.; Dyer, M.; Alavi, A. J. Phys. Chem. B 2005, 109, 22089-
22091.
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(13) Juza, R.; Opp, K. Z. Anorg. Allg. Chem. 1951, 266, 313-324.
10.1021/ic060769y CCC: $37.00
Published on Web 12/22/2006
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 2, 2007 517

Wu et al.
However, because of the lack of ternary amides, few ternary
imides have been synthesized and reported in the literature.13-17
The novel synthetic route involving reaction of a binary
amide with a hydride under appropriate conditions has proven
to be a viable method for making a variety of novel ternary
imides.8,9 In this paper, the synthetic approach mentioned
above was applied to the preparation of the ternary imide of
Li2Ca(NH)2. The synthesis was carried out at relatively low
temperatures because of the poor thermal stability of Li2-
Ca(NH)2. As a consequence, only microcrystalline imides
may be obtained. Determination of the structure merely by
powder XRD alone may not be sufficiently accurate.
Therefore, combined characterizations by means of XRD,
vibrational spectra, X-ray absorption spectra, and nuclear
magnetic resonance (NMR) were performed to acquire
structural information on both the bulk and the atomic scales
with the intention of obtaining a better understanding of the
composition, structure, and properties of Li2Ca(NH)2.
Figure 1. TPD (a) and volumetric release (b) measurements on the post-
milled 2LiNH2-CaH2 samples.
and then coated with solid wax in the glovebox to avoid air
contamination during XAFS measurements. The sample thickness
was adjusted to achieve the X-ray absorption edge jump of
approximately one for each sample.
Experimental Section
Preparation of Li2Ca(NH)2. LiNH2 and CaH2 were synthesized
by reacting metallic Li (99.9%, Aldrich Chemicals) with ammonia
(99.98%, BOC Gases) and by reacting metallic Ca (99.9%, Aldrich
Chemicals) with hydrogen (99.99%, National Oxygen), respectively.
LiNH2 and CaH2 at a molar ratio of 2:1 were thoroughly mixed by
a SPEX 8000M mixer/mill. Li2Ca(NH)2 was synthesized by heating
the post-milled mixture in a flow of purified argon from room
temperature to 300 °C at a ramping rate of 1 °C/min. Gaseous
products were analyzed by an on-line HPR20 mass spectrometer
(MS) in tandem with a gas chromatograph (GC). For the quantitative
analysis of ammonia, gaseous products were slowly introduced to
distilled water. A Metrohm 781 pH/ion meter equipped with an
NH3-selective electrode was used to detect the concentration of
ammonia in distilled water. A quantitative measurement of hydrogen
desorption was conducted in a commercial pressure composition
isotherm (PCI) unit from Advanced Materials Corporation.
Structure Determination. XRD data were collected using a
BRUKER D8 Advance X-ray diffractometer with Cu KR radiation
at a power of 40 kV × 40 mA. Stepwise scans were performed in
the 2θ range of 10-90° at steps of 0.05°. The obtained data were
indexed using the TREOR or DICVOL program. Lattice parameters
were refined by a least-squares refinement method using the
CELREF program. Rietveld refinement structural analyses were
performed using the Rietica program.18
Analysis of the XAFS data followed the standard procedures
using the WINXAS code.20 After normalization, transformation
from energy space to momentum (k) space, and extraction of the
background absorption were carried out, the ø(k) function was
extracted in the range of 2.2-9.2 Å-1 and weighted by k3. Fourier
transform of k3ø(k) into R space was performed using the Bessel
function. The data fit of the first coordination shell was performed
using CaNH as the reference.
6Li magic-angle spinning (MAS) NMR measurements were
carried out at room temperature in a Bruker Advance 400
spectrometer with a 4 mm broadband cross-polarization (CP)/MAS
probe operating at a 6Li frequency of 58.8 MHz. Each sample was
packed in a zirconia rotor with a Kel-F cap and rotated at a rotor-
spinning rate of 10 kHz. Chemical shifts were reported with respect
to a 1 M LiCl aqueous solution. The peaks in the NMR spectra
were fitted to obtain the values of the full width at half-maximum
and chemical shift.
As the starting materials and products were sensitive to air, all
sample loadings were performed in an MBRAUN glovebox. The
glovebox was filled with purified argon gas. Water and oxygen
concentrations were below 10 ppm.
Results and Discussion
N-H vibration was recorded using a Perkin-Elmer Fourier
transform infrared (FTIR) spectroscopy 2000 spectrometer equipped
with a diffuse reflectance infrared Fourier transform (DRIFT) cell.
Temperature programmed desorption (TPD) of the post-
milled 2LiNH2 + CaH2 sample was performed by using
purified argon as a carrier gas. The temperature was raised
from room temperature to 300 °C at a ramping rate of
1 °C/min. The sample (100.2 mg) was loaded, and plots of
temperature versus the mass spectrometer signals of the outlet
gas were obtained (Figure 1a). Ammonia was undetectable
by MS during the whole testing period, indicating that the
amount of coproduced ammonia was negligible. Notably, the
The scan range was 400-4000 cm-1, and the resolution was 4 cm-1
.
Transmission X-ray absorption fine structure (XAFS) spectra
were collected in the vicinity of the Ca K edge (4.038 keV) at
room temperature at the XDD beamline of the Singapore Synchro-
tron Light Source (SSLS, Singapore).19 Each sample was ground
and mixed with LiF powder (99%, Fluka) at a weight ratio of 1:10.
The mixture was pressed into a pellet under a pressure of 2.5 tons
(14) Juza, R.; Schumacher, H. Z. Anorg. Allg. Chem. 1963, 324, 278-
286.
(15) Jacobs, H.; Schmidt, D. Curr. Top. Mater. Sci. 1982, 8, 381-427.
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515.
(17) Palvadeau, P.; Rouxel, J. Bull. Soc. Chim. Fr. 1970, 2, 480-485.
(18) All of the programs download from http://www.ccp14.ac.uk.
(19) Moser, H. O.; Casse, B. D. F.; Chew, E. P.; Cholewa, M.; Diao, C.
Z.; Ding, S. X. D.; Kong, J. R.; Li, Z. W.; Hua, M.; Ng, M. L.; Saw,
B. T.; bin Mahmood, S.; Vidyaraj, S. V.; Wilhelmi, O.; Wong, J.;
Yang, P.; Yu, X. J.; Gao, X. Y.; Wee, A. T. S.; Sim, W. S.; Lu, D.;
Faltermeier, R. B. Nucl. Instrum. Methods Phys. Res., Sect. B 2005,
238, 83-86.
(20) Ressler, T. J. Phys. IV 1997, 7, 269-270.
518 Inorganic Chemistry, Vol. 46, No. 2, 2007

Synthesis and Characterization of Li2Ca(NH)2
Table 1. Li2Ca(NH)2 Crystallographic Dataa
space group
unit cell dimensions
cell volume
P3hm1
a ) 3.5664(3) Å, c ) 5.9540(8) Å
V ) 65.585(11) Å3
Z ) 1, d ) 2.126 g/cm3
Z, density
atom
position
x
y
z
Ca
Li
N
1b
2d
2d
0
1/3
1/3
0
2/3
2/3
0.5
0.8841(22)
0.2565(15)
H
not included
Interatomic Distances
Ca-N
Li-Li
Li-Ca
2.518 Å × 6
2.479 Å
3.077 Å
Li-N
Ca-Ca
2.223 Å × 3
2.217 Å × 1
3.566 Å
a Rp ) 7.37%, Rwp ) 9.54%.
Figure 2. XRD patterns of the ternary imide Li2Ca(NH)2. Legend: +,
observed; s, calculated; bottom line, the difference; and vertical tick, Bragg
peak positions.
Because the scattering factor of hydrogen is very small
compared to those of the other elements, hydrogen has little
effect on XRD and can therefore be neglected. In order to
locate hydrogen, neutron diffraction of a deuterium-
substituted sample should be performed. Li+, Ca2+, and N3-
positions in the initial model for Rietveld refinement were
derived from published structures of ternary nitrides. Ternary
lithium nitrides can be divided mainly into two groups, that
is, those with anti-fluorite type crystal structures represented
by a general formula Li2n-3Mn+Nn-1 (n g 2) and those with
the anti-La2O3 type structure represented by a general formula
Li2MN2.1 As evidenced by the indexing result of the TREOR
program, Li2Ca(NH)2 should possess the anti-La2O3 type
structure, which belongs to the hexagonal structure. CaMg2N2
(JCPDS-ICDD card no. 83-0042)21 has the anti-La2O3 type
structure with lattice constants of a ) 3.5405 Å and c )
6.0908 Å. Since the ionic radii of Li+ and Mg2+ are almost
identical (0.68 and 0.67 Å),22 the structure of CaMg2N2 was
adopted as the starting model to study Li2Ca(NH)2, wherein
Mg2+ was substituted by Li+. The experimental powder XRD
pattern of Li2Ca(NH)2 was in excellent agreement with the
calculated pattern derived from the starting model (Figure
2), which implied that the structural model adopted in the
present simulation was reasonable and acceptable. A series
of parameters, including background, zero shift, scale, lattice
parameter, graphic parameter (U, V, W), and Li and N
coordinates, were refined by the Rietica program (Table 1).
Li2Ca(NH)2 belongs to the space group P3hm1 with lattice
constants of a ) 3.5664(4)Å and c ) 5.9540(8) Å. Ca
occupies the 1b site (0, 0, 1/2), Li occupies the 2d site (1/3,
2/3, 0.8841(22)), and N occupies the 2d site (1/3, 2/3, 0.2565-
(15)). The regular octahedral and tetrahedral holes of the
approximately cubic closed-packed matrix of nitrogen in the
Li2Ca(NH)2 lattice (Figure 3) are the occupation sites for
Ca and Li, respectively.
NH3 concentration in the desorbed hydrogen was below 100
ppm according to the amount of ammonia detected by the
Metrohm 781 pH/ion meter. Hydrogen appeared at temper-
atures above 50 °C and peaked at 140 °C and 206 °C. After
the dehydrogenation, 95.7 mg of the solid residue was
obtained, indicating a release of 4.5 mg of hydrogen which
was equivalent to ∼4.0 H atoms per 2LiNH2 + CaH2.
Volumetric measurement of the amount of hydrogen des-
orbed during the thermal treatment also gave a similar result.
As shown in Figure 1b, about 4.3 wt % of hydrogen was
released from the starting materials, which was equivalent
to 3.8 H atoms per 2LiNH2 + CaH2. The slight difference
between the results from these two aforementioned methods
may be due to differences in the hydrogen pressure in the
two testing systems. In the TPD measurement, the partial
pressure of H2 in the gaseous phase was negligible. However,
in the volumetric release measurement, hydrogen was
gradually accumulated in the sample chamber, reaching 0.3
bar at the end of the test. Part of the hydrogen could not be
released because of the reaction equilibrium limit. On the
basis of the amount of hydrogen detached from the starting
chemicals, the chemical composition of the dehydrogenated
solid product should be Li2CaN2H2. The rehydrogenation of
the solid residue was achieved when applying 80 bar of
hydrogen to the sample in the temperature range of 20 to
200 °C.8
The dehydrogenated solid product was characterized by
XRD and identified by searching and matching the JCPDS-
ICDD cards. The characteristic diffraction peaks of the
starting amide and hydride disappeared (Figure 2). Instead,
a new set of peaks positioned at 32.5°, 42.1°, 51.1°, and so
forth emerged, which could not be matched with any known
compounds containing Li, Ca, N, and H elements.8 After
indexing with the TREOR program, the XRD pattern of the
solid product matched well with a single phase having the
hexagonal structure with lattice constants of a ) 3.566 Å
and c ) 5.954 Å. To be a neutral molecule, H in the Li2-
CaN2H2 phase should be positively charged and bond with
N. Therefore, this newly developed compound is a new
ternary imide, represented by Li2Ca(NH)2.
6
High-resolution Li MAS NMR was employed to study
the chemical environment of Li in the ternary imide. The
chemical shift values are directly related to the shielding of
the nucleus by the electronic structure of its local environ-
(21) Schultz-Coulon, V.; Schnick, W. Z. Naturforsch., B: Chem. Sci. 1995,
50, 619-622.
Rietveld refinement of the XRD data was performed by
using a structure model containing Li+, Ca2+, and N3-.
(22) Juza, R.; Langer K.; von Benda, K. Angew. Chem., Int. Ed. Engl. 1968,
7, 360-370.
Inorganic Chemistry, Vol. 46, No. 2, 2007 519

Wu et al.
Figure 3. Crystal structure of the ternary imide Li2Ca(NH)2. Li, green
spheres; Ca, blue spheres; N, purple spheres. The unit cell is delineated by
the dark yellow line. Li and Ca are linked to N coordinates by black sticks.
Figure 5. XANES transmission spectra at the Ca K edge of the Ca3N2,
CaNH, and Li2Ca(NH)2 samples.
Figure 4. Solid state 6Li MAS NMR spectra of Li2Ca(NH)2, Li2NH,
LiNH2, and Li3N.
ment.23 For example, there is a proportional correlation
between the 6Li chemical shifts and the oxygen coordination
number in lithium silicate.23 As shown in Figure 4, Li in
Li2Ca(NH)2, Li2NH, Li3N, and LiNH2 has the chemical shifts
of 3.433, 3.455, 7.493, and 2.748 ppm, respectively. Similar
chemical shifts of Li in Li2Ca(NH)2 and Li2NH indicate the
identical chemical environment of Li in these two com-
pounds. Li2NH has a simple face-centered cubic (fcc)
structure with Fm3hm symmetry.13 Each lithium cation in Li2-
NH is coordinated with four imide anions. It is likely that
the coordination number of the lithium cation in Li2Ca(NH)2
is the same as that in Li2NH, in agreement with the XRD
results.
X-ray absorption near-edge structure (XANES) spectros-
copy is a powerful technique in determining metal atom/ion
coordination number, especially when an appropriate model
compound with well-defined coordination structure is avail-
able for comparison. In the present study, Ca3N2 was used
as the reference for Ca in tetrahedral coordination,24 and
CaNH was used as the reference for Ca in octahedral
coordination.14,25 Figure 5 shows the XANES spectra of
Ca3N2, CaNH, and Li2Ca(NH)2 samples at the Ca K edge.
The peak at 4.050 keV is ascribed to the Ca 1s to 4p dipole
transition. The peak at 4.038 keV is the pre-edge transition,
Figure 6. Fourier transforms of the EXAFS spectra of ternary imide Li2-
Ca(NH)2.
which is related to the direct 1s to 3d quadrupole transition
or the dipole transitions due to the d-p mixing between the
3d orbitals and the 4p orbitals of the central atom or
neighboring atoms.26,27 Therefore, the intensities of the pre-
edge features are sensitive to the local coordination geometry
of the metal atom.26,27 It can be seen in Figure 5 that the
intensity of the peak at 4.038 keV is much higher for Ca in
the tetrahedral site than that in the octahedral site. Similar
to that of CaNH, the intensity of the peak at 4.038 keV for
Li2Ca(NH)2 is very weak, indicating an octahedral coordina-
tion.
The Fourier transform of the extended XAFS (EXAFS)
spectrum for Li2Ca(NH)2 is presented in Figure 6. The peak
at the radial distance of about 2.0 Å corresponds to the above-
mentioned Ca-N coordination of the first shell. For the data
evaluation, the peak between 1 Å and 2.5 Å was isolated by
(23) Xu, Z.; Stebbins, J. F. Solid State Nucl. Magn. Reson. 1995, 5, 103-
112.
(24) Heyns, A. M.; Prinsloo, L. C.; Range, K. J.; Stassen, M. J. Solid State
Chem. 1998, 137, 33-41.
(25) Sichla, T.; Jacobs, H. Z. Anorg. Allg. Chem. 1996, 622, 2079-2082.
(26) Chen, L. X.; Liu, T.; Thurnauer, M. C.; Csencsits, R.; Rahj, T. J.
Phys. Chem. B 2002, 106, 8539-8546.
(27) de Groot, F. Chem. ReV. 2001, 101, 1779-1808.
520 Inorganic Chemistry, Vol. 46, No. 2, 2007

Synthesis and Characterization of Li2Ca(NH)2
randomized over the 192l sites above that temperature.30
1
According to the H NMR measurement result of the solid
lithium imide, the N-H vector is oriented to the middle point
of two adjacent lithium sites.31 The first-principles calcula-
tions based on the density functional theory (DFT) were used
to identify H positions in Li2Ca(NH)2 (details in Supporting
Information). Our detailed DFT calculations on Li2Ca(NH)2
reveal that H atoms may distribute over the 6i equivalent
sites (0.1838, 0.8162, 0.1723) around nitrogen atoms, and
the N-H vector orients toward the middle point of two
adjacent lithium sites (Li-Li distance 3.566 Å). Further
investigations by neutron powder diffraction, NMR, and
neutron inelastic scattering are needed to provide more
information on the position and dynamics of hydrogen in
Li2Ca(NH)2.
Figure 7. FTIR spectra of the ternary imide Li2Ca(NH)2.
means of a filter function, and fitted with the amplitude
reduction factor of S02 ) 0.73. The value of 0.73 is derived
from the fitting results of the EXAFS data of CaNH using
the coordination number and distance of the first shell from
crystallographic data.14,25 The coordination number of Ca
obtained from the EXAFS data of Li2Ca(NH)2 is 6.3, which
indicates that each Ca is coordinated with six N, in good
agreement with the XRD results.
To investigate the position of H in the lattice and the nature
of the N-H bond, Li2Ca(NH)2 was further analyzed by FTIR
spectroscopy. A broad absorbance in the range of 3050 cm-1
to 3200 cm-1 was detected (Figure 7), which correlates with
the stretch of the N-H bond in imide ions.7,28 For example,
N-H stretches in Li2NH and CaNH are 3160 cm-1 and 3183
cm-1, respectively.7,28 The broad peak is characteristic of
imide ions, which may be the result of dynamic or static
disorder of hydrogen over the equivalent sites that broadens
the localized vibrational states into bands.11,25 The overall
absorbance of the ternary imide resembles that of Li2NH
except the asymmetric nature caused by the presence of Ca.
Thus, the hydrogen site in Li2Ca(NH)2 can refer to that in
the Li2NH lattice. In the crystal structure model of Li2NH,
the hydrogen site is 16e (F4h3m) or 48h (Fm3hm).29 The recent
neutron diffraction and XRD results and the DFT calculations
revealed that Li2ND can have a disordered cubic (Fd3hm)
structure with partially occupied Li 32e sites below 360 K
and have the disordered cubic (Fm3hm) structure with D atoms
Conclusions
Thermal dehydrogenation of a mixture of LiNH2 and CaH2
at a molar ratio of 2:1 led to the formation of a new ternary
imide Li2Ca(NH)2. The crystal structure of Li2Ca(NH)2 was
determined by powder XRD. It adopts the trigonal anti-La2O3
structure (space group P3hm1). The cell parameters and atomic
fractional coordinates were refined by the Rietveld method.
Furthermore, local coordination states around Li and Ca in
the Li2Ca(NH)2 crystal were investigated by 6Li MAS NMR
and by XAFS at the Ca K edge, respectively, and the results
were in good agreement with the results of the XRD
measurement.
Acknowledgment. The present work is financially sup-
ported by the Agency of Science, Technology, and Research,
Singapore (A*STARS). Work of XAFS was performed at
Singapore Synchrotron Light Source (SSLS) under NUS
Core Support C-380-003-003-001, A*STAR/MOE RP
3979908M, and Grant A*STAR 12 105 0038. We thank Dr.
Ping Yang for the technical support at the XDD beamline.
Supporting Information Available: H positions in Li2Ca(NH)2
were investigated by the first-principles calculations based on the
density functional theory (DFT). This material is available free of
charge via the Internet at http://pubs.acs.org.
IC060769Y
(30) Balogh, M. P.; Jones, C. Y.; Herbst, J. F.; Hector, L. G.; Kundrat, M.
J. Alloys Compd. 2006, 420, 326-336.
(31) Haigh, P. J.; Forman, R. A.; Frisch, R. C. J. Chem. Phys. 1966, 45,
812-816.
(28) Linde, G.; Juza, R. Z. Anorg. Allg. Chem. 1974, 409, 119-214.
(29) Ohoyama, K.; Nakamori, Y.; Orimo, S.; Yamada, K. J. Phys. Soc.
Jpn. 2005, 74, 483-487.
Inorganic Chemistry, Vol. 46, No. 2, 2007 521