286
J. Appl. Crvst. (1991). 24, 286-292
Structure Determination of Magnesium Boron Nitride, Mg3BNa, from X-ray Powder
Diffraction Data
BY HIDEOHIRAGUCHI AND HIROOHASHIZUME*
Research Laboratory of Engineering Materials, Tokyo Institute of Technology, Nagatsuta, Midori,
Yokohama 227, Japan
OSAMU FUKUNAGA
Department of Inorganic Materials, Faculty of Engineering, Tokyo Institute of Technology, Ookayama,
Meguro, Tokyo 152, Japan
AKIO TAKENAKA
Department of Life Science, Faculty of Bioscience and Biotechnology, Nagatsuta, Midori, Yokohama 227,
Japan
MAKOTO SAKATA
Department of Applied Physics, Faculty of Engineering, Nagoya University, Furocho, Chikusa, Nagoya 454,
Japan
(Received 13 October 1990; accepted 24 January 1990)
Abstract
Knowledge of the crystal structure of magnesium
boron nitride, Mg3BN 3, is essential to the understanding of the catalytic property of this material in
the reaction converting hexagonal boron nitride
(hBN) into a cubic form (cBN) under high pressuretemperature conditions (Wentorf, 1961; Endo,
Fukunaga & Iwata, 1979; Sato, 1986). The latter
material cBN has a zinc blende structure, is next to
diamond in mechanical hardness and non-corrodible
in ferric environments and so of wide practical use in
the machining and polishing industry. Mg3BN3 is
known to exist in two polymorphs (Sato, 1986;
Hohlfeld, 1989), called low- and high-pressure phases
in the present paper.t To date no single crystal of
appropriate size has been grown for either phase.
X-ray Debye-Scherrer lines from Mg3BN3(L) were
indexed on a hexagonal cell with a = 3-544, c =
16.04 A (Sato, 1986). The same author restricted the
possible space groups to P63mc and P63/mmc from
systematic absences in X-ray and electron diffraction
data. Marginal anomalies in DTA (differential thermal analysis) curves observed during a synthesis of
Mg3BN3 from Mg3N2 and hBN under ambient
pressure led Sato (1986) to a belief that Mg3BN3(L)
has a derivative structure of hBN and is an intercalation compound of the latter. The detailed crystal
structure has yet to be determined, however.
Quite a few structures have already been solved ab
initio from powder diffraction data (Berg & Werner,
1977; Clearfield, McCusker & Rudolf, 1984;
Cheetham, David, Eddy, Jakeman, Johnson &
Torardi, 1986; Attfield, Sleight & Cheetham, 1986;
* To whom all correspondence should be addressed.
t Hereafter low-pressure Mg.~BN3is expressed as Mg3BN3(L).
The crystal structure of magnesium boron nitride in
the low-pressure phase, Mg3BN3(L), has been solved
ab initio from X-ray powder data. The cell is hexagonal (space group P63/mmc, Z = 2) with a =
3.54453 (4), c = 16-03536 (30)/k. Initial positional
parameters for the Mg atoms were obtained from
Patterson functions generated by 50 integrated intensities derived from a whole-powder pattern
decomposition. The remaining atoms were located
by trial-and-error model building, followed by Rietveld refinements (R,p = 8.5%). The structure can be
described as consisting of ABB'BACC'CA... layers
perpendicular to the c axis with linear N-~-B~N
molecular anions at position A, Mg 2+ at positions B
and C and Mg 2+ with three coordinating N atoms at
positions B' and C', although Mg3BN3(L) is not a
layer compound. A very similar structure has also
been obtained by applying standard direct methods
to the same intensity data. A high-quality electrondensity map has been calculated from the structurefactor data using the maximum-entropy method.
Introduction
0021-8898/91/040286-07503.00
© 1991 International Union of Crystallography
H. HIRAGUCHI, H. HASHIZUME, O. FUKUNAGA, A. TAKENAKA AND M. SAKATA
McCusker, 1988, and references therein; Attfield,
1988; Le Bail, Duroy & Fourquet, 1988; Marezio,
Cox, Rossel & Maple, 1988; Plevert, Lou~r & Lou~r,
1989; Laligant, Le Bail & Ferey, 1989; Honda, Goto
& Kurahashi, 1990; Lafontaine, Le Bail & Ferey,
1990). Some of these investigations relied on synchrotron radiation to obtain highly resolved data
from complex structures like those of zeolites,
although overlapping reflections were not eliminated.
Comparable or higher resolutions were achieved at
high-intensity neutron sources. Other studies used
Guinier cameras and counter diffractometers at conventional X-ray sources for the analyses of less complex structures. In many cases Pawley's method
(Pawley, 1981) played a central role in the decomposition of overlapping reflections. The method uses
variable Bragg intensities in calculating a whole
powder pattern from known cell parameters and
symmetry information and determines individual
Bragg intensities by fitting the observed diffraction
profile. Once reliable integrated intensity data have
been extracted, standard single-crystal structure solving techniques involving Patterson and direct
methods are followed. The structure obtained can be
refined by different methods including the Rietveld
method (Rietveld, 1969), which refers to the original
diffraction profile.
In the present paper this approach is applied to an
ab initio structure determination of Mg3BN3(L) from
X-ray powder data collected at a laboratory source.
An attempt is also described to calculate high-quality
electron-density maps using the maximum-entropy
principle.
Experimental
Mg3BN3(L) is conventionally synthesized by heating
a mixture of hBN and Mg3N2 or metallic Mg
powders in a nitrogen atmosphere. The optimum
mixing molar ratio and heating temperature are
reported to be hBN/Mg= 0-6-1 and 13731433K, respectively (Sato, Endo & Fukunaga,
1984). Materials thus prepared, however, tend to
include appreciable amounts of unreacted or byproduct compounds. To avoid this, we mixed hBN
powder (Denka GP, purity 99-5%) and flaked magnesium metal in a l:x (x = 3) molar ratio and contained the mixture in a loosely capped molybdenum
capsule (volume - 8 cm 3) placed in a nitrogen oven.
Trial syntheses and X-ray diffractometer scans of the
products discovered a procedure for preparing
Mg3BN3(L) samples with a minimal content of
unwanted compounds. This procedure uses a starting
hBN-Mg mixture of x = 3-45 and keeps it at 873 K
for 4 h and then at 913 K for 4 h to promote the
reaction 3Mg + N2--" Mg3N2. Temperature should be
raised slowly ( - 100 K/2.5 h) to avoid rapid evapor-
287
ation of magnesium. The mixture is subsequently
heated to 1413 K and held at this temperature for
6 h. A final heat treatment is performed at 1473 K
for 6 h, which completes the reaction Mg3N2 + hBN
---,Mg3BN3 and at the same time allows unreacted
Mg3N2 to escape the capsule. Chemical analyses of a
yellow product showed an Mg:B:N molar ratio of
2.83:1:2-85 with some oxygen content.
Transmission-electron-microscope
pictures
of
pulverized Mg3BN3(L) samples revealed irregularly
shaped particles with a size distribution ranging from
0.5 to 5 ~m in diameter. Many particles exhibited
well defined single-crystal electron diffraction diagrams, but some showed diffuse spots or even ringlike patterns.
X-ray diffraction data were collected on a JEOL
0-0 diffractometer using pyrolytic graphite for the
reflected-beam analyzer at a Cu tube source. The
sample was lightly packed on a flat glass holder and
kept stationary in a horizontal position while the 0-0
motion of the X-ray tube and the analyzer-Nal
detector assembly scanned a range 20 < 20 < 120 ~ in
the vertical plane in steps of 0.04 '~, with a count time
of 20 s per step. The powder data obtained showed
sharp Mg3BN3(L) reflections with a few low hBN
and Mg(OH)2 peaks. The peak width of the
Mg3BN3(L) reflections was 0.2 ~' 20 at 2 0 = 50 °,
which increased to 0.38 ~ at 20 = 100 ~. The instrumental resolution, measured using standard a-quartz
powder, was 0.1Y at 20 = 50 ~ and 0.185"" at 100%
Structure determinations
The intensity data, with the angular ranges contaminated by the Mg(OH)2 and hBN reflections
excluded, were input to the whole-powder-pattern
decomposition program WPPF (Toraya, 1986) along
with the initial values of the cell parameters and the
reflection conditions (hhil: l = 2n, 000l: l = 2n
common to P63mc and P63/mmc). The refined
parameters included Bragg intensities, the 20 zero
offset, the unit-cell parameters, the background
parameters and the peak-shape parameters. Also
refined were parameters adjusting the wavelength
separation and intensity ratio of the Ka doublet. The
profile fit converged to R,.p = 7-07% and all 69
symmetry-allowed reflections were resolved. Heavily
overlapping reflections were associated with high
e.s.d.'s because of the ill conditioning of the leastsquares procedure. There were 50 reflections
observed with intensities I > 3o-(/) and these were
used in the structure determination.
Patterson junctions and model building
After Lorentz-polarization and multiplicity corrections, the 50 integrated intensities were used to
288
MAGNESIUM BORON NITRIDE
generate Patterson functions. Ten well resolved interatomic vectors were recognized on the (1120)
Patterson m a p and these were assumed to represent
M g - X pair correlations where X is Mg, N or B
atoms. Structure models were built by trial and error
with guidance from the s y m m e t r y i n f o r m a t i o n and
chemical knowledge, as well as the discovered interatomic vectors, and considered c a t i o n - a n i o n interaction. It was readily seen that Z = 2 (two Mg3BN3
formula units per unit cell) and that the structure
sought should be centrosymmetric, setting the space
group to P63/mmc. M g atoms could only be located
at the special positions 2(b), (0,0,J) and 4 ( f ) , (~,~,z)
~2
with z - 0 - 1 2 5 . More than ten chemically feasible
models were consistent with the Patterson peak positions. They were classified according to the presence
or absence o f the B3N3 hexagon layers reported for
the hBN structure (Pease, 1952). A calculation o f a
powder diffraction pattern from each model and
c o m p a r i s o n with the observation suggested that
Mg3BN3(L) has a B - N structure totally different
from that for hBN. In fact, the plausible models were
characterized by B3, N3, Mg3 a n d Mg3N 3 layers
stacked at unequal spacings along the c axis. Leastsquares structure refinements were performed on
most plausible models using the Rietveld p r o g r a m
PFLS (Toraya & M a r u m o , 1980). The unit cell,
instrumental, positional and overall thermal p a r a m eters were refined simultaneously (Rwp 12.3%). A
visual inspection o f the calculated and observed
Bragg-peak heights suggested a weak preferred
orientation parallel to the (1010) plane present in the
sample. F u r t h e r refinements were carried out with
variable orientational (p), M g site occupancy (g) and
individual isotropic thermal (B,.) parameters. A
G a u s s i a n correction factor e x p ( - p ~ o 2) was assumed
for the incomplete r a n d o m orientation (Rietveld,
1969). This resulted in a satisfactory profile fit with
Table 1. Crystallographic data at 298 K
P63/mrnc
Space group
a (/~)
c (/k)
z
Wavelength (A)
Pattern 20 range (~)
Step size (~)
Number of observation points
Number of contributing reflections
Number of structural parameters
Number of profile parameters
Rt
RF
Rr
R,,p
R,.
3"54453 (4)
16"03536 (30)
2
1.540562 (Kay)
1.544390 (Ka2)
20-120
0.04
2311
69
12
11
0"0410
0"0392
0"0622
0"0852
0"0381
Table 2. Rietveld-refined positional, thermal and occupancy parameters for low-pressure Mg3BN3
Numbers in parentheses are the e.s.d.'s in units of the leastsignificant digit given. Those parameters without e.s.d.'s were held
fixed in least-squares refinement.
Mg(1)
Mg(2)
N(I)
N(2)
2(b)
400
2(d)
4(e)
x
0
~
]
0
y
0
3
_32
0
z
]
0.1228(0)
4j
0-0851(2)
Bi (/~fl)
1"10 (6)
1.05 (3)
0"42 (8)
1.64 (8)
g
0"894(5)
0.933(4)
1"0
1.0
B
2(a)
0
0
0
1"7 (2)
1"0
=
~0.5
z_
1'-4
3
0.0
I
IIII i i I
I II1|11111 II I IIIIII1'111' IIIII II1'i I',11
07 0.0
2o
3o
so
5o
60 70 so 90 loo ,o 12o
TWO-IHEIA (*)
Fig. 1. Observed (cross), calculated (full line) and difference
(bottom trace) X-ray diffraction profiles for low-pressure
Mg3BN3 powder. The difference trace plots y,(obs.)- y,(cal.),
where y,(obs.) and y,(cal.) are observed and calculated intensities at step i. Short vertical bars mark the Bragg peak positions.
The maximum observed X-ray count is 41 798.
Rwp = 8"5%. The observed, calculated and difference
profiles are shown in Fig. 1.* The crystallographic
details are s u m m a r i z e d in Table 1 and the final
structural parameters in Table 2. The refined orientation p a r a m e t e r p -- 0-135 (4) d e g - 2 represents a really
weak preferred orientation.
Solution by direct methods
54 structure-factor amplitudes IFI [>3~r(IFI)]
were used as input to the MULTAN78 directmethods p r o g r a m and a phase d e t e r m i n a t i o n was
attempted a s s u m i n g each o f the P63/mmc and P6mc
space-group symmetries. However, P63/mmc alone
led to a reasonable solution. With this space group,
the m o d u l e N O R M A L o f M U L T A N gave 49 reflections having IEI=IFI/(IFE2)>_O.13, which were
used in the phase determination. The total n u m b e r o f
triplet phase relations generated was 940, and four
reflections were selected for the starting set. These
* Data for the observed and calculated profiles for low-pressure
Mg3BN~ have been deposited with the British Library Document
Supply Centre as Supplementary Publication No. SUP 53841 (20
pp.). Copies may be obtained through The Technical Editor,
International Union of Crystallography, 5 Abbey Square, Chester
CH! 2HU, England. The intensity data are also available from the
JCPDS Diffraction File, JCPDS International Centre for Diffraction Data, Swarthmore, PA, USA.
H. HIRAGUCHI, H. HASHIZUME, O. FUKUNAGA, A. TAKENAKA AND M. SAKATA
led to 16 phase sets with a highest combined figure of
merit, C, equal to 2-6. Four highest peaks in the E
map derived from the phase set with the highest C
were assigned to Mg, N and B with one more peak
missing for N to give a chemically consistent solution. The other peaks were regarded as noise as there
was a large gap between the fourth and fifth peaks.
The four peaks assigned closely matched the Mg(2),
Mg(1), N(1) and B positions given in Table 2. The
remaining N atoms were found in a difference Fourier map. These atoms were already visible in a
Fourier map calculated from the observed structure
factors with phases given from the four E-map
peaks. Two other M U L T A N phase sets led to the
identical solution. No chemically consistent structure
was obtained from the other sets.
It may be worth mentioning at this point that
N O R M A L yielded a negative B factor in normalizing
F's to obtain E's. Clearly, the small number of
Bragg reflections used invalidated the Wilson statistics. Nevertheless, a successful scaling was accomplished by using a temporal value of 1.154 A 2 for the
B factor, which was confirmed by the resultant
(IEI 2) = 1.0. Also, there were a few reflections with
smallest E values, making the figure of merit ~o, and
hence C, unreliable. One can learn from this experience that for small data sets M U L T A N can give
correct solutions with unreliable values for the
defined figures of merit.
A least-squares refinement of the atomic coordinates and anisotropic temperature factors was carried out on the integrated intensity data using the
program L S A P 8 0 (Takenaka & Sasada, 1980). The
refined atomic coordinates were in very good
agreement with those of the Rietveld refinement and
anisotropic thermal vibration was found to be
unimportant for each atom. Fig. 2(a) shows an
electron-density map Fourier synthesized from the 54
observed structure factors with phases from the
refined model. The close similarity of this model to
that from the real-space analysis (Table 2) is demonstrated by the fact that the two models give a
common phase, 0 or rr, for each of the 49 structure
factors used in solving the structure.
Maximum-entropy electron-density distributions
Maximum-entropy (ME) calculations of the electron-density distribution were carried out on the
phased structure factors using the procedure
described by Sakata & Sato (1990). We minimized
the Shannon-Kullback relative information (Shannon, 1948; Kullback & Leibler, 1951; Hobson &
Cheng, 1973; Levine, 1980; Steenstrup & Wilkins,
1984; Wilkins, Steenstrup & Varghese, 1985)
I = Zpiln(pi/mi),
i
289
subject to the weak constraint (Gull & Daniell, 1978;
Collins, 1982; Wilkins, Varghese & Lehmann, 1983;
Livesey & Skilling, 1985)
N~
C, = (1/2N,) Z
]Fh(obs.)- F~(cal.)]2/o.~,,
h-I
where mi = "ri/S'.7"i and Pi = Pi/~Pi are the normalized
prior and posterior electron densities in pixel i,
F~(obs.) is the hth observed structure factor (with
phase given from the model), F~(cal.) is the hth structure factor calculated from {Pi}, NI and o .2 are the
total number and variance of iF~(obs.)l, respectively.
"'
:
.,i
,.,
.+ " " .
".
i ",::.....:
.-
..--.~- .
t
o:..-..
1:
'~-........
.. ,,,,
;
.+
--.
: J'"--;
.--..
.+
.."
....
I :-...:,--.
"'"
,
:
',,,:
" :-/.'" ". ,,'
~_......
.
....: : . " ~
..--:
.-~
',. " - . .
~ ~ ~ _ _ _
(" :. "',,
,,"
-,,
/"
:
:
.:
.... ± . . . . .
-.
\"
.,
(a)
Mg
~
N
B
N
>< ILct ~ - ~ F , > tc~)~-~
!5
<-J-~ !
(b)
(c)
Fig. 2. Sectioned electron-density map for the (11]~0) plane of
hexagonal Mg3BN3(L). The c axis is parallel to the horizontal
direction. (a) Conventional Fourier map from 54 reflections and
refined phases. Contour levels are in steps of 2.5 e A-3. Broken
lines represent negative densities; (b) maximum-entropy reconstruction calculated on a 30 × 30 x 60 pixel grid using the same
structure-factor data as for (a). Contour lines in steps of
0"05eA3cover up to 1.0eA-3; (c) same as (b) but for
high-density regions. Contour lines are drawn in steps of
2.5 e A 3 for a density range 1-0-26.0e A 3 and in steps of
10.0e A -3 for a range 26.0-86.0eA J. Density levels at the
Mg(1), Mg(2), N(I), N(2) and B atomic positions are 87-05,
74"39, 25"66, 19.14 and 10.54 e A -3, respectively.
290
MAGNESIUM BORON NITRIDE
The more popular Shannon-Jaynes entropy S
(Jaynes, 1968) is related to I through S = - I . The
factor 2 appears on the constraint function CI as we
assume Friedel's law for Fh(obs.). This is a justifiable
assumption since Mg, B and N atoms show negligible anomalous dispersion for Cu Ktr X-rays. Minimization of I under the constraint C1 leads to an
exponential expression for p;, which includes r; as a
pre-exponential factor. Starting from a uniform density distribution with all {r;} set equal on 30 ×
30 x 60 pixels (over the whole unit cell), we calculated p; by repeating an iterative procedure until the
condition C~ <- 1 was reached (Sakata & Sato, 1990).
At each iteration cycle, an explicit normalization Yp;
= 1 was applied. This confined the total number of
electrons in the unit cell to 124 [= F(000)]. Figs.
2(b),(c) show an ME electron-density map {p;} for
the (1120) plane of Mg3BN3(L) calculated from the
same structure-factor data as used for Fig. 2(a). The
ME reconstruction is free from unphysical negative
densities and includes no spurious peak higher than
0.22 e ,~-3. Also, many more electrons are concentrated at the atomic positions than in the density
map obtained from the conventional Fourier synthesis (Fig. 2a). These features arise from the exponential representation of density, which is superior to the
truncated Fourier series, and demonstrate the higher
resolution achievable with the ME method.
Another feature of the calculated ME map is that
it is least affected by statistical errors in IFh(obs.)l.
This arises from the fact that we terminated the
calculation at C~ = 1.00 (Gull & Daniell, 1978). This
is the expected value of the ,t,2 distribution which Cl
possesses. The factor
RI = Y l & ( o b s . ) -
these reflections. The phase-extension procedure
failed in an early cycle when strong reflections with
large IFh(obs.)l's were assigned wrong phases
because no algorithm was available to refine once
determined phases, i.e. signs of Fh in our case of a
centrosymmetric structure. Also, no successful result
was obtained in attempts to derive an ME density
map by using a constraint on phaseless structure
factors similar to C2 of Sakata & Sato (1990) in
conjunction with G .
Discussion
The crystal structure of Mg3BN3(L) can be described
as consisting of A B B ' B A C C ' C A . . . layers parallel to
the (0001) plane with vertically extended linear NBN
units at position A, Mg at positions B and C, and
MgN units at positions B' and C' (Fig. 3). Each Mg
atom in the MgN layer is coordinated by three N
atoms. This description, however, is misleading as it
implies a layer-like nature of the crystal. It is more
appropriate to describe Mg3BN3(L) as an ionic crystal composed of [ N - - B z N ] 3- molecular anions,
Mg 2÷ cations and N 3- anions, although this expression assumes nominal ionic charges for individual
species. One can estimate the actual valency situation
by counting electrons in appropriate volumes around
individual atoms or groups of atoms in the ME
density map (Sakata, Mori, Kumazawa, Takata &
Toraya, 1990). Attempts to do so in the case of
Mg3BN3(L) revealed that all the Mg, N and N B N
ions are positively charged or neutral. This interesting result is not acceptable, however, before the
accuracy of the ME map calculated is assessed in
&(cal.)l=/lK(obs.)l 2
h
would thus represent the overall error in the intensity
measurement, which amounts to 3.22% for the
present data.
The role of the ME principle in crystallography is
not just to present a tool for inversion problems.
There is currently considerable interest in the application of this principle to the phase problem. We
report here on our experience of using the procedure
by Gull, Livesey & Sivia (1987) to solve the crystal
structure of Mg3BN3(L). We assigned phase 0 to the
origin-defining 217 reflection, which was selected by
M U L T A N and given phase 0, and generated an ME
electron-density map from this reflection alone.
Structure factors Fh(cal.) for all other reflections were
calculated from the obtained map through inverse
Fourier transformation. After Gull et al. (1987), we
regarded &(cal.)'s with amplitudes greater than one
half of their measured values, LFh(obs.)l, as correct
inferences of 'true' structure factors and gave their
signs to IFh(obs.)l's to form a new map including
A
B
B"
C
C'
i
i
A
A
C
(
C'
C
( )))
A
B
B'
B
Fig. 3. Crystal structure of low-pressure MgsBN3 sectioned by the
(1 I~0) plane. The c axis is parallel to the vertical direction. Mg:
large open circles, B: small hatched circles, N: medium open
circles.
H. HIRAGUCHI, H. HASHIZUME, O. FUKUNAGA, A. TAKENAKA AND M. SAKATA
terms of the limited amount of structure-factor data
used and their errors.
The presence of N = B = N double bonds is confirmed by the pronounced infrared absorption
observed at 1780 c m - ~ i n Fig. 4. Since von Goubeau
& Anselment (1961), the N = B - - N anion is known
to show an absorption band at 1660-1900 cm-~ and
the high stability of this bond has been discussed in
terms of BN rr back bonding (K611e & N6th, 1985).
The observed B - - N interatomic distance, 1.365 A
(Rietveld refinement) or 1.337 A (integrated-intensity
refinement), in the linear symmetric NBN anions
significantly differs from the B - - N bond length in
hBN [1-446 ,/k (Pease, 1952)], but very close to those
found in bis(benzyl-tert-butylamino)boron
tetrachloroaluminate [1.331 (5) and 1.334 (5)/k (K611e &
N6th, 1986)], Li3BN2 [1.3393 (5) and 1.3361 (5)
for the fl-phase (Yamane, Kikkawa, Horiuchi &
Koizumi, 1986); !.339 (2)/k for the a-phase (Yamane, Kikkawa & Koizumi, 1987)] and Na3BN2
[1.345 (6) and 1.344 (6) A (Evers, Miinsterk6tter,
Oehlinger, Polborn & Sendlinger, 1990)]. All these
compounds were reported to include quasi-linear
N - - B - - N bonds. Further support to the doublebond N = B = N
anions is found in the electrondensity maps of Fig. 2, where the appreciably
shallow valleys between the B and N(2) atoms are
ascribable to bonding electrons.
The structural features of Mg3BN3(L) suggest that
a synthesis of Mg3BN3(L) from Mg3N2 and hBN at
high temperatures proceeds via the following scheme:
291
this property of Mg3N2 that Mg3BN3(L) has a quite
different structure from hBN.
The authors thank S. Kumazawa for adapting the
ME program to hexagonal structures. Thanks are
also due to I. Minato for taking electron-microscope
photographs of the sample, T. Sato for taking IR
spectra and S. Nakano, H. Sakuma and O. Sakata
for assistance in data collection and analysis. Discussions with T. Sato, H. Toraya, S. Wilkins, A. K.
Livesey and the high-pressure team at N I R I M are
appreciated. This work is supported by The Asahi
Glass Foundation.
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Mg3N2 + hBN---, 3MR 2+ + N 3- + [ N = B - - N ] 3 GULL, S. F. & DANIELL, G. J. (1978). Nature (London),
--~ Mg3BN3(L).
272, 686-690.
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