OH-F disorder in non-centrosymmetric Zn2(BO3)(OH)0.75F0.25: Ab initio structure determination and NMR study; Comparison with tridymite and fluoride borates

Article abstract of DOI:10.1016/S0925-8388(01)00889-1

The crystal structure of Zn₂(BO₃)(OH)_0.75F_0.25 is determined ab initio from X-ray and neutron powder data. The unit cell is monoclinic, space group P2₁ (no. 4), Z = 2, with a = 6.8738(4) angstroms, b = 4.9178(2) angstroms, c = 5.7018(3) angstroms, β = 98.829(3) °, V = 190.46(3) angstroms³ (neutron powder data). Alternating up and down ZnO₃(OH,F) tetrahedra and triangular BO₃^3- groups built corrugated layers. Their connection by hydroxyl ions gives a 3D network similar to that of tridymite. The statistical distribution of fluorine and hydroxyl in infinite chains is evidenced by magical angle spinning-NMR spectroscopy.

Full text of DOI:10.1016/S0925-8388(01)00889-1

Journal of Alloys and Compounds 315 (2001) 287–295
L
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OH–F disorder in non-centrosymmetric Zn2(BO3)(OH)0.75F0.25: ab initio
structure determination and NMR study; comparison with tridymite and
fluoride borates
b
c
a
G. Corbela , , E. Suard , J. Emery , M. Leblanc
*
a
´
Laboratoire des Fluorures, UPRES-A 6010, Universite du Maine, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France
bInstitut Laue-Langevin, Avenue des Martyrs, B.P. 156, 38042 Grenoble Cedex 9, France
c
´
´
Laboratoire de Physique de l’Etat Condense, UPRES-A 6087, Universite du Maine, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France
Received 17 May 1999; received in revised form 5 January 2000; accepted 7 January 2000
Abstract
The crystal structure of Zn2(BO3)(OH)0.75F0.25 is determined ab initio from X-ray and neutron powder data. The unit cell is
3
˚
˚
˚
˚
monoclinic, space group P21 (no. 4), Z52, with a56.8738(4) A, b54.9178(2) A, c55.7018(3) A, b598.829(3) 8, V5190.46(3) A
(neutron powder data). Alternating up and down ZnO3(OH,F) tetrahedra and triangular BO332 groups built corrugated layers. Their
connection by hydroxyl ions gives a 3D network similar to that of tridymite. The statistical distribution of fluorine and hydroxyl in infinite
chains is evidenced by magical angle spinning-NMR spectroscopy.
2001 Elsevier Science B.V. All rights reserved.
Keywords: Hydroxyfluoride borate; Ab initio structure determination; Non-centrosymmetry; Nuclear magnetic resonance
1. Introduction
2. Experimental
According to Xia [1], the polar orientation of inorganic
triangular groups in crystals leads to acentric structures and
consequently favours non linear optical properties. Numer-
ous borates are investigated: KBe2(BO3)F2 (KBBF) [2],
Sr2Be2(BO3)2O (SBBO) [3], Ca4Gd(BO3)3O (GdCOB)
Powder of Zn2(BO3)(OH)0.75F0.25 was prepared through
hydrothermal process from a mixture of Na2B4O7?10 H2O,
ZnF2 and H2O in the molar ratio 1:1:55, heated in a
PTFE-lined Parr autoclave at 453 K for 24 h and cooled at
about 6 K/h. The pH decreased from 9 to 8 during the
reaction. The resulting product was washed with distilled
hot water in order to dissolve NaF, then with acetone and
finally dried at 373 K. A small amount of monoclinic
b-HBO2 was found as impurity and cannot be removed
from the sample. The most intense peaks are indicated on
the X-ray diffraction pattern (Fig. 1).
The X-ray diffraction pattern of the sieved powder was
recorded on a Siemens D501 diffractometer. The neutron
diffraction pattern, recorded on the D1A high resolution
powder diffractometer at the I.L.L., Grenoble (France),
was used for locating boron and hydrogen atoms. The
conditions of data collection are reported in Table 1.
Structural calculations were performed with TREOR-90
[10], DICVOL-91 [11], FULLPROF [12], SHELX-76 [13] and
SHELXS-86 [14] programs. The program DIAMOND [15] was
used for the structure drawings.
[4]
and
recently
several
fluoride
carbonates
K4Ln2(CO3)3F4 (Ln5Pr, Nd, Sm, Eu, Gd) [5] were
evidenced. All these phases present parallel layers of
triangular BO332 or CO322 groups and the acentric character
results from the similar orientation of the triangles in the
layers. In this paper, we present a new acentric borate,
obtained by hydrothermal synthesis in sub-critical con-
ditions. 11B and 19F MAS-NMR spectroscopic study is
performed and structural comparisons with tridymite SiO2
[6,7] and fluoride borates Be2(BO3)F [8], NaBe2(BO3)F2
[9] and KBe2(BO3)F2 [2] are discussed.
*Corresponding author. Correspondence address: Department of
Chemistry, University of Cambridge, Lensfield Road, CB2 1EW Cam-
bridge, UK.
An optical second-harmonic generation test was per-
formed by the Kurtz–Perry method [16] on a powder
E-mail address: gc260@cus.cam.ac.uk (G. Corbel).
0925-8388/01/$ – see front matter
PII: S0925-8388(01)00889-1
2001 Elsevier Science B.V. All rights reserved.

288
G. Corbel et al. / Journal of Alloys and Compounds 315 (2001) 287–295
Fig. 1. Observed (dots) and calculated (line) X-ray (upper) and neutron (lower) powder diffraction diagrams of Zn2(BO3)(OH)0.75F0.25. The difference
pattern (Observed2Calculated) is shown at the bottom, as well as the peak positions.
sample using the 1.06 mm line of a YAG:Nd31 laser. A
signal was detected, implying the non-centrosymmetry of
the crystal structure.
Thermal analysis was conducted on a coupled TGA-
DTA, TA Instruments SDT 2960, under argon flow
(heating rate 10 K/min, temperature range 300–1173 K).
Above 723 K, decomposition occurs and the observed
weight loss, 4.61%, corresponds to the departure of 0.25
water molecule and 0.25 hydrofluoric molecule per mole
of fluoride-hydroxide borate (theoretical 4.59%). The
resulting products are Zn3(BO3)2 and ZnO according to
the hypothetical reaction:
Energy dispersive X-ray analysis (EDX) with a Link
EDX spectrometer coupled with a scanning electron
microscope (SEM), Hitachi S2300, allowed to check the
presence of zinc, oxygen and a small amount of fluorine.
The IR spectrum, realized on an Ati Mattson Genesis
series FTIR apparatus, is consistent with the presence of
borate groups BO332 (lines between 685 and 1345 cm21
)
2 Zn2(BO3)(OH)0.75F0.25 → Zn3(BO3)2 1 ZnO
1 0.5 H2O 1 0.5 HF
and hydroxyl ions (two intense peaks at 3365 and 3404
cm21 in Fig. 7), respectively [17].
The fluoride content (wt.%), estimated by ionometry
using an Orion 720A potentiometer equipped with a
specific LaF3 electrode, is 1.3 (60.5)% (theoretical 2.3%
for 0.25 F2) and corresponds to 0.15 (60.05) fluoride ions
per formula unit. This value is approximative as a reason
of the very small fluorine weight in the solid.
The volume weight r53.68(2) g cm23, measured on
400 mg of sample with a pycnometer Accupyc 1330, is in
good agreement with the theoretical value (r53.610(1)
g cm23).
11B and 19F high resolution MAS (magical angle

G. Corbel et al. / Journal of Alloys and Compounds 315 (2001) 287–295
289
Table 1
Crystallographic data and conditions of data collection and refinement for Zn2(BO3)(OH)0.75F0.25
a
Diffractometer, geometry
Temperature
Radiation, monochromator
Takeoff angle (8)
2u range (8)
Siemens D501
293 K
CuKa, graphite
–
D1A
293 K
1.909 A, Ge (115)
120
10–140
˚
10–130
Step scan
Time per step
Slit aperture
Symmetry
Space group
0.028 (2u)
38 s
0.15 mm
Monoclinic
P21 (no. 4)
2
0.058 (2u)
65 s (10 scans)
–
Monoclinic
P21 (no. 4)
2
Z
Parameters
˚
a (A)
6.8872(4)
4.9277(3)
5.7122(3)
98.840(3)
191.56(3)
207.07
3.610(1), 3.68(2)
371
6.8738(4)
4.9178(2)
5.7018(3)
98.829(3)
190.46(3)
207.07
˚
b (A)
˚
c (A)
b (8)
3
˚
Volume (A )
Molar weight (g mol21
)
Calculated and measured density (g cm23
Number of reflexions
)
3.610(1), 3.68(2)
221
Number of refined parameters
Zero point 8 (2u)
h (pseudo-Voigt)
13
0.092(2)
0.341(6)
44
20.032(3)
0.18(4)
h angular variation
–
0.0040(6)
Halfwidth parameters (U,V,W)
Asymmetry parameters (P1, P2)
Reliability factors
0.748(30), 20.315(18), 0.087(3)
0.054(4), 0.028(1)
0.263(14), 20.357(19), 0.283(7)
0.150(9), 0.050(4)
For points with Bragg contribution
Rp 5 0.131, Rwp 5 0.161,
R
Rp 5 0.179, Rwp 5 0.164,
exp 5 0.017, x2 5 88.42
R
exp 5 0.109, x2 5 2.255
Structure reliability factors
RB50.076, RF50.059
RB50.070, RF50.041
a Note: A pseudo-Voigt function was used to describe individual line profiles in Rietveld refinements. Background intensities were selected by hand or
obtained by using a fifth-order polynomial function in the analyses of X-ray and neutron diffraction data, respectively.
spinning) NMR spectra were recorded, at several spinning
rates and under static conditions, on a Bruker MSL-300
spectrometer. The resonance frequency was 96.216 MHz
for 11B and 282.282 MHz for 19F. The 11B and 19F
chemical shifts were referenced to Et2OBF3 and C6F6 at 0
ppm, respectively. Static and MAS experiments were
performed at room temperature by using a commercial
double bearing probe with 4-mm rotors at a maximum 15
kHz spinning speed. A Hahn spin echo sequence with
sixteen phase cycling was applied for refocusing mag-
netisation lost in the dead time and distorted by the signal
probe. The signal acquisition starts at the top of the echo.
The main recording parameters, pulse length pl1 (pl2 52
pl1), delay time t and spinning frequencies ns are reported
in Table 2. 11B (I 5 3/2) is a quadrupolar nucleus, conse-
quently, a selective irradiation technique was used in order
to irradiate only the 1/2→21/2 transition. The isotropic
chemical shift (diso), the quadrupolar coupling CQ 5
eQVZZ /h and the asymmetry parameter hQ were deter-
mined by fitting the experimental spectra with an extended
version of Bruker WINFIT program [18].
3. Structure determination
Indexing of the powder X-ray diffraction pattern was
performed by using the program TREOR-90 [10] and by the
successive dichotomy method of the program DICVOL-91
[11] applied to the first sixteen lines, excluding second and
third harmonics. A solution, with high figures of merit
[19,20], was obtained by TREOR-90 (M(16)562, F(16)5
71) and confirmed by DICVOL-91. The symmetry is mono-
Table 2
˚
clinic and the cell parameters are: a56.879(2) A, b5
Conditions of NMR experiments for Zn2(BO3)(OH)0.75F0.25
˚
˚
4.922(1) A, c55.705(1) A and b598.80(3)8 (TREOR-90
solution). The cell dimensions were refined by using the
program CELREF [21]. Existence conditions of reflections
(0k0: k 5 2n) lead to two possible space groups: centric
P21 /m (no. 11), excluded by SHG test, and P21 (no. 4),
which was used for the structure determination.
Nucleus
(spin)
Liquid p/2
pulse (ms)
t
ns
pl1
(ms)
Spinning
speed (kHz)
(ms)
(kHz)
11B (3/2)
19F (1/2)
19F (1/2)
6.8
4.5
4.5
100
69
70
10
200
200
2
4.5
4.5
10
10
Static

290
G. Corbel et al. / Journal of Alloys and Compounds 315 (2001) 287–295
Table 4
Selected
In a first stage, a partial structure determination was
˚
interatomic
distances
(A)
and
angles
(8)
in
conducted from X-ray diffraction data. The presence of
b-HBO2 was not taken into account in the refinement
owing to the weak intensity of observed reflections. The
observed structure factors were extracted with FULLPROF
[12] in the ‘Pattern matching’ mode. The TREF option of
SHELXS-86 program [14] gave the positions of two zinc
atoms on 2a crystallographic sites. Successive Fourier
difference syntheses (SHELX-76 [13]) allowed to locate four
oxygen (R1 5 0.12). However, the calculated zinc–oxygen
distances deduced from the refinement of zinc and oxygen
Zn2(BO3)(OH)0.75F0.25
Zn(1) polyhedron
Zn–O(4) 1.92(1)
Zn–O(4) 1.93(1)
Zn–O(3) 1.94(1)
Zn–O(2) 1.95(1)
Zn(2) polyhedron
Zn–O(1) 1.92(2)
Zn–O(2) 1.94(1)
Zn–O(3) 1.97(1)
Zn–O(1) 1.99(1)
˚
˚
kZn(1)–Ol51.94 A
kZn(2)–Ol51.96 A
Borate BO33– ions
B–O(1) 1.37(1)
B–O(4) 1.39(1)
B–O(3) 1.40(2)
O(1)–B–O(3) 121(1)
O(3)–B–O(4) 117(1)
O(4)–B–O(1) 122(1)
˚
coordinates, in FULLPROF, are either too short (1.78 A) or
˚
kB–Ol51.39 A
˚
too long (2.44 A) in spite of good reliability factors.
Hydroxide ion OH2
O(2)–H 0.99(1)
Consequently, the distance soft constraint option of
˚
FULLPROF was applied (dZn–O 5 1.95(5) A and dB–O 5
˚
1.39(5) A).
X-ray diffraction data, leads to the reliability factors Rp 5
0.131 and Rwp 5 0.161 (Table 1).
The observed and calculated X-ray and neutron patterns
are shown in Fig. 1; the final atomic coordinates and
temperature factors are reported in Table 3 and selected
interatomic distances are listed in Table 4.
A bond valence analysis was calculated using the
scheme proposed by Zachariasen [22] and Brown [23]. The
bond valence, s, is calculated as a function of interatomic
distances, di, and summed in the coordination sphere of a
Then, from the analysis of IR spectrum and the consid-
eration of oxygen–oxygen distances, boron atoms were
assumed to be in triangular coordination with the oxygen
atoms O(1), O(3) and O(4). As a consequence, hydrogen
atoms should be associated with O(2) atoms. An approx-
imative position (1/2, 0, 1/2) is imagined for this hydro-
gen atom in order to point towards the center of the most
distorted hexagonal rings of oxygen atoms.
In a second step, neutron diffraction data were used in
order to locate accurately the hydrogen and boron atoms.
The previous set of atomic coordinates was refined without
distance constraints (Table 3). Oxygen, boron and hydro-
gen thermal motion parameters were refined to the same
value. It must be noted that fluorine and oxygen are not
distinguished by neutron (and X-ray) diffraction. As a
reason for OH2↔F2 substitution, fluorine atoms occupy
statistically the O(2) position; the proportion of fluorine
was obtained from NMR experiments. Subsequently, the
O(2) label, in Table 3 and the figures, refers to
(O0.75F0.25). Correlatively, the site occupancy factor of
hydrogen site was fixed to 0.75. The resulting reliability
factors are quite high (Rp 5 0.179 and Rwp 5 0.164) and
this can be explained by a poor counting statistic due to the
presence of boron and hydrogen, strong absorbers of the
neutron scattering. This final set of coordinates (with
given
atom.
The
calculated
Ss
values
for
Zn2(BO3)(OH)0.75F0.25 (Table 5) are consistent with the
proposed formulation.
4. Structure description
In the crystal structure of Zn2(BO3)(OH)0.75F0.25, Zn21
cations reside on slightly distorted tetrahedral sites while
boron atoms adopt a triangular coordination. The arrange-
ment of boron atoms of BO332 groups in this structure can
be described by an hexagonal lattice with the cell parame-
˚
ters aH ¯ bH 5 b 5 4.9 A (Fig. 2 dashed lines).
Borate ions lie in the (2202) planes which define dense
2
˚
layers of quasi flat lying triangles [24]. The oxygen atoms
B
overall 5 0.80(2) A ), introduced in the refinement of
Table 3
Atomic coordinates and temperature factors in Zn2(BO3)(OH)0.75F0.25
2
a
˚
Atom
Site
x
y
z
Biso (A )
S.O.F.
Zn(1)
Zn(2)
O(1)
O(2), F
O(3)
O(4)
B
2a
2a
2a
2a
2a
2a
2a
2a
0.145(1)
0.375(1)
0.643(1)
0.613(1)
0.806(1)
0.918(1)
0.212(1)
0.533(2)
0
0.552(1)
0.068(1)
0.030(1)
0.584(1)
0.161(1)
0.350(1)
0.824(1)
0.517(3)
0.5(1)
0.5(1)
1.0
1.0
1.0
0.75, 0.25
1.0
1.0
1.0
0.75
0.983(3)
0.856(2)
0.482(2)
0.277(2)
0.862(2)
0.494(2)
0.327(3)
0.61(7)
0.61(7)
0.61(7)
0.61(7)
0.61(7)
0.61(7)
H
a Biso [26].

G. Corbel et al. / Journal of Alloys and Compounds 315 (2001) 287–295
291
Table 5
a
Valence bond analysis of Zn2(BO3)(OH)0.75F0.25
Zn(1)
Zn(2)
B
H
Ss
Expected
2
O(1)
0.558
0.462
0.501
0.487
1.003
2.02
O(2), F
O(3)
O(4)
0.488
0.528
0.558
0.543
0.673
1.66
1.94
2.05
1.75
2
2
0.925
0.950
Ss
expected
2.12
2
2.01
2
2.88
3
0.67
0.75
a B–O s 5 exp[(1.371 2 di)/0.37] Zn–O s 5 exp[(1.704 2 di)/0.37]
H–O s 5 exp[(0.95 2 di)/0.37] Zn–F s 5 exp[(1.620 2 di)/0.37]
(values from Ref. [22]).
of hydrogen-bonded hydroxyl ions, extending along the b
of three adjacent borate groups form the triangular bases of
anionic tetrahedra in which Zn21 are inserted (S1 and S2
positions in Fig. 2).
axis (dashed line in Fig. 4). This hydrogen bond (2.01(2)
˚
A) formation strengthens the stability of the structure and
leads to strongly distorted hexagonal tunnels, centred on
the b axis.
The Zn(1)O3(OH,F) and Zn(2)O3(OH,F) tetrahedra,
alternately up and down as indicated by (1) or (2) in Fig.
2, form, respectively, two infinite ZnO O OH,F
chains running along the b axis. These chains, linked
s
d
g`
f
2 / 2
5. Structural correlations
together by oxygen O(3), build up infinite layers
ZnO OH,F . Then, adjacent layers, shifted one from
3 / 2s
d
g`
f
¢
The structural arrangement of the tetrahedra in
Zn2(BO3)(OH)0.75F0.25 can be compared with that found
in tridymite SiO2 [6,7] (Fig. 3). Two modifications of the
tridymite network lead to the topology present in
¢
each other (translation vector aH 1 1/2 bH), are connected
by the hydroxyl anions O(2)H2 and the resulting 3D
network is ZnO OH,F
.
d1 / 2
g
3 / 2s
f
All OH2 groups (dO–H 50.99(1) A) point towards the
˚
Zn2(BO3)(OH)0.75F0.25
:
[0–10] direction in order to build up perfect zigzag chains
Fig. 2. Layer of tetrahedra and hexagonal lattice of borate ions in Zn2(BO3)(OH)0.75F0.25 (a), rotations of the tetrahedra in the (2202) layers (b) and tilting
of these tetrahedra (c).

292
G. Corbel et al. / Journal of Alloys and Compounds 315 (2001) 287–295
• alternating rotations (ar 5 37.78) of the tetrahedra
around the hexagonal c axis. The threefold coordination
of boron atoms is then ensured.
• tilting of the tetrahedra from the hexagonal axis. Large
reductions from 1808 of the Zn–O–Zn angles occur:
b1 5 110.68, b2 5 106.28, b3 5 108.18 (Fig. 2). More-
over, the shift of the apical oxygen O(2) causes a strong
corrugation of the layers ZnO OH,F : b4 5119.78
3 / 2s
d
f
g`
(Fig. 2).
It was recently mentioned by Xia [1] that similar layers
of tetrahedra and borate groups can be only found in
beryllates MBe2(BO3)F2 (M5Na [9],
K
[2]),
Be2(BO3)OH, H2O [25] and Be2(BO3)F [8] with beryl-
lium in fourfold coordination. All the structures, deter-
mined in acentric space groups, present up and down
tetrahedra. Consequently, the acentric character, in a first
approximation, can be due to the borate groups except in
berborite polytypes, Be2(BO3)OH, H2O, for which the
ordering of hydroxyl ions and water molecules implies the
non-centrosymmetry. The orientation of BO332 triangles
can be represented by a vector (or spin), parallel to one of
the B–O bonds. A parallel (ferromagnetic) setting of the
vectors in successive layers leads to acentric structures
Fig. 3. Projection, on the (a,b) plane, of the layers of tetrahedra found in
tridymite SiO2.
Fig. 4. Perspective view, along the b axis, of the corrugated sheets and of the zigzag chains of hydrogen-bonds (dashed lines) in Zn2(BO3)(OH)0.75F0.25
.
Fig. 5. Acentric (a) and centric (b) arrangement of borate layers in Zn2(BO3)(OH)0.75F0.25, NaBe2(BO3)F2 or KBe2(BO3)F2 and Be2(BO3)F, respectively.

G. Corbel et al. / Journal of Alloys and Compounds 315 (2001) 287–295
293
Table 6
Structural comparison of fluoride beryllates with Zn2(BO3)(OH)0.75F0.25
Compound
Tetrahedron
formulation
ar
Stacking of
borate layers
Translation
between layers
(8)
¢
¢
Zn2(BO3)(OH)0.75F0.25
Be2(BO3)F
NaBe2(BO3)F2
KBe2(BO3)F2
ZnO3(OH,F)
BeO3F
BeO3F
37.7
33.4
34.8
33.4
/A(1)/B(1)/
/A(1)/B(2)/
/A(1)/B(1)/
/A(1)/B(1)/C(1)/
a
H 1 1/2 bH
¢
¢
a
H 1 1/2 bH
¢
1/2 bH
¢
¢
2/3 aH 1 1/3 bH
BeO3F
while antiparallelism (antiferromagnetism) gives centric
structures (Fig. 5). Then, a simple notation can be defined:
the layers, which are shifted one from each other, are
represented by A or B or C and the symbols (1) or (2)
indicate the orientation of the vectors. The stacking of the
layers, given in Table 6, shows that NaBe2(BO3)F2,
KBe2(BO3)F2 and Zn2(BO3)(OH)0.75F0.25 are acentric.
Surprisingly, Be2(BO3)F is described by a centric stacking
of the layers. This simple observation allowed us to find
that a translation (21/4, 20.19, 21/4) of the atomic
coordinates, given per the authors in [8], leads to a centric
solution, within 60.01 for the atomic coordinates. This
structure should be revised to a C2/n space group with 2
axis at (1/4, y, 1/4) and n plane at (x, 0, z).
6. NMR results
The
11B
MAS-NMR
spectrum
of
Zn2(BO3)(OH)0.75F0.25 (Fig. 6), contaminated with a
small amount of b-HBO2, exhibits two contributions,
broadened by second order quadrupolar perturbation. The
main component, due to Zn2(BO3)(OH)0.75F0.25, is typical
of 11B nucleus in axial symmetry (diso 5 19.360.5 ppm,
CQ 5 2620650 kHz, hQ 5 0.0060.01). The second com-
ponent, characterized by a small peak at 20.5 ppm, can be
attributed to tetrahedrally coordinated boron atom of b-
HBO2 [27,28]. This phase presents also two boron atoms
in triangular coordination. The 11B MAS-NMR spectrum
of this phase was recorded and is given, at an enlarged
scale, at the bottom of Fig. 6. Only the tetrahedral boron
contribution appears clearly in the spectrum of the mixture.
The fluorine content of Zn2(BO3)(OH)0.75F0.25 was
estimated with a static NMR experiment by using the
compound Na3Al2(PO4)2F3 [29] as a standard. The 19F
Fig. 6. 11B MAS-NMR spectra of (a) Zn2(BO3)(OH)0.75F0.25 (contami-
nated with a small amount of monoclinic b-HBO2) and (b) monoclinic
b-HBO2.
Fig. 7. 19F MAS-NMR spectrum (arrows indicate spinning sidebands) and a part of IR spectrum of Zn2(BO3)(OH)0.75F0.25
.

294
G. Corbel et al. / Journal of Alloys and Compounds 315 (2001) 287–295
Table 7
optically active for second harmonic generation. MAS-
NMR experiments confirm the triangular coordination of
boron atoms and the presence of fluoride ions. The infinite
19F MAS-NMR parameters for Zn2(BO3)(OH)0.75F0.25
Environment
type
diso
(ppm)
Line width
(ppm)
Relative intensity
ZnO OH,F
3 / 2s
layers can be compared with that found
d
f
g`
Exp. (65%)
Calc.
in SiO2 tridymite and in several beryllates. A simple
notation of the borate layers is introduced in order to
compare their stacking in centric and acentric compounds.
1
2
3
27.0
216.8
255.4
2.0
2.4
9.6
14
36
50
6
38
56
MAS-NMR spectrum of Zn2(BO3)(OH)0.75F0.25 exhibits
three distinct and isotropic peaks, centered at 27.0, 216.8
and 255.4 ppm (Fig. 7); their relative intensity, calculated
without taking into account the spinning side bands, is
14/36/50 (65%) (Table 7). They can be attributed to
three distinct environments of fluorine atoms which substi-
tute with hydroxyl groups (Fig. 8). A statistical repartition
of xF2 and (1 2 x)OH2 can be assumed. The probability
P(m, x) that one F2 ion is surrounded by m F2 as first
neighbours is given by the binomial distribution function:
Acknowledgements
The authors thank Dr. V. Maisonneuve for the Energy
Dispersive X-ray analysis, Dr. P. Aschehoug (LCAES,
ENSC Paris) for the SHG test and S. Grolleau (IMN,
Nantes) for the density measurements.
References
2!
P(m,x) 5 xm ? (1 2 x)22m
?
( m 5 2, 1 or 0)
]
m!(2 2 m)!
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