Hydrothermal synthesis and structural characterization of new lanthanide coordination polymers with pimelic acid and 1,10-phenanthroline

Article abstract of DOI:10.1016/j.molstruc.2004.01.027

This paper presents three new lanthanide coordination polymers [Pr ₂(pim)₃(phen)₂(H₂O)] _n·3nH₂O (1) (H₂pim=pimelic acid; phen=1,10-phenanthroline) and [Ln(Hpim)(pim)(phen)]_n·1. 5nH₂O (Ln=Nd, 2; Er, 3) prepared by the hydrothermal reaction. The structural details of complexes 1, 2 and 3 are reported. In complex 1, the Pr(III) ions are bridged by pim in three modes to form 2D layers. Adjacent layers are assembled by hydrogen bonds and π-π stacking between phen ligands into a 3D network. In complex 2, the Nd(III) ions are connected by pim and Hpim in two modes into 1D chains. Hydrogen bonding and π-π stacking between chains result in a 3D supramolecular structure. Single crystal X-ray diffraction analysis shows that complex 3 is isostructural with complex 2.

Full text of DOI:10.1016/j.molstruc.2004.01.027

Journal of Molecular Structure 692 (2004) 169–175
www.elsevier.com/locate/molstruc
Hydrothermal synthesis and structural characterization of new lanthanide
coordination polymers with pimelic acid and 1,10-phenanthroline
a,
Liang Huang , Li-Ping Zhang , Lin-Pei Jin
a
b
*
a
Department of Chemistry, Anyang Teachers College, Anyang 455000, China
b
Department of Chemistry, Beijing Normal University, Beijing 100875 China
Received 25 November 2003; revised 25 November 2003; accepted 21 January 2004
Abstract
This paper presents three new lanthanide coordination polymers [Pr
·1.5n H
2
(pim)
3
(phen)
2
(H
2
O)]
n
·3n H
2
O (1) (H
2
pim ¼ pimelic acid;
phen ¼ 1,10-phenanthroline) and [Ln(Hpim)(pim)(phen)]
n
2
O (Ln ¼ Nd, 2; Er, 3) prepared by the hydrothermal reaction. The
structural details of complexes 1, 2 and 3 are reported. In complex 1, the Pr(III) ions are bridged by pim in three modes to form 2D layers.
Adjacent layers are assembled by hydrogen bonds and p–p stacking between phen ligands into a 3D network. In complex 2, the Nd(III) ions
are connected by pim and Hpim in two modes into 1D chains. Hydrogen bonding and p–p stacking between chains result in a 3D
supramolecular structure. Single crystal X-ray diffraction analysis shows that complex 3 is isostructural with complex 2.
q 2004 Elsevier B.V. All rights reserved.
Keywords: Lanthanide; Coordination polymer; Hydrothermal synthesis; Crystal structure; Infrared
1
. Introduction
and phen [9]. To the best of our knowledge, binary
lanthanide pimelates have been reported [10]. However,
no lanthanide complex containing both the ligands of
pimelic acid and 1,10-phenanthroline has been reported.
Herein, we report the synthesis and crystal structures of three
In recent years, the design and synthesis of metal–
organic coordination polymers have received increasing
interest because such supramolecular assembly is a central
theme in the design of new materials with unusual structures
and properties [1–3]. The multifunctional rigid ligands such
as 1,2,4,5-benzenetetracarboxylic acid [4], 1,3,5-benzene-
tricarboxylic acid [5], and 1,4-benzenedicarboxylic acid [6]
are used extensively as linkers in constructing coordination
polymers. Despite examples of coordination polymers
containing flexible ligands have appeared in the literature
in recent years, much of the work has so far been focused on
coordination polymers containing transition metal and post-
transition metal elements including Fe, Co, Ni, Cu, Ag, Zn,
Cd, and Hg [7]. Lanthanide metal complexes have been
rarely investigated [8]. In this work, we select pimelic acid
and 1,10-phenanthroline as ligands and adopt hydrothermal
technique to construct new lanthanide complexes with
characteristic structures and get better understanding of
ternary lanthanide complexes with a,v-dicarboxylic acid
lanthanide complexes [Pr (pim) (phen) (H O)] ·3nH O (1)
2
3
2
2
n
2
and [Ln(Hpim)(pim)(phen)] ·1.5nH O (Ln ¼ Nd, 2; Er, 3),
n
2
which have supramolecular architecture assembled from 1D
chain or 2D layer via hydrogen bonding and p–p stacking.
2. Experimental
2.1. Materials and apparatus
LnCl ·nH O (Ln ¼ Pr, n ¼ 7; Ln ¼ Nd and Er, n ¼ 6)
3
2
were prepared by dissolving their oxides in dilute
hydrochloric acid, respectively, and then dried. All other
chemicals were purchased and used as received without
further purification. C, H and N data were obtained using an
American PE 2400 II CHNS/O elemental analyzer. Infrared
spectra were recorded with an American Nicolet Avatar 360
FT-IR spectrometer.
*
Corresponding author. Tel./fax: þ86-037-22902048.
E-mail address: lhuang@aytc.edu.cn (L. Huang).
0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2004.01.027

170
L. Huang et al. / Journal of Molecular Structure 692 (2004) 169–175
2
.2. Preparation of complexes
1407(vs), 1346(w), 1310(w), 1283(w), 1259(w), 1209(w),
1
105(w), 1091(w), 851(s), 732(s), 635(w).
LnCl ·nH O (0.3 mmol; Pr, n ¼ 7, 0.1120 g; Nd, n ¼ 6,
3
2
2
.3. Single-crystal X-ray diffraction
0
0
.1076 g; Er, n ¼ 6, 0.1145 g) and H pim (0.3 mmol,
2
.0481 g) were dissolved in 10 ml deionized water,
Single-crystal X-ray data were collected on a Bruker
SMART 1000 CCD diffractometer equipped with graphite
respectively, to which phen·H O (0.3 mmol, 0.0595 g) was
2
added and the pH value was adjusted to about four with
NaOH aqueous solution. The mixture was placed in a
Teflon-lined stainless steel vessel (23 ml). The vessel was
sealed and heated (Pr, at 160 8C for 3 d; Nd, at 150 8C for
˚
monochromatized Mo Ka radiation (l ¼ 0:71073 A).
Semiempirical absorption corrections were applied using
the SADABS program. All calculations were carried out
with use of SHELXS-97 and SHELXL-97 programs [11]. The
structures were solved by the direct methods. All structures
3
d and Er, at 190 8C for 3 d) under autogeneous pressure
and then cooled to room temperature. After filtering, the
product was washed with ethanol and then dried under
ambient. Columnlike crystals of the complex 1 were
collected in yield (0.0681 g, 38.2%). After evaporation of
the filtrates for several days at room temperature, rhombus
platelike crystals of the complexes 2 and 3 were obtained,
respectively (yield: 4.9% for 2; 5.1% for 3).
2
were refined on F by full-matrix least-squares methods.
The crystallographic data of the complexes are summarized
in Table 1 and the coordination and hydrogen bond lengths
in Table 2.
Table 2
˚
Coordination and hydrogen bond lengths (A) for complexes 1, 2 and 3
[Pr (pim) (phen) (H O)] ·3n H O 1. Found (calcd., %):
C, 45.24(45.46); H, 4.52(4.58); N, 4.26(4.17). IR(KBr
2 3 2 2 n 2
Coordination bonds in complex 1
2
pellet, cm ): 3448(s, br), 3073(w), 3058(w), 2933(m),
1
Pr(1)–O(6)#1
Pr(1)–O(9)
Pr(1)–O(1)
Pr(1)–O(2)#1
Pr(1)–O(13)
Pr(1)–O(5)
Pr(1)–O(6)
Pr(1)–N(1)
Pr(1)–N(2)
2.408(8)
2.422(8)
2.444(7)
2.468(8)
2.565(7)
2.569(8)
2.689(8)
2.698(9)
2.667(9)
Pr(2)–O(7)
2.436(8)
2.458(8)
2.457(9)
2.509(8)
2.539(9)
2.575(8)
2.616(8)
2.666(10)
2.695(10)
Pr(2)–O(4)#2
Pr(2)–O(8)#2
Pr(2)–O(11)#3
Pr(2)–O(3)
2
1
7
860(w), 1578(vs), 1560(vs), 1421(vs), 1346(w),
309(w), 1220(w), 1139(w), 1102(w), 1088(w), 848(s),
32(s), 637(m).
Pr(2)–O(12)#3
Pr(2)–O(4)
[
C, 46.72(46.69); H, 5.21(4.82); N, 4.04(4.19). IR(KBr
Nd(Hpim)(pim)(phen)] ·1.5nH O 2. Found (calcd., %):
n 2
Pr(2)–N(3)
2
pellet, cm ): 3423(m, br), 3059(w), 2934(s), 2860(w),
1
Pr(2)–N(4)
1
1
6
718(s), 1596(vs), 1587(vs), 1542(vs), 1424(vs), 1347(w),
311(w), 1142(w), 1103(w), 1088(w), 850(s), 731(s),
38(w).
Hydrogen bonds in complex 1
O(13w)–H· · ·O(14w)#4 2.939
O(15w)–H· · ·O(5)#1
O(15w)–H· · ·O(11)
2.866
2.852
O(14w)–H· · ·O(12)
2.886
2.821
[
C, 45.37(45.14); H, 4.85(4.66); N, 3.97(4.05). IR(KBr
Er(Hpim)(pim)(phen)] ·1.5nH O 3. Found (calcd., %):
O(16w)–H· · ·O(10)#5
C(36)–H· · ·O(15w)#6 3.214
n
2
2
1
Coordination bonds in complex 2
pellet, cm ): 3386(m, br), 3082(w), 3060(w), 2934(s),
856(w), 1716(s), 1610(vs), 1554(s), 1439(vs), 1424(vs),
Nd(1)–O(3)#1
Nd(1)–O(4)#2
Nd(1)–O(1)
2.395(5)
2.413(5)
2.484(5)
2.511(5)
Nd(1)–O(5)
Nd(1)–O(6)#3
Nd(1)–N(1)
Nd(1)–N(2)
2.407(5)
2
2.521(5)
2.679(6)
2.696(6)
Nd(1)–O(2)
Table 1
Crystallographic data for complexes 1, 2 and 3
Hydrogen bonds in complex 2
O(7)–H· · ·O(9w)#4
O(9w)–H· · ·O(1)#3
O(9w)–H· · ·O(6)#5
2.620
O(10w)–H· · ·O(8)#6
C(17)–H· · ·O(8)#7
2.861
3.424
1
2
3
2.754
2.794
Formula
2
Pr C
45
H
54
N
4
O
16 NdC26
H
32
N
2
O
32 2 9.5
9.5 ErC26H N O
Formula weight 1188.74
668.78
Triclinic
ꢀ
P1
691.80
Triclinic
ꢀ
P1
Coordination bonds in complex 3
Er(1)–O(5)
Crystal system
Space group
Triclinic
ꢀ
P1
2.259(10) Er(1)–O(6)#3
2.294(11) Er(1)–O(2)
2.406(10)
2.416(9)
Er(1)–O(3)#1
Er(1)–O(4)#2
Er(1)–O(1)
˚
a (A)
˚
b (A)
˚
c (A)
12.267(4)
13.903(5)
15.348(5)
89.008(7)
86.468(7)
64.754(6)
2
11.170(3)
11.720(3)
12.562(4)
11.044(6)
11.588(6)
12.608(6)
110.474(7)
100.420(8)
110.389(8)
2
2.295(10) Er(1)–N(1)
2.386(11) Er(1)–N(2)
2.563(11)
2.612(11)
a (8)
b (8)
g (8)
Z
110.454(5)
99.217(5)
111.514(5)
2
Hydrogen bonds in complex 3
O(7)–H· · ·O(9w)#4
O(9w)–H· · ·O(1)#3
O(9w)–H· · ·O(6)#5
2.615
O(10w)–H· · ·O(8)#6
C(17)–H· · ·O(8)#7
2.906
3.314
2.780
2.811
3
˚
V (A )
2363.0(14)
1.671
1352.8(7)
1.642
1330.1(11)
1.727
3
dcalcd (g/cm )
Symmetry transformations used to generate equivalent atoms in
complex 1: #1 2x þ 1; 2y þ 2; 2z þ 1; #2 2x þ 1; 2y þ 1; 2z; #3 x; y;
z 2 1; #4 x; y þ 1; z; #5 x; y 2 1; z; #6 x 2 1; y; z 2 1: Symmetry
transformations used to generate equivalent atoms in complexes 2 and 3: #1
Temperature (K) 293(2)
293(2)
676
293(2)
Fð000Þ
1196
692
2
1
m (mm
R[I . 2s(I)] R
wR
)
2.112
0.0657
0.1242
1.976
3.213
2
x; 2y þ 2; 2z þ 1; #2 x; y 2 1; z; #3 2x; 2y þ 1; 2z þ 1; #4 2x þ 1;
y þ 2; 2z þ 1; #5 2x þ 1; 2y þ 1; 2z þ 1; #6 2x þ 1; 2y þ 2; 2z þ 2;
1
0.0489
0.0867
0.0801
2
2
0.1917
#
7 2x þ 1; 2y þ 3; 2z þ 2:

L. Huang et al. / Journal of Molecular Structure 692 (2004) 169–175
171
2 3 2 2 n 2
Fig. 1. The asymmetric unit of [Pr (pim) (phen) (H O)] ·3nH O (1) showing 50% thermal ellipsoids. All hydrogen atoms are omitted for clarity.
3
. Results and discussion
ring of chelating–bridging coordination. The average length
˚
of Pr–O bonds is 2.511 A. Three kinds of crystallographi-
3
.1. Structure of [Pr (pim) (phen) (H O)] ·3nH O 1
2
cally dependent pimelate ligands exist in complex 1
(
3
2
2
n
2
Scheme 1): (i) The two carboxylate groups of the pimelate
In the asymmetric unit of complex 1, there are two types
coordinate to Pr(III) ions in chelating–bridging tridentate
and bridging bidentate modes (Scheme 1a). From C(2) to
C(6), only methylene group of C(5)–C(6) adopts a gauche
conformation [torsion angle: C(4)–C(5)–C(6)–C(7),
of Pr(III) ion environment. As shown in Fig. 1, Pr(1) is
coordinated by two nitrogen atoms from phen ligand and
seven oxygen atoms, of which, one oxygen atom [O(13)]
comes from a water molecule and six from five different
pimelate ligands. Pr(2) is also nine-coordinated and
possesses an N O environment, two nitrogen atoms come
69.4(15)8] and the other three pairs of CH groups adopt
2
anti-conformation [torsion angles: C(1)–C(2)–C(3)–C(4),
2175.9(11)8; C(2)–C(3)–C(4)–C(5), 2174.8(12)8; C(3)–
C(4)–C(5)–C(6), 178.0(11)8]. (ii) Similar to (i), the two
carboxylate groups of the pimelate coordinate to Pr(III) ions
also in chelating–bridging tridentate and bridging bidentate
modes (Scheme 1b). But the alkyl chain conformation of
2
7
from a chelating phen molecule and seven oxygen atoms
from five pimelate ligands. The Pr–N bond lengths range
˚
˚
from 2.666 to 2.698 A. The average bond length is 2.682 A.
˚
The Pr–O bond lengths range from 2.408 to 2.689 A, of
˚
˚
which the Pr(1)–O(6) (2.689 A) and Pr(2)–O(4) (2.616 A)
are quite long while the other twelve Pr–O bond lengths are
typical. The reason is probably due to the four-membered
the pimelate is different. In particular, the CH group of
2
C(11)–C(12) adopts an unfavorable eclipsed conformation
[torsion angle: C(10)–C(11)–C(12)–C(13), 2113(2)8]
Scheme 1. Coordination modes of pimelate groups in complexes 1 and 2, showing different alkyl chains conformation of pimelate [a, b, c and d (gauche-form);
e (anti-form)].

172
L. Huang et al. / Journal of Molecular Structure 692 (2004) 169–175
Fig. 2. Projection down the a-axis showing 2D layer structure connected by three types of pimelate ligands (All H atoms and C atoms of phen ligands are
omitted for clarity).
and the other three pairs of CH2 groups are anti-
conformation [torsion angles: C(8)–C(9)–C(10)–C(11),
adopt anti-conformation [torsion angles: C(16)–C(17)–
C(18)–C(19), 178.9(11)8; C(17)–C(18)–C(19)–C(20),
172.2(11)8]. Pr(III) ions are bridged by pimelate ligands
into 2D layer structure (Fig. 2). There are two types of
crystallographically dependent phen ligands in complex 1.
They coordinate to Pr(III) ions in chelating mode and
protrude from the polymeric layer in two different
directions. One type of p–p interaction is observed
between two adjacent layers (Scheme 2a). The average
distance between two parallel phen ligands from two
1
78.4(13)8; C(9)–C(10)–C(11)–C(12), 2179.8(15)8;
C(11)–C(12)–C(13)–C(14), 168.4(19)8]. So, it is easy to
understand that C(11), C(12) and C(13) have large ellipsoids
duo to the eclipsed conformation. (iii) One carboxylate
group of the pimelate ligand coordinates to Pr(III) ion in
chelating bidentate mode and the other coordinates to Pr(III)
ion in monodentate mode (Scheme 1c). In the pimelate alkyl
chain, two pairs of CH groups adopt gauche conformation
2
˚
adjacent layers is 3.55 A. Lattice water molecules are
stabled in the crystal by hydrogen bondings between
[torsion angles: C(15)–C(16)–C(17)–C(18), 68.8(15)8;
C(18)–C(19)–C(20)–C(21), 63.7(15)8] and the other two
Scheme 2. Packing of phen ligands in complexes 1 and 2.

L. Huang et al. / Journal of Molecular Structure 692 (2004) 169–175
173
coordinated water and lattice water, carboxylate oxygen and
lattice water, and C–H of phen and lattice water (Table 2).
A triple hydrogen bond is formed by O(15w) from lattice
hydrogen bonds to lattice water molecules and C–H of
phen from adjacent chain (Scheme 1e). The alkyl chain
conformation of Hpim is anti-form [torsion angles: C(8)–
C(9)–C(10)–C(11), 179.4(7)8; C(9)–C(10)–C(11)–C(12),
2175.8(7)8; C(10)–C(11)–C(12)–C(13), 171.6(8)8;
C(11)–C(12)–C(13)–C(14), 2177.4(8)8;]. Phen ligands
coordinate to Nd(III) ions in chelating mode and locate at
both sides of the chain. p–p interactions exist between
phen ligands from adjacent chains with the average distance
#
1
water, O(15w)–H· · ·O(5) , O(15w)–H· · ·O(11) and
#
7
C(36) –H· · ·O(15w) (#1: 2x þ 1; 2y þ 2; 2z þ 1; #7:
x þ 1; y; z þ 1). The triple hydrogen bonding and p–p
stacking between polymeric layers result in 3D supramo-
lecular structure.
˚
of 3.43 A (Scheme 2b). There are two kinds of crystal-
3
.2. Structure of [Ln(Hpim)(pim)(phen)] ·1.5n H O
n 2
lographically dependent lattice water molecules in complex
2
(
Ln ¼ Nd, 2; Er, 3)
, one of them forms a triple hydrogen bond shown as dotted
line in Fig. 4 and the other due to its partial occupancy forms
a weak hydrogen bond to oxygen atom of carbonyl group
from Hpim ligand. The relevant hydrogen bond lengths are
listed in Table 2. As shown in Fig. 4, 2D network parallel to
b–c plane is formed via p–p interactions between chains.
Then 2D networks are interconnected into 3D supramole-
cular structure via three hydrogen bond bridging centers and
C–H· · ·O hydrogen bonds between chains.
Single-crystal X-ray diffraction studies reveal that
complexes 2 and 3 are isostructural. The asymmetric unit
of 2 is shown in Fig. 3. There is one type of Nd(III) ion
environment in the asymmetric unit of 2. As shown in Fig. 3,
Nd(1) is eight-coordinated and surrounded by two nitrogen
atoms from a phen molecule and six carboxylate oxygen
atoms, four of which come from pim ligands and two from
Hpim ligands. The Nd–O bond distances range from
˚ ˚
.395(5) to 2.521(5) A. The average distance is 2.455 A.
2
˚
The Nd–N bond distances are 2.679(6) and 2.696(6) A,
respectively. The pim ligands coordinate to four Nd(III)
ions in chelating bidentate and bridging bidentate modes
3.3. IR spectra
The IR spectrum of complex 1 shows characteristic
bands of the carboxylate groups in the usual region at
(
Scheme 1d). From C(2) to C(6), only methylene group of
2
1578(vs) and 1560(vs) cm for asymmetric stretching and
1
C(2)–C(3) adopts a gauche conformation [torsion angle:
C(1)–C(2)–C(3)–C(4), 271.6(8)8] and the other CH2
groups adopt anti-conformation [torsion angles: C(2)–
C(3)–C(4)–C(5), 169.5(6)8; C(3)–C(4)–C(5)–C(6),
2
1
at 1421(vs) cm for symmetric stretching (Shoulders not
listed) [12]. In both spectra of complexes 2 and 3, the
characteristic bands of the carboxylate groups occur within
2
1
1
75.0(7)8; C(4)–C(5)–C(6)–C(7) 166.5(6)8]. Nd(III) ions
are connected by pim ligands into 1D chain structure. In the
D structure, all Nd(III) ions are coplanar. The nearest
the range 1542–1610 cm for asymmetric stretching and
1
2
the range 1407–1439 cm
Comparing the spectra of three complexes, C–H stretching
for symmetric stretching.
1
2
1
˚
Nd· · ·Nd distance is 4.0503(11) A, indicating the lack of
direct metal–metal interaction. The carboxylate group of
Hpim ligand coordinates to two Nd(III) ions in bridging
bidentate mode and the undissociated carboxylic group of
Hpim ligand protrudes from polymeric chain forming
vibrations appear above 3000 cm [13,14] and character-
istic C–H out-of-plane bending vibrations are seen at about
2
1
731 and 850 cm , indicating the presence of phen ligands
[13]. The O–H (from the coordinated and lattice water
molecules or the hydroxyl of Hpim ligand) stretching bands
n 2
Fig. 3. The asymmetric unit of [Nd(Hpim)(pim)(phen)] ·1.5nH O 2 showing 50% thermal ellipsoids. The hydrogen atoms attached to C atoms are omitted for
clarity.

174
L. Huang et al. / Journal of Molecular Structure 692 (2004) 169–175
Fig. 4. Packing of the chains showing the 3D network via hydrogen bonding and p–p stacking in 2 viewed down the b-axis.
2
1
centered at 3400 cm are broadened by hydrogen bonding
14]. Asymmetric and symmetric stretching of CH from
5. Supporting information available
[
pimelate occurs at about 2900 cm [7,8,13]. In comparison
2
2
1
The crystallographic data have been deposited at Cam-
bridge Crystallographic Data Centre, CCDC Nos 224870
for 1, 224871 for 2 and 224872 for 3. Copies of this
information may be obtained free of charge from the
director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK
(E-mail: deposit@ccdc.cam.ac.uk; fax: þ44-1223-336033;
http://www.ccdc.cam.ac.uk).
of the spectrum of complex 1 with those of complexes
2
and 3, characteristic bands of CvO double bond are
2
1
21
observed at 1718(s) cm for 2 and 1716(s) cm for 3,
indicating the occurrence of Hpim ligand [14,15], which is in
accordance with the results of single crystal X-ray analysis.
4
. Conclusion
References
First examples of lanthanide coordination polymers
[1] P.J. Hagrman, D. Hagrman, J. Zubieta, Angew. Chem. Int. Ed. 38
(
1999) 2638.
constructed by H pim and phen, [Pr (pim) (phen)
2
2
2
3
[
[
2] B. Moulton, M.J. Zaworotko, Chem. Rev. 101 (2001) 1629.
3] M. Eddaoudi, D.B. Moler, H. Li, B. Chen, T.M. Reineke, M.
O’Keeffe, O.M. Yaghi, Acc. Chem. Res. 34 (2001) 319.
(
H O)] ·3nH O 1 and [Ln(Hpim)(pim)(phen)] ·1.5n H O
2 n 2 n 2
(
Ln ¼ Nd, 2; Er, 3) have been obtained by hydrothermal
synthesis and characterized by single crystal X-ray analysis.
Both the average Ln–O bond distance and coordination
number of the Ln(III) ions show lanthanide contraction in
the series. The Pr(III) ions in 1 are bridged by pim ligands
forming a layer structure, while chains in 2 and 3 are formed
by the coordination of pim and Hpim to Ln(III) ions and are
further assembled via p–p stacking and a variety of
hydrogen bonds to give a 3D network. In addition, IR
spectra of the complexes 1, 2 and 3 support the results of the
X-ray diffraction analysis.
[4] (a) R. Cao, D.F. Sun, Y.C. Liang, M.C. Hong, K. Tatsumi, Q. Shi,
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