High Fraction of Penta-coordinated Aluminum and Gallium in Lanthanum–
Aluminum–Gallium Borates
Flaviu R. V. Turcu,
‡
Ago Samoson,
§
Maria Maier,
‡
Diana L. Trandafir,
‡
and Simion Simon
‡,†
‡
Faculty of Physics, Institute of Interdisciplinary Research in Bio-Nano-Sciences, National Centre of Magnetic Resonance,
Babes ß-Bolyai University, Cluj-Napoca, Romania
§
NMR Institute, Tallinn University of Technology, Tartu Teaduparks, Tehnomeedikum, Tallinn, Estonia
This study is focused on structural changes induced by increas-
ing treatment temperature of sol-gel–derived La
2
O
3
∙Al
2
O
3
∙
Ga
2
O
3
∙5B
2
O
3
system. The structure of samples heated for
30 min up to 900°C was investigated by X-ray diffraction
(XRD), Fourier transform infrared (FTIR) spectroscopy, and
magic-angle spinning nuclear magnetic resonance (MAS-NMR)
analysis of
27
Al,
11
B, and
71
Ga nuclei. The vitreous structure is
preserved inclusively after 800°C treatment, and starting with
850°C the only crystalline phase evidenced in XRD patterns is
of LaAl
2.03
B
4
O
10.54
type, of La(Al,Ga)
2.03
B
4
O
10.54
composi-
tion. The FTIR results point out the presence of BO
3
, AlO
4
,
and AlO
6
, and starting with 800°C treatment also of BO
4
and
AlO
5
structural units, but more detailed information related to
boron, aluminum, and gallium environments is obtained from
the analysis of MAS-NMR data. These data evidenced in both
amorphous xerogels and in crystallized samples a high fraction
of penta-coordinated aluminum and gallium.
Keywords: borates; crystals/crystallization; Fourier transform
infrared spectroscopy; gallium/gallium compounds; nuclear
magnetic resonance
I. Introduction
L
anthanum- and gallium-aluminoborate systems attracted
special interest first for optical applications and then, by
tailoring their structure, they were also considered for poten-
tial applications as support materials for catalysis.
1–5
The
presence of rare earths in these materials induces beside
catalytic properties interesting magnetic properties.
6
All these
properties strongly depend on materials structure. At the
same time it is was shown that the borate glass systems
containing aluminum and gallium prove a more stable glass
network on account of AlO
4
and GaO
4
structural units
which may cross-link the neighboring borate chains.
7
It is
expected that both in amorphous and crystalline porous
materials, the presence of unusual coordinated cations, like
penta-coordinated aluminum or/and gallium, would lead to
more catalytically active sites.
The aim of this study is to characterize the evolution of
the local structure in amorphous and crystallized lanthanum–
aluminum–gallium borates of La
2
O
3
∙Al
2
O
3
∙Ga
2
O
3
∙5B
2
O
3
composition, using X-ray diffraction, and infrared and
nuclear magnetic resonance data.
II. Experimental Procedure
The sol-gel–derived La
2
O
3
∙Al
2
O
3
∙Ga
2
O
3
∙5B
2
O
3
system was
prepared from aquous solutions of La(NO
3
)
3
Á6H
2
O, Al
(NO
3
)
3
Á9H
2
O, Ga(NO
3
)
3
ÁxH
2
O, and H
3
BO
3
of analytical
purity grade, by thermal decomposition of starting nitrates
and boric acid sustained by simultaneous oxidation of glyc-
erol (10 wt%). The mixture solved in desalinized water
appears as a transparent solution at room temperature. A
viscous gel is obtained after 2 h heating at 95°C. The further
heating at this temperature leads to a spongious solid sample.
Xerogel samples were obtained by heating for 30 min at dif-
ferent treatment temperatures, T
t
, between 250°C and 900°C.
After thermal treatment, the samples were removed from the
furnace and cooled down to room temperature.
The differential thermal analysis (DTA) and thermogravi-
metric analysis (TGA) runs were recorded with DTG-60H
Shimadzu (Shimadzu Corporation, Kyoto, Japan) derivato-
graph at a heating rate of 10°C/min from room temperature
to 1000°C. Alumina open crucibles and a-alumina powder as
reference material were used, and the measurement was made
in flowing atmosphere of nitrogen/air at a flow rate of
70 mL/min.
X-ray diffraction (XRD) analyses were made on a Shi-
madzu XRD-6000 diffractometer using Ni-filtered CuK
a
radi-
ation (k = 1.5418
A) at a scanning speed of 2°/min. The
chemical analysis of the crystalline phase identified by XRD
was carried out by energy-dispersive X-ray (EDX) spec-
troscopy with a FEI Quanta 3D FEG dual-beam scanning
electron microscope using EDAX Genesis FEI software
package for SEM–EDX system.
Fourier transform infrared (FTIR) spectra were recorded
in reflection configuration in the range 4000–400 cm
À1
with
spectral resolution of 4 cm
À1
using a Jasco FT-IR-6000 spec-
trometer (JASCO Corporation, Tokyo, Japan) and KBr pel-
let technique.
Magic-angle spinning nuclear magnetic resonance (MAS-
NMR) spectra were recorded at room temperature for the
11
B,
27
Al, and
71
Ga nuclei in glass and glass–ceramic powdered
samples spun at the magic angle 54.74° with a frequency of 20,
30, and 40 kHz, respectively, using a Bruker AVANCE 600
MAS-NMR (Karlsruhe, Germany). The deconvolution of
spectra was performed by using Dmfit program.
8
III. Results and Discussion
The thermal analysis traces (Fig. 1) recorded after drying at
95°C of sol-gel–derived sample evidence several events. First
weight losses observed in TG curve around 100°C are due to
the loss of free water and physically adsorbed water, accom-
panied by endothermic peaks in DTA trace.
The weight loss between 150°C and 300°C corresponds to
removal of water and glycerol entrapped in the pores, and to
decomposition of glycerol and residual nitrates, accompanied
E. C. Dickey—contributing editor
Manuscript No. 37925. Received December 15, 2015; approved March 25, 2016.
†
Author to whom correspondence should be addressed. e-mail: simons@phys.
ubbcluj.ro
1
J. Am. Ceram. Soc., 1–6 (2016)
DOI: 10.1111/jace.14265
© 2016 The American Ceramic Society
J
ournal