Synthesis of scandium orthoborate powders

Article abstract of DOI:10.1134/S0020168506020117

Two polymorphs of scandium orthoborate, ScBO₃, are synthesized by adding aqueous ammonia to aqueous solutions of scandium nitrate and boric acids and calcining the resulting precipitates. Dehydration of the precipitates reaches completion below

Full text of DOI:10.1134/S0020168506020117

ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 2, pp. 171–175. © Pleiades Publishing, Inc., 2006.
Original Russian Text © E.A. Tkachenko, P.P. Fedorov, S.V. Kuznetsov, V.V. Voronov, S.V. Lavrishchev, V.E. Shukshin, I.V. Yarotskaya, N.G. Kononova, 2006, published in Neor-
ganicheskie Materialy, 2006, Vol. 42, No. 2, pp. 207–211.
Synthesis of Scandium Orthoborate Powders1
E. A. Tkachenko*, P. P. Fedorov*, S. V. Kuznetsov*,
V. V. Voronov*, S. V. Lavrishchev*, V. E. Shukshin*,
I. V. Yarotskaya**, and N. G. Kononova***
* Prokhorov General Physics Institute, Russian Academy of Sciences, ul. Vavilova 38, Moscow, 119991 Russia
** Lomonosov State Academy of Fine Chemical Technology, pr. Vernadskogo 86, Moscow, 119571 Russia
*** Institute of Mineralogy and Petrography (Novosibirsk Branch), Siberian Division, Russian Academy of Sciences,
ul. Russkaya 43, Novosibirsk, 630058 Russia
e-mail: tkatchenko@ns.crys.ras.ru
Received June 23, 2005
Abstract—Two polymorphs of scandium orthoborate, ScBO3, are synthesized by adding aqueous ammonia to
aqueous solutions of scandium nitrate and boric acids and calcining the resulting precipitates. Dehydration of
the precipitates reaches completion below 300°C, and further heating leads to highly exothermic crystallization
near 750°C. The synthesized ScBO3 powders consist of submicron-sized particles.
DOI: 10.1134/S0020168506020117
1
INTRODUCTION
mediate grindings. The effectiveness of this process is
limited by the high volatility of boric anhydride above
1000°C, which impedes the preparation of single-phase
material.
Indium and rare-earth orthoborates (including
ScBO3) activated with Ce3+, Pr3+, Tb3+, or Eu3+ have
attracted considerable attention as materials for scintil-
lators [1, 2] and phosphors [3–5], in particular for the
ultraviolet spectral region. Scandium rare-earth borates
are attractive host materials for diode-pumped lasers
[6–8]. Cr3+-doped scandium borate is of interest as a
gain medium for 787- to 892-nm tunable lasers [9].
Orthoborate-based materials offer a high UV transmit-
tance, high optical damage threshold, and good chemi-
cal stability and mechanical strength, in particular, at
high temperatures [10, 11].
Steele and Decius [21] reported the synthesis of
granular lanthanum, scandium, and indium orthobo-
rates via dissolution of appropriate oxides in nitric acid
with boric acid additions, followed by boiling down the
solution.
Lou et al. [10] and Boyer et al. [11] investigated in
detail the classic sol–gel process for the preparation of
rare-earth borates from appropriate metal alkoxides
(based on isopropanol) and a so-called “sol–gel min-
eral” process: precipitation of hydrous borates from
acid solutions via neutralization with ammonia, fol-
lowed by calcination. Reports on ScBO3 synthesis by
these processes are not available in the literature.
Scandium borate is the only compound in the
Sc2O3–B2O3 system [12]. It crystallizes in the calcite
structure (sp. gr. R3 Ò [10, 13–15]), is isostructural with
InBO3 and the high-temperature form of LuBO3 [15],
and melts congruently at 1600–1610°C [6, 12].
Newnham et al. [13] reported the preparation of
rare-earth borates from rare-earth nitrate solutions by
adding an ammoniacal solution of ammonium penta-
borate, followed by filtration and calcination of the pre-
cipitate at 1200°C. The excess of the forming boron
oxide volatilized at 1400°C.
Levin [12] described another form of ScBO3, pre-
pared by liquid quenching, but failed to index its x-ray
diffraction (XRD) pattern. We will designate it
β-ScBO3, in contrast to the calcite phase α-ScBO3.
Single crystals of α-ScBO3 are commonly grown from
high-temperature solutions [6, 9, 16].
Using indium borate as a test system, we developed
a method for the preparation of nanocrystalline pow-
ders through so-called borate rearrangement [17, 18].
During heating, hydrous borate precipitates loose water
and turn into an amorphous state. As a result, crystalli-
zation occurs upon a significant superheating, is
accompanied by active heat release, and leads to the
formation of very small particles.
Techniques for the preparation and characterization
of RBO3 powders were described in [5, 10–13, 17–21].
Rare-earth orthoborates can be synthesized by react-
ing rare-earth oxides with boric anhydride or boric acid
at 850–1300°C [5, 20, 21] for several hours with inter-
1
Presented in part at the XI National Conference on Crystal
Growth, Moscow, Russia, December 14–17, 2004.
171

172
pH
TKACHENKO et al.
An Sc(NO3)3 solution (0.029 0.001 M) was pre-
pared by heating scandium nitrate powder in distilled
water (50–60°C). Boric acid solutions were prepared
by dissolving B2O3 in boiling water (0.161 M in terms
of H3BO3) and by dissolving HBO2 in cold water
(0.227 M). The scandium nitrate solution and one of the
acid solutions were poured together, and the mixture
was titrated by aqueous ammonia until the formation of
a gel-like precipitate. During titration, we monitored
the solution pH (Hanna Instruments Checker) as a func-
tion of the volume of added NH4OH (Fig. 1). The pre-
cipitate was collected on a two-layer ashless blue rib-
bon filter paper, repeatedly washed with water, and left
to dry in air for 2–3 days.
10
8
6
4
2
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Volume, ml
Fig. 1. Solution pH as a function of the volume of added
aqueous ammonia for the titration of a scandium borate
nitrate solution (precipitation of a precursor to α-ScBO ).
Thermal analysis (DTA + TG + DTG) was carried
out with a MOM Q-1500 system in air. Samples were
placed in alundum crucibles and heated to 1000°C at a
rate of 10°C/min. XRD measurements were performed
on a DRON-2 powder diffractometer with CuKα radia-
tion. Raman spectra were recorded on a Spex Ramalog
1403 (1-41) spectrometer under excitation with the
488-nm argon laser (ILA 120) line. Powder morphol-
ogy was examined by scanning electron microscopy
(SEM) on a JEOL-5910. To prevent specimen charging,
a gold layer was evaporated onto the powder.
3
The purpose of this work was to prepare scandium
orthoborate powders by this method.
EXPERIMENTAL
As starting chemicals, we used reagent-grade scan-
dium nitrate Sc(NO3)3 · 4H2O, reagent-grade boric acid
H3BO3, OSCh 12-3 boric anhydride B2O3, analytical-
grade aqueous ammonia (RF State Standard GOST
3760-79), and distilled water. Metaboric acid, HBO2,
was prepared by dehydrating boric acid at 150°C. We
obtained the metastable form HBO2-II with lattice
parameters a = 7.12 Å, b = 8.84 Å, c = 6.77 Å, and β =
93.3° (JCPDS PDF, 22-1109).
RESULTS AND DISCUSSION
Our results demonstrate that both forms (α and β) of
ScBO3 can be prepared by precipitation from aqueous
solutions followed by calcination, depending on the
DTG
TG
DTG
TG
755°C
730°C
813°C
DTA
DTA
632°C
300°C
150°C
266°C
140°C
T
T
Fig. 2. Thermal analysis of solvated scandium borate pre-
Fig. 3. Thermal analysis of solvated scandium borate pre-
pared using metaboric acid; sample weight of 207 mg, heat-
ing to 1000°C.
pared using B O ; sample weight of 304.5 mg, heating to
2
3
1000°C.
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SYNTHESIS OF SCANDIUM ORTHOBORATE POWDERS
173
150–300°C. The sample prepared with the use of B2O3
(Fig. 2) looses 94 wt % on heating to 300°C and an
additional 2 wt % on heating to 632°C. At 755°C, an
exothermic event begins. For the sample prepared using
HBO2 (Fig. 3), the weight loss in the range 150–300°C
is 96%, and the exotherm begins at 730°C.
The XRD patterns taken after heating the samples to
1000°C in DTA scans are displayed in Fig. 4. The sam-
ple prepared with the use of B2O3 has the calcite struc-
ture (α-ScBO3). The lattice parameters computed with
1
Powder-2 in space group R3 c (rhombohedral symme-
try) agree with those reported by Levin [12]: a =
4.736 Å, c = 15.34 Å. The XRD pattern of the sample
prepared with the use of HBO2 is very similar to that
reported by Keszler and Sun [15] for β-ScBO3, but we
failed to index it or to determine the lattice parameters.
2
10
20
30
40
50
2θ, deg
Fig. 4. XRD patterns of scandium orthoborate samples:
(1) α-ScBO , prepared using HBO ; (2) β-ScBO , pre-
The Raman spectra of the two scandium borate
polymorphs are shown in Fig. 5. The spectrum of
α-ScBO3 (spectrum 2) shows peaks at 237, 365, 644,
3
2
3
pared using B O .
2
3
936, 1220, and 1470 cm–1, which correspond to the
vibration frequencies of the [BO3]3– ion in the structure
of α-ScBO3 (calcite type). As shown by Voron’ko et al.
[22], the Raman spectrum of InBO3 differs in the rela-
tive intensities of peaks due to [BO3]3– from the spectra
of other orthoborates, e.g., LuBO3. In particular, the
acid solution used. We obtained α-ScBO3 using HBO2
and β-ScBO3 using B2O3 as a starting reagent.
Titration of the nitrate solution of scandium and
boric acids with aqueous ammonia leads to the forma-
tion of a white gel-like precipitate, which settles to the
bottom in several hours. The process reaches comple- totally symmetric mode of the [BO3]3– anion near
930 cm–1 is comparatively weak, which is attributable
to the significant covalent contribution to the indium–
oxygen bonds. This makes the free [BO3]3– anion
approximation inapplicable in the case of InBO3. As
tion at pH 8.2 (Fig. 1). The precipitate thus obtained is
x-ray amorphous.
According to thermal analysis results (Figs. 2, 3),
the precipitate looses nearly all of the water in the range
1
2
3
4
500
1000
1500
Raman shift, cm–1
Fig. 5. Raman spectra of (1) InBO , (2) α-ScBO , (3) LuBO (calcite structure), and (4) β-ScBO .
3
3
3
3
INORGANIC MATERIALS Vol. 42 No. 2 2006

174
TKACHENKO et al.
ScBO3 synthesis through precipitation of hydrous
borates from aqueous solutions followed by calcination
was not reported previously. This approach has the
advantages of ease for implementation and low energy
consumption. The powders thus synthesized can be
used both in fabricating phosphors and in preparing
charges for the crystal growth of ScBO3 and scandium
rare-earth borates.
Thus, borate rearrangement appears to be a viable
approach to producing nanoparticles and fine powders
of various borates.
1 µm
(a)
ACKNOWLEDGMENTS
We are grateful to A.A. Sobol’ (Prokhorov General
Physics Institute, Russian Academy of Sciences) for
useful discussions and to Rachid Mahiou (Université
Blaise Pascal, Clermont-Ferrand, France) for his con-
tinuous interest in our work.
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