ORIGINAL ARTICLE
High niobium oxide content in germanate glasses: Thermal,
structural, and optical properties
Lia Mara Marcondes
1
| Sergio Maestri
1
| Bianca Sousa
1
| Rogeria Rocha Gonc ßalves
2
|
Fabia Castro Cassanjes
1
| Gael Yves Poirier
1
1
Grupo de Quımica de Materiais,
Universidade Federal de Alfenas, Poc ßos
de Caldas, Brazil
2
Departamento de Quımica, Faculdade de
Filosofia, Ci^encias e Letras de Ribeir~ao
Preto, Universidade de S~ao Paulo,
Ribeir~ao Preto, Brazil
Correspondence
Lia Mara Marcondes, Instituto de Ci^encia
e Tecnologia, Campus de Poc ßos de
Caldas–UNIFAL-MG, Cidade
Universitaria, Poc ßos de Caldas, Brazil.
Email: lia-marcondes@hotmail.com
Funding information
FAPEMIG; FINEP; CNPq; CAPES
Abstract
Niobium alkali germanate glasses were synthesized by the melt-quenching tech-
nique. The ternary system (90-x)GeO
2
–xNb
2
O
5
–10K
2
O forms homogeneous
glasses with x ranging from 0 to 20 mol%. Samples were investigated by DSC
and XRD analysis, FTIR and Raman spectroscopy, and optical absorption. Struc-
tural and physical features are discussed in terms of Nb
2
O
5
content. The niobium
content increase in the glass network strongly modifies the thermal, structural and
optical properties of alkali germanate glasses. DSC, Raman and FTIR analysis
suggest niobium addition promotes NbO
6
groups insertion close to GeO
4
units of
the glass network. XRD analysis also pointed out that samples containing high
niobium oxide contents exhibit preferential niobium oxide-rich phase after crystal-
lization after heat treatment, which is similar to orthorhombic Nb
2
O
5
. Absorption
spectra revealed high transmission range between 400 nm to 6.2 lm, added to a
considerably decreased hydroxyl group content as the addition of niobium in the
alkali germanate network. The niobium oxide-rich phase crystallization process
was studied and activation energy was determined, as well as nucleation and crys-
tal growth temperatures and time for obtaining transparent glass-ceramics.
KEYWORDS
crystallization, germanates, glass, glass-ceramics, niobium
1 | INTRODUCTION
Germanate glasses are promising materials for optical
applications for their high rare earth solubility,
1-4
wide
transparence window from the UV to the infrared region,
low photon energy (800 cm
1
) when compared to silicate
(1150 cm
1
),
5,6
high refractive index and high thermal sta-
bility.
7
For applications in photonics, glassy systems with
low phonon energy are required to minimize nonradiative
processes in rare earth-doped glasses.
8
Besides, the inser-
tion of heavy transition metal oxides can improve the lumi-
nescent features thanks to their relatively lower phonon
energy and higher refractive index.
9
In addition, glasses
containing transition-metal oxides in the amorphous
network are interesting for their ability to produce
nanocrystalline transparent glass-ceramics after suitable
heat treatments.
10
Recently, structural changes due to the
addition of Ta
2
O
5
,
11
V
2
O
5
,
12
TiO
2
,
13
WO
3
,
14
Nb
2
O
5
,
15
into
germanate glasses has been reported.
Niobium germanate glasses can be formed in the ternary
system GeO
2
–Nb
2
O
5
–A
2
O, where A is usually an alkali
metal used to lower the melting point and viscosity of the
melt. Glasses in the binary system GeO
2
–Nb
2
O
5
are not
reported since homogeneous glasses cannot be obtained
under laboratory conditions, but a glass-ceramics sponta-
neously formed under quenching were reported in the
phases diagram by E.M. Levin.
16
Glasses samples in the
ternary system GeO
2
–Nb
2
O
5
–K
2
O with different molar
Received: 3 April 2017
|
Accepted: 30 August 2017
DOI: 10.1111/jace.15215
J Am Ceram Soc. 2017;1–11. wileyonlinelibrary.com/journal/jace © 2017 The American Ceramic Society
|
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