Structural and electrical transport studies of reduced in hydrogen surface of bismuth germanate glass

Article abstract of DOI:10.1016/S0038-1098(03)00008-5

Bismuth nanoclusters embedded in germanate glass matrices and surface layer of bismuth grains have been obtained by thermal treatment in hydrogen atmosphere of Bi_0.33Ge_0.67O_1.83 glass. Confirmed by AFM and XRD measurements a simple model of two conducting layers is proposed. The influence of time and temperature of reduction on the properties of the reduced samples have been studied. A change in surface conductivity caused by melting of Bi granules has been observed in all studied samples. The possibility of existence of a superconducting state in reduced bismuth germanate glasses has been discussed.

Full text of DOI:10.1016/S0038-1098(03)00008-5

Solid State Communications 125 (2003) 623–627
www.elsevier.com/locate/ssc
Structural and electrical transport studies of reduced
in hydrogen surface of bismuth germanate glass
*
B. Kusz
´
Faculty of Applied Physics and Mathematics, Gdansk University of Technology, Ul. Narutowicza 11/12, 80-952 Gdansk, Poland
Received 21 August 2002; received in revised form 23 December 2002; accepted 5 January 2003 by K.-A. Chao
Abstract
Bismuth nanoclusters embedded in germanate glass matrices and surface layer of bismuth grains have been obtained by
thermal treatment in hydrogen atmosphere of Bi0.33Ge0.67O1.83 glass. Confirmed by AFM and XRD measurements a simple
model of two conducting layers is proposed. The influence of time and temperature of reduction on the properties of the reduced
samples have been studied. A change in surface conductivity caused by melting of Bi granules has been observed in all studied
samples. The possibility of existence of a superconducting state in reduced bismuth germanate glasses has been discussed.
q 2003 Elsevier Science Ltd. All rights reserved.
PACS: 73.61.Jc
Keywords: A. Disordered systems; A. Nanostructures; C. X-ray scattering; D. Tunnelling
1. Introduction
2. Experiment
Bismuth germanate Bi0.33Ge0.67O1.83 glass was syn-
thesized as follows. Milled mixture of powdered GeO2 and
Bi nitrate, placed into a platinum crucible, was decomposed
at 1000 K for 1 h. After the decomposition, the mixture was
ground again and submitted to a gradual heating from room
temperature to 1500 K. Melted glass was homogenized by
mechanical stirring and than quenched by pouring onto a
steel plate. Before further treatment the surface of samples
was polished and cleaned carefully. The nominal oxygen
content in the glass was calculated within the assumption
that the original glass has a composition determined by the
valency 3þ of Bi and 4þ of Ge.
Bismuth silicate glasses find a lot of industrial and
special applications [1,2]. On the other hand, bismuth
germanate glasses are less known and their applications are
still rare. The interesting property of bismuth germanate
glasses is that in the course of heat treatment in hydrogen
atmosphere they change surface conductivity by several
orders. Such a thermal procedure reduces Biþ3 ions into
neutral atoms. Only a few works have been done on physical
properties of non-reduced [3] and reduced [4,5] bismuth
germanate glasses and there still remain many questions
about conductivity and structures of these materials.
In this paper, we report the results of studies on bismuth
germanate Bi0.33Ge0.67O1.83 glass reduced under different
conditions. The resulting glass–metal composite has been
used for investigations of electrical properties of metallic
bismuth in its nanocrystalline state. Possibility of existence
of superconducting state in reduced bismuth germanate
glasses have been discussed.
Reduction process was carried out in the temperature
range from 573 to 673 K in hydrogen atmosphere.
Surface morphology of non-reduced and reduced
samples was tested by working in air AFM microscope.
The glass and its reduced surface layer were examined by
X-ray diffraction analysis with the use of Philips X’Pert
diffractometer system.
Measurements of sample conductivity were made in
the temperature range from 5.8 to 613 K in ambient
atmosphere using two- or four-terminal methods. The
*
Tel.: þ48-44-58-3471486; fax: þ48-44-58-3472587.
E-mail address: bodzio@mif.pg.gda.pl (B. Kusz).
0038-1098/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0038-1098(03)00008-5

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B. Kusz / Solid State Communications 125 (2003) 623–627
surface conductivity of the samples has been calculated from
equation: sA ¼ R21d=l; where R is the resistivity of sample,
d the distance between the electrodes and l their length.
In this paper, we present results obtained for
Bi0.33Ge0.67O1.84 samples reduced at 613 K for 0.8 h (G1),
2 h (G2), 7 h (G3), 12 h (G4), 24 h (G5) and 44 h (G6). For
comparison, samples reduced at 663 K for 0.12 h (H1),
0.22 h (H2), 1.4 h (H3), 24 h (H4) and 44 h (H5) also were
studied.
3. Results
The plots of surface conductivity versus reduction time
of Bi0.33Ge0.67O1.83 sample reduced at five different
temperatures (563, 588, 613, 643 and 663 K) are presented
in Fig. 1. The behaviour seen in the figure is characteristic of
bismuth germanate glasses subjected to the reduction
process [5]. First, after some time a rapid of a few orders
of magnitude increase in the surface conductivity of sample
Bi0.33Ge0.67O1.83 appears. Next, the surface conductivity of
glasses attains a maximum (about 1026 V21) and in a
pronounced way decreases to a minimum. Further reduction
causes increasing of conductivity of a few orders. The
results confirmed that when reduction temperature increases
the speed of changes of surface conductivity and the depth
of the minimum increases too, but character of the
dependence is still the same.
Fig. 2. X-ray diffraction spectra of rhombohedral bismuth, G2, G5,
G6 and G60 samples. The sample G60 is G6 after removing outer
Bi–Ge layer. (†) marks Ge peaks. The plots are shifted for better
view.
Fig. 3. The flat surface of the glass (G1), a layer of
connected or disconnected granules (G3) and multi-layer
granular systems (G5) are visible in Fig. 3. We have
obtained similar results for the samples reduced at 663 K.
The results of measurements of surface conductivity
versus temperature in Bi0.33Ge0.67O1.83 samples reduced for
various times at 613 and 663 K are shown in Figs. 4–6.
The results of X-ray diffraction measurements of studied
samples and rhombohedral bismuth are shown in Fig. 2. The
spectra of as-quenched glass and reduced G1 and G2
samples show a halo pattern with no peaks (only G2 sample
is shown in Fig. 2). Glasses annealed in hydrogen for 24 and
44 h (G5 and G6 samples) exhibit a series of peaks
characteristic of rhombohedral crystalline Bi. They also
show some peaks, which correspond to germanium.
4. Discussion
Properties of the bismuth germanate glasses reduced in
AFM pictures of G1, G3 and G5 samples are presented in
Fig. 1. The time dependence (log–log scale) of surface conductivity
of Bi0.33Ge0.67O1.83 glasses during heat treatment in five different
temperatures (the letters G1…G6 and H1…H6 denote studied
samples).
Fig. 3. Examples of AFM pictures of studied Bi0.33Ge0.67O1.83 glass
samples.

B. Kusz / Solid State Communications 125 (2003) 623–627
625
Fig. 6. The plots of surface conductivity versus temperature in
Bi0.33Ge0.67O1.83 samples reduced for 24 h (H5) and 44 h (H6) at the
temperature 613 K. Measurements range is from 5.8 to 560 K.
Fig. 4. The plots of surface conductivity versus T 21/4 in
Bi0.33Ge0.67Oy samples reduced for 0.8 h (G1), 2 h (G2), 7 h (G3)
at the temperature 613 K and 0.12 h (H1), 0.22 h (H2), 1.4 h (H3) at
the temperature 663 K. Measurements range is from 300 to 573 K.
the potential barrier between metallic granules appears and
the conductivity rapidly increases (Fig. 1). It means that the
2D layer of Bi granules embedded in GeO2 matrices is
created. Further reduction causes rising of layer thickness.
In reduced bismuth germanate glasses, apart from the
processes described above, both the migration of bismuth
atoms to the surface of samples and reduction of GeO2 is
observed. This phenomenon is a reason of changing surface
morphology and decreasing to minimum of surface
conductivity of studied glasses. Up to this point (sample
G3) the conductivity by electron tunneling dominates in
reduced bismuth germanate samples. The electrical trans-
port by tunneling of electrons in granular systems has been a
subject of intensive experimental and theoretical studies for
more than tree decades [7–10]. It well known that the n ¼
1=2 index in ln½sðTÞꢀ , 2T2n function is characteristic in
disordered granular systems in the case of Coulomb
interaction (e.g. in cermets [7]). It is known also that some
materials (diamond-like carbon films containing tungsten
[15], disordered semiconductors [16]) exhibit a dependence
ln½sðTÞꢀ , 2T21=2 at low temperatures with the cross-over
to T 21/4 at high temperatures. Sandoe et al. [17] found
n ¼ 1/3 in reduced lead–silicate glasses (where a layer of
lead embedded in SiO2 matrices is created) which he
hydrogen atmosphere depend both on time and temperature
of reduction. We propose a simple model of the glass
subjected to the reduction process, which is illustrated in
Fig. 7. Forming of two conductive layers can be distin-
guished. One is the layer, which contains Bi particles
embedded in a GeO2 glass matrix, and the latter is the very
top layer containing either connected or not grains of Bi and
a small amount of Ge. The both layers are created
simultaneously during annealing of the samples in hydrogen
in appropriately high temperatures. First, on account of
reduction, neutral bismuth atoms occur and next they
agglomerate into clusters. As long as the distance between
Bi nanostructures is too large for an electron tunnelling to
take place, the conductivity is determined by ionic mobility
and does not change with reduction time. While the
reduction is carried on, the concentration and dimension
of Bi clusters grow. When the Bi concentration on a surface
layer of glasses is sufficient, an electrons tunnelling trough
Fig. 5. The plots of surface conductivity versus temperature in
Bi0.33Ge0.67O1.83 samples reduced for 24 h (G5) and 44 h (G6) at the
temperature 613 K. Measurements range is from 5.8 to 560 K.
Fig. 7. Model of two-layered structure of reduced in hydrogen
bismuth–germanate glasses.

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B. Kusz / Solid State Communications 125 (2003) 623–627
interpreted as VRH in 2D system. Our results suggest that
G1 and G2 samples are 3D systems with n ¼ 1/4 in high
temperature regime but because of too narrow measurement
temperature range (300–520 K) n ¼ 1/2 cannot be
excluded. The measurements down to cryogenic tempera-
tures are needed to resolve the problem. Detailed analysis of
surface conductivity in a layer of Bi clusters embedded in
GeO2 matrices will be a subject of the next paper.
bismuth changes 2.3 times [6] during the phase
transition. The jumps (sliquid/ssolid) in reduced samples
are practically the same or a little smaller than 2.3 (see
H6 in Fig. 6). We think that a process of partial damage
of the Ag electrodes by the melting of bismuth layer is
responsible for the smaller value of the jump. An abrupt
conductivity decrease (about 2.3 times) on cooling as a
result of solidification of overcooled bismuth is also
visible in Figs. 5 and 6 (e.g. at 479 K in H5 sample).
The phenomena of changing of conductivity at Bi phase
transition support the thesis that outer layer contain
metallic granules of bismuth. Our investigations by
differential scanning calorimetry (DSC) confirmed that
large changes in surface conductivity of reduced glasses
are induced by melting and solidification of bismuth
nanocrystals [18].
The migration of bismuth atoms to the surface of
samples cause that the very top layer containing either
connected or not connected grains of Bi is created. When the
concentration of Bi nanoclusters attains a percolation
threshold (near point G3 in Fig. 1) the increase in
conductivity is observed. The continuous layer of Bi
granules is created and its electrical properties determine
surface conductivity of the reduced samples. Further
reduction causes increase of thickness of this layer.
Another thing characteristic of the temperature
dependence of conductivity of G5 and G6 samples is
its activated character (Fig. 5), whereas in the bulk
bismuth a metallic dependence would be expected. As
our results show the top conductive layer contains
weakly connected Bi granules, which, we believe,
determines the character of sA (T) function in reduced
bismuth germanate glasses. We think that a difference in
thermal expansion coefficients of surface of reduced glass
and that of a layer of granular bismuth is responsible for
activated character of conductivity. Probably the dimen-
sions of Bi granules decrease quicker than dimensions of
glass matrices with decreasing temperature. This may
disconnect some paths of conducting granules and cause
a decrease in conductivity during cooling (and inverse
process during heating). When reduction temperature
increases the thickness of outer layer increases and Bi
granules conglomerate into bigger system. The sA (T)
function changes into typical metallic character (Fig. 6 in
temperature range from 5 to 290 K).
The model agrees very well with results of XRD
analysis, which can observe both the layers. The creation
of the very surface layer of Bi and Ge granules in G5 and G6
samples is detected (Fig. 2). The inner layer containing
bismuth nanoparticles embedded in GeO2 matrixes was
studied after removing the outer one (Fig. 2 sample G60).
The analysis of broadening of XRD peaks shows that
diameter of embedded Bi nanoparticles in reduced sample
G60 is less than 10 nm. The thickness of the inner layer of G5
sample is 1.6 mm, whereas that of G6 sample is 4.2 mm. In
the case of samples reduced at 663 K the thickness of inner
layer of H5 i H6 samples are 3.4 and 5.2 mm, respectively.
Further support for the above model comes from AFM
results (Fig. 3) where the outer layer of Bi granules is
visible. The diameter and height of granules on the surface
of G3 sample are about 30 and 10 nm, respectively. The
granules in the layer of G5 sample have diameter about 30–
50 nm. The AFM pictures of cross-section of G5 and G6
samples show that thickness of granular layer is about 50
and 170 nm, respectively.
Rhombohedral bulk bismuth is a semimetal, which do
not display superconductivity under ordinary circumstances.
However, under pressure (over 25 kbars) bulk Bi shows
superconductivity with a Tc ranging from 3.9 to 8.6 K [11].
Bi in amorphous state also shows superconductivity with a
Tc ¼ 6.2 K [12]. The superconductivity with a Tc up to
,5.8 K in granular Bi films prepared from Bi clusters with
well-defined size was confirmed by some authors [13,14].
Heat treatment in hydrogen of bismuth germanate glasses
creates Bi granular layer. When these granules are relatively
big they have non-superconducting rhombohedrical form.
The dimensions of granules depend on Bi contents in glass,
on temperature and time of annealing in hydrogen. We think
that smaller reduction temperature, shorter time and changes
of Bi content in BixGe12xO220.5x glasses could lead to
creating smaller bismuth granules. In such a 2D monolayer
of distorted rhombohedrical granules existence of super-
conductivity is possible [14]. The measurements at lower
temperature are necessary to confirm this hypothesis.
In the temperature dependence of conductivity of the
layer of bismuth granules embedded in glass matrices
(sample G2, H1, H2 and H3) interesting phenomena can be
observed. The characteristic changes in the slope of sA (T)
plots around melting point of Bi (544 K) are seen (Fig. 4).
The surface conductivity decreases and changes the slope
during the heating at melting temperature. We think that
phenomenon is caused by decreasing volume of Bi
nanoparticles during melting which leads to growing of
s=d ratio where s is a distances between granules and d their
diameter. So that the probability of activated tunnelling
between grains decreases which is seen as a decrease of
conductivity. More pronounced changes of sA (T) in the
samples reduced at 663 K (H1, H2 and H3) are connected
with larger dimensions of bismuth granules in the glass
reduced at higher temperature.
In contrary to G2, H2 and H3 sample the increase of
conductivity in the case of G5, H5, G6 and H6 samples
near the melting point is observed. Conductivity of bulk

B. Kusz / Solid State Communications 125 (2003) 623–627
627
´
[3] B. Kusz, K. Trzebiatowski, J.R. Barczynski, Solid State Ionic
5. Conclusion
(2003) in press.
[4] A. Witkowska, J. Rybicki, A. Di, Cicco Phys. Chem. Glasses
43 (2002) 124.
The interesting property of bismuth germanate glasses is
that in the course of reduction in hydrogen atmosphere they
develop two conducting layers at their surface. One of them
is a layer of Bi nanoclusters embedded in germanate glass
matrix. The second is a mixture of Bi and Ge granules on
the surface.
[5] B. Kusz, K. Trzebiatowski, J. Non-Cryst. Solids (2003) in
press.
[6] N.F. Mott, H. Jones, The theory of the properties of metals and
alloys, Dover Publications, Inc, New York, 1958.
[7] B. Abeles, P. Sheng, M.D. Coutts, Y. Arie, Adv. Phys. 24
(1975) 406.
A gradual change of the slope or a jump of conductivity
observed during heating or cooling in all studied samples
near the temperature of 544 K is caused by a phase transition
of the Bi granules.
[8] C.-H. Lin, G.Y. Wu, Phys. B 279 (2000) 341.
[9] I.P. Zvyagin, R. Keiper, Phil. Mag. B 81 (9) (2001) 997.
[10] A.G. Hunt, Phil. Mag. B 81 (9) (2001) 875.
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[12] D.B. Haviland, Y. Liu, A.M. Goldman, Phys. Rev. Lett. 62
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Acknowledgements
[13] B. Weitzel, H. Micklitz, Phys. Rev. Lett. 66 (3) (1991) 385.
[14] C. Vossloh, M. Holdenried, H. Micklitz, Phys. Rev. B 58 (18)
(1998) 12422.
We would like to thank Dr Konrad Trzebiatowski for
sample preparation and Prof. Leon Murawski and Dr Maria
Gazda for their fruitful discussion.
[15] A. Bozko, S. Chudinov, M. Evangelisti, S. Stizza, V. Dorfman,
Mater. Sci. Forum 235–238 (1997) 955.
[16] T.G. Castner, in: Pollak, B. Shklovskii (Eds.), Hoppin
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