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Local arrangement of amorphous GeSe2 from the viewpoint of crystallization sequence by electron microscopic observations
Journal of Non-Crystalline Solids 117/118 (1990) 215-218 North-Holland
215
LOCAL ARRANGEMENT OF AMORPHOUS GeSe2 FROM THE VIEWPOINT OF CRYSTALLIZATION SEQUENCE BY ELECTRON MICROSCOPIC OBSERVATIONS Toshio OKABE, K i m i h i r o NAKAMURA and Masayuki NAKAGAWA Department o f Physics, Toyama U n i v e r s i t y ,
Toyama 930, Japan
The c r y s t a l l i z a t i o n sequence o f amorphous GexSel-x (O.l
I.
INTRODUCTION
v i t r e o u s GeSe2 seems t o possess a s i m i l a r s t r u c -
A g r e a t deal of e x p e r i m e n t a l e f f o r t
has been
expended to determine the s t r u c t u r e o f amorphous
t u r e as glassy GeS2, There are two c r y s t a l s t r u c t u r e s in GeS2 and
chalcogenides by d i f f r a c t i o n
methods I and by i n 2 f r a r e d and Raman spectroscopy, From the i r and
GeSe2: a l o w - t e m p e r a t u r e (LT-) form 8 ' 9 and a
Raman spectroscopy on GexSel_ x glasses, Tronc 3 et al. have confirmed the s t r u c t u r a l model of
dimen-sional w i t h 24 formula u n i t s per u n i t c e l l ,
a c h e m i c a l l y ordered continuous network in which
per l a y e r - u n i t
8-N valence c o - o r d i n a t i o n r u l e i s s a t i s f i e d
for
h i g h - t e m p e r a t u r e (HT-) one, lO~12the former t h r e e the l a t t e r
a l a y e r s t r u c t u r e w i t h 8 formula u n i t s cell.
In the former the t e t r a -
hedra share corners, w h i l e in the l a t t e r
half
both components and t h e r e is chemical preference
the t e t r a h e d r a share only corners w h i l e h a l f
f o r the bonds between u n l i k e atoms,
share one edge,
Raman
In both s t r u c t u r e s the c o - o r d i -
s t u d i e s 4 on bulk GexSl_ x have i n d i c a t e d the
n a t i o n of Ge is 4 and t h a t o f S and Se is 2.
presence of a l a r g e r molecules, namely S3Ge-GeS3,
However, only the HT-phase o f GeSe2 has been
as w e l l as the e x i s t e n c e of GeS4 t e t r a h e d r a ,
observed on the s t u d i e s 13'14 of the c r y s t a l l z -
The o b s e r v a t i o n o f the s p l i t t i n g
a t i o n process of amorphous GexSel_ x f o r x < 0,3.
o f Raman l i n e
has been i n t e r p r e t e d as an i n d i c a t i o n o f u n i t s
In t h i s paper we r e p o r t the c r y s t a l l i z a t i o n
w i t h layered s t r u c t u r e , 5
quence o f the amorphous f i l m s ,
By both Raman and
EXAFS measurements, however, Nemanich e t a l , 6
O.l
se-
investigated for
by t r a n s m i s s i o n e l e c t r o n microscopy,
have suggested no evidence f o r m o l e c u l a r s t r u c -
f i n d i n g the f o r m a t i o n o f the LT-phase as well
t u r e s in GeSe2 but r a t h e r i t is more random, 7 Recently, F e l t z e t a l , have r e p o r t e d the p r e s -
as the HT-one w i t h s p e c i f i c c o n d i t i o n s ,
ence o f a remarkable number o f edge-shared GeS4/2
GexSel_ x c o n s i s t s o f the edge-shared GeSe4/2
u n i t s in the t h r e e d i m e n s i o n a l l y connected n e t -
t e t r a h e d r a l u n i t in the whole range of composi-
work by m o d e l l i n g the glass s t r u c t u r e of GeS2
t i o n s w i t h 0. I < x < 0.4.
in o r d e r t o i n t e r p r e t
the e x p e r i m e n t a l RDF, They
also suggest from the comparison o f the RDFs t h a t
0022-3093/90/$03.50 ~) Elsevier Science Publishers B.V. (North-Holland)
particular
In
we suggest the s t r u c t u r e of amorphous
T. Okabe et al. / Local arrangement of amorphous GeSe2
216
2. EXPERIMENTAL PROCEDURES Samples o f amorphous GexSel_ x were prepared in t h i n - f i l m
form, p a r t l y by thermal f l a s h evapo-
r a t i o n and p a r t l y by c o - e v a p o r a t i o n o f the c o n s t i t u e n t s from two sources in a vacuum system evacuated to < 1 x 10- 4 and maintained at < 7 x 10-4 Pa throughout the e v a p o r a t i o n .
The f i l m s were
grown on mica s u b s t r a t e s a t room temperature on monitoring their oscillating
thickness by q u a r t z c r y s t a l
microbalances; l i k e w i s e carbon f i l m s .
amorphous Ge-Se a l l o y s and again carbon f i l m s , resp. ~ 7, ~ 30 and ~ 7 nm t h i c k s u c c e s s i v e l y . The f i l m was w e t - s t r i p p e d from the s u b s t r a t e and mounted on a molybdenum e l e c t r o n microscope g r i d s . Carbon f i l m s were used to p r o t e c t the amorphous a l l o y s from the i m p u r i t y c o n t a m i n a t i o n or the selenium s u b l i m a t i o n during the o b s e r v a t i o n made in the hot stage o f an e l e c t r o n microscope.
The
composition o f the f i l m s was determined by energy
FIGURE 1 Transmission e l e c t r o n micrograph of annealed GeO.3SeO.7 f i l m : amorphous phase ( a ) , high-tempe r a t u r e GeSe2 phases ( b , c ) and l o w - t e m p e r a t u r e GeSe2 phase ( d ) .
d i s p e r s i v e x - r a y a n a l y s i s w i t h an ORTEC EEDS-II system attached to the e l e c t r o n microscope.
High
r e s o l u t i o n e l e c t r o n micrographs were taken using a I0 nm- I
r a d i u s o b j e c t i v e a p e r t u r e under a x i a l
illumination
w i t h a JEM-2OOCX e l e c t r o n microscope
by d i f f r a c t i o n analysis.
The c r y s t a l l i z a t i o n
temperature varies depending on the f i l m s , presumably on the rate of heating.
In the narrow
range of compositions with 0.3 < x < 0.4, some
equipped w i t h a high r e s o l u t i o n side e n t r y g o n i o -
HT-GeSe2 c r y s t a l l i t e s grow r a d i a l l y from a point
meter stage and an o b j e c t i v e lens w i t h Cs = 1.9
in the amorphous matrix, followed by the formation of a LT-GeSe2 crystal adjacent to the HT-
mm, operated a t 200 kV.
one, as shown in figure I. 3. RESULTS AND DISCUSSION 3.1. C r y s t a l l i z a t i o n
No independent forma-
tion of the LT-phase is v i s i b l e in the amorphous matrix.
behavior
Transmission e l e c t r o n micrographs f o r the asdeposited GexSel_ x (O.l
3.2. Orientation relationships The observed topotaxial relations between the
homogeneous and f e a t u r e l e s s s t r u c t u r e on a m i c r o -
HT- and the LT-GeSe2 crystals are analyzed from
scopic scale and the corresponding d i f f r a c t i o n
the electron d i f f r a c t i o n patterns of the i n t e r -
p a t t e r n s are broad, h a l o - t y p e , t y p i c a l
face of two phases as,
phous s t r u c t u r e .
of amor-
In s i t u o b s e r v a t i o n o f the
amorphous f i l m s w i t h x < 0.3 in the heating stage
(O01)LT / / (O01)HT and [ I I O ] L T / / [020]HT where the l a t t i c e parameters are chosen a f t e r
of e l e c t r o n microscope show t h a t a s e p a r a t i o n
r e f . 8 f o r the LT-GeSe 2 and r e f .
of the o r i g i n a l
GeSe2.
amorphous phase i n t o two v i t r e o u s
phases is appearing so as to form condensed
I0 f o r the HT-
The former is orthorhombic w i t h a =
0.694. b = 1.220 and c = 2.299 nm and the l a t t e r
regions r i c h in Ge and t h i n regions r i c h in Se.
also orthorhombic w i t h a = 0.704, b = 1.183 and
The condensed r e g i o n s , then. t u r n to c r y s t a l l i z e
c = 1.682 nm.
above 350 °C t o the HT-GeSe2 phase, i d e n t i f i e d
determined to m o n o c l i n i c 9 or pseudo-orthorhom-
These s t r u c t u r e s has been r e -
T. Okabe et al./ Local arrangement of amorphous GeSe2
217
FIGURE 2 High-resolution electron microscopic images of the interface between HT-GeSe2 and LT-GeSe2, viewed along the [001] common axis (a) and along the o f f - a x i s (b). The positions of the interphase boundary are arrowed. bic 12 with the space groups.
which may be s t i l l
3,3. High-resolution electron microscopy Figure 2a shows an example of the high-resolut i o n electron microscopic images of the i n t e r fa c e viewed along the common [001] axis.
In the
amorphous because the compo-
s i t i o n s d i f f e r from the stoichiometry. 3.4. Local arrangement of amorphous GeSe2 The present observations of successive format i o n of a LT-phase c r y s t a l , adjacent to a grown
figure, the 0.59 nm {020} planes of the HT-GeSe2
HT-phase crystal in the amorphous matrix with
are p a r a l l e l to the 0.60 nm { I I 0 }
planes of the
a s p e c i f i c o r i e n t a t i o n r e l a t i o n s h i p , provide en-
LT-GeSe2 and the interface is seen to be nearly
hanced insight into the local arrangement of the
f l a t on the atomic scale.
amorphous structure, because c r y s t a l l i z a t i o n
The l a t t i c e match
is c l e a r l y i l l u s t r a t e d by figure 2b, which is
processes are i n t i m a t e l y related with the short
taken by t i l t
range order present in the glassy state.
o r i e n t a t i o n of the [001] axis to
the electron beam.
The 0.59 nm {021} planes
The
HT-phase contains h a l f the edge-shared and h a l f
of the LT-phase are well f i t t e d to the 0.65 nm
the corner-shared tetraheda while the LT-phase
{I01} planes of the HT-one at the boundary with
only the edge-shared tetrahedra.
making angles of 20 degree, including a few d i s -
of these results in comparison with the atomic
locations to accomodate the m i s f i t of t h e i r
arrangement in the LT- and the HT-phases, we
spacings.
coclude the existence of the structural u n i t of
On the {020} planes of the HT-phase,
On the basis
t h e i r exists the layer structure consists of the
edge-shared tetrahedron GeSe4/2 in the amorphous
edge-shared tetrahedron as well as the corner-
structure with near the stoichiometric composi-
shared one.
Moreover, on the f i g u r e 2b, the HT-
crystals contain considerable defect regions,
t i o n GeSe2., which is in agreement with results of infrared and Raman measurements15 and also
218
T. Okabe et al. / Local arrangement of amorphous GeSe2
with X-ray measurements7,
8. Liu Ch'un-hua, A.S. Pashinkin and A.V. Nevoselova, Russian J. Inorg. Chem. 7 (1962) 1117.
REFERENCES I. See, for example, A.C. Wright and A.J. Leadbetter. Phys. Chem. Glasses 17 (1976) 122. 2. See the standard text. N.F. Mott and E.A. Davis, Electronic Processes in Non-Crystalline Materials, 2nd edition (Clarendon Press, Oxford 1979) pp. 445 - 452.
9. G. Dittmar and H. Schafer. Acta Cryst. B32 (1976) 1188. I0. J. Burgeat, G.Le Roux and A. Brenac, J. Appl Cryst. 8 (1975) 325. I I . G. Dittmar and H. Schafer, Acta Cryst. B31 (1975) 2060.
3. P. Tronc, M. Bensoussan and A. Brenac, Phys. Rev. B8 (1973) 5947.
12. G° Dittmar and H. Schafer, Acta Cryst. B32 (1976) 2726.
4. G. Lucovsky, F.L. Galeener, R.C. Keezer, R.H. Geils and H.A. Six, Phys. Rev. BIO (1973) 5134.
13. R. Azoulat, H. Thibierge and A. Brenac, J. Non-Cryst. Solids 18 (1975) 33.
5. P.M. Bridenbaugh, G.P. Espinosa, J.E. G r i f f i t h s . J.C. P h i l l i p s and J.P. Remeika, Phys. Rev. B20 (1979) 4140.
14. M. Esquerre, J.C. Carballes, J.P. Audiere and C. Mazieres, J. Mat. Science 13 (1978) 1217.
6. R.J. Nemanich, G.A.N. Connell, T.M. Hayes and R.A. Street, Phys. Rev. BI8 (1978) 6900.
15. Y. Kawamoto and C. Kawashima, Mat. Res. Bull 17 (1982) 1511.
7. A. Feltz. M. Pohle. H. Steil and G. Herms, J. Non-Cryst. Solids 69 (1985) 271.
215
LOCAL ARRANGEMENT OF AMORPHOUS GeSe2 FROM THE VIEWPOINT OF CRYSTALLIZATION SEQUENCE BY ELECTRON MICROSCOPIC OBSERVATIONS Toshio OKABE, K i m i h i r o NAKAMURA and Masayuki NAKAGAWA Department o f Physics, Toyama U n i v e r s i t y ,
Toyama 930, Japan
The c r y s t a l l i z a t i o n sequence o f amorphous GexSel-x (O.l
I.
INTRODUCTION
v i t r e o u s GeSe2 seems t o possess a s i m i l a r s t r u c -
A g r e a t deal of e x p e r i m e n t a l e f f o r t
has been
expended to determine the s t r u c t u r e o f amorphous
t u r e as glassy GeS2, There are two c r y s t a l s t r u c t u r e s in GeS2 and
chalcogenides by d i f f r a c t i o n
methods I and by i n 2 f r a r e d and Raman spectroscopy, From the i r and
GeSe2: a l o w - t e m p e r a t u r e (LT-) form 8 ' 9 and a
Raman spectroscopy on GexSel_ x glasses, Tronc 3 et al. have confirmed the s t r u c t u r a l model of
dimen-sional w i t h 24 formula u n i t s per u n i t c e l l ,
a c h e m i c a l l y ordered continuous network in which
per l a y e r - u n i t
8-N valence c o - o r d i n a t i o n r u l e i s s a t i s f i e d
for
h i g h - t e m p e r a t u r e (HT-) one, lO~12the former t h r e e the l a t t e r
a l a y e r s t r u c t u r e w i t h 8 formula u n i t s cell.
In the former the t e t r a -
hedra share corners, w h i l e in the l a t t e r
half
both components and t h e r e is chemical preference
the t e t r a h e d r a share only corners w h i l e h a l f
f o r the bonds between u n l i k e atoms,
share one edge,
Raman
In both s t r u c t u r e s the c o - o r d i -
s t u d i e s 4 on bulk GexSl_ x have i n d i c a t e d the
n a t i o n of Ge is 4 and t h a t o f S and Se is 2.
presence of a l a r g e r molecules, namely S3Ge-GeS3,
However, only the HT-phase o f GeSe2 has been
as w e l l as the e x i s t e n c e of GeS4 t e t r a h e d r a ,
observed on the s t u d i e s 13'14 of the c r y s t a l l z -
The o b s e r v a t i o n o f the s p l i t t i n g
a t i o n process of amorphous GexSel_ x f o r x < 0,3.
o f Raman l i n e
has been i n t e r p r e t e d as an i n d i c a t i o n o f u n i t s
In t h i s paper we r e p o r t the c r y s t a l l i z a t i o n
w i t h layered s t r u c t u r e , 5
quence o f the amorphous f i l m s ,
By both Raman and
EXAFS measurements, however, Nemanich e t a l , 6
O.l
se-
investigated for
by t r a n s m i s s i o n e l e c t r o n microscopy,
have suggested no evidence f o r m o l e c u l a r s t r u c -
f i n d i n g the f o r m a t i o n o f the LT-phase as well
t u r e s in GeSe2 but r a t h e r i t is more random, 7 Recently, F e l t z e t a l , have r e p o r t e d the p r e s -
as the HT-one w i t h s p e c i f i c c o n d i t i o n s ,
ence o f a remarkable number o f edge-shared GeS4/2
GexSel_ x c o n s i s t s o f the edge-shared GeSe4/2
u n i t s in the t h r e e d i m e n s i o n a l l y connected n e t -
t e t r a h e d r a l u n i t in the whole range of composi-
work by m o d e l l i n g the glass s t r u c t u r e of GeS2
t i o n s w i t h 0. I < x < 0.4.
in o r d e r t o i n t e r p r e t
the e x p e r i m e n t a l RDF, They
also suggest from the comparison o f the RDFs t h a t
0022-3093/90/$03.50 ~) Elsevier Science Publishers B.V. (North-Holland)
particular
In
we suggest the s t r u c t u r e of amorphous
T. Okabe et al. / Local arrangement of amorphous GeSe2
216
2. EXPERIMENTAL PROCEDURES Samples o f amorphous GexSel_ x were prepared in t h i n - f i l m
form, p a r t l y by thermal f l a s h evapo-
r a t i o n and p a r t l y by c o - e v a p o r a t i o n o f the c o n s t i t u e n t s from two sources in a vacuum system evacuated to < 1 x 10- 4 and maintained at < 7 x 10-4 Pa throughout the e v a p o r a t i o n .
The f i l m s were
grown on mica s u b s t r a t e s a t room temperature on monitoring their oscillating
thickness by q u a r t z c r y s t a l
microbalances; l i k e w i s e carbon f i l m s .
amorphous Ge-Se a l l o y s and again carbon f i l m s , resp. ~ 7, ~ 30 and ~ 7 nm t h i c k s u c c e s s i v e l y . The f i l m was w e t - s t r i p p e d from the s u b s t r a t e and mounted on a molybdenum e l e c t r o n microscope g r i d s . Carbon f i l m s were used to p r o t e c t the amorphous a l l o y s from the i m p u r i t y c o n t a m i n a t i o n or the selenium s u b l i m a t i o n during the o b s e r v a t i o n made in the hot stage o f an e l e c t r o n microscope.
The
composition o f the f i l m s was determined by energy
FIGURE 1 Transmission e l e c t r o n micrograph of annealed GeO.3SeO.7 f i l m : amorphous phase ( a ) , high-tempe r a t u r e GeSe2 phases ( b , c ) and l o w - t e m p e r a t u r e GeSe2 phase ( d ) .
d i s p e r s i v e x - r a y a n a l y s i s w i t h an ORTEC EEDS-II system attached to the e l e c t r o n microscope.
High
r e s o l u t i o n e l e c t r o n micrographs were taken using a I0 nm- I
r a d i u s o b j e c t i v e a p e r t u r e under a x i a l
illumination
w i t h a JEM-2OOCX e l e c t r o n microscope
by d i f f r a c t i o n analysis.
The c r y s t a l l i z a t i o n
temperature varies depending on the f i l m s , presumably on the rate of heating.
In the narrow
range of compositions with 0.3 < x < 0.4, some
equipped w i t h a high r e s o l u t i o n side e n t r y g o n i o -
HT-GeSe2 c r y s t a l l i t e s grow r a d i a l l y from a point
meter stage and an o b j e c t i v e lens w i t h Cs = 1.9
in the amorphous matrix, followed by the formation of a LT-GeSe2 crystal adjacent to the HT-
mm, operated a t 200 kV.
one, as shown in figure I. 3. RESULTS AND DISCUSSION 3.1. C r y s t a l l i z a t i o n
No independent forma-
tion of the LT-phase is v i s i b l e in the amorphous matrix.
behavior
Transmission e l e c t r o n micrographs f o r the asdeposited GexSel_ x (O.l
3.2. Orientation relationships The observed topotaxial relations between the
homogeneous and f e a t u r e l e s s s t r u c t u r e on a m i c r o -
HT- and the LT-GeSe2 crystals are analyzed from
scopic scale and the corresponding d i f f r a c t i o n
the electron d i f f r a c t i o n patterns of the i n t e r -
p a t t e r n s are broad, h a l o - t y p e , t y p i c a l
face of two phases as,
phous s t r u c t u r e .
of amor-
In s i t u o b s e r v a t i o n o f the
amorphous f i l m s w i t h x < 0.3 in the heating stage
(O01)LT / / (O01)HT and [ I I O ] L T / / [020]HT where the l a t t i c e parameters are chosen a f t e r
of e l e c t r o n microscope show t h a t a s e p a r a t i o n
r e f . 8 f o r the LT-GeSe 2 and r e f .
of the o r i g i n a l
GeSe2.
amorphous phase i n t o two v i t r e o u s
phases is appearing so as to form condensed
I0 f o r the HT-
The former is orthorhombic w i t h a =
0.694. b = 1.220 and c = 2.299 nm and the l a t t e r
regions r i c h in Ge and t h i n regions r i c h in Se.
also orthorhombic w i t h a = 0.704, b = 1.183 and
The condensed r e g i o n s , then. t u r n to c r y s t a l l i z e
c = 1.682 nm.
above 350 °C t o the HT-GeSe2 phase, i d e n t i f i e d
determined to m o n o c l i n i c 9 or pseudo-orthorhom-
These s t r u c t u r e s has been r e -
T. Okabe et al./ Local arrangement of amorphous GeSe2
217
FIGURE 2 High-resolution electron microscopic images of the interface between HT-GeSe2 and LT-GeSe2, viewed along the [001] common axis (a) and along the o f f - a x i s (b). The positions of the interphase boundary are arrowed. bic 12 with the space groups.
which may be s t i l l
3,3. High-resolution electron microscopy Figure 2a shows an example of the high-resolut i o n electron microscopic images of the i n t e r fa c e viewed along the common [001] axis.
In the
amorphous because the compo-
s i t i o n s d i f f e r from the stoichiometry. 3.4. Local arrangement of amorphous GeSe2 The present observations of successive format i o n of a LT-phase c r y s t a l , adjacent to a grown
figure, the 0.59 nm {020} planes of the HT-GeSe2
HT-phase crystal in the amorphous matrix with
are p a r a l l e l to the 0.60 nm { I I 0 }
planes of the
a s p e c i f i c o r i e n t a t i o n r e l a t i o n s h i p , provide en-
LT-GeSe2 and the interface is seen to be nearly
hanced insight into the local arrangement of the
f l a t on the atomic scale.
amorphous structure, because c r y s t a l l i z a t i o n
The l a t t i c e match
is c l e a r l y i l l u s t r a t e d by figure 2b, which is
processes are i n t i m a t e l y related with the short
taken by t i l t
range order present in the glassy state.
o r i e n t a t i o n of the [001] axis to
the electron beam.
The 0.59 nm {021} planes
The
HT-phase contains h a l f the edge-shared and h a l f
of the LT-phase are well f i t t e d to the 0.65 nm
the corner-shared tetraheda while the LT-phase
{I01} planes of the HT-one at the boundary with
only the edge-shared tetrahedra.
making angles of 20 degree, including a few d i s -
of these results in comparison with the atomic
locations to accomodate the m i s f i t of t h e i r
arrangement in the LT- and the HT-phases, we
spacings.
coclude the existence of the structural u n i t of
On the {020} planes of the HT-phase,
On the basis
t h e i r exists the layer structure consists of the
edge-shared tetrahedron GeSe4/2 in the amorphous
edge-shared tetrahedron as well as the corner-
structure with near the stoichiometric composi-
shared one.
Moreover, on the f i g u r e 2b, the HT-
crystals contain considerable defect regions,
t i o n GeSe2., which is in agreement with results of infrared and Raman measurements15 and also
218
T. Okabe et al. / Local arrangement of amorphous GeSe2
with X-ray measurements7,
8. Liu Ch'un-hua, A.S. Pashinkin and A.V. Nevoselova, Russian J. Inorg. Chem. 7 (1962) 1117.
REFERENCES I. See, for example, A.C. Wright and A.J. Leadbetter. Phys. Chem. Glasses 17 (1976) 122. 2. See the standard text. N.F. Mott and E.A. Davis, Electronic Processes in Non-Crystalline Materials, 2nd edition (Clarendon Press, Oxford 1979) pp. 445 - 452.
9. G. Dittmar and H. Schafer. Acta Cryst. B32 (1976) 1188. I0. J. Burgeat, G.Le Roux and A. Brenac, J. Appl Cryst. 8 (1975) 325. I I . G. Dittmar and H. Schafer, Acta Cryst. B31 (1975) 2060.
3. P. Tronc, M. Bensoussan and A. Brenac, Phys. Rev. B8 (1973) 5947.
12. G° Dittmar and H. Schafer, Acta Cryst. B32 (1976) 2726.
4. G. Lucovsky, F.L. Galeener, R.C. Keezer, R.H. Geils and H.A. Six, Phys. Rev. BIO (1973) 5134.
13. R. Azoulat, H. Thibierge and A. Brenac, J. Non-Cryst. Solids 18 (1975) 33.
5. P.M. Bridenbaugh, G.P. Espinosa, J.E. G r i f f i t h s . J.C. P h i l l i p s and J.P. Remeika, Phys. Rev. B20 (1979) 4140.
14. M. Esquerre, J.C. Carballes, J.P. Audiere and C. Mazieres, J. Mat. Science 13 (1978) 1217.
6. R.J. Nemanich, G.A.N. Connell, T.M. Hayes and R.A. Street, Phys. Rev. BI8 (1978) 6900.
15. Y. Kawamoto and C. Kawashima, Mat. Res. Bull 17 (1982) 1511.
7. A. Feltz. M. Pohle. H. Steil and G. Herms, J. Non-Cryst. Solids 69 (1985) 271.