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The amorphization of the Cd-Sb system by high pressure
Scripta METALLURGICA et MATERIALIA
Vol. 26, pp. 621-626, 1992 Printed in the U.S.A.
Pergamon Press plc All rights reserved
qTqE /hM{~PHIZATION OF THE Cd-Sb SYSTEM BY HIGH PRESSURE D.J. Li J.T. Wang B.Z. Ding State Key Lab of RSA, Institute of Metal Reserach, Academia Sinica 110015, Shenyang, P.R. of CHINA
(Received (Revised
September 25, 1991) December 13, 1991)
1.Introduction
i.I
Amorphization by high pressure
Amorphization by high pressure is a new subject which infers that crystalline alloys under high pressure are heated to a temperature below the melting point, kept for some time, and quenched to liquid nitrogen temperature; then the high pressure is removed and the temperature rises to the room temperature, which results in the crystalline alloy transformation to amorphous state spontaneously. At present, the subject of amorphization by high pressure has just begun at the Institute of Solid Physics(USSR)[1], and no reports yet appear elsewhere. Thus "the amorphization process and mechanism induced by high pressure are not clear yet. Althottgh amorphous alloys are of great importance because of their excellent properties, none of the existing amorphization methods can produce amorphous bulk material that is large in three dimensions, which greatly limits the application of amorphous alloys. At present, various methods are used to produce amorphous bulk'by pressing amorphous fine tapes or powders, such as material pressed from amorphous powders by quasi-hydrostatic pressure or explosion, etc, but because these methods all involve pressing from the amorphous state, the properties of the amorphous bulk are affected by the interfaces among the original amorphous powder or tapes. Amorphization by high pressure can produce amorphous bulk directly from crystalline particles. Thus amorphlzation by high pressure is hoped to open vast vistas for application of an amorphous state. At present, there are nominally two kinds of amorphization methods: one involves the particles disordered state at high temperature being rapidly quenched to room temperature and transformed to the amorphous state before crystallization, e.g. RQ(rapidly quenching technique), sputtering, etc; Another method is to achieve the disordered state from the crystalline phase directly at a temperature(below the crystallization temperature), such as MA(mechan~cal alloying) and SSAR(solid state amorphization reaction). The mechanism of amorphization by high pressure is different from the above-mentioned, and is classified as a third amorphization method. So the study of phase transformations under high pressure and the amorphization process are helpful in gaining a clear understanding of the mechanism of amorphization by high pressure. 1.2.
Brief introduction of Cd-Sb system Three stoichiometric compounds, CdSb, CdaSb z, Cd4Sb 3, can be obtained in the Cd-Sb system
at
atmospheric pressure, depending on the composition and beat treatment. CdSb is a stable phase, but Cd3Sb 2 and Cd4Sb 3 are metastable at room temperature. CdSb has a narrow homogeneity region and is an anisotropic semiconductor with remarkable thermoelectric and photoelectric properties. It has a complex orthorhombic structure with 16 atoms~der unit cell[2]. I n view of its relatively low density, it might be expected that its melting point will decrease with pressure and that it will undergo a transition into phases of higher density as in the case with AI~tB v and A ~ B vl compounds. ]~ae T-P phase diagram of CdSb system(see fig. 1.) shows that the melting point of the Cd
621 0036-9748/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press plc
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AMORPHIZATION OF Cd-Sb
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phase increases with increasing pressure, and the addition of Sb to Cd makes the melting point of the alloy decrease with increasing pressure at low pressure(see fig.l.(a)). The critical pressure increases with an increasing of the content of Sb, and if Sb >505, the melting point decreases with increase of pressure throughout the process(see fig. l.(b))[3]. 2.Experimental Specimen(Cd43Sbsv) antimony(99.99~),
was
prepared
by
melting
the
components,
cadmium(99.99%)
and
in evacuated silica tubes. The tube was heated to 670 °C, kept for 5 min at this
temperature, then quenched in water to avoid segregation. The specimen was annealed at lO0h.
250°C
for
The " pressure quenching " method consists of the following stages: I). Thermal treatment of the alloy of the given composition at given values of parameters P and T, providing transition of the alloy.to the high-pressure phase. The parameters of pressure and temperature for forming the new phases during the process were usually detemined by both DTA and resistometry 10 z
02). Rapid cooling of an alloy under pressure to liquid nitrogen temperature at rates of about C/s.
3). Decreasing the high p r e s s u r e to the atmospheric one, d i s m a n t l i n g chamber and s t o r i n g the sample between experiments i n l i q u i d n i t r o g e n .
the
high
pressure
The amorphous specimen was examined by a micro d i f f r a c t i o n u n i t a t room temperature, then analysed by DSC-II to observe the c r y s t a l l i z a t i o n process. The specimen i n s e v e r a l s t a g e s of c r y s t a l l i z a t i o n process i s a l s o examined by xvray, then observed by t r a n s m i s s i o n e l e c t r o n microscopy(TEM). 3.Results A c r y s t a l l i n e sample, t r e a t e d by the p r e s s u r e quenching technique, w i l l be amorphizated spontaneously when i t i s kept a t room temperature. The x - r a y p a t t e r n s i n f i g . 2 show the t r a n s f o r m a t i o n process, (a) i s the x - r a y p a t t e r n of the o r i g i n a l c r y s t a l sample, ( b ) - ( e ) are those of the h i g h l y p r e s s u r i z e d m e t a s t a b l e phase a t a s e l e c t e d time a t room temperature. The high p r e s s u r i z e d m e t a s t a b l e phase(HPMP)(see f i g . 2 (b)) i s d i f f e r e n t from the o r i g i n a l sample. Apparently, the p r e s s u r e quenching process r e s u l t s i n the production of a new phase, :which shows t h a t the phase t r a n s f o r m a t i o n in the p r e s s u r e quenching process i s d i f f e r e n t from t h a t a t atmospheric p r e s s u r e . The HPMP product, which i s c a l l e d the ~ phase f o r the Cd-Sb system, has a simple hexagonal structure(a=o.3182nm, c = o . 2 9 3 9 n m ) , and could be kept under l i q u i d n i t r o g e n temperature for a long time, but would d i s s o l v e when the temperature i n c r e a s e d . The d i f f e r e n c e of p a t t e r n s ( b ) - ( e ) shows t h a t the c r y s t a l l i n e peak~ of ~ p~ase g r a d u a l l y decrease with time, and are replaced by two i n c r e a s i n g amorphous peaks a t 25- and 45-. When the specimen has been kept a t the room temperature for 6Oh, i t i s almost completely amorphous(see f i g . 2 ( e ) ) . This suggests t h a t a phase has been transformed i n t o another metastable phase--an amorphous phase i n s t e a d of a c r y s t a l l i n e one. The amorphous specimen obtained by the p r e s s u r e quenching technique was thermo-analysed by DSC-lI(see f i g . 3 ) in order to understand i t s t h e r m o - s t a b l i t y . The specimen has an exothermic r e a c t i o n peak a t 385K, and an endothermic r e a c t i o n a t 727K. ThE specimens were heated to 50OK, 8O0K, r e s p e c t i v e l y , and r a p i d l y quenched to room temperature, then analysed by x - r a y ( s e e f i g . 4 ) and T ~ ( s e e f i g . 5 ) i n order to know what the peaks are. The x - r a y p a t t e r n (a) i n f i g . 4 i s t h a t of the o r i g i n a l c r y s t a l specimen, (b)-(d) are those of the amorphous specimen, p a r t i a l l y c r y s t a l l i z e d specimen, and c r y s t a l l i z e d specimen a t 500K, r e s p e c t i v e l y , (e) i s t h a t of the specimen a t 800K. The x - r a y r e s u l t s i n d i c a t e t h a t the exothermic peak a t 385K i s the c r y s t a l l i z a t i o n peak of the amorphous specimen. The endothermic peak a t 727K i s a phase t r a n s f o r m a t i o n peak of the c r y s t a l l i z e d specimen, where the c r y s t a l l i z e d specimen transforms to a new unknown phase i n s t e a d of m e l t i n g . The c r y s t a l l i z a t i o n temperature of amorphous Cd-Sb specimen induced by high p r e s s u r e i s only 385K. This shows t h a t the amorphous phase i s u n s t a b l e , which corresponds with the experimental
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AMORPHIZATION
OF Cd-Sb
623
r e s u l t t h a t the amorphous Cd-Sb specimen would be c r y s t a l l i z e d a t room temperature a f t e r 2-4 weeks. The c r y s t a l l i n e p e a k ( f i g . 4 ( c ) ) i s almost the same as t h a t of the o r i g i n a l c r y s t a l specimen, except f o r a l i t t l e d i f f e r e n c e in r a t i o between the i n t e n s i t i e s o f some c r y s t a l l i n e peaks. This i n d i c a t e s t h a t the amorphous specimen w i l l r e t u r n to i t s o r i g i n a l c r y s t a l l i n e phase a f t e r i t s c r y s t a l l i z a t i o n , and t h a t the ~ phase or amorphous phase ar e j u s t the i n t e r m e d i a t e m e t a s t a b l e phases in the high p r e s s u r e p r o c e s s . TEM photos c l e a r l y show the amorphous r i n g of the amorphous s p e c i m e n ( f i g . 5 ( a ) ) ; d i f f r a c t i o n p a t t e r n s on the amorphous r i n g of p a r t i a l l y c r y s t a l l i z e d s p e c i m e n ( f i g . 5 ( b ) ) ; d i f f r a c t i o n p a t t e r n s of c r y s t a l l i z e d s p e c i m e n ( f i g . 5 ( c ) ) . All t h e s e TEM r e s u l t s are in good agreement with the x - r a y r e s u l t s in f i g . 4 . 4.Discussion
The r e s u l t s show t h a t a c r y s t a l l i n e phase under high p r e s s u r e can transform to an amorphous phase when i t undergoes h e a t i n g and quenching. According to our experiment, i t i s impossible to g e t the amorphous phase from the c r y s t a l l i n e phase i f the high p r e s s u r e were not a p p l i e d to the samples. Apparently, high p r e s s u r e p la y s an important r o l e in the amorphization p r o c e s s . The c r y s t a l becomes dense a t extremely high p r e s s u r e , and atoms in the c r y s t a l become d i f f i c u l t to move because of the i n c r e a s e in a c t i v a t i o n volume work in d i f f u s i o n , which i s h e l p f u l in d e c r e a s i n g the needed c r i t i c a l c o o l i n g r a t e , and makes the amorphous phase formed a t high temperature and high p r e s s u r e easy to keep a t room temperature. But the ~ phase i s a l s o a c r y s t a l l i n e phase d i f f e r e n t from the e q u i l i b r i u m phase a t 350°C, 75GPa[3], and i t i s a ¥ phase achieved i n s t e a d of amorphous phase by p r e s s u r e quenching. This s u g g e s t s t h a t p r e s s u r e quenching r e s u l t s in a new phase t r a n s f o r m a t i o n d i f f e r e n t from t h a t a t atmospheric p r e s s u r e . The HPMP phase induced by p r e s s u r e quenching i s a dense phase a t l i q u i d n i t r o g e n temperature, and atom m o b i l i t y i n c r e a s e s with the r i s i n g of temperature. The atoms on the c r y s t a l l a t t i c e of the ~ phase w i l l become s u f f i c i e n t l y mobile to make the HPMP--~ phase 16se i t s s t a b l i t y g r a d u a l l y and transform to o t h e r new phases. But when the ~ phase i s dense, and the temperature i s low(room t e m p e r a t u r e ) , atoms cannot d i f f u s e a long d i s t a n c e , and have to move in a s h o r t d i s t a n c e t h e i r i t s o r i g i n a l l a t t i c e s i t e s . These reasons impede the formation of phases which need l o n g - d i s t a n c e d i f f u s i o n , such as the s t a b l e c r y s t a l l i n e phase. Although the energy of the s t a b l e phase i s lower than t h a t of the amorphous phase, the dynamic c o n d i t i o n s of formation of the s t a b l e phase ar e not p r e s e n t . I f t h e r e i s a s i m i l a r m e t a s t a b l e phase s t r u c t u r e as t h a t o f the HPMP phase in the experiment system, the }TPMP phase w i l l transform to t h i s new m e t a s t a b l e phase, then transform to the amorphous s t a t e [ 4 ] ; I f t h e r e i s not, the HPMP phase w i l l transform to the amorphous s t a t e directly. Amorphization by high p r e s s u r e i s a s o l i d s t a t e r e a c t i o n p r o cess of s t r u c t u r e t r a n s i t i o n under a c e r t a i n c o n d i t i o n . Ponyatovskii considered t h a t the systems which c o n t a i n semiconductor compounds or phases can transform to the amorphous s t a t e under high p r e s s u r e [ I ] . I t must be as a r e s u l t of t h e i r s p e c i a l s t r u c t u r e , and f u r t h e r d i s c u s s i o n s ar e g i v en as f o l l o w s : The Clapeyron e q u a t i o n s t a t e s t h a t m e l t i n g p r e s s u r e acco r d i n g t o : dT(P) - dP
AVf ~ Z~S r
point
of
a
crystal
under
hydrostatic
TmAVr ,~ll r
Where AV r i s the volume change upon m e l t i n g . Apparently, i f AVr)0, i n c r e a s e s with p r e s s u r e ;
varies
in o t h e r words, p r e s s u r e h i n d e r s the
the
melting
melting reaction;
temperature
T
If
Tm
AVr(O,
d e c r e a s e s with i n c r e a s i n g p r e s s u r e , and the m e l t i n g r e a c t i o n i s a c c e l e r a t e d by the p r e s s u r e . These c o n c l u s i o n s a r e s u i t a b l e to the m e l t i n g r e a c t i o n , but in the s o l i d f i c a t i o n p r o cess the p r e s s u r e has an o p p o s i t e e f f e c t . I f the p r e s s u r e i s high enough to r e s t r a i n the s o l i d f i c a t i o n proc e s s c o m p l e t e l y a t atmospheric p r e s s u r e , the o r i g i n a l s o l l d i f l c a t i o n p r o c e s s would have to change, which r e s u l t s in the formation o f a m e t a s t a b l e phase. For most s u b s t a n c e s , ~ V t > 0 , although among n o n - m e t a l l i c m a t e r i a l s , water(H20) i s
a
notable
e x c e p t i o n . Whalley observed t h a t hexagonal i c e w i l l transform to cubic i c e when i t i s cooled to 135K a t atmospheric p r e s s u r e , but the p r o c e s s does not occur i f the p r e s s u r e a p p l i e d to the
624
AMORPHIZATION
OF Cd-Sb
Vol.
26, No.
4
reaction reaches 0.6-0.7 GPa[5]; and under this condition, amorphous ice is formed instead of cubic ice. This suggests that if the pressure is higher than a critical one, the nucleation of cubic ice would be suppressed and hexagonal ice would transform to amorphous ice directly. Thus it can be said that Whalley's experimental result is evidence for ourinference. For semi-metals or semiconductors, e.g. Si, Ge, Sb, Bi, which form covalent crystals but melt to metallic liquids[6], one might expect that semiconductor compounds having relatively low coordination number might also display such behavior because ~Vf
to a stable crystalline phase,
and
results
According to the above analyses, a system which contains semiconductor compounds or phases can transform to the amorphous state under high pressure because its ~Vr<0, the high pressure would hinder the solidfying process and would constrain the nucleation of the stable crystalline phase. This is proved by the HaO and Cd-Sb systems, etc. Further work is needed for other systems.
5.Conc lusions
I). The Cd-Sb system can transform to the amorphous state under a high pressure-quenching technique because of the formation of a HPMP phase which then transforms to the amorphous phase. 2). The systems with phase transition volume changes AVr<0 can transform state under the
pressure-quenching technique.
3). This may be a new way to produce amorphous bulk material.
6.Reference 1. I.T. Belash, V.F. Degtyareva, E.G. Ponyatovskii, and
Y.I.
Rashchpkin,
Soy.
Phys. Solid S t a t e , 29(1987)1028-1031 2. K.E. Almin, Acta Chem. Scand., 2(1984)400-407 3. I.T. Belash, E.G. Ponyatovskii, High Temp.-High Pressure, 6(1974)241-244 4. 0 . I . Barkalov, I.T. Belash, V.T. Degtyarev~, and E.G. Ponyatovskii, Soy. Phys. Solid S t a t e , 29(1987)1138-1140 5. E. Whalley, J. of the Less-Common Metals, 140(1988)361-373 6. J. Lees, High Temp.-High Pressure, 1(1969)601
to
the
amorphous
Vol.
26,
No.
4
AMORPHIZATION
OF C d - S b
625
'°°I 200
,~
3oo [
"s61c
200 L
,
I~
!~CdSb ~ C d + S b
" ~"
(a)
O
I0
20
30
e (108Pa)
40
50
(b)
0
10
20
30
i
~(108pa)
40
50
Fig.1 T-P phase diagram of Cd-Sb. (a). Sb<50%, (b). Sb>50%
f A >_
300
4O3
500
6OO
7O0
8OO
Temp.(K) I 2O
° 50
100 20
140
Fig.2 X-ray p a t t e r n s of high pressure metastable phase(Micro D i f f r a c t i o n Unit, Cu, K~) (a). O r i g i n a l c r y s t a l sample ( b ) - ( f ) : High p r e s s u r e metastable phase kept a t room temperature for 12h, 24h, 36h, 48h, 6Oh, r e s p e c t i v e l y
Fig.3 DSC r e s u l t of c r y s t a l l i z a t i o n of Cd-Sb amorphous sample. The exothermic peak i s a t 385K, and the endothermic peak, 727K. The h e a t i n g r a t e i s 5 K/min.
626
AMORPHIZATION
20
~
~
~
OF Cd-Sb
~ 29
Vol.
26, No.
70
Fig.4 X-ray p a t t e r n s of amorphous specimen in i t s c r y s t a l l i z a t i o n process.(Cu, KK) (a) i s the X - r a y
pattern
of
original
crystal
specimen.
(b)-(d) are the X-ray p a t t e r n s of the amorphous specimen, p a r t i a l l y crystallized specimen, crystallized specimen at 500K, respectively. (e) is the X-ray pattern of the specimen at 800K.
I
@ @
@
L
(a) Fig.5 TEM (a) (b) (c)
(b)
(c)
photos of amorphous specimen in its crystallization process. The amorphous ring of amorphous specimen; Diffraction patterns on the amorphous ring of partially crystallized specimen; Diffraction patterns of crystallized specimen;
4
Vol. 26, pp. 621-626, 1992 Printed in the U.S.A.
Pergamon Press plc All rights reserved
qTqE /hM{~PHIZATION OF THE Cd-Sb SYSTEM BY HIGH PRESSURE D.J. Li J.T. Wang B.Z. Ding State Key Lab of RSA, Institute of Metal Reserach, Academia Sinica 110015, Shenyang, P.R. of CHINA
(Received (Revised
September 25, 1991) December 13, 1991)
1.Introduction
i.I
Amorphization by high pressure
Amorphization by high pressure is a new subject which infers that crystalline alloys under high pressure are heated to a temperature below the melting point, kept for some time, and quenched to liquid nitrogen temperature; then the high pressure is removed and the temperature rises to the room temperature, which results in the crystalline alloy transformation to amorphous state spontaneously. At present, the subject of amorphization by high pressure has just begun at the Institute of Solid Physics(USSR)[1], and no reports yet appear elsewhere. Thus "the amorphization process and mechanism induced by high pressure are not clear yet. Althottgh amorphous alloys are of great importance because of their excellent properties, none of the existing amorphization methods can produce amorphous bulk material that is large in three dimensions, which greatly limits the application of amorphous alloys. At present, various methods are used to produce amorphous bulk'by pressing amorphous fine tapes or powders, such as material pressed from amorphous powders by quasi-hydrostatic pressure or explosion, etc, but because these methods all involve pressing from the amorphous state, the properties of the amorphous bulk are affected by the interfaces among the original amorphous powder or tapes. Amorphization by high pressure can produce amorphous bulk directly from crystalline particles. Thus amorphlzation by high pressure is hoped to open vast vistas for application of an amorphous state. At present, there are nominally two kinds of amorphization methods: one involves the particles disordered state at high temperature being rapidly quenched to room temperature and transformed to the amorphous state before crystallization, e.g. RQ(rapidly quenching technique), sputtering, etc; Another method is to achieve the disordered state from the crystalline phase directly at a temperature(below the crystallization temperature), such as MA(mechan~cal alloying) and SSAR(solid state amorphization reaction). The mechanism of amorphization by high pressure is different from the above-mentioned, and is classified as a third amorphization method. So the study of phase transformations under high pressure and the amorphization process are helpful in gaining a clear understanding of the mechanism of amorphization by high pressure. 1.2.
Brief introduction of Cd-Sb system Three stoichiometric compounds, CdSb, CdaSb z, Cd4Sb 3, can be obtained in the Cd-Sb system
at
atmospheric pressure, depending on the composition and beat treatment. CdSb is a stable phase, but Cd3Sb 2 and Cd4Sb 3 are metastable at room temperature. CdSb has a narrow homogeneity region and is an anisotropic semiconductor with remarkable thermoelectric and photoelectric properties. It has a complex orthorhombic structure with 16 atoms~der unit cell[2]. I n view of its relatively low density, it might be expected that its melting point will decrease with pressure and that it will undergo a transition into phases of higher density as in the case with AI~tB v and A ~ B vl compounds. ]~ae T-P phase diagram of CdSb system(see fig. 1.) shows that the melting point of the Cd
621 0036-9748/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press plc
622
AMORPHIZATION OF Cd-Sb
Vol.
26, No.
4
phase increases with increasing pressure, and the addition of Sb to Cd makes the melting point of the alloy decrease with increasing pressure at low pressure(see fig.l.(a)). The critical pressure increases with an increasing of the content of Sb, and if Sb >505, the melting point decreases with increase of pressure throughout the process(see fig. l.(b))[3]. 2.Experimental Specimen(Cd43Sbsv) antimony(99.99~),
was
prepared
by
melting
the
components,
cadmium(99.99%)
and
in evacuated silica tubes. The tube was heated to 670 °C, kept for 5 min at this
temperature, then quenched in water to avoid segregation. The specimen was annealed at lO0h.
250°C
for
The " pressure quenching " method consists of the following stages: I). Thermal treatment of the alloy of the given composition at given values of parameters P and T, providing transition of the alloy.to the high-pressure phase. The parameters of pressure and temperature for forming the new phases during the process were usually detemined by both DTA and resistometry 10 z
02). Rapid cooling of an alloy under pressure to liquid nitrogen temperature at rates of about C/s.
3). Decreasing the high p r e s s u r e to the atmospheric one, d i s m a n t l i n g chamber and s t o r i n g the sample between experiments i n l i q u i d n i t r o g e n .
the
high
pressure
The amorphous specimen was examined by a micro d i f f r a c t i o n u n i t a t room temperature, then analysed by DSC-II to observe the c r y s t a l l i z a t i o n process. The specimen i n s e v e r a l s t a g e s of c r y s t a l l i z a t i o n process i s a l s o examined by xvray, then observed by t r a n s m i s s i o n e l e c t r o n microscopy(TEM). 3.Results A c r y s t a l l i n e sample, t r e a t e d by the p r e s s u r e quenching technique, w i l l be amorphizated spontaneously when i t i s kept a t room temperature. The x - r a y p a t t e r n s i n f i g . 2 show the t r a n s f o r m a t i o n process, (a) i s the x - r a y p a t t e r n of the o r i g i n a l c r y s t a l sample, ( b ) - ( e ) are those of the h i g h l y p r e s s u r i z e d m e t a s t a b l e phase a t a s e l e c t e d time a t room temperature. The high p r e s s u r i z e d m e t a s t a b l e phase(HPMP)(see f i g . 2 (b)) i s d i f f e r e n t from the o r i g i n a l sample. Apparently, the p r e s s u r e quenching process r e s u l t s i n the production of a new phase, :which shows t h a t the phase t r a n s f o r m a t i o n in the p r e s s u r e quenching process i s d i f f e r e n t from t h a t a t atmospheric p r e s s u r e . The HPMP product, which i s c a l l e d the ~ phase f o r the Cd-Sb system, has a simple hexagonal structure(a=o.3182nm, c = o . 2 9 3 9 n m ) , and could be kept under l i q u i d n i t r o g e n temperature for a long time, but would d i s s o l v e when the temperature i n c r e a s e d . The d i f f e r e n c e of p a t t e r n s ( b ) - ( e ) shows t h a t the c r y s t a l l i n e peak~ of ~ p~ase g r a d u a l l y decrease with time, and are replaced by two i n c r e a s i n g amorphous peaks a t 25- and 45-. When the specimen has been kept a t the room temperature for 6Oh, i t i s almost completely amorphous(see f i g . 2 ( e ) ) . This suggests t h a t a phase has been transformed i n t o another metastable phase--an amorphous phase i n s t e a d of a c r y s t a l l i n e one. The amorphous specimen obtained by the p r e s s u r e quenching technique was thermo-analysed by DSC-lI(see f i g . 3 ) in order to understand i t s t h e r m o - s t a b l i t y . The specimen has an exothermic r e a c t i o n peak a t 385K, and an endothermic r e a c t i o n a t 727K. ThE specimens were heated to 50OK, 8O0K, r e s p e c t i v e l y , and r a p i d l y quenched to room temperature, then analysed by x - r a y ( s e e f i g . 4 ) and T ~ ( s e e f i g . 5 ) i n order to know what the peaks are. The x - r a y p a t t e r n (a) i n f i g . 4 i s t h a t of the o r i g i n a l c r y s t a l specimen, (b)-(d) are those of the amorphous specimen, p a r t i a l l y c r y s t a l l i z e d specimen, and c r y s t a l l i z e d specimen a t 500K, r e s p e c t i v e l y , (e) i s t h a t of the specimen a t 800K. The x - r a y r e s u l t s i n d i c a t e t h a t the exothermic peak a t 385K i s the c r y s t a l l i z a t i o n peak of the amorphous specimen. The endothermic peak a t 727K i s a phase t r a n s f o r m a t i o n peak of the c r y s t a l l i z e d specimen, where the c r y s t a l l i z e d specimen transforms to a new unknown phase i n s t e a d of m e l t i n g . The c r y s t a l l i z a t i o n temperature of amorphous Cd-Sb specimen induced by high p r e s s u r e i s only 385K. This shows t h a t the amorphous phase i s u n s t a b l e , which corresponds with the experimental
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AMORPHIZATION
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r e s u l t t h a t the amorphous Cd-Sb specimen would be c r y s t a l l i z e d a t room temperature a f t e r 2-4 weeks. The c r y s t a l l i n e p e a k ( f i g . 4 ( c ) ) i s almost the same as t h a t of the o r i g i n a l c r y s t a l specimen, except f o r a l i t t l e d i f f e r e n c e in r a t i o between the i n t e n s i t i e s o f some c r y s t a l l i n e peaks. This i n d i c a t e s t h a t the amorphous specimen w i l l r e t u r n to i t s o r i g i n a l c r y s t a l l i n e phase a f t e r i t s c r y s t a l l i z a t i o n , and t h a t the ~ phase or amorphous phase ar e j u s t the i n t e r m e d i a t e m e t a s t a b l e phases in the high p r e s s u r e p r o c e s s . TEM photos c l e a r l y show the amorphous r i n g of the amorphous s p e c i m e n ( f i g . 5 ( a ) ) ; d i f f r a c t i o n p a t t e r n s on the amorphous r i n g of p a r t i a l l y c r y s t a l l i z e d s p e c i m e n ( f i g . 5 ( b ) ) ; d i f f r a c t i o n p a t t e r n s of c r y s t a l l i z e d s p e c i m e n ( f i g . 5 ( c ) ) . All t h e s e TEM r e s u l t s are in good agreement with the x - r a y r e s u l t s in f i g . 4 . 4.Discussion
The r e s u l t s show t h a t a c r y s t a l l i n e phase under high p r e s s u r e can transform to an amorphous phase when i t undergoes h e a t i n g and quenching. According to our experiment, i t i s impossible to g e t the amorphous phase from the c r y s t a l l i n e phase i f the high p r e s s u r e were not a p p l i e d to the samples. Apparently, high p r e s s u r e p la y s an important r o l e in the amorphization p r o c e s s . The c r y s t a l becomes dense a t extremely high p r e s s u r e , and atoms in the c r y s t a l become d i f f i c u l t to move because of the i n c r e a s e in a c t i v a t i o n volume work in d i f f u s i o n , which i s h e l p f u l in d e c r e a s i n g the needed c r i t i c a l c o o l i n g r a t e , and makes the amorphous phase formed a t high temperature and high p r e s s u r e easy to keep a t room temperature. But the ~ phase i s a l s o a c r y s t a l l i n e phase d i f f e r e n t from the e q u i l i b r i u m phase a t 350°C, 75GPa[3], and i t i s a ¥ phase achieved i n s t e a d of amorphous phase by p r e s s u r e quenching. This s u g g e s t s t h a t p r e s s u r e quenching r e s u l t s in a new phase t r a n s f o r m a t i o n d i f f e r e n t from t h a t a t atmospheric p r e s s u r e . The HPMP phase induced by p r e s s u r e quenching i s a dense phase a t l i q u i d n i t r o g e n temperature, and atom m o b i l i t y i n c r e a s e s with the r i s i n g of temperature. The atoms on the c r y s t a l l a t t i c e of the ~ phase w i l l become s u f f i c i e n t l y mobile to make the HPMP--~ phase 16se i t s s t a b l i t y g r a d u a l l y and transform to o t h e r new phases. But when the ~ phase i s dense, and the temperature i s low(room t e m p e r a t u r e ) , atoms cannot d i f f u s e a long d i s t a n c e , and have to move in a s h o r t d i s t a n c e t h e i r i t s o r i g i n a l l a t t i c e s i t e s . These reasons impede the formation of phases which need l o n g - d i s t a n c e d i f f u s i o n , such as the s t a b l e c r y s t a l l i n e phase. Although the energy of the s t a b l e phase i s lower than t h a t of the amorphous phase, the dynamic c o n d i t i o n s of formation of the s t a b l e phase ar e not p r e s e n t . I f t h e r e i s a s i m i l a r m e t a s t a b l e phase s t r u c t u r e as t h a t o f the HPMP phase in the experiment system, the }TPMP phase w i l l transform to t h i s new m e t a s t a b l e phase, then transform to the amorphous s t a t e [ 4 ] ; I f t h e r e i s not, the HPMP phase w i l l transform to the amorphous s t a t e directly. Amorphization by high p r e s s u r e i s a s o l i d s t a t e r e a c t i o n p r o cess of s t r u c t u r e t r a n s i t i o n under a c e r t a i n c o n d i t i o n . Ponyatovskii considered t h a t the systems which c o n t a i n semiconductor compounds or phases can transform to the amorphous s t a t e under high p r e s s u r e [ I ] . I t must be as a r e s u l t of t h e i r s p e c i a l s t r u c t u r e , and f u r t h e r d i s c u s s i o n s ar e g i v en as f o l l o w s : The Clapeyron e q u a t i o n s t a t e s t h a t m e l t i n g p r e s s u r e acco r d i n g t o : dT(P) - dP
AVf ~ Z~S r
point
of
a
crystal
under
hydrostatic
TmAVr ,~ll r
Where AV r i s the volume change upon m e l t i n g . Apparently, i f AVr)0, i n c r e a s e s with p r e s s u r e ;
varies
in o t h e r words, p r e s s u r e h i n d e r s the
the
melting
melting reaction;
temperature
T
If
Tm
AVr(O,
d e c r e a s e s with i n c r e a s i n g p r e s s u r e , and the m e l t i n g r e a c t i o n i s a c c e l e r a t e d by the p r e s s u r e . These c o n c l u s i o n s a r e s u i t a b l e to the m e l t i n g r e a c t i o n , but in the s o l i d f i c a t i o n p r o cess the p r e s s u r e has an o p p o s i t e e f f e c t . I f the p r e s s u r e i s high enough to r e s t r a i n the s o l i d f i c a t i o n proc e s s c o m p l e t e l y a t atmospheric p r e s s u r e , the o r i g i n a l s o l l d i f l c a t i o n p r o c e s s would have to change, which r e s u l t s in the formation o f a m e t a s t a b l e phase. For most s u b s t a n c e s , ~ V t > 0 , although among n o n - m e t a l l i c m a t e r i a l s , water(H20) i s
a
notable
e x c e p t i o n . Whalley observed t h a t hexagonal i c e w i l l transform to cubic i c e when i t i s cooled to 135K a t atmospheric p r e s s u r e , but the p r o c e s s does not occur i f the p r e s s u r e a p p l i e d to the
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AMORPHIZATION
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reaction reaches 0.6-0.7 GPa[5]; and under this condition, amorphous ice is formed instead of cubic ice. This suggests that if the pressure is higher than a critical one, the nucleation of cubic ice would be suppressed and hexagonal ice would transform to amorphous ice directly. Thus it can be said that Whalley's experimental result is evidence for ourinference. For semi-metals or semiconductors, e.g. Si, Ge, Sb, Bi, which form covalent crystals but melt to metallic liquids[6], one might expect that semiconductor compounds having relatively low coordination number might also display such behavior because ~Vf
to a stable crystalline phase,
and
results
According to the above analyses, a system which contains semiconductor compounds or phases can transform to the amorphous state under high pressure because its ~Vr<0, the high pressure would hinder the solidfying process and would constrain the nucleation of the stable crystalline phase. This is proved by the HaO and Cd-Sb systems, etc. Further work is needed for other systems.
5.Conc lusions
I). The Cd-Sb system can transform to the amorphous state under a high pressure-quenching technique because of the formation of a HPMP phase which then transforms to the amorphous phase. 2). The systems with phase transition volume changes AVr<0 can transform state under the
pressure-quenching technique.
3). This may be a new way to produce amorphous bulk material.
6.Reference 1. I.T. Belash, V.F. Degtyareva, E.G. Ponyatovskii, and
Y.I.
Rashchpkin,
Soy.
Phys. Solid S t a t e , 29(1987)1028-1031 2. K.E. Almin, Acta Chem. Scand., 2(1984)400-407 3. I.T. Belash, E.G. Ponyatovskii, High Temp.-High Pressure, 6(1974)241-244 4. 0 . I . Barkalov, I.T. Belash, V.T. Degtyarev~, and E.G. Ponyatovskii, Soy. Phys. Solid S t a t e , 29(1987)1138-1140 5. E. Whalley, J. of the Less-Common Metals, 140(1988)361-373 6. J. Lees, High Temp.-High Pressure, 1(1969)601
to
the
amorphous
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No.
4
AMORPHIZATION
OF C d - S b
625
'°°I 200
,~
3oo [
"s61c
200 L
,
I~
!~CdSb ~ C d + S b
" ~"
(a)
O
I0
20
30
e (108Pa)
40
50
(b)
0
10
20
30
i
~(108pa)
40
50
Fig.1 T-P phase diagram of Cd-Sb. (a). Sb<50%, (b). Sb>50%
f A >_
300
4O3
500
6OO
7O0
8OO
Temp.(K) I 2O
° 50
100 20
140
Fig.2 X-ray p a t t e r n s of high pressure metastable phase(Micro D i f f r a c t i o n Unit, Cu, K~) (a). O r i g i n a l c r y s t a l sample ( b ) - ( f ) : High p r e s s u r e metastable phase kept a t room temperature for 12h, 24h, 36h, 48h, 6Oh, r e s p e c t i v e l y
Fig.3 DSC r e s u l t of c r y s t a l l i z a t i o n of Cd-Sb amorphous sample. The exothermic peak i s a t 385K, and the endothermic peak, 727K. The h e a t i n g r a t e i s 5 K/min.
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AMORPHIZATION
20
~
~
~
OF Cd-Sb
~ 29
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Fig.4 X-ray p a t t e r n s of amorphous specimen in i t s c r y s t a l l i z a t i o n process.(Cu, KK) (a) i s the X - r a y
pattern
of
original
crystal
specimen.
(b)-(d) are the X-ray p a t t e r n s of the amorphous specimen, p a r t i a l l y crystallized specimen, crystallized specimen at 500K, respectively. (e) is the X-ray pattern of the specimen at 800K.
I
@ @
@
L
(a) Fig.5 TEM (a) (b) (c)
(b)
(c)
photos of amorphous specimen in its crystallization process. The amorphous ring of amorphous specimen; Diffraction patterns on the amorphous ring of partially crystallized specimen; Diffraction patterns of crystallized specimen;
4