Vol. 32, No. 9
ELECTRON PHASE TRANSITION IN TiNi?
737
temperature dependencei (T) in B2—struc—
observed decrease ofJ1 in martensite
2 0.8) is connected with the
decrease of s—d scatterin g .
ture at 80—400°C, in B19’—struoture at
(1P~,~7j?8
=
1
2—1O0° ~a nd in the MT region was inves—
t1gate~by Faraday’s method. The obtai-
ned results are given in Fig.2. It is
seen that 2 depends on temperature in
33.
Optical Investigations
The optical constants n and k were
obtained by polarintetric B eattie method
in the energy range 0.07—5ev. It ia f o -
und that only an ifra-red. (IR )part (0.07—
0
.8ev) is sensitive to MT. This p art w ill
be discussed here. In more detail these
results and the investigation method were
presented in[7J.
Figure 3 shows optical conductivity
t~(W)= nk’~ in both phases. One can see
that IR of optical conductivity which is
stip u lated by an electron transition in
1/0
the Fermi—level region changes cardinally
at MT. In au sten ite�~ (ꢀ )in creases conti—
p~q
nuously with energy reduction displaying
peaks of the interband absorption (0.085,
0
i13, 0.17,0.24, 0.34ev) which do not
allow the contribution from free carriers
S
on the Fermi surface to be separated out.
2
In m artensite the absorption edge sh ifts
to higher energies where a new 8trong
41/
peak appears near 0.52ev. The growth ofØ
a t h a ~ 0.2ev is caused by free carriers
of the Drude—Zinner type.
The strong interband absorption in
/
IR of 0’ in austenite shows high DOS in
10
the kT vicinity of Ep. (However, some
peaks can be associated with many—body
effects). The extinction of these bands
I
I
I
I
I
I
.
1 .2 .J .4 .5 .bhtiJ,cV
and the appearance of a strong peak at
0
.52ev are connected with the fact that
Ep gets into the gap which may occur
Fig. 2. Temperature dependence of the
TiNI magnetic susceptibility,
from any degenerate states splitting in
the high—symmetry points of the Brillo—
ui.n zone
•
In the band structure calcula—
,
tion[8Jof TiNi the states ( ‘7,~’
which could be sp litte d in the rhombic
symmetry lattice B19’ are 0.5ev higher
both structures. The points of strong
changes of 2 (T) coincide with the ~—
racteristic temperatures defined by the
X—ray method. The 2 CT) slope has a di—
over Ep and form the BCE) peak there[9J.
The same peak but sharper (the width
0.05ev) is in selfconsistent calculati—
on of BCE) [4] • The strong interband ab—
fferent sign in both phases. It is
sorption in far IR and also the obser—
known that the slope sign of 2 CT) is
ved. temperature dependence of magnetic
determined by the Ep position with res—
pect to the extrema of DOS [6]• The p0—
sitive slope will occur if ~p is in ml—
nimum, and negative if By is in maxi—
mum of BCE). Prom this fact it follow s
that in the structures B2 and B19’E~
is in the region of maximum and mini-
stunt of N CE), respectively,
On the assumption of’ the unchanged
diam agnetic contribution the decrease
of 2 in B19’by a factor of two is caused
by both spin— and o rb ital su scep tib ility
susceptibility can be understood if E ~
lies in the kT vicinity of this peak.
Such Ep position in TLNi provides the
tendency to stru ctu ral transform ation
as it occurs in A—15 compounds [1OJ.
From this point of view one can
understand the data on decrease of the
tem perature of the MT sta rt .A~ sw ith
alloying [ii
,
1 2J or cycling. Admixtures
and la ttic e defects cause d—electron
scatterin g which resu lts in excitation
lifetim e decrease and N(E) peak smoo—
decrease that may be associated with the
decrease of d—electrori contribution to
density of states at Ep. This is in ag—
reeinent with the XI’S data given above.
Based on these results, on the as—
thing.
it should be noted that the gap
around Epin martensite only tentative—
ly stabilizes the structure. It follows
from magnetic susceptibility data that
sumption of the main contribution to elec— the change rate of 2 is 0.1 units per
trical resistance from s—d scattering one
can understand the character of the TiNi
e le c tric a l resistance change at MT. The
degree in B19~ that indicates a very
steep gap edges. Lomer noted that such
an extremal position of E ~ in the vi—