chapter vi - shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/387/12/12_chapter 6.pdf · l2 a...
TRANSCRIPT
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CHAPTER V I
L W TEMPERATURE THERMAL EXPANSION OF YBa2Cu307 AND
GdBa Cu 0 . 2 3 7
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6.1 INTRODUCTION
A method of calculation of Grunei sen
parameters (GPs) from third order elastic constants is
given. A brief introduction to finite strain
elasticity theory and wave propagation in a
homogeneously deformed elastic medium is also presented
here to obtain the G P s .
6.2 INTRODUCTION TO FINITE STRAIN ELASTICITY THEORY AND
CALCULATION OF THE GENERALIZED GRUNEISEN PARAMETERS FOR
ACOUSTIC WAVES IN ORTHORHOMBIC CRYSTALS FROM THIRD ORDER
ELASTIC CONSTANTS.
Low temperature limits of the effective
- - Gruneisen functions yII '-3) and y1 - a of a uniaxial
crystal depend on the generalized GPs Y ;
tB,@ and Y ;
t B , @ of the acoustic modes propagating in different
directions in the crystal.
Let the position coordinates of a material
particle in the unstrained or natural state be a n
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=r,z,s ) . Let the coordinates of the material particle
in the strained state be X (L= 1.2.3). Consider two 1
material particles located at a and a + da . Let their 1 L L.
coordinates in the deformed state be X and X + dx. L 1 1
The elements dx are related to da by the equation L L
dx = ( a x / a a . ) d a . L 1 J J
The convention that repeated indices indicates
summation over the indices will be followed here. 6 L J
is the kronecker delta symbol and c are the i j
deformation parameters. The Jacobian of the
transformation
J = Det ( a x . / a a ) , L j
is taken to be positive for all real
transformations. If dva is a volume element in the
natural state and dvx its volume after deformation
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where p and p are the densities in the natural and 0
deformed states respectively . The square of the length
of arc from Q Lo a + da be de in 1 L 0
the L
unstrained state and de in the strained state
Then
Here nJk
are the Lagrangian strain
components. They are symmetric with respect to an
interchange of the indices J and k in terms of E ~k '
The internal energy function u ( 6 . 1 ) . for the 1 k
material is a function of the entropy and the Lagrangian
strain components. u refers to unit volume of the
undeformed state. u can be expanded in power of the
strain parameters about the undeformed state.
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1 U - - u = - ( a 2 u / a V
0 1 J " *
L2 a " k l 10,s 1 )
The linear term in strain is absent because
the natural state is one where u isminimum. Following
Brugger [1] we define the elastic constants of different
orders referred to the natural state.
cS 2
I J . k l = ( a u / a I ) . . a V k l
L J
These are the adiabatic elastic constants of
second and third orders respectively. They are tensors
of fourth and sixth ranks. The number of independent
nonvanishing second order elastic constants and third
order elastic constants for different crystal systems
are tabulated by Bhagavantam [Z]. Starting from the free
energy F ( T, ) ,one can define isothermal elastic rs
constants as derivatives of F with respect to
V P i Murnaghan [ 3 ] gives the following expression for
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stress:
The stress tensor T is referred to the deformed ~k
state of the medium . The conditions for equilibrium
require that the stress tensor be symmetric
A medium is said to be homogeneously deformed
if the components of the strain tensor Q do not vary Pq
from point to point in the medium . The homogeneously
deformed state is called the initial state and the
coordinates in this state are referred to by x;. When
the particles are given infinitesimal displacements u i
from this state , the resulting state , termed as the
final state , is-referred to by the coordinates
X - - x: + ui. The equationof motion is i I
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using the result
i a x 6.11
B x k
8 a P
Thurston and Brugger [ 4 ] arrive at the
following wave equation in terms of the displacements u l
For a homogeneously strained medium,
, where
The denotes that the quantities have to
be evaluated in the homogeneously strained state of the
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-6 medium . Using (6.1) and (6.5) for u we get A t o
~ k . p m
the first order in s as J k
we now substitute plane wave solutions in(6.12). We may
write the solution in deformed coordinates as
where w is the actual velocity of the wave in the
deformed state and n are the direction cosines of wave 1
propagation . However , i t is more advantageous to write
the displacements as
where v is called the natural velocity and N are the L
direction cosines of the wave in the undeformed state.
Let ho be the wavelength of a given wave in the
undeformed state travelling along a direction having
direction cosines N . After the deformation the wave i
length of the wave changes to A and the wave-
propagation direction is also changed. The direction
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cosines are now n , . The frequency of the wave changes 1
from o to o . In.the unstrained state the velocity w in 0 0
i the direction N is
w = w k / z n 0 0 0
6 . 1 7
In the strained state the actual velocity w
of the wave is
w = w X / z n
and the natural velocity of the wave is
v = a h / 2 n 6 . 1 9 0
ie the ratio o / w directly gives v / o without 0 0
involving the changes in the dimensions of the specimen
Substituting (6.16) in ( 6 . 1 2 )
2 0 = is 0
Po v U N N U
J j k , p m P m k
These three linear homogeneous equations
corresponding to j = 1.2,s can be solved only i f
giving the three natural velocities for any
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direction of wave propagation. The GPs y " (8 ,$) and 1
t h y ' ( 8 .@) for the J acoustic mode propagating in the
J
direction I , are
and
- y ; 8 , = -a Log Uq.)) / a Log A
= - 8 Log V (8,@ / a Log A J
v (8.4) is the natural velocity of the J th
J
acoustic mode propagating in the direction (8 ,@).
Referring to the c- axis as the z-axis when the
medium is homogeneously deformed by
(1) a c~niform longitl~dinal s t r a i n t V = dlogc along
the c-auis, or
(2) a uniform areal strain c'= d L o g ~ in the plane
perpendicular to the c axis.
The direction cosines are N = s in 8 con @, X
N = srn@sin@, N = c o s 8 . To the first order in .& , the Y z '-1
uniform longitudinal strain t" corresponds to c 3 3
and
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the uniform areal strain c, is equivalent to c = 11
The rest of the c are zero . Thp expression for ' J - 6
D : A N N 6 . 2 3 ~k ik ,pm P m
under the strains c., and C , are writ ten down
below for the orthorhombic system taking into account
the nonvanishing second order elastic constants and
third order elastic constants .
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6 . 2 4
Putting po v2 = X, the determinantal equation can he
expanded to give t h ~ ci~hic equation
9 2 X - A X + I I x - - c = 0
where A = C Oii
L
The A ,B , c are function of &, and r. . When c,
- - - and r'. are zero , their val~les are A , E. and c and
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- - - the roots of the e q u a l ion are x , x and x
1 2 3
The derivatives of A,B and c are to be taken at
zero values of c and 6". After getting the individual
GPs of the acoustic modes, the low temperature limits
- - y,,'-3) and yL'-3) are calculated using (6.28). In
orthorhombic crystals the acoustic wave velocities and
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( :Ps d e p ~ n c l only on d and not on the azimuth 0. whelp (8.
+ ) ~ i v e s the direct ion of wave prop~gat ion for the
elastic waves. The low temperature limits for thr
orthogonal crystals are then calculated using the
following expressions
Herr 1 is the index of the acoustic branch
Burgger and Fritz [ 5 ] have described a
+ . perturbation method for calculating ylm(q,~) from the
third order elastic constants suitable for machine
computation. A part from the fact that we do not need
i to know a1 1 the components YL,(q,j) c1.m = i to 9) the
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application of the strain a ' and c " in the present
method does not alter the symmetry of the crystal and
hence the factorizability of the determinant in certain
directions which reduces the lahour of the computation.
In anorthorhombic crystal the velocity w is independent 0
of 4 and we need consider the calcl~lation of w only in 0
the plane q5 = o. In this plane, for any direction 8 of
propagation. the determinant lactorises into a quadrntir
eq'lnt i o n
and a linear equation
This is true even for v in the presence of the strains
& ' and 6" so that the calculation of y i ( 6 , o )and y ;
( 8 , o ) are considerably simplified. The numer i ca 1
integration suggested by Ramji Rao and Srinivasan [6,7]
and Rao and Menon [8,9] has been used to calculate the
- - low trmperattlrr limit y (-3) and yL(-3) of the thermal
I1
expansion of tho orthorhombic crystals in this thesi~.
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6.3 LOW TEMPERATURE THERMAL EXPANSION OF YHa2C1~307
The third order elastic constants ohtained in
tahle (4.1) have been used to obtain the Gruneisen
parameters (GPS) for the acoustic modes and.the low
- - temperature limits Y I ( - 3 ) and y , ,<-3) has been
calculated. In orthorhombic crystals the acoustic wave
velocities and the GPs depend only on 8 and not on the
azimuth $ where (8, $ ) gives the direction of wavc
propagating in the crystal. Using the second and third
order elnstic constnnts of YRa C u 0 from tahle (3.1) of 2 3 7
rhapter 1 1 1 and table (4.1) of chapter I V , the wav?
velocities and GPs for the elastic waves propagating at
different angles 8 to the z-axis in the xz planes are
calc~~lated using the procedure described in detail in
sertion (6.2) of this chapter. The calculated values
are given in table(6.1). Generalized GPs lor the
elastic waves propagating at different angle 8 to the
orthorhomhic axis in YBa Cu 0 are also given in 2 3 7
table(6.1). Po
is the density of the unstrained
crystal.
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Fig.(6.1) gives the plot of the variation of
y . for the three acoustic branches as a function of the
angle (8). Fig.(6.2) gives the variation of ).,, for the
three acoustic hranches as a function of 8 , which is the
angle the direction of propagation makes with the
c-axis. Also the variation of the generalized GPs
).,and y " of these elastic modes with the direction of
propagation are shown hy polar diagrams. Fig.
. 3 , . 4 , ( 6 . 5 , and (6.6) are the polar diagram
showing the variation of GPs y for the three acoustic
branches as a Pltnction of the angle 8 which the
direction of propagation makes with the orthorhomhic
axis.
- - The low temperature 1 imi ts yL(-3) and y,,(-3)
are obtained using equation (6.28) hy numerical
integration procedure using the data from tahle(6.1).
Since the solid angles of the cone of semi-vertical
angle 8 is proportional to srn8 , the value of Y1'
-9/2 -9/2 -9/2 x . '1 X j
and x . at a n g l e 8 is multiplied by J J
--3/2 srn8 and the sum y , x srn Qn over all the 8
J J
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PIG.6.1. GPS Y ~ O R THE THREE ACOUSTIC BRANCHES AS A FUNCTION OP ANGLE FOR YBa2Cu3O7.
- - -
-1.20 - - - - - - - - - -
- 1.60
0 0 0 0 0 - f+ I * * * * - ri' A A A A A - r;
~ ~ I I I I I ~ ~ ~ I I I I I I I I I ~ I I I I I I I I I ~ I ~ ~ ~ ~ ~ ~ ~ ~ I ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
0.00 20.00 40.00 60.00 80.00 100.00 Angle (degree)
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0.80
0.60
0.40
0.20 0) .- : E' 0 a 0.00 C al m .- al C 3
& -0.20
-0.40
-0.60 A A A A A -
Angle (degree)
fl
PIG.6.2. GPa FOR THE THREE ACOUSTIC BRANCHES A S A FUNCTION OF ANGLE FOR YBa2Cu307.
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, PIG.6.3. POLAR DIAGRAM SHOWING THE PLOT OF THE GPS AND 'rs'
FOR THE ACOUSTIC BRANCHES AS A FUNCTION OF THE ANGLE WHICH THE DIRECTION O F PROPAGATION MAKES W I T 6 THE ORTHORHOMBIC AXIS
FOR YBa2Cu307-
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I ,
FIG.6.5. POLAR DIAGRAM SHOWING THE PLOT OF THE GPS 'r, FOR THE ACOUSTIC BRANCH AS A FUNCTION OF THE ANGLE WHICH THE
DIRECTION OF PROPAGATION MAKES WITH THE ORTHORAOMBIC AXIS FOR YBa2C?0,.
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133
--9/2 values are taken to be proportional t o fy,x do.
J J - -
Thus the low temperature limit ~ ~ ( - 9 ) and y ll(-9)are
obtained as
9 - --3/2 ~ ~ ( - 9 ) = C ( C Yz. X . ) sin e n
J J
- - yI (-9) and yII (-9) have been thus obtained for YBa Cu 0
2 3 7
are - . 3 6 1 and . 2
The lattice thermal expansion coefficients at
various temperatures can be expressed in terms of the
- - effective Gruneisen function Y ~ ( T ) and yII(~) as
follows
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Here S . , are the elastic compliance LJ
coefficients; v is the molar volume and x is the LOO
-B r isothermal compressibility ; L B r and y I , are the
average Gruneisen functions used by Brugger and Fritz
1 5 1 . From (6.30) we may calculate
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A t low temperature,equation ( 6.31) give
The low temperature limit of the volume
Gruneisen function is then obtained as
- - The Values of
- ~r - Br Y1'-3', Y I I 9 . YL ( - 3 , y (( (-9)-
- Y~
for YBa Cu 0 are reported in table (6.2) 2 3 7
6.4 LOW TEMPERATURE THERMAL EXPANSION OF GdBa Cu 0 2 3 7
Using the second and third order elastic
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constants of GdBa Cu 0 from table (3.2) of chapter111 2 3 7
and table (4.2) of chapter IV respectively the wave
velocities and the GPs for the elastic waves
propagating at different angles 8 to the z-axis in the
xz planes are calculated using the procedure given in
section 6.2 of this chapter. The calculated values are
given in table (6.3). Po is the density of the
unstrained crystal.
Fig.(6.7) gives the plot of the variation of y ,
for the three acoustic branches as a function of the
angle (8). Fig.(6.8) gives the variation of y., for the
three acoustic branches as a function of 8 , which is the
angle the direction of propagation makes with the
c-axis. Figs.(6.9), (6.10), (6.11) and (6.12) are the
polar diagrams showing the variation of the GPs y for
the three acoustic branches as a function of the angle e
which the direction of propagation makes with the
orthorhombic axis.
- - y1(-3) and y I1( -3) obtained for GdBa Cu 0
2 3 7 are
- -.404 and .I75 respectively . The values of yl (-3),
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-2.00 &--,--, , , , , , , , , , , , , , , , I , , , , , , , , , I , , , , , , , , , , 000 20.00 40.00 60.00 80.00 100.00
Angle (degree)
FIC.6 .7 . GPO r FOR TRE TRREK A C O U g R C B R A N C R W A S A FUNCTION OF ANGLE FOR C d ~ C u 3 0 , .
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- 0 80 0 0 0 20.00 40.00 60.00 80 00 100.00
Angle (degree)
. rIG.6.8 GPs r FOR THE THREE ACOUSTIC BRANCHES
A S A FUNCTION OP ANGLE FOR Gd-Cu.,O,
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CIG.6.9. POLAR DUCRAIl SROWMG TEE PLOT OF TALI GPs f4 AND 7;' OR THE A c o m m c BRANCRW AS A r m c n o N or r n ~ ANGLE WRICA
THE DIRECTION O r PROPAGATION MAKES WITH TRI! ORTAORflOIlBIC AXIS roll Gd~ClL ,O7-
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FIG.6.10. POLAR DIAGRAH SAOWMG THE PLOT OF THE G P s yr FOR THE ACOUSTIC BRANCH A S A FUNCTION OF THE ANGLE WAICA TI
DIRECTION OF PROPAGATION MAKES WITH T E E ORTAORAOMBIC AXIS FOR GdBa2Cu307.
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PIG.6 .11 . POLAR DIAGRAM SHOWING THE P L O T O F THE G P s \ r N FOR THE ACOUSTIC B R A N c H x AS A FUNCTION O F THE ANGLE WHICH %HE
DIRECTION O F PROPAGATION UAKES WITH THE ORTHORAOMBIC A X I S FOR GdBa2Cu30,.
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,, r IG .6 .U . POLAR DIAGRAM SHOWING T I IE PC., OF F I E GPO rs AND r'
FOR THE A c O a S n C BRANCHES AS A FUNCTION OF TEE ANGLE *HXCn THE DIRECTION OF PROPAGATION M A K W WITH TFIE ORTROREOMBIC AX=
FOR GdBa2CU,0,.
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- - E r - B r - Y i l (-3) , Y 1 '-3'' 11 - 3 , and L
of GdBa Cu 0 arr 2 3 7
also collected in tnhle ( 6 . 4 ) .
6.5. RESULTS AND DISCUSSIONS.
From the fig~~(6.1) and (6.7) of YBa CU 0 and 2 3 7
GdBa CII 0 we can see that anisotropy in are quite 2 3 7
large compared to Y ; and Y ; , Y; reaches a minimum
0 at po , which means that the value of y in this branch
increases continuously as we move towards the c-axis.
Also i t can he seen that y; and y arc negative whilr y . 2 6 4
is positive .
Fips.(6.2).and (6.8) give the variation of y.
of YHa Cu 0 and GdBa Cu 0 . The anisotropy in y - and 2 3 7 2 3 7 4
y5" are large compared to y , ' . Half the gammas in the 6
y branches are positive while all the gammas in y6 5
branches are negative.
The symmetry in y , , is quite marked even though 4
all the y ' s are positive. One may infer from the
negative values of the large number of elastic y 's that
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a t high temperature a l l the plastic y ' s may hecome
negative when the crystal is subjrctrd to a transverse
thprmal strain perpendicular to the c-axis which may
lead to lattice instability and phase transformation in
YBa Cu 0 and GdBa Cu 0 . 2 3 7 2 3 7
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tu Llw orLhorhonlbic axis in YBa. Cu. 0-. 2 .3 d
is the density ol' Llle ct-ystal
in the ur~-ztrained state
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'I'aL7le<6.4.~
- - - R I CalculaLr~.I Values 01 yL(-3). 1 ,, (3) . l i (-3),
- Ur - ). ,, (-3) alid y ,,I (3dE)n Cu. 0
2 . 3 7
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REFERENCES
1. K.Bruggcr Physical Review A.133, 1611 (1964).
2. S.Bllagavantarn Crystal symmetry and Physical
Properties Chapter 11 , Academic Press (1966).
3. F.U.Mur11agha11 , Finite deformation of an elastic
solid . John Wiley and Sons N.Y.( 1951).
4. R. N. Thurston and K. Brugger Physical Review A
133, 1604 (1964).
5. K.Brugger and T.C. Fritz Physical Review 157.
A524 (1967).
6. K.Rarnji Kao and K.Srinivasan Physics Status Solidi
29, 865 (1968).
7. R.Ramji Rao and R.Srinivasan Proceedings of indian
National Academy of science ,A36,97 (1970).
8. R.Ramji Rao and C.S. Menon, Journal. of.Low.Temp.
Phys.20,563(1975).
9. R.Ramji Rao and C.S.Menon, Journal. of
Low.Ternp.Phys. 22,325(1976).