PREFACE The solid undergoes a phase

PREFACE
The solid undergoes a phase transformation when the particular phase
of the solid becomes unstable under a given set of thermodynamic conditions.
Phase transformation in solid is associated with change in material properties.
The free energy varies continuously if any of the thermodynamic variables
like temperature or pressure is varied and the rate of variation is system and
structure dependent.
This
thesis
entitled
"INVESTIGATION
ON
PHASE
TRANSFORMATIONS O F SOME IONIC CONDUCTORS AND 11-VI
SEMICONDUCTORS'consists of two sections. Section A containing
Chapter 2 to Chapter 6 deals with the experimental investigations of
temperature induced phase transformations in some ionic conductors.
Section B containing Chapter 7 to Chapter 9 deals with the theoretical
investigations on the high pressure phase transformations of some 11-VI
semiconductors.
Chapter 1 gives a general introduction about the phase transformation
in solids and the thermodynamics of phase transformation. Various kinds of
phase transformations and properties of solids at phase transformation are also
discussed in this chapter.
Chapter 2 deals with the experimental techniques utilised to
investigate the phase transformation in solids. In particular the advantage of
thermo-Raman
spectroscopy over the conventional thermal analysis
i
techniques; DTA, DSC and temperature dependent ionic conductivity in
probing the structural phase transformation are discussed in detail.
Chapter 3 deals with the ionic conductivity and Raman investigations
on the phase transformations of sodium pyrophosphate Na4P207.
TO
understand the ionic conductivity of mixed pyrophosphate systems, the
detailed investigations on ionic conductivity and phase transformations of
NQP~O,are very much essential. Earlier investigations on the polymorphism
~
that there were at least five to six polymorphic
of N Q P ~ O indicated
structures. The ionic conductivity and themo- Raman spectra of anhydrous
Na4P207 were measured dynamically in the temperature range from 25 to
600°C with heating rate of 2OC min" to understand the structural evolution and
phase transformations involved. The spectral variations observed in the
thermo-Raman investigation indicated the transformation of Na4P207 from
low temperature phase
(E)
to high temperature phase (a)proceeded through
pre-transformational region from 75 to 410°C before the major orientational
disorder at 420°C and minor structural modifications at 51 1, 540 and 560°C.
The activation energies and enthalpies of the proposed phase transformations
were determined.
Chapter 4 deals with the investigation on anion reorientation in
Na3P04during the phase transformation. Alkali-metal super ionic conductors
(SIC'S) based on Na3P04 structure received considerable attention in recent
times. Na3P0, has also been an important example for a study of the interplay
of anion rotation and cation hopping, i.e. the importance of the "PaddleWheel" mechanism.
To understand the ionic conductivity of SIC'S with
Na3P04 structure, the detailed investigations on ionic conductivity and phase
transformations of NapP04 are very much essential. Although the phase
transformation of Na3P04 were studied by several techniques the nature and
sequence of the phase transformations are not known completely yet.
Thermo-Raman spectroscopic (TRS) studies were carried out on anhydrous
Na3P04and dehydrated Na;P04 in order to probe dynamics of the
PO^" during
the temperature induced phase transformation. Thermal analysis techniques;
differential thermal analysis (DTA), differential scanning calorimetry (DSC)
and temperature dependent electrical conductivity measurements were also
carried out to derive the nature of phase transformation. The spectral variation
observed in the theno-Raman spectra of low temperature a-phase of
anhydrous Na3P04 and dehydrated Na3P04strongly suggests the possibility of
the local structural difference between these two samples. The dramatic
changes in the spectral protile of the v; mode and the sudden increase in the
line width of the v l mode of pod3'observed in the temperature interval from
33 1 to 345" for anhydrous Na3P04revealed the collapse of the structure and
the high degree of disorder nature of the ions during a-y Na3P04 phase
transformation.
The jump in the dc conductivi? and the corresponding
decrease in the activation energy upon transformation to the high temperature
y-Na3P04 phase attributed to the rotational motion of the translationally fixed
~
0 The~ rotational
~ ~ motion
. of the translationally fixed PO:
enhances the
Nat diffusion and results in to high conductivity during the a-y Na3P04 phase
transformation. The reorientational motion of the phosphate anions revealed
by TRS studies and their coupling with sodium motion inferred from electrical
conductivity measurements strongly suggests a correlation between the
structural and dynamic changes in the case of the a-y Na3P04 phase
transformation.
Chapter 5 deals with the investigations on temperature dependent
structural evolution of sodium metaphosphate NaP03 glass. Phosphates are
among the best glass formers and have been widely investigated in recent
times.
The understanding of molecular level structural information of
phosphate glasses is very much essential.
The unique microwave-absorbing ability of NaH2P04'2H20was found
to be very useful for preparing crystal and glassy sodium super ionic
conductors as a component of batch mixtures. In this work NaP03 glass was
prepared by both conventional melt quench and microwave heating from
NaHzPO4'2Hz0as a starting material. The structure of NaPOJ glass and their
structural evolution upon heating through glass transformation were probed by
combination
of complementary techniques like differential scanning
calorimetry (DSC),powder X-ray diffraction (PXRD),Fourier transform
infrared (FT-IR) and thenno-Raman spectroscopy (TRS).The presence of the
prominent bending mode of P01' in the FT-IR spectrum of NaP03 glass phase
strongly signaled the network features of phosphate species in the glass phase.
Therrno-Raman studies on NaP03 glass clearly indicate the removal of water in
the temperature interval from 85 to 145OC, glass transformation at around
280°C and the crystallization process at around 330°C. The thermo-Rarnan
spectrum of crystalline Nap03 obtained after the glass transformation indicated
that this crystalline phase is more close to phase I of NaP03.
Chapter 6 deals with the investigation on structural changes in the
phase transformations of y-Bi2Mo06.
Bismuth molybdates, represented
stoichiometrically as Bi203.nMo03 fall into an unusual category of
compounds, the ternary bismuth oxide systems Bi-M-0 (where M=Mo, W, V:
Nb and Ta) and all exhibit interesting physical properties. Bismuth molybdates
figure prominently on an industrial scale as heterogeneous catalysts in
reactions such as selective oxidation or ammoxidation processes. The main
characteristic of Bi-Mo mixed oxides utilized in these reactions are their abiliy
to use lattice oxygen to oxidize hydrocarbons and to be reoxidized in presence
of gaseous oxygen. These catalysts can form several phases depending on the
B i N o ratio and reaction temperature. Of the various known bismuth
molybdate phases, only the a, P and y phases are recognized as active catalysts
for selective oxidation and ammoxidation. In order to understand the structure
and its relationship to the observed catalytic activities, bismuth moiybdare
phases have been investigated using various techniques. Despite their broad]!.
similar catalytic performance, the a and
P -phases differ substantially in
structure from they- phase.
An extensive thermo-Raman investigations have been carried out on
y(L)-Bi2Mo06 in an effort to gain an understanding the structural evolution and
phase transformation involved during a dynamical thermal process. The room
temperature Raman spectrum of y(L)-Bi2Mo06phase indicated the presence of
comer sharing distorted Moo6 octahedra. The spectral variations, shift in the
band position, decrease in intensities and broadenings of the Mo-0 modes
observed in thermo-Raman spectra and the thermal evolution of Mo-0 bonds
in the temperature interval from 25 to 610°C strongly suggest that the
tnnsfonnation From y(L)-BizMo06 to y(1)-Bi2Mo06 phase involves a gradual
anisotropic thermal response of Mo-0 bonds initially upon heating, and further
appreciable increase in octahedral distortion from 310°C onwards, until the
phase is on the verge of becoming composed of distoned tetraheda. The
spectral variations in the thermo-Raman
spectra observed for the
transformation to y(H)-Bi2Mo06 starting at around 62OoC and ending at 660°C
revealed the sluggish nature of the transformations and also clearly indicated
the transformation from highly distorted Moo6 octahedra to distorted
tetrahedra.
The initial anisotropic thermal response of the Mo-0 bonds and
further appreciable distortions in the Moo6 octahedra caused by decrease in
bond length of the apical Mo-0 distance in temperature interval from 311) to
610°C leads to charge transfer benveen [ ~ i 2 0 2 1 ~and
'
MOO^]^' layers, and this
interlayer charge transfer may be a factor contributing to the catalytic
performance of this material.
To understand the physical and chemical properties of solid state
materials one has to do the band structure calculations. The linear methods
are relatively faster and accurate. The linear muffin tin orbital (LMTO) is
widely used method for the band structure determination by which the self
consistent electronic structure problem can be solved in highly efficient
manner.
Chapter 7 discuss the self consistent tight binding linear muffin tin
orbital (TB-LMTO) method used for the calculation of the energy band
structure of ambient and high pressure phases of some 11-VI semiconductors.
Chapter 8 deals with the pressure induced structural transformations of
zinc chalcogenides ZnX (X
=
S, Se, Te). High pressure studies of 11-VI
semiconductors have considerable attention due to their polymorphic structural
transformation.
The generally accepted view of 11-VI compounds was that
these compounds transform from the zinc blende (ZB) or wwtzite to the rock
salt (RS) and then to the P-Sn phase. However, recent reports indicated
appreciable alteration to these generally accepted structural systematics. The
structural phase transformations of ZnS, ZnSe and ZnTe under high pressure
are studied by tight binding linear muffin tin orbital (TB-LMTO) method. A
simple cubic 16 (SC16) phase is observed in all the three zinc chalcogenides
ZnX (X= S, Se, Te). In ZnS and ZnSe, the sequence of transformation is
similar as zinc blende (ZB)
+ SC16 + rock
salt (RS) and in ZnTe the
transformation sequence is different namely the SC16 phase is observed above
the cinnabar phase. The ground state properties of the zinc chalcogenides ZnX
(X= S, Se, Te) are also calculated. The two high pressure phases cinnabar and
SC16 are found to be very close in energy. Present theoretical calculations
confirm the experimentally observed structural phase transformations of ZnTe
and also indicated the possibility of the existence of the SC16 phase. The
calculated transformation pressures and transformation sequences are in good
agreement with the experimental results.
Chapter 9 deals with the pressure induced structural transformations of
alkaline earth chalcogenides AX (A= Be, Mg, Ca; X = S, Se, Te). Among the
chalcogenides of Be, Mg, Ca, Sr, Ba; beryllium chalcogenides (BeS. BeSe.
BeTe) crystallizes in fourfold cubic zinc blende (ZB) structure. The remaining
chalcogenides have NaCl structure at ambient pressure except B e 0 and MgTe.
which adopt the fourfold coordinated (hexagonal) wurtzite structure. Tne
sequence of pressure induced phase transformation and the nature of hi&
pressure phases of the alkaline earth chalcogenides (AX, where A= Be, big.
Ca; X = S, Se, Te) are studied theoretically by TB-LMTO method.
Recent
experimental investigations indicated that the NiAs structure, the hexagonal
analogue of the NaCl structure was found to be the high pressure phase in the
alkaline earth chalcogenides. The high pressure experimental investigations
indicated the beryllium chalcogenides transforms from zinc blende strucue to
nickel arsenide structure. The calculated transition pressure of ZB to NiAs
phase for benylium chalcogenides are in good agreement with the earlier
reported experimental values. The theoretical calculation shows the NiAs and
NaCl phases are very close in energy and the NiAs phase is found to be more
stable than the NaCl phase in beryllium chalcogenides, which agrees with the
earlier experimental studies.
The theoretical calculations have carried out for four phases such as
NaCI, NiAs, FeSi, and CsCl structures of magnesium chalcogenides. Total
energy calculations for MgS and MgSe shows that the stable phase at ambient
conditions is NaC1. Under the application of pressure FeSi structure is found
intermediate between NaCl and high pressure CsCl structure. Calculations for
MgTe show the NiAs phase is more stable than the NaCl phase.
Calcium chalcogenides crystallizes in the NaCl (Bl) strucrure. All the
three compounds CaS, Case and CaTe are expected to undergo NaCl
-
CsCl
(B2) transformation under pressure, following the sequence found in the most
alkaline earth chalcogenides.
The TB-LMTO calculations for calcium
chalcogenides are found to have the possibility of the NiAs phase as
intermediate high pressure phase. The calculated transformation pressures,
transformation sequences, lattice parameters and bulk moduli of alkaline earth
chalcogenides are compared with the available experimental data.