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Chapter 1: Lanthanide Intermetallic Compounds

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CHAPTER 1

Lanthanide Intermetallic Compounds

Binary rare earth compounds as well as their derivatives have gained lot of

attention during past decades after the discovery of giant magnetocaloric effect and

simultaneous discovery of giant magnetoresistance effect. In extension to this, rare

earth intermetallic compounds with transition metals prove to be important class of

materials due to their possible potential applications towards the fabrication of

magnetic sensors, magnetic memory devices, automobile devices operated at high

temperatures, aerospace applications like the development of aircraft turbines for

commercial use. The intermetallic compounds of lanthanide with nickel are ideal

ferromagnetic materials because of their extraordinary magnetic properties used in

the generation of magnetocaloric devices, which are active materials in eco-friendly

magnetic refrigeration process. Also they have use in permanent magnets, magnetic

lasers, medical field for MRI and many more.

In this chapter we have introduced the solid state physics and introduction of

lanthanide intermetallic compounds with transition metals and their peculiar

technical applications. A description of origin of magnetism, the spin interaction

within the compounds and concept of magnetocaloric effect is included. The chapter

also includes the motivation, objectives behind the target and aim of the present

thesis.


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1.1 Solid State Physics

Science is a systematic attempt to understand natural phenomena and use the

knowledge so gained to predict, modify and control phenomena. The curiosity to

learn about the world, unravelling the secrets of nature is the first step towards the

discovery of science. Unifying concepts that offered a genuine ability to calculate the

properties of solids had to await the coming of quantum mechanics. In order to

compute the properties of solids we can use our knowledge of atoms, Quantum

Mechanics and Statistical physics to explain what we call condensed matter physics.

Condensed matter Physics also known as solid state physics deals with the

macroscopic and microscopic physical properties of matter. It classifies materials

into two categories: Crystalline materials and Non- crystalline materials

(Amorphous materials). Solid State Physics is concerned with study of crystal

structure and behaviour of electrons in crystals. It began with the discovery of X-ray

diffraction and calculation and predictions of various crystal properties. Basically the

structure of solid can be defined in terms of lattice with a group of atoms attached to

every lattice points called basis. Such type of groups of basis, when repeated in a

space forms a crystal lattice.

In comprise manner, crystal structure can be obtained by attaching atoms,

groups of atoms or molecules which are called basis (motif) to the lattice sides of the

lattice point. The smallest component of the crystal (group of atoms, ions or

molecules) when stacked together with pure translational repetition reproduces the

whole crystal is called unit cell.


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1.2 Types of Crystal Lattice

The study of condensed matter physics begins in the early years of the

century following the discovery of X-ray diffraction (XRD) by crystals leads to the

successful predictions of the various properties of crystal [1]. In a crystal structure

the lattice points group provides the information about the collection of symmetry

operations carried about the lattice point. There are 32 classes of crystal systems

based on their geometrical considerations (symmetry and internal system).

To understand the various types of lattices, one has to learn elements of group

theory:

Point group consists of symmetry operations in which at least one point

remains fixed and unchanged in space.

Space group consists of both translational and rotational symmetry operations

of a crystal.

For that reason, crystal systems are categorised into 7 groups on the basis on

angles between the three internal axes and intercepts of the faces along them. The

basic crystal systems are: cubic, tetragonal, orthorhombic, monoclinic, triclinic,

trigonal, and hexagonal. These seven basic crystal lattices are further divided into 14

crystal lattices by Bravais and are commonly known as Bravais Lattices. A Bravais

Lattice is a three dimensional lattice. A Bravais Lattice tiles space without any gaps

or holes. There are 14 ways in which Bravais Lattices can be accomplished. They are

shown in Figure 1.1.


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Figure 1.1: Seven basic crystal lattices classified into Bravais Lattices

1.3 Brillouin Zone

A Brillouin Zone is defined as a Wigner-Seitz primitive cell in the reciprocal

lattice. Reciprocal lattice is a concept which is devised for tabulating both the slopes

and the interplaner spacing of the planes of the crystal lattice. If the length assigned

to each normal is proportional to the reciprocal of the interplaner spacing of the

plane, then the points at the end of their normal drawn from a common origin is

called reciprocal lattice.

Brillouin zones for a crystal lattice can be obtained by constructing the

reciprocal lattice. Then, use the same algorithm as for finding the Wigner-Seitz

primitive cell in real space (draw vectors to all the nearest reciprocal lattice points

and then bisect them. The resulting figure is gives Wigner-Seitz cell. The nice result

of this, it has a direct relation to the Bragg‟s diffraction condition.


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22K.G G 0 or 22K.G G (1.1)

Where, K is the wave vector of an X-ray measured from the origin of reciprocal

lattice, can be written as G and –G are also vectors in the reciprocal lattice related to

original lattice structure.

Therefore, equation 1.1 concludes that the Brillouin Zone exhibits all wave vectors

K in reciprocal lattice G , which can be Bragg-reflected by a crystal.

The first Brillouin zone is the smallest volume entirely enclosed by the planes

that are perpendicular bisectors of the reciprocal lattice vectors drawn from

the origin. Or we can also define it as the volume encompassed around a

lattice point without crossing any Bragg planes.

Second Brillouin zone is the volume obtained by crossing only one plane.

Continue to higher orders…

1.4 Symmetry of a Crystal

In all the crystal lattices it is found that the angles between corresponding

faces of the lattice have same value. It means the regularity of the external structure

implies regularity of internal structure. This leads to the sense of symmetry within

the crystal lattice which is a powerful tool to study the internal structure of crystal

structures. The symmetry possessed by the crystal lattices is described by symmetry

operations.

A symmetry operation is one which carries the crystal structure into itself i.e.

leaves the crystal and its environment invariant [2]. If a body attains its position after

an operation the body is said to possesses a symmetry corresponding to that

operation. Symmetry operations performed about a point or a line define point group

symmetries and if translation is also added then the combination of rotation and


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translation define space group symmetry elements. There are 32 point groups which

cover all the possible symmetries of a crystal with respect to a point in space which

does not move during the symmetry operations [3].

Basically there are three point group symmetry elements:

Rotation axes of symmetry: If a body remains invariant after a rotation

through any angle θ, the body is said to possess rotational symmetry.

Inversion symmetry: A crystal is said to possess centre of symmetry also

known as inversion centre or inversion symmetry, if for every lattice point at

position r, there must present an another lattice point –r.

Reflection symmetry: In this symmetry operation, a line or plane exists which

divides the crystal into two exactly identical halves.

1.5 Rare Earth Compounds

The term „rare earths‟ was proposed in 1794 [4]. The term „rare‟ was used

because when they were found they were thought to be present in the earth‟s crust in

only small amounts, and the term „earths‟ was used because as oxides they have an

earthy appearance. The rare earth elements are the 15 lanthanide elements (Ln) with

atomic numbers 57 to 71. In order of increasing atomic number, they are Lanthanum

(La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd), Promethium (Pm),

Samarium (Sm), Europium (Eu), Gadolinium (Gd), Terbium (Tb), Dysprosium (Dy),

Holmium (Ho), Erbium (Er), Thulium (Tm), Ytterbium (Yb) and Lutetium (Lu).

Yttrium (Y), Scandium (Sc) and Thorium (Th), they are f-block elements except

La57

, which belongs to d-block element with electronic configuration [Xe] 5d1

6s2.

Electronic configuration of lanthanides may be represented by a general formula

[Xe] 4f n

5dm

6s2 (n= 1, 2, 3, --- 14; m= 0 or 10). Ln‟s are classified into two groups:


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light Ln‟s or cerium group (Lanthanum to Europium) as well as yttrium and

scandium and the heavy Ln‟s comprising Gadolinium through Lutetium. The light

Ln‟s are more abundant than the heavy Ln‟s. In these elements the 4f electrons are

deeply embedded within the atoms and encapsulated by 5s and 5p states situated

around them. The conduction bands of the metals are formed by the 5d and 6s

electrons and since these electrons encase the 4f electrons, they are drawn closer

toward the nucleus because of the increased shielding of the increasing nuclear

charge by the 4f electrons (when moving across the series from lanthanum to

lutetium). Some of the structural parameters of Lanthanide ions at room temperature

[1] are shown in Table1.1.

Table 1.1 The crystal structure, lattice constants and atomic radius of the elements

of lanthanide series at room temperature

Lanthanide Lattice structure

Lattice constants (Å) Atomic radius (a.u.)

A c

La dhcp 3.774 12.171 3.92

Ce () dhcp 3.681 11.857 3.83

Ce () fcc 5.161 3.81

Ce(α) fcc 4.850 (77K) 3.58

Pr dhcp 3.672 11.833 3.82

Nd dhcp 3.658 11.797 3.80

Pm dhcp 3.650 11.650 3.78

Sm rhom 3.629 26.207 3.77

Eu bcc 4.583 -- 4.26

Gd hcp 3.634 5.781 3.76

Tb hcp 3.606 5.697 3.72

Dy hcp 3.592 5.650 3.70

Ho hcp 3.578 5.618 3.69

Er hcp 3.559 5.585 3.67

Tm hcp 3.538 5.554 3.65

Yb fcc 5.485 4.05

Lu hcp 3.505 5.549 3.62


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Lanthanide elements have numerous, diverse, highly specialised applications.

The largest use of lanthanide oxides is in mixed forms, principally in petroleum

fluid-cracking catalysts and in lanthanide-phosphors for television, X-ray

intensifying, and fluorescent and incandescent lighting [5]. Ln‟s have great use as

catalysts, mainly in the refining of crude oil to improve cracking efficiencies and in

automobiles to improve oxidation of pollutants. They are used in the glass and

ceramics industry as glass-polishing compounds, decolourising agents, UV

absorbers, colouring agents, in optical lenses and glasses, and additives to structural

ceramics [6]. They also have potentials applications to be used as alloying agents to

improve the properties of superalloys and magnesium, aluminium and titanium

alloys. Ln‟s and their alloys form an important class of materials due to the presence

of localized f-band electrons in these materials. These compounds have partially

localized f states which get delocalized under pressure into d states of Ln–ion and

make them strongly-correlated. Their physical properties are different from other

materials because of large atomic number. The Lanthanides have long been an

interesting and challenging subject to physicists due to their unique magnetic,

electric, thermal and optical properties. They show various fascinating physical

phenomena, such as magnetic-optical effect, heavy-fermion state, dense Kondo

effect, magnetic polaron effect, etc [7]. In addition, they are also of great importance

to the industries due to their numerous technological applications.

1.6 Lanthanide – Nickel Intermetallic Compounds

High pressure research on compounds with lanthanide elements has drawn

great attention during last decades because of their peculiar properties [8-14]. Their

interesting features have been correlated with the existence of unfilled f-electron


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shells of the lanthanide ions, which are highly delocalized and interact strongly with

the lattice [15-16]. Correlation between f-electrons of lanthanide elements and d-

electrons of transition elements is the origin of high magnetic moments and magnetic

anisotropy and semi-metallic, half metallic and metallic character of a particular

material [17-18]. Their intermetallics with transition metals are known as promising

materials for hydrogen storage purpose which is one of the recent topic of interest in

fundamental as well as applied researches.

Intermetallic compounds in which the magnetism of the lanthanide (Ln) ions

with their partially filled localized 4f shell is combined with that of the itinerant 3d

transition (T) metals form an important class of materials, both for fundamental

studies in magnetism as well as from an applications point of view. The 4f and 3d

electron spins are coupled by exchange interactions for which three different types

are distinguished: T-T, Ln-T and Ln-Ln interactions [19]. For the iron, cobalt and

nickel rich Ln-T compounds, the T-T interaction is the strongest interaction and

primarily governs the Curie temperature. The Ln-Ln interaction is weak, although its

effect contributes to a characteristic variation of the Curie temperature with the

lanthanide element in an iso-structural series. The Ln-T interaction, which is

intermediate between the two former ones, plays an important role in the magnetism

of LnT compounds, since it couples the strongly anisotropic Ln-sublattice

magnetization to the less anisotropic T- sublattice magnetization. In this way, some

of the LnT compounds exhibit large magnetic anisotropies even at room temperature,

one of the prerequisites for potential application as permanent-magnet material. The

exchange coupling between the Ln and T electron spins is indirect. There is an intra-

atomic, ferromagnetic exchange interaction between the 4f and 5d spins of the


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lanthanide ions and an interatomic interaction between the itinerant 5d and 3d spins.

For electrons in a less than half-filled d band (the 5d electrons of the lanthanide ions)

interacting with electrons in a more than half-filled d band (3d electrons of the

transition metal), the exchange interaction is, in general, found to lead to an anti

parallel coupling between the 5d and 3d spins. Taking into account the coupling

between the spin and orbital moments of the 4f electrons, it can be explained that the

magnetic order is ferromagnetic (parallel Ln and T magnetic moments) in LnT

compounds with light and heavy lanthanide elements. The exchange interaction

between the 3d and 4f electrons is usually represented by a molecular-field

parameter, nLnT, by which the 4f and 3d sublattice magnetic moments are coupled

[20]. Values for this molecular field for the iron, nickel or cobalt-rich LnT

compounds are typically of the order of 100 T and large magnetic fields are required

in order to induce changes in the magnetic moment configuration of the two

sublattices [3].

Here, in the present work, we considered a set of some lanthanide intermetallic

compounds; LnNi‟s (where, Ln= Ce, Pr, Nd, Sm, Gd and Dy) to perform first

principle study on their structural, magnetic, electronic and thermal properties.

1.7 Crystal Properties of LnNi’s

1.7.1 Structural Properties

When the material changes from one lattice structure to another state then

they creates minimum energy configuration. The change in crystal structure with

change in internal energy leads to the phase transformation phenomenon. Phase

transformation of a thermodynamic system causes a sudden change in one of the


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physical quantities like specific heat or volume due to change in thermodynamic

variable like pressure or temperature. At the point of phase transition, two phases of

a substance have identical free energies and therefore are equally likely to exist. (The

process and phenomenon of phase transitions are discussed in detail in chapter 6).

The present chosen set of lanthanides intermetallic compounds i.e. CeNi,

PrNi, NdNi, SmNi, GdNi and DyNi, the lanthanide ion exists in trivalent form (Ln3+

)

[21-22] with Ni. These lanthanide intermetallic compounds exhibits base centred

chromium boride structure with space group Cmcm and No. 63. CrB structure except

DyNi compound whose ground state is orthorhombic FeB structure with space group

(62-Pnma) [23-29]. The unit cell representation of CrB and FeB structure is shown in

Figure 1.2.

Figure 1.2 The unit cell structure of CrB and FeB in which lanthanide intermetallics

compounds exists.


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Effect of Pressure on Structural Properties

The applications of high pressure in solid state physics have become a topic

of great interest from past few decades. In crystal structure, the distribution of atoms

and / or molecules may be homogenous or non-homogeneous. Its homogenous

assembly is called phase and characterized by many thermodynamic quantities, like

volume, pressure, temperature, energy, etc. A phase is said to be meta stable, if it is

present with the intermediate minimum free energy and the actual phase is found at

further lower energy under the same thermodynamic conditions [30-31]. If such

phases do not exist, then the crystal state becomes unstable and the system

transforms to the other stable or equilibrium phase at lower Gibb‟s free energy. The

two phases of a system are distinguished from each other, if they crystallize in

different compositions or crystal structures. The phase transition in a solid occurs

when the variation of Gibb‟s free energy is associated with some changes in

structural details (atomic or electronic configuration) and the variation of energy

takes place, if the thermodynamic conditions acting on the system, like pressure,

temperature, electric or magnetic field are varied and causes smooth variation in the

Gibb‟s free energy.

1.7.2 Electronic Properties

Electronic properties of solids deals with energy band structures of the

electronic states of the valence shell electrons. It provides an idea about the basic

nature of the material whether it is metallic characteristics, insulator or semi

conductor.

Every solid has its own characteristic energy band structure. A solid can have

large number of bands. In theory, a solid can have infinitely many bands (just as an


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atom has infinitely many energy levels). However, all but a few of these bands lie at

energies so high that any electron that attains those energies will escape from the

solid. These bands are usually disregarded. Bands have different widths, based upon

the properties of the atomic orbitals from which they arise. Also, allowed bands may

overlap, producing (for practical purposes) a single large band. On the basis of

energy band structures solids are categorise into three types shown in figure given

below:

Figure 1.3 Classification of materials on the basis of energy band gaps

Metals

Metals have free electrons and partially filled valence bands. Metals have

overlapping valence and conduction bands, therefore they are highly conductive.

They possess high density of states at the Fermi energy Level.

Semi Metals

It is the sub branch of metals. Semimetals have their highest valance band

filled. The filled valance band, however, are overlapped with the next higher band

(conduction band), therefore they are conductive but with slightly higher resistivity


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than normal metals. They possess very low density of states at the Fermi energy

Level. Semi metals do not have free electrons.

Semi conductors

Semiconductors have similar band structure as insulators but with a much

smaller band gap (< 4 eV). Some electrons can jump to the empty conduction band

by thermal or optical excitation. Semiconductors have resistivity in between those of

metals and insulators.

Insulators

Insulators have filled valence bands and empty conduction bands, separated

by a large energy band gap (>4eV), which is a “forbidden” range of energies.

Electrons must be promoted across the energy gap to conduct, but the materials

having energy gap typically > 4eV have very high resistivity.

The knowledge of the density of states, that is, what is the probability of an

electron in an energy state, also tells about the magnetic and non magnetic behaviour

of solids.

Effect of Pressure on Electronic Properties

The high pressure applications related to electronic structure of solids become

more important phenomenon to understand the electron transfer from one orbital to

another orbital. When the pressure of higher range is applied on the solids then the

average distance between the molecules decreases which increases the tunnelling to

the mobile electrons [32-33]. As a result of which both mobility and carrier

concentration of the electrons increases. Therefore, electronic structure of the solids

can be modified by applying the pressure of appropriate magnitude.


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1.7.3 Magnetic Properties and different types of magnetism

The magnetism is a phenomenon in which the materials assert an attractive or

repulsive force on other materials existing nearby them. Some elements like iron,

cobalt and nickel and their alloys behave like magnets. It is a property of materials

that respond to an applied magnetic field. The magnetic state / phase of a material

depends on temperature, pressure etc. So a material may exhibit more than one form

of magnetism depending on its temperature etc. When magnetic materials are placed

in magnetic field they show one of the following behaviour,

Diamagnetic: Such materials show a net but weak magnetic moment

opposite to an applied magnetic field.

Paramagnetic: Such materials show a net magnetic moment in the direction

of an applied field.

Ferromagnetic: Such materials possess a net magnetic moment even in zero

applied magnetic fields.

Antiferromagnetic: in such type of materials the magnetic moments of

atoms or molecules, usually related to the spins of electrons, align in a regular

pattern with neighbouring spins (on different sublattices) pointing in opposite

directions.

Ferrimagnetic: in such materials the magnetic moments of the atoms on

different sublattices are opposed as in antiferromagnetism but the opposing

moments are unequal and a spontaneous magnetization remains.

Magnetism in Lanthanide Compounds

The atomic configurations of the lanthanide metals are characterized by

partially filled 4f shells, which like the partially filled d shells of the transition

metals, can lead to a variety of magnetic effects. Depending upon the immediate

environment of the lanthanide atom, the magnitude of the effective magnetic


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moments of the atom can vary, that is why a collection of lanthanide atoms generally

represent slightly different magnetic properties than in the case of a free lanthanide

ion.

The magnetic properties of the lanthanides, as well as those interesting many-

body phenomena, are intimately related to the highly localized 4f electrons of the

open shell. As early as seventy years ago, Van Vleck et al. had studied the magnetic

properties of lanthanide ions [7]. They have suggested that the 4f electrons

responsible for the magnetism of the lanthanides are sequestered in the interior of the

atom and so experience only a small crystalline field [34]. They explained why

Hund‟s rule with L-S coupling could give values of the magnetic moments of

lanthanide ions very close to experimental observations, at least in the high-

temperature region. The lanthanides generally have large magnetic moments, yet the

exchange interactions between local spins are relatively weak. Thus other factors,

such as magnetic dipole interactions, may also have significant influence on their

magnetic orderings.

Almost the entire lanthanides exhibit complicated forms of magnetism. The

most widely recognised materials are the ferromagnetic. Nickel containing

lanthanide elements belong to same class.

1.7.4 Thermal Properties

The change in the temperature of a material affects its dynamic properties.

The branch of science “Thermodynamics”, impart great help to explain the internal

characteristics of solids which cannot be explained by the transport of single particle.

Lattice dynamics is an important aspect to study properties of materials. It concerns

with the vibrations of the atoms about their mean position. These vibrations are


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entirely responsible for thermal properties heat capacity, thermal expansion, entropy,

phonons and thermal conductivity etc [35]. The concept of phonons assumes that the

atomic vibrations are harmonic in nature, which is strictly valid at low temperatures,

typically below the Debye temperature and specific heat of solid.

In the present thesis, we have also analysed the thermal behaviour of chosen

set of lanthanide nickel intermetallic compounds (i.e CeNi, PrNi, NdNi, SmNi, GdNi

and DyNi). Various phenomenons related to thermal properties like thermal

expansion, specific heat, Gruneisen parameter, bulk modulus and equilibrium

volume by varying temperature and pressure range are studied.

1.8 Review of Literature

A plenty of literature is available based on experimental techniques as well as

theoretical approaches on lanthanide compounds. But as concerned with the topic

intermetallics of lanthanide elements they need more attention because meager

information is available about these compounds. Earlier studies on such compounds

contain the structural parameters of some Gadolinium and Dysprosium intermetallics

have been reported by Baenziger and Moriarty Jr. [23]. They have concluded that

the lanthanide intermetallics exist with either CrB structure (B33) or FeB structure

(B27) in their equilibrium state. CrB (B33) phase have orthorhombic structure with

space group Cmcm. It is denoted by space group number 63. The lattice co-ordinates

occupied by Cr and B atoms are (0, y1, 1/4) and (0, y2, 1/4) respectively. On the other

side FeB structure (B27) is also orthorhombic but with space group Pnma (space

group number 62). The atomic positions of Fe atoms in a unit cell are found at (x1,

1/4, z1) and for B (x2, 1/4, z2) atom respectively.


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A sample of polycrystalline DyNi has been reported by Tripathy et.al [24] by

arc melting stoichio-metric proportions of the starting materials (of at least 99.9%

purity) on a water-cooled copper hearth under high purity argon atmosphere. The

crystal structure and the phase purity of the sample were analyzed from the rietveld

refinement of the powder x-ray diffraction data. In purpose to analyse the magneto

caloric effect, Magnetization measurements, in the temperature range 4-150 K were

carried out on pieces of the annealed sample using a vibrating sample magnetometer.

The study on magnetoclaoric effect concludes that the compound is ferromagnetic

with a Curie temperature (TC) of 59 K. Also the effective magnetic moment

calculated from the high temperature susceptibility is found to be 10.6μB, which is

almost equal to the free ion magnetic moment of 10.3 μB of the Dy ion. Later, the

magnetic properties of lanthanides–nickel intermetallic compounds and their relative

hydrides have been studied by Yaropolov et.al [25-26]. The intermetallic compounds

were synthesized by arc melting under argon atmosphere in a furnace with a non

consumable tungsten electrode and water-cooled copper tray. Nickel (purity of

99.99%) and lanthanide metals (99.9%) were used as a starting components and

titanium sponge as a getter. Hydrogen absorption properties were investigated on a

Sievert type volumetric apparatus at room temperature and hydrogen pressures up to

1 MPa. Authors discovered that LnNi alloys easily interact with hydrogen at room

temperature and hydrogen pressure about 0.1 MPa. But In case of GdNi and SmNi

introduction of hydrogen atoms leads to expansion of the unit cells without structure

transformation. In case of TbNi and DyNi the ternary hydrides formation is

accompanied by metal sublattice structure transformation (FeB–CrB structure

transition). The conclusion of their study is that the transition temperatures of the

LnNi intermetallic compounds and ternary hydrides appeared to be lower than liquid


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nitrogen temperature (78 K). In 2008, Durga Paudyal and her co-workers computed

the magnetoelastic behaviour of GdNi [27]. The work has been done by using both

experimental and theoretical approaches. First of all the intermetallic compound was

prepared by arc melting of the pure metals under argon atmosphere The x-ray

powder-diffraction study at room temperature was performed using a Bragg-

Brentano diffractometer. The variation of lattice parameters and interatomic

distances between atoms with varying temperature has been analysed. All the

experimental results have also been compared with the computational studies. The

computation part of the work was based on LAPW method within the framework of

DFT by performing tight binding linear muffin tin orbital method. H. Drulis et.al,

have studied the magnetocaloric effect in magnetic SmNi by evaluating

magnetization and heat capacity measurements [28]. SmNi samples were synthesized

by the arc melting in an argon gas atmosphere of nickel (purity of 99.99%) and

samarium (99.9%) metals. The X-ray diffraction studies shows that the material is

single phase of CrB type structure with the lattice constants a = 3.782(3) Å, b =

10.375(4) Å and c = 4.301(2) Å, respectively. Magnetic measurements were carried

out in the temperature range of 1.7–300 K in an applied magnetic field up to 5 T

using a Quantum Design superconducting quantum interference device (SQUID)

magnetometer. The magnetization curve indicates the ferromagnetic nature at

temperatures lower than about 43 K. The magnetic and transport properties of PrNi

were studied by S. Matar et. al [30]. The single crystal was grown in a tri-arc furnace

by the Czochralski method under an argon atmosphere. The orthorhombic CrB-type

structure was confirmed by X-ray diffraction having unit cell dimensions, a =

0:38307, b = 1:0543, c = 0:4369 nm. They showed that the nonmagnetic singlet

ground state PrNi undergoes a ferromagnetic transition at TC = 20 K.


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In recent decades, many efforts have been made on the investigation of

magnetism in compounds with lanthanide (Ln) elements due to the possibility of

technological application in industry (laser, luminescent materials, permanent

magnets, glasses) and medicine (contrast agents) [36], as well as the interest in basic

scientific research, because of its electric and magnetic properties and its non-usual

structures [37]. A great deal of work has been reported recently on the magnetic

properties of lanthanides and its compounds [21, 22]. On the other hand, the study of

the origin of magnetism and formation of magnetic moments in Ln atoms in such

compounds, as well as their interactions with neighbours atoms have become one of

the main objectives of basic research in magnetism.

1.9 Motivation

Lanthanide intermetallics have become a topic of great interest because of

their peculiar properties which are helpful in fabrication of lasers, luminescent

materials, permanent glasses, permanent magnets having high coercive field, nuclear

batteries etc. They are found having great variety in their structural, magnetic, and

electrical and phonon properties as they exhibit many diverse and unusual physical

properties such as large magnetic anisotropy, complex magnetic phase diagram, a

very small crystal field splitting. On the other hand the intermetallics are also used

in industrial, technological and medicinal field. They are also known as promising

materials for hydrogen storage purposes. Some of these materials are also used as

contract agent in magnetic resonance imaging (MRI) studies, X-ray tubes etc. On

realizing the importance due to their useful applications it is realized that these

systems still need to be studied widely to analyze variation in their structural,

magnetic, electronic, thermal properties.


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1.10 Aim

Due to the advancement of the quantum mechanical approach, it is now

possible to compute various crystal properties of solids with high accuracy from

first-principles methods [38-42]. In this thesis, first principles study on the structural,

electronic, magnetic and thermal properties of LnNi (i.e. CeNi, PrNi, NdNi, SmNi,

GdNi and DyNi) compounds. The emphasis is given to study the interplay between

the localized f-electrons of lanthanide atoms and d-electrons of transition metals.

Such interactions are responsible for the high magnetic moments. It will provide the

next generation of magnetic refrigeration materials. The high pressure effect has also

been included to analyze the change in the crystallography of the structure from one

phase to another. The lanthanide compounds with Ni composition are also studied

under high temperature conditions. An understanding of the modifications of various

properties of the materials under certain conditions is desirable for novel

technological applications.

All the theoretical calculations are based on the full potential linearized

augmented plane wave approximation within the framework of density functional

theory. The complete and detailed overview about the theoretical tools and

approximations used in the present study is given in chapter 2 and 3.


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