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CHAPTER 1 INTRODUCTION The name garnet comes from the Latin granatus, a grain possibly in reference to malum garanatum (pomegranate) a plant with red seeds similar in shape, size and color to some garnet crystals. Garnet exists both in amorphous and polycrystalline form. The garnets are classified as naturally occurring garnet and chemically synthesized garnet [1*]. 1.1 Natural garnets Garnets are neo-silicates of general formula C3A2(SiO4)3, and have 8 formula units in a unit cell. The spac
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  1 CHAPTER 1   INTRODUCTION   The name garnet comes from the Latin granatus , a grain possibly in reference to malumgaranatum (pomegranate) a plant with red seeds similar in shape, size and color to somegarnet crystals. Garnet exists both in amorphous and polycrystalline form. The garnets areclassified as naturally occurring garnet and chemically synthesized garnet [1 * ]. 1.1 Natural garnets Garnets are neo-silicates of general formula C 3 A 2 (SiO 4 ) 3 ,   and have 8 formula units in a unitcell. The space group of garnet is ‘Ia3d’ i.e. a body centered cubic lattice. The C- site isusually occupied by divalent cations (Ca 2+ , Mg 2+ , Fe 2+ ) and the A - site by trivalent cations(Al 3+ , Fe 3+ , Cr 3+ ) in an octahedral/tetrahedral framework with Si occupying the tetrahedralsite. ã   c-site (dodecahedral) is the largest cation site. In this site eight oxygen ions inpositions 96 h-sites form the corners of dodecahedral configuration which amounts toa cube with the faces slightly bent along one diagonal of each. Each unit cell has 24 c-site with a orthoromhbic point group symmetry 222. ã   a-site (octahedral) is the next largest cation site. In this site six oxygen ions in position96 h-sites forn an octahedron streched along one three fold axis. Each unit cell has 16a-site in rohmbohedral point group symmetry 3 bar. ã   d-site (tetrahedral) is the smallest cation site. In this site four oxygen ions in positions96 h-sites form the corners of tetrahedral configuration. Each unit cell has 24(tetrahedral)-site with a tetrahedral point group symmetry 4 bar. * Note : The referencrces cited from internet, not being peer reviewed are * markedand are given seperately in Bibliography  2 Figure 1.1 (a) Dodecahedron geometry of c-site, (b) octahedron shape of a-site and (c) tetrahedronshape of d-site. Oxygen occupies the corners and cations occupy the center of polyhedra [3*]. 96 h-sites with a point symmetry 1 bar and are occupied by oxygen ions. In this structuretetrahedron shares two edges with neighoring dodecahdeons, the octanhedron shares six edgeswith dodecahedrons, and each dodechedron shares ttwo edges with tetrahedrons, four edgeswith other dodechedrons. The octahedron and tetrahedra do not share a common edge. Alledge-sharing involves at least one dodecahdron [1]. Figure 1.2 Garnet unit cell (BCC), as network of tetrahedral (bluish), octahedral (pinkish) anddodecahedral (circular) sites, contains 160 atoms (64 cations and 96 oxygen ions) [2*].    3 1.1.1 Applications of natural garnets ã   Garnet sand is a good abrasive and a common replacement for silica sand in sandblasting. ã   Garnet sand is also used for water filtration media. ã   For water jet cutting, garnet extracted from hard rock is suitable since it is moreangular in form, therefore more efficient in cutting. 1.2 Synthetic garnet The general formula is A 3 B 2 (CO 4 ) 3 . As in case of natural garnet we have silicon at C site buthere we can put large no. of material including Ge, Ga, Al, V and Fe and can have thevariable properties. This is a class of garnet which is prepared in laboratory both in pure anddoped with other ions form. We need to dope the garnet in order to have the variableproperties so that it can be used for various applications like in electronics and magneto-opticsindustry. There are two very important synthetic garnets that we will discuss in later sections.These are yttrium iron garnet (YIG) and yttrium aluminium garnet (YAG) [1*]. 1.2.1 Yttrium iron garnet (YIG) Yttrium iron garnet (YIG) is a kind of synthetic garnet which is ferrimagnetic with chemicalcomposition Y 3 Fe 2 (FeO 4 ) 3 , or Y 3 Fe 5 O 12 and Curie temperature 550 K. I t has advantageousproperties like Faraday rotation: -3000-(-4000) degrees/cm (Ce-doped YIG films), Saturationmagnetization: 1800-2500 G (bulk YIG at room temperature), High Quality factor inmicrowave frequencies, low absorption of infrared wavelengths up to 600 nm and Very smallline-width in electron spin resonance [5*]. 1.2.1.1 Crystalline structure of YIG YIG belongs to space group O h10 -Ia3d. It is a body centered cubic bravis lattice. In YIG, thefive Fe (III) ions occupy two octahedral and three tetrahedral sites, with the Y (III) ionscoordinated by eight oxygen ions in an irregular cube. The iron ions in the two coordinationsites exhibit different spins, resulting in magnetic behavior [1].    4 Figure 1.3 Arrangement of sites in YIG [6*]. It has three different crystallographic sites with 16Fe 3+ cations in octahedral [a] sites, 24Fe 3+  cations in the tetrahedral (d) sites and 24 Y 3+ cations in the dodecahedral{c} sites. Neither of these polyhedra is regular and oxygen lattice is much distorted. The magnetic contributionarises from the antiparallel alignment of the Y 3+ magnetic moments in the{c} site to theresultant of the antiferromagnetically coupled magnetic moments in the [a] and (d) sites. Thecation distribution at the[a] and (d) sites of garnet is expected to play the most important rolein controlling its magnetic properties. The strongest magnetic interactions in pure YIG isrelated to the inter-sublattice exchange, i.e. super exchange interaction betweenFe +3 iron inoctahedral and tetrahedral through intervening O 2- ions [16]. Figure 1.4 (a) distribution of sites in unit cell and (b) distribution of sites in an octant of unit cell with О tetrahedral cation, ∆ octahedral cation, ã dodecahedral cation [1].
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