Block 8 Materials Chemistry 1

Block 8 Materials Chemistry 1

1. Structures of metals

   • Close packed structure
     • Illustrates the simple sphere packing arrangements. Al, Cu and gold are examples of close packed structures. The close packing model treats atoms as if they were hard spheres - this fits with current thinking.
     • CP in 2 dimensions
       • Circles that are close packed touch more other circles (6) than square packing (4) More can be fitted in in close packing.
     • CP in 3 dimensions -hpc
       • See video Hexagonal close packing. Two possible positions for second layer. Either in triangle pointing away or in triangles pointing towards you - the choice is arbitary but the next laer will go in the oppposite direction triangles to ensure the 3rd row aligns with the first. Repeating pattern of layers is ababab with alternating identical layers. In hpc some of the holes are the same through all the layers so that a wire could be drawn through them.
       • tetrahedral hole
         • 4 spheres enclose it. Three are 6 tetrahedral holes ( or intertsices) between layers two and three. Tetrahedral hole is much smaller than octahedral hole.
       • octrahedral hole
         • 6 spheres enclose the octahedral hole. There are 2 octahedral hole between the 2nd and third layers
       • Size of holes
         • If radius of a sphere is r radius of octahedral bead is 0.414 r. radius of tetrahedral bead is 0.225 r
       • number of holes
         • There are twice as many tetrahedral holes as there are spheres. But there are the same number of octahedral holes as there are spheres.
       • Coordination
         • Around a sphere there are 6 touching it in the same layer as well as three below and three above. The number of spheres in contact is known as the coordination number.
       • Cubic close packing
         • THe first and second layers are the same as in hexagonal close packing but for the third layer you must cover triagles pointing away from you to produce another different layer. THey follow an abcabc pattern.
         • Gold
           • Like many metallic elements it displays ccp. Nano particles should have at least one dimension less then 100 nm. Gold nanoparticles produced by reducing aqueous solutions of gold trichloride with reducing agents (like iron sulphate.) With other reagents can produce ruby glass.
       • Body centred structures bcc
         • sphere is in the centre of a cube surrounded by 8 equidistant neighbours at the corners of the cube.(e.g. group 1 metals) There are 6 other spheres slightly further away at the centre of other cubes. bcc occupies 68 % of the available volume compared to 74 % for hcp.
         • pythagoras's theorum
           • P's theorum can be used to work out the distances inside the cube. See section 1.3
       • Primitive cubic structure
         • Is the siplest and only polonium has this structure. They are paced at the four corners of a cube, directly on top of one another. Each is in contact with 6 others. The coordination number is therefore 6 and the geometrical arrangement is octahedral.
       • Lanthaniods/actinoids
         • Some of these have a mixed hcp and ccp structure.
       • irregular structures
         • Mn Ga In and Sn
       • Fig 1.13 -table of structures

2. interfacial angles

   • interfacial angle are constant for a crystal of a given type. This reflects the internal order of the crystal or a periodicity.
   • Crystal lattice
     • A regular repeating structure within the crystal. regular arrys of atoms are separated by a distance of 100 pm. The atoms can be replaced by lattice points in the crystal lattice.

3. x-ray crystallography

   • .As crystall structure is regular (periodicity) it can act as a diffraction grating. For diffraction to occur the wavelength must be of a similar magnitude to the grating. The scattering is caused by electrons in the atoms of the sample therefore it is difficult to distinguish between atoms of similar electron configuration or light atoms such as H.

4. The Unit Cell

   • The unit cell should be a completely representive bite of the structure of the crystal. The surroundings at each of the lattice points should be identical so that when it is repeated each unit cell is identical to the last. The unit cell chosen is usually the one that shows the full symmetry of the structure.
   • Paralellapieped
     • A 3D paralellagram. Many unit cells of crystals take this shape. It is defined by the 3 lengths a,b,c and the 3 angles alpha, beta and gamma.
     • Cubic
     • tetragonal
     • orthorhombic
     • hexagonal
     • monoclinic
     • trogonal
     • triclinic
   • primitive unit cell
   • face centred unit cell
   • body centred unit cell
   • calculating atoms in a unit cell
     • Face centred atom is shared by two unit cells. An edge atom is shared by four unit cells and a corner one is shared by eight.
     • calculating density
       • COunt number of atoms eg 8x half for corners. Work out sum of  RAM for each atom. RAM over avogadro number to find mass. Mass over volume (taken from lengths of unit cell, give density.

5. Ionic solids

   • Ions achieve noble gas configuration
   • Cations
     • Groups 1 and 2
   • anaions
     • Groups 16 and 17
   • ions form other groups
     • group 13 like Al3+ transition metals -groups 3-12 high atomic number elements in group 14 such as lead or tin (2+ or 4+) group 15 elements such as nitrogen N3-
   • Ionic bonds
     • Formed between two oppositely charged ions. Bond between them is given by the balance of the two opposite charges present.
     • Coulcomb's Law
       • The electrostatic forces between two oppositely charged ions is governed by Coulcomb's law and proportional to the product of the two charges. It is inversely proportional to the distance between them.
       • inverse sq law
         • also applies to repulsive forces
     • Degrees of covalency
   • MX - sodium chloride
     • Zinc Blende structure
       • Cubic close packed array of sulphur atoms with zinc in alternate tetrahedral holes. 1:1 stiochemistry
     • Wurtzite
       • hexagonal close packed array of sulphur with zinc occupying alternate tetrahedral holes. Compounds that adopt the wurtzite structure include BeO ZnO and NH4F.
       • polymorphs
         • polymorphs are alternate structures with the same chemical formula
     • Caeseum Chloride
       • 2 interpenetrating primitive cubic arrays. (The caeseum occupying the corners of one and the chloride occupying the corners of the other so that the opposite ion is in the centre in each case. coordination of each ion is 8 and each unit cell contains one of each type of ion (8 x1/8) it is not body centred.
   • representations
     • Different representations bring out diffeent features. 1. In terms of close packing and in terms of the other ions filling the types of holes. 2. Using a picture of a unit cell 3. Thinking of the Na surrounded by an octahedron of Cl ions and seeing how they fit together. The octahedron has twelve edges and each edge is shared by two octahedrons.
     • unit cell projections
       • Also called packing diagrams. Used to represent 3D structures in 2D. Fractional coordinates are used to describe the different heights with zero at the bottom of the unit cell and 1 at the top. 0.5 is half way up, 0.25 a quater way up etc.
   • MX2
     • Compounds with the MX2 formula include fluorite, cadmium Chloride, cadmium iodide and rutile.
     • fluorite/antifluorite
       • Lithium oxide LiO2 Oxide ions form a close packed array with the Li going in every tetrahedral hole. (antifluorite) Fluorite structure named after CaF2 fluorspar. The anions form the cubic close packed array. Antifluorite occurs when the tiny cations seem to be the ones that form the close packed array and the anions try to fit into the tiny tetrahedral holes. The octahedral holes are empty in both fluorite and antifluorite.
     • Cadmium chloride and iodide
       • CdCl2 is a cubic close packed array of Cl- ions with half the octahedral holes filled by Cd+ ions. Only alternate layers of octahedral holes are filled. CdI2 is a hexagonal close packed array also with alternate layers of octahedral holes filled.
       • not wholly ionic?
       • BiI3
         • Also hexagonal close packing with the bismuths occupying one third of the octahedral holes. Layers of unoccupied octahedral holes are alternated with layers where two thirds of octahedral holes are filled. Coordination of the bismuth is octahedral. So if 6 iodides coordinate one bismuth, then 3 bismuths must coordinate one iodide to maintain the correct ratio.
     • rutile
       • The rutile structure is named after one of the TiO2 structures. The unit cell is tetragonal. It demonstrates 6:3 coordination with each Ti ion being coordinated by 6 oxide ions in a octhedron and each oxide ion coordinated by 3 Ti ions on an equilateral triangle. It is not possible for the shapes to be perfect. The structure can be viewed as chians of linked octahedra of TiO6 where each octahedron shares opposite edges and the chains are linked by shared verticies.
   • inorganic pigments
   • corundum structure
     • Such as alpha Al2O3 is an hcp array of oxygen ions with the Al ions occupying two thirds of the octahedral holes.
   • perovskite structure
     • ABX3 with the A ion in the centre. The CaTiO3 - the Ca is coordinated to 12 O ions and 8 Ti ions. Can be thought of as a ccp array of Ca and O ions with the Ti occupying the octahedral holes. Often ferroelectric compounds that display spontaneous electric polarisation. If an ion is too small for the hole it occupies it can move off centre causing a net polaristaion. After an electric field is applied the aligned domains can remain - useful for superconducters at higher temps.
   • The spinial structure
   • different crystaline forms
     • Aluminium oxide and other co,pounds such as iron(iii) oxide, quartz and silver Chloride can exsist in more than one form. Each form is designated a greek letter.

6. Table 3.1 overview

7. ionic radii

   • r - ionic radius -distance from nucleus to end of electron density.
   • hard sphere model
     • The hard sphere model is not unreasonable but not entirely accurate either. When pne cation is replaced by another larger cation one might expect the size increase of internuclear distance to go up by the same amount. It goes up by similar amount by=ut not identical. Electron density maps also show that density never falls to zero wich would represent a definate end to the ion.
   • trends in ionic radii
     • ionic radii decreases as positive charge increases for isoelectronic cations. ionic radii increases slighty as negative charge increases on anions. Cations are smaller than parent atoms and anions are larger. Therefore cations tend to be smaller than anions for similar sized atoms.
   • Determining ionic radii
     • The Lande method
   • Coulcomb energy
     • The structure with the lowest energy is the one where any ion is in contact with the largest number of oppositely charged ions. If the size of the cation is steadily decreased a point will come when the anions touch and not further increase in attraction can take place without the coordination of the ions changing. This is bacuse the cations and anions would cease to touch.
     • geometric considerations
       • polarisability
         • With increasing r there is greater polarisability as there is greater compressability and therefore a greater ability for the electron density to be distorted.
         • polarising power
           • An ions ability to polarise another ion. Small ions and those with a high ionic charge have most polarising power

8. moving away from ionic model