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Crystal structure

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Crystal structure

1. 1. UEEP2024 Solid State Physics Topic 1 Crystal Structure
2. 2. Introduction Solid state physics : ‒ The physics in condensed matter "condensed matter" : ‒ a collection of atoms (or molecules) arranged in a well defined lattice with long range order.
3. 3. Definitions • Crystal structure = lattice + basis – A lattice is a set of regular and periodic geometrical points in space – A basis is a collection of atoms or molecules at a lattice point A crystal is a collection of atoms or molecules arranged at all the lattice points.
4. 4. Crystal structure
5. 5. Crystal structure Basic
6. 6. Example Given that the grains in a polycrystalline metal are typically 50 m across and that metal ions have a radius of 0.15 nm, estimate the average number of ions in a grain and the proportion of these ions which are adjacent to a grain boundary. (Assume the grain is roughly cubic in shape)
7. 7. Solution Volume of grain is = (50.0×10-6 m)3 = 1.25×10-13m3. Volume of ion is = (0.30×10-9 m)3 = 2.70×10-29m3. Number of ions per gain is Surface area of grain≈(6)(50× 10-6 )2 = 1.5×10-8m2. Area corresponding to one ion ≈(0.3× 10-9 )2= 9×10-20m2. Number of ions adjacent to surface of grain is Proportion of ions adjacent to grain boundary .1063.4 1070.2 1025.1 15 329 313       m m .1067.1 109 105.1 11 220 28       m m .106.3 1063.4 1067.1 5 15 11     
8. 8. translation operation • long range order – One symmetry that all lattices must have is the translation symmetry. This means that if one moves along some axis by a certain distance, one reaches another lattice point which looks the same as the first point in all respects. This movement is known as the translation operation and is also the definition of long range order in a crystal
9. 9. Translation vectors Mathematically, the crystal translation operation may be defined as: r’ = r + l a1 + m a2 + n a3 (l, m, n are integers) The quantities a1, a2 and a3 are the smallest vectors called the primitive translation vectors. T = l a1 + m a2 + n a3 T is the translation vector and any two points are connected by a vector of this form
10. 10. T = -a1 + 2 a2 r r T
11. 11. primitive and conventional cells • A lattice can be formed by repetition of a cell and the cell can be either primitive or conventional Note : The ways to define a primitive cell or conventional cell are not unique
12. 12. Primitive Lattice cell • A primitive cell is a minimum-volume cell. • There is always one lattice point per primitive cell. • The volume of primitive cell with axes a1, a2 and a3 is • The basis associated with a primitive cell is called a primitive basis. 321 aaa cV
13. 13. Wigner-Seitz cell • A Wigner-Seitz cell is a primitive cell constructed by the following method: – (i) draw lines to connect a given lattice point to all nearby lattice points; – (ii) at the mid point and normal to these lines, draw new lines or planes; – (iii) the smallest volume enclosed by these new lines or planes is the Wigner-Seitz cell.
14. 14. 3-D Lattice types • seven major groups of lattice types
15. 15. • if non-primitive cells (or conventional cell) are allowed, the number will expand to 14 Bravais lattice diamond
16. 16. Diamond lattice Zincblende lattice Si, Ge GaAs, GaP, AlAs, InSb
17. 17. Lattice planes • Lattice planes are flat parallel planes separated by equal distance. All the lattice points lye on these lattice planes
18. 18. Miller indices • Orientation of the lattice planes is specified by the Miller indices (hkl). • To determine the Miller indices: 1. Find the intercepts on the axes in terms of the lattice constants a1, a2, a3. (The axes may be those of a primitive or nonprimitive cell.) 2. Take the reciprocals of these numbers. 3. Reduce the numbers to three smallest integers by multiplying the number with the same integral multipliers. 4. The results, enclosed in parenthesis (hkl), are called the Miller indices.
19. 19. Miller indices This plane intercepts the a, b, c axes at 3a, 2b, 2c. The reciprocals of these numbers are 1/3,1/2, 1/2. The smallest three integers having the same ratio are 2, 3, 3, and thus the Miller indices of the plane are (233).
20. 20. Cubic crystal system Simple cubic Body-centered cubic Face-centered cubic
21. 21. Simple Cubic (SC) • Primitive translation vectors zac yab xaa ˆ' ' ˆ'      
22. 22. Body-centered cubic (BCC) • Primitive translation vectors )ˆˆˆ( 2 ' )ˆˆˆ( 2 ' )ˆˆˆ( 2 ' zyx a c zyx a b zyx a a       orthogonal vectors of unit length
23. 23. Face-centered cubic (FCC) • Primitive translation vectors ).ˆˆ( 2 ' )( 2 ' );ˆˆ( 2 ' xz a c zy a b yx a a      
24. 24. Example Determine the actual volume occupied by the spheres in a simple cubic structure as a percentage of the total volume.
25. 25. Solution • Volume of cube is Vc = (2r)3 = 8r3. • There are eight spheres of radius r each of volume 1/8 of the sphere. • Volume of sphere Vs = • Percentage of volume occupied   3 4 3 4 8 1 8 33 rr              %4.52%100 8 3 4 %100 3 3  r r V V c s 
26. 26. Real Crystal Structures (NaCl) • FCC • The basis consists of one Na atom and one Cl atom Na Cl
27. 27. Real Crystal Structures (NaCl) A 3x3x3 lattice of NaCl
28. 28. Real Crystal Structures (NaCl) The (111) plane of NaCl The (100) plane of NaCl
29. 29. Real Crystal Structures (CsCl) • Simple cubic A single unit cell of CsCl
30. 30. Real Crystal Structures (CsCl) A 3x3x3 lattice of CsCl
31. 31. Real Crystal Structures (CsCl) The (111) plane of CsCl The (100) plane of CsCl
32. 32. • Zincblende Real Crystal Structures (GaAs) A single unit cell of GaAs
33. 33. Real Crystal Structures (GaAs) A 3x3x3 lattice of GaAs
34. 34. Real Crystal Structures (GaAs) The (111) plane of GaAs The (110) plane of GaAs
35. 35. X-ray Diffraction • X-ray diffraction is the most commonly used technique for studying the structure of crystal lattice.
36. 36. X-ray Diffraction Typical x-ray diffraction data
37. 37. X-ray Diffraction (Bragg Law) • For constructive interference  md sin2
38. 38. X-ray Diffraction (Bragg Law) • the longest possible wavelength is for sinθ=1 and m = 1, λmax = 2d • This means that no diffraction is possible if the wavelength is greater than this maximum. This explains why we cannot study crystal structure with visible or I.R. radiation.
39. 39. X-ray Diffraction For crystal f(r) = f(r+T) T = l a1 + m a2 + n a3 • Any local physical property of the crystal is invariant under T, such as the electron number density, magnetic moment density and etc.
40. 40. X-ray Diffraction 1-D system n(x) = n(x+pa) p : arbitraty integers Expand n(x) in a Fourier series  p p apxinxn )/2exp()( 
41. 41. X-ray Diffraction 3-D system n(r) = n(r+T) Expand n(r) in a Fourier series     cellc G G G rGirdVn V n rGinrn )exp()( 1 )exp()(  
42. 42. Reciprocal lattice vectors If a1, a2 and a3 are primitive vectors of crystal lattice, then b1, b2 and b3 are primitive vectors of reciprocal lattice. Reciprocal lattice vector G= v1 b1 + v2 b2 + v3 b3 (v1, v2, v3 are any integers) b1 = 2π(a2xa3)/(a1•a2xa3) b2 = 2π(a3xa1)/(a1•a2xa3) b3 = 2π(a1xa2)/(a1•a2xa3) primitive vectors of the reciprocal lattice
43. 43. Reciprocal lattice vectors • Properties of Reciprocal lattice vector G . From these equations we observe the following properties: 1. The vector b1 is normal to both a2 and a3 . This is particularly simple for a cubic system in which case we can see that the reciprocal lattice is also a cubic system. • For any component, say b1 , we have the relations: 211  ab 01312  abab
44. 44. X-ray Diffraction • Every crystal structure has two lattices associated with it, the crystal lattice and the reciprocal lattice. A diffraction pattern of a crystal is a map of the reciprocal lattice of the crystal. A microscope image is a map of the crystal structure in real space.
45. 45. Diffraction condition • Theorem- The set of reciprocal lattice vectors G determines the possible x-ray reflections. • The Diffraction condition is written as where k is the wavevector of the beam. 2 2 2 02 G or G   Gk Gk
46. 46. X-ray Diffraction • diffraction pattern of salt crystal
47. 47. X-ray Diffraction • Diffraction pattern of crystallized enzyme.
48. 48. Brillouin Zones • Brillouin zones is defined as a Wigner-Seitz primitive cell in reciprocal lattice. • The central cell in the reciprocal lattice is of special importance in theory of solids, and is called the first Brillouin zone. • The first Brillouin zone is the smallest volume entirely enclosed by planes that are the perpendicular bisectors of the reciprocal lattice vectors drawn from the origin. • Only waves whose wavevector k drawn from the origin terminates on a surface of the Brillouin zone can be difffracted by the crystal.
49. 49. First Brillouin Zones
50. 50. Second Brillouin Zones
51. 51. Third Brillouin Zones
52. 52. Higher Brillouin Zones
53. 53. Reciprocal lattice to SC lattice • The primitive translation vectors of a simple cubic lattice may be taken as • are orthogonal vectors of unit length. • The volume of the cell is • The primitive translation vectors of reciprocal lattice are .,, 321 zayaxa  aaa  zyx  ,, .3 321 a aaa . 2 , 2 , 2 321 zbybxb                     aaa 
54. 54. Reciprocal lattice to SC lattice • The reciprocal lattice is itself a simple cubic lattice of lattice constant 2/a. • The boundaries of the first Brillouin zones are planes normal to the six reciprocal lattice vectors b1, b2, b3 at their midpoints: • The six planes bound a cube of edge 2/a and of volume (2/a)3. This cube is the first Brillouin zone of the sc crystal lattice. . 2 1 , 2 1 , 2 1 321 zbybxb                     aaa 
55. 55. Reciprocal lattice to bcc lattice • Primitive translation vectors of bcc lattice are where a is the side of the conventional cube. • The volume of primitive cell is • The primitive translations of the reciprocal lattice are );( 2 1 );( 2 1 );( 2 1 321 zyxazyxazyxa   aaa . 2 1 3 321 aV  aaa      . 2 ; 2 ; 2 321 yxbzxbzyb                     aaa 
56. 56. Reciprocal lattice to fcc lattice • Primitive translation vectors of fcc lattice are where a is the side of the conventional cube. • The volume of primitive cell is • The primitive translations of the reciprocal lattice are );( 2 1 );( 2 1 );( 2 1 321 yxazxazya   aaa . 4 1 3 321 aV  aaa      . 2 ; 2 ; 2 321 zyxbzyxbzyxb                     aaa 
57. 57. Crystal binding • What holds a crystal together? – Electrons and electrostatic forces play an important role in binding atoms together to form a solid (crystal). • Common types of crystal bindings: – (i) Ionic bonding – (ii) Covalent bonding – (iii) Metallic bonding – (iv) Hydrogen bonding – (v) Van der Waals interaction
58. 58. Crystal binding • Cohesive energy (u) – the energy required to disassemble the solid into its constituent part (e.g. atoms of the chemical elements out of which the solid is composed) • For a stable, the cohesive energy has an attractive term when the inter atomic distance is large (so that the crystal can be formed), and a repulsive term when the inter atom distance is short (so the crystal will not collapse).
59. 59. Crystal binding The equilibrium distance between two atoms is given by
60. 60. Ionic bonding • When the difference in electronegativity between two different types of atom is large, electrons will be transferred from the low electronegative atom to the high electronegative atom. The low electronegative atom will become a positive ion and the high electronegative atom will become a negative ion (e.g. Na + Cl → Na+ + Cl-). These ions will attract each other by electrostatic force to form a solid. • The repulsive force is due to the Pauli exclusion principle – this prevents the crystal from collapsing. • The attractive force is due to the Coulomb attraction between the ions.
61. 61. electronegativity • Electronegativity is the average of the first ionization energy and the electron afinity. It is the measure of the ability of an atom or molecule to attract electrons in the context of a chemical bond.
62. 62. Covalent bonds • When the electronegativiy between two atoms is small, the two atoms can form covalent bond by sharing a pair of electrons (one from each atom). • Most atoms can form more than one covalent bond. For example, C has four outer electrons and hence it can form 4 covalent bonds. • A crystal can be formed with one atom forming covalent bonds with several other atoms.
63. 63. Metallic bonding • Atoms bounded by “free electrons”. Good example is alkali metals (Li, K, Na, etc.)
64. 64. Van der Waals interaction • Coulomb attraction can occur between two neutral spheres, as long as they have some “internal charges” so that the neutral spheres can be polarized. • The repulsive force is due to the Pauli exclusion principle. • The attractive force is due to the Coulomb attraction
65. 65. Van der Waals interaction • Larger molecule ⇒ stronger Van der Waals force ⇒ higher melting point. For example: He Ne Ar Kr Xe Rn Increasing melting point This is also true for many organic molecules.
66. 66. Hydrogen bonding • First ionization energy of atomic hydrogen is very high (13.6 eV). It is highly unlikely for hydrogen to form ionic bonding. • The complete shell of hydrogen atom is 2 electrons and a hydrogen atom has only one electron. It can form only one covalent bond and it does not have sufficient bond to bind the whole crystal together with covalent bond. • However, the covalent bond between hydrogen and the other atom (e.g. oxygen) can often be polarized,
67. 67. Hydrogen bonding • These polarized molecules will “stick” to each other by Coulomb attraction. This is possible because the hydrogen size is very small. For example, for water:
68. 68. Hydrogen bonding