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Introduction: X-Ray Diffraction

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Introduction: X-Ray Diffraction Introduction: X-Ray Diffraction • XRD is a powerful experimental technique used to determine the – crystal structure and its lattice parameters (a,b,c,a,b,g) and – spacing between lattice planes (hkl Miller indices)→ this interplanar spacing (dhkl) is the distance between parallel planes of atoms or ions. • Diffraction is result of radiation’s being scattered by a regular array of scattering centers whose spacing is about same as the l of the radiation. • Diffraction gratings must have spacings comparable to the wavelength of diffracted radiation. • We know that atoms and ions are on the order of 0.1 nm in size, so we think of crystal structures as being diffraction gratings on a sub-nanometer scale. • For X-rays, atoms/ions are scattering centers (photon interaction with an orbital electron in class24/1 the atom). Spacing (dhkl) is the distance between parallel planes of atoms…… XRD to Determine Crystal Structure & Interplanar Spacing (dhkl) Recall incoming X-rays diffract from crystal planes: reflections must be in phase for q is scattering a detectable signal (Bragg) angle i.e., for diffraction to occur, x-rays scattered off adjacent crystal planes extra l must be in phase: distance traveled q q by wave “2” spacing Adapted from Fig. 3.37, d between Callister & Rethwisch 3e. hkl planes Measurement of critical angle, qc, allows computation of interplanar spacing (d) X-ray l d = intensity 2 sinqc a (from Bragg’s Law(1) dhkl = cubic h2 + k 2 + l2 (Bragg’s Law is detector) (2) not satisfied) q 2 qc The interplanar (dhkl) spacings for the 7 crystal systems •As crystal symmetry decreases, the number of XRD peaks observed increases: •Cubic crystals, highest symmetry, fewest number of XRD peaks, e.g. (110)=(101)=(011), since a=b=c → all 3 have same interplanar (dhkl) spacing. •Orthorhombic crystals, lower symmetry, larger number of XRD peaks, e.g. (110)≠(101) ≠(011), since a≠b≠c → all 3 have different interplanar (dhkl) spacings class24/3 Geometry of XRD (F.Y.I.) •A single (monochromatic) wavelength of x-ray radiation is often used to keep the number of diffraction peaks to a small workable number, since samples often •The diffracted beam intensity is monitored consists of many small crystal grains electronically by a mechanically driven scanning orientated randomly. radiation detector. Usually graphite to X-ray focus beam like a Source parabolic mirror (fixed) High Z to max. absorption. Removing Collimate q 2q q unwanted radiation beam q Bragg angle = q Diffraction angle, what’s measured experimentally = 2q •Counter/detector is rotated about O-axis; 2q is its angular position. •Counter and specimen are mechanically coupled such that a rotation of the specimen through q is accompanied by a 2q rotation of the counter. class24/4 •This assures that the incident and reflection angles are maintained equal to one another. More on Bragg’s Law •Bragg’s Law is a necessary but insufficient condition for diffraction. •It only defines the diffraction condition for primitive unit cells, e.g. P cubic, P tetragonal, etc., where atoms are only at unit cell corners. •Crystal structures with non-primitive unit cells have atoms at additional lattice (basis) sites. •These extra scattering centers can cause out-of-phase scattering to occur at certain Bragg angles. •The net result is that some of the diffraction predicted by Bragg’s Law (eqn. 1) does not occur, i.e. certain sets of planes do not exist (forbidden reflections). •Selection (or Reflection) rules: Bravais Lattice Example Compounds/Structures Allowed Reflections Forbidden Reflections Primitive (P) Cubic Simple Cubic (a-Po) Any h,k,l None Body-centered (I) Cubic Body-Centered Cubic metal h+k+l even h+k+l odd Face-centered (F) Cubic Face-Centered Cubic metal h,k,l all odd or all even h,k,l mixed odd or even Face-centered (F) Cubic NaCl-rocksalt, ZnS-zincblende h,k,l all odd or all even h,k,l mixed odd or even Face-centered (F) Cubic C, Si, Ge - Diamond cubic As FCC, but if all even h,k,l mixed odd or even then h+k+l=4n (n is and if all even and integer) h+k+l≠4n Primitive (P) Hexagonal Hexagonal closed packed metal All other cases h+2k=3n, l odd These rules are calculated based on atomic scattering factors (f) and structure factors (F), which we will discuss next class (you will also class24/5 see them in HW4 and possibly on the final exam). Selection Rules for Cubic P I F a a dhkl = = S (hkl) S (hkl) S (hkl) h2 + k 2 + l 2 S 1 100 - - - - Odd+even where “S” is line #, 2 110 2 110 - - Odd+even e.g. for (100) S=1 and for (200) S=4 3 111 - 3 111 All odd 4 200 4 200 4 200 All even 5 210 - - - - Odd+even 6 211 6 211 - - Odd+even As we will determine 7 - - - - - later when we calculate the structure factors, 8 220 8 220 8 220 All even these selection rules 9 221,300 - - Odd+even also hold for the other 5 non-primitive Bravais 10 310 10 310 Odd+even lattices, e.g. 11 311 - - 11 311 All odd I-tetragonal, All even F-orthorhombic, etc. 12 222 12 222 12 222 class24/6 Example of XRD Pattern •The unit cell size and geometry may be resolved from the angular positions of the diffraction peaks. •When the distance between the peak spacings are all pretty much the same (2q) it is likely cubic. •The arrangement of atoms within the unit cell is associated with the relative intensities of these peaks z z z (200) c (110) c c (211) y (110) y y a b a b a b x x x (211) Diffraction d-spacing decreases pattern for according to Bragg’s Law polycrystalline (200) Intensity (relative) Intensity a-iron (BCC) Diffraction angle 2q h+k+l even are only allowed according to previously shown class24/7 selection rules for BCC metal. Review of Systematic Absences in the Diffraction Patterns of 4 Cubic Structures =S= → When indexing XRD data for your material always try cubic first (least amount of diffracting planes since most symmetric lattice parameters). class24/8 X-Ray Diffraction for Tetragonal, Hexagonal and Orthorhombic Crystals Plane spacings for: Bragg’s Law (1): (4) l d = (1) 2 sinq c (5) (2) Plane spacing for cubic crystals (6) Combined (1) and (2): 2 2 2 If crystal is tetragonal with a=a≠c then (1) and (4) become: nl sin q 2 l 2 2 2 = or sin q = (h + k + l ) 2 2 2 2 2 2 l l 2a h + k + l 4a (3) sin 2 q = (h2 + k 2 )+ (l 2 ) (7) 4a2 4c2 For a particular incident x-ray wavelength and cubic crystal of unit cell size a, this equation predicts all If crystal is hexagonal with a=a≠c then (1) and (5) become: 2 2 possible Bragg angles at which diffraction can occur 2 l 2 2 l 2 sin q = 2 (h + k + hk)+ 2 (l ) (8) from planes (hkl). 3a 4c If crystal is orthorhombic with a≠b≠c then (1) and (6): Diffraction planes are determined solely by the l2 l2 l2 sin 2 q = (h2 ) + (k 2 ) + (l 2 ) (9) shape and size (lattice parameters) of the unit cell. 4a 2 4b2 4c 2 •Next class we will determine how intensities of diffracted beams are determined by positions of the atoms within the unit cell. Thus, we must measure the intensities if we are able to obtain any information at all about atom positions. (Intensitiesa (I) are related to Structure Factor (F), i.e., dhkl = I is proportional to F). h2 + k 2 + l2 •We will determine that for many crystals there are particular atomic arrangements which reduce the intensities of some diffracted beams to zero, i.e. no diffracted beam at the angle ( ) predicted q class24/9 by Equations (3), (7), (8), (9), etc., which means F=0. The Effect of Cell Distortion (Symmetry) on XRD Patterns •As the symmetry of crystal decreases more peaks are seen, e.g. cubic → triclinic. •Similar with cell distortion. If distort I cubic along [001] by 4% so c is now 4.16Å get I tetragonal. •This decreases symmetry more, and shifted diffraction lines are formed (shown in the middle for tetragonal). •If now stretch 8% along [010] axis with a=4.00Å, b= 4.32Å, and c= 4.16Å to get I orthorhombic which lowers symmetry even more thus adding more diffraction lines (shown far right). •The increase in number of lines is due to the introduction of new hkl plane d-spacings, caused by non-uniform distortion. •Thus in cubic cell, the (200), (020) and (002) planes all have the same spacing and only one line is formed, called the (200) line. However, this line splits into two when the cell becomes tetragonal since now the (002) plane spacing differs from the other two. •When the cell becomes orthorhombic, all three spacings are different and three lines are formed. •Common in phase transformations, e.g. carbon steel slowly cooled get Ferrite (BCC) →Cementite (orthor.), and quench from austentite (FCC) →martensite (I-tetragonal). •In disorder-order reactions (CuAu), cubic in disordered 10 state but becomes either tetragonal or orthorhombic class24/10 (depending on temperature) when ordered. Example: Phase Transformations in ZrO2 Low-pressure forms of ZrO2. Red=O Cyan=Zr Property Cubic ZrO2 Tetragonal ZrO2 Monoclinic ZrO2 Lattice parameters (Å) XRD scan of a tetragonal ZrO2 a 5.034 5.037 5.098 b 5.034 5.037 5.171 thin film taken from UNT’s c 5.034 5.113 5.264 Rigaku Ultima III high- Temperature (K) >2570 1400 to 2570 <1400 Coordination Zr=8; Zr=8; Zr=7; resolution XRD O1=4; O2=4 O1=4; O2=4 O1=3; O2=4 Volume (Å3) 127.57 129.73 136.77 Density (g/cc) 6.17 6.172 5.814 class24/11 Space Group Fm-3m P42/nmc P21/c Monoclinic Cubic class24/12.
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