The Basis of X-Ray Powder Diffraction

Teresa Pi PuigInstituto de Geologia UNAM

Applications

Bragg´s Law & Manual Identification of PD patterns

Sample Preparation

Sample Mounting

Collecting Data: diffractometer

Software & Databases

Common Errors in XRPD

X-Ray diffraction

• Scattering of X-rays by the atoms of a crystal that produces an interference effect. The diffraction pattern of a substance is its "fingerprint" allowing us to identify the substance and determine its crystalline structure.

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• Crystalline solids, when exposed to monochromatic X-rays will diffract according to the principles of Bragg’s law.

http://www.scienceiscool.org/solids/intro.html

http://www.crystalsoftcorp.com/featuresv15.php

X Ray Powder diffraction in Mineralogy

• The vast majority of minerals (>90%) are crystalline solids made of periodic arrays of atoms.

• Is particularly useful for identifying fine-grained minerals (clays) and mixtures or intergrowths of minerals (rocks).

• In a rock, XRD data can be analyzed to determine the proportion of the minerals.

http://www.icdd.com/resources/tutorials/pdf/HowtoAnalyzeMinerals

The diffraction pattern of a mixture is a simple sum of the scattering from each component phase

X Ray Powder diffraction in Mineralogy

Other information obtained can include:

• degree of crystallinity of the mineral • presence of element substitutions and

solid solutions• structural state of the minerals (which

can be used to deduce temperatures and (or) pressures of formation

• degree of hydration for mineralsPotts (1987) Hanbook of silicate rocks analysis

Applications of XRPD

Cell parametersin ALUNITES

Qualitative analysis for rocksof sedimentary basin

Applications of XRPD

Illite crystallinity

Structure refinement of Struvite with TOPASApplications of XRPD

Apatite crystallinity

Applications: Modeling complex structures:

USGS image

Diffraction Pattern

http://micro.magnet.fsu.edu/primer/java/diffraction/basicdiffraction/ Images of Panalytical brochure

Phillips Debye-Scherrer Camera

X-Radiation (X-RAYS)

Electromagnetic ionizing radiation, similar to light but of shorter wavelength (0.01–10 nm ) and

capable of penetrating solids

Bragg’s Law

http://www.icdd.com/resources/tutorials/pdf/HowtoAnalyzeMinerals

Since we know λ and we can measure 2θ , we can calculate the d-spacings.

The characteristic set of d-spacings provides a unique "fingerprint" of themineral or minerals present in the sample that we can interpret bycomparison with standard reference patterns

Where:λ= the wavelength of the x-rayd= the spacing of the crystal layers (path difference) θ= the incident angle (the angle between incident rayand the scatter plane) n = an integer

Bragg’s Law

Single crystal specimen in a Bragg-Brentano diffractometer would produce only one family of peaks in the diffraction pattern.

http://prism.mit.edu/xray

A polycrystalline sample should contain thousands of crystallites. All possible diffraction peaks should be observed.

http://prism.mit.edu/xray

1. Collect the XRD pattern.

2. Calculate the values of experimentalspacing d (Bragg law) of the moreintense lines in the pattern.

3. Estimate its relative intensity on a scalein which the more strong line isconsidered 100.

X-ray Powder Diffraction: Manual Identification

http://prism.mit.edu/xray

6. Examine the precision attributed to dataand enter the tab's ID number PDF.

X-ray Powder Diffraction: Manual Identification4. Find the Hanawalt group based on the spacing d of more intense reflection.

5. Identify the substance based on the value of of the reflections of second and third order

within the group.

Example of a PDF-2 image for quartz. The star in the upper right corner indicates a pattern of high quality.

Faber & Fawcett

Volume 58 | Part 3 | June 2002 | Pages 325–332 | 10.1107/S0108768102003312

7.Comparison of experimental data with the corresponding PDF pattern

Sample Preparation

• We need a representative sample that will produce a representative XRD pattern.

• 1 g of sample is perfect, but we can work with less.

• The rock/mineral sample will need to be crushed into small pieces and then ground into a fine powder and sieved to obtain homogeneous particle size.

• If the mineral is soft an agate mortar and pestle can be used to produce a fine powder. If the mineral is hard mechanical grinding with Micronizing Mill may be required.

Rio tinto Ore deposits

Paricutín Volcano

Alumina mortar

sieve

Micronizing Mill

Sample Preparation

Sample MountingTo obtain a randomly oriented powder the particle size needs to be below 10 micron.

The powder needs to be packed tightly in the sample holder.

Zero background holder: is an off‐cut single

piece of quartz or silicon that will not produce any background in the pattern. It is very useful for small specimens.

For clay minerals we use oriented fractions (<2µm)

Transmission

• Foil Holder• Capilar

Sample Mounting

Dispersing a thin powder layer on a smooth surface (glass slide or ZBH) may be used to hold a thin layer of powder

a) dispersing the powder with alcohol onto the sample holder

b) powder may be gently sprinkled onto a piece of double-sided tape or a thin layer of Vaseline to adhere it to the sample holder.

These methods help alleviate problems with preferred orientation but the constant volume assumption is not valid and quantitative and Rietveld analysis are difficult

Data collection: Diffractometer

X-Ray Generation

U. S. Geological Survey Open-File Report 01-041

X Ray Spectrum

http://prism.mit.edu/xray

X Ray Spectrum

X Ray Spectrum

Continuos radiation Characteristic radiation

1. Selection of instrument variables: X-ray source, geometry, kV, mA, divergence and reiciving slits, PSD…

2. Programa a measurement routine: range, stepsize , count time…

Data collection: Diffractometer

EMPYREAN Configurations, courtesy of Panalytical

Divergent X-Ray beam(Large amounts of sample)

Parallel X-Ray beam(small amounts of sample)

Reflexion

Transmision

Data collection: Diffractometer

U. S. Geological Survey Open-File Report 01-041

Data collection: X-Ray filters and monochromators

Shimadzu XRD6000 Brochure

Data Collection: Calibración

Q as internal standard

Data collection: Calibración

Acurate XRDP

1.Large Dynamic range in Intensity2. Large range in two theta (5-100 degrees)3.Small step size (0.01). Peaks well defined.4. We can see the small peaks, expanding the image.

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4

3

Pattern reduction with XRD Software

1. Import data

2. Calculate background

3. Smooth data if needed

4. Remove K alpha 2 if needed

5. Find peaks (The software needs to be able to distinguish between low intensity peaks and noisy background).

6. Identify peak heights areas and widths

7. Compare mineral data to reference data. Search Match efficiency is greatly improved by using a database.

Pattern Identification using database

1. The data in the database have an evaluated and assigned quality that relates to the accuracy of the reference.

2. The database should be organized in a way that facilitates the identification process.

3. The database should be comprehensive –containing all known materials, in all forms of solid state matter (i.e. crystalline, non crystalline, nanocrystalline, amorphous)

icsd.fiz-karlsruhe.de/$$$$$

PDF: Powder Diffraction File of the International Centre for Diffraction Datahttp://www.icdd.com/$$$$$$

AMCSD: American Mineralogist Crystal Structuredatabase: http://rruff.geo.arizona.edu/AMS/amcsd.php

COD: Crystallographic Open Databasehttp://www.crystallography.net/

BCS: Bilbao Crystallogrpahic server of crystallographic symmetry informationhttp://www.cryst.ehu.es/

Inorganic Crystal Structure database

Using database:

XRD Software

Bruker

Crystal Impact

Panalytical

Pattern Identificationusing database: Match

http://web.mineral

Cuantitative analysis1. RIR: The Reference Intensity Ratio or RIR method

calculates an estimated concentration based on scaling all data to the intensity of corundum (I/Ic).

2- Pattern fitting methods: digital patterns would be required for all phases

3. Rietveld method: needs atomic coordinates for each mineral structure. Tha data was fit and refined.Employs a least squares matching algorithm refinement based on sample parameters and instrumental parameter

X-Powder Diffraction Errors

Peak position ( qualitative analysis):

1. Zero point error: use a NIST standard periodically to check it

2. Sample displacement: too high/low? (0.1 mm ~ 0.045°)

3. Sample Transparency: if X-rays penetrate a long way into the sample can get a sample displacement.

Parallel-beam immune to sample displacement & transparency but has worse peak resolution

X-Powder Diffraction Errors

Relative Intensity (Quantitative analysis)

1. Grain size and particle statistics : 600 mesh sieve = <20 mm– Increase the area illuminated by X-rays (Divergence angle)– Rotate samples – Use a PSD (programmable divergence slit)

2. Microabsortion : Higher energy X-rays often less problematic

3. Prefered orientation: is most often seen in samples that contain crystallites with a platy or needle-like morphology like micas.

CONCLUSIONS

1. Select a representative sample

2. Prepare well the sample (be careful with granulometry and size distribution)

3. Mount the sample in the correct holder ( reflexion, transmission)

4. Collect data fit to the propose of the study ( quantitative analysis required longer measurement times)

5. Use a Proved , well organized and comprensive database

6. Bee critic and use all the complementary information about the sample ( chemistry, paragenesis, texture…)

THANKS FOR YOU ATENTION