lesson 6 “how-to” session -...

Lesson 6“How -To” Session

Nicola DöbelinRMS Foundation, Bettlach, Switzerland

October 16 – 17, 2013, Uppsala, Sweden

Refinement Strategy: Words of Wisdom

2

Release parameters one by one.When the fit doesn’t improve anymore,

don’t try to extract more information.

Always refining everythingmay lead to good fits,

but the results may be useless.

Chose your refinement strategy wisely.Ask yourself if the results make

physical sense.

Examples

3

Lesson 5: Example 1: Simple phase quantification

Lesson 5: Example 2: Phase quantification with size and micro-strain

Lesson 5: Example 5: Batch processing

Lesson 6: Example 1: Texture, preferred orientation

Lesson 6: Example 2: Anisotropic crystallite sizes

Lesson 6: Example 3: Non-existent phases

Lesson 6: Example 4: Micro-absorption and Brindley correction

Lesson 6: Example 5: Amorphous Content

Texture, Preferred Orientation

4

Smooth, but non-continuousdiffraction rings

Reason:

- Orientation of crystallitesis not random.

- Some orientations areover-represented,others are under-represented.

Texture, Preferred Orientation

5

Images: L. Galea, RMS Foundation

Plateletslying flat

Needles, Fibers, Whiskerslying flat

may point in one direction (bundles)

Random orientation Preferred orientation

Texture: Symmetrized Spherical Harmonics

6

Järvinen, M. Materials Science Forum [278-281], 1998, 184-199.

In structure files (*.str) change:

PARAM=GEWICHT=0.1_0

to

GEWICHT=SPHARn(n=0, 2, 4, 6, 8, 10)

No preferred orientation:

PARAM=GEWICHT=0.1_0

GEWICHT=SPHAR0

GEWICHT=SPHAR2

GEWICHT=SPHAR4

GEWICHT=SPHAR6

GEWICHT=SPHAR8

GEWICHT=SPHAR10

Lesson 6: Example 1 - Texture

7

PARAM=GEWICHT=0.1_0

GEWICHT=SPHAR0

Instrument: pw1800-fds-0250

Phases: Corundum, Fluorite

RP=4 PARAM=k1=0_0^1 PARAM=k2=0_0 PARAM=B1=0_0^0.03 PARAM=GEWICHT=0.1_0 //Both phases:

GEWICHT=SPHAR2


8

GEWICHT=SPHAR4

Instrument: pw1800-fds-0250

Phases: Corundum, Fluorite

RP=4 PARAM=k1=0_0^1 PARAM=k2=0_0 PARAM=B1=0_0^0.03 GEWICHT=SPHAR4 //Both phases:

GEWICHT=SPHAR6

GEWICHT=SPHAR8

GEWICHT=SPHAR10


9

!

Automatic refinement strategy of BGMNprevented over-interpretation!

Reduction depends on phase abundance andquality of diffraction pattern.


10

Recommendation:

- Use a moderate order of SPHAR polynomes in yourstructure files (e.g. SPHAR4)

- Let BGMN reduce the order if necessary

- Only increase the order if the fit really improves

- Refining «GEWICHT» with symmetrized spherical harmonicsfunctions allows to model texture / preferred orientation.

- Complexity of the polynome can be set in structure file (SPHARn).

- High order introduce large number of refined parameters.(� slow refinement, may get unstable)

- Automatic refinement strategy will protect from over-interpretation.

Anisotropic Crystallite Sizes

11

Images: L. Galea, RMS Foundation

Platelets Needles, Fibers, Whiskers

Small sizeStrong peak broadening

Large sizeWeak peak broadening

Lesson 6: Example 2 – Anisotropic Crystallite Size

12

Wide peaksSharp peak(002) (004)


13

Change:

RP=4 PARAM=k1=0_0^1 PARAM=k2=0_0 PARAM=B1=0_0^0.1 GEWICHT=SPHAR6 //

To:

RP=4 PARAM=k1=0_0^1 k2=ANISO4 B1=ANISO^0.1 GEWICHT=SPHAR6 //

Remove «PARAM=» keyword and lower limit

Use ANISO4 for k2, and ANISO for B1


14

Needle length: 35.4 nm

Diameter: 11.2 nm


15

Slightly improved fit withtexture model «SPHAR10»


16

Automatic refinement strategy sometimesreduces anisotropic parameters to isotropy


17

Recommendation:

- Do not refine micro-strain anisotropically unless it improves the fit

- Refine peak broadening anisotropically(B1=ANISO^0.01)let BGMN handle the reduction to isotropy

- Check if the upper limit of B1 was reached. If yes:

- increase the limit…

- … or see next example (non-existent phases)

Refine anisotropic crystallite sizes with «B1=ANISO»

Refine anisotropic micro-strain with «k2=ANISO4»

Lesson 6: Example 3 – Non-existent Phases

18

Background information:

Step 1:

- α-TCP prepared at 1350 °C

- Traces of β-TCP may have formed during cooling

Step 2:

- α-TCP hydrated to CDHA

- β-TCP (if present) remains

Consequence:

- If β-TCP is present, it has formed at ~1000°C

- Must be highly crystalline with large crystallites


19

Main Phase: Hydroxyapatite

Question: Does it contain β-TCP?


20

Background curve goes wild

No distinct diffraction peaks from β-TCP

Does not generate a diffraction patternTotally wrong!


21

Problem:

Refining non-existing phases=

Fitting a calculated patternto random noise

Solution 1:

Reducing intensityConstant peak width

Result:

Refined quantity � 0 %

Correct result


22

Solution 2:

Increase peak width

Result:

Crystallite Size � 0 nmOver-estimated quantity

Problem:

Refining non-existing phases=

Fitting a calculated patternto random noise

Wrong result


23

Even worse:

No anchor points forbackground anymore

Background ispoorly/randomly defined

Massive over-estimationof non-existing phase

Result:

Totally wrong!Unpredictable!


24

Background curve goes wild

No distinct diffraction peaks from β-TCP

� 0 nmTotally wrong!


25

Solutions:

- Use a reasonable upper limit for B1 (peak broadening, crystallite size)

- Don’t trust very small crystallite sizes (e.g. < 20 nm)

- Repeat the refinement without the questionable phase(Does the fit really look worse? Or just as good?)

- Use additional information:

- Sintered samples: very small crystallites are unlikely

- Cement samples: very small crystallites are reasonable

Trying to identify very poorly crystalline precipitated phases?Good luck!


26

1. Edit betaTCP.str

2. Change

PARAM=B1=0_0

to

PARAM=B1=0_0^0.005

3. Run Refinement…


27

< 1 % vs. 23.8 %!

= upper limit of ^0.005(random choice by operator)


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Upper limit B1 Crystallite Size β-TCP Quantity β-TCP

None 2 nm 23.8 wt-%

0.1 4 nm 14.6 wt-%

0.05 8 nm 7.0 wt-%

0.01 42 nm 0.4 wt-%

0.005 85 nm 0.8 wt-%

0.001 424 nm 0.2 wt-%

0.0005 849 nm 0.2 wt-%

0 ∞ 0.2 wt-%

How to choose the upper limit for B1?

Sample was sintered at 1350°C:

� Crystallites of several100 nm diameter expected

� Any other useful data available?

� Other samples which docontain β-TCP?

� Before cement reaction?

Educated Guess!

Lesson 6: Example 5 – Amorphous Content

29

Question: Does this sample contain amorphous material?

Refinement = 100% β-TCP

«Bump» in the background= sign for amorphous phase


30

Problem: Amorphous phases

- Don’t procude a distinct diffraction pattern

- Create a broad bump around 30° 2θ

10 15 20 25 30 35 40 45 50 55

Diffraction Angle (°2θ)

Most common solution:

Internal Standard


Am

orph

ous

β-T

CP

True composition:

64% β-TCP36% amorphous Visible to XRD:

100% β-TCP

Invisibleto XRD A

mor

phou

sβ

-TC

P

True composition:

30% int. Std.44.8% β-TCP25.2% amorphous

31

Add known amountof a fully crystallinereference powder(internal standard)


Am

orph

ous

β-T

CP

True composition:

30% standard44.8% β-TCP25.2% amorphous

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Visible to XRD

Std

β-T

CP

Visible to XRD:

59.9% β-TCP40.1% int. Std.

(=100%)

Std


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Phase Mixed RefinedNormalized to

int. Std.

Normalized

w/o int. Std.

Amorphous ? -Fill up to 100% =

25.2 wt-% 36 wt-%

β-TCP ? 59.9 wt-%59.9 * 0.748 =

44.8 wt-% 64 wt-%

Internal

Standard30.0 wt-% 40.1 wt-%

40.1 * 0.748 =

30.0 wt-% -

Σ = 70.0 wt-%

0 wt-%

100 wt-%

30 wt-%


34

Example 5 File 2

Sample β-TCP + amorphous

phase

70 wt-%

Internal Standard Monetite 30 wt-%


35


36

Particle size, to correct formicro-absorption

Phase Mixed RefinedNormalized to

int. Std.

Normalized

w/o int. Std.

Amorphous ? - 25.22 36.03

b-TCP ? 59.88 44.78 63.97

Internal Standard 30.00 40.12 30.00 -


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Challenge: Selection of internal standard material:

- Must be 100% crystalline

- Simple structure (cubic)

- No texture or micro-absorption problems

- Absorption coefficient similar to matrix

- Absolutely homogeneous mixing

- Must not react with sample matrix

Common materials:

- Si

- LiF

Monetite was a bad choice:

- Triclinic

- Large crystals (micro-absorption)

- Severe texture effects

Summary

38

- First contact established

- Try to repeat the simple examples(Lesson 5) in a quiet moment

- Ask your more experienced colleagues

- Read:

- http://www.bgmn.de/program.html

- Slides of Lessons 5 and 6

Micro-absorption and Brindley Correction

39

Weak attenuation by phase 2

� Large volume of interaction

Strong attenuation by phase 1

Large particles absorb significantpart of the radiation.

� Small volume of interaction

Phase quantification biased for phase 2!

Micro-absorption and Brindley Correction

40

Micro-absorption can be corrected,but mean particle* size must be known.

*not crystallite size

0 50 100 150 200 2500

1

2

3

4

5

Vol

umen

[%]

Diameter [um]

Lesson 6: Example 4 – Micro-Absorption

41

Reference mixture:

33.33 wt-% Corundum (Al2O3)

33.33 wt-% Fluorite (CaF2)

33.33 wt-% Griceite (LiF)

Wrong phasequantities


42

Add mean particle diameter (µm) to structure files:

Corundum: 12 µmFluorite: 10 µmLiF: 9 µm

my (µ) = mass absorption coefficient(calculated automatically by BGMN)


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Micro-Absorption and Brindley correction:

- Try to avoid the problem in the first place (keep particlesize close to 1 µm)

- Additional information (particle size from SEM, PSD analysis)required for all refined phases!

- Large particles still lead to grainy diffraction patterns. Brindley-correction does not solve this problem!

lesson 6 “how-to” session -...

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