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Phase Analysis – A Primer

Dr Nik Reeves-McLaren

One of the most fundamental questions we can ask about a material is ‘What is it?’ This short

primer is intended to give people new to X-ray diffraction an understanding of what phase

analysis involves, and how to perform it. Throughout this guide, instructions for the user to

choose in software tend to be given in bold.

Contents

Introduction ..................................................................................................................................... 2

Data Collection Strategies ............................................................................................................... 2

References & Databases .................................................................................................................. 3

Quality Ratings ............................................................................................................................ 5

Alternative References ................................................................................................................ 5

Data Processing ............................................................................................................................... 6

Data Importing ............................................................................................................................ 7

Viewing the Data ......................................................................................................................... 8

Publishing data from WinXPOW

................................................................................................... 9

The Search/Match Process – Qualitative Phase Analysis ............................................................. 10

Using Subfiles ........................................................................................................................... 11

Searching for specific elements ................................................................................................. 12

Strong peak searches ................................................................................................................. 13

Looking for specific PDF cards ................................................................................................ 14

Text string searches ................................................................................................................... 14

Strategies, hints and tips (work in progress) ........................................................................... 15

Common problems ........................................................................................................................ 17

‘I have found a card with peaks in similar positions, but shifted to higher/lower angle’ ......... 17

‘The intensities on the PDF card are odd – they’re all multiples of 10, or 20!’ ........................ 17

Phase analysis in action: Examples ............................................................................................... 18

Example 1: Titanium dioxide .................................................................................................... 18

References ..................................................................................................................................... 20

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Introduction

Every solid crystalline compound gives its own unique X-ray diffraction pattern, consisting of a

set of ‘Bragg peaks’. The positions of the peaks are related to the unit cell dimensions and

symmetry. Peak intensities are determined by the ‘distribution of atoms in the unit cell of every

crystalline phase present in the sample’. The diffraction pattern for a compound can, therefore,

be considered analogous to a fingerprint, or barcode. When we collect XRD data, we can use

these ‘fingerprints’ to identify what phases are present in our sample(s).

Data Collection Strategies

Phase analysis does not require data of very high quality. It is sufficient to have s good signal-

to-noise ratio, and a relatively flat background. For phase analysis, there are three main

decisions that should be made before carrying out experimental work:

- Equipment: Certain diffractometers are significantly better than others for phase analysis

work. Bragg-Brentano (also known as reflection) geometry instruments (i.e. the Philips

PW1825, Siemens D500 and D5000) or the STOE imaging plate systems are suitable

choices; the higher resolution STOE PSD detectors should NOT be used, as they do not

give sufficiently good counting statistics to yield data reliable for phase analysis.

- Scan range: Phase analysis does not usually require data to be collected over a wide

angular range. A scan range of ~ 50 º2 is usually more than sufficient – also, try to

target angles where both primary and secondary phase peaks are likely to be found.

Avoid collecting unnecessary angular ranges where only background noise will occur – it

is likely that you may need to run some initial scans over a wider angular range to find

where peaks occur for your samples, and then focus on those regions.

- Scan rate: You should be aiming to collect data for phase analysis relatively quickly. Try

to collect at the fastest rate possible, and then see how good your data are:

o For Bragg-Brentano geometry instruments, start with a scan rate of 2º per minute.

You should aim to record 1000-5000 counts on your largest peaks.

o On the STOE imaging plate, try a total collection time of around 20 minutes to

begin with – you should aim for a maximum intensity of between 30,000 and

40,000 counts in each data range.

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- Step size: For phase analysis, peak resolution (i.e. being able to distinguish two peaks that

are very close together) is often not critical. Consider using larger step sizes of ~ 0.05

º2 to improve the signal-to-noise ratio in datasets.

It is also usually a good idea to also collect XRD data for the reagents used in the synthesis of

samples. Collecting data for sample holders, the Apiezon putty used to mount solid samples etc.

should also be considered.

The most common pitfall with Bragg-Brentano experiments is specimen height displacement.

Loading the sample too high, with respect to the sample holder, will cause any Bragg peaks to be

shifted to higher angles; if the sample is too low, the peaks will be shifted to lower angles. The

effect on peak positions is almost linear with 2. It is always worth making sure your sample

preparation is as good as possible!

References & Databases

Analysing XRD data to identify the phases present in a sample of course requires that the

researcher has something to compare against. The most useful and powerful source of assistance

available in the Department of Engineering Materials is a database made by the International

Center for Diffraction Data (ICDD) called the Powder Diffraction File (PDF); the first edition

was published in 1941. The software we use to access this database is made by STOE, and is

called WinXPOW

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The PDF contains a mixture of experimentally-determined and calculated X-ray diffraction

patterns. In the 2004 edition licensed by Engineering Materials, there are a total of 134,000

patterns belonging to inorganic and organic compounds. Historically, the PDF was published

annually in huge sets of postcards, with the information for each individual compound being

published on one side of one of the cards – today, we still refer to each individual record as a

‘PDF card’, though the data is now in a much more accessible electronic format! Each PDF card

is number according to the format [xx-yyyy], where xx = set of cards, and yyyy = card number

within set xx.

A PDF Card entry looks like this example, PDF Card No. [27-1402], for Silicon:

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Reference to original diffraction work & how data were obtained

Card no., Quality, wavelength used for calculation of peak positions

Chemical name, stoichiometry; any mineral names

Crystallographic data: space group, lattice parameters, etc.

Sample colour and properties.

Comments. Often includes source/prep. of sample.

Information for peak search

routines

Peak listings – information about position, intensity and (where known) the Miller indices for each of the observed Bragg

reflections.

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Quality Ratings

Each PDF card contains a wealth of information, though they are not always complete, and the

data can be of variable reliability. To this end, the ICDD editors award four quality ratings to

each card:

* = A quality entry; a good instrument, a well characterised sample and a good operator.

I = The pattern has been indexed, with values of {hkl} assigned to the Bragg reflections

O = An entry which may be unreliable, the reason will be given on the card……not

necessarily rubbish

C = Calculated from structural data, rather than experimentally determined

If no quality rating is given, the data is probably quite unreliable! Try to use * quality data,

though this is not always possible.

One other useful piece of information to check for is how the original data were collected. The

entries ‘d-sp’ and ‘Int.’ show how the d-spacings (peak positions) and peak intensities were

calculated – if these are listed as ‘Hagg-Guinier’ (or ‘Debye-Scherrer’) and Visual respectively

(or similar to these) then these data were collected using old photographic film techniques. The

data may be reliable - on a film, a reflection is either present or absent, whereas with modern

digitised data it can be harder to discern a peak from the background. However, the researcher’s

ability to quantify the data is limited; since positions must be measured with a ruler, and

intensities guessed by line darkness both may be a little inaccurate. The data may still be of

value though!

Alternative References

Of course, no single database can be fully exhaustive, and this is true for the PDF. There are

plenty of other places to turn to when in need: your research supervisor, the scientific literature

obviously. There are other databases one can check, also:

- ICSD: Inorganic Crystal Structure Data (website)

- Mineralogy Database: http://webmineral.com/

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Data Processing

You should now be able to collect diffraction data good enough for phase analysis, and also

understand the nature and structure of the databases of reference materials – now for the

software!

The software available in Engineering Materials for performing the phase identification

processes is STOE’s WinXPOW

package. Double-clicking on the WinXPOW

icon on the desktop

will open up the console menu, from which all the other programs can be accessed:

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Data Importing

None of the diffractometer control packages record the initial data file in a format suitable for

immediate phase analysis, so we must first process the data. Note that this process is slightly

different for data collected from each different diffractometer:

For data collected on the Siemens D500 or Philips PW1825:

To import *.csv files into WinXPOW

:

1. Go to Raw Data/Raw Data Handling

2. Go to File/Import

3. Change the entry in the ‘Files of Type’ box to ‘ASCII X/Y Files (*.*)’, select the

data file you wish to import, then click ‘OK’.

4. Enter the desired filename, and click ‘Save’.

5. By default, copper is chosen as the radiation source used for the experiment. If you

collected your data using a different source, e.g. cobalt, then go to

Parameters/Wavelength and select the correct radiation.

6. If you wish to edit, or add, a title for your experiment (which will appear later when

data is displayed), then this may be done in Parameters/Title.

7. If you have changed the Wavelength or Title, click on File/Save As… and overwrite

the *.raw file created earlier to save the changes.

For data collected on the Siemens D5000:

1. Go to Raw Data/Raw Data Handling

2. Go to File/Import

3. Change to ‘Files of Type’ = Diffrac Raw Files

4. Select your Siemens *.raw file

5. Save, WITH A DIFFERENT FILE NAME, as a STOE *.raw file

6. Change the Wavelength to Cobalt.

7. Click File / Save As, and overwrite the STOE *.raw file.

Note: it may be appropriate to have 1 folder for your raw data, and 1 for your imported

data – otherwise things will get pretty complicated!

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For data collected on the STOE Image Plate:

1. Go to Raw Data/Raw Data Handling

2. Go to File/Open

3. Click Ranges/Add Ranges – add ranges 2 and above to range 1.

4. Click Ranges/Extract Ranges – extract range 1, to a file with format

<filename>.ra1.

Note: STOE PSD files do not need any processing, but PSDs are not suitable for phase analysis

work.

Viewing the Data

The ‘Graphics’ package can be accessed via the ‘Raw Data’ menu. This package can be used to

view the raw data, to compare datasets and most importantly, to print out the final analysis! It

can be particularly helpful during phase analysis to keep a ‘Graphics’ window open in the

background to use as a ‘work-in-progress’ area – opening raw data files and adding in selected

PDF cards as and when appropriate.

To open a file in ‘Graphics’, simply click File, then Open. Select the file you want to view, and

press Open.

The right-click menu contains a number of the most useful options:

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Here, it is possible to add further raw data patterns, card records from the PDF, and also to

change how the data are presented (scaling axes) or to remove data or change the displayed title

(Edit Curves…).

The buttons along the top contain some other useful controls:

- If this button is ‘depressed’, the status bar at the bottom of the Graphic window will show

the position and intensity information for wherever the tip of the mouse cursor is pointed. To be

more accurate, one can press and hold the left mouse button, and drag a box around a particular

peak or area to zoom in.

- When zoomed in, press to reset the graphics window to show the full diagram.

- These buttons allow easy switching between Absolute and Relative intensities on the y-

axis. Absolute = number of X-ray counts detected; Relative = everything is out of 100%.

The main lesson is that you cannot break the software, so play around – see what options do for

yourself!

Publishing data from WinXPOW Once you’ve finally finished analysing your data, and have it presented exactly as you wish,

click File/Write Metafile. These *.emf files can be inserted into MS Word or Powerpoint as

pictures; they can also be manipulated in a graphic editing package (I use Paint.net) to remove

the WinXPOW

header bar that the software places above the data, but which really is unsuitable

for publication.

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The Search/Match Process – Qualitative Phase Analysis

WinXPOW

has a specific module for carrying out phase analysis, called ‘Search/Match’. This can

be accessed from the main WinXPOW

console by clicking ‘Phase Analysis’ and then

‘Search/Match’.

Within the XRD laboratory, there are diffractometers using three different radiation sources –

Cu, Co and Mo. The database stores peak position information in the form of d-spacings, so it is

important that you ensure that the Search/Match software is using the appropriate wavelength

during your analysis. To do this, in Search/Match:

1. Select Options, and then Find.

2. Under ‘General’, use the ‘Wavelength’ drop-down menu to select the wavelength you

used to collect your diffraction data.

3. Click OK.

The next step is to load the data file that you wish to perform data analysis on. In Search/Match:

1. Go to File/Open

2. Change to ‘Files of Type’ = Raw Data Files (*.raw, *.r*)

3. Select the file, and press Open.

You will notice a new window appears, showing your diffraction data overlaid with some lines –

these lines are the reference diffraction data from the first PDF card record in the Current Set.

This will probably be one of the very first cards in the database. Note that you can use the < and

> buttons at the top of this ‘graphical data’ window to navigate through the PDF cards, and that

the peaks overlaid on your trace will change accordingly. The aim of phase analysis is to be

able to identify what is causing ALL the Bragg peaks in the experimental dataset; i.e. to

match all of the observed reflections against lines from card entries in the PDF.

Next, you will note that the main Search screen appears empty. That is not the case! To see the

database card entries, click View and then Names. Note also, at the bottom of the screen the

number of cards selected in the ‘Current Set’ is shown. This is the number of cards in the

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current working set, i.e. how many cards will be shown on the screen. At the start of your

analysis session, this will be very large and would take quite some to navigate through! Double-

clicking on any of the entries in the list will open that second (graphical data) window again –

but it will also open the card text window, giving access to all the valuable data in that particular

PDF card.

While there is an automated search routine in WinXPOW

, it is not very reliable. Instead, it is

recommended to use combinations of the five main ways of manually selecting portions of the

database:

1. Using subfiles

2. Performing an element search

3. Strong peak searches

4. Looking for specific Cards

5. Text string searches

We will now look at each of these in turn. Each option is available from the ‘Select’ menu in

Search/Match. Bear in mind – the objective is always to minimise the number of cards you need

to look through, while avoiding excluding likely candidates!

Using Subfiles

Historically, the PDF database was sectioned by material type – inorganic materials or organics,

cements, superconductors, etc. It is possible to search by these sub-divisions, should you wish,

by:

1. Go to the Select menu, and choose Subfiles.

2. Tick or untick the appropriate subfile boxes.

3. Press OK.

4. If you have chosen a different selection from that previously active, you will be asked

whether you want to Replace, Add to, Match with, or Subtract from the previously

selected working set. In the early stages of your search, this will be the full database – so

you would want to Replace this full list with the new selected options.

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One note of caution, however: what is a ceramic to one researcher may well be a cement or a

superconductor to another…! In modern analyses, this tool is probably no longer in such

common usage.

Searching for specific elements

Clearly, have some form of elemental analysis – i.e. knowing what is in your sample – will be

extremely useful when it comes to identifying what phases are present! For example, if you

know that you have made your sample of BaTiO3 using BaCO3 and TiO2, it is unlikely that you

will have formed NiMnO3! You may also get elemental analyses from other techniques, such as

EDX.

To search for PDF cards obtained from samples containing specific elements:

1. Go to the Select menu, and choose Elements.

2. Colour in the periodic table according to the likelihood of each element being in none

(blue), some (i.e. optional, grey), or all (red) of the phases in your sample. Pressing

Reset will make all elements optional, while one can also Toggle between optional and

omitted.

a. For the previous example of a sample of BaTiO3 prepared from BaCO3 and TiO2,

we would Toggle all the elements to omitted (blue), we would then click the All

req. button and select oxygen, since this element is very likely to be in all phases

encountered. We would then click the Optional button and choose Ba, Ti and C,

which would then cover all possible combinations for our sample: Ba-O, Ba-Ti-O,

Ba-C-O, Ba-Ti-C-O, Ti-O, Ti-C-O, C-O and O

3. When a suitable selection has been made, press OK.

4. The software will then ask for the required number of elements to be found in each result.

Enter a minimum and maximum number, and press Range.

a. For the above example, we do not wish to find elemental oxygen, since this will

not occur in our sample! Neither would we expect any Ba-Ti-C-O phase; we

would choose a range of 2 to 3 for this sample.

5. If you have chosen a different selection from that previously active, you will be asked

whether you want to Replace, Add to, Match with, or Subtract from the previously

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selected working set. In the early stages of your search, this will be the full database – so

you would want to Replace this full list with the new selected options.

At this stage, a search can still return a large number of results – ideally, we don’t want to have

to look through any more cards than we have to or it will take a very long time!

Try to select a limited number of more likely elements to begin with.

XRD is actually fairly insensitive to small amounts of material – if a phase makes up less

than a few percent of the volume of your sample, it is unlikely you will spot it.

Look for likely majority phases first.

Elemental analyses are very useful – particularly if your sample is a complete unknown!

Strong peak searches If you are trying to do phase analysis, the chances are you have a very good source of data on

your sample already – the diffraction data itself! When matched with an element search, this can

be one of the most powerful tools in phase analysis. The tool is not perfect though, and can

return some false positives, i.e. phases which do not contain a strong peak in the specified

position! To search for phases containing a particular peak, or peaks:

1. Identify the position of a strong peak in your dataset. You can do this in ‘Graphics’ or

‘Search/Match’ by hovering the tip of the mouse cursor over a Bragg peak of interest.

2. In ‘Search/Match’, go to the Select menu, and choose Strong Peaks.

3. Choose the appropriate Method (are you looking for a peak which is likely to be one of

the 3 Strongest, or over 30 or 70% relative intensity compared with the strongest peak

from that phase? 3 Strongest is the most common search).

4. Choose whether you are going to enter the peak in degrees 2theta or as a D-space, by

clicking the appropriate button.

5. In the ‘Line or Region (Begin, End)’ box, enter the peak position.

a. i.e. if you have an observed peak at 28.443 º2θ, input the 2θ range as:

28.33 28.53, or

28.43

The second option automatically enters a 2θ window of 0.1 either side; you may

want to replace this with a bigger window and click ‘Replace’.

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6. You can enter a list of peaks, should you wish. Bear in mind, however, that not all of the

reflections will necessarily be from the same phase. It is often better to enter only a

single peak initially.

7. Press OK.

8. You will then be asked whether you want to Replace, Add to, Match with, or Subtract

from the previously selected working set. If you have already done an ‘Elements’ search,

and have then attempted a ‘Strong Peaks’ search:

a. Replace: replace the Elements search with this new search

b. Add: show the results from both the Elements and Strong Peaks searches

c. Match: show only the results which contain BOTH the chosen Elements AND a

peak in the specified position – this is a very powerful combination! If no results

are found to Match, this option will not be available – try to widen the 2θ window

around the peak.

d. Subtract: omit cards with the specified peak

Looking for specific PDF cards

It is possible that you will have learned from your supervisor, from previous work, or from the

literature, that your samples should contain a particular phase, and you may already know the

specific card number for that phase. You can search for a specific card by:

1. Go to the Select menu, and choose Cards.

2. Enter the number(s) of the card(s) you wish to find, in the format xx yyyy or xx-yyyy.

3. Press the Add button.

4. Repeat steps 2 and 3 as necessary. When done, press OK.

Text string searches

The final type of search can again be a very powerful tool – to search for specific text within

PDF cards. To perform a text search:

1. Go to the Select menu, and choose Text Search.

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2. Choose the ‘type’ of text you wish to search for. Is it a chemical Formula, or a Mineral

or Chemical name? Or would you prefer just to search for a specific string of text

anywhere within a PDF card – for this, choose Pdf2.dat.

3. In the text fragment box, enter the text you wish to choose from.

4. If you wish to search the full database, tick the Search Full Database option.

Otherwise, it will just search the currently selected Working Set.

5. Press Start. It may take a little time!

Strategies, hints and tips (work in progress)

You now have all the tools at your disposal to perform phase analysis, so good luck!

The underlying principle in phase identification is to match the positions and intensities of Bragg

reflections (i.e. the sharp peaks) from the unknown sample, with those in the reference pattern(s)

of a known material, or materials. Selection of suitable matches from the Powder Diffraction

File database can be tricky, but bear in mind the following criteria when choosing:

The Bragg peaks in your diffraction pattern may come from more than one phase. It will

not always be possible to fully ‘index’ the peaks from one single PDF card - for multi-

phase samples, more than one card will be required!

You should try to match both the position and relative intensities of all the peaks in the

appropriate PDF card to those in the diffraction pattern for your sample.

If more than one phase is present in the sample, it is likely that some of the smaller Bragg

reflections will be lost amongst the background noise.

However, all of the most intense peaks should be observed. For example, a simple search

for a peak at 28.433 °2θ will return 3,065 cards. Many of these will contain intense

reflections at other Bragg angles, not observed in our experimental dataset, which

therefore precludes the possibility of those phases being in our sample. Analysis would

also take forever! Instead, combining this search with a search for a second reflection

(e.g. 47.303 °2θ) or a specific element (e.g. Si) will return far fewer cards – 92 or 1,

respectively!

Good presentation of the data is extremely important, and there are some simple tips you

can follow to help make viewing your data easier. Some people like to mark each peak

with a symbol, perhaps using a different symbol for each phase present – this can be

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achieved in ‘Graphic’ in the right-click menu, by choosing ‘Insert Note…’. Notes can be

deleted by double-clicking on them, and choosing ‘Delete’.

Alternatively, the lines for each PDF Card added can be scaled to fit the peaks they are

being matched against. To do this, choose an Absolute scale in ‘Graphic’ right-click on

the background and choose ‘Edit Curves’. From the list, double-click on the PDF card

entry you wish to scale. Under scaling, tick the box and choose the absolute intensity

value that you wish to scale to. Press OK twice and check the results – repeat and adjust

as necessary!

In the final section of this guide, we will look at a few examples of well and poorly identified

samples to round off your understanding of what you are trying to achieve in your phase

analysis!

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Common problems

‘I have found a card with peaks in similar positions, but shifted to higher/lower angle’

This can have three possible causes:

1. Sample height displacement – even a small error in the height of the sample will have an

impact on peak position. This error may appear fairly linear with diffracted angle. This

is a very common, but largely avoidable, problem. Run the sample again, with more

attention to correct sample height.

2. Wrong wavelength selected – either when importing the data (copper is the default), or in

Search/Match (other people may have changed this). If the peak shift is dramatically

different, this may be the cause.

3. Change in chemistry – if you can replace an element in the crystal structure with another,

this is likely to cause changes in the shape and size of the unit cell. You may get the

same pattern of peaks, but shifted to higher or lower angles. In this case, the shift is

likely to be small at low angles, and significantly larger at higher angles.

‘The intensities on the PDF card are odd – they’re all multiples of 10, or 20!’

This would indeed be quite odd for a real sample! Most likely the data have been collected using

an old photographic film technique; check the card text entry for the method used – perhaps it

says a Guinier diffractometer was used, or that intensities were obtained via ‘visual’ or

‘densitometer’ methods. Such techniques are extremely accurate; however, the ability of the

researcher to accurately interpret the data is more limited! Use such cards with caution.

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Phase analysis in action: Examples

Example 1: Titanium dioxide

It can often be a good idea to perform XRD on your reagents before you begin your sample

preparation work; any problems can be spotted before they ruin hours of good work! Here, I was

checking that my reagent, TiO2, was what it said on the bottle - and in the crucible after it had

been dried at 900 °C! There are two known polymorphs of TiO2: anatase and rutile. The data

were collected on the STOE Imaging Plate detector, with a total scan time of around 38 minutes.

An element search for both Ti and O, omitting all other elements, returns 121 cards including

many such as TiO, Ti2O3 etc. Scanning through the list highlighted quite a few anatase and rutile

phases – in this first plot, I compare my experimental data (in green) with PDF card number [21-

1272], a *-quality entry, for anatase:

It can immediately be seen that the sample does not contain anatase – none of the most intense

Bragg reflections for anatase, e.g. the (101) peak at 25.281 °2θ, are observed in the experimental

dataset. If we compare instead with rutile:

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All of the Bragg reflections, including all those of low intensity, for rutile are observed in our

dataset; there are no additional reflections. We can be confident that this sample is the rutile

phase of TiO2. What was apparent, however, was a small linear peak shift across the diffraction

pattern – the experimental reflections were always at a slightly higher angle than those from the

PDF card. This is likely caused by a small misalignment of the diffractometer.

Had the data been collected on a Bragg-Brentano geometry instrument, this peak shift would

more likely have arisen from a specimen height displacement error. Remember: all experiments

have errors! Finally, had the peak shift been non-linear, i.e. a small shift at low it most likely

have been a sample effect – some change in the chemistry of the sample causing a change in the

unit cell dimensions.

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References

West, A.R., (1988). Basic Solid State Chemistry. John Wiley & Sons Ltd: Chichester.

Pecharsky, V.K. & Zavalij, P.Y., (2005). Fundamentals of Powder Diffraction and Structural

Characterization of Materials. Springer: New York.

Clearfield, A., Reibenspies, J. & Bhuvanesh, N., (2008). Principles and Applications of Powder

Diffraction. John Wiley & Sons Ltd: Chichester.

Hyatt, N.C., (2008). MAT255: X-ray Diffraction. Lecture course notes.