August 1997 - techniques

• Darts - A new service
• Epdic 5 Report
• Help! = Reference Standards for routine diffractome ter checks
• Intensity Round Robin Update
• XRPD European Standards Initiative

DARTS - a new service for Industry

CCLRC Daresbury Laboratory's new service for Industry, DARTS, was successfully launched at an event held at the laboratory on May 20th. Over thirty representatives of Industry from all over the UK attended, to hear presentations about the capabilities of Daresbury 's unique Synchrotron Radiation Source (SRS). DARTS offers Industry quick, cost effective and user-friendly access to the benefits offered by Synchrotron Radiation (SR). Through its flexible structure, Industrial customers need not become users of the SRS, merely users of DARTS, as DARTS staff will perform the necessary SR work in consultation with the customer.

Powder Diffraction and its related reflectivity and energy dispersive methods, are among the techniques offered by the service, as is single crystal diffraction using Daresbury's new small crystals facility. Use of SR, with its tunable wavelength, high intensity and low beam divergence (meaning high resolution in the powder pattern), offers measurable benefits over conventionally collected data for applications such as indexing, difficult QPA tasks, stress and strain, and structure determination. Non-ambient temperatures are also available, from cryogenic to elevated temperatures and for certain kinds of experiments, non-ambie nt pressures also. The flexible, consultative approach of DARTS means that customers can tap into the depth of expertise available at Daresbury, if required, for experimental design and data processing.

After three full months operation, the number of customers is already well into double figures.

For further information, contact:
CCLRC Daresbury Laboratory
Daresbury
Warrington WA4 4AD
Tel: 01925 603141 e-mail: [email protected]
Website: http://www.dl.ac.uk/SRS/ DARTS

Steve Maginn

5th European Powder Diffraction Meeting at Parma (EPDIC-5)

In late May I found myself in Parma, northern Italy, in the select company of two or three hundred powder diffractionists from around Europe. The venue, weather, entertainments, food and wine were all very enjoyable - the conference was pretty good too. I was just a little disappointed to find how select was the UK industrial represe ntation. For most of the meeting I thought there were only two of us, but my estimate had doubled before the close.

It has to be admitted that industrial crystallographers were in pretty short supply from any country. During a 'round table on powder diffraction in industry' it quickly became apparent how fortunate we are to have both the BCA and an Industrial Group within it. Academics in other countries are still wondering how to involve the industrial community.

All this might s uggest a heavy academic bias to the conference. In terms of the participants that was certainly true, but the subject matter was much closer to home. Judge for yourself from the list of sessions: disordered structures, accuracy, catalysts, in situ studies, materials, HT-HP studies, structure solution (X2), instruments/methods (X2), quantitative analysis, kinetic studies, size-strain / stress, databases / software, texture analysis, structure refinement, inorganics / organics, thin films. I found mos t presentations stimulating and many very relevant. The list of speakers was impressive with many from USA and beyond in addition to Europeans who would quickly be placed in the "world leader" class.

There were many excellent posters presented by scientists delighted to have an interest taken by industrial colleagues. The commercial exhibition was inevitably much more comprehensive than anything you are likely to see in the UK apart from the 1999 IUCr Congress. Many manufacturers' experts w ere immediately to hand for discussion.

EPDIC-4 at Chester in 1995 was an excellent meeting, attended by many from the Industrial Group, which led me to expect more of us to turn up at Parma. EPDIC-6 is to be in Budapest during August 22-25 1998 with an expanded range of topics. You could do worse than fit it into a summer holiday. Far be it from me to suggest that you will find anything to beat our magnificent BCA meetings (especially the IG bits). But if you can make it I would commend E PDIC-6 to you on the basis of numbers 4 and 5. I hope to see you there.

Steve Norval

Reference Standards for Routine Diffractometer Checks

Mention of diffraction reference standards always seems to provoke interest and it is obvious that the materials supplied by NIST only go part way to meeting the average lab's needs. Several people have asked for details of the samples we use at Wilton. Here is the basic information (with apologies to those expecting an earlier reply from me).

We occasionally use most of the NIST Standard Reference Materials for XRD including, most recently, SRM 1976 alumina plate for the BCA round robin. These all have their place, but for routine checks we need samples that do not have to be remade before each use. That would be not only time-consuming, but introduce a set of reproducibility problems.

To maximise the value of each measurement, we use lanthanum hexaboride (SRM 660) embedded in resin as our main standard (figure 1). A paste of SRM660 is mixed with low viscosity epoxy resin (as used for embedding microscopy samples). This is back-loaded into a suitable xrd sample holder with its reference surface held firmly against a glass plate (level!) coated with a very thin layer of release agent. Warming the epoxy while setting speeds up the process and helps the LaB₆ to sediment o ut to the reference surface. This layer is kept pretty thin (0.5 mm is plenty) to economise on SRM660 and a thicker backing layer of straight epoxy resin added to give stability.

When fully set, the glass is removed and the surface cleaned. You may find the finish quite adequate for your needs as it is. If your diffractometer positions the sample directly from the holder surface, the entire surface can be machined to an appropriate tolerance (ideally <10um). Alternatively, a standard/res in block could be produced and machined without the sample holder, then glued into position. This avoids machining the sample holder surface e.g. for D5000 instruments where the sample surface is at a fixed height above the machine reference surface. (Take care in machining SRM660 - LaB₆ is toxic!)

Why use lanthanum hexaboride? It is primarily intended as a standard for measuring the instrument peak shape in relation to line profile analysis. It is also has a certificated lattice parameter (cubic) which means that peak positions can be checked. One standard therefore allows simultaneous measurement of peak area (to monitor tube intensity), peak position (to check 2 theta alignment) and peak width (dependent on tube focus and theta alignment). Full scans are useful occasionally for a thorough check.

Analysing one or two peaks at least weekly for area, position and width should give confidence in your instrument performance and help spot developing problems.

For those who need a lower angle alignment check (<20 ° 2 theta for CuKa), the options are limited, especially if you need a traceable standard (NIST is usually the nearest thing). A silver behenate standard is being prepared by NIST that should help, but the currently available material is a mica, SRM675. This SRM has been criticised, but if properly handled it works well. The trick is to use a very small amount of the mica platelets aligned against a flat surface (figure 2), rather than try to prepare a bulk sample.

A single drop of dilute alcohol suspension of the mica dried onto a polished low background holder should give at least six measurable orders of the mica basal spacing (lowest peak at about 8° 2 Theta ). A certificated spacing is provided but you can also check the goniometer alignment by the relative positions of the various (00l) peaks.

Steve Norval

ftp via whatever network you use to the computer at: 

iona.cryst.bbk.ac.uk 

using the username: anony

mous and password: your own email 

address

type: cd pub/BCA

set the file transfer type to binary by entering: type image 

then retrieve the file by: get rrobin.zip