Scintag Pad V

Powder X-ray Diffractometer

(stored in 110 Beckman Institute)

This XRD is for teaching only.

General Summary

An x-ray powder diffractometer is primarily used for the identification of phases in powder form. An x-ray beam of known wavelength is focused on a powdered sample and x-ray diffraction peaks are measured using a germanium detector; the d-spacing of the observed diffraction peaks is calculated using Bragg's Law [n*lamda = 2dsin(theta)]. The Scintag Pad V automated powder diffractometer uses a Cu x-ray tube with variable filters, a four-sample changer, and a low-noise, high efficiency, liquid-nitrogen cooled germanium detector. The goniometer is automated and software packages are run from a PC running Windows NT 4. A number of Scintag software packages are available for routine powder diffraction data acquisition, background correction and peak identification. Unknowns can be matched to JCPDS cards in an on-line database. Other software is available for quantitative analysis of powder mixtures, unit cell refinement, Rietveld analysis, and GSAS structural analysis. Samples should be prepared as powders with a grain size of 10 um (approximately), and typically about 100 milligrams of sample is required.

Instrument Summary

The Division Analytical Facility Scintag Pad V X-ray Powder Diffractometer is a theta-2theta goniometer instrument. It is outfitted with a Cu x-ray tube with two beam slits, a four-sample automated sample changer with sample spinning capability, and a low-noise liquid-nitrogen cooled germanium detector outfitted with two detector slits. The x-ray tube has a filter wheel allowing one to manually select a single filter. The wheel has filters made of vanadium, manganese, iron, nickel (this is the default filter which is used to remove Cu k-beta radiation), zirconium, or open tube (i.e. no filter).

The goniometer can be scanned in normal mode (i.e. theta and 2theta are scanned in sync), or independent theta and 2theta scans can be performed. Rocking curve scans can be acquired using this capability of independently scanning. The sample is mounted on a 1 inch glass round disc for a diffraction run. The sample can be spun in the plane of the glass disc in order to increase the random orientation of powder grains in the mount.

The instrument is currently outfitted with a set of beam and detector slits that balance the needs of resolution and intensity for typical runs. The slits that are in use are selected for somewhat better resolution than one would find on other instruments.

The pulse-height analyzer which processes the detected x-ray pulses from the germanium detector is set for narrow discrimination of Cu k-alpha pulses. All other energy ranges are rejected, and in this way any x-ray fluorescence emanating from the sample is rejected via pulse-height analysis.

The instrument is capable of very rapid scans for identification of powders that are simple in composition and abundant on the mount. Much slower scans are usually required for the analysis of complex mixed phases, high resolution work, cell refinement, or identification of trace impurities. Two scan modes are possible, a continuous scan mode which integrates counts over a small angular range (for rapid scans), and a step-scan mode that is used for slower scans and any precision work.

Our recently installed Scintag PAD-V XRD has the ability to perform very rapid phase identifications of powdered specimens in a fully automated mode. The XRD has a low noise Ge detector that greatly improves sensitivity. Data collection and manipulation is under control of a MicroVax computer which contains a data base of the JCPDS powder diffraction files. Diffraction spectra are plotted and can be compared (in whole or in selected portions) to specified JCPDS cards or automatically matched to the most similar spectra in the data base. This instrument is very easy to learn to use and operates very rapidly and efficiently. There is an active research effort in the Division to use this instrument for quantitative mixture analysis as well as for phase identification.

The laboratory also has a Rigaku Rotaflex high-intensity, rotating anode X-ray source with Debye-Scherrer and Gandolfi cameras for characterization of very small samples. This instrument is currently mothballed.

Software Summary

The Scintag Pad V X-ray Powder Diffractometer is automated with a brand new PC running Windows NT 4.

DMSNT

The software package used to perform most diffraction work is the Scintag program DMSNT (Diffraction Management System for NT). The DMSNT program allows one to set up a sequence of automated tasks related to either collection of data or processing of the data already collected. This portion of the software package also allows one to perform search match procedures using the ICDD database (JCPDS cards from the ICDD cd-rom). The DMSNT help file can be downloaded from this web page; it is a PC help file. Download the DMSNT Help File.

Crystallography

The Crystallography package allows one to perform crystallographic analysis to powder diffraction data, and is used to identify the crystal system and other symmetry parameters of the unknown phase.

Quantitative Analysis

The Quantitative Analysis package allows one to analyze powders that contain a mixture of phases, for example, a rock powder. This package uses peak deconvolution to determine the peak area and, via correction parameters, the volumetric abundance of that phase in the mixture.

Rietveld Analysis

A public domain version of Rietveld cell refinement software is available on the system, with supporting documentation.

GSAS Analysis

The GSAS package from Lawrence Livermore National Labs has been placed on the computer, and is available for structural analysis. Supporting documentation is also available.

Sample Requirements

The sample preparation step is important for a successful powder diffraction run. The sample should be ground to an approximately 100 um powder; this is usually done in a mortar under acetone or isopropanol, and the sample is most easily loaded onto the 1 inch round glass disc while still a paste. A minimum of 100 milligrams is necessary for a good run, and although one can run with a smaller amount of sample it will require much longer count times to get good data.

Powders that have a very small grain size (i.e. nanometer-sized grains) will be very difficult to obtain diffraction data from since the grain size is of the same order as the wavelength of x-ray radiation being used.

Single-crystals, films on substrates, and all other types of samples can be run as long as they fit onto the 1 inch round glass disc.

The diffractometer has been calibrated using NBS Silicon powder, on a set of mounts that have a weighed amount of powder. Be aware that differences in your sample mass on the glass disc from this amount that was used for calibration will introduce a systematic peak shift in your diffraction spectra due to the different z-axis position of the surface of your mount relative to that used for calibration. Keep this in mind when evaluating the absolute position of diffraction peaks and any observed differences between observed peak positions and those expected from the book values.

Use Policy

Here is a summary of the policy for use of the Division Analytical Faciltiy Scintag Pad V X-ray Powder Diffractometer.

The diffractometer is available on a first-come, first-serve basis. If necessary, users can sign up for time on the diffractometer using the signup sheet.
Users must have a Radiation Safety Worker classification from the Safety Office in order to use the diffractometer unsupervised (see below).
The manager will run samples for you on a workload permitting basis. Generally, If you have more than one sample you will need to get an instrument checkout and run the samples yourself.
The manager is available for any kind of help, but it is requested that you contact him to arrange a time if you need more than a few minutes help (The manager may be scheduled with another user on another instrument and is not available if working with someone else).
Users are responsible for filling the liquid nitrogen dewar on the germanium detector prior to their run. Bear in mind that the dewar has to be cooled down on Monday morning, or any time that it dries out during the week. The system needs 1.5 hours to get down to temperature before use.
Users are also responsible for setting the x-ray tube back to standby settings after a run.

Checkout Procedure

The diffractometer is easy to use, especially with our new PC computer system. You can obtain an instrument checkout as follows:

First obtain a Radiation Safety Worker classification from the Safety Office (located in 25 Keith Spalding Bldg., contact Haick Issaian, Radiation Safety Officer, at x6727) in order to use the diffractometer unsupervised. This entails about 30 minutes and will require that you achieve a level of understanding of radiation safety issues; you will take a quiz administered by Safety in order to become a Radiation Safety Worker. If you already have Radiation Safety Worker status, this needs to be documented for the safety records kept in the Division Analytical Facility. Either have a copy of your paperwork made, or verify that your paperwork is on file in the Safety office.
Obtain a copy of the Scintag software manual, or log on to the computer and download the help file for use with the DMSNT program. Read this so that you will know what is being done during your checkout.
Schedule time with lab manager to get a diffractometer checkout. This will require about an hour for discussion of safety issues related to the Scintag Pad V, sample preparation, and use of the software.
Lab manager will place your name on the list of authorized users if and when you are able to correctly use the instrument and the software in an unsupervised capacity.

Writeup Summary

Here is a short summary of typical run parameters.

X-ray powder diffraction runs are acquired on a Scintag Pad V X-ray Powder Diffractometer using Cu Ka radiation on a theta-2theta goniometer equipped with a germainium solid-state detector. Acquisition conditions are 35 KV and 30 mA. Scans are obtained typically from 10 to 70 degrees 2theta, with step size or integration range of 0.05 degrees 2theta, with a count time of 5 seconds. Raw diffraction scans are stripped of ka2 component, background corrected with a digital filter (or fourier filter), and peaks are identified using a variety of algorithms. Observed peak positions are matched against the ICDD JCPDS database on CDROM (currently up through volume 47).

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