X-RAY DIFFRACTION TECHNIQUES IN THE ANVIL HIGH CELL I. Spain

To cite this version:

I. Spain. X-RAY DIFFRACTION TECHNIQUES IN THE DIAMOND ANVIL HIGH PRESSURE CELL. Journal de Physique Colloques, 1984, 45 (C8), pp.C8-395-C8-397. ￿10.1051/jphyscol:1984870￿. ￿jpa-00224372￿

HAL Id: jpa-00224372 https://hal.archives-ouvertes.fr/jpa-00224372 Submitted on 1 Jan 1984

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL DE PHYSIQUE Colloque C8, supplément au n°ll, Tome 45, novembre 1984 page C8-395

X-RAY DIFFRACTION TECHNIQUES IN THE DIAMOND ANVIL HIGH PRESSURE CELL

I.L. Spain

Department of Physios, Colorado State University, Fort Collins, Colorado 80523, U.S.A.

Résumé - Les techniques expérimentales applicables à la diffraction X dans les dispositifs à enclumes de diamant, sont examinées. La compa­ raison (a) de la dispersion d'énergie avec la dispersion angulaire et (b) des expériences en rayonnement avec les sources conven­ tionnelles sera faite. Les techniques expérimentales basse température (%10-300 K) seront aussi discutées. Abstract - Experimental techniques for x-ray diffraction in the diamond anvil high pressure cell will be discussed. A comparison (a) of energy dispersive and angle dispersive and (b) of synchrotron and conventional source experiments will be made. Techniques for experiments at low tem­ perature (1-10-300 K) are also discussed. I - INTRODUCTION

A brief review of methods used by us for x-ray diffraction experiments with the diamond anvil cell will be given. This includes the use of both conventional and synchrotron radiation, and experiments at low . The brevity of the paper allows only conclusions to be given and mainly references from our own work. Discussion will be limited to powdered samples.

II - X-RAY DIFFRACTION USING SYNCHROTRON RADIATION

The arrangement used by us for energy-dispersive x-ray diffraction (EDXRD) measure­ ments using synchrotron radiation is illustrated in Fig. 1, and is similar to that used by other researchers /1,2/ with some differences. The main problems which are encountered in using this brilliant energy source of wide energy range are: (1) The beam is narrow in the vertical direction, so that precise positioning of the diamond cell is mandatory. This has been achieved by us through the use of an on-line detector, with the diamond cell mounted in a carriage whose position can be adjusted and monitored to ±20 urn (see later). (2) A collimator is needed to direct the incident x-rays onto the sample, avoiding the gasket. A two-slit collimator has allowed the adjustment time to be kept to a minimum (approximately a few minutes) which is important, since beam time is valuable, and the beam height varies rapidly during set-up, and can drift slowly during an experiment /3/.

Detector slit /CZ^>V

Vs~- -^- Sol id state \ "J \.

beam [IIP ~rfS VL S^K''""' \" ^ \ \ (^^nT^r l^^~j Noise reducing On-line \ ^MI— \C^^ slit proportional t ^^ \^ counter Al filter X-Y-Z stage

Incident beam collimators Fig- 1 - Sketch of the energy-dispersive x-ray diffraction arrangement using synchrotron radiation.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984870 C8-396 JOURNAL DE PHYSIQUE

(3) It is very important to reduce detector counts from spurious sources as much as possible. An A1 filter and the diamond anvils reduce low energy photons, while shielding and a double slit system on the detector reduce high-energy counts. Even when background noise is reduced as much as possible, the detector, and more importantly the detector electronics, can be overloaded. It was estimated during recent runs at CHESS that data could have been detected approximately 10 times faster if detector electronics could be improved, and approximately 100 times faster if both detector and electronics could be improved sufficiently. During these experiments the solid angle subtended by the detector slit was %2-3 x steradians, whereas a value of %2 x 10-4 steradians could have been arranged without loss of measurement precision.

A preliminary experiment has been carried out to test whether a detector could be used to replace the solid state detector. Calculations show that such an arrangement could give higher signal-to-noise ratio and better peak resolution. The ideal arrangement is one in which both a solid state and a crystal detector are used, with the latter programmed to search only near peak positions.

Sample heating can be severe in a diamond anvil cell if the sample is immersed in a fluid such as alcohol (thermal conductivity 'l.0.15 W/mK). For instance, a temper- ature rise of 10-50 K is possible using typical conditions at CHESS (5 GeV, 10 mA) 4 The heating effect can be particularly severe at low temperature (T 5 20 K).

I11 - X-RAY DIFFRACTION USING CONVENTIONAL RADIATlON

Angle-dispersive x-ray diffraction (ADXRD) has been carried out with MOK" radiation using both film and position sensitive proportional detectors. EDXRD has been carried out using W-Bremsstrahlung and a Si(Li) detector. The PSPD is generally the preferred technique giving approximately 10-20 times the counts in diffraction peaks for a given time exposure compared to EDXRD, while sensitivities are com- parable. A more detailed comparison is given elsewhere 151.

A comparison of EDXRD at CHESS to that on a conventional source shows the former to give approximately 5000 times the count rate. Accordingly, it is anticipated that ADXRD using a PSPD and a rotating anode source should produce 'l.0.05 - 0.1 times the count rate in diffraction peaks compared to CHESS. Apparatus for this experiment is being set up.

IV - X-RAY DIFFRACTION EXPERIIIENTS AT LOW TENPERATURE

Early experiments were carried out by us in small diamond cells using a LHe-flow cryostat 161. It has been found more convenient (300 > T > 10 K) to use an Air Products Displex refrigerator with Mylar windows. The temperature of the sample can be controlled to 20.2 K provided appreciably heating by the x-ray beam does not occur.

In experiments with a conventional source of x-radiation, a pinhole collimator is located behind the incident side diamond anvil. The pinhole diameter is usually approximately two-thirds that of the gasket hole, so that precise positioning of the diamond cell in the beam is required. For this purpose, the carriage discussed earlier has been modified to provide x-y-z linear adjustments and angular adjust- ments (iO.ZO) about the z and y axes (x-axis parallel to the beam).

Pressure measurement has been accomplished using NaCl on CsCl x-ray scales. For this purpose Decker's /7/ scales have been extended to low temper- ature 181.

V - DIAMOND ANVIL CELLS

Diamond anvil cells have been modified and improved by us over a period of time using the Bassett cell /9/ as a basis. Modifications have included the use of carbide rockers /lo/, rocker adjustment plates Ill/, tougher, Body

Diamonds and Bellevilla sample spring washers \ I

Rockers' ' Piston Ball race (WCI

Fig. 2 - Sketch of a diamond anvil cell used for low-temperature measurements. low-temperature alloys 1121, pneumatic/hydraulic piston activators, improved collimators for enhanced radiation intensity.

Experiments have mostly been carried out in the range up to 500 kbar, with some to %700 kbar. Until recently, 4:l methano1: was used as the pressure trans- mitting medium. Later studies with Ar as pressure transmitting medium have con- firmed the importance of the state of stress condition on the details of transitions. Figure 2 shows a cross section of a diamond anvil cell used in the refrigerator.

ACKNOWLEDGEMENTS

Thanks are due to NASA, Ames Laboratory, for a grant supporting this work (iiNAG2-157).

REFERENCES

BAUBLITZ It., ARNOLD V., RUOFF, A. L. , Rev. Sci. Instr. 52 (1981) 1616. SKELTON, E.F., QADRI, S.B., WEBB, A.W., LEE, C.W., KIRKLAND, J.P., Rev. Sci. Instr. 56 (1983) 403. SPAIN, I.L., BLACK, D.R., MENONI, C.S., Rev. Sci. Instr. (in press). MENONI, C.S., SPAIN, I.L., High Temp.-High Press. 16 (1984) 157. SPAIN, I. L., BLACK, D.R., MERKLE, L.D., HU, J.Z., MENONI, C.S., Proceedings of International Workshop on X-ray Diffraction at High Pressure, Karlsruhe, West Germany, August 1984 (to be published). SKELTON, E.F., SPAIN, I.L., YU, S.C., LIU, C.Y., CARPENTER, E.R., Rev. Sci. Instr. 48, (1977) 879. DECKER, D.L., J. Appl. Phys. 42, (1971) 3239. PIENONI, C.S., SPAIN, I.L., High Temp.-High Press. 16 (1984) 119. BASSETT, W.A., TAKAHASHI, T., STOOK, P.W., Rev. Sci. Instr. 38 (1967) 37. SKELTON, E.F., LIU, C.Y., SPAIN, I.L., Rev. Sci. Instr. 9 (1977) 19. YU, S.C., LIU, C.Y., SPAIN, I.L., SKELTON, E.F., "High Pressure Science and Technology, Vol. 1," p. 274 (1979) (ed. K. D. Timmerhaus and M. S. Barber, Plenum Press). SPAIN, I.L., SKELTON, E.F., RACHFORD, F., "High Pressure Science and Tech- nology, Vol. 1," p. 150 (1980) (ed. Vodar and Marteau, Pergamon Press).