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3.10 a. A diamond-anvil cell for single-crystal X-ray diffraction studies to pressures in excess of 10 GPa (D.R. Allan, R. Miletich and R.J. Angel)

Single-crystal X-ray diffraction remains the most effective technique for the study of structural pressure dependence in a significant class of problems - namely, where high precision on atomic coordinates is required to pressures well above 3-4 GPa, and where the use of single-crystal methods is not prevented by any pulverising phase transition. With the ongoing development of X-ray diffraction facilities at the Bayerisches Geoinstitut, we have been able to make significant improvements to the precision and accuracy of lattice parameter determinations, which has led, in turn, to an accompanying sensitivity of the measurements to small drifts in pressure within the diamond-anvil cell (DAC). To counteract this problem, and so gain the full precision available from our diffractometer, we have developed a new DAC design that enables a more accurate and stable radial and parallel alignment of the diamond culets. This not only helps to prevent premature failure of both the diamond anvils and the gasket but also allows a given sample volume to be taken to a significantly higher pressure. The cell, shown in Figure 3.10-1, is composed of two platens (A and B) machined from RHF steel which are drawn together by two pairs of oppositely threaded bolts. The platens are kept parallel by four guide pins which have been increased in diameter and moved further apart so that the cell has a greater stability than the Mao-Bell design, on which this cell is based. The platen thickness has also been increased so that a simple mechanism can be included to allow parallel alignment of the culets and, as the guide pins now pass through longer holes, this has the additional benefit of further increasing the stability.

The parallel alignment mechanism is based on a design similar to those used in other DACs and is composed of a 'ball-and-socket' arrangement. By adjusting the three M2 screws holding the RHF steel hemisphere into the socket of the 'upper' platen (labelled A in Fig. 3.10-1) the diamond mounted to the hemisphere can be adjusted so that it is parallel to the opposing diamond. As for the Mao-Bell cell, radial alignment is achieved by adjusting the four M2 set-screws holding the beryllium backing disc into the 'lower' platen (labelled B in Fig. 3.10-1). The backing discs themselves have also been thickened from 3 mm to 4 mm so that higher pressures can be achieved without premature failure of the beryllium.

The cell has been tested successfully to a pressure of 25 GPa with 600 µm culet diamonds and a single-crystal sample contained within a 90/10 tantalum-tungsten alloy gasket. We expect, though, that the cell will be of most practical use to pressures somewhat less than this as the hydrostatic conditions required for single-crystal studies cannot be maintained to pressures much in excess of approximately 10 GPa with a "standard" 4:1 methanol/ethanol pressure transmitting medium. Successful high-pressure structure determinations and equation of state measurements have been undertaken with the cell on a variety of systems and these studies are briefly summarised in other sections of this annual report.

Although initially intended for single-crystal diffraction, we are planning to use the cell for angle-dispersive powder diffraction studies with the image plate area detector system. Similar designs of 'beryllium backed' cells, which were also originally intended for single-crystal work, have reached pressures in excess of 80 GPa with suitably small (200 µm) diamond culets and we anticipate that our present cell will be capable of taking powdered samples well into this pressure region.

Fig. 3.10-1: An axonometric view (a) and a multi-layer cross-sectional view (b) of the diamond anvil cell showing; the upper platen (A); the lower platen (B); the mounting bracket (C): right-handed M5 bolts (1); left-handed M5 bolts (2); 5 mm diameter guide pins (3); M2 parallel alignment screws (4); M2 radial alignment set-screws (5); beryllium backing discs (6); mounting pin (7) and M2 mounting-bracket securing screws (8). The radius of the hemisphere is indicated by the arrow and the faint sector in (b).

Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
Tel: +49-(0) 921 55 3700 / 3766, Fax: +49-(0) 921 55 3769, E-mail: bayerisches.geoinstitut(at)