A high pressure X-ray cell for liquid crystals: Pressure dependence of the smectic A layer spacing of 8 OCB in relation to its re-entrant behaviour

Abstract

An opposed diamond anvil high pressure cell is described with special reference to its applicability to X-ray (as well as optical) studies of liquid crystals at high pressures. By using the diamond anvils in a gasketed configuration and by suitably adjusting the areas of the anvil faces, it has been possible to achieve two important functions (a) the liquid crystalline sample is maintained in hydrostatic conditions for long periods of time without any pressure leaks, and (b) the cell, normally considered useful for pressures beyond a few kilobars, is suitable for pressures as low as a few hundred bars. Using this cell an X-ray investigation has been made of the pressure variation of the layer thickness in the smectic A phase of 8 OCB. It is found that as the pressure is increased, the spacing decreases more or less linearly at the rate of about 0.,6 A/kbar till the pressure attains about 1.4 kbar, after which it increases. This result shows conclusively that the re-entrant behaviour of this compound which occurs at higher pressures is associated with an expansion of the smectic A layer spacing with increasing pressure, as was predicted by Cladis et al.