Ruby fluorescence scale

For pressure measurement we used the ruby fluorescence scale, calibrated under quasi-hydrostatic load conditions to 80 GPa with the known equations of state of two metals [7]. The pressure was measured at room temperature using micronsized ruby crystals, distributed on the edge of the sample volume (Fig. 2) to avoid chemical reaction with the sample material during heating. The ruby fluorescence was excited by Ar-laser radiation (Fig. 2). 

The linear calibration of the ruby fluorescence emission, initially based on the equation of state of sodium cloride [71, 89, 96, 103, 237], is reliable up to 30 GPa for a quasi-hydrostatic environment. The quasi-hydrostatic calibration was extended up to 80 GPa [157], and a slight nonlinearity of the calibration curve at high pressure was found. Calibration of the ruby scale against primary... 

Calibration of the pressure is best accomplished using the Ruby scale. Generally this is done using a metal gasket between the two diamond windows surrounding the sample in which a liquid (such as Nujol or Teflon oil) is added to produce hydrostatic pressure. The technique measures the pressure dependence of the sharp Ruby Ri fluorescence transition at 692.8 nm, although the R2 band at 694.2 nm can also be used. The Ruby fluorescence is induced by the blue excitation of the Ar+ (488.0 nm) or the He- C d (441.6nm) lasers. 

Since diamond is transparent to visible light, the easiest method of measuring the pressure is with the ruby-fluorescence (Al203 Cr ) scale. The fluorescence associated with the Rj and R2 transitions of the ion around 14400 cm at ambient pressure shifts by —7.57 cm GPa under pressure, and pressure variations of —100 GPa can be measured with a modest spectrometer. Excitation-argon-laser powers of —10 mW are sufficient to obtain measurable signals from ruby chips with a volume of —1000 (cm. A chromium content in the ruby of anywhere between 500 and 5000 p.p.m. 

At room temperature, pressures in excess of 500 GPa can be attained using the diamond anvil cell (DAC) technique [1]. This technique, described in a number of comprehensive reviews [2- ], has found broad application in the high pressure sciences, because diamond serves as an optical window from the far infrared to the near ultraviolet wavelength regime and is transparent to X-rays [4,5]. Development of the convenient ruby pressure scale, where the red shift of the Ri-fluorescence line with pressure is used in situ [6,7], led to a much wider use of this experimental technique. 

All specimens were tested in the as-received condition (cold-rolled). Scale characterisation was carried out by metallographic investigations on the respective cross-sections using conventional light and electron optical techniques. Additionally, residual compressive stresses in the oxide scales were determined using the ruby fluorescence technique. Details on the latter can be found in [3]. 

Pressure calibration is necessary in pressure work, and this is accomplished by incorporating a small ruby crystal with the sample under study. The Ruby scale (4) was developed by the National Bureau of Standards (now the NIST) in 1972, and the sharp Ruby Ri fluorescent line has been calibrated vs. pressure by NIST, and is suitable even up to megabar pressures (5). 

For monitoring the pressure in anvil cells we use the frequncy shift of internal, chemically inert pressure calibrants. For Raman spectroscopic measurements, the most commonly used method is based on the pressure-induced frequency shift of the fluorescence line of a small piece of ruby that is placed in the sample compartment of the cell, next to the sample [1]. For infrared spectroscopic measurements, we have developed a quartz pressure scale [9], a BaSO pressure scale [10], and an HOD pressure scale [11], In the case of the first two techniques, a small amount of powdered quartz or BaSO powder are placed in the sample hole on the gasket, together with the sample under investigation. The infrared spectra of quartz or BaSO, which are relatively simple, are recorded simultaneously with the spectrum of the sample and the pressure on the sample b then determined from the frequency shift of the infrared bands of quartz or BaSO. The HOD pressure scale was developed specifically for aqueous solutions. In this case, the pressure in solution is determined from the frequency shift of the uncoupled O-H stretching band of residual HOD in DjO solutions, or from the uncoupled O-D stretching band of residual HOD in HjO solutions [11]. 

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