Editing Diamond anvil cell (section)

==Measuring pressure==
The two main pressure scales used in [[Statics|static]] [[high-pressure]] experiments are [[X-ray diffraction]] of a material with a known [[equation of state]] and measuring the shift in ruby [[fluorescence]] lines. The first began with NaCl, for which the compressibility has been determined by first principles in 1968. The major pitfall of this method of measuring pressure is that the use of X-rays is required. Many experiments do not require X-rays and this presents a major inconvenience to conduct both the intended experiment and a diffraction experiment. In 1971, the NBS high pressure group was set in pursuit of a [[spectroscopic]] method for determining [[pressure]]. It was found that the [[wavelength]] of ruby [[fluorescence]] [[emission (electromagnetic radiation)|emissions]] change with pressure; this was easily calibrated against the NaCl scale.<ref>{{cite journal |last1=Mao |first1=H.K. |last2=Bell |first2=P.M. |last3=Shaner |first3=J.W. |last4=Steinberg |first4=D.J. |date=June 1978 |title=Specific volume measurements of Cu, Mo, Pd, and Ag and calibration of the ruby R1 fluorescence pressure gauge from 0.06 to 1&nbsp;Mbar |journal=[[Science (journal)|Journal of Applied Physics]] |volume=49 |issue=6 |pages=3276–3283 |doi=10.1063/1.325277 |bibcode=1978JAP....49.3276M }}
</ref><ref>{{cite journal |last1=Mao |first1=H.K. |last2=Xu |first2=J. |last3=Bell |first3=P.M. |date=April 1986 |title=Calibration of the ruby pressure gauge to 800&nbsp;kBar under quasi-hydrostatic conditions |journal=[[Science (journal)|Journal of Geophysical Research]] |volume=91 |issue=B5 |pages=4673–4676 |doi=10.1029/JB091iB05p04673 |bibcode=1986JGR....91.4673M}}</ref>

Once pressure could be generated and measured it quickly became a competition for which cells can go the highest. The need for a reliable [[pressure]] scale became more important during this race. Shock-wave data for the [[compressibility|compressibilities]] of Cu, Mo, [[Palladium|Pd]], and Ag were available at this time and could be used to define equations of states up to Mbar [[pressure]]. Using these scales these pressures were reported:
{| class="wikitable"
|+ Highest reported cell pressure
! Year
! Pressure
|-
| 1976
| 1.2&nbsp;Mbar (120&nbsp;GPa)
|-
| 1979
| 1.5&nbsp;Mbar (150&nbsp;GPa)
|-
| 1985
| 2.5&nbsp;Mbar (250&nbsp;GPa)
|-
| 1987
| 5.5&nbsp;Mbar (550&nbsp;GPa)
|}

Both methods are continually refined and in use today. However, the ruby method is less reliable at high temperature. Well defined equations of state are needed when adjusting [[temperature]] and [[pressure]], two parameters that affect the lattice parameters of materials.