New research led by Dr. Lei Su from the Center of High Pressure Science and Technology Advanced Research (HPSTAR) developed a new approach to expand the pressure of Grüneisen parameter measurement. Their work, published in Physical Review Letters, measured the Grüneisen parameter of sodium chloride up to 20 GPa using their developed method and state that this technique could be applied on more materials for the Grüneisen parameter direct measurement in a wide pressure range.

Grüneisen parameter is an important parameter of condensed matter, which is of great significance for studying the thermodynamic properties, elasticity and non-resonant properties of matter. Grüneisen parameters correlates elastic properties with thermal properties, which can be used to determine the equation of state of and study the thermodynamic effects under high temperature and high-pressure conditions. However, the isentropic compression conditions required for the direct measurement of Grüneisen parameters under high pressure are pretty challenged, especially in a wide pressure range.

The research team co-led by prof. Xiao Dong from Nankai University and Prof. Guoqiang Yang from University of Chinese Academy of Sciences, Chinese Academy of Sciences developed a new technique called dynamic diamond anvil cell combined with in-situ temperature measurement which could realize rapid compression on the sample and in-situ pressure-temperature measurement on the sample. They used the technique on sodium chloride and directly measured the Grüneisen parameter up to 20 GPa for the first time."There are several methods to measure the Grüneisen parameter, such as ultrasonic, thermodynamic etc, comparing to these methods, dynamic compression  a compression between static compression and shock compression, is the mostly used one to directly measure the Grüneisen parameter under high pressure conditions. But there is one important precondition for dynamic compression is that the compression rate should be fast enough, faster than the thermal transformation to make sure the compression is under an iso-isentropic adiabatic (no heat lost) environment,” explained Dr. Su. “For normal static compression, the compression rate is quite slow, the temperature change on the sample from compression will be quickly transported outside, so the static compression is isothermal compression. The dynamic compression method was proposed by Boehler et al in 1977, to measure the Grüneisen parameter under compression. Then several dynamic methods were proposed, but mostly are based on large volume press, in which the compression range is just within several gigapascals. So up to now, how to increase the compression range, to improve the temperature measurement of the Grüneisen parameter is still a long-standing question.”

The team used their developed dynamic compression method combined with in-situ pressure-temperature measurement technique to rapid compress the sodium chloride sample up to 20 GPa and directly determined the Grüneisen parameter of the sample. The compression rate is 0.5 ms, so the compression is iso-isentropic. They also used the obtained Grüneisen parameter to calculate the Hugoniot curve of sodium chloride, which is well consistent with that from shock compression, and thus prove that the Grüneisen parameter determined from their method is reliable.

"Our developed dynamic diamond anvil cell makes sure we could compress the sample up a higher pressure and iso-isentropic condition and the designed temperature measurement realize precise, simultaneous temperature measurement with pressure on the sample," said lead author Jun Kong, a joint PhD student of HPSTAR . "This method could be applied to other more materials and the pressure range could be extended to megabar pressure rang in our next studies.

Caption: a. Schematic illustration of the programmable automatic high pressure jump and temperature detection apparatus; b. A typical record of temperature and pressure for sodium chloride before and after the pressure jump; c. Grüneisen parameter vs pressure for sodium chloride from our data and previously published data; d. The pressure along the Hugoniot curve of NaCl.

由北京高压科学研究中心、南开大学和中国科学院化学研究所组成的研究小组（通讯作者：苏磊、董校和杨国强），提出了一种可以实现宽压力范围内格林艾森参数测量的新方法。相关成果以“Expanding the pressure frontier in Grüneisen parameter measurement: Study of sodium chloride”为题，发表于近期的《物理评论快报》（Physical Review Letters）上。