What thermodynamic process is being simulated here?


 High pressure physics involves the study of materials under extreme conditions of compression. By extreme conditions of compression it is meant pressures of order 100-1000 GPa, that is, about 1,000-10,000 times greater the pressure that we experiment on the surface of the Earth. Extreme pressures and also temperatures are found in the interior of planets and celestial bodies, so knowing the properties of materials at these conditions provides us with accurate ways of inferring their internal composition and past and future physical/chemical evolution. From a technological point of view, high pressures are also relevant since they can be used to alter and finely tune the energetic, structural and electronic properties of materials for realization of specific applications. For instance, a large number of metals are observed to undergo normal-to-superconductor phase transitions (i.e. they become materials with nominally zero electrical resistance) upon application of large or moderate pressures. Furthermore, large pressure and temperature swings can induce unique structural solid-solid, solid-liquid and liquid-liquid phase transitions on materials that otherwise would not be observed in ordinary life thermodynamic conditions.

 So far, my research on materials at high-P has focused on the study of transition metals (metals on the third and fourth row of the periodic table of elements with electronic d valence orbitals). The physics of transition metals (TM) at low temperatures and pressures is well understood as result of an early combination of experiments, first-principles calculations and simple models. Recent advances on experimental techniques and first-principles calculations might lead to similar understanding of TM at high temperatures and Mbar pressures but present information on the phase diagrams of interest is rather fragmentary and conflicting. Many years ago, shock wave (SW) measurements provided the first experimental P-T points on the melting lines of TM (Fe, Mo, Ta and W) in the Mbar regime. Lately, static compression techniques based on the diamond anvil cell (DAC) were used to fully map out the solid-liquid phase boundary of Fe, Mo, Ta, W, V and Y at pressures up to 100 GPa and temperatures T< 4000 K. Strikingly, huge discrepancies appeared among the sets of SW and DAC melting data amounting in some cases to several thousand K and DAC providing always the lowest melting curves. On the theoretical side, modeling of melting curves from first-principles (FP) started more than 10 years ago. Metals like Fe, Mo and Ta have been investigated with these techniques and so far FP calculations appear to support the correctness of SW measurements. Explanations based on quite diverse arguments have been proposed to rationalize the origins for these dramatic disagreements however there is still lack of overall consensus on this matter. Among all the studied TMs, Mo and Ta appear to be particularly interesting since they present the largest discrepancies between DAC measurements and ab initio-based predictions.

 In my work I use computational FP techniques like density functional theory (DFT) and tight-binding approaches to understand and predict the energetic and melting properties of transition metals at low and extreme compressions ( 0< P< 400 GPa).  In the last few years, I also have investigated on the nature of highly compressed rare-gases elements and mixtures (Ne-He, Ar-He and Ar-H2).

 

1. Physical Review B 75, 214103 (2007)
S. Taioli, C. Cazorla, M. J. Gillan and D. Alfe
"Melting curve of Tantalum from first principles calculations"

2. Journal of Chemical Physics 126, 194502 (2007)
C. Cazorla, M. J. Gillan, S. Taioli and D. Alfe
"Ab initio Melting Curve of Molybdenum by the Phase Coexistence Method"

3. Physical Review Letters 101, 049601 (2008)
C. Cazorla, D. Alfe and M. J. Gillan
"Comment on Molybdenum at high pressure and temperature: Melting from another solid phase"

4. Physical Review B 77, 224103 (2008)
C. Cazorla, D. Alfe and M. J. Gillan
"Zero-temperature generalized phase diagram of the 4d transition metals under pressure"

5. Physical Review B 80, 064105 (2009)
C. Cazorla, D. Errandonea and E. Sola
"High-pressure phases, vibrational properties and electronic structure of Ne(He)2 and Ar(He)2 : A first-principles study"

6. Journal of Physical Chemistry C 117, 11292 (2013)
C. Cazorla and D. Errandonea
"The high-pressure high-temperature phase diagram of calcium fluorite from classical atomistic simulations"
 

7. Physical Review Letters 113, 235902 (2014)
C. Cazorla and D. Errandonea
"Superionicity and polymorphism in calcium fluoride at high pressure"