Hybrid ab-initio/experimental high temperature equations of state: Application to the NaCl pressure scale

Abstract

Equations of state (EoS) are a fundamental subject in high pressure/temperature (PT) research. Ab initio calculations based on density functional theory (DFT) can provide valuable information about a material's EoS at PT conditions that cannot be easily accessed experimentally. However, these calculations have systematic errors due to (1) a lack of a precise description of the exchange correlation energy, (2) methodological limitations in the way temperature is addressed, for instance, anharmonicity at high temperatures in quasiharmonic calculations. To address the first issue, we have improved, developed, and tested correction schemes aiming to remove DFT errors and to produce predictive low temperature EoS with accuracy comparable to experiments. We have investigated four schemes and applied them to three different functionals. The second issue has been addressed with a simple anharmonic correction that effectively removed high temperature anharmonic errors.

Importance of van der Waals interaction on structural, vibrational, and thermodynamic properties of NaCl

“…Subsequently, the VDoS was computed at each volume using the same functional adopted for each static calculation, and the Helmholtz free energy was obtained. This procedure contrasts with that in reference [19], where the PBE VDoS was used in all cases. However, the goal of that calculation was to develop a scheme to combine ab initio results with experimental compression curves to produce thermal EoSs with optimal accuracy to be used as pressure scale.…”

“…In general, EoS for alkali halides are not well calculated when compared to strongly bonded ionic compounds with the same crystalline structure. For instance, MgO EoS parameters at 300 K deviate by at most 0.6% from experimental values [21], while for NaCl this deviation can be 6% [19].…”

Toward an international practical pressure scale: A proposal for an IPPS ruby gauge (IPPS-Ruby2020)

“…The obtained A-value may be compared with those estimated by using the low-pressure data of the ruby fluorescence shifts against specific volumes of NaCl [8,24]. By converting the V-Δλ data to P-Δλ relations using the EOS of NaCl [21,22,64], we obtain A = 1890(5) GPa from Brown's EOS for NaCl [21], A = 1848(6) GPa from Marcondes and Wentzcovitch's EOS [22], and A = 1833(4) GPa from Decker's EOS [64], all with B fixed at 5.5 and a similar pressure range of 0-12 GPa. Our constrained A-value, 1.87(1)x10 3 GPa, is generally consistent with those from NaCl data within the uncertainties in the EOS from different sources.…”

“…The ramp compression technique offers knowledge of the EOS of solid materials at multi-Mbar pressures. Lastly, theoretical calculations have become increasingly accurate tools for predicting the EOS of materials [19][20][21][22].…”

Temperature Dependent Equation of State for Solids

“…An equation of state can be applied successfully to explain the behavior of solids under varying pressure and temperature and helps to obtain the extrapolated values in the regions where the experimental data are not available 6,7 . Several theoretical and experimental studies have been performed during past [8][9][10][11][12][13][14][15][16] to explain the behavior of solids under varying temperature pressure conditions. However, there is always a scope for the better approach which may be more efficient and applicable to predict the better results and therefore a more efficient equation of state is required to explain the temperature and pressure dependent anharmonic behavior of solids under extreme temperature and pressure conditions.…”