Experimental Study On High - Speed Sound Velocity And Structural Stability Of LiF Single Crystal

Structural stability of crystalline materials has been a common topic of intensive studies in physics, materials science, mechanics and others. Study on phase transitions and other physical properties of materials under high pressure, especially under high shock pressure, is of significance to the understanding of their dynamic behaviors and is of practical value to their applications in engineering, and has been an important research subject in shock wave physics. For example, phase transitions occurring in a material may substantially change its dynamic properties and influence its performance in a material application. Hence, study on phase-transition and structural stability plays an important role, and is of values in engineering application.LiF single crystal is an important material used frequently as a transparent window in pyrometric and velocimetric measurements for opaque samples under shock compression. It has also broad applications in electronic probes, fluorescence analyzers and large-scale optical instruments. Much attention has been focused on its equation-of-state, optical refractive index and elastic-plastic transition, which have been well documented. However, its phase-transitions and structural stability under high pressures are still not clear. The limited dataset from first-principles calculations and molecular dynamics simulations shows a big discrepancy about its phase-transition behavior at high-pressures. Different authors reported different results regarding whether a phase-transition occurs in LiF within the 10-Mbar pressure range, or at what the onset pressure of the phase-transition is. Structural stability of this material under high-pressure is still not well understood, and this is a crucial issue deserving experimental examination. In this work, shock-induced melting and structural stability in LiF shocked along its [100] crystallographic orientation are studied by wave profile and sound velocity measurements in combination of shock temperature and melting curve calculations.Shock compression experiments have been performed on [100] LiF single crystals with the planar impact method at a one-stage light-gas (10 mm bore diameter) gun and a two-stage light-gas gun (25 mm or 30 mm bore diameter). Interfacial velocities at two interfaces (impact and sample/window) are obtained with a Dopler Pin System (DPS) to investigate its Gruneisen equation of state and shock-induced melting. Shock pressure ranges from 2 GPa to 152 GPa. Longitudinal and bulk sound velocities of LiF are obtained along its Hugoniot via the Lagrangian analysis. A drop in the longitudinal sound velocity to bulk sound velocity appearing between 134 GPa and 152 GPa, suggestings that shock-induced melting initiates at 134-152 GPa. The Gruneisen parameter as a function of density in shock-states is determined. Given high-pressure Gruneisen parameters, the solid Hugoniot temperature and the melting curve of B1 phase LiF are calculated, which are consistent with previous molecular dynamics and ab initio calculations, as well as diamond-anvil cell and shock wave measurements. Our calculation suggests that shock-induced melting initiates at 142 GPa, in agreement with our sound velocity measurements.In this work, based on the sound velocities data and the calculations of shock temperature and melting the curve of B1 phase LiF single crystal, we successfully identify the onset pressure of shock-induced melting. Moreover, the new method developed in this work for accurate sound velocity measurements can be extended to other transparent materials.