The Litao <sub> 3 </ Sub> Single Crystal Of The Impact Phase Transition

Shock-induced phase transformation is a crucial subject of study in the high-pressre science field,by which we could explore and discover structural transition of materials at high pressure and high temperature.Because of abundant phase transformation of planety materials and metals at extreme condition,high-pressure phase transition investigations are of significance to shock wave physics,material science,geophysics and planet science,and weapon physics.LiTaO₃ is an important functional material with a variety of technological applications in optical,electro-optic,piezoelectric,surface acoustic wave devices and infrared thermal detectors,is also a model single-crystal material in the ABO₃-type.However,there are big divergences about its high-pressure phase-transition behaviors for the limited data obtained from static and dynamic high-pressure experiments,such as large discrepancy about reported results of onset pressure of phase transition,and ambiguous cognition of high-pressure phase structure.In this paper,high pressure phase transformation behavior in z-cut LiTaO₃ crystal has been studied by a combination method of shock wave profile mesurements,Hugoniot measurements and the first-principle calculations.At our two-stage light-gas gun(35 mm bore diameter),shock compression data(shock velocity versus particle velocity relation)and time-resolved shock wave profile measurements of z-cut LiTaO₃ crystal were performed by a combined diagnostic of multi-channel pyrometer and velocity interferometer system for any reflector(VISAR).A distinct discontinuity was discovered on the measured D-u relation,and the linear fitting in high pressure region is D=2.748+1.577u.Time-resolved shock wave profiles showed a two-wave structure at low pressures(below 37.9 GPa)and three-wave structure at higher pressures.Experimental facts indicated a shock-induced phase transformation in LiTaO₃ crystal.Onset pressure of the phase transformation(37.9 GPa)redefined by our new shock compression data is quite higher than the early shock wave value(19 GPa)reported by Stanton et al.[J.Appl.Phys.50,6892 (1979)].In order to better understand the high-pressure phase structure of LiTaO₃,the zero temperature compression curves(P-V/V₀)and enthalpies(H-P)were calculated with ab initio plane wave pseudopotential local density approximation method.Result show that,the first-priciples calculation of zero temperature compression curve(P-V)for the rhombohedral phase(R3c space group)is in good agreement with DAC and our low-pressure experimental data,while that for orthorhombic phase(Pbnm space group)is in concord with our high-pressure shock compression data deducted thermal pressure contribution.Orthorhombic phase was much incompressible,and the zero-pressure density of orthorhombic phase was 24% higher than that of rhombohedral phase.It is concluded that the shock high-pressure phase of LiTaO₃ has an orthorhombic symmetry,which is similar to the room-temperature and high-pressure phase of LiNbO₃ crystal.Based on the analysis above-mentioned about shock wave experimental and theorically calculated resluts,we successfully clarify the onset pressure of phase transformation and high pressure structure of z-cut LiTaO₃ crystal,and confirm the high-pressure phase tansformation properties be similar with those of isomorphic LiNbO₃ crystal.Moreover,this work provide good ideas and methods for the investigations of shock-induced phase-transition for similar single crystal materials.