High Pressure Structure And Properties Of Silicon Hydride

In 1935 Wigner proposed that hydrogen atoms should display metallic properties at high pressure.Since the prediction was proposed,much investigations has been paid to metallic hydrogen by different theoretical and experimental study groups.The hydrogen was attractive because of the predicted have very high critical temperature for superconductivity and remain an important significance in modern physics and materials science.In 2017,R.P.Dias and Isaac F.Silvera of Harvard university declaring that the metallic hydrogen was obtained under the pressure of 495 GPa.However,this is a controversial result because the sample disappeared.M.I.Eremets et al.show that at a pressure of 350–360 GPa and temperatures below 200 K,the hydrogen starts to conduct,and that the temperature dependence of the electrical conductivity is typical of a semimetal.Paul Loubeyre et al.have observed the hydrogen insulator-to-metal transition above 425 GPa.Here a discontinuous change of the direct bandgap of hydrogen,from 0.6 electronvolts to below 0.1 electronvolts,is observed.The metallic hydrogen has not been achieved until now.It is still a great challenge to get metallic hydrogen.In 2004,N.W.Ashcroft proposed that heavier atom introduced in the lattice of hydrogen will provide a precompression effect on the hydrogen atom,and this precompression effect could lower the pressure of metallization.The concept of chemically precompressed was frist proposed at the hydrides of group IVa elements.Recent theoretical studies predict that silane（SiH₄）,disilane（Si₂H₆）,germane（GeH₄）,and stannane（SnH₄）superconduct at much higher temperature.However,until now the high-pressure research on silane（SiH₄）and disilane（Si₂H₆）remain controversy for either experimentally or theoretically.For example,there are several phase structures of silane at high-pressure and which structure is the most stable.For the theoretical studies,the structure of the high-pressure phase of silane is obtained via various of calculation methods clearly different from each other.Experiment as a powerful technique,was expect can be able to distinguish those proposed structures and even give a cleary and definitely result,unfortunately,the obtained consequence are also divergent.In this paper,we have a series of detailed investigated of SiH₄ and Si₂H₆under high pressure and temperature.The main achievements are as follows:1、The high-pressure studies of disilane（Si₂H₆）by in-situ X-ray diffraction,Raman and Brillouin spectral measurements below 50 GPa were performed,Seven phase transitions of Fluid Phase I,Phase I’,Phase II,Phase III,Phase IV,Phase V,Phase VI,Phase VII were revealed,respectively.Under 20 GPa,disilane can be recovered to ambient pressure.Under further compression above 35.3 GPa,the Si₂H₆completely decomposed in to elemental Si and H₂.With pressure down loading to ambient pressure,the disilane did not occur again.Additionally,raman spectral measurements at low temperature,the structure behavior of disilane is consistent with that of normal temperature and high pressure.2、The high-pressure studies of Silane（SiH₄）by in-situ X-ray diffraction and Raman spectral measurements at room temperature were performed.Silane starts to dehydrogenate through dimerization processes at room temperature at 12.9GPa.The transition sequence and phase transition pressure of silane were determined.The resulting Si₂H₆ is verified experimentally and the Si₂H₆ will be further decomposed into simple silicon and hydrogen at 23.6 GPa.Pressure-induced decomposition process of SiH₄ is irreversible.We report the upon decompression studies of Silane（SiH₄）by Raman and in-situ X-ray diffraction spectral measurements at room temperature.The Raman spectrum of Si-XII/Si-III and a-Si:H were obtained at ambient pressure.At ambient conditions,laser annealing lead to a a-Si:H→Si-I phase transition by light-induced degradation.3、The high temperature and high-pressure studies of Silane（SiH₄）by Brillouin spectral measurements were performed.Here we report a first high temperature and high pressure Brillouin scattering measurements on condensed fluid SiH₄,velocities and refractive index n of fluid SiH₄ along 300 K,359 K and 407 K isothermals were all calculated and presented here.High temperature weakens the stability of SiH₄molecular leading to partial decomposition of fluid SiH_4.The velocity decrease with increasing temperature at a similar constant pressure both silane and PEEK films,which speculated that silane soften phenomenon by high temperature would be ignored after the application of pressure.The partial structural transformation can be identified from velocity measurement,which is consistent with the raman results.