| Temperature, Pressure and composition are three parameters of the material state. To change the temperature and composition is a common method to study the characters and to improve the performances of the material. With the help of the high pressure technique, we can obtain new materials which cannot be prepared at ambient conditions. The essential effect of pressure is to reduce the inter-atomic distance, which leads to the modification of lattice constants and changes in atomic positions, and it can also change the Atomic Force Constance and the overlapping of electronic orbits which brings the differences of the electronic properties and optical properties. So the study of materials under high pressure is quite abundant. Moreover high pressure can help us to understand the interaction between the molecule and atom, and to study the utility conditions and limits of some models in normal condition, thus improving us the understanding of the physical law.Metal hydrides are of great scientific and technological interest in view of their potential applications for hydrogen storage, and are also of fundamental importance because of their intriguing electronic, structural, and dynamical properties associated with the hydrogen atoms. In particular, those metal hydrides with high hydrogen to metal ratio, such as the group II hydrides (MgH2,CaH2,SrH2and BaH2), are desirable for real applications. The utilization of high-pressure technology has made considerable progress in experimental studies of hydrogen storage materials. It has been demonstrated that the application of high pressure is an effective tool for producing vacancies in the host metallic matrix for a number of metal–hydrogen systems, which in turn leads to various novel properties. Therefore, in order to improve the hydrogen storage properties of metals or metal alloys, an understanding of the structural stability and bonding nature of metal hydrides is considered to be essential.At ambient conditions, in group II hydrides, MgH2 has a Rutile-type structure (α-MgH2) with the space group P42/mnm, while CaH2, SrH2 and BaH2 adopt an orthorhombic structure with the space group Pnma. The high-pressure behavior of MgH2 has been studied extensively by using first-principles simulations, x-ray diffraction measurement, and their combination. Although the pressure-induced structural phase transition sequence of MgH2 under high pressure is still under debate, it is clear that new structural modifications (such as Pbcn and Pbca phases, etc) form under pressure. However, there have been no reports on high-pressure research on CaH2, SrH2 and BaH2 in the literature. Therefore, we present in situ Raman scattering measurements on CaH2 to probe its high-pressure behavior and also we perform first-principles calculations to aid the unambiguous assignment of the measured Raman vibrational modes and to understand better the physics behind the experimental observation. Indeed, a pressure-induced structural transition was identified at～16GPa and the phase transition was found to complete at～21 GPa with a new phase being formed. The current exploration of CaH2 also sheds strong light on the high-pressure behavior of SrH2 and BaH2, as they share the same ground-state structure.We have tried four different excitation lines of the Raman spectra to get the better one; it is found that at least nine Raman peaks are clearly revealed in the CaH2 Raman measurement under ambient condition. However, it is almost impossible to identify the vibrational modes without the help of theoretical analysis. One observes that the overall agreement between theory and experimental is excellent by evidence of a largest deviation of 3.6% in the low-frequency B1g mode (169 cm?1). The high accuracy in the theoretical calculation allows us to sign the Raman vibrational modes unambiguously. Eigenvector analysis suggests that the strongest Ag Raman peaks centered at 194 cm?1 are attributed to Ca/diagonal motion within a–c plane.In situ Raman spectrum under high pressures is one of the most sensitive measurements to detect the dynamical and structural properties of materials, moreover with this in situ measurement; we can give the simultaneous structure dynamic changes of the materials under high pressure. One qualified High-pressure in situ Raman scattering measurements were performed at up to 25.6 GPa with a Raman excitation of 514 nm from an argon ion laser. It is found that there are several obvious changes in the Raman spectra with the application of pressure. Firstly, the Raman peak centered at 152 cm?1 (Ag) shows a blue-shift and increased peak intensity below 15.6 GPa. When the pressure goes beyond 15.6 GPa, the Raman peak position remains nearly unchanged and its intensity decreases until it disappears beyond 21.3 GPa. Secondly, the two Raman bands centered at 196 cm?1 (Ag) and 226 cm?1 (B2g) move to higher energy and lose their intensities with pressure. Thirdly, on uploading pressure, the shoulder B3g band at 708 cm?1 and the broad band (B2g + Ag) at 752 cm?1 shift to higher energy and gain intensity under pressure below 14.5 GPa. However, the shoulder band moves faster, thus it merges into the nearby strong Raman band at 7.8 GPa. It is very interesting to note that at 14.5 GPa, the broad Raman band (B2g + Ag) split into two bands, losing their intensities with a main peak softening and a shoulder peak hardening. Finally, it is very important to note that a new Raman peak at 15.6 GPa centered at 212 cm?1 starts to appear and all the other Raman bands become weaker and weaker and eventually vanish. Beyond 21.3 GPa, only the new Raman band is visible in the energy range 100–1200cm?1. This Raman feature on uploading pressure clearly reveals a pressure-induced phase transition in CaH2. The coexistence of the new and old Raman peaks suggests a mixed phase in the pressure range 15.6–21.3 GPa. During the process of downloading pressure, the spectrum gradually changes back to a pattern similar to that of the starting material. Thus a reversible phase transition is evidenced. However, the characteristic Raman mode for the new phase disappears at about 12 GPa, which is much lower than the transition pressure of 15.6 GPa when uploading. This behavior clearly suggests a hysteresis in the reversible transition.In conclusion, the vibrational properties of CaH2 have been studied by in situ Raman scattering under high pressure. The results suggest that a pressure-induced phase transition occurred at about 16GPa and completed at～21 GPa, and that this phase transition is reversible, with a hysteresis, to～12 GPa. The new high-pressure phase was found to be stable up to at least 42 GPa. First-principles calculations help to assign the measured Raman modes accurately and to understand the pressure dependence of the Raman frequency shift. Our results are published in J. Phys.: Condens. Matter 19 (2007) 226205, and are confirmed by another group Prof. J. S. Tse in the department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan, Canada, P.R.B.75, 134108 (2007).
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