Ab Initio Study Of Pressure-Induced Phase Transition In Alkali Hydrides

The properties of materials under high pressure and temperature have attracted much attention because of their relevance for understanding the compositions of the earth's interior and other planetary interiors. Accurate first-principles methods can complement and help to interpret high-pressure experiments, which can provide a detailed deCsClription of the structural and bonding changes that a material undergoes under extreme conditions. The behavior of hydrogen at high pressure is central to a number of fundamental problems in condensed matter and planetary science.The possibility of the formation of the proton hydride at high pressure has recently been raised.2 Moreover, the metal-hydrogen systems have received wide attention partly due to the large number of technical applications.3 Among the different classes of hydrogen compounds, the alkali hydrides (LiH, NaH, KH, RbH, and CsH) form"so-called"ionic hydrides. Support for this view can be obtained from the observed crystal structures of Rocksalt (RS) and CsCl for these compounds which are typical for ionic systems.It is known that the alkali hydrides crystallize with the RS structure at ambient pressure. Except for LiH, under high pressure, NaH, KH, RbH, and CsH were observed to transform to CsCl structure at pressures of 29.3 GPa, 4.0 GPa, 2.2 GPa, and 0.83 GPa, respectively. Also, Ghndehari et al. observed a further transformation of CsCl to a CrB structure (Cmcm space group) at a higher pressure of 17.5GPa for CsH, while no experimental measurements suggest the existence of such phase transition for NaH, KH, and RbH. On the theoretical side, Kuiljov et al. using an empirical equation of states (EOS) firstly predicted a RS→CsCl transition in LiH with a transition pressure in the range of 50-100GPa. Hammerberg et al. using a Heine-Abarenkov type pseudopotential with an empty core for Li~+ ion predicted also a RS→CsCl transition in LiH at about 200GPa. Later, Martins et al. using ab inito pseudopotential method within local density approximation (LDA) calculated the EOS of LiH, NaH, and KH and suggested that the transition in LiH would occur only at very high pressures of 450 to 500GPa. Recently, Ahuja et al. theoretically predicted the similar phase transition sequence of CsCl to CrB for KH and RbH by means of the total-energy calculations within LDA using the full-potential linear muffin-tin orbital (LMTO) method. More recently, Saitta et al.successfully demonstrated that the CsCl to CrB phase transition in CsH is attributable to the combination of a shear deformation and an atomic distortion associated with an M 2? phonon mode.However, to the best of our knowledge, the physically driven mechanism of the pressure-induced structural phase transition of RS→CsCl in NaH, KH, RbH, and CsH is still elusive. Dynamical and elastic instabilities are often responsible for phase transitions under pressure. Lattice dynamics and elastic behaviors play an important role in understanding the mechanisms of the phase transitions. To probe the physically driven mechanism of the phase transition in the RS alkali hydrides, detailed ab initio calculations of the lattice dynamics and elastic constants for these compounds are, thus, motivated.Pseudopotential plane-wave ab initio calculations were performed within the framework of density functional theory. The generalized gradient approximation (GGA) exchange-correlation functional was employed. The nom-conserving scheme is used to generate the pseudopotentials for Li, Na, K, Rb, Cs, and H, respectively. A non-linear core correction to the exchange-correlation energy functional was introduced to generate pseudopotentials for Li, Na, and Cs. Instead, 3p and 4p semicore states are incorporated into the valence electrons for K and Rb, respectively. The core radii for H, Li, Na, K, Rb, and Cs are chosen to be sufficiently small to guarantee the core non-overlapping under compression in this study. Convergence tests gave a kinetic energy cutoff, Ecutoff, as 70Ry and a 8×8×8 Monkhorst-Pack (MP) grid (k-mesh) for the electronic BZ integration. The lattice dynamics of these compounds were investigated by using the linear-response method. A 12×12×12 MP k-mesh was found to yield phonon frequencies converged to within 0.05 THz. A 4×4×4 q mesh in the first BZ was used in the interpolation of the force constants for the phonon dispersion curve calculations. The elastic constant tensors were calculated as a function of pressure using the stress-strain relations. Elastic constants were obtained from evaluations of the stress tensor generated by small strains using the density-functional plane wave technique as implemented in the CASTEP code.In this work, we have extensively studied the lattice dynamics and elastic constants of RS alkali hydrides under pressure using the ab initio pseudopotential plane-wave method. We predicted a universal pressure-induced soft TA phonon mode at the zone boundary X point for these compounds. Moreover, a softening behavior in C44 shear modulus with pressure is verified for NaH, KH, RbH, and CsH, while it is absent for LiH. Analysis of the calculated results suggested that with increasing pressure the predicted TA phonon softening behaviors, instead of C44 shear modulus instability, is closely related to the pressure-induced structural phase transition from RS to CsCl. From the currently predicted phonon softening behaviors for LiH, One might also expect a similar RS→CsCl phase transition in LiH which did not confirm by experiments yet. The calculated pressure for TA(X) phonon mode softening to zero frequency in LiH is ~200GPa as deduced, which is now accessible in the experiment.