Phonon Softening and Elastic Instabilities in the Cubic-to-Orthorhombic Structural Transition of CsH

Abstract

The cubic-to-orthorhombic structural transition occurring in CsH at a pressure of about 17 GPa is studied by ab initio calculations. The relative stability of the competing structures and the transition pressure are correctly predicted. We show that this pressure-induced first-order transition is intimately related to a displacive second-order transition which would occur upon application of a shear strain to the (110) planes. The resulting instability is rationalized in terms of the pressure-induced modifications of the electronic structure. [S0031-9007(97)03462-5] PACS numbers: 64.70. Kb, 62.50. + p, 63.20.Dj, 63.70. + h In recent years the availability of the diamond anvilcell technology [1] has renewed the interest in the highpressure properties of alkali halides which are the simplest and prototypical among ionic solids. In the case of cesium halides, these studies have been carried out especially in the search of band-overlap metallization which occurs at pressures in the Mbar range. In the quest of metallization, unexpected phase transformations to tetragonal and orthorhombic structures have been observed experimentally [2-6] and studied theoretically [6][7][8][9].Metal hydrides are considerably more covalent than the corresponding halides. In fact, transition-metal hydrides are considered to be covalent materials, whereas alkali hydrides are structurally rather similar to the corresponding halides. Alkali hydrides at zero pressure crystallize in the rocksalt (cubic B1) structure, while most of them undergo a transition to the cubic B2 (CsCl-like) structure at an applied pressure of a few GPa [10 -12]. Recently, a second transition from the B2 structure to a new orthorhombic phase has been observed to occur in CsH at an applied pressure of about 17 GPa [13]. The new phase has been assigned the CrB structure which belongs to the D 17 2h space group. In order to assess the driving mechanisms of this transition we have undertaken a series of first-principles calculations of the electronic, structural, elastic, and vibrational properties of CsH as well as of their dependence upon pressure. To this end, we have employed density-functional theory (DFT) within the local-density approximation (LDA) [14], and its gradient-corrected (GC) generalizations [15]. Our calculations have been performed using norm-conserving pseudopotentials [16]. In the case of Cs, 5s and 5p semicore states have been treated as valence states. For LDA calculations we have used the same Cs potential as in Ref.[8], while hydrogen is described by a norm-conserving pseudopotential constructed for the 1s wave function, so as to smoothen it somewhat and make our calculations well converged with the 25 Ry kinetic-energy cutoff which we use. New potentials have been generated for GC calculations. The vibrational properties are determined using the densityfunctional perturbation theory (DFPT) [17].In Table I we report our results for some structural properties of CsH in the three phases studied here: B1, B2, and CrB. The LDA fails to account for...

Observation of the high-pressure Pmma phase in InAs: A combined X-ray absorption and diffraction study

“…Moreover, in these calculations, the CsCl phonon instability that leads to P mma is predicted to set in after 135 GPa [4], whereas our transition is at much lower pressure. However, we underline again that the observed transition is from the orthorhombic Cmcm phase rather than from the cubic CsCl phase, so the much lower transition pressure value is not surprising: other works [16] have shown that phonon softening in distorted phases sets in at much lower pressures than in closely related high-symmetry phases. The mechanism responsible for the Cmcm → P mma transition is not yet clear, but calculations are underway to suggest what it is.…”

The study of structural, elastic, electronic and optical properties of CsYx I(1 − x)(Y = F, Cl, Br) using density functional theory

“…Cesium halides studies have been carried out to search for the band-overlap metallization which occurs at pressures in the Mbar range. Unexpected phase transformations to tetragonal and orthorhombic structures in the quest of metallization have been observed experimentally and theoretically [14][15][16].…”

Phonons and related crystal properties from density-functional perturbation theory

“…A second transition from the B2 structure to a new orthorhombic phase (assigned to a CrB structure, space group D 17 2h ) has been observed in CsH at P ∼ 17 GPa (Ghandehari et al 1995). In Saitta et al (1997) it is shown that this first-order transition is intimately related to a displacive second-order transition (driven by a soft phonon at the M point of the BZ, M − 2 ) which would occur upon application of a shear strain to the (110) planes. Baroni and Giannozzi (1998).…”