Theoretical and computational study of high pressure structures in barium

Recent experimental results on two hcp phases of barium under high pressure show interesting variation of the lattice parameters. They are here interpreted in terms of electronic structure calculation by using the LMTO method and generalized pseudopotential theory (GPT) with a NFE-TBB approach. In phase II the dramatic drop in c/a is an instability analogous to that in the group II metals but with the transfer of s to d electrons playing a crucial role in Ba. Meanwhile in pha...

Group I/II materials exhibit unexpected structural phase transitions at high pressures, providing potential insight into the origins of elemental superconductivity. We present here a computational study of elemental barium and binary sodium-calcium alloys to identify both known and unknown phases of barium under pressure, as well as stable high-pressure compounds in the immiscible Na-Ca system. To predict stability, we performed density functional theory calculations on rando...

Using a membrane-driven diamond anvil cell and both ac magnetic susceptibility and electrical resistivity measurements, we have characterized the superconducting phase diagram of elemental barium to pressures as high as 65 GPa. We have determined the superconducting properties of the recently discovered Ba-VI crystal structure, which can only be accessed via the application of pressure at low temperature. We find that Ba-VI exhibits a maximum Tc near 8 K, which is substantial...

At high pressure, bismuth acquires a complex incommensurate host-guest structure, only recently discovered. Characterizing the structure and properties of this incommensurate phase from first principles is challenging owing to its non-periodic nature. In this study we use large scale DFT calculations to model commensurate approximants to the Bi-III phase, and in particular to describe the atomic modulations with respect to their ideal positions, shown here to strongly affect ...

Alkaline-earth metal oxides, in particular MgO and CaO dominate Earths lower mantle, therefore, exploring high pressure behavior of this class of compounds is of significant geophysical research interest. Among all these compounds, BaO exhibits rich polymorphism in the pressure range of 0-1.5 Mbar. Static enthalpy calculations revealed that BaO undergoes a pressure induced structural phase transition from NaCl-type (B1) $\rightarrow$ NiAs-type (B8) $\rightarrow$ distorted CsC...

Boron is an element of fascinating chemical complexity. Controversies have shrouded this element since its discovery was announced in 1808: the new 'element' turned out to be a compound containing less than 60-70 percent of boron, and it was not until 1909 that 99-percent pure boron was obtained. And although we now know of at least 16 polymorphs, the stable phase of boron is not yet experimentally established even at ambient conditions. Boron's complexities arise from frustr...

Following the discovery of high-temperature superconductivity in the La-H system, where for the recently discovered fcc-LaH10 a record critical temperature Tc = 250 K was achieved [Drozdov et al., Nature, 569, 528 (2019) and Somayazulu et al., Phys. Rev. Lett. 122, 027001 (2019)], we studied the formation of new chemical compounds in the barium-hydrogen system at pressures up to 173 GPa. Using in situ generation of hydrogen from NH3BH3, we synthesized previously unknown super...

Recent high-pressure x-ray diffraction studies of alkali metals revealed unusual complex structures that follow the body-centered and face-centered cubic structures on compression. The structural sequence of potassium under compression to 1 Megabar is as follows: bcc - fcc - h-g (tI19*), hP4 - oP8 - tI4 - oC16. We consider configurations of Brillouin-Jones zones and the Fermi surface within a nearly-free-electron model in order to analyze the importance of these configuration...

Experimental studies established that calcium undergoes several counterintuitive transitions under pressure: fcc \rightarrow bcc \rightarrow simple cubic \rightarrow Ca-IV \rightarrow Ca-V, and becomes a good superconductor in the simple cubic and higher-pressure phases. Here, using ab initio evolutionary simulations, we explore the behavior of Ca under pressure and find a number of new phases. Our structural sequence differs from the traditional picture for Ca, but is simila...

We use Density Functional Theory (DFT) and structure searching methods to show that the ground state of elemental sulfur is a Barium-IVa type incommensurate host-guest (HG) phase between $386$-$679$ GPa, becoming the first group-VI element predicted to possess such a structure. Within the HG phase, sulfur undergoes a series of transitions in which adjacent guest chains are not aligned but rather offset by different amounts, which can be described as a rearrangement of the sta...