March 9, 2020.
Extreme condition research has played an important role in material sciences and geophysical research for more than three decades. However, it was not until very recently that the high pressure techniques have been used to explore the new properties of existing materials. Quenching technique is now available in several materials fabrication process where the starting raw materials have been put under high pressure after a certain period of time to undergo the structural phase transition and after taken down to ambient pressure the modified properties remain unchanged. An obvious example of this technique is the fabrication of synthetic diamond. In this process, high pressure and high temperature are required to transform carbon-rich substances intothe synthetic diamond. After the transformation, this newly formed diamond structure is a stable phase under ambient pressure and room temperature. The nano-polycrystalline diamond, fabricated from this technology, is now available for commercial used. Recent advance on the use of high pressure to enhance properties of materials is the emerging of superconductivity in H2S at 203 K which is the warmest superconductor ever been discovered. This frontier research can be clearly seen from several extreme condition research centers are now working in close collaboration with frontier material manufacturers and international companies.
Metal hydride materials under high pressure have been attracted both experimental and theoretical researchers since the discovery of superconductivity at 203 K and 150 GPa in H2S . Given that the higher hydrogen content in many simple hydride materials is predicted to give still higher Tc values . Therefore, several metal superhydrides are expected to develop a similar property under high pressure as also recently reports [3-6]. The highest Tc to date, is the LaH10 with the world record 3f 250 K at 170 GPa . Another aspect of metal hydride is for storing H in solid form which is reliable and highly safe for transportation. However, the relationship between the transition mechanism under high pressure and the effect of hydrogen atoms in those superhydride compounds have never been systematically studied. This is of crucial importance to understand the nature of emergence of high Tc in this family of materials as also proposed by N.W. Ashcroft .
Extreme Conditions Physics Research Laboratory focus our work on two aspect for metal hydride, application for hydrogen storage and superconductivity. For the hydrogen storage application, ScH3 has been studied for structural phase transition under high pressure. The wave-like arrangements of H atoms around metals plane have been observed. The wave-like arrangement allows the off-site symmetry positions of the H atoms, and leads to substantial changes in the pair distribution between Sc and H atoms which are associating with the changes in the electronic structure in such a way that the total energy is lowering. The symmetry breaking from P63mmc to P63 is also responsible for the band gap opening. In the P63 structure, the calculated band gap is 0.823 eV and is dynamically stable. This stability comes from sufficiently strong interactions between two neighboring H atoms at their off-site symmetry positions . Further work for superconductivity under high pressure in various metal hydrides is currently under investigation.
Fig. 1 Hydrogen atoms which arranged in the wave-like positions under high pressure in ScH3 (blue circles represent H, brown circles represent Sc and grey circles represent H atoms from HCP phase) .
Moreover, the recent interest in hybrid organic–inorganic perovskites (HOIPs) originates from their potential applications in optoelectronic, thermoelectric, and photovoltaic technologies, owing to their favorable electronic and optical properties [9-11]. In only a decade, reported power conversion efficiencies of perovskite solar cells have risen from the first record of 4% in 2009, to 22.1% in 2016 [12-14]. Intensive researches have rendered perovskite solar cells into devices that are both easy to fabricate and have high cell power conversion efficiency, on par with those of silicon solar cells. But a marketable perovskite photovoltaic device is still several years down the road, due to its stability issues [15-18]. Surprisingly, there are no high pressure investigation on this family of advanced materials. Therefore, full investigation on high pressure structure and physical properties are urgently required both from experimental and theoretical approach. This would not only provide a clear guidance for the experimental approach but also reveal future mechanism for the new generation of energy materials fabrication process. Our recent theoretical investigation reveal a significance in the choice of functional used in DFT. Effects of electronic nonlocality in density functional theory study of structural and energetic properties of a pseudocubic CH3NH3PbI3 are investigated by considering coherent rotation around C–N axis of a CH3NH3 cation. This rotation in CH3NH3PbI3 have been observed by neutron diffraction experiment. Our investigation exploit a number of truly non-local and semi-local exchange correlation density functionals and comparing calculated structural parameters with available experimental results. The vdW-DF-cx which takes into account the non-local van der Waals correlation and consistent exchange shows the best overall performance for density functional theory study of this particular system. Remarkable distinctions between results from vdW-DF-cx and those from PBEsol exchange correlation functionals are observed and indicate the need of including the non-local interaction in the study of this system, especially its dynamical properties. Interestingly, the maximally localised Wannier function analysis shows the hydrogen bonding assisted covalent character of two iodide anions at a moderate rotational angle which can lead to I2 formation and losing the original crystal structure which cause a degradation in this type of materials .
Fig. 2 Potential barrier for various types of functional used for DFT calculation .
The second part of our investigation extended to the dynamical properties of formamidinium lead iodide. The crucial role of the organic cation on a cubic perovskite structure of formamidinium lead iodide (FAPI) were investigated deploying the state-of-the-art density functional theory including the spin−orbit coupling (SOC). Equipped with Euler’s rotations, energy landscapes corresponding to the different orientations of formamidinium (FA) cation were calculated. From the energy landscapes, the flipping energy barriers are interpreted to be thermal agitations required to flip over for FA. The highest energy barrier is 24.7 meV, which is equivalent to T ∼ 286 K, the temperature over which the FA molecules are randomly oriented .
Fig. 3 An example of energy landscape for the rotation of organic molecule in formamidinium lead iodide .
 Drozdov, A.P. et al., Nature, 2015. 73, p. 525.
 Ashcroft, N.W. et al., Physical Review Letter, 2004. 92, p. 187002.
 Liu, H. et al., Proceedings of the National Academy of Sciences, 2017. 114, p. 6990.
 Peng, F. et al., Physical Review Letter, 2017. 119, p. 107001.
 Bi, T. et al., et al, arXiv: 1806.00163.
 Liu, H. et al., Physical Review B, 2018. 98: p. 100102(R).
 Somayazulu, M., et al., Physical Review Letter, 2019. 112, p. 027001.
 T. Pakornchote, T. Bovornratanaraks, S. Vannarat and U. Pinsook, Solid State Commun. 225, 48-55 (2016).
 M. A. Green et al., Nature Photonics, vol. 8, no. 7, pp. 506–514, 2014.
 T. M. Brenner, et al., Nature Reviews Materials, vol. 1, p. 15007, jan 2016.
 F. Zheng, et al., Journal of Physical Chemistry Letters, vol. 6, no. 23, pp. 4862–4872, 2015.
 J. Even et al., Journal of Physical Chemistry C. vol. 119, pp. 1016–10177, 2015.
 A. Kojima, et al., Journal of the American Chemical Society, vol. 131, no. 17, pp. 6050–6051, 2009.
 G. Hodes, Science (New York, N.Y.), vol. 342, no. 2013, pp. 317–8, 2013.
 J.-P. Correa-Baena, et al., Energy Environmental Science, vol. 10, no. 3, pp. 710–727, 2017.
 J. S. Manser, et al., Accounts of Chemical Research, vol. 49, no. 2, pp. 330–338, 2016.
 M. Shahbazi and H. Wang, Solar Energy, vol. 123, pp. 74–87, 2016.
 S. Pathak, et al., ACS Nano, vol. 9, no. 3, pp. 2311–2320, 2015.
 R. Klinkla, V. Sakulsupich, T. Pakornchote, U. Pinsook and T. Bovornratanaraks, Sci. Report 8, 13161 (2018).
 W. Sukmas, U. Pinsook, P. Tsuppayakorn-aek, T. Pakornchote, A. Sukserm and T. Bovornratanaraks, J. Phys. Chem. C, 123 (27), 16508-16515 (2019).
Assoc. Prof. Dr. Thiti Bovornratanaraks
Dept. of Physics, Fac. of Science, Chulalongkorn University, Bangkok 10330, Thailand