The formamidinium lead iodide hybrid perovskite is studied using first principles molecular dynamics simulations and further analyzed using group theory. The simulations are performed on large supercells containing 768 atoms under isothermal and fully anisotropic isobaric conditions. Two trajectories, one at 300 K and another at 450 K, were extended for over 50 ps in order to perform a detailed assessment of the rotational dynamics of organic cations. The characteristic rotations of the cation are analyzed by defining two rotation axes. It is found that the formamidinium molecules rotate preferentially around the direction parallel to the line connecting the two nitrogen atoms. The rotational dynamics shows some characteristics already observed in methylammonium lead iodide, like the heterogeneous dynamics at room temperature that disappears at 450 K. The orientational probability of the molecules is explored in terms of an expansion in cubic harmonics up to the 12th order. It reveals a strong directionality at room temperature that relaxes when increasing the temperature. These findings are further rationalized using Landau and group theories suggesting a mixed displacive/order-disorder structural instability at lower temperatures.

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Vertical stacking of two-dimensional materials into layered van der Waals heterostructures has recently been considered as a promising candidate for photocatalytic and optoelectronic devices because it can combine the advantages of the individual 2D materials.

Layered materials are the best candidates for thermoelectric application due to their in-plane low thermal conductivity that is a key property to achieve high efficiency. Owing to that, here we present our investigations on electronic as well as thermal transport of bulk and monolayer MX3 compounds (M = Ti, Zr, and Hf and X = S and Se) based on density functional and semiclassical Boltzmann theories. The values of the bandgap are rather similar for bulk and the monolayer, with only a slight change in the shape of bands near the Fermi level that results in a different effective mass. We found that the monolayer MX3 compounds are better thermoelectric materials than bulk. Also, the p-type monolayer of TiS3 has a high power factor at 600 K that doubles its room-temperature value. The monolayer of the Zr/HfSe3 compounds shows a promising behavior as a n-type thermoelectric materials at 600 K. In-plane tensile strain could be used to further tune the TE properties of the monolayers to obtain high-performance TE materials.

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We discuss the thermoelectric and optical properties of layered KxRhO2 (x = 1/2 and 7/8) in terms of the electronic structure determined by first principles calculations as well as Boltzmann transport theory. Our optimized lattice constants differ significantly from the experiment, but result in optical and transport properties close to the experiment. The main contribution to the optical spectra are due to intra and inter-band transitions between the Rh 4d and O 2p states. We find a similar power factor for pristine KxRhO2 at low and high cation concentartions. Our transport results of hydrated KxRhO2 at room temperature show highest value of the power factor among the hole-type materials. Specially at 100 K, we obtain a value of 3×10 −3 K −1 for K 7/8 RhO2, which is larger than that of Na0.88CoO2 [M. Lee et al., Nat. Mater. 5, 537 (2006)]. In general, the electronic and optical properties of KxRhO2 are similar to NaxCoO2 with enhanced transport properties in the hydrated phase.

In this work, we have studied new double perovskite materials, A21+B2+B3+X61−, where A21+ = Cs, B2+ = Li, Na, B3+ = Al, Ga, In, and X61−.

We report a first-principles study of structural and phase stability in three different structures of perovskite-types KMgH(3) according to H position. While electronic and optical properties were measured only for stable perovskite-type KMgH(3), our calculated structural parameters are found in good agreement with experiment and other theoretical results. We also study the electronic charge density space distribution contours in the (200), (101), and (100) crystallographic planes, which gives better insight picture of chemical bonding between K-H, K-Mg-H, and Mg-H. Moreover, we have calculated the electronic band structure dispersion, total, and partial density of electron states to study the band gap origin and the contribution of s-band of H, s and p-band of Mg in the valence band, and d-band of K in the conduction band. Furthermore, optical features such as dielectric functions, refractive indices, extinction coefficient, optical reflectivity, absorption coefficients, optical conductivities, and loss functions of stable KMgH(3) were calculated for photon energies up to 40 eV.