First‐principles calculations within density functional theory were performed on a series of halide perovskite compounds ABX3(A: Cs or Rb; B:Pb or Sn). Their electronic structure, lattice dynamics, and dielectric properties were studied in relationship with the change in atom species at each one of the three inequivalent crystallographic sites, to explain the origin of these properties. Thus, the variation of the bandgap with the overlap between the B cation lone pair and the electronic states of halide atoms, as well as with the distortion of the BX6 octahedra network is discussed. It is shown that the vibrational modes, phonon frequencies, atomic displacements, and the possible ferroelectric instability in these compounds are dependent on masses of atoms, volume of AX12 polyhedron, as well as on streoactivity of Pb lone pair. Also, the Born effective charges, dielectric constant, spontaneous polarization, and infrared spectra are calculated. The relation between these dielectric properties and the ions dynamics is discussed.

Using first‐principles lattice dynamics calculations, new phase transitions driven by soft phonon modes in CsPbBr3, from the high‐symmetry cubic structure Pm3m to hypothetical structures, are predicted. The structural, electronic, and dielectric properties of the new phases are determined. A ferroelectric phase with Pmc21 symmetry is found to be more stable than the experimentally observed phases. It is suggested that it is a good candidate as a ground state. It is demonstrated that the out‐of‐phase rotations of the PbBr6 octahedra determine a more stable crystal lattice than the in‐phase rotations. All predicted structures show semiconductor character, with bandgap values between 1.44 and 1.87 eV. The bandgap increases with increasing PbBr bond length and the angle of rotation of the PbBr6 octahedron. The dielectric tensor, Born effective charges, and spontaneous polarization are also calculated. The optical dielectric constant shows low sensitivity to unit cell volume and symmetry of the structures. The Born effective charges are sensitive to lattice symmetry, as they are related to the directions of charge transfer between ions, whereas the cell volume has no significant effect on their values. The infrared and Raman vibration modes of the three structures are also determined.

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New phase transitions driven by the soft mode condensation from the Pm3m structure to related subgroup variants are predicted for CsPbBr3 from DFT calculations, performed by Raouia Ben Sadok and co‐workers (see article number http://doi.wiley.com/10.1002/pssb.202000289). The Pb2+ cation has only a small contribution to the structural distortions, which leaves an important role to Br– and Cs+ displacements in the structure stabilization. The ferroelectric distortion is found less favorable than the antiferrodistortive one, where the out‐of‐phase rotations of the PbBr6 octahedra may determine more stable structures than the in‐phase rotations. Nevertheless, the polar structure Pmc21 appears to be a candidate as ground state. All predicted structures show semiconductor character, with the band gap increasing when increasing the Pb–Br bond length and the angle of rotation of the PbBr6 octahedra. The optical dielectric constant and Born effective charges show low sensitivity to unit cell volume.