Crystal structure and phase transition of TlReO₄: a combined experimental and theoretical study

Abstract

The present work describes a density-functional theory (DFT) study of TlReO4 in combination with powder x-ray diffraction experiments as a function of temperature and Raman measurements at ambient temperature. X-ray diffraction measurements reveal three different structures as a function of temperature. A monoclinic structure (space group P21/c) is observed at room temperature while two isostructural tetragonal structures (space group I41/a) are found at low- and high-temperature. In order to complement the experimental results first-principles DFT calculations were performed to compute the structural energy differences. From the total energies it is evident that the monoclinic structure has the lowest total energy when compared to the orthorhombic structure, which was originally proposed to be the structure at room temperature, which agrees with our experiments. The structural and vibrational properties of the low- and room-temperature phase of TlReO4 have been calculated using DFT. Inclusion of van der Waals correction to the standard DFT exchange correlation functional is found to improve the agreement with the observed structural and vibrational properties. The Born effective charge of these phases has also been studied which shows a combination of ionic and covalent nature, resembling metavalent bonding. Calculations of zone-center phonon frequencies lead to the symmetry assignment of previously reported low-temperature Raman modes. We have determined the frequencies of the eight infrared-active, 13 Raman-active and three silent modes of low-temperature TlReO4 along with 105 infrared-active and 108 Raman-active modes for room-temperature TlReO4. Phonons of these two phases of TlReO4 are mainly divided into three regions which are below 150 cm−1 due to vibration of whole crystal, 250 to 400 cm−1 due to wagging, scissoring, rocking and twisting and above 900 cm−1 due to stretching in ReO4 tetrahedron. The strongest infrared peak is associated to the internal asymmetric stretching of ReO4 whereas the strongest Raman peak is associated to the internal symmetric stretching of ReO4. We have also measured the room-temperature Raman spectra of monoclinic TlReO4 identifying up to 28 modes. This Raman spectrum has been interpreted by comparison with the previously reported Raman frequencies of the low-temperature phase and our calculated Raman frequencies of low- and room-temperature phases of TlReO4.

Understanding the Re-entrant Phase Transition in a Non-magnetic Scheelite

“…The DFT calculations also predicted that the equivalent volume of the monoclinic cell is smaller than that of the tetragonal cell. 41 This symmetry lowering upon heating in the tetragonal−monoclinic transition of TlReO 4 is in contradiction to the observations in RbReO 4 and perovskite oxides, where increases in temperature reduce the magnitude of cooperative rotations of the polyhedral, ultimately increasing symmetry.…”

“…First principle DFT calculations show that ordering of the Tl 6s 2 electrons, as occuring in the monoclinic structure, lowers the energy of the system compared to a structure with fully disordered Tl 6s 2 electrons and, as experimentally observed, this is the favored long-range structure at room temperature. The energy difference between the two structures is however small (Table S1).…”

“…We propose that at a critical temperature, this increased flexibility, due to thermal expansion, facilitates long-range ordering of the Tl 6s 2 lone pairs and cooperative tilting of the tetrahedra, resulting in the monoclinic structure with increased packing efficiency of the unit cell. The DFT calculations also predicted that the equivalent volume of the monoclinic cell is smaller than that of the tetragonal cell . This symmetry lowering upon heating in the tetragonal–monoclinic transition of TlReO 4 is in contradiction to the observations in RbReO 4 and perovskite oxides, where increases in temperature reduce the magnitude of cooperative rotations of the polyhedral, ultimately increasing symmetry.…”

DFT study of electronic structure and mobility of pristine and fluorinated methylammonium lead halide perovskites (CH 3 NH 3 PbX 3 , X = I, Br, Cl)

“…Spin‐orbit coupling (SOC) effect is not considered in our calculations. This effect is important for compounds containing lead but it is not expected to impact our crystal structures and in similar studies 51 . It is a well‐established fact that SOC impacts the bandgap and the GGA method usually underestimates the bandgap.…”

“…This effect is important for compounds containing lead but it is not expected to impact our crystal structures and in similar studies. 51 It is a well-established fact that SOC impacts the bandgap and the GGA method usually underestimates the bandgap. In this way, the impact in the bandgap value due to the non-consideration of SOC effect is close to the value underestimated by the GGA method due to the error cancellation effect.…”