Theoretical and Experimental Study of the Crystal Structures, Lattice Vibrations, and Band Structures of Monazite-Type PbCrO₄, PbSeO₄, SrCrO₄, and SrSeO₄

Abstract

The crystal structures, lattice vibrations, and electronic band structures of PbCrO4, PbSeO4, SrCrO4, and SrSeO4 were studied by ab initio calculations, Raman spectroscopy, X-ray diffraction, and optical-absorption measurements. Calculations properly describe the crystal structures of the four compounds, which are isomorphic to the monazite structure and were confirmed by X-ray diffraction. Information is also obtained on the Raman- and IR-active phonons, with all of the vibrational modes assigned. In addition, the band structures and electronic densities of states of the four compounds were determined. All are indirect-gap semiconductors. In particular, chromates are found to have band gaps smaller than 2.5 eV and selenates higher than 4.3 eV. In the chromates (selenates), the upper part of the valence band is dominated by O 2p states and the lower part of the conduction band is composed primarily of electronic states associated with the Cr 3d and O 2p (Se 4s and O 2p) states. Calculations also show that the band gap of PbCrO4 (PbSeO4) is smaller than the band gap of SrCrO4 (SrSeO4). This phenomenon is caused by Pb states, which, to some extent, also contribute to the top of the valence band and the bottom of the conduction band. The agreement between experiments and calculations is quite good; however, the band gaps are underestimated by calculations, with the exception of the bang gap of SrCrO4, for which theory and calculations agree. Calculations also provide predictions of the bulk modulus of the studied compounds.

Pressure-induced phase transition and band-gap collapse in the wide-band-gap semiconductorInTaO4

“…For the LP wolframite phase, at ambient pressure the absorption spectrum shows a steep absorption, which corresponds to the overlapping of the fundamental absorption plus a low-energy absorption band. This low-energy absorption corresponds to the typical Urbach tail observed in related ternary oxides [51] plus a small diffusion of light between the grains of the polycrystalline sample used in the measurements. The nature of the low-energy tail has been the subject of considerable debate [52] and is beyond the scope of this paper.…”

High-Pressure Crystal Structure, Lattice Vibrations, and Band Structure of BiSbO4

“…However these differences are similar to those observed between experiments and calculations in other oxides. 17 The information obtained from our calculations is not only useful to interpret the present experiment, but also future experiments in which the frequency region would be extended below 400 cm -1 .…”

“…On top of that, ab initio simulations have shown to be an accurate method to study the physical and chemical properties of compounds related to BiSbO 4 . 16,17 Usually these calculations provide an accurate description of such oxides as well as a better understanding of the experimental results and accurate predictions. Based upon the facts described above we consider timely to perform and experimental and theoretical study of BiSbO 4 to deepen the knowledge on its properties and its behavior under compression.…”

High‐pressure phase transitions and properties of MTO4 compounds with the monazite‐type structure

“…Typical values of the bulk modulus of monazites are summarized in Table . In this table the results obtained from phosphates , vanadates , chromates , selenates , and sulphates are shown. Notice that chromates and selenates appear to be the most compressible members of the monazite family.…”

“…Raman and infrared (IR) spectroscopic investigations have been performed on lanthanide orthophosphates and in other monazites . Under compression only the behavior of the Raman‐active modes has been experimentally studied.…”