First-principles calculations and Raman scattering evidence for local symmetry lowering in rhombohedral ilmenite: temperature- and pressure-dependent studies

Abstract

ATiO 3-type materials may exist in two different crystalline forms: the perovskite and ilmenite. While many papers have devoted their attention to evaluating the structural properties of the perovskite phase, the structural stability of the ilmenite one still remains unsolved. Here, we present our results based on the lattice dynamics and first-principles calculations (density functional theory) of the CdTiO 3 ilmenite phase, which are confronted with experimental data obtained through micro Raman spectroscopy that is a very good tool to probe the local crystal structure. Additional Raman bands, which are not foreseen from group-theory for the ilmenite rhombohedral structure, appeared in both low temperature (under vacuum condition) and high-pressure (at room temperature) spectra. The behavior can be explained by considering the local loss of inversion symmetry operation which reduces the overall space group from R3 (C 2 3i) to R3 (C 4 3). Our results can also be extended to other ilmenite-type compositions.

Lattice dynamics and spontaneous magnetodielectric effect in ilmenite CoTiO$_3$

“…Figure 6 shows the ionic displacement patterns for 5 and 5 Raman active modes in ilmenite CoTiO 3 which are similar to those for isostructural CdTiO 3 [33] and MgTiO 3 [29]. According to symmetry analysis, displacements of Co and Ti ions occur in the plane for all modes, while for modes these ions move along the axis.…”

“…But further intensive research of this promising material is inhibited by the lack of complete information about the phonon spectrum. It is worth noting that up to now, most experimental studies of the lattice dynamics of ATiO 3 materials with ilmenite structure were performed on the poly-and microcrystalline samples using unpolarized far infrared [20,21,22,23,24,25] and Raman [26,27,28,29,30,31,32,33] spectroscopy.…”

Optical phonon modes in 1:2 ordered trigonal Ba3MgNb2O9 perovskite: Synergy of both classical and quantum methods

“…In such a way, the first‐order vibrational spectra can be approached using the first‐neighbor approximation based on the local atomic structure that obeys the Hooke's law: the valence ( K i )‐, repulsive ( F i )‐, and angle ( H i )‐type force constants, that is, the SRFFM. This method has proven its effectiveness in describing the vibrational properties of several compounds, including rare earth orthovanadates AVO 4 , 38 alkaline earth molybdates AMoO 4 , 39 cubic pyrochlore, 40 CdTiO 3 ilmenite, 28 and bismuth oxyhalide as BiOBr 33 …”

“…37 The secular equation carries information on the chemical bonds by means of the potential energy matrix F, besides atomic data by the kinetic energy matrix G. In such a way, the first-order vibrational spectra can be approached using the firstneighbor approximation based on the local atomic structure that obeys the Hooke's law: the valence (K i )-, repulsive (F i )-, and angle (H i )-type force constants, that is, the SRFFM. This method has proven its effectiveness in describing the vibrational properties of several compounds, including rare earth orthovanadates AVO 4 , 38 alkaline earth molybdates AMoO 4 , 39 cubic pyrochlore, 40 CdTiO 3 ilmenite, 28 and bismuth oxyhalide as BiOBr. 33 Here, valence and repulsive forces (both stretching) accounting the chemical bonds in the F matrix are not enough to describe the transverse vibrations in the lattice, explaining the use of bending forces (angles).…”

“…Besides, to our knowledge until now, literature does not provide any work concerning the force field studies on the optical modes at Γ‐point in 1:2 ordered trigonal Ba 3 MgNb 2 O 9 perovskites. Indeed, we already described the vibrational spectra in some compounds including NdAlO 3 , 27 CdTiO 3 , 28,29 Ba 2 MgWO 6 , 30 Ba 3 CaNb 2 O 9 , 31 Fe 2 TiO 5 , 32 and BiOBr 33 . We aim to combine both classical and quantum methodologies to get further insights into the optical phonon description in barium magnesium niobate, which is a promising material for microwave applications.…”