First-principles study of the orthorhombicCdTiO3perovskite

Abstract

In this work we perform an ab-initio study of CdTiO3 perovskite in its orthorhombic phase using FLAPW method. Our calculations help to decide between the different cristallographic structures proposed for this perovskite from X-Ray measurements. We compute the electric field gradient tensor (EFG) at Cd site and obtain excellent agreement with available experimental information from a perturbed angular correlation (PAC) experiment. We study EFG under an isotropic change of volume and show that in this case the widely used "point charge model approximation" to determine EFG works quite well.PACS numbers: 77.84. Dy, 71.15.Nc Perovskites materials of the form ABO 3 have been intensively studied last years. In addition to the interest that present the technological aplications of ferroelectric materials, perovskites are known to undergo several phase transitions as a function of temperature that make them appealing for theoreticians and experimentalists. The transitions between different structures usually involve very little atomic displacements and are difficult to characterize, requiring some times several experimental techniques. First principles calculations have been playing an increasing role in the understanding of the these systems 1 . These studies not only try to give theoretical explanations to the observed phenomena, but usually give valuable information that help in the interpretation of the experimental results.CdTiO 3 perovskite has been the object of several studies using X-Ray diffraction 2,3,4 , dielectric properties methods 4,5,6 , infrared spectroscopy 7 and PAC technique 8,9 and there have been some difficulties and controversies in determining the symmetry group of it's orthorhombic structure at room temperature. In 1957 H.F.Kay and J.L.Miles established through XRay measurements that CdTiO 3 perovskite should have orthorhombic symmetry and non-centrosymmetric P bn2 1 10 space group with a large displacement of Ti atoms along the b axis 3 . However, the measurement of the electrical properties of CdTiO 3 single crystals couldn't resolve whether they were ferroelectric or not, then, the displacement of Ti atoms from their symmetric positions couldn't be confirmed 6 . In Reference 8 Baumvol and coworkers study the EFG at Cd in CdTiO 3 through PAC measurements. Using the Point Charge Model approximation (PCM) to estimate theoretically the EFG they infer that the structure of the perovskite couldn't be the one corresponding to P bn2 1 space group but should be another one with centrosymmetric space group, also considered and discarded by Kay&Miles. 4 performed new XRay studies on CdTiO 3 . They obtained that two orthorhombic structures with very similar coordinates and different space groups (centrosymmetric P bnm and noncentrosymmetric P bn2 1 ) fitted equally well the X-ray data. Analizing some structural and physical properties, they finally argued that it was more suitable to assign the space group P bnm to CdTiO 3 .The main objective of the present work is to clarify this picture, studying ...

Ab initio calculations of phonon spectra in ATiO3 perovskite crystals (A = Ca, Sr, Ba, Ra, Cd, Zn, Mg, Ge, Sn, Pb)

“…This conclusion was also made in Ref. 36. According to our data, the energy of this phase is lower than that of the cubic phase by 1.28 eV (Table III), which is somewhat higher than the value obtained in Ref.…”

“…According to our data, the energy of this phase is lower than that of the cubic phase by 1.28 eV (Table III), which is somewhat higher than the value obtained in Ref. 36 Although the ferroelectric instability associated with the Γ 15 mode is not very important in the cubic CdTiO 3 , it is known that this instability exists in the P bnm phase and results in the ferroelectric phase transition at 80 K. The first-principles calculations of the properties of the orthorhombic P bnm phase 36 did not found a stable ferroelectric distortion in it. In contrast, our calculations of the phonon spectrum at the Γ point of the orthorhombic CdTiO 3 reveal two unstable B 1u and B 2u modes resulting in the formation of polar P b2 1 m and P bn2 1 phases, respectively.…”

First-principles calculations of lattice dynamics in CdTiO3and CaTiO3: Phase stability and ferroelectricity

“…The energy difference between the orthorhombic and cubic phases of CdTiO 3 is calculated to be E = −989 meV/f.u., in good agreement with previous calculations. 21 Experimentally, cubic CdTiO 3 has not been observed, consistent with the transition temperature being calculated to exceed the melting point. 21 In the case of CaTiO 3 , which exhibits P m3m symmetry above 1,635 K, the energy difference between the orthorhombic phase and cubic phase is E = −410 meV/f.u.…”

“…21 Experimentally, cubic CdTiO 3 has not been observed, consistent with the transition temperature being calculated to exceed the melting point. 21 In the case of CaTiO 3 , which exhibits P m3m symmetry above 1,635 K, the energy difference between the orthorhombic phase and cubic phase is E = −410 meV/f.u. In contrast, for CdTiO 3 , the energy differences between the experimentally reported paraelectric phase (Pnma) and proposed ferroelectric phases are E = −0.23 meV/f.u.…”

“…20 The first theoretical study using density functional theory (DFT) predicted the Pna2 1 structure to be unstable compared to the Pnma structure, based on the full-potential linearized augmented plane wave (FLAPW) method. 21 Lebedev 22 later calculated phonon modes at the point of the paraelectric phase, concluding that the Pna2 1 phase was the stable form at 0 K. Halilov et al 23 also used DFT to study changes in the stability of the cubic phase as Cd is replaced by Pb. In this study, to shed further light on the phase stability of the various structures, we performed a series of first-principles calculations of the different phases of CdTiO 3 , including a detailed examination of their vibrational modes for a range of wave vectors.…”