High temperature neutron diffraction studies of PrInO3 and the measures of perovskite structure distortion

Experimental and Theoretical Study of the Electronic Structures of Lanthanide Indium Perovskites LnInO3

Ternary lanthanide indium oxides LnInO₃ (Ln = La, Pr, Nd, Sm) were synthesized by high-temperature solid-state reaction and characterized by X-ray powder diffraction. Rietveld refinement of the powder patterns showed the LnInO₃ materials to be orthorhombic perovskites belonging to the space group Pnma, based on almost-regular InO₆ octahedra and highly distorted LnO₁₂ polyhedra. Experimental structural data were compared with results from density functional theory (DFT) calculations employing a hybrid Hamiltonian. Valence region X-ray photoelectron and K-shell X-ray emission and absorption spectra of the LnInO₃ compounds were simulated with the aid of the DFT calculations. Photoionization of lanthanide 4f orbitals gives rise to a complex final-state multiplet structure in the valence region for the 4f ⁿ compounds PrInO₃, NdInO₃, and SmInO₃, and the overall photoemission spectral profiles were shown to be a superposition of final-state 4f ^n-1 terms onto the cross-section weighted partial densities of states from the other orbitals. The occupied 4f states are stabilized in moving across the series Pr-Nd-Sm. Band gaps were measured using diffuse reflectance spectroscopy. These results demonstrated that the band gap of LaInO₃ is 4.32 eV, in agreement with DFT calculations. This is significantly larger than a band gap of 2.2 eV first proposed in 1967 and based on the idea that In 4d states lie above the top of the O 2p valence band. However, both DFT and X-ray spectroscopy show that In 4d is a shallow core level located well below the bottom of the valence band. Band gaps greater than 4 eV were observed for NdInO₃ and SmInO₃, but a lower gap of 3.6 eV for PrInO₃ was shown to arise from the occupied Pr 4f states lying above the main O 2p valence band.

Magnetism and hybrid improper ferroelectricity in LaMO3/YMO3 superlattices

Using first-principles calculations, we investigate the structural, electronic, and magnetic properties of perovskite LaMO3/YMO3 superlattices (M = Cr, Mn, Co and Ni). It is found that ferroelectricity can emerge in LaMO3/YMO3 superlattices (M = Cr, Mn, Co), allowing them to be promising multiferroic candidates, while no ferroelectricity is found in the LaNiO3/YNiO3 superlattice. The electronic structure calculations indicate that the LaCrO3/YCrO3, LaMnO3/YMnO3, and LaCoO3/YCoO3 superlattices are insulators, and their magnetic ground states exhibit G-type antiferromagnetic (AFM), A-type AFM, and G-type AFM order, respectively, while the LaNiO3/YNiO3 superlattice is however a half-metallic ferromagnet. The electronic structure and magnetic ground state are discussed, based on the projected density of states data and Heisenberg model, respectively, and the magnetic phase transition temperature is evaluated based on mean-field theory. In the meantime, the spontaneous ferroelectric polarization of the LaMO3/YMO3 superlattices (M = Cr, Mn, Co) is determined respectively using the Born effective charge model and Berry phase method, and their hybrid improper ferroelectric character is predicted, with the net polarization mainly from the different displacements of the LaO layers and YO layers along the b-axis. It is suggested that alternative multiferroic materials can be obtained by properly designing superlattices that consist of two non-polar magnetic materials but exhibit tunable magnetic ground states and transition temperature and hybrid improper ferroelectricity.

New insights into BaTi1–x Sn x O3 (0 ≤ x ≤ 0.20) phase diagram from neutron diffraction data

Neutron powder diffraction (NPD) was employed to further investigate the BaTi1−x Sn x O3 (BTS) system previously studied by X-ray diffraction. The room-temperature phase compositions and crystal structures of BTS samples with x = 0, 0.025, 0.05, 0.07, 0.10, 0.12, 0.15 and 0.20 were refined by the Rietveld method using NPD data. It is well known that barium titanate powder (x = 0) crystallizes in the tetragonal P4mm space group. The crystal structures of the samples with 0.025 ≤ x ≤ 0.07 were refined as mixtures of P4mm and Amm2 phases; those with x = 0.1 and 0.12 show the coexistence of rhombohedral R3m and cubic phases, while the samples with x = 0.15 and 0.20 crystallize in a single cubic Pm{\overline 3}m phase. Temperature-dependent NPD was used to characterize the BaTi0.95Sn0.05O3 sample at 273, 333 and 373 K, and it was found to form single-phase Amm2, P4mm and Pm{\overline 3}m structures at these respective temperatures. The NPD results are in agreement with data obtained by differential scanning calorimetry and dielectric permittivity measurements, which show a paraelectric–ferroelectric transition (associated with structural transition) from Pm{\overline 3}m to P4mm at about 353 K followed by a P4mm to Amm2 phase transition at about 303 K.

Electronic, optical and thermoelectric properties of PrMO3 (M = Al, Ga, In) from first-principles calculations

Though PrInO₃ is structurally anisotropic it is found to be optically isotropic which is the basic requirement for ceramic scintillators.