Structural properties of PbVO3 perovskites under hydrostatic pressure conditions up to 10.6 GPa

Intermetallic Charge Transfer in V-Substituted PbCrO3

PbCrO₃ features an unusual charge distribution Pb_0.5²⁺Pb_0.5⁴⁺Cr³⁺O₃ with Pb charge disproportionation at ambient pressure. A charge transfer between Pb and Cr is induced by the application of pressure resulting in Pb²⁺Cr⁴⁺O₃ charge distribution and a large volume collapse. Here, structural and charge distribution changes in PbCr_1-xV_xO₃ are investigated. Despite a cubic crystal structure in 0 ≤ x ≤ 0.60, discontinuous reduction in the unit cell volume was observed between x = 0.35 and 0.40. Hard X-ray photoemission spectroscopy confirmed the change in Pb charge state from the coexisting Pb²⁺ and Pb⁴⁺ at x = 0.35 to single Pb²⁺ at x = 0.40. This indicates that V substitution stabilizes the high pressure cubic Pb²⁺Cr⁴⁺O₃-type phase. With further increase in the V substitution, the PbVO₃-type polar tetragonal phase appeared at x = 0.80.

Giant Magnetoelectric Coupling in Multiferroic PbTi1-x V x O3 from Density Functional Calculations

Giant magnetoelectric coupling is a very rare phenomenon that has gained much attention in the past few decades due to fundamental interest as well as practical applications. Here, we have successfully achieved giant magnetoelectric coupling in PbTi1-x V x O3 (x = 0-1) using a series of generalized gradient-corrected GGA (generalized gradient approximation), including on-site Coulomb repulsion (U)-corrected spin-polarized calculations based on accurate density functional theory. Our total energy calculations show that PbTi1-x V x O3 stabilizes in C-type antiferromagnetic ground state for x > 0.123. With the substitution of V into PbTiO3, the tetragonal distortion is highly enhanced accompanied by a linear increase in polarization. In addition, our band structure analysis shows that for lower x values, the tendency to form two-dimensional magnetism of PbTi1-x V x O3 decreases. The orbital magnetic polarization was calculated with self-consistent field method by including orbital polarization correction in the calculation as well as from the computed X-ray magnetic dichroism spectra. A nonmagnetic metallic ground state is observed for the paraelectric phase for V concentration (x) = 1 competing with a volume change of 10% showing a large magnetovolume effect. Our orbital-projected density of states as well as orbital ordering analysis suggest that the orbital ordering plays a major role in the magnetic-to-nonmagnetic transition when going from ferroelectric to paraelectric phase. The calculated magnetic anisotropic energy shows that the direction [110] is the easy axis of magnetization for x = 1 composition. The partial polarization analysis shows that the Ti/V-O hybridization majorly contributes to the total electrical polarization. The present study adds a new series of compounds to the magnetoelectric family with rarely existing giant coupling between electric- and magnetic-order parameters. These results show that such kind of materials can be used for novel practical applications where one can change the magnetic properties drastically (magnetic to nonmagnetic, as shown here) with external electric field and vice versa.

Stability of Polar Structure in Filling-Controlled Giant Tetragonal Perovskite Oxide PbVO3

The crystal structure and stability of a giant tetragonal phase in electron-doped Pb_{1- x}Bi _xVO₃ ( x = 0.1, 0.2, and 0.3) and hole-doped Pb_{1- x}Na _xVO₃ ( x = 0.1, 0.2, and 0.3) were studied. Electron doping effectively destabilized the tetragonal structure. The c/ a ratio, spontaneous polarization, and tetragonal-to-cubic phase transition pressure systematically decreased with increasing Bi³⁺ substitution. In contrast, hole doping hardly affected the tetragonal distortion and structural stability. We showed that electron doping is an effective way to control the stability of the tetragonal phase of PbVO₃ with a 3d¹ electronic configuration.

Colossal Negative Thermal Expansion in Electron-Doped PbVO3 Perovskites

Colossal negative thermal expansion (NTE) with a volume contraction of about 8 %, the largest value reported so far for NTE materials, was observed in an electron-doped giant tetragonal perovskite compound Pb_1-x Bi_x VO₃ (x=0.2 and 0.3). A polar tetragonal (P4mm) to non-polar cubic structural transition took place upon heating. The coefficient of thermal expansion (CTE) and the working temperature could be tuned by changing the Bi content, and La substitution decreased the transition temperature to room temperature. Pb_0.76 La_0.04 Bi_0.20 VO₃ exhibited a unit cell volume contraction of 6.7 % from 200 K to 420 K. Interestingly, further gigantic NTE of about 8.5 % was observed in a dilametric measurement of a Pb_0.76 La_0.04 Bi_0.20 VO₃ polycrystalline sample. The pronounced NTE in the sintered body should be attributed to an anisotropic lattice parameter change.

Structural Transformation in Inverse-Perovskite REPt3B (RE = Sm and Gd-Tm) Associated with Large Volume Reduction

In this work, we report the structural phase transformation of tetragonal inverse-perovskite REPt₃B (RE = Sm, and Gd-Tm) compounds to cubic perovskite structure, with a large volume reduction of about 9% (reduction of the c axis, ∼17%; increase in the a axis, ∼5%). The structural stability of the cubic phase, however, could only be maintained by lowering the lattice parameter of the off-stoichiometric REPt₃B_x (x < 1), formed in the process of annealing. The combined effect of phase transformation and stoichiometric defects is argued to be responsible for the observed volume collapse. Unexpectedly, the application of a large hydrostatic pressure of ∼20 GPa does not have any significant effect on the crystal structure. Neutron diffraction studies and heat capacity measurements unambiguously confirm different magnetic transition temperatures in the tetragonal and cubic phases. The different physical properties of these two phases demonstrate the interrelationship between the crystal chemistry and the physics of the system. The synthetic route to cubic REPt₃B_x identified in this work may be utilized to prepare new ternary rare-earth intermetallics in a cubic perovksite form, which was previously found to facilitate unconventional superconductivity.