Open Access

Show Abstract ▼

Abstract: The origin of the large electrostrain in BiFeO3-BaTiO3 (BF-BT) ceramics is controversial and has been attributed to either a field-induced transition to a long-range ferroelectric (FE) state or to multi-symmetry, polar nanoregions within a pseudocubic matrix whose vectors approximately align with the direction of the applied field. The (1-x)BiFeO3-xSrTiO3 (BF-xST) solid solution is structurally and microstructurally similar to BF-BT and provides a further case study to assess the origin of electrostrain. In BF-xST, electrostrain is optimised at x = 0.4 (0.15%) which zero field, room temperature full-pattern X-ray diffraction (XRD) Rietveld refinement and scanning/transmission electron microscopy suggest is composed of 15% rhombohedral (R) cores, surrounded by 85% pseudocubic (PC) shells. In-situ poling synchrotron XRD revealed that all peaks remained singlet and exhibited no change in full width half maximum up to 100 kV cm-1, confirming the absence of long-range FE order and the retention of short-range polar order, despite the large applied field. Strain anisotropy (calculated from individual peaks) of ε220 > ε111 > ε200 and the associated strain orientation distribution however, indicate the existence of local orthorhombic (O), rhombohedral (R) and tetragonal (T) symmetries. The data therefore imply the existence under poling of multi-symmetry polar nanoregions in BF-0.4ST rather than a long FE phase, supporting the model described by Wang and co-workers (2019) for BF-BT compositions.

Show Abstract ▼

Abstract: Vanadium is reported to undergo a pressure-induced bcc-rhombohedral phase transition at 30–70 GPa, with a transition pressure that is sensitive to the hydrostaticity of the sample environment. However, the experimental evidence for the structure of the high-pressure phase being rhombohedral is surprisingly weak. We have restudied vanadium under pressure to 154 GPa using both polycrystalline and single-crystal samples, and a variety of different pressure transmitting media (PTM). We find that only when using single-crystal samples does one observe a rhombohedral high-pressure phase; the high-pressure diffraction profiles from the polycrystalline samples do not fit a rhombohedral lattice, irrespective of the PTM used. The single-crystal samples reveal two rhombohedral phases, with a continuous transition between them, and distortions from cubic symmetry are much smaller than previously calculated.

Show Abstract ▼

Abstract: Angle-dispersive x-ray powder diffraction experiments have been performed on neodymium metal to a pressure of 302 GPa. Up to 70 GPa we observe the h P 4 → c F 4 → h R 24 → o I 16 → h P 3 transition sequence reported previously. At 71(2) GPa we find a transition to a phase which has an orthorhombic structure ( o F 8 ) with eight atoms in the unit cell, space group F d d d . This structure is the same as that recently observed in samarium above 93 GPa, and is isostructural with high-pressure structures found in the actinides Am, Cf, and Cm. We see a further phase transition at 98(1) GPa to a phase with the orthorhombic α -U ( o C 4 ) structure, which remains stable up to 302 GPa, the highest pressure reached in this study. Electronic structure calculations find the same structural sequence, with calculated transition pressures of 66 and 88 GPa, respectively, for the h P 3 → F 8 and o F 8 → o C 4 transitions. The calculations further predict that o C 4 -Nd loses its magnetism at 100 GPa, in agreement with previous experimental results, and it is the accompanying decrease in enthalpy and volume that results in the transition to this phase. Comparison calculations on the o F 8 and o C 4 phases of Sm show that they both retain their magnetism to at least 240 GPa, with the result that o C 4 -Sm is calculated to have the lowest enthalpy over a narrow pressure region near 200 GPa at 0 K.

Show Abstract ▼

Abstract: Cadmium arsenide ( Cd 3 As 2 ) hosts massless Dirac electrons in its ambient-condition tetragonal phase. We report x-ray diffraction and electrical resistivity measurements of Cd 3 As 2 upon cycling pressure beyond the critical pressure of the tetragonal phase and back to ambient conditions. We find that, at room temperature, the transition between the low- and high-pressure phases results in large microstrain and reduced crystallite size, both on rising and falling pressure. This leads to nonreversible electronic properties, including self-doping associated with defects and a reduction of the electron mobility by an order of magnitude due to increased scattering. This paper indicates that the structural transformation is sluggish and shows a sizable hysteresis of over 1 GPa. Therefore, we conclude that the transition is first-order reconstructive, with chemical bonds being broken and rearranged in the high-pressure phase. Using the diffraction measurements, we demonstrate that annealing at ∼ 200 ∘ C greatly improves the crystallinity of the high-pressure phase. We show that its Bragg peaks can be indexed as a primitive orthorhombic lattice with a HP ≈ 8.68 Å , b HP ≈ 17.15 Å , and c HP ≈ 18.58 Å . The diffraction study indicates that, during the structural transformation, a new phase with another primitive orthorhombic structure may also be stabilized by deviatoric stress, providing an additional venue for tuning the unconventional electronic states in Cd 3 As 2 .

Show Abstract ▼

Abstract: Local distortions in perovskite-like A-site-deficient (Sr,La)TiO3 solid solutions have been determined by refining large-scale atomic configurations against neutron/X-ray total-scattering and extended-X-ray-absorption-fine-structure data. Structural relaxations in this system are driven by the competing bonding requirements of Sr, La, and the undercoordinated oxygen atoms that surround vacant A-sites, which form upon substitution of La for Sr. La cations exhibit significant, disordered off-center displacements within their oversized oxygen cages required by the larger Sr cations. The resulting split-site probability density distributions of La vary with the Sr/La ratio and the state of the A-site ordering, which together modify the structure's ability to relieve the tensile bond strain around La through octahedral rotation and displacements of oxygens surrounding the vacancies. The displacive disorder of La can provide a hitherto overlooked mechanism for reducing the thermal conductivity, which is relevant to thermoelectric properties of this system. A comparison of the local structural behaviors in (Sr,La)TiO3 and the previously studied (Na,Bi)NbO3 solid solutions permits generalizations about A-site deficient perovskites. We find that A-site vacancies provide the nearest-neighbor oxygens with a degree of freedom to mediate the strain in the system, and their effects on local structural relaxations are determined by cation chemistry and stoichiometry.