PbMn(IV)TeO₆: A New Noncentrosymmetric Layered Honeycomb Magnetic Oxide

PbMnTeO₆, a new noncentrosymmetric layered magnetic oxide was synthesized and characterized. The crystal structure is hexagonal, with space group <i>P</i>6̅2<i>m</i> (No. 189), and consists of edge-sharing (Mn⁴⁺/Te⁶⁺)­O₆ trigonal prisms that form honeycomb-like two-dimensional layers with Pb²⁺ ions between the layers. The structural difference between PbMnTeO₆, with disordered/trigonal prisms of Mn⁴⁺/Te⁶⁺, versus the similar chiral SrGeTeO₆ (space group <i>P</i>312), with long-range order of Ge⁴⁺ and Te⁶⁺ in octahedral coordination, is attributed to a difference in the electronic effects of Ge⁴⁺ and Mn⁴⁺. Temperature-dependent second harmonic generation by PbMnTeO₆ confirmed the noncentrosymmetric character between 12 and 873 K. Magnetic measurements indicated antiferromagnetic order at <i>T</i>_N ≈ 20 K and a frustration parameter (|θ|/<i>T</i>_N) of ∼2.16

A(II)GeTeO₆ (A = Mn, Cd, Pb): Non-Centrosymmetric Layered Tellurates with PbSb₂O₆‑Related Structure

A­(II)­GeTeO₆ (A = Mn, Cd, Pb), new non-centrosymmetric (NCS) honeycomb-layered tellurates, were synthesized and characterized. A­(II)­GeTeO₆ (A = Mn, Cd, Pb) crystallize in trigonal space group <i>P</i>312 (No. 149) of edge-sharing Ge⁴⁺O₆ and Te⁶⁺O₆ octahedra, which form honeycomb-like-layers in the <i>ab</i>-plane with A­(II) (A = Mn, Cd, Pb) cations located between the layers. Their crystal structures are PbSb₂O₆-related, and the ordering of Ge⁴⁺ and Te⁶⁺ in octahedral environment breaks the inversion symmetry of the parent PbSb₂O₆ structure. The size of A­(II) cation in six coordination is an important factor to stabilize PbSb₂O₆-based structure. Temperature-dependent optical second harmonic generation measurements on A­(II)­GeTeO₆ confirmed non-centrosymmetric character in the entire scanned temperature range (0 to 600 °C). The materials exhibit a powder SHG efficiency of ∼0.37 and ∼0.21 times of KH₂PO₄ for PbGeTeO₆ and CdGeTeO₆, respectively. Magnetic measurements of MnGeTeO₆ indicate anti-ferromagnetic order at <i>T</i>_N ≈ 9.4 K with Weiss temperature of −22.47 K

PbMn(IV)TeO₆: A New Noncentrosymmetric Layered Honeycomb Magnetic Oxide

PbMnTeO₆, a new noncentrosymmetric layered magnetic oxide was synthesized and characterized. The crystal structure is hexagonal, with space group <i>P</i>6̅2<i>m</i> (No. 189), and consists of edge-sharing (Mn⁴⁺/Te⁶⁺)­O₆ trigonal prisms that form honeycomb-like two-dimensional layers with Pb²⁺ ions between the layers. The structural difference between PbMnTeO₆, with disordered/trigonal prisms of Mn⁴⁺/Te⁶⁺, versus the similar chiral SrGeTeO₆ (space group <i>P</i>312), with long-range order of Ge⁴⁺ and Te⁶⁺ in octahedral coordination, is attributed to a difference in the electronic effects of Ge⁴⁺ and Mn⁴⁺. Temperature-dependent second harmonic generation by PbMnTeO₆ confirmed the noncentrosymmetric character between 12 and 873 K. Magnetic measurements indicated antiferromagnetic order at <i>T</i>_N ≈ 20 K and a frustration parameter (|θ|/<i>T</i>_N) of ∼2.16

Continuously Tuning Epitaxial Strains by Thermal Mismatch

Strain engineering of thin films is a conventionally employed approach to enhance material properties and to energetically prefer ground states that would otherwise not be attainable. Controlling strain states in perovskite oxide thin films is usually accomplished through coherent epitaxy by using lattice-mismatched substrates with similar crystal structures. However, the limited choice of suitable oxide substrates makes certain strain states experimentally inaccessible and a continuous tuning impossible. Here, we report a strategy to continuously tune epitaxial strains in perovskite films grown on Si(001) by utilizing the large difference of thermal expansion coefficients between the film and the substrate. By establishing an adsorption-controlled growth window for SrTiO₃ thin films on Si using hybrid molecular beam epitaxy, the magnitude of strain can be solely attributed to thermal expansion mismatch, which only depends on the difference between growth and room temperature. Second-harmonic generation measurements revealed that structure properties of SrTiO₃ films could be tuned by this method using films with different strain states. Our work provides a strategy to generate continuous strain states in oxide/semiconductor pseudomorphic buffer structures that could help achieve desired material functionalities

Polar Oxides without Inversion Symmetry through Vacancy and Chemical Order

One synthetic modality for materials discovery proceeds by forming mixtures of two or more compounds. In transition metal oxides (TMOs), chemical substitution often obeys Vegard’s principle, and the resulting structure and properties of the derived phase follow from its components. A change in the assembly of the components into a digital nanostructure, however, can stabilize new polymorphs and properties not observed in the constituents. Here we formulate and demonstrate a crystal-chemistry design approach for realizing digital TMOs without inversion symmetry by combining two centrosymmetric compounds, utilizing periodic anion-vacancy order to generate multiple polyhedra that together with cation order produce a polar structure. We next apply this strategy to two brownmillerite-structured TMOs known to display centrosymmetric crystal structures in their bulk, Ca₂Fe₂O₅ and Sr₂Fe₂O₅. We then realize epitaxial (SrFeO_2.5)₁/(CaFeO_2.5)₁ thin film superlattices possessing both anion-vacancy order and Sr and Ca chemical order at the subnanometer scale, confirmed through synchrotron-based diffraction and aberration corrected electron microscopy. Through a detailed symmetry analysis and density functional theory calculations, we show that <i>A</i>-site cation ordering lifts inversion symmetry in the superlattice and produces a polar compound. Our results demonstrate how control of anion and cation order at the nanoscale can be utilized to produce acentric structures markedly different than their constituents and open a path toward novel structure-based property design

Interfacial Octahedral Rotation Mismatch Control of the Symmetry and Properties of SrRuO₃

Epitaxial strain can be used to tune the properties of complex oxides with perovskite structure. Beyond just lattice mismatch, the use of octahedral rotation mismatch at heterointerfaces could also provide an effective route to manipulate material properties. Here, we examine the evolution of the structural motif (i.e., lattice parameters, symmetry, and octahedral rotations) of SrRuO₃ films grown on substrates engineered to have the same lattice parameters, but different octahedral rotations. SrRuO₃ films grown on SrTiO₃ (001) (no octahedral rotations) and GdScO₃-buffered SrTiO₃ (001) (with octahedral rotations) substrates are found to exhibit monoclinic and tetragonal symmetry, respectively. Electrical transport and magnetic measurements reveal that the tetragonal films exhibit higher resistivity, lower magnetic Curie temperatures, and more isotropic magnetism as compared to those with monoclinic structure. Synchrotron-based quantification of the octahedral rotation network reveals that the tilting pattern in both film variants is the same (albeit with slightly different magnitudes of in-plane rotation angles). The abnormal rotation pattern observed in tetragonal SrRuO₃ indicates a possible decoupling between the internal octahedral rotation and lattice symmetry, which could provide new opportunities to engineer thin-film structure and properties

Single-Crystal Silicon Optical Fiber by Direct Laser Crystallization

Semiconductor core optical fibers with a silica cladding are of great interest in nonlinear photonics and optoelectronics applications. Laser crystallization has been recently demonstrated for crystallizing amorphous silicon fibers into crystalline form. Here we explore the underlying mechanism by which long single-crystal silicon fibers, which are novel platforms for silicon photonics, can be achieved by this process. Using finite element modeling, we construct a laser processing diagram that reveals a parameter space within which single crystals can be grown. Utilizing this diagram, we illustrate the creation of <i>single-crystal</i> silicon core fibers by laser crystallizing amorphous silicon deposited inside silica capillary fibers by high-pressure chemical vapor deposition. The single-crystal fibers, up to 5.1 mm long, have a very well-defined core/cladding interface and a chemically pure silicon core that leads to very low optical losses down to ∼0.47–1 dB/cm at the standard telecommunication wavelength (1550 nm). It also exhibits a photosensitivity that is comparable to bulk silicon. Creating such laser processing diagrams can provide a general framework for developing single-crystal fibers in other materials of technological importance

Fast Magnetic Domain-Wall Motion in a Ring-Shaped Nanowire Driven by a Voltage

Magnetic domain-wall motion driven by a voltage dissipates much less heat than by a current, but none of the existing reports have achieved speeds exceeding 100 m/s. Here phase-field and finite-element simulations were combined to study the dynamics of strain-mediated voltage-driven magnetic domain-wall motion in curved nanowires. Using a ring-shaped, rough-edged magnetic nanowire on top of a piezoelectric disk, we demonstrate a fast voltage-driven magnetic domain-wall motion with average velocity up to 550 m/s, which is comparable to current-driven wall velocity. An analytical theory is derived to describe the strain dependence of average magnetic domain-wall velocity. Moreover, one 180° domain-wall cycle around the ring dissipates an ultrasmall amount of heat, as small as 0.2 fJ, approximately 3 orders of magnitude smaller than those in current-driven cases. These findings suggest a new route toward developing high-speed, low-power-dissipation domain-wall spintronics

Competing Structural Instabilities in the Ruddlesden–Popper Derivatives HRTiO₄ (R = Rare Earths): Oxygen Octahedral Rotations Inducing Noncentrosymmetricity and Layer Sliding Retaining Centrosymmetricity

We report the observation of noncentrosymmetricity in the family of HRTiO₄ (R = Eu, Gd, Dy) layered oxides possessing a Ruddlesden–Popper derivative structure, by second harmonic generation and synchrotron X-ray diffraction with the support of density functional theory calculations. These oxides were previously thought to possess inversion symmetry. Here, inversion symmetry is lifted by rotations of the oxygen-coordinated octahedra, a mechanism that is not active in simple perovskites. We observe a competition between rotations of the oxygen octahedra and sliding of a combined unit of perovskite–rocksalt–perovskite blocks at the proton layers. For the smaller rare earth ions, R = Eu, Gd, and Dy, which favor the octahedral rotations, noncentrosymmetricity is present but the sliding is absent. For the larger rare earth ions, R = Nd and Sm, the octahedral rotations are absent, but the sliding at the proton layers is present to optimize the length and direction of hydrogen bonding in the crystal structure. The study reveals a new mechanism for inducing noncentrosymmetricity in layered oxides, and chemical–structural effects related to rare earth ion size and hydrogen bonding that can turn this mechanism on and off. We construct a phase diagram of temperature versus rare earth ionic radius for the HRTiO₄ family