ABSTRACT
Hybrid improper ferroelectricity, which utilizes nonpolar but ubiquitous rotational/tilting distortions to create polarization, offers an attractive route to the discovery of new ferroelectric and multiferroic materials because its activity derives from geometric rather than electronic origins. Design approaches blending group theory and first principles can be utilized to explore the crystal symmetries of ferroelectric ground states, but in general, they do not make accurate predictions for some important parameters of ferroelectrics, such as Curie temperature ( TC). Here, we establish a predictive and quantitative relationship between TC and the Goldschmidt tolerance factor, t, by employing n = 2 Ruddlesden-Popper (RP) A3B2O7 as a prototypical example of hybrid improper ferroelectrics. The focus is placed on an RP system, (Sr1- xCa x)3Sn2O7 ( x = 0, 0.1, and 0.2), which allows for the investigation of the purely geometric (ionic size) effect on ferroelectric transitions, due to the absence of the second-order Jahn-Teller active (d0 and 6s2) cations that often lead to ferroelectric distortions through electronic mechanisms. We observe a ferroelectric-to-paraelectric transition with TC = 410 K for Sr3Sn2O7. We also find that the TC increases linearly up to 800 K upon increasing the Ca2+ content, i.e., upon decreasing the value of t. Remarkably, this linear relationship is applicable to the suite of all known A3B2O7 hybrid improper ferroelectrics, indicating that the TC correlates with the simple crystal chemistry descriptor, t, based on the ionic size mismatch. This study provides a predictive guideline for estimating the TC of a given material, which would complement the convergent group-theoretical and first-principles design approach.
ABSTRACT
Soft chemical reactions such as ion-exchange and acid-base reactions have been extensively investigated to synthesize novel metastable layered inorganic solids, to exfoliate them into individual nanosheets, and to re-assemble them as thin films and nanocomposite materials. These reactions proceed at relatively low temperature and enable the synthesis of a rich variety of structures by stepwise reactions. In recent years, the toolbox of soft chemical reactions has been utilized to rationally design and tailor the properties of functional layered transition metal oxides. Layer-by-layer assembly and intercalation chemistry have provided insight into covalent interactions that stabilize oxide-supported nanoparticle catalysts. In addition, topochemical reactions have been utilized to tune the compositions of layered perovskite oxides in order to break inversion symmetry, resulting in piezoelectric and ferroelectric properties. This review focuses on the use of soft chemical approaches to design functional layered transition metal oxides with tunable properties. Soft chemical reactions enable the design of functional materials for diverse applications that include artificial photosynthesis, catalysis, energy storage, fuel cells, optical sensors, ferroics, and high-k dielectrics.
ABSTRACT
Double corundum-related polar magnets are promising materials for multiferroic and magnetoelectric applications in spintronics. However, their design and synthesis is a challenge, and magnetoelectric coupling has only been observed in Ni3TeO6 among the known double corundum compounds to date. Here we address the high-pressure synthesis of a new polar and antiferromagnetic corundum derivative Mn2MnWO6, which adopts the Ni3TeO6-type structure with low temperature first-order field-induced metamagnetic phase transitions (T N = 58 K) and high spontaneous polarization (~ 63.3 µC·cm-2). The magnetostriction-polarization coupling in Mn2MnWO6 is evidenced by second harmonic generation effect, and corroborated by magnetic-field-dependent pyroresponse behavior, which together with the magnetic-field-dependent polarization and dielectric measurements, qualitatively indicate magnetoelectric coupling. Piezoresponse force microscopy imaging and spectroscopy studies on Mn2MnWO6 show switchable polarization, which motivates further exploration on magnetoelectric effect in single crystal/thin film specimens.
ABSTRACT
PbMnTeO6, a new noncentrosymmetric layered magnetic oxide was synthesized and characterized. The crystal structure is hexagonal, with space group P6Ì 2m (No. 189), and consists of edge-sharing (Mn(4+)/Te(6+))O6 trigonal prisms that form honeycomb-like two-dimensional layers with Pb(2+) ions between the layers. The structural difference between PbMnTeO6, with disordered/trigonal prisms of Mn(4+)/Te(6+), versus the similar chiral SrGeTeO6 (space group P312), with long-range order of Ge(4+) and Te(6+) in octahedral coordination, is attributed to a difference in the electronic effects of Ge(4+) and Mn(4+). Temperature-dependent second harmonic generation by PbMnTeO6 confirmed the noncentrosymmetric character between 12 and 873 K. Magnetic measurements indicated antiferromagnetic order at T(N) ≈ 20 K and a frustration parameter (|θ|/T(N)) of â¼2.16.
ABSTRACT
Rotations of oxygen octahedra are ubiquitous, but they cannot break inversion symmetry in simple perovskites. However, in a layered oxide structure, this is possible, as we demonstrate here in A-site ordered Ruddlesden-Popper NaRTiO4 (R denotes rare-earth metal), previously believed to be centric. By revisiting this series via synchrotron x-ray diffraction, optical second-harmonic generation, piezoresponse force microscopy, and first-principles phonon calculations, we find that the low-temperature phase belongs to the acentric space group P42(1)m, which is piezoelectric and nonpolar. The mechanism underlying this large new family of acentric layered oxides is prevalent, and could lead to many more families of acentric oxides.
