Non-centrosymmetric Crystal Structure Research Articles

Magnetization‐Induced Second Harmonic Generation (MSHG) in a Pentacyanidonitrosylmolybdate‐Based Piezoelectric Ferrimagnet

The pentacyanidonitrosylmetalate [M(CN)5(NO)] (M = metal ion) is an attractive building block for bimetal assemblies exhibiting a long‐range magnetic ordering. In this work, a cyanido‐bridged bimetal assembly based on a pentacyanidonitrosylmolybdate, Cs1.26Mn0.87[Mo(CN)5(NO)]·0.6H2O, was prepared. This compound has a noncentrosymmetric cubic crystal structure in the F43m space group and a lattice constant of 10.8970(3) Å. It exhibits ferrimagnetism with a Curie temperature (TC) of 65 K due to the antiparallel ordering of the MnII (S = 5/2) and MoI (S = 1/2) magnetic spins. Owing to the piezoelectric crystal structure, this compound exhibits second harmonic generation (SHG) and magnetization‐induced second harmonic generation (MSHG).

Electro-optic Response in Germanium Halide Perovskites.

Electro-optic materials that can be solution-processed and provide high-crystalline quality are sought for the development of compact, efficient optical modulators. Here we present density functional theory investigations of the linear electro-optic coefficients of candidate materials cesium and methylammonium germanium halide perovskites. As with their lead halide counterparts, these compounds can be solution-processed, but in contrast, they possess the noncentrosymmetric crystal structures needed to provide a linear electro-optic effect. We find substantial electro-optic responses from these compounds; in particular, for the r51 tensor element of CsGeI3, we predict an electro-optic coefficient of 47 pm·V-1 at the communications wavelength of 1550 nm, surpassing the strongest coefficient of LiNbO3 at 31 pm·V-1. The strong electro-optic responses of the germanium compounds are driven by high nonlinear susceptibilities and dynamics of the germanium atoms that ultimately arise from the distorted crystal structures. Alongside the electro-optic coefficient calculations, we provide the frequency responses for the linear and nonlinear electronic susceptibilities.

Superconductivity in non-centrosymmetric ThNiSi

We report on the synthesis of ThNiSi and the characterization of its physical properties by means of powder x-ray diffraction, magnetization, electrical resistivity and heat capacity measurements. The compound is confirmed to crystallize with a non-centrosymmetric tetragonal crystal structure of the LaPtSi-type (space group I41md). Below Tc = 0.85 K, it exhibits a BCS-type superconductivity with moderately anisotropic superconducting gap.

Fermi surfaces of the pyrite-type cubic AuSb2 compared with split Fermi surfaces of the ullmannite-type cubic chiral NiSbS and PdBiSe

We grew high-quality single crystals of AuSb2 with the pyrite (FeS2)-type cubic structure by the Bridgman method and studied the Fermi surface properties by the de Haas-van Alphen (dHvA) experiment and the full potential LAPW band calculation. The Fermi surfaces of AuSb2 are found to be similar to those of NiSbS and PdBiSe with the ullmannite (NiSbS)-type cubic chiral structure because the crystal structures are similar each other and the number of valence electrons is the same between two different compounds. Note that each Fermi surface splits into two Fermi surfaces in NiSbS and PdBiSe, reflecting the non-centrosymmetric crystal structure.

EO properties and photorefractive two-beam coupling in AlN at 375 nm

Aluminium nitride (AlN) is a wide bandgap semiconductor with an intrinsic band-to-band absorption edge at ∼200 nm wavelength. Therefore, the material is promising for optical use in the ultraviolet region. With a wurtzite-type, noncentrosymmetric crystal structure, AlN is also interesting for second order nonlinear optics and electro-optics. As grown crystals of AlN show relatively large absorption starting below 650 nm. With the material properties of AlN supporting the photorefractive effect we examined AlN as a photorefractive beam coupler. No effect was observed in the visible spectral range. Using the UV wavelength of 375 nm, where the absorption constant is (17 ± 2) cm-1 for the extraordinary polarization beam coupling was realized. The results show relatively weak and slow photorefractive effect.

Solution-Processed Tellurium Nanowires for Stretchable and Wearable Piezoelectric Device

Nanowires made of materials with non-centrosymmetric crystal structure are under investigation for their piezoelectric properties and suitability for next-generation self-powered nanodevices. In this work, we report a systematic study on synthesis of trigonal tellurium nanowires with well-controlled dimensions. After assembling the tellurium nanowires on the stretchable substrates, a stretchable, soft piezoelectric nanogenerator has been developed with the capability to efficiently convert mechanical stimuli into electric power. We revealed the correlation between the piezoelectric outputs from the integrated devices and the diameters of as-produced tellurium nanowires. We further studied the feasibility of using the as-fabricated piezoelectric device for biomedical applications, such as gesture recognition. This work establishes the process-structure-property-performance relationship in the piezoelectric nanogenerator that may pave way for the nanomanufacturing and practical application of piezoelectric nanomaterials.

Magnetoelectric effect in ytterbium aluminum borate YbAl3(BO3)4

The anisotropic magnetoelectric properties of an ytterbium aluminum borate YbAl (BO single crystal having noncentrosymmetric crystal structure (space group R32) are studied, including the orientational, field, and temperature dependences of the polarization in magnetic fields up to 5 T in the temperature range of 2–300 K. It has been shown experimentally for the first time that the symmetry of the observed magnetoelectric effects exactly corresponds to the trigonal structure of the crystal and is characterized by two quadratic magnetoelectric constants. The polarization in the basal plane P a, b is a quadratic function of the field at low fields and reaches 250–300 μC/m2 in a field of 5 T at a temperature of 2 K, almost an order of magnitude exceeding the previously reported values. A theoretical model based on the spin Hamiltonian of the ground Kramers doublet of Yb3+ ions in the crystal field is proposed including magnetoelectric interactions allowed by the symmetry. This model makes it possible to quantitatively describe all observed magnetic and magnetoelectric properties of YbAl3(BO3)4.

Temperature and angular dependence of the upper critical field in K2Cr3As3

We report measurements of the upper critical field $H_{\mathrm{c2}}$ as functions of temperature $T$, polar angle $\theta$ (of the field direction with respect to the crystallographic $c$ axis), and azimuthal angle $\phi$ (of the field direction relative to the $a$ axis within the $ab$ plane) for the Cr-based superconductor K$_{2}$Cr$_{3}$As$_{3}$ with a quasi-one-dimensional and non-centrosymmetric crystal structure. We confirm that the anisotropy in $H_{\mathrm{c2}}(T)$ becomes inverse with decreasing temperature. At low temperatures, $H_{\mathrm{c2}}(\theta)$ data are featured by two maxima at $\theta$ = 0 ($\mathbf{H}\parallel c$) and $\pi/2$ ($\mathbf{H}\bot c$), which can be quantitatively understood only if uniaxial effective-mass anisotropy and absence of Pauli paramagnetic effect for $\mathbf{H}\bot c$ are taken simultaneously into consideration. The in-plane $H_{\mathrm{c2}}(\phi)$ profile shows a unique threefold modulation especially at low temperatures. Overall, the characteristic of the $H_{\mathrm{c2}}(\theta, \phi, T)$ data mostly resemble those of the heavy-fermion superconductor UPt$_3$, and we argue in favor of a dominant spin-triplet superconductivity with odd parity in K$_{2}$Cr$_{3}$As$_{3}$.

Synthesis, Crystal Structure, and Properties of Bi3 TeBO9 or Bi3 (TeO6 )(BO3 ): A Non-Centrosymmetric Borate-Tellurate(VI) of Bismuth.

Pale-yellow single crystals of the new borate tellurate(VI) Bi3 TeBO9 were obtained by reaction of stoichiometric amounts of Bi2 O3 , B2 O3 , and Te(OH)6 at 780 °C. The non-centrosymmetric crystal structure (P63 , Z=2, a=8.7454(16), c=5.8911(11) Å, 738 refl., 43 param, R1=0.037, wR2=0.093) contains isolated trigonal-planar BO3 units and nearly undistorted TeO6 octahedra. The Bi3+ cations are located in between in octahedral voids. The BiO6 octahedra are significantly distorted to a [3+3] pattern (2.25/2.50 Å) due to the ns2 configuration. According to the structural features, the formula can be written as Bi3 (TeO6 )(BO3 ). Alternatively, the structure can also be described as hcp of oxygen with TeVI and BiIII in octahedral voids and BIII in trigonal- planar voids. The vibrational spectra show the typical features of BO3 and TeO6 units with a significant 10 B/11 B isotopic splitting of the IR-active B-O valence mode (1248 and 1282 cm-1 ). The UV/Vis spectrum shows an optical band edge with an onset around 480 nm (2.6 eV). MAS-NMR spectra of 11 B show an anisotropic signal with a quadrupole coupling constant of CQ =2.55 MHz. and a very small deviation from rotational symmetry (η=0.2). The isotropic chemical shift is 20.1 ppm. The second harmonic generation (SHG) test was positive with an activity comparable to potassium dihydrogen phosphate (KDP). Bi3 TeBO9 decomposes in air at 825 °C to Bi2 TeO5 .

Spin wave excitations in the pyrovanadate α−Cu2V2O7

The pyrovanadate alpha-Cu2V2O7 belongs to the orthorhombic (Fdd2) class of crystals with noncentrosymmetric crystal structure. Recently, the compound has been identified to be a magnetic multiferroic with a substantial electric polarization below the magnetic transition temperature TC = 35 K. Here we report the results of our inelastic neutron scattering (INS) studies on a polycrystalline sample of alpha-Cu2V2O7. Our INS data clearly show the existence of dispersive spin wave excitations below TC with a zone-boundary energy of 11 meV at 5 K.We have analyzed the data using linear spin wave theory, which shows good agreement between the experiment and calculation. The analysis is consistent with the third nearest neighbor exchange interaction playing a dominant role in the magnetism of the material.

Scaling Effects in Perovskite Ferroelectrics: Fundamental Limits and Process‐Structure‐Property Relations

Ferroelectric materials are well‐suited for a variety of applications because they can offer a combination of high performance and scaled integration. Examples of note include piezoelectrics to transform between electrical and mechanical energies, capacitors used to store charge, electro‐optic devices, and nonvolatile memory storage. Accordingly, they are widely used as sensors, actuators, energy storage, and memory components, ultrasonic devices, and in consumer electronics products. Because these functional properties arise from a noncentrosymmetric crystal structure with spontaneous strain and a permanent electric dipole, the properties depend upon physical and electrical boundary conditions, and consequently, physical dimension. The change in properties with decreasing physical dimension is commonly referred to as a size effect. In thin films, size effects are widely observed, whereas in bulk ceramics, changes in properties from the values of large‐grained specimens is most notable in samples with grain sizes below several micrometers. It is important to note that ferroelectricity typically persists to length scales of about 10 nm, but below this point is often absent. Despite the stability of ferroelectricity for dimensions greater than ~10 nm, the dielectric and piezoelectric coefficients of scaled ferroelectrics are suppressed relative to their bulk counterparts, in some cases by changes up to 80%. The loss of extrinsic contributions (domain and phase boundary motion) to the electromechanical response accounts for much of this suppression. In this article, the current understanding of the underlying mechanisms for this behavior in perovskite ferroelectrics is reviewed. We focus on the intrinsic limits of ferroelectric response, the roles of electrical and mechanical boundary conditions, grain size and thickness effects, and extraneous effects related to processing. In many cases, multiple mechanisms combine to produce the observed scaling effects.

Peculiar Piezoelectric Properties of Soft Two-Dimensional Materials

Group II–VI semiconductor honeycomb monolayers have a noncentrosymmetric crystal structure and therefore are expected to be important for nano piezoelectric device applications. This motivated us to perform first-principles calculations based on density functional theory to unveil the piezoelectric properties (i.e., piezoelectric stress (e11) and piezoelectric strain (d11) coefficients) of these monolayer materials with chemical formula MX (where M = Be, Mg, Ca, Sr, Ba, Zr, Cd and X = S, Se, Te). We found that these two-dimensional materials have peculiar piezoelectric properties with d11 coefficients 1 order of magnitude larger than those of commercially utilized bulk materials. A clear trend in their piezoelectric properties emerges, which originates mainly from their mechanical properties. We establish a simple correlation between the piezoelectric strain coefficients and the physical properties, as the natural elemental polarizabilities, the Bader charges, and lattice constants of the individual M/X a...

Comparative study of the exciton states in CdSe/ZnS core-shell quantum dots under applied electric fields with and without permanent electric dipole moment

Due to its non-centrosymmetric wurtzite crystal structure, the CdSe dot presents a permanent electric dipole moment. In this paper we study the effect of an electric applied field on the emission wavelength of a CdSe/ZnS core-shell quantum dot with a permanent electric dipole. The electron and hole single-particle energy and wave function in the presence of an electric dipole are obtained in the effective-mass and parabolic-band approximation for various electric field strengths. The Schrodinger equation was solved by use of the finite element method. The exciton binding energy is calculated in the first-order perturbation theory and the optical emission wavelengths are found and compared to the experimental values. We find that the photoluminescence emission can be tuned by varying the electric dipole size, the electric field strength and by an appropriate orientation between the permanent dipole moment and applied electric field.

Single Crystal Growth and Physical Property Characterization of Non-centrosymmetric Superconductor PbTaSe2**Supported by the National Basic Research Program of China under Grant Nos 2015CB921303, 2011CBA00100 and 2012CB821404, the Strategic Priority Research Program (B) of Chinese Academy of Sciences under Grant Nos XDB07020100 and XDB07020200, and the National Natural Science Foundation of China under Grant No 11174350.

We report on the single crystal growth and superconducting properties of PbTaSe2 with the non-centrosymmetric crystal structure. By using the chemical vapor transport technique, centimeter-size single crystals are successfully obtained. The measurement of temperature dependence of electrical resistivity ρ(T) in both normal and superconducting states indicates a quasi-two-dimensional electronic state in contrast to that of polycrystalline samples. Specific heat C(T) measurement reveals a bulk superconductivity with Tc ≃ 3.75 K and a specific heat jump ratio of 1.42. All these results are in agreement with a moderately electron–phonon coupled, type-II Bardeen–Cooper–Schrieffer superconductor.

Superconductivity in the Hexagonal Ternary Phosphide ScIrP

We report the discovery of a bulk superconducting transition at 3.4 K in the ternary phosphide, ScIrP, which crystallizes in a hexagonal ZrNiAl-type structure without spatial inversion symmetry. On the basis of heat capacity data in a zero magnetic field, ScIrP is suggested to be a weakly-coupled Bardeen-Cooper-Schrieffer superconductor. Alternatively, experimental results under magnetic fields indicate that this material is a type-II superconductor with an upper critical field Hc2 at magnetic fields above 5 T at zero temperature. This moderately high Hc2 does not violate the Pauli limit, but it does imply that there is a significant effect from the strong spin-orbit interaction of Ir 5d electrons in the noncentrosymmetric crystal structure. Electronic structure calculations show an interesting feature of ScIrP, where both the Sc 3d and Ir 5d orbitals contribute to the electronic density of states at the Fermi level.

Multi-functional supramolecular building blocks with hydroxy piperidino groups: new opportunities for developing nonlinear optical ionic crystals

We report on multi-functional supramolecular building blocks for obtaining a non-centrosymmetric crystal structure, which is the essential requirement for achieving a macroscopic second-order nonlinear optical activity. The supramolecular building blocks PM (piperidin-4-ylmethanol) and PO (piperidin-4-ol) consist of a piperidino group acting as a strong electron-donating group and a hydroxyl group acting as a strong hydrogen-bonding group, which also may act as a ‘site-isolation’ group. The quinolinium crystals PMQ-T and POQ-T that introduce PM and PO blocks, respectively, exhibit a non-centrosymmetric crystal structure with nonlinear optical activity, while analogous quinolinium DAQ-T crystal with the widely used N,N-dimethylamino (DA) group exhibits a centrosymmetric crystal structure without nonlinear optical activity. Compared to the DAQ-T crystal, the PMQ-T and POQ-T crystals exhibit a lower tendency to form a hydrate phase in the crystalline state. The PMQ-T crystal exhibits a higher molecular optical nonlinearity in the crystalline state than the DAQ-T crystal, which is attributed to the ‘site-isolation’ effect resulting from the bulky PM block. In addition, the PMQ-T crystal exhibits large macroscopic optical nonlinearities with more than 3 times higher off-diagonal effective hyperpolarizability than that of benchmark stilbazolium crystals. Therefore, multi-functional supramolecular building blocks PM and PO can give new opportunities for developing non-centrosymmetric ionic crystals for nonlinear optical applications.

Itinerant antiferromagnetism inFeMnP0.8Si0.2single crystals

Compounds based on the Fe2P structure have continued to attract interest because of the interplay between itinerant and localized magnetism in a non-centrosymmetric crystal structure, and because of the recent developments of these materials for magnetocaloric applications. Here we report the growth and characterization of mm size single crystals of FeMnP0.8Si0.2. Single crystal x-ray diffraction, magnetization, resistivity, Hall and heat capacity data are reported. Surprisingly, the crystals exhibit itinerant antiferromagnetic order below 158 K with no hint of ferromagnetic behavior in the magnetization curves and with the spins ordered primarily in the ab plane. The room temperature resistivity is close to the Ioffe-Regel limit for a metal. Single crystal x-ray diffraction indicates a strong preference for Mn to occupy the larger pyramidal 3g site. The cation site preference in the as-grown crystals and the antiferromagnetism are not changed after high temperature anneals and a rapid quench to room temperature.

Promising Piezoelectric Performance of Single Layer Transition-Metal Dichalcogenides and Dioxides

Piezoelectricity is a unique material property that allows one to convert mechanical energy into electrical one or vice versa. Transition metal dichalcogenides (TMDC) and transition metal dioxides (TMDO) are expected to have great potential for piezoelectric device applications due to their noncentrosymmetric and two-dimensional crystal structure. A detailed theoretical investigation of the piezoelectric stress (e11) and piezoelectric strain (d11) coefficients of single layer TMDCs and TMDOs with chemical formula MX2 (where M= Cr, Mo, W, Ti, Zr, Hf, Sn and X = O, S, Se, Te) is presented by using first-principles calculations based on density functional theory. We predict that not only the Mo- and W-based members of this family but also the other materials with M= Cr, Ti, Zr and Sn exhibit highly promising piezoelectric properties. CrTe2 has the largest e11 and d11 coefficients among the group VI elements (i.e., Cr, Mo, and W). In addition, the relaxed-ion e11 and d11 coefficients of SnS2 are almost the sa...

Piezotronic Effect in Strain-Gated Transistor of a-Axis GaN Nanobelt.

Due to the non-centrosymmetric crystal structures, wurtzite family semiconducting materials possess piezoelectric properties and exhibit polarizations along certain directions upon straining. Utilizing strain-induced piezoelectric polarization charges to modulate the energy band structures and thus to tune/control the transport processes of charge carriers is referred to as the piezotronic effect. Distinct from the previous studies of c-axis GaN nanowires, here we systematically study the piezotronic-effect-induced modifications of energy band structures and the corresponding influence on electronic transport properties of a-axis GaN nanobelts. The physical mechanism is carefully illustrated and further confirmed by theoretical simulations via finite element analysis. The spatial distributions of local carrier concentration and the energy band diagrams of a-axis GaN under various straining conditions are calculated. This work provides a thorough understanding of strain-gated transport properties of a-axis GaN piezotronic transistors and its future applications in semiconductor devices.

Split Fermi Surface Properties in Ullmannite NiSbS and PdBiSe with the Cubic Chiral Crystal Structure

We grew single crystals of ullmannite NiSbS and PbBiSe with the cubic chiral structure and carried out electrical resistivity, specific heat, and de Haas–van Alphen (dHvA) experiments to clarify their Fermi surface properties. The Fermi surfaces were found to split into two, reflecting the non-centrosymmetric crystal structure. The splitting energies between the two nearly spherical electron Fermi surfaces named α and α′ were determined as 220 K in NiSbS and 1050 K in PdBiSe for H || [100] or [001]. This difference in splitting energies between the two compounds originates mainly from the fact that the spin–orbit interactions of Ni-3d, Sb-5p, and S-3p electrons in NiSbS are smaller in magnitude than those of Pd-4d, Bi-6p, and Se-4p electrons in PdBiSe, respectively.