SEARCH · Search NASA
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
We report the synthesis of layered anisotropic semiconductor GeSe and GeSe 2 nanomaterials through low temperature and atmospheric pressure chemical vapor deposition using halide based precursors. The crystal phase is controlled by simply changing selenium vapor pressure.
The photoluminescence properties of thallium-containing vitreous semiconductor systems with stoichiometric and nonstoichiometric compositions (GeSe{sub 2}){sub 1–x}Tl{sub x} and (GeSe{sub 3}){sub 1–x}Tl{sub x} (0 ⩽ x ⩽ 0.1) are studied at a temperature of T = 77 K. Intrinsic defects with negative correlation energy are responsible for the Gaussian shape of the photoluminescence spectra. It is established that an increase in x in the systems does not affect the shape of the spectrum, does not generate new emission bands, shifts the photoluminescence spectra to the region of low energies, reduces the intensity of radiation, and increases its half-width. Kinetics of the fatigue of photoluminescence is different for both systems and is characterized by one curve irrespective of Tl content in the systems.
Here, we have used highly accurate quantum Monte Carlo methods to determine the chemical structure and electronic band gaps of monolayer GeSe. Two-dimensional (2D) monolayer GeSe has received a great deal of attention due to its unique thermoelectric, electronic, and optoelectronic properties with a wide range of potential applications. Density functional theory (DFT) methods have usually been applied to obtain optical and structural properties of bulk and 2D GeSe. For the monolayer, DFT typically yields a larger band-gap energy than for bulk GeSe but cannot conclusively determine if the monolayer has a direct or indirect gap. Moreover, the DFT-optimized lattice parameters and atomic coordinates for monolayer GeSe depend strongly on the choice of approximation for the exchange-correlation functional, which makes the ideal structure-and its electronic properties-unclear. In order to obtain accurate lattice parameters and atomic coordinates for the monolayer, we use a surrogate Hessian-based parallel line search within diffusion Monte Carlo to fully optimize the GeSe monolayer structure. The DMC-optimized structure is different from those obtained using DFT, as are calculated band gaps. The potential energy surface has a shallow minimum at the optimal structure. This, combined with the sensitivity of the electronic structure to strain, suggests that the optical properties of monolayer GeSe are highly tunable by strain.
The extraordinary properties and the novel applications of black phosphorene induce the research interest in the monolayer group-IV monochalcogenides. Here using first-principles calculations, we systematically investigate the electronic, transport, and optical properties of monolayer α- and β-GeSe, revealing a direct band gap of 1.61 eV for monolayer α-GeSe and an indirect band gap of 2.47 eV for monolayer β-GeSe. For monolayer β-GeSe, the electronic/hole transport is anisotropic, with an extremely high electron mobility of 2.93×104cm2/Vs along the armchair direction, comparable to that of black phosphorene. However, for β-GeSe, robust band gaps nearly independent of the applied tensile strain along the armchair direction are observed. Both monolayer α- and β-GeSe exhibit anisotropic optical absorption in the visible spectrum.
GeSe is a IV–VI semiconductor, like the excellent thermoelectric materials PbTe and SnSe. Orthorhombic GeSe has been predicted theoretically to have good thermoelectric performance but is difficult to dope experimentally. Like PbTe, rhombohedral GeTe has a multivalley band structure, which is ideal for thermoelectrics and also promotes the formation of Ge vacancies to provide enough carriers for electrical transport. Herein, we investigate the thermoelectric properties of GeSe alloyed with AgSbSe 2 , which stabilizes a new rhombohedral structure with higher symmetry that leads to a multivalley Fermi surface and a dramatic increase in carrier concentration. The z T of GeAg 0.2 Sb 0.2 Se 1.4 reaches 0.86 at 710 K, which is 18 times higher than that of pristine GeSe and over four times higher than doped orthorhombic GeSe. Our results open a new avenue towards developing novel thermoelectric materials through crystal phase engineering using a strategy of entropy stabilization of high-symmetry alloys.
Abstract Performance of the group IV monochalcogenide GeSe in solar cells, electronic, and optoelectronic devices is expected to improve when high‐quality single crystalline material is used rather than polycrystalline films. Crystalline flakes represent an attractive alternative to bulk single crystals as their synthesis may be developed to be scalable, faster, and with higher overall yield. However, large – and especially large and thin – single crystal flakes are notoriously hard to synthesize. Here it is demonstrated that vapor‐liquid‐solid growth combined with direct lateral vapor‐solid incorporation produces high‐quality single crystalline GeSe ribbons with tens of micrometers size and controllable thickness. Electron microscopy shows that the ribbons exhibit perfect equilibrium (AB) van der Waals stacking order without extended defects across the entire thickness, in contrast to the conventional case of substrate‐supported flakes where material is added via layer‐by‐layer nucleation and growth on the basal plane. Electrical measurements show anisotropic transport and a high Hall mobility of 85 cm 2 V −1 s −1 , on par with the best single crystals to date. Growth from mixed GeSe and SnSe vapors, finally, yields ribbons with unchanged structure and composition but with jagged edges, promising for applications that rely on ample chemically active edge sites, such as catalysis or photocatalysis.
The emergence of atomically thin crystals has allowed extending materials integration to lateral heterostructures where different 2D materials are covalently connected in the plane. The concept of lateral heterostructures can be generalized to thicker layered crystals, provided that a suitably faceted seed crystal presents edges to which a compatible second van der Waals material can be attached layer by layer. Here, we examine the possibility of integrating multilayer crystals of the group IV monochalcogenides SnS and GeSe, which have the same crystal structure, small lattice mismatch, and similar bandgaps. In a two-step growth process, lateral epitaxy of GeSe on the sidewalls of multilayer SnS flakes (obtained by vapor transport of a SnS 2 precursor on graphite) yields heterostructures of laterally stitched crystalline GeSe and SnS without any detectable vertical overgrowth of the SnS seeds and with sharp lateral interfaces. Combined cathodoluminescence spectroscopy and ab initio calculations show the effects of small band offsets on carrier transport and radiative recombination near the interface. Further, the results demonstrate the possibility of forming atomically connected lateral interfaces across many van der Waals layers, which is promising for manipulating optoelectronics, photonics, and for managing charge- and thermal transport.
Historically PbSe has underperformed PbTe in thermoelectric efficiency and has been regarded as an inferior relative to its telluride congener. However, the fifty-fold greater natural abundance of Se relative to Te makes PbSe appealing as a thermoelectric material. We report that the n-type GeSe-alloyed PbSe system achieves a peak figure of merit, ZT, of ~1.54 at 773 K and maintains ZT values above 1.2 over a broad temperature range from 623 K to 923 K. The highest performing composition is Sb-doped PbSe–12%GeSe, which exhibits an ultralow lattice thermal conductivity of ~0.36 W m –1 K –1 at 573 K, close to the limit of amorphous PbSe. Theoretical studies reveal that the alloyed Ge 2+ atoms prefer to stay at off-center lattice positions, inducing low frequency modes. The Ge atoms also cause the unexpected behavior where the next nearest atom neighbors (6 Pb atoms) oscillate at lower frequencies than in pure PbSe leading to a large reduction of the Debye temperature and acoustic phonon velocity. The Pb 0.9955 Sb 0.0045 Se–12%GeSe system also shows Ge-rich precipitates and many aligned dislocations within its microstructure which also contribute to phonon scattering. The resultant average ZT (ZT avg ), a broad measure of the material's potential for functional thermoelectric modules, is 1.06 from 400 K to 800 K, the highest among all previously reported n- and p-type PbSe. Furthermore, this value matches or exceeds even those of the best n-type PbTe-based thermoelectric materials.
GeSe crystallizes in the orthorhombic Pnma space group. The structure is two-dimensional and consists of two GeSe sheets oriented in the (0, 0, 1) direction. Ge2+ is bonded in a 3-coordinate geometry to three equivalent Se2- atoms. There are one shorter (2.59 Å) and two longer (2.61 Å) Ge–Se bond lengths. Se2- is bonded in a distorted trigonal non-coplanar geometry to three equivalent Ge2+ atoms.
Ni17(GeSe)4 crystallizes in the tetragonal P4mm space group. The structure is two-dimensional and consists of one Ni17(GeSe)4 sheet oriented in the (0, 0, 1) direction. there are twelve inequivalent Ni sites. In the first Ni site, Ni is bonded to eight Ni and four Ge atoms to form NiNi8Ge4 cuboctahedra that share corners with four equivalent NiNi8Ge4 cuboctahedra, edges with eight NiNi8Ge4 cuboctahedra, edges with eight GeNi12 cuboctahedra, faces with four GeNi12 cuboctahedra, and faces with twelve NiNi8Ge4 cuboctahedra. There are a spread of Ni–Ni bond distances ranging from 2.50–2.55 Å. There are two shorter (2.53 Å) and two longer (2.54 Å) Ni–Ge bond lengths. In the second Ni site, Ni is bonded to eight Ni and four Ge atoms to form NiNi8Ge4 cuboctahedra that share corners with four equivalent NiNi8Ge4 cuboctahedra, edges with eight NiNi8Ge4 cuboctahedra, edges with eight GeNi12 cuboctahedra, faces with four GeNi12 cuboctahedra, and faces with twelve NiNi8Ge4 cuboctahedra. There are four shorter (2.51 Å) and four longer (2.55 Å) Ni–Ni bond lengths. There are two shorter (2.54 Å) and two longer (2.55 Å) Ni–Ge bond lengths. In the third Ni site, Ni is bonded to eight Ni and four Ge atoms to form NiNi8Ge4 cuboctahedra that share corners with four equivalent NiNi8Ge4 cuboctahedra, edges with eight NiNi8Ge4 cuboctahedra, edges with eight GeNi12 cuboctahedra, faces with four GeNi12 cuboctahedra, and faces with twelve NiNi8Ge4 cuboctahedra. There are two shorter (2.50 Å) and two longer (2.51 Å) Ni–Ni bond lengths. There are two shorter (2.53 Å) and two longer (2.54 Å) Ni–Ge bond lengths. In the fourth Ni site, Ni is bonded in a 12-coordinate geometry to eight Ni, two equivalent Ge, and two equivalent Se atoms. There are a spread of Ni–Ni bond distances ranging from 2.53–2.64 Å. Both Ni–Ge bond lengths are 2.48 Å. Both Ni–Se bond lengths are 2.43 Å. In the fifth Ni site, Ni is bonded in a 2-coordinate geometry to two equivalent Ni, two equivalent Ge, and two equivalent Se atoms. Both Ni–Ni bond lengths are 2.54 Å. Both Ni–Ge bond lengths are 2.50 Å. Both Ni–Se bond lengths are 2.33 Å. In the sixth Ni site, Ni is bonded in a 2-coordinate geometry to two equivalent Ni, two equivalent Ge, and two equivalent Se atoms. Both Ni–Ni bond lengths are 2.54 Å. Both Ni–Ge bond lengths are 2.51 Å. Both Ni–Se bond lengths are 2.32 Å. In the seventh Ni site, Ni is bonded in a 2-coordinate geometry to two equivalent Ni, two equivalent Ge, and two equivalent Se atoms. Both Ni–Ni bond lengths are 2.53 Å. Both Ni–Ge bond lengths are 2.52 Å. Both Ni–Se bond lengths are 2.33 Å. In the eighth Ni site, Ni is bonded to eight Ni and four equivalent Ge atoms to form NiNi8Ge4 cuboctahedra that share corners with eight NiNi8Ge4 cuboctahedra, edges with four equivalent GeNi12 cuboctahedra, edges with eight NiNi8Ge4 cuboctahedra, edges with four equivalent NiNi4Se5 square pyramids, faces with four equivalent GeNi12 cuboctahedra, and faces with nine NiNi8Ge4 cuboctahedra. All Ni–Ge bond lengths are 2.55 Å. In the ninth Ni site, Ni is bonded to eight Ni and four equivalent Ge atoms to form NiNi8Ge4 cuboctahedra that share corners with eight NiNi8Ge4 cuboctahedra, edges with four equivalent GeNi12 cuboctahedra, edges with eight equivalent NiNi8Ge4 cuboctahedra, faces with four equivalent GeNi12 cuboctahedra, and faces with nine NiNi8Ge4 cuboctahedra. All Ni–Ge bond lengths are 2.55 Å. In the tenth Ni site, Ni is bonded to eight Ni and four equivalent Ge atoms to form NiNi8Ge4 cuboctahedra that share corners with eight NiNi8Ge4 cuboctahedra, edges with four equivalent GeNi12 cuboctahedra, edges with eight equivalent NiNi8Ge4 cuboctahedra, faces with four equivalent GeNi12 cuboctahedra, and faces with nine NiNi8Ge4 cuboctahedra. All Ni–Ge bond lengths are 2.55 Å. In the eleventh Ni site, Ni is bonded to eight Ni and four equivalent Ge atoms to form NiNi8Ge4 cuboctahedra that share corners with eight NiNi8Ge4 cuboctahedra, edges with four equivalent GeNi12 cuboctahedra, edges with eight NiNi8Ge4 cuboctahedra, faces with four equivalent GeNi12 cuboctahedra, and faces with nine NiNi8Ge4 cuboctahedra. All Ni–Ge bond lengths are 2.55 Å. In the twelfth Ni site, Ni is bonded to four equivalent Ni and five Se atoms to form distorted NiNi4Se5 square pyramids that share corners with four equivalent GeNi12 cuboctahedra, corners with four equivalent NiNi4Se5 square pyramids, edges with four equivalent NiNi8Ge4 cuboctahedra, a faceface with one GeNi12 cuboctahedra, and faces with four equivalent NiNi4Se5 square pyramids. There are one shorter (2.28 Å) and four longer (2.56 Å) Ni–Se bond lengths. There are four inequivalent Ge sites. In the first Ge site, Ge is bonded to twelve Ni atoms to form GeNi12 cuboctahedra that share corners with eight GeNi12 cuboctahedra, corners with four equivalent NiNi4Se5 square pyramids, edges with twelve NiNi8Ge4 cuboctahedra, faces with five GeNi12 cuboctahedra, faces with eight NiNi8Ge4 cuboctahedra, and a faceface with one NiNi4Se5 square pyramid. In the second Ge site, Ge is bonded to twelve Ni atoms to form GeNi12 cuboctahedra that share corners with eight GeNi12 cuboctahedra, edges with twelve NiNi8Ge4 cuboctahedra, faces with five GeNi12 cuboctahedra, and faces with eight NiNi8Ge4 cuboctahedra. In the third Ge site, Ge is bonded to twelve Ni atoms to form GeNi12 cuboctahedra that share corners with eight GeNi12 cuboctahedra, edges with twelve NiNi8Ge4 cuboctahedra, faces with five GeNi12 cuboctahedra, and faces with eight NiNi8Ge4 cuboctahedra. In the fourth Ge site, Ge is bonded to twelve Ni atoms to form GeNi12 cuboctahedra that share corners with eight GeNi12 cuboctahedra, edges with twelve NiNi8Ge4 cuboctahedra, faces with five GeNi12 cuboctahedra, and faces with eight NiNi8Ge4 cuboctahedra. There are four inequivalent Se sites. In the first Se site, Se is bonded in a 8-coordinate geometry to eight Ni atoms. In the second Se site, Se is bonded in a 4-coordinate geometry to four equivalent Ni atoms. In the third Se site, Se is bonded in a 5-coordinate geometry to five Ni atoms. In the fourth Se site, Se is bonded in a 4-coordinate geometry to four equivalent Ni atoms.
The structure of glassy GeSe{sub 9} was investigated by combining neutron diffraction with density-functional-theory-based first-principles molecular dynamics. In the simulations, three different models of N = 260 atoms were prepared by sampling three independent temporal trajectories, and the glass structures were found to be substantially different from those obtained for models in which smaller numbers of atoms or more rapid quench rates were employed. In particular, the overall network structure is based on Se{sub n} chains that are cross-linked by Ge(Se{sub 4}){sub 1/2} tetrahedra, where the latter are predominantly corner as opposed to edge sharing. The occurrence of a substantial proportion of Ge–Se–Se connections does not support a model in which the material is phase separated into Se-rich and GeSe{sub 2}-rich domains. The appearance of a first-sharp diffraction peak in the Bhatia-Thornton concentration-concentration partial structure factor does, however, indicate a non-uniform distribution of the Ge-centered structural motifs on an intermediate length scale.
GeSe is Halite, Rock Salt structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Ge2+ is bonded to six equivalent Se2- atoms to form a mixture of corner and edge-sharing GeSe6 octahedra. The corner-sharing octahedral tilt angles are 0°. All Ge–Se bond lengths are 2.83 Å. Se2- is bonded to six equivalent Ge2+ atoms to form a mixture of corner and edge-sharing SeGe6 octahedra. The corner-sharing octahedral tilt angles are 0°.
Co2(GeSe)3 is Hausmannite-derived structured and crystallizes in the trigonal R3 space group. The structure is three-dimensional. there are four inequivalent Co sites. In the first Co site, Co is bonded to three equivalent Ge and three equivalent Se atoms to form CoGe3Se3 octahedra that share corners with six equivalent CoGe3Se3 octahedra, corners with six GeCo2Se2 tetrahedra, and corners with six SeCo2Ge2 tetrahedra. The corner-sharing octahedra tilt angles range from 52–58°. All Co–Ge bond lengths are 2.35 Å. All Co–Se bond lengths are 2.37 Å. In the second Co site, Co is bonded to three equivalent Ge and three equivalent Se atoms to form CoGe3Se3 octahedra that share corners with six equivalent CoGe3Se3 octahedra, corners with six GeCo2Se2 tetrahedra, and corners with six SeCo2Ge2 tetrahedra. The corner-sharing octahedra tilt angles range from 51–58°. All Co–Ge bond lengths are 2.35 Å. All Co–Se bond lengths are 2.36 Å. In the third Co site, Co is bonded to three Ge and three Se atoms to form CoGe3Se3 octahedra that share corners with six CoGe3Se3 octahedra, corners with six GeCo2Se2 tetrahedra, and corners with six SeCo2Ge2 tetrahedra. The corner-sharing octahedra tilt angles range from 51–58°. There are two shorter (2.33 Å) and one longer (2.35 Å) Co–Ge bond lengths. There are a spread of Co–Se bond distances ranging from 2.37–2.41 Å. In the fourth Co site, Co is bonded to three Ge and three Se atoms to form CoGe3Se3 octahedra that share corners with six CoGe3Se3 octahedra, corners with six GeCo2Se2 tetrahedra, and corners with six SeCo2Ge2 tetrahedra. The corner-sharing octahedra tilt angles range from 51–58°. There are one shorter (2.32 Å) and two longer (2.34 Å) Co–Ge bond lengths. There are a spread of Co–Se bond distances ranging from 2.37–2.40 Å. There are four inequivalent Ge sites. In the first Ge site, Ge is bonded to two Co and two Se atoms to form distorted GeCo2Se2 tetrahedra that share corners with four CoGe3Se3 octahedra, corners with four GeCo2Se2 tetrahedra, corners with six SeCo2Ge2 tetrahedra, and an edgeedge with one GeCo2Se2 tetrahedra. The corner-sharing octahedra tilt angles range from 70–71°. There are one shorter (2.57 Å) and one longer (2.75 Å) Ge–Se bond lengths. In the second Ge site, Ge is bonded to two Co and two Se atoms to form distorted GeCo2Se2 tetrahedra that share corners with four CoGe3Se3 octahedra, corners with four GeCo2Se2 tetrahedra, corners with six SeCo2Ge2 tetrahedra, and an edgeedge with one GeCo2Se2 tetrahedra. The corner-sharing octahedra tilt angles range from 69–72°. There are one shorter (2.58 Å) and one longer (2.74 Å) Ge–Se bond lengths. In the third Ge site, Ge is bonded to two Co and two Se atoms to form distorted GeCo2Se2 tetrahedra that share corners with four CoGe3Se3 octahedra, corners with four GeCo2Se2 tetrahedra, corners with six SeCo2Ge2 tetrahedra, and an edgeedge with one GeCo2Se2 tetrahedra. The corner-sharing octahedra tilt angles range from 69–72°. There are one shorter (2.58 Å) and one longer (2.73 Å) Ge–Se bond lengths. In the fourth Ge site, Ge is bonded to two Co and two Se atoms to form distorted GeCo2Se2 tetrahedra that share corners with four CoGe3Se3 octahedra, corners with four GeCo2Se2 tetrahedra, corners with six SeCo2Ge2 tetrahedra, and an edgeedge with one GeCo2Se2 tetrahedra. The corner-sharing octahedra tilt angles range from 69–71°. There are one shorter (2.58 Å) and one longer (2.74 Å) Ge–Se bond lengths. There are four inequivalent Se sites. In the first Se site, Se is bonded to two Co and two Ge atoms to form distorted SeCo2Ge2 tetrahedra that share corners with four CoGe3Se3 octahedra, corners with four SeCo2Ge2 tetrahedra, corners with six GeCo2Se2 tetrahedra, and an edgeedge with one SeCo2Ge2 tetrahedra. The corner-sharing octahedra tilt angles range from 69–74°. In the second Se site, Se is bonded to two Co and two Ge atoms to form distorted SeCo2Ge2 tetrahedra that share corners with four CoGe3Se3 octahedra, corners with four SeCo2Ge2 tetrahedra, corners with six GeCo2Se2 tetrahedra, and an edgeedge with one SeCo2Ge2 tetrahedra. The corner-sharing octahedra tilt angles range from 70–74°. In the third Se site, Se is bonded to two Co and two Ge atoms to form distorted SeCo2Ge2 tetrahedra that share corners with four CoGe3Se3 octahedra, corners with four SeCo2Ge2 tetrahedra, corners with six GeCo2Se2 tetrahedra, and an edgeedge with one SeCo2Ge2 tetrahedra. The corner-sharing octahedra tilt angles range from 69–73°. In the fourth Se site, Se is bonded to two Co and two Ge atoms to form distorted SeCo2Ge2 tetrahedra that share corners with four CoGe3Se3 octahedra, corners with four SeCo2Ge2 tetrahedra, corners with six GeCo2Se2 tetrahedra, and an edgeedge with one SeCo2Ge2 tetrahedra. The corner-sharing octahedra tilt angles range from 69–73°.
The layered semiconductor tin selenide (SnSe) has received extensive interest due to its promising thermoelectric and ferroelectric properties. Integrating SnSe with other layered crystals in heterostructures can enable the modification of charge- and thermal transport, electrical polarization, and other properties such as chemical stability, optoelectronics, and photonics. Here, we demonstrate the vapor transport synthesis of single-crystalline SnSe monochalcogenide flakes that are spontaneously encapsulated in a thin layered SnSe 2-x S x dichalcogenide shell. In a second growth step, such SnSe-SnSe 2-x S x heterostructures are integrated with the monochalcogenide GeSe, which is laterally stitched to the SnSe side facets while preserving the dichalcogenide shell across the basal facets. This architecture is confirmed by optical microscopy, electron microscopy and diffraction, energy dispersive X-ray and Raman spectroscopies, as well as cathodoluminescence spectroscopy. Furthermore, the results extend our capabilities for materials integration by forming complex heterostructures with both vertical van der Waals interfaces and covalent lateral interfaces between layered semiconductors.
The 3D integration technology in semiconductor fabrication requires a key component, the ovonic threshold switching (OTS) selector, to suppress the current leakage. The As doped amorphous (a-) GeSe glass is a commercialized OTS material in 3D phase-change memory, but the understanding of such a doping mechanism is still inadequate. Here we systematically explore the effect of As doping on the structural, bonding, and dynamics properties of a-GeAsSe using ab initio molecular dynamics simulations. The results reveal that As atoms form strong bonds with both Ge and Se atoms. The distorted octahedral structures and the 5-fold rings linked by atoms are increased. All of these structural features lead to a more disordered configuration. Moreover, as atoms have notably slowed down the atomic mobility, rendering a-GeAsSe a high stability. Overall, our studies offer insightful understanding of As-doping in OTS materials, paving the way for the design and application of advanced selector devices.
Analyses and modeling of the T -dependent thermal properties of Zn 2 GeSe 4 reveal an ultra-low thermal conductivity over a large temperature range due to low-frequency soft modes as well as strong lattice anharmonicity.
We analyse the functional capabilities of new crystals, BaGa{sub 2}GeS{sub 6} (BGGS) and BaGa{sub 2}GeSe{sub 6} (BGGSe), which are used for nonlinear optical frequency conversion in their transparency range. The wavelengths at which maximum conversion efficiencies can be obtained and the tuning range for difference-frequency generation are found. It is shown that there are wavelength combinations at which the effective nonlinearity coefficient varies only slightly in a wide frequency band. (nonlinear optical phenomena)