Hexagonal Polymorph Research Articles

B-site engineering in rare earth ferrites: Composition and polymorphic control of electrical and photocatalytic functionalities

Rare-earth perovskites possess structural features conducive to co-existence of interesting functionalities. This study establishes structural tunability of B-site tailored GdFeO3 and its implications. Isovalent introduction of In3+ at B-site by hybrid synthesis approach yielded GdInxFe1-xO3 (0.0 ≤ x ≤ 1.0) system. The synthesis-route adopted could enable stabilization of metastable C-type and hexagonal (H) polymorphs that eventually led to thermodynamically stable phases in H (hexagonal), O (orthorhombic) and biphasic (H + O) phases. Gradual evolution of phases with respect to temperature as well as composition were studied with XRD, HT-XRD and Raman spectroscopy. Density functional theory was employed to explain prevalence of wide biphasic-field and it was attributed to non-favourable energetics of Fe3+ at trigonal-bipyramidal site in H-polymorph. The system exhibits structure and composition-dependent electrical behaviour and tunable band gap. Detailed study highlighted a very low leakage current(I) and low dielectric loss in In3+-rich polymorphs(H). Typically, GdInO3 shows I of 4 × 10−9A/cm2, high dielectric constant (ε) of 106 with a stable loss of 0.009. The energy storage performance showed decrease in recoverable E-density (Wrec) and increase in energy storage efficiency with In3+-content. Optimum properties were observed for equimolar composition, GdIn0.5Fe0.5O3 (Wrec= 560 mJ/cm3; ɳ= 82 %). An intriguing observation is effect of structure on visible light-catalysed degradation of methylene blue with O-polymorphs showing excellent photodegradation (̴ 99 % dye/180 min) while H-polymorphs performing abysmally. This is explained based on band gaps, surface charges and plausible difference in generation of photo-induced charge carriers.

Anomalous Raman Response in 2D Magnetic FeTe Under Uniaxial Strain: Tetragonal and Hexagonal Polymorphs.

2D Fe-chalcogenides emerge with rich structures, magnetisms, and superconductivities, which spark the growing research interests in the torturous transition mechanism and tunable properties for their potential applications in nanoelectronics. Uniaxial strain can produce a lattice distortion to study symmetry breaking induced exotic properties in 2D magnets. Herein, the anomalous Raman spectrum of 2D tetragonal (t-) and hexagonal (h-) FeTe is systematically investigated via uniaxial strain engineering strategy. It is found that both t- and h-FeTe keep the structural stability under different uniaxial tensile or compressive strain up to ± 0.4%. Intriguingly, the lattice vibrations along both in-plane and out-of-plane directions exceptionally harden (softened) under tensile (compressive) strain, distinguished from the behaviors of many conventional 2D systems. Further, the difference in thickness-dependent strain effect can be well explained by their structural discrepancy between two polymorphs of FeTe. These results can supply a unique platform to explore the vibrational properties of many novel 2D materials.

Compressibility and pressure-induced structural evolution of kokchetavite, hexagonal polymorph of KAlSi3O8, by single-crystal X-ray diffraction

Abstract Compressibility and pressure-induced structural evolution of kokchetavite, the hexagonal polymorph of KAlSi3O8, has been studied up to 11.8 GPa using synchrotron single-crystal X-ray diffraction. Two phase transitions were observed at pressures of ~0.3 and 10.4 GPa. Kokchetavite-I (as-synthesized, P6/mcc) transforms into kokchetavite-II with the P6c2 space group. Kokchetavite-II → kokchetavite-III phase transition at ~10.4 GPa is accompanied by a change of symmetry to probably orthorhombic. After pressure release, kokchetavite reverts to the initial single-crystal state with P6/mcc space group. A second-order Birch-Murnaghan equation of state was calculated for phase kokchetavite-II with coefficients V0 = 1486(3) Å3, K0 = 59(2) GPa.

A robust hydrothermal synthesis method of NH4Zr2(PO4)3, a precursor for HZr2(PO4)3 proton conductor

Hydrogen zirconium phosphate (HZP) is a ceramic material with the formula HZr2(PO4)3 that can potentially offer anhydrous protonic conductivity, with thermostability for the intrinsic protons up to 650 °C, and good chemical stability in aqueous environments or acid gases. Despite the attraction of these features for electrolyte applications, such as gas separation, fuel cells or electrolysers, work on this material remains in its infancy. A possible reason for this lack of attention may be related to the complexity to obtain the HZP material as a pure phase. For this reason, the current article examines a new hydrothermal synthesis method to produce the ammonium precursor, NH4Zr2(PO4)3, in its phase pure form, and consequently a refined overall route to produce the desired HZP target material. In the synthesis of the ammonium precursor, we underscore the precise control of reaction conditions that are necessary to obtain the pure NH4Zr2(PO4)3 material, both in its cubic and hexagonal structural polymorphs. For reaction times of 10 h, the cubic phase is obtained by employing a hydrothermal temperature of 150 °C and an aqueous ammonia volume of 2.1 mL, while the hexagonal phase is obtained for a hydrothermal temperature of 200 °C and an aqueous ammonia volume between 2.3 and 2.4 mL. Subsequently, we investigate the required conditions to convert hexagonal NH4Zr2(PO4)3 fully to hexagonal HZr2(PO4)3 via thermal treatment at 600 °C for 5 h, assessed by a combination of X-ray diffraction and infrared spectroscopy.In this way this article aims to open up further research on this interesting HZP material by documenting a robust route for its preparation.

Synergistic Effect of Dual Phases to Improve Lithium Storage Properties of Nb2O5.

Niobium pentoxides (Nb2O5) present great potential as next-generation anode candidates due to exceptional lithium-ion intercalation kinetics, considerably high capacity, and reasonable redox potential. Although four phases of Nb2O5 including hexagonal, orthorhombic, tetragonal, and monoclinic polymorphs show diverse characteristics in electrochemical performance, stable lifetime, high specific capacity, and fast intercalation properties cannot be delivered simultaneously with a single phase. Herein, this issue is addressed by generating a homogeneous mixture of orthorhombic and monoclinic crystals at the nanoscale. Reversible lithium-ion intercalation/deintercalation of the monoclinic phase is achieved, and exceptional lithium storage sites are created at the interface of the two phases. As a result, electrochemical features of stable lifetime from the orthorhombic phase and high specific performance from the monoclinic phase are harmoniously combined. This dual-phase Nb2O5/C nanohybrids deliver as high as 380 mA h g-1 (0.01-3.0 V) and 184 mA h g-1 (1.0-3.0 V) after 200 cycles. The essential principle of property enhancement is further confirmed through in situ XRD measurements and DFT calculations. The dual-phase concept can be further applied on electrodes with multiphases to achieve high electrochemical performance.

Epitaxial Stabilization and Persistent Nucleation of the 3C Polymorph of Ba0.6Sr0.4MnO3.

Ba-rich compositions in the BaxSr1-xMnO3 (BSMO) cubic perovskite (3C) system are magnetic ferroelectrics and are of interest for their strong magnetoelectric coupling. Beyond x = 0.5, they only form in hexagonal polymorphs. Here, the 3C phase boundary is pushed to Ba0.6Sr0.4MnO3 for the thin films. Using regular pulsed laser deposition (rPLD), 3C Ba0.6Sr0.4MnO3 could be epitaxially stabilized on DyScO3 (101)o substrates by using a 0.1% O2/99.9% N2 gas mixture. However, the 3C phase was mixed with the 4H polymorph for films 24 nm thick and above, and the films were relatively rough. To improve flatness and phase purity, changes in growth kinetics were investigated and interval PLD (iPLD) was especially effective. In iPLD, deposition is interrupted after completion of approximately one monolayer, and the deposit is annealed for a specific period of time before repeating. Both film flatness and, more importantly, the volume of the 3C polymorph improved with iPLD, resulting in 40 nm single-phase films. The results imply that iPLD improves the persistent nucleation of highly metastable phases and offers a new approach to epitaxial stabilization of novel materials, including more Ba-rich BSMO compositions in the 3C structure.

Seed-assisted crystallization of high-silica cubic and hexagonal faujasite polymorphs in the presence of the tetraethylammonium (TEA+) cation

Silica-rich Y (FAU topology) and EMC-2 (EMT topology) zeolites have been obtained in the presence of the tetraethylammonium (TEA+) cation as a structure-directing molecule and the corresponding protonic zeolites as seeds.

Phase-Modulated Elastic Properties of 2D Magnetic FeTe: Hexagonal and Tetragonal Polymorphs.

2D layered magnets, such as iron chalcogenides, have emerged these years as a new family of unconventional superconductors and provided the key insights to understand the phonon-electron interaction and pairing mechanism. Their mechanical properties are of strategic importance for the potential applications in spintronics and optoelectronics. However, there is still a lack of efficient approach to tune the elastic modulus despite the extensive studies. Herein, the modulated elastic modulus of 2D magnetic FeTe and its thickness-dependence is reported via phase engineering. The grown 2D FeTe by chemical vapor deposition can present various polymorphs, that is tetragonal FeTe (t-FeTe, antiferromagnetic) and hexagonal FeTe (h-FeTe, ferromagnetic). The measured Young's modulus of t-FeTe by nanoindentation method shows an obvious thickness-dependence, from 290.9±9.2 to 113.0±8.7 GPa when the thicknesses increased from 13.2 to 42.5 nm, respectively. In comparison, the elastic modulus of h-FeTe remains unchanged. These results can shed light on the efficient modulation of mechanical properties of 2D magnetic materials and pave the avenues for their practical applications in nanodevices.

Raman spectroscopic study of the transformation of nitrogen‐bearing K‐cymrite during heating experiments: Origin of kokchetavite in high‐pressure metamorphic rocks

AbstractNitrogen‐bearing K‐cymrite was synthesized in experiments at high pressures but so far has not been found/identified as inclusions in minerals subducted and subsequently exhumed from mantle depths. K‐cymrite is a potential carrier of nitrogen; it can store and transport up to 4 wt% of nitrogen in the deep mantle, thus contributing to the global nitrogen cycle. The stability of nitrogen‐bearing K‐cymrite synthesized at pressure of 6.3 GPa, upon exhumation to the Earth's surface, remains unknown. Here, we report an in situ Raman spectroscopic study of nitrogen‐bearing K‐cymrite, using Linkam heating and freezing microscope stage (THMS600). The transition induced in this experiment corresponds to the retrograde PT‐path of deeply subducted rocks. Our results demonstrate that (i) nitrogen‐bearing K‐cymrite structure is more stable upon heating than pure H2O bearing specimens (KAlSi3O8·H2O), (ii) phase transformation (e.g., H2O, N2, and NH3 release) starts at 550°C leading to the formation of kokchetavite (an anhydrous hexagonal polymorph of KAlSi3O8) frequently observed in high‐pressure metamorphic rocks, and (iii) Raman spectra of kokchetavite obtained from different precursors (e.g., nitrogen‐bearing and H2O‐bearing K‐cymrite) are identical. Thus, nitrogen‐bearing K‐cymrite can be considered a potential host phase for nitrogen in subduction‐related environments, and it can be preserved as inclusions in refractory minerals during the exhumation of deeply subducted rocks.

Predicted novel Janus γ-Ge2 XY ( S, Se, Te) monolayers with Mexican-hat dispersions and high carrier mobilities

This work is motivated by the recent fabrication of a new four-atom-thick hexagonal polymorph from group IV monochalcogenide, so-called γ-GeSe (Lee et al 2021 Nano Lett. 21 4305). In this paper, we propose and examine the structural characteristics, electronic properties, and carrier mobility of monolayers Janus γ-Ge2 XY ( S, Se, or Te) based on comprehensive first-principles calculations. Monolayers γ-Ge2 XY are confirmed to be structurally stable. Our calculations reveal that γ-Ge2 XY monolayers are indirect semiconductors with Mexican-hat-like dispersions in the top valence band. While the effect of the electric field on the energy band dispersions of γ-Ge2 XY monolayers is weak, the energy band dispersions are changed drastically in the presence of strain, especially compressive strain. Interestingly, a structural phase transition from semiconductor to metal is observed in γ-Ge2 XY under compressive strain. γ-Ge2STe and γ-Ge2SeTe possess high electron mobility with values of and cm2 V−1 s−1, respectively. Our findings not only explore the fundamental physical properties of γ-Ge2 XY but also open up new opportunities in the design of high-performance electronic nanodevices based on layered nanomaterials with Mexican-hat-like dispersions.

Magnetism and hyperfine interactions in the hexagonal polymorph of delafossite CuFeO[formula omitted

The magnetic susceptibility of and hyperfine interactions in the hexagonal 2H polymorph of delafossite CuFeO2 were investigated by SQUID magnetometry and Mössbauer spectroscopy, respectively, at low temperatures. The hydrothermally synthesized 2H-CuFeO2 sample contained a 10 vol-% α-Fe2O3 impurity detected by X-ray diffraction, whose contribution to the susceptibility and hyperfine interactions were easily distinguishable from the major 2H-CuFeO2 one. Morphology and indirect optical band gap investigated by scanning electron microscopy and diffuse reflectance measurements showed well expected results for a hydrothermally synthesized delafossite sample. The magnetic susceptibility of 2H-CuFeO2 revealed a first antiferromagnetic like transitions at 16 K and a second transition at 13.5 K or 10 K depending on measurement protocol, which points towards modified exchange interactions as compared to the 3R polymorph. Complementary Mössbauer measurements revealed complicated spectral shapes at low temperatures indicating rather complex magnetic structures and magnetic relaxations above 20 K.

Pursuing colloidal diamonds.

The endeavor to selectively fabricate a cubic diamond is challenging due to the formation of competing phases such as its hexagonal polymorph or others possessing similar free energy. The necessity to achieve this is of paramount importance since the cubic diamond is the only polymorph exhibiting a complete photonic bandgap, making it a promising candidate in view of photonic applications. Herein, we demonstrate that due to the presence of an external field and delicate manipulation of its strength we can attain selectivity in the formation of a cubic diamond in a one-component system comprised of designer tetrahedral patchy particles. The driving force of such a phenomenon is the structure of the first adlayer which is commensurate with the (110) face of the cubic diamond. Moreover, after a successful nucleation event, once the external field is turned off, the structure remains stable, paving an avenue for further post-synthetic treatment.

Phase evolution, electrical properties, and conductivity mechanism in LiNbWO6

The tetragonal LiNbWO 6 (α−LiNbWO 6 ) with a trirutile structure was previously reported to have good lithium ion conductivity. However, the electrical properties of the hexagonal LiNbWO 6 (β−LiNbWO 6 hereafter) polymorph have not been reported yet mainly due to the difficulty to obtain single-phase. Herein, by quenching the sample from high temperature to room temperature at ambient air condition, a series of pure-phase β−LiNb 1- x Ta x WO 6 ( x = 0, 0.05, 0.1) materials were successfully prepared. The phase evolutions and conductivities of these materials were investigated by variable temperature X-ray diffraction and alternating current impedance spectroscopy techniques, respectively. The results revealed that lithium ion conduction also dominated in this hexagonal polymorph of LiNbWO 6 and an interesting great ionic conduction enhancement behavior was observed at a temperature region of 660-800 ºC due to the minor phase transition from hexagonal to tetragonal. In addition, the possible ionic conducting mechanisms in both the tetragonal and hexagonal LiNbWO 6 materials were studied by the bond-valence-based method for the first time. The results disclosed two-dimensional and three-dimensional lithium ion migration pathways for the tetragonal and hexagonal LiNbWO 6 , respectively. • Hexagonal LiNb 1-x Ta x WO 6 (0≤x≤0.1) was prepared through solid-state reaction. • Lithium ion conduction was identified in hexagonal LiNb 1-x Ta x WO 6 . • Phase evolutions of LiNb 1-x Ta x WO 6 were investigated by variable temperature XRD. • Great conduction increase at 650-800 ºC was due to a composite electrolyte effect. • Conducting mechanisms for both the tetragonal and hexagonal LiNbWO 6 was studied.

Structural Diversity in Dimension-Controlled Assemblies of Tetrahedral Gold Nanocrystals.

Polyhedron packings have fascinated humans for centuries and continue to inspire scientists of modern disciplines. Despite extensive computer simulations and a handful of experimental investigations, understanding of the phase behaviors of synthetic tetrahedra has remained fragmentary largely due to the lack of tetrahedral building blocks with tunable size and versatile surface chemistry. Here, we report the remarkable richness of and complexity in dimension-controlled assemblies of gold nanotetrahedra. By tailoring nanocrystal interactions from long-range repulsive to hard-particle-like or to systems with short-ranged directional attractions through control of surface ligands and assembly conditions, nearly a dozen of two-dimensional and three-dimensional superstructures including the cubic diamond and hexagonal diamond polymorphs are selectively assembled. We further demonstrate multiply twinned icosahedral supracrystals by drying aqueous gold nanotetrahedra on a hydrophobic substrate. This study expands the toolbox of the superstructure by design using tetrahedral building blocks and could spur future computational and experimental work on self-assembly and phase behavior of anisotropic colloidal particles with tunable interactions.

Role of lone-pair electron localization in temperature-induced phase transitions in mimetite.

The crystal structure of mimetite Pb5(AsO4)3Cl, a phosphate with apatite structure-type has been investigated in situ at 123, 173, 273, 288, 353 and 393 K. A careful inspection of the diffraction pattern and subsequent structure refinements indicated that mimetite transforms from the monoclinic to the hexagonal polymorph with increasing temperature. At 123 K, a monoclinic superstructure, mimetite-2M, with cell parameters a = 20.4487 (9), b = 7.4362 (2), c = 20.4513 (9) Å, β = 119.953 (6)°, V = 2694.5 (2) Å3 and space group P21 was observed. From 173 to 353 K, the reflections of the supercell were evident only along one direction of the corresponding hexagonal apatite-cell and the structure transforms to the polymorph mimetite-M with space group P21/b and unit-cell parameters a = 10.2378 (3), b = 20.4573 (7), c = 7.4457 (2) Å, β = 120.039 (5)°, V = 1349.96 (9) Å3. Only at higher temperature, i.e. 393 K, does mimetite adopt the hexagonal space group P63/m characteristic of apatite structure-types. The role of the electron lone pairs of Pb atoms in the phase transition was investigated through the analysis of the electron localization function (ELF) calculated based on the DFT-geometry optimized structures of the three polymorphs. The changes in spatial distribution of the 6s2 electron density during the phase transitions were explored by means of the Wannier Function Centres (WFCs) derived from ab initio molecular dynamics trajectories. In the high-temperature hexagonal structure the 6s2 electrons are spherically symmetric relative to the position of Pb atoms. At low temperature the maximum of 6s2 electron density is displaced relative to the position of Pb atom contributing to the polar interaction in the monoclinic polymorphs.

Microwave-Assisted Solution Synthesis of Metastable Intergrowth of AgInS2 Polymorphs.

The intergrowth of stable and metastable AgInS2 polymorphs was synthesized using a microwave-assisted synthesis. The samples were synthesized in water and in a deep eutectic solvent (DES) consisting of choline chloride and thiourea. An increase in the metal precursor concentration improved the crystallinity of the synthesized samples and affected the particle size. AgInS2 cannot be synthesized from crystalline binary Ag2S or In2S3 via this route. The solution synthesis reported here results in the intergrowth of the thermodynamically stable polymorph (space group I2d, chalcopyrite structure) and the high-temperature polymorph (space group Pna21, wurtzite-like structure) that is metastable at room temperature. A scanning transmission microscopy (STEM) study revealed the intergrowth of tetragonal and orthorhombic polymorphs in a single particle and unambiguously established that the long-thought hexagonal wurtzite polymorph has pseudo-hexagonal symmetry and is best described with the orthorhombic unit cell. The solution-synthesized AgInS2 polymorphs intergrowth has slightly lower bandgap values in the range of 1.73 eV–1.91 eV compared to the previously reported values for tetragonal I2d (1.86 eV) and orthorhombic Pna21 (1.98 eV) polymorphs.

Effects of acceptor doping and oxygen stoichiometry on the properties of sputter-deposited p-type rocksalt NixZn1-xO (0.3[formula omitted]x[formula omitted]1.0) alloys

Reliable bipolar carrier transport remains a challenge in most wide-gap oxide semiconductors, and this limits the extensive adoption of oxides in transparent optoelectronic devices. For instance, it has been difficult to efficiently dope ZnO p-type in its hexagonal wurtzite polymorph. However, metastable octahedral rocksalt (RS) polymorph of ZnO has been predicted to be p-type dopable. Previously, we showed that RS-ZnO can be stabilized by alloying with octahedral RS- NiO forming Zn-rich RS-NixZn1-xO alloys which exhibit p-type conductivity due to their much higher valence band maximum with respect to vacuum. Here, we compare experimental results with computations by first-principles methods based on density functional theory (DFT) and Green’s function-based Koringa-Kohn-Rostoker (KKR), and confirm that NixZn1-xO alloy assumes the octahedral rocksalt structure at alloy composition concentration x ~ 0.32. We further explore the electrical and optical properties of RS-NixZn1-xO (0.3≤x≤1.0) alloys with Li doping synthesized by magnetron sputtering under different growth conditions. Our data show that Li doping in O-rich RS alloys (NixZn1-xO1+δ) can lead to reliable p-type conductivity. In the Zn-rich NixZn1-xO1+δ (0.3≤x≤0.5) region, with Li doping the electrical resistivity, ρ decreases from 397 Ω-cm (x = 0.3) to 108 Ω-cm (x = 0.5), and the hole concentration increased from 1.1 × 1017 cm−3 to 3.8 × 1018 cm−3. The electrical properties of these alloys further improve as the Ni content x increases. This can be attributed to the higher Li doping and upward lifting of the valence band maximum (VBM), making the Li acceptor level shallower. These p-type Li doped Zn-rich alloy oxides are desirable for the formation of “quasi-homojunction” in ZnO-based devices for potential transparent optoelectronic applications.

Atomic insights into the ordered solid solutions of Ni and Au in η-Cu6Sn5

The Cu6Sn5 intermetallic, which commonly forms at the solder interconnects, is a critical component contributing to the reliability of today's electronic products. It has been established that the structural control of its hexagonal η-Cu6Sn5 polymorph can be achieved over a wide temperature range of service conditions via chemical doping with Ni or Au, effectively suppressing the undesirable hexagonal to monoclinic (ƞ → ƞ′) phase transition at 186 °C and its associated volume change. In this study, we further investigate the effects of Ni (26.5 at%) and Au (9 at%), with high doping/alloying contents, on the atomic-scale structure of η-Cu6Sn5 using a suite of microscopy techniques including atomic-resolution imaging, chemical mapping, electron diffraction, and in-situ heating, coupled with advanced data informatics. Our study reveals that while Ni occupancy takes place in both the Cu1 and Cu2 sites of η-Cu6Sn5 in substantial amounts, Au is mostly substituted at the Cu1 sites of η-Cu6Sn5. Most interestingly, characteristic occupational modulations of the Cu6Sn5 structure arise with each type of dopants: a three-fold ordered structure for Ni accompanied by a displacive modulation of the constituent atoms, but a two-fold layer-like structure for Au. Moreover, with a high content of Ni, the unit cell of η-Cu6Sn5 is found to contract along its hexagonal ah axis relative to the Ni-dilute case, but anisotropically expands the ch axis in a bimodal fashion; in contrast, the effect of Au appears to be of an isotropically expanding nature.

Low-temperature phase transition and magnetic properties of K3YbSi2O7

The new ambient-temperature hexagonal (space group P63 /mmc) polymorph of tripotassium ytterbium(III) disilicate (β-K3YbSi2O7) has been synthesized by the high-temperature flux method and subsequently structurally characterized. In the course of the temperature-dependent single-crystal diffraction experiments, a phase transformation of β-K3YbSi2O7 to a novel low-temperature orthorhombic phase (β′-K3YbSi2O7, space group Cmcm) has been observed at about 210 K. β-K3YbSi2O7 is isostructural with K3ErSi2O7, whereas β′-K3YbSi2O7 adopts a new type of structure. Both compounds can be built up from a regular alternation of layers of two types, which are parallel to the (001) plane. In the octahedral layer, YbO6 octahedra are isolated and linked by K1O6+3 polyhedra. The second, slightly thicker sorosilicate layer is formed by a combination of Si2O7 dimers and K2O6+3 polyhedra. The boundary between the layers is a pseudo-kagome oxide sheet based on 3.6.3.6 meshes. The phase transition is due to a tilt of the two SiO4 tetrahedra forming a single dimer which induces a decrease of the Si—O—Si angle between bridging Si—O bonds from 180° (dictated by symmetry in space group P63/mmc) to ≃164°. Magnetic characterization indicates that K3YbSi2O7 remains paramagnetic down to 2 K, showing no apparent influence of the phase transformation on its magnetic properties. Analysis of the magnetization data revealed the positions of the three lowest crystal field levels of the Yb3+ cations, as well as the corresponding projections of their angular momentum on the direction of the magnetic field.

γ-GeSe: A New Hexagonal Polymorph from Group IV–VI Monochalcogenides

The family of group IV-VI monochalcogenides has an atomically puckered layered structure, and their atomic bond configuration suggests the possibility for the realization of various polymorphs. Here, we report the synthesis of the first hexagonal polymorph from the family of group IV-VI monochalcogenides, which is conventionally orthorhombic. Recently predicted four-atomic-thick hexagonal GeSe, so-called γ-GeSe, is synthesized and clearly identified by complementary structural characterizations, including elemental analysis, electron diffraction, high-resolution transmission electron microscopy imaging, and polarized Raman spectroscopy. The electrical and optical measurements indicate that synthesized γ-GeSe exhibits high electrical conductivity of 3 × 105 S/m, which is comparable to those of other two-dimensional layered semimetallic crystals. Moreover, γ-GeSe can be directly grown on h-BN substrates, demonstrating a bottom-up approach for constructing vertical van der Waals heterostructures incorporating γ-GeSe. The newly identified crystal symmetry of γ-GeSe warrants further studies on various physical properties of γ-GeSe.