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Emerging Frontiers in Metastable Crystalline Solids
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Emerging Frontiers in Metastable Crystalline Solids

A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Inorganic Chemistry".

Deadline for manuscript submissions: closed (31 December 2022) | Viewed by 15959

Special Issue Editor

Prof. Dr. Paul A. Maggard

E-Mail Website
Guest Editor
Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, USA
Interests: solid state chemistry; metal oxides; metal oxide/organic hybrids; aerogels; nanochemistry; semiconductors; flux synthesis; hydrothermal synthesis; photocatalysis; and photoelectrochemistry

Special Issue Information

Dear Colleagues,

Current research into addressing major bottlenecks in solid-state chemistry are shifting toward the challenges involved in the investigation of new crystalline solids that may reside near to or over the edge of thermodynamic stability. Many of the most highly desired solid-state compounds, as predicted in silico, have presented severe synthesis and characterization challenges, especially because the frequency with which many of these phases have limited or no thermodynamic stability, and thus require techniques to kinetically stabilize them. Metastable solids can often show (or are predicted to show) superior physical properties. Their importance to the future attainment of exceptional, technologically-desirable properties has been growing in many diverse research fields, for uses such as solar energy conversion, ferroelectric devices, and quantum information processing. Addressing the fundamental scientific challenges of kinetically stabilizing, characterizing, and measuring the physical properties of metastable crystalline solids represents the main overarching theme of this Special Issue.

The goal of the present Special Issue of Molecules is to provide a forum for the discussion of the recent developments in the research area of metastable solids, including the syntheses, physical properties, and theoretical results.  This invitation encompasses original research articles, reviews, and commentaries.

Prof. Dr. Paul A. Maggard
Guest Editor

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Keywords

  • metastability
  • solid-state chemistry
  • crystalline structure
  • thermodynamics
  • kinetic stabilization
  • electronic structure

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Published Papers (7 papers)

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Research

19 pages, 3518 KiB  
Article
Facile Solvent-Free Synthesis of Metal Thiophosphates and Their Examination as Hydrogen Evolution Electrocatalysts
by Nathaniel Coleman, Jr., Ishanka A. Liyanage, Matthew D. Lovander, Johna Leddy and Edward G. Gillan
Molecules 2022, 27(16), 5053; https://doi.org/10.3390/molecules27165053 - 9 Aug 2022
Cited by 7 | Viewed by 2237
Abstract
The facile solvent-free synthesis of several known metal thiophosphates was accomplished by a chemical exchange reaction between anhydrous metal chlorides and elemental phosphorus with sulfur, or combinations of phosphorus with molecular P2S5 at moderate 500 °C temperatures. The crystalline products [...] Read more.
The facile solvent-free synthesis of several known metal thiophosphates was accomplished by a chemical exchange reaction between anhydrous metal chlorides and elemental phosphorus with sulfur, or combinations of phosphorus with molecular P2S5 at moderate 500 °C temperatures. The crystalline products obtained from this synthetic approach include MPS3 (M = Fe, Co, Ni) and Cu3PS4. The successful reactions benefit from thermochemically favorable PCl3 elimination. This solvent-free route performed at moderate temperatures leads to mixed anion products with complex heteroatomic anions, such as P2S64−. The MPS3 phases are thermally metastable relative to the thermodynamically preferred separate MPx/ MSy and more metal-rich MPxSy phases. The micrometer-sized M-P-S products exhibit room-temperature optical and magnetic properties consistent with isolated metal ion structural arrangements and semiconducting band gaps. The MPS3 materials were examined as electrocatalysts in hydrogen evolution reactions (HER) under acidic conditions. In terms of HER activity at lower applied potentials, the MPS3 materials show the trend of Co > Ni >> Fe. Extended time constant potential HER experiments show reasonable HER stability of ionic and semiconducting MPS3 (M = Co, Ni) structures under acidic reducing conditions. Full article
(This article belongs to the Special Issue Emerging Frontiers in Metastable Crystalline Solids)
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15 pages, 10455 KiB  
Article
Microwave-Assisted Solution Synthesis of Metastable Intergrowth of AgInS2 Polymorphs
by Adedoyin N. Adeyemi, Rae Ann Earnest, Tori Cox, Oleg I. Lebedev and Julia V. Zaikina
Molecules 2022, 27(6), 1815; https://doi.org/10.3390/molecules27061815 - 10 Mar 2022
Cited by 2 | Viewed by 2462
Abstract
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 [...] Read more.
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 I4¯2d, 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 I4¯2d (1.86 eV) and orthorhombic Pna21 (1.98 eV) polymorphs. Full article
(This article belongs to the Special Issue Emerging Frontiers in Metastable Crystalline Solids)
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12 pages, 16034 KiB  
Article
Metastable Kitaev Magnets
by Faranak Bahrami, Mykola Abramchuk, Oleg Lebedev and Fazel Tafti
Molecules 2022, 27(3), 871; https://doi.org/10.3390/molecules27030871 - 27 Jan 2022
Cited by 8 | Viewed by 2590
Abstract
Nearly two decades ago, Alexei Kitaev proposed a model for spin-1/2 particles with bond-directional interactions on a two-dimensional honeycomb lattice which had the potential to host a quantum spin-liquid ground state. This work initiated numerous investigations to design and synthesize [...] Read more.
Nearly two decades ago, Alexei Kitaev proposed a model for spin-1/2 particles with bond-directional interactions on a two-dimensional honeycomb lattice which had the potential to host a quantum spin-liquid ground state. This work initiated numerous investigations to design and synthesize materials that would physically realize the Kitaev Hamiltonian. The first generation of such materials, such as Na2IrO3, α-Li2IrO3, and α-RuCl3, revealed the presence of non-Kitaev interactions such as the Heisenberg and off-diagonal exchange. Both physical pressure and chemical doping were used to tune the relative strength of the Kitaev and competing interactions; however, little progress was made towards achieving a purely Kitaev system. Here, we review the recent breakthrough in modifying Kitaev magnets via topochemical methods that has led to the second generation of Kitaev materials. We show how structural modifications due to the topotactic exchange reactions can alter the magnetic interactions in favor of a quantum spin-liquid phase. Full article
(This article belongs to the Special Issue Emerging Frontiers in Metastable Crystalline Solids)
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17 pages, 4793 KiB  
Article
Investigation of Metastable Low Dimensional Halometallates
by Navindra Keerthisinghe, Matthew S. Christian, Anna A. Berseneva, Gregory Morrison, Vladislav V. Klepov, Mark D. Smith and Hans-Conrad zur Loye
Molecules 2022, 27(1), 280; https://doi.org/10.3390/molecules27010280 - 3 Jan 2022
Cited by 2 | Viewed by 2150
Abstract
The solvothermal synthesis, structure determination and optical characterization of five new metastable halometallate compounds, [1,10-phenH][Pb3.5I8] (1), [1,10-phenH2][Pb5I12]·(H2O) (2), [1,10-phen][Pb2I4] (3), [1,10-phen] [...] Read more.
The solvothermal synthesis, structure determination and optical characterization of five new metastable halometallate compounds, [1,10-phenH][Pb3.5I8] (1), [1,10-phenH2][Pb5I12]·(H2O) (2), [1,10-phen][Pb2I4] (3), [1,10-phen]2[Pb5Br10] (4) and [1,10-phenH][SbI4]·(H2O) (5), are reported. The materials exhibit rich structural diversity and exhibit structural dimensionalities that include 1D chains, 2D sheets and 3D frameworks. The optical spectra of these materials are consistent with bandgaps ranging from 2.70 to 3.44 eV. We show that the optical behavior depends on the structural dimensionality of the reported materials, which are potential candidates for semiconductor applications. Full article
(This article belongs to the Special Issue Emerging Frontiers in Metastable Crystalline Solids)
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12 pages, 3064 KiB  
Article
Study of Integer Spin S = 1 in the Polar Magnet β-Ni(IO3)2
by Ebube E. Oyeka, Michał J. Winiarski and Thao T. Tran
Molecules 2021, 26(23), 7210; https://doi.org/10.3390/molecules26237210 - 28 Nov 2021
Cited by 5 | Viewed by 1951
Abstract
Polar magnetic materials exhibiting appreciable asymmetric exchange interactions can potentially host new topological states of matter such as vortex-like spin textures; however, realizations have been mostly limited to half-integer spins due to rare numbers of integer spin systems with broken spatial inversion lattice [...] Read more.
Polar magnetic materials exhibiting appreciable asymmetric exchange interactions can potentially host new topological states of matter such as vortex-like spin textures; however, realizations have been mostly limited to half-integer spins due to rare numbers of integer spin systems with broken spatial inversion lattice symmetries. Here, we studied the structure and magnetic properties of the S = 1 integer spin polar magnet β-Ni(IO3)2 (Ni2+, d8, 3F). We synthesized single crystals and bulk polycrystalline samples of β-Ni(IO3)2 by combining low-temperature chemistry techniques and thermal analysis and characterized its crystal structure and physical properties. Single crystal X-ray and powder X-ray diffraction measurements demonstrated that β-Ni(IO3)2 crystallizes in the noncentrosymmetric polar monoclinic structure with space group P21. The combination of the macroscopic electric polarization driven by the coalignment of the (IO3)− trigonal pyramids along the b axis and the S = 1 state of the Ni2+ cation was chosen to investigate integer spin and lattice dynamics in magnetism. The effective magnetic moment of Ni2+ was extracted from magnetization measurements to be 3.2(1) µB, confirming the S = 1 integer spin state of Ni2+ with some orbital contribution. β-Ni(IO3)2 undergoes a magnetic ordering at T = 3 K at a low magnetic field, μ0H = 0.1 T; the phase transition, nevertheless, is suppressed at a higher field, μ0H = 3 T. An anomaly resembling a phase transition is observed at T ≈ 2.7 K in the Cp/T vs. T plot, which is the approximate temperature of the magnetic phase transition of the material, indicating that the transition is magnetically driven. This work offers a useful route for exploring integer spin noncentrosymmetric materials, broadening the phase space of polar magnet candidates, which can harbor new topological spin physics. Full article
(This article belongs to the Special Issue Emerging Frontiers in Metastable Crystalline Solids)
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12 pages, 3287 KiB  
Article
A Metastable p-Type Semiconductor as a Defect-Tolerant Photoelectrode
by Zahirul Sohag, Shaun O’Donnell, Lindsay Fuoco and Paul A. Maggard
Molecules 2021, 26(22), 6830; https://doi.org/10.3390/molecules26226830 - 12 Nov 2021
Cited by 2 | Viewed by 2060
Abstract
A p-type Cu3Ta7O19 semiconductor was synthesized using a CuCl flux-based approach and investigated for its crystalline structure and photoelectrochemical properties. The semiconductor was found to be metastable, i.e., thermodynamically unstable, and to slowly oxidize at its surfaces upon [...] Read more.
A p-type Cu3Ta7O19 semiconductor was synthesized using a CuCl flux-based approach and investigated for its crystalline structure and photoelectrochemical properties. The semiconductor was found to be metastable, i.e., thermodynamically unstable, and to slowly oxidize at its surfaces upon heating in air, yielding CuO as nano-sized islands. However, the bulk crystalline structure was maintained, with up to 50% Cu(I)-vacancies and a concomitant oxidation of the Cu(I) to Cu(II) cations within the structure. Thermogravimetric and magnetic susceptibility measurements showed the formation of increasing amounts of Cu(II) cations, according to the following reaction: Cu3Ta7O19 + x/2 O2 → Cu(3−x)Ta7O19 + x CuO (surface) (x = 0 to ~0.8). With minor amounts of surface oxidation, the cathodic photocurrents of the polycrystalline films increase significantly, from <0.1 mA cm−2 up to >0.5 mA cm−2, under visible-light irradiation (pH = 6.3; irradiant powder density of ~500 mW cm−2) at an applied bias of −0.6 V vs. SCE. Electronic structure calculations revealed that its defect tolerance arises from the antibonding nature of its valence band edge, with the formation of defect states in resonance with the valence band, rather than as mid-gap states that function as recombination centers. Thus, the metastable Cu(I)-containing semiconductor was demonstrated to possess a high defect tolerance, which facilitates its high cathodic photocurrents. Full article
(This article belongs to the Special Issue Emerging Frontiers in Metastable Crystalline Solids)
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10 pages, 4432 KiB  
Article
Synthesis of Metastable Ternary Pd-W and Pd-Mo Transition Metal Carbide Nanomaterials
by James M. Thode, Daniel P. Harris, Cheng Wan and Brian M. Leonard
Molecules 2021, 26(21), 6650; https://doi.org/10.3390/molecules26216650 - 2 Nov 2021
Cited by 1 | Viewed by 1652
Abstract
Research and catalytic testing of platinum group transition metal carbides have been extremely limited due to a lack of reliable, simple synthetic approaches. Powder samples have been reported to phase separately above 1%, and only thin-film samples have been reported to have appreciable [...] Read more.
Research and catalytic testing of platinum group transition metal carbides have been extremely limited due to a lack of reliable, simple synthetic approaches. Powder samples have been reported to phase separately above 1%, and only thin-film samples have been reported to have appreciable amounts of precious metal doping. Herein, we demonstrated, through the simple co-precipitation of Pd and W or Mo precursors and their subsequent annealing, the possibility to readily form ternary carbide powders. During the investigation of the Pd-W ternary system, we discovered a new hexagonal phase, (PdW)2C, which represents the first non-cubic Pd ternary carbide. Additionally, the solubility of Pd in the Pd-W-C and Pd-Mo-C systems was increased to 24 and 32%, respectively. As a potential application, these new materials show an enhanced activity for the methanol oxidation reaction (MOR) compared to industrial Pd/C. Full article
(This article belongs to the Special Issue Emerging Frontiers in Metastable Crystalline Solids)
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