Ba₂HgS₅A Molecular Trisulfide Salt with Dumbbell-like (HgS₂)^2– Ions

Ba₂HgS₅ was synthesized by cooling a molten mixture of BaS, HgS, and elemental sulfur. It crystallizes in the orthorhombic <i>Pnma</i> space group with <i>a</i> = 12.190(2) Å, <i>b</i> = 8.677(2) Å, <i>c</i> = 8.371(2) Å, and <i>d</i>_calc = 4.77 g cm^–3. Its crystal structure consists of isolated dumbbell-shaped (HgS₂)^2– and v-shaped S₃^2– ions. These molecular anions are charge-balanced by Ba²⁺ cations. Raman spectroscopy shows three strong bands originating from symmetric, asymmetric, and bending vibrational modes of the S₃^2– ions. X-ray photoelectron spectroscopic analysis confirms the presence of the trisulfide species. Ba₂HgS₅ has a bandgap of ∼2.4 eV. Electronic band structure calculations show that the bandgap is defined essentially by the p-orbitals of the sulfur atoms of the S₃^2– group

Antagonism between Spin–Orbit Coupling and Steric Effects Causes Anomalous Band Gap Evolution in the Perovskite Photovoltaic Materials CH₃NH₃Sn_1–<i>x</i>Pb_<i>x</i>I₃

Halide perovskite solar cells are a recent ground-breaking development achieving power conversion efficiencies exceeding 18%. This has become possible owing to the remarkable properties of the AMX₃ perovskites, which exhibit unique semiconducting properties. The most efficient solar cells utilize the CH₃NH₃PbI₃ perovskite whose band gap, <i>E</i>_g, is 1.55 eV. Even higher efficiencies are anticipated, however, if the band gap of the perovskite can be pushed deeper in the near-infrared region, as in the case of CH₃NH₃SnI₃ (<i>E</i>_g = 1.3 eV). A remarkable way to improve further comes from the CH₃NH₃Sn_1–<i>x</i>Pb_<i>x</i>I₃ solid solution, which displays an anomalous trend in the evolution of the band gap with the compositions approaching <i>x</i> = 0.5 displaying lower band gaps (<i>E</i>_g ≈ 1.1 eV) than that of the lowest of the end member, CH₃NH₃SnI₃. Here we use first-principles calculations to show that the competition between the spin–orbit coupling (SOC) and the lattice distortion is responsible for the anomalous behavior of the band gap in CH₃NH₃Sn_1–<i>x</i>Pb_<i>x</i>I₃. SOC causes a linear reduction as <i>x</i> increases, while the lattice distortion causes a nonlinear increase due to a composition-induced phase transition near <i>x</i> = 0.5. Our results suggest that electronic structure engineering can have a crucial role in optimizing the photovoltaic performance

Semiconducting [(Bi₄Te₄Br₂)(Al₂Cl_6–<i>x</i>Br_<i>x</i>)]Cl₂ and [Bi₂Se₂Br](AlCl₄): Cationic Chalcogenide Frameworks from Lewis Acidic Ionic Liquids

Lewis acidic organic ionic liquids provide a novel synthetic medium to prepare new semiconducting chalcogenides, [(Bi₄Te₄Br₂)­(Al₂Cl_5.46Br_0.54)]­Cl₂ (<b>1</b>) and [Bi₂Se₂Br]­(AlCl₄) (<b>2</b>). Compound <b>1</b> features a cationic [(Bi₄Te₄Br₂)­(Al₂Cl_5.46Br_0.54)]²⁺ three-dimensional framework, while compound <b>2</b> consists of cationic layers of [Bi₂Se₂Br]²⁺. Spectroscopically measured band gaps of <b>1</b> and <b>2</b> are ∼0.6 and ∼1.2 eV, respectively. Thermoelectric power measurements of single crystals of <b>1</b> indicate an n-type semiconductor

Possible <i>n–</i>type carrier sources in In₂O₃(ZnO)_k

Homologous compounds with the formula In₂O₃(ZnO)_k, where k is an integer, have potential applications as transparent conducting oxides and high temperature thermoelectric materials. In this study, we focus on the defect properties. Using the <i>k</i> = 3 phase as a prototype, we calculate with the first-principles method the defect formation energies and transition levels of the most probable <i>n</i>-type carrier producers, which include oxygen vacancy (V_O), indium antisite on zinc (In_Zn), indium interstitial (In_i), and zinc interstitial (Zn_i). The site-preference of these defects has been explored by comparing the total energies of defects at different sites. Under the <i>n</i>-type environment, In_Zn has a low formation energy and meanwhile a transition energy level close to the conduction band minimum (CBM); V_O also has a lower formation energy, however a deep transition energy level in the band gap; the cation interstitials have high formation energies, although their defect transition energy levels are quite shallow. Besides, we find that V_O and In_Zn tend to form a defect complex when the two isolated defects take the nearest-neighboring atomic sites in the same <i>ab</i>-plane. We conclude that In_Zn and its related defect-complex are the possible <i>n</i>–type carrier sources in In₂O₃(ZnO)_k. Besides, we found that V_O has a significant site-preference, which can modify the site-preference of In_Zn by forming defect-complexes. This may lead to high anisotropy in relaxation time, and then the experimentally reported strong anisotropy in electrical conductivities in In₂O₃(ZnO)₅

Semiconducting [(Bi₄Te₄Br₂)(Al₂Cl_6–<i>x</i>Br_<i>x</i>)]Cl₂ and [Bi₂Se₂Br](AlCl₄): Cationic Chalcogenide Frameworks from Lewis Acidic Ionic Liquids

Lewis acidic organic ionic liquids provide a novel synthetic medium to prepare new semiconducting chalcogenides, [(Bi₄Te₄Br₂)­(Al₂Cl_5.46Br_0.54)]­Cl₂ (<b>1</b>) and [Bi₂Se₂Br]­(AlCl₄) (<b>2</b>). Compound <b>1</b> features a cationic [(Bi₄Te₄Br₂)­(Al₂Cl_5.46Br_0.54)]²⁺ three-dimensional framework, while compound <b>2</b> consists of cationic layers of [Bi₂Se₂Br]²⁺. Spectroscopically measured band gaps of <b>1</b> and <b>2</b> are ∼0.6 and ∼1.2 eV, respectively. Thermoelectric power measurements of single crystals of <b>1</b> indicate an n-type semiconductor

LiPbSb₃S₆: A Semiconducting Sulfosalt with Very Low Thermal Conductivity

The new semiconductor LiPbSb₃S₆ crystallizes in the space group <i>P</i>2₁/<i>c</i>. The structure is a member of the lillianite homologous series and is composed of layers of PbS archetype Sb/Li–S separated by trigonal-prismatic-coordinated Pb/Li. Electronic band structure calculations indicate an indirect band gap, with direct gaps lying very close in energy. LiPbSb₃S₆ has one of the lowest thermal conductivities seen in a crystalline material, ∼0.24 W m^–1 K^–1 at room temperature, and a high resistivity, ∼4 × 10⁹ Ω·cm, and exhibits strong light absorption with a nearly direct band gap of 1.6 eV

LiPbSb₃S₆: A Semiconducting Sulfosalt with Very Low Thermal Conductivity

The new semiconductor LiPbSb₃S₆ crystallizes in the space group <i>P</i>2₁/<i>c</i>. The structure is a member of the lillianite homologous series and is composed of layers of PbS archetype Sb/Li–S separated by trigonal-prismatic-coordinated Pb/Li. Electronic band structure calculations indicate an indirect band gap, with direct gaps lying very close in energy. LiPbSb₃S₆ has one of the lowest thermal conductivities seen in a crystalline material, ∼0.24 W m^–1 K^–1 at room temperature, and a high resistivity, ∼4 × 10⁹ Ω·cm, and exhibits strong light absorption with a nearly direct band gap of 1.6 eV

An Unusual Crystal Growth Method of the Chalcohalide Semiconductor, β‑Hg₃S₂Cl₂: A New Candidate for Hard Radiation Detection

We assess the mercury chalcohalide compound, β-Hg₃S₂Cl₂, as a potential semiconductor material for X-ray and γ-ray detection. It has a high density (6.80 g/cm³) and wide band gap (2.56 eV) and crystallizes in the cubic <i>Pm</i>3̅<i>n</i> space group with a three-dimensional structure comprised of [Hg₁₂S₈] cubes with Cl atoms located within and between the cubes, featuring a trigonal pyramidal SHg₃ as the main building block. First-principle electronic structure calculations at the density functional theory level predict that the compound has closely lying indirect and direct band gaps. We have successfully grown transparent, single crystals of β-Hg₃S₂Cl₂ up to 7 mm diameter and 1 cm long using a new approach by the partial decomposition of the quaternary Hg₃Bi₂S₂Cl₈ compound followed by the formation of β-Hg₃S₂Cl₂ and an impermeable top layer, all happening in situ during vertical Bridgman growth. The decomposition process was optimized by varying peak temperatures and temperature gradients using a 2 mm/h translation rate of the Bridgman technique. Formation of the quaternary Hg₃Bi₂S₂Cl₈ followed by its partial decomposition into β-Hg₃S₂Cl₂ was confirmed by in situ temperature-dependent synchrotron powder diffraction studies. The single crystal samples obtained had resistivity of 10¹⁰ Ω·cm and mobility-lifetime products of electron and hole carriers of 1.4(4) × 10^–4 cm²/V and 7.5(3) × 10^–5 cm²/V, respectively. Further, an appreciable Ag X-ray photoconductivity response was observed showing the potential of β-Hg₃S₂Cl₂ as a hard radiation detector material

CsSnI₃: Semiconductor or Metal? High Electrical Conductivity and Strong Near-Infrared Photoluminescence from a Single Material. High Hole Mobility and Phase-Transitions

CsSnI₃ is an unusual perovskite that undergoes complex displacive and reconstructive phase transitions and exhibits near-infrared emission at room temperature. Experimental and theoretical studies of CsSnI₃ have been limited by the lack of detailed crystal structure characterization and chemical instability. Here we describe the synthesis of pure polymorphic crystals, the preparation of large crack-/bubble-free ingots, the refined single-crystal structures, and temperature-dependent charge transport and optical properties of CsSnI₃, coupled with <i>ab initio</i> first-principles density functional theory (DFT) calculations. <i>In situ</i> temperature-dependent single-crystal and synchrotron powder X-ray diffraction studies reveal the origin of polymorphous phase transitions of CsSnI₃. The black orthorhombic form of CsSnI₃ demonstrates one of the largest volumetric thermal expansion coefficients for inorganic solids. Electrical conductivity, Hall effect, and thermopower measurements on it show p-type metallic behavior with low carrier density, despite the optical band gap of 1.3 eV. Hall effect measurements of the black orthorhombic perovskite phase of CsSnI₃ indicate that it is a p-type direct band gap semiconductor with carrier concentration at room temperature of ∼ 10¹⁷ cm^–3 and a hole mobility of ∼585 cm² V^–1 s^–1. The hole mobility is one of the highest observed among p-type semiconductors with comparable band gaps. Its powders exhibit a strong room-temperature near-IR emission spectrum at 950 nm. Remarkably, the values of the electrical conductivity and photoluminescence intensity increase with heat treatment. The DFT calculations show that the screened-exchange local density approximation-derived band gap agrees well with the experimentally measured band gap. Calculations of the formation energy of defects strongly suggest that the electrical and light emission properties possibly result from Sn defects in the crystal structure, which arise intrinsically. Thus, although stoichiometric CsSnI₃ is a semiconductor, the material is prone to intrinsic defects associated with Sn vacancies. This creates highly mobile holes which cause the materials to appear metallic

Cs₂M^IIM^IV₃Q₈ (Q = S, Se, Te): An Extensive Family of Layered Semiconductors with Diverse Band Gaps

Flame-melting rapid-cooling reactions were used to synthesize a number of pure phases of the Cs₂M^IIM^IV₃Q₈ family (M^II = Mg, Zn, Cd, Hg; M^IV = Ge, Sn; Q = S, Se, Te) whereas the more toxic members were synthesized using a traditional tube furnace synthesis. All Cs₂M^IIM^IV₃Q₈ compounds presented here crystallize in the noncentrosymmetric space group <i>P</i>2₁2₁2₁, except for Cs₂ZnGe₃S₈, which crystallizes in the centrosymmetric space group <i>P</i>2₁/<i>n</i>. The structures contain chains of corner-sharing M^IIQ₄ and M^IVQ₄ tetrahedra linked by edge-sharing M^IV₂Q₆ dimers to give a two-dimensional structure. All phases are structurally similar to the AM^IIIM^IVQ₄ (A = alkali metal, Tl; M^III = Al, Ga, In; M^IV = Si, Ge, Sn; Q = S, Se) phases; however, the members of this family have completely ordered M^II and M^IV sites as opposed to the occupational disorder of M^III and M^IV over all tetrahedral sites present in AM^IIIM^IVQ₄. The structural trends of the Cs₂M^IIM^IV₃Q₈ family are discussed, along with a systematic study of their optical properties. Density functional theory (DFT) electronic structure calculations were performed using the projector augmented wave method to further investigate the trends in the band gaps of the Cs₂M^IIM^IV₃Se₈ (M^II = Mg, Zn; M^IV = Ge, Sn) compounds. The experimental diffuse reflectance UV–vis spectroscopy results show that the Mg compounds have smaller band gaps than those containing Zn for both the Ge and the Sn families whereas the DFT calculations show the opposite trend. Cs₂HgSn₃Se₈ was studied as a representative example of this family using differential thermal analysis and melts congruently at 595 °C. Crystal growth of this compound using the Bridgman method resulted in a polycrystalline ingot from which plate crystals ∼2 mm × 3 mm could be cleaved. The band gap of the compounds varies from a narrow 1.07 eV for Cs₂ZnGe₃Te₈ to a wide 3.3 eV for Cs₂ZnGe₃S₈ and Cs₂CdGe₃S₈ making this family a potentially useful source of materials for a variety of electronic applications. Cs₂HgSn₃Se₈ crystals exhibit photoconductivity response where the photoexcited electron and hole show mobility-lifetime products on the order of 3.69 × 10^–5 cm²/V and (<i>μτ</i>)_h∥ = 7.78 × 10^–5 cm²/V, respectively