Born Criteria Research Articles

Graphsene as a novel porous two-dimensional carbon material for enhanced oxygen reduction electrocatalysis

Graphene allotropes with varied carbon configurations have attracted significant attention for their unique properties and chemical activities. This study introduces a novel two-dimensional carbon-based material, termed Graphsene (GrS), through theoretical study. Comprising tetra-, penta-, and dodeca-carbon rings, GrS’s cohesive energy calculations demonstrate its superior structural stability over existing graphene allotropes, including graphyne and graphdiyne families. Phonon dispersion analysis confirms GrS’s dynamic stability and its relatively low thermal conductivity. All calculated GrS elastic constants meet the Born criteria, ensuring mechanical stability. Ab-initio molecular dynamic simulations show GrS maintains its structure at 300 K. HSE06 calculations reveal a narrow electronic bandgap of 20 meV, with the electronic band structure featuring a highly anisotropic Dirac-like cone due to its intrinsic structural anisotropy along armchair and zigzag directions. Notably, GrS is predicted to offer exceptional catalytic performance for the oxygen reduction reaction, favoring the four-electron reduction pathway with high selectivity under both acidic and alkaline conditions. This discovery opens promising avenues for developing metal-free catalyst materials in clean energy production.

A pressure tunable optical and thermoelectric properties of Rb2CuSbX6 (X = cl, Br, and I) by FPLAPW+lo method

The pressure dependencies of the structural, electrical, elastic, optical, and thermoelectric characteristics of the Halide double perovskites (HDPs) Rb2CuSbX6 (X = Cl, Br, and I) compounds were computed using the FP-LAPW method within the density functional theory framework, under high pressure conditions. HDPs received a lot of attention due to their potential application in optoelectronic and thermoelectric devices. The Perdew-Burke-Ernzerhof (PBEsol-GGA) is used to investigate the structural properties of selected compounds, while the modified Becke-Johnson (mBJ) functional is used to obtain a better agreement between the calculated optoelectronic and electronic properties and experimental observations. The Born criteria and formation energy are explored for the stability of all selected HDPs. The elastic constants of cubic symmetry are investigated to determine the difference between ductile and brittle nature, anisotropy, and bulk modulus. The bandgaps for Rb2CuSbCl6, Rb2CuSbBr6, and Rb2CuSbI6 are 1.08 eV, 0.72 eV, and 0.33 eV, respectively. Decrease band gap with increase in pressure. The density of states plots under zero pressure exhibit a combination of ionic and covalent bonding, whereas an increase in pressure results in a reinforcement of covalent bonding. The absorption spectra are observed in the infrared region. All HDPs were further analyzed with respect to optical absorption, refractive index, and dielectric constants for the energy range 0–10 eV. Additionally, Boltzmann classical theory explains the lower lattice temperature, greater Seebeck coefficient, thermal conductivity, and electrical conductivity of HDPs, making them for thermoelectric applications.

Computational study of stability, photovoltaic, and thermoelectric properties of new inorganic lead-free halide perovskites

Abstract Due to their rich and extraordinary properties, halide perovskites have gained attention over time for their applications in thermoelectric and solar cells. Here, several physical properties (stability, photovoltaic, and thermoelectric) of inorganic halide perovskites XZnI3 (X = Na, K, Rb, Cs) are predicted using the density functional theory (DFT) within the Wien2k code. The optimization of structural parameters has been calculated using PBE-GGA approach. The tolerance factor, Born criteria, phonon dispersion, and negative formation energy show the formation and stability of these studied materials in the ideal cubic structure. Additionally, the modified Becke-Johnson method is applied for optoelectronic and transport properties. All compounds exhibit the nature of indirect band gap semiconductors with better absorption in the visible and ultraviolet regions 10^{5} \textrm {cm}^{-1})$ ?> . The transport properties present high electrical conductivity, large Seebeck coefficient, and good (PF, ZT) factors for all these materials. Finally, all these properties of inorganic halide perovskites open up new possibilities for efficient applications in thermoelectric and solar cells.

A DFT insight into the physical features of alkaline based perovskite compounds AInBr3 (A = K, Rb)

Owing to their proper characteristic, simple halide perovskites became engaged over time for thermoelectric applications. In this work, several physical properties of simple halide perovskite materials AInBr3 (A = K, Rb) are predicted based on density functional theory (DFT) within the Wien2k code. The tolerance factor and negative formation energy demonstrate the stability and formation of these compounds, and the Born criteria shows their mechanical stability and inherent ductility. Notably, the modified Becke-Johnson potential (mBJ) was used to calculate the optoelectronic characteristics. The AInBr3 (A = K, Rb) perovskites present semiconducting aspects. Analyses of their optical characteristics, including refractive index, reflectivity, were performed. AInBr3 (A = K, Rb) transport properties were studied. The highest figure of merit was attained by AInBr3 (A = K, Rb) which had a high Seebeck coefficient and high electrical conductivity. Finally, all studied properties of simple halide perovskites give a new way for efficient applications in thermoelectric.

Revisiting Lindemann’s criterion from a minimal viscosity perspective

A revisiting of the Lindemann criterion under the recently established minimal viscosity formulation is proposed which uncovers intriguing insights into the melting process. The approach suggests that melting involves competition between the electron rest energy and the thermal one caused by temperature increase when considering a defect-free crystal structure. However, when accounting for the role of vacancies in the melting process, the analysis suggests that the key competing factors are the thermal energy and the one needed for vacancy migration. The estimated value of the Lindemann constant, in this case, is close to the reported values and with recent simulations studying the correlation between Born and Lindemann melting criteria.

Theoretical investigations on the structural, elastic and thermodynamic properties of dysprosium hafnate

The lattice structure and elastic properties of Dy2Hf2O7 at 0 K and 0–40 GPa are calculated by using first-principles methods. The results show that the calculated structural parameters are in good agreement with the experimental values. The elastic constants increase with the increase of pressure, and all the elastic constants satisfy the Born criterion, that is, the Dy2Hf2O7 crystal is kinetically stable within the range of the calculated conditions. In addition, the thermodynamic properties of Dy2Hf2O7 crystal at different temperatures (0–2000 K) and pressures (0–40 GPa) are calculated by using the quasi-harmonic Debye model, and the temperature-pressure dependence of thermodynamic properties, such as the heat capacity, coefficient of thermal expansion, and Debye temperatures, is obtained and compared with the experimental values. We also obtain the melting point and hardness of Dy2Hf2O7 compound by using empirical formulas in the literature.

An in-depth investigation of lead-free KGeCl3 perovskite solar cells employing optoelectronic, thermomechanical, and photovoltaic properties: DFT and SCAPS-1D frameworks.

Potassium germanium chloride (KGeCl3) has emerged as a promising contender for use as an absorber material for lead-free perovskite solar cells (PSCs), offering significant potential in this domain. In this study, we conducted a density functional theory (DFT) investigation to analyze and assess the structural, electronic, thermomechanical, and optical characteristics of the cubic KGeCl3 absorber. The positive phonon dispersion curve confirmed the dynamical stability of KGeCl3. The elastic constant satisfied the Born criteria, validating the mechanical stability and ductility of solid KGeCl3. The electronic band structure and density of states (DOS) confirmed that the KGeCl3 material is a semiconductor with a direct band gap of 0.754 eV (GGA) and 0.803 eV (mGGA-RSCAN). The study identified key optical parameters, including absorption, conductivity, reflectivity, dielectric function, refractive index, and loss function, revealing the potential suitability of KGeCl3 for solar applications. The Helmholtz free energy (F), internal energy (E), entropy (S), and specific heat capacity (Cv) are computed based on the phonon density of states. Additionally, we investigated twenty-four configurations comprising different combinations of electron transport layers (ETLs) and hole transport layers (HTLs) in SCAPS-1D software. For this purpose, ETLs such as Ws2, ZnSe, PCBM, and C60 and HTLs such as CBTS, CdTe, CFTS, Cu2O, P3HT, and PEDOT:PSS are employed. The highlighted structure, ITO/CBTS/KGeCl3/Ws2/Ni, demonstrates remarkable performance with an efficiency of 22.01%, a Voc of 0.6799 V, a Jsc of 41.439 mA cm-2, and a FF of 78.12%. To analyze photovoltaic (PV) performance, we chose the top four solar cell (SC) configurations. Moreover, a comprehensive examination was conducted to assess the impact of various factors, including the thickness of different layers, capacitance, Mott-Schottky behavior, series and shunt resistance, temperature, and generation-recombination rates, as well as J-V (current-voltage density) and quantum efficiency (QE) characteristics.

A DFT study of structural, electronic, mechanical, phonon, thermodynamic, and H2 storage properties of lead-free perovskite hydride MgXH3(X=Cr, Fe, Mn)

Perovskite hydride materials have gained significant attention for their potential in hydrogen storage, a key component of renewable energy systems. In this study, we used density functional theory (DFT) to investigate the structural, vibrational, electronic, mechanical, thermodynamic, and hydrogen storage properties of MgXH3 (X = Cr, Fe, Mn) hydrides. The energy volume curves were used to determine optimized lattice parameters of MgCrH3(3.4590 Å), MgFeH3(3.0284 Å), and MgMnH3(3.3485 Å). The Born criteria for elastic constant (Cij) showed that the hydrides are mechanically stable, whereas the phonon dispersion curves showed their dynamic stability. The calculated electronic properties revealed the metallic nature of hydrides under investigation. The Cauchy pressure (Cp) revealed the brittle and ductile behavior. The calculated gravimetric hydrogen storage capacity of MgCrH3, MgFeH3, and MgMnH3 hydrides was 3.771, 3.606, and 3.643 wt% respectively. The electronic and thermodynamic properties suggest that MgXH3 hydrides can conduct electrical as well as thermal energy. Our results provide insight into the potential of MgXH3(X = Cr, Fe, Mn) perovskite hydride material for hydrogen storage application.

Ab initio insight into the physical properties of MgXH3 (X = Co, Cu, Ni) lead-free perovskite for hydrogen storage application.

Renewable energy systems are vital for a sustainable future, where solid-state hydrogen storage can play a crucial role. Perovskite hydride materials have attracted the scientific community for hydrogen storage applications. The current work focuses on the theoretical study using density functional theory (DFT) to evaluate the characteristics of MgXH3 (X = Co, Cu, Ni) hydrides. The structural, vibrational, electronic, mechanical, thermodynamic, and hydrogen storage properties of these hydrides were investigated. The equilibrium lattice parameters were calculated using the Birch-Murnaghan equation of state-to-energy volume curves. The elastic constants (Cij) and relevant parameters, such as Born criteria, were calculated to confirm the mechanical stability of the hydrides. The Cauchy pressure (Cp) revealed brittle or ductile behavior. The outcomes of the Pugh ratio, Poisson ratio, and anisotropy were also calculated and discussed. The absence of negative lattice vibrational frequencies in phonon dispersion confirmed the lattice's dynamic stability. The heat capacity curves of thermodynamic properties revealed that hydrides can conduct thermal energy. The metallic character and ample interatomic distances of hydrides were confirmed by the band structure and population analysis, which confirmed that hydrides can conduct electrical energy and adsorb hydrogen. The density of state (DOS) and partial DOS unveiled the role of specific atoms in the DOS of the crystal. The calculated gravimetric hydrogen storage capacity of MgCoH3, MgCuH3, and MgNiH3 hydrides was 3.64, 3.32, and 3.49wt%, respectively. Our results provide a deeper understanding of its potential for hydrogen storage applications through a detailed analysis of MgXH3 (X = Co, Cu, Ni) perovskite hydride material.

DFT calculations of optoelectronic and thermoelectric properties of K2NaTlX6 (X = Cl, Br, I) halide double perovskites for energy harvesting applications

The DFT approach was employed to investigate the mechanical, optical and thermoelectric properties of double perovskites (DPs) compounds K2NaTlX6 (X = Cl, Br, I). PBEsol-GGA approximation along with birch-Murnaghan equation is used to calculate the lattice constant, other structural and ground state parameters. The structural, thermodynamic and mechanical stability of these compounds was demonstrated by computing tolerance factor, formation energy and Born criteria. Poisson and Pugh ratio are analyzed to describe the brittle or ductile nature of these studied double perovskites compounds. The anion Cl, Br and I-based double perovskites exhibited direct bandgap as determined from band structure calculations. The study further examined the optical absorption and dielectric constant of the compounds across the energy range 0–10 eV confirming their ability to absorb light in the infrared to visible spectrum. Furthermore, the suitability of the studied double perovskites for thermoelectric applications was assessed using BoltzTraP coding. The Seebeck coefficient, electric conductivity and figure of merit were analyzed, suggesting that these compounds hold promise as viable candidates for thermoelectric applications.

Investigation of mechanical, optoelectronic, and thermoelectric properties of double perovksites Li2CuBiZ6 (Z = Cl, Br, I) for solar cell applications

Due to their potential applications for optoelectronic and thermoelectric devices, Li-based double perovskites (DPs) have been widely investigated. In current article, we theoretically examined the mechanical, optoelectronic and thermoelectric features of DPs Li2CuBiZ6 (Z = Cl, Br, I). We employ the generalized gradient approximation (PBEsol) for examining the structural and elastic characteristics of these compounds, while the modified Becke-Johnson (mBJ) functional is used for obtaining an improved accord of the thermoelectric and electronic characteristics with experimental observations. For the stability of all DPs, the Born Criteria and formation energy are investigated. The cubic symmetry's elastic constants are examined to determine the difference between ductile and brittle nature, anisotropy, and Debye temperatures. The bandgaps are 2.0 eV, 1.7 eV, and 1.3 eV, for Li2CuBiCl6, Li2CuBiBr6, and Li2CuBiI6, respectively, as determined by the mBJ potential. All DPs were also examined regarding optical absorption, refractive index, and dielectric constants for the energy range 0–10 eV, ensuring the absorption among visible regions. Furthermore, all DPs are appropriate candidates for thermoelectric applications due to the lower lattice thermal, higher Seebeck coefficient, thermal conductivity, and Figure of merit (ZT), which are all elucidated by Boltzmann classical theory.

Investigating the phase transition and properties of CaSiN2 under pressure based on first-principles calculations

In this study, we apply first-principles calculations to examine the pressure-induced phase transformation of CaSiN2 in a range of pressure of 0–100 GPa. Its pressure-induced transitions at 1.3 GPa, 15.3 GPa, and 55.8 GPa followed the order of α- CaSiN2→β- CaSiN2→α- CaSiN2→γ- CaSiN2, for α- CaSiN2→β- CaSiN2, β- CaSiN2→α- CaSiN2, and α- CaSiN2→γ- CaSiN2, respectively. The stability of the phases of CaSiN2 was confirmed based on calculations of the Born criterion of elastic stability. Its behavior transitioned in the sequence of brittle (0–1.3 GPa) → ductile (1.3–55.8 GPa) → brittle (55.8–100 GPa). The structure of its projected orbital band reflected insulating behavior by CaSiN2 under a range of pressure of 0–55.8 GPa with a direct band gap, which transformed into metallic behavior by the γ- CaSiN2 phase under pressures higher than 55.8 GPa, due to a shift in energy to higher levels around the Γ point of the N p orbitals and Si p orbitals. The Si-N bonds in CaSiN2 were found to be covalent, while ionic bonding dominated the Ca-Si and Ca-N bonds in the range of pressure of 0–100 GPa. We also investigate and discuss its mechanical properties, Vickers hardness (Hv), average sound velocity vm, and Debye temperature (θD).

Computational insight into the fundamental physical properties of ternary ABCl3 chloroperovskites compounds using the DFT approach

In this research, the ternary non-centrosymmetric chloroperovskites compounds of the form ABCl3 (A = Rb and B = Be, Mg) are investigated extensively to predict the structural, mechanical, and optoelectronic properties with DFT incorporated in WIEN2K code. The crystalline structure of interested chloroperovskites is identified to be cubic, non-centrosymmetric, and stable. The elastic constants Cij, bulk modulus, criteria of Pugh ratio, and the Born criteria confirm the ductility and mechanical stability of ternary RbBeCl3 and RbMgCl3 materials. Electronic properties such as the band structures and density of states are examined with the most widely recognized TB-mBJ potential approximation. RbBeCl3 shows semiconducting behaviour with an indirect wide band gap energy of 3.74 eV from R-Γ symmetries points, while RbMgCl3 is assumed to be an insulator that possesses indirect wide band gap energy of 6.28 eV from R-Γ. It is identified that the ABCl3 (A = Rb and B = Be, Mg) non-centrosymmetric compounds change the behavior from wide band gap semiconductors to perfect insulators when the ‘B’ site in ABCl3 varies from ‘Be’ to ‘Mg’ element. In the electromagnetic range from 0 eV to 40 eV of incident photons energy, several parameters in optical properties that includes the dielectric function, refractive index, absorption coefficient, optical conductivity, extinction coefficient, and energy loss function are investigated for the quest of potential applications of interested non-centrosymmetric cubic systems in modern photovoltaic technologies. These outcomes may add inclusive understanding within ultraviolet ranges for photovoltaic applications.

First-principles investigations of arsenate doping into the ettringite lattice

Hazardous wastes are commonly immobilized using cementitious materials. Ettringite (Ca6Al2(OH)12(SO4)3∙26H2O) is one of the main components of hydrated cement and is known to stabilize anionic species. Thus, it is a promising material for the stabilization of arsenate (As(V)) in cementitious waste streams. However, the effects of As(V) incorporation into the ettringite structure on the structural properties of As and ettringite remain unclear. Herein, As(V) doping into the structure of ettringite was investigated based on the first-principles calculations using density functional theory (DFT), and the results suggested that the mechanism involving the substitution of AsO43− by SO42− was energetically favorable and that As doping increased the total bond order of ettringite, leading to a more stable structure. The elastic properties and anisotropy of ettringite and As-doped ettringite were also investigated using DFT calculations. The results showed that the mechanical properties of the As-doped ettringite structure were stable according to Born's criterion. Moreover, As doping increased the elastic constants and elastic modulus of ettringite but reduced its elastic anisotropy. These results provide valuable insights into the long-term stabilization of As species and their migration in cementitious wastes.

Mechanical, vibration, and optical properties of IrAl intermetallic compound via DFT calculations: high-pressure effect

In this study, the effect of pressure on the structural, elastic, anisotropy, electronic, phonon, and optical properties of the IrAl compound in B2 structure was investigated by the first-principles method. The lattice constant, volume, density, and bulk modulus parameters were compared with theoretical and experimental data at zero pressure, and their variations with pressure effect were also examined. The IrAl compound fullfills the Born criteria at all pressure values, indicating structural stability, as no instability is observed even with increasing pressure. The compound exhibits ductile properties according to Paugh ratio (B/G), Cauchy pressure (C12–C44, C’), and Poisson ratio (λ) criteria. Electronically metallic in nature, it has maintained this nature with pressure variation. The absence of negative frequencies in the phonon dispersion curve explains that the compound is dynamically stable. Mulliken Atom Populations analysis was used to understand the bond properties between Ir-Al atoms, focusing on charge transfer and ionicity. The complex dielectric function was employed to determine the optical properties and evaluate their variations under pressure. Finally, the effect of pressure on the physical and electronic properties of the IrAl compound useful for coating applications were evaluated.

Study of optoelectronic, thermoelectric, mechanical, and thermodynamic properties of Cs2NaTlX6 (X = Cl, Br, I) for energy harvesting

The inorganic double perovskites are remarkable materials for renewable energy which can be realized through solar cells and thermoelectric generators. Here, we have comprehensively elaborated the stability, mechanical, thermodynamic, optical and transport characteristics. The formation energy is computed to ensure thermodynamic validity and the tolerance factor is assessed for structural existence. The elastic constants satisfied the Born criteria and mechanical stability. Naviera's velocities have been used to study the Debye temperature and directional lattice conductivity. Modifying the band gap from 3.1 to 0.58 eV by halide ions (Cl to I) probes the distinct optoelectronic characteristics. Absorption bands, dispersion of light energy, refraction, and optical loss explain the optical characteristics. In addition, the above thermoelectric factors like conductivities, Seebeck effect, and performance are studied in the temperature range of 100–600 K. Large figure of merit and extremely low lattice vibration at room temperature indicate their significance for thermoelectric devices.

First-principles calculations to investigate mechanical, thermoelectric and optical performance of inorganic double perovskites Rb2AgAlZ6 (Z = Br, I) for energy harvesting

The clean and green energy is a merging technology of advanced era to achieve the demands of energy requirements. For this purpose, the double perovskites Rb2AgAlZ6 (Z = Br, I) are explored comprehensively in terms of optical, mechanical, thermoelectric, and thermodynamic characteristics. The stability of structures, elastic and thermodynamic have been ensured through tolerance factor, Born Criteria, and formation energy. The elastic constants of cubic symmetry are analyzed to distinguish ductile/brittle nature, anisotropy, Harness, minimum lattice thermal, conductivity, melting and Debye temperatures. The anions (Br, I) modified the band gaps (2.60, 1.08) eV which turned the absorption of light energy from ultraviolet to visible portion of spectrum. The absorption band of Rb2AgAsI6 in the energy range 1.7eV–3.4 eV is particularly significant for solar cells. The large Seebeck coefficient, and electrical conductivity while particularly low lattice thermal conductivity has been increased the figure of merit. The large figure of at low temperature open the new finding roots for realization of these DPs for thermoelectric and other energy applications.

Structural and thermal properties of H6Ta2O17

The structural, electronic, mechanical, and thermal properties of H6Ta2O17 are studied using density functional theory. The Helmholtz free energy is computed for various symmetry constraints on the H6Ta2O17 structure during lattice structure optimization and atomic configuration relax- ation. The Helmholtz free energies calculated from Γ-point phonon frequencies demonstrate a pref- erence for slightly lowered symmetry, preferring triclinic symmetry rather than the experimentally determined orthorhombic symmetry. Elastic constants are obtained for the symmetry-enforced (i.e., orthorhombic) structure and are used to calculate the structure’s mechanical and thermal properties including its bulk, shear, and Young’s moduli, Poisson ratio, Pugh ratio, longitudinal and shear sound velocities, and Debye temperature. Using the Born criteria, the symmetry-enforced structure is shown to be mechanically stable. The electronic band structure for H6Ta2O17 is pre- sented and found to exhibit a large indirect band gap.

Effects of Fe occupancy on physical properties of non-magnetic GeGa2O4 spinel oxide

This study investigated the structural, elastic, electronic, thermal, magnetic, and optical behavior of GeGa2O4, FeGa2O4, GeFe2O4, and FeFe2O4 spinel oxides based on ab-initio calculations. The results showed that the non-magnetic spinel oxide GeGa2O4 exhibited remarkable physical properties due to Fe occupancy at different lattice sites within the spinel lattice. The Fe containing spinel oxides (FeGa2O4, GeFe2O4, and FeFe2O4) were found to be stable in the magnetic phase in contrast to the host GeGa2O4 spinel oxide. The spin polarizations of FeGa2O4, GeFe2O4, and FeFe2O4 oxides were calculated as 98.5%, 98.4%, and 100%, respectively. Analysis of the elastic and mechanical properties showed that all of the spinel oxides followed the Born and Huang criteria for mechanical stability. The highest Debye temperature (θD = 663.77 K) was determined for the FeGa2O4 spinel oxide. For GeGa2O4, FeGa2O4, GeFe2O4, and FeFe2O4 oxides, the calculated structural parameters (cation–anion bond lengths and average values of tetrahedral and octahedral radii) predicted the following cation distributions among the A and B atomic sites: (Ge2+)[Ga23+]O4−2, (Fe2+)[Ga23+]O4−2, (Ge4+)[Fe22+]O4−2, and (Fe+2)[Fe23+]O4−2. The calculated electronic structures indicated that the host GeGa2O4 spinel oxide and FeFe2O4 spinel ferrite were semi-conducting and half-metallic ferromagnetic, respectively, whereas FeGa2O4 and GeFe2O4 oxides exhibited semi-metallic behavior. The GeFe2O4 spinel oxide was found to have the highest saturation magnetization (185.79 emu g−1) and total magnetic moment (8.26 ΣBfu−1). The calculated optical properties indicated the suitability of these oxides for use in optical filters and sensors. The electronic and magnetic characteristics of these spinel oxides with excellent thermal and elastic stability suggest that they have potential uses in diluted magnetic semiconductor devices.

First-Principles Study of Structural and Electronic Properties of Monolayer PtX2 and Janus PtXY (X, Y = S, Se, and Te) via Strain Engineering.

In this work, the structural parameters and electronic properties of PtX2 and Janus PtXY (X, Y = S, Se, and Te) are studied based on the density functional theory. The phonon spectra and the Born criteria of the elastic constant of these six monolayers confirm their stability. All PtX2 and Janus PtXY monolayers show an outstanding stretchability with Young's modulus ranging from 61.023 to 82.124 N/m, about one-fifth that of graphene and half that of MoS2, suggesting highly flexible materials. Our first-principles calculations reveal that the pristine PtX2 and their Janus counterparts are indirect semiconductors with their band gap ranging from 0.760 to 1.810 eV at the Perdew-Burke-Ernzerhof level (1.128-2.580 eV at the Heyd-Scuseria-Ernzerhof level). By applying biaxial compressive and tensile strain, the electronic properties of all PtX2 and Janus PtXY monolayers are widely tunable. Under small compressive strain, PtX2 and Janus PtXY structures remain indirect semiconductors. PtTe2, PtSeTe, and PtSTe monolayers undergo a semiconducting to metallic transition when the strain reaches -6, -8, and -10%, respectively. Interestingly, there is a transition from the indirect semiconductor to a quasi-direct one for all PtX2 and Janus PtXY monolayers when the tensile strain is applied.