Jennifer A. Aitken

Second-Harmonic Generation and Crystal Structure of the Diamond-like Semiconductors Li2CdGeS4 and Li2CdSnS4

The semiconductors Li(2)CdGeS(4) and Li(2)CdSnS(4), which are of interest for their nonlinear optical properties, were synthesized using high-temperature solid-state and polychalcogenide flux syntheses. Both compounds were found to crystallize in Pmn2(1), with R1 (for all data) = 1.93% and 1.86% for Li(2)CdGeS(4) and Li(2)CdSnS(4), respectively. The structures of both compounds are diamond-like with the tetrahedra pointing in the same direction along the c axis. The alignment of the tetrahedra results in the structure lacking an inversion center, a prerequisite for second-harmonic generation (SHG). A modified Kurtz nonlinear optical powder technique was used to determine the SHG responses of both compounds. Li(2)CdGeS(4) displayed a type I phase-matchable response of approximately 70x alpha-quartz, while Li(2)CdSnS(4) displayed a type I non-phase-matchable response of approximately 100x alpha-quartz. Diffuse-reflectance spectroscopy was used to determine band gaps of 3.10 and 3.26 eV for Li(2)CdGeS(4) and Li(2)CdSnS(4), respectively.

Outstanding Laser Damage Threshold in Li2MnGeS4 and Tunable Optical Nonlinearity in Diamond-Like Semiconductors

The new Li2MnGeS4 and Li2CoSnS4 compounds result from employing a rational and simple design strategy that guides the discovery of diamond-like semiconductors (DLSs) with wide regions of optical transparency, high laser damage threshold, and efficient second-order optical nonlinearity. Single-crystal X-ray diffraction was used to solve and refine the crystal structures of Li2MnGeS4 and Li2CoSnS4, which crystallize in the noncentrosymmetric space groups Pna21 and Pn, respectively. Synchrotron X-ray powder diffraction (SXRPD) was used to assess the phase purity, and diffuse reflectance UV-vis-NIR spectroscopy was used to estimate the bandgaps of Li2MnGeS4 (Eg = 3.069(3) eV) and Li2CoSnS4 (Eg = 2.421(3) eV). In comparison with Li2FeGeS4, Li2FeSnS4, and Li2CoSnS4 DLSs, Li2MnGeS4 exhibits the widest region of optical transparency (0.60-25 μm) and phase matchability (≥1.6 μm). All four of the DLSs exhibit second-harmonic generation and are compared with the benchmark NLO material, AgGaSe2. Most remarkably, Li2MnGeS4 does not undergo two- or three-photon absorption upon exposure to a fundamental Nd:YAG beam (λ = 1.064 μm) and exhibits a laser damage threshold > 16 GW/cm(2).

Optical Nonlinearity in Cu2CdSnS4 and α/β-Cu2ZnSiS4: Diamond-like Semiconductors with High Laser-Damage Thresholds

Cu2CdSnS4 and α/β-Cu2ZnSiS4 meet several criteria for promising nonlinear optical materials for use in the infrared (IR) region. Both are air-stable, crystallize in noncentrosymmetric space groups, and possess high thermal stabilities. Cu2CdSnS4 and α/β-Cu2ZnSiS4 display wide ranges of optical transparency, 1.4-25 and 0.7-25 μm, respectively, and have relatively large second-order nonlinearity as well as phase matchability for wide regions in the IR. The laser-damage threshold (LDT) for Cu2CdSnS4 is 0.2 GW/cm(2), whereas α/β-Cu2ZnSiS4 has a LDT of 2.0 GW/cm(2) for picosecond near-IR excitation. Both compounds also exhibit efficient third-order nonlinearity. Electronic structure calculations provide insight into the variation in properties.

Field-Induced Spin-Flop in Antiferromagnetic Semiconductors with Commensurate and Incommensurate Magnetic Structures: Li2FeGeS4 (LIGS) and Li2FeSnS4 (LITS)

Li2FeGeS4 (LIGS) and Li2FeSnS4 (LITS), which are among the first magnetic semiconductors with the wurtz-kesterite structure, exhibit antiferromagnetism with TN ≈ 6 and 4 K, respectively. Both compounds undergo a conventional metamagnetic transition that is accompanied by a hysteresis; a reversible spin-flop transition is dominant. On the basis of constant-wavelength neutron powder diffraction data, we propose that LIGS and LITS exhibit collinear magnetic structures that are commensurate and incommensurate with propagation vectors km = [1/2, 1/2, 1/2] and [0, 0, 0.546(1)], respectively. The two compounds exhibit similar magnetic phase diagrams, as the critical fields are temperature-dependent. The nuclear structures of the bulk powder samples were verified using time-of-flight neutron powder diffraction along with synchrotron X-ray powder diffraction. (57)Fe and (119)Sn Mössbauer spectroscopy confirmed the presence of Fe(2+) and Sn(4+) as well as the number of crystallographically unique positions. LIGS and LITS are semiconductors with indirect and direct bandgaps of 1.42 and 1.86 eV, respectively, according to optical diffuse-reflectance UV-vis-NIR spectroscopy.

Highly efficient infrared optical nonlinearity of a wide-bandgap chalcogenide Li(2)CdGeS(4).

A quaternary chalcogenide Li(2)CdGeS(4) is an excellent candidate for a nonlinear optical (NLO) material exhibiting wide transparency spanning from its fundamental band edge (3.15 eV) to the terahertz regime (23.5 μm). Strong optical nonlinearity of Li(2)CdGeS(4) has been investigated over a wide spectral range (λ=1.064-3.3  μm) based on second- and third-harmonic generation. The compound has a high damage threshold at λ=1.064  μm because of saturable three-photon absorption, and is phase-matchable for λ>1.5  μm with χ(2) ≃50  pm/V. It also exhibits strong third-order nonlinearity of χ(3) ≃10(5) pm(2)/V(2). Li(2)CdGeS(4) is promising for high-power NLO applications in the broad infrared spectrum.

Suppression of antiferromagnetic interactions through Cu vacancies in Mn-substituted CuInSe2 chalcopyrites

Stoichiometric and Cu-poor Cu(0.95-x)Mn(0.05)InSe(2) (x = 0-0.20) compounds were synthesized by high-temperature, solid-state reactions. The presence of copper vacancies is revealed by Rietveld refinements of combined neutron and x-ray powder diffraction data. The antiferromagnetic interaction is depressed by the copper deficiency, which may be explained as the competition between the antiferromagnetic Mn-Se-Mn superexchange interaction and the hole-mediated ferromagnetic exchange induced by the copper vacancy. The introduction of copper vacancies is proposed to be a viable route to impart carrier-mediated ferromagnetic exchange in the chalcopyrite-based dilute magnetic semiconductors.

Li2CdGeSe4 and Li2CdSnSe4: biaxial nonlinear optical materials with strong infrared second-order responses and laser-induced damage thresholds influenced by photoluminescence

Two new biaxial, diamond-like semiconductors, Li2CdGeSe4 and Li2CdSnSe4, were prepared via high-temperature, solid-state synthesis. Single crystal X-ray diffraction and X-ray powder diffraction coupled with Rietveld refinement were used to refine the crystal structures and assess the phase purity, respectively. Both compounds adopt the lithium cobalt(II) silicate structure type. Strong second-order nonlinear optical (NLO) susceptibility, phase matchability, relatively high thermal stability, and excellent transparency deem both materials potential infrared (IR) NLO candidates. Li2CdGeSe4 and Li2CdSnSe4 display optical bandgaps of approximately 2.5 and 2.2 eV, respectively. Li2CdSnSe4 exhibits a strong, red-light emission under 1064 nm excitation, allowing the compound to release energy that accumulates by two-photon absorption under Nd:YAG laser radiation. Therefore, Li2CdSnSe4 shows a high laser-induced damage threshold (LIDT) of 0.7 GW cm−2. This special phenomenon is remarkable and may open a new avenue in searching for promising IR NLO materials with large LIDTs.

Bis[2,2'-(2-amino-ethyl-imino)-di(ethyl-ammonium)] di-μ-sulfido-bis[disulfido-stannate(IV)].

The asymmetric unit of the title compound, (C6H20N4)2[Sn2S6], comprises half of a [Sn2S6]4− anion and a diprotonated tris(2-aminoethyl)amine cation. The anion lies on an inversion center, while the atoms of the cation occupy general positions. An intramolecular N—H⋯N hydrogen bond is observed in the cation. In the crystal, strong N—H⋯S hydrogen bonding between the terminal sulfur atoms of the anion and the protonated amine N atoms of the cations result in a three-dimensional network.

EuSe2: A novel antiferromagnetic rare-earth polychalcogenide

The reaction of Eu with a molten mixture of Li2Se and excess Se has produced a new rare earth dichalcogenide, EuSe2, with a type “a” antiferromagnetic structure. The compound crystallizes in the tetragonal space group I4/mcm forming into small single crystal rectangular blocks with their long axis along c. The high temperature magnetic susceptibility exhibits Curie–Weiss behavior with a ferromagnetic Θ=15±1 K and an effective moment μeff=7.9±0.1 Bohr magnetons consistent with the theoretical value of 7.94 for the Eu2+ free ion. There is a peak in χc at Tn=8.0±0.5 K which is attributed to an antiferromagnetic transition. At T=2.5 K with the magnetic field, H, applied along c, M(H) exhibits a sharp increase near 1.5 T attributed to a metamagnetic transition. With H along a the moments saturate at about 5 T. A simple mean field model in which the Eu ions in the ab plane are coupled into ferromagnetic sheets, with alternate sheets along c coupled antiferromagnetically to form a bulk antiferromagnet is used to...

Insights on the Synthesis, Crystal and Electronic Structures, and Optical and Thermoelectric Properties of Sr1–xSbxHfSe3 Orthorhombic Perovskite

Single-phase polycrystalline powders of Sr1- xSb xHfSe3 ( x = 0, 0.005, 0.01), a new member of the chalcogenide perovskites, were synthesized using a combination of high temperature solid-state reaction and mechanical alloying approaches. Structural analysis using single-crystal as well as powder X-ray diffraction revealed that the synthesized materials are isostructural with SrZrSe3, crystallizing in the orthorhombic space group Pnma (#62) with lattice parameters a = 8.901(2) Å; b = 3.943(1) Å; c = 14.480(3) Å; and Z = 4 for the x = 0 composition. Thermal conductivity data of SrHfSe3 revealed low values ranging from 0.9 to 1.3 W m-1 K-1 from 300 to 700 K, which is further lowered to 0.77 W m-1 K-1 by doping with 1 mol % Sb for Sr. Electronic property measurements indicate that the compound is quite insulating with an electrical conductivity of 2.9 S/cm at 873 K, which was improved to 6.7 S/cm by 0.5 mol % Sb doping. Thermopower data revealed that SrHfSe3 is a p-type semiconductor with thermopower values reaching a maximum of 287 μV/K at 873 K for the 1.0 mol % Sb sample. The optical band gap of Sr1- xSb xHfSe3 samples, as determined by density functional theory calculations and the diffuse reflectance method, is ∼1.00 eV and increases with Sb concentration to 1.15 eV. Careful analysis of the partial densities of states (PDOS) indicates that the band gap in SrHfSe3 is essentially determined by the Se-4p and Hf-5d orbitals with little to no contribution from Sr atoms. Typically, band edges of p- and d-character are a good indication of potentially strong absorption coefficient due to the high density of states of the localized p and d orbitals. This points to potential application of SrHfSe3 as absorbing layer in photovoltaic devices.