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Optically anisotropic materials are sought after for tailoring the polarization of light. Recently, colossal optical anisotropy (Δn = 2.1) was reported in a quasi-one-dimensional chalcogenide, Sr9/8TiS3. Compared to SrTiS3, the excess Sr in Sr9/8TiS3 leads to periodic structural modulations and introduces additional electrons, which undergo charge ordering on select Ti atoms to form a highly polarizable cloud oriented along the c-axis, hence resulting in the colossal optical anisotropy. Here, further enhancement of the colossal optical anisotropy to Δn = 2.5 in Sr8/7TiS3 is reported through control over the periodicity of the atomic-scale modulations. The role of structural modulations in tuning the optical properties in a series of SrxTiS3 compounds with x = [1, 9/8, 8/7, 6/5, 5/4, 4/3, 3/2] is investigated using density-functional-theory (DFT) calculations. The structural modulations arise from various stacking sequences of face-sharing TiS6 octahedra and twist-distorted trigonal prisms and are found to be thermodynamically stable for 1 < x < 1.5. As x increases, an indirect-to-direct band gap transition is predicted for x ≥ 8/7 along with an increased occupancy of Ti-dz2 states. Together, these two factors result in a theoretically predicted maximum birefringence of Δn = 2.5 for Sr8/7TiS3. Single crystals of Sr8/7TiS3 were grown using a molten-salt flux method. Single-crystal X-ray diffraction measurements confirm the presence of long-range order with a periodicity corresponding to Sr8/7TiS3, which is further corroborated by atomic-scale observations using scanning transmission electron microscopy. Polarization-resolved Fourier-transform infrared spectroscopy of Sr8/7TiS3 crystals shows Δn ≈ 2.5, in excellent agreement with the theoretical predictions. Overall, these findings demonstrate the compositional tunability of optical properties in SrxTiS3 compounds by control over atomic scale modulations and suggest that similar strategies could be extended to other compounds having modulated structures. 

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane-spanning lipids synthesized by bacteria in numerous substrates. The degree of methylation of the five methyl brGDGTs in both soils and lake sediments, described by the MBT′5Me index, is empirically related to surface atmospheric temperature. This relationship in lakes is generally assumed to reflect lake surface temperatures captured by brGDGT production in the water column and exported to lake sediments, and the MBT′5Me index has been applied to brGDGTs in lake sediment successions to reconstruct changes in temperature through time. We analyzed the relationship between MBT′5Me and the isomerization of brGDGTs (IR6Me) in globally distributed surficial lake sediments and demonstrated that the relationship, and calibrations, of MBT′5Me and temperature in middle and high latitude lakes are sensitive to incompletely understood factors related to IR6Me. IR6Me does not appear to track a non-thermal influence of brGDGT methylation in tropical lakes, but this could change as the data set is expanded. We address ongoing challenges in the application of the MBT′5Me paleothermometer in middle and high latitude lakes with new MBT′5Me-temperature calibrations based on grouping lakes by IR6Me. We demonstrate how IR6Me can distinguish samples with a significant non-thermal influence on MBT′5Me by targeting anomalously warm temperatures during the Last Glacial Maximum from newly analyzed piston and gravity core samples from Lake Baikal, Russia. 

Abstract Noncollinear ferroic materials are sought after as testbeds to explore the intimate connections between topology and symmetry, which result in electronic, optical, and magnetic functionalities not observed in collinear ferroic materials. For example, ferroaxial materials have rotational structural distortions that break mirror symmetry and induce chirality. When ferroaxial order is coupled with ferroelectricity arising from a broken inversion symmetry, it offers the prospect of electric‐field‐control of the ferroaxial distortions and opens up new tunable functionalities. However, chiral multiferroics, especially ones stable at room temperature, are rare. A strain‐stabilized, room‐temperature chiral multiferroic phase in single crystals of BaTiS₃is reported here. Using first‐principles calculations, the stabilization of this multiferroic phase havingP6₃space group for biaxial tensile strains exceeding 1.5% applied on the basalab‐plane of the room temperatureP6₃cmphase of BaTiS₃is predicted. The chiral multiferroic phase is characterized by rotational distortions of TiS₆octahedra around the longc‐axis and polar displacement of Ti atoms along thec‐axis. The ferroaxial and ferroelectric distortions and their domains inP6₃‐BaTiS₃are directly resolved using atomic resolution scanning transmission electron microscopy. Landau‐based phenomenological modeling predicts a strong coupling between the ferroelectric and the ferroaxial order makingP6₃‐BaTiS₃an attractive test bed for achieving electric‐field‐control of chirality. 

Numerous temperature and environmental proxies are based on glycerol dialkyl glycerol tetraethers (GDGTs), which are membrane lipids commonly found in the water columns and sediments of lakes. The TEX86 temperature proxy is based on isoprenoid GDGTs, which are produced by members of the archaea, and is used to reconstruct lake surface temperature. Branched GDGTs are lipids produced by bacteria and form the basis of the MBT′5ME temperature proxy. Although many outstanding questions still exist regarding proxies based on isoprenoid and branched GDGTs, both compound classes have been relatively well-studied in lakes. More recently, other types of GDGTs and related compounds are increasingly being reported from lacustrine sediments including hydroxylated GDGTs (OH-GDGTs) and glycerol monoalkyl glycerol tetraethers (GMGTs). In the process of generating lacustrine TEX86 or MBT′5ME temperature records, we noted that OH-GDGTs or GMGTs (or both) are frequently present. The RI-OH, based on OH-GDGTs, recently has been proposed as a temperature proxy in lakes while GMGTs are associated with oxygen-deficient environments. Here we examine distributions of OH-GDGTs and GMGTs in a variety of lakes that also have existing TEX86 or MBT’5ME temperature reconstructions. These lakes range from small to large, shallow to deep, tropical to arctic, differ in oxygenation state, and have sedimentary records covering timespans from the Holocene to multiple glacial-interglacial cycles. Study lakes include El’gygytgyn (arctic Russia), Malawi (tropical southeast Africa), Issyk Kul (Kyrgyzstan), Lake 578 (Greenland), and high elevation lakes in the central Andes (South America). We explore the presence/absence of these compounds in contrasting depositional environments and examine their GDGT distributions in relationship to temperature variability, oxic/anoxic conditions, hydroclimate fluctuations, and other geochemical/environmental parameters. 

Numerous temperature and environmental proxies are based on glycerol dialkylglycerol tetraethers (GDGTs), which are membrane lipids commonly found in thewater columns and sediments of lakes. The TEX86 temperature proxy is based onisoprenoid GDGTs, which are produced by members of the archaea, and is used toreconstruct lake surface temperature. Branched GDGTs are lipids produced bybacteria and form the basis of the MBT′5ME temperature proxy. Although manyoutstanding questions still exist regarding proxies based on isoprenoid and branchedGDGTs, both compound classes have been relatively well-studied in lakes. Morerecently, other types of GDGTs and related compounds are increasingly beingreported from lacustrine sediments including hydroxylated GDGTs (OH-GDGTs) andglycerol monoalkyl glycerol tetraethers (GMGTs). In the process of generating lacustrine TEX86 or MBT′5ME temperature records, we noted that OH-GDGTs orGMGTs (or both) are frequently present. The RI-OH, based on OH-GDGTs, recentlyhas been proposed as a temperature proxy in lakes while GMGTs are associatedwith oxygen-deficient environments. Here we examine distributions of OH-GDGTs and GMGTs in a variety of lakes that also have existing TEX86 or MBT′5ME temperature reconstructions. These lakes range from small to large, shallow to deep,tropical to arctic, differ in oxygenation state, and have sedimentary records coveringtimespans from the Holocene to multiple glacial-interglacial cycles. Study lakesinclude El’gygytgyn (arctic Russia), Malawi (tropical southeast Africa), Issyk Kul(Kyrgyzstan), Lake 578 (Greenland), and high elevation lakes in the central Andes (South America). We explore the presence/absence of these compounds incontrasting depositional environments and examine their GDGT distributions inrelationship to temperature variability, oxic/anoxic conditions, hydroclimatefluctuations, and other geochemical/environmental parameters. 

Global warming during the Last Glacial Termination was interrupted by millennial-scale cool intervals such as the Younger Dryas and the Antarctic Cold Reversal (ACR). Although these events are well characterized at high latitudes, their impacts at low latitudes are less well known. We present high-resolution temperature and hydroclimate records from the tropical Andes spanning the past ~16,800 y using organic geochemical proxies applied to a sediment core from Laguna Llaviucu, Ecuador. Our hydroclimate record aligns with records from the western Amazon and eastern and central Andes and indicates a dominant long-term influence of changing austral summer insolation on the intensity of the South American Summer Monsoon. Our temperature record indicates a ~4 °C warming during the glacial termination, stable temperatures in the early to mid-Holocene, and slight, gradual warming since ~6,000 y ago. Importantly, we observe a ~1.5 °C cold reversal coincident with the ACR. These data document a temperature change pattern during the deglaciation in the tropical Andes that resembles temperatures at high southern latitudes, which are thought to be controlled by radiative forcing from atmospheric greenhouse gases and changes in ocean heat transport by the Atlantic meridional overturning circulation. 

Abstract BaTiS₃, a quasi-1D complex chalcogenide, has gathered considerable scientific and technological interest due to its giant optical anisotropy and electronic phase transitions. However, the synthesis of high-quality BaTiS₃crystals, particularly those featuring crystal sizes of millimeters or larger, remains a challenge. Here, we investigate the growth of BaTiS₃crystals utilizing a molten salt flux of either potassium iodide, or a mixture of barium chloride and barium iodide. The crystals obtained through this method exhibit a substantial increase in volume compared to those synthesized via the chemical vapor transport method, while preserving their intrinsic optical and electronic properties. Our flux growth method provides a promising route toward the production of high-quality, large-scale single crystals of BaTiS₃, which will greatly facilitate advanced characterizations of BaTiS₃and its practical applications that require large crystal dimensions. Additionally, our approach offers an alternative synthetic route for other emerging complex chalcogenides. Graphical Abstract