Single crystals of SnSe have gained considerable attention in thermoelectrics due to their unprecedented thermoelectric performance. However, polycrystalline SnSe is more favorable for practical applications due to its facile chemical synthesis procedure, processability, and scalability. Though the thermoelectric figure of merit (zT) of p‐type bulk SnSe polycrystals has reached >2.5, zT of n‐type counterpart is still lower and lies around ≈1.5. Herein, record high zT of 2.0 in n‐type polycrystalline SnSe0.92 + x mol% MoCl5 (x = 0–3) samples is reported, when measured parallel to the spark plasma sintering pressing direction due to the simultaneous optimization of n‐type carrier concentration and enhanced phonon scattering by incorporating modular nano‐heterostructures in SnSe matrix. Modular nanostructures of layered intergrowth [(SnSe)1.05]m(MoSe2)n like compounds embedded in SnSe matrix scatters the phonons significantly leading to an ultra‐low lattice thermal conductivity (κlat) of ≈0.26 W m−1 K−1 at 798 K in SnSe0.92 + 3 mol% MoCl5. The 2D layered modular intergrowth compound resembles the nano‐heterostructure and their periodicity of 1.2–2.6 nm in the SnSe matrix matches the phonon mean free path of SnSe, thereby blocking the heat carrying phonons, which result in low κlat and ultra‐high thermoelectric performance in n‐type SnSe.

The structural and magnetic behavior of Mn-site doped intermetallic manganese silicide alloys of nominal compositions Mn5−xAxSi3 (x = 0.05, 0.1, 0.2 and A = Ni, Cr) have been investigated with a focus to the inverted hysteresis behavior and thermomagnetic irreversibility. Room temperature x-ray powder diffraction data confirm that all the doped alloys crystallize in hexagonal D88 type structure with space group P 63/mcm. The doped alloys are found to show paramagnetic (PM) -collinear antiferromagnetic (AFM2) -noncollinear antiferromagnetic (AFM1) transitions during cooling from room temperature. A significant decrease in the critical values of both AFM1-AFM2 transition temperatures and fields have been observed with the increasing Ni/Cr concentration. Inverted hysteresis loop, field-induced arrest, and thermomagnetic arrest, the key features of the undoped Mn5Si3 alloy, are found to be significantly affected by the Mn-site doping and eventually vanishes with 4% doping.

The ground-state magnetic properties of a hexagonal equiatomic alloy of nominal composition Mn0.8Fe0.2NiGe were investigated through dc magnetization and heat capacity measurements. The alloy undergoes a first-order martensitic transition below 140 K with simultaneous development of long-range ferromagnetic ordering from the high-temperature paramagnetic phase. The undoped compound MnNiGe has an antiferromagnetic ground state and it shows martensitic-like structural instability well above room temperature. Fe doping at the Mn site not only brings down the martensitic transition temperature, but it also induces ferromagnetism in the sample. Our study brings out two important aspects regarding the sample, namley i) the observation of exchange bias at low temperature, and ii) spin-glass–like ground state which prevails below the martensitic and magnetic transition points. In addition to the observed usual relaxation behavior, the spin glass state is confirmed by the zero-field–cooled memory experiment, thereby indicating cooperative freezing of spin and/or spin clusters rather than uncorrelated dynamics of superparamagnetic-like spin clusters. We believe that doping disorder can give rise to some islands of antiferromagnetic clusters in the otherwise ferromagnetic background which can produce interfacial frustration and exchange pinning responsible for spin glass and exchange bias effect. A comparison is made with doped rare-earth manganites where a similar phase separation can lead to a glassy ground state.

2D layered Ruddlesden–Popper (RP) perovskites gained significant attention owing to their unique optoelectronic properties and ultralow thermal conductivity; however, in comparison to the hybrid Pb-based RP phases, their Pb-free all-inorganic analogues are rarely explored. Herein, we demonstrate the optical and magnetic properties of Pb-free RP-type layered Rb2CuCl2Br2, which was synthesized by liquid-assisted mechanochemistry at ambient temperature and pressure. The dark brown colored material displayed band gap of ∼1.88 eV accompanied by a room-temperature band-edge photoluminescence (PL) at ∼1.97 eV. Moreover, Raman scattering at low-frequency region from the [CuCl4Br2]4– unit and reasonable thermal and environmental stability were manifested. The Rb2CuCl2Br2 exhibited an excitonic absorption which is characteristic of layered perovskites accompanied by a high exciton binding energy. Large Stokes-shifted PL emission, negligible self-absorption, and fast charge carrier lifetimes were evidenced for Rb2CuCl2Br2. Low-temperature (77 K) measurement revealed red-shifted and intense dual emission in contrast to the room-temperature emission. Finally, the temperature-dependent magnetization measurement indicates a paramagnetic to antiferromagnetic (AFM) transition at ∼16.1 K in Rb2CuCl2Br2, primarily related to the interlayer AFM exchange interaction. Competition between the interlayer AFM interaction and the intralayer ferromagnetic interaction determine the nature of magnetic ground state in Rb2CuCl2Br2.

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Two-dimensional layered tin selenide (SnSe) has attracted immense interest in thermoelectrics due to its ultralow lattice thermal conductivity and high thermoelectric performance. To date, the majority of thermoelectric studies of SnSe have been based on single crystals. However, because synthesizing SnSe single crystals is an expensive, time-consuming process that requires high temperatures and because SnSe single crystals have relatively weaker mechanical stability, they are not favorable for scaling up synthesis, commercialization, or practical applications. As a result, research on nanocrystalline SnSe that can be produced in large quantities by simple and low-temperature solution-phase synthesis is needed. In this Perspective, we discuss the progress in thermoelectric properties of SnSe with a particular emphasis on nanocrystalline SnSe, which is grown in solution. We first describe the state-of-the-art high-performance single crystal and polycrystals of SnSe and their importance and drawbacks and discuss how nanocrystalline SnSe can solve some of these challenges. We illustrate different solution-phase synthesis procedures to produce various SnSe nanostructures and discuss their thermoelectric properties. We also highlight a unique solution-phase synthesis technique to prepare CdSe-coated SnSe nanocomposites and its unprecedented thermoelectric figure of merit (ZT) of 2.2 at 786 K, as reported in this issue of ACS Nano. In general, solution synthesis showed excellent control over nanoscale grain growth, and nanocrystalline SnSe shows ultralow thermal conductivity due to strong phonon scattering by the nanoscale grain boundaries. Finally, we offer insight into the opportunities and challenges associated with nanocrystalline SnSe synthesized by the solution route and its future in thermoelectric energy conversion.

Single crystals of tin selenide (SnSe), a layered material, have recently drawn massive attention in the field of thermoelectrics for its low thermal conductivity and high thermoelectric figure of merit (zT). However, nanocrystalline SnSe always remains a better choice for thermoelectric applications owing to its simple synthesis and machinability. On the other hand, enhancement of the thermoelectric performance can be achieved by the incorporation of magnetic nanoprecipitates in a thermoelectric host matrix. Herein, we have demonstrated the significant enhancement in the thermoelectric performance of the two-dimensional (2D) nanoplates of SnSe by introducing magnetic Gd dopants, which are synthesized and scaled up (∼10 g) by a low-temperature hydrothermal method. The p-type carrier concentration increases significantly upon 3 mol % Gd addition in SnSe nanoplates due to phase separation of Gd 2 Se 3 nanoprecipitates (2−5 nm) and subsequent Sn 2+ vacancy formation. Thus, the thermoelectric power factor has been markedly enhanced to 6.7 μW/(cm K 2 ) at 868 K compared to that of the pristine SnSe nanoplates. The presence of magnetic fluctuations induced by small nanoprecipitates of Gd 2 Se 3 provides additional scattering of the phonons in SnSe, which reduces the lattice thermal conductivity significantly to 0.41 W/(m K) at 868 K in Sn 0.97 Gd 0.03 Se. We have achieved a zT of ∼1 at 868 K for the spark plasma sintered (SPS) Sn 0.97 Gd 0.03 Se nanoplates along the perpendicular to the pressing direction.

Magnetic and magneto-functional behavior of a Fe-doped MnNiGe alloy with nominal composition Mn0.85Fe0.15NiGe have been investigated in ambient as well as in high pressure condition. The alloy undergoes first order martensitic phase transition (MPT) around 200 K and also shows large conventional magnetocaloric effect (MCE) (∆S ∼ -21 J/kg-K for magnetic field (H) changing from 0-50 kOe) around the transition in ambient condition. Application of external hydrostatic pressure (P ) results a shift in MPT towards the lower temperature and a clear decrease in the saturation moment of the alloy at 5 K. The peak value of MCE is also found to decrease with increasing external P (∼ 18 J/kg-K decrease in ∆S has been observed for P = 12.5 kbar). The most interesting observation is the occurance of exchange bias effect (EBE) on application of external P . The competing ferromagnetic and antiferromagnetic interaction in presence of external P plays the pivotal role towards the observation of P induced EBE.