Polycrystalline SnSe was synthesized by a melting-annealing-sintering process. X-ray diffraction reveals the sample possesses pure phase and strong orientation along [h00] direction. The degree of the orientations was estimated and the anisotropic thermoelectric properties are characterized. The polycrystalline sample shows a low electrical conductivity and a positive and large Seebeck coefficient. The low thermal conductivity is also observed in polycrystalline sample, but slightly higher than that of single crystal. The minimum value of thermal conductivity was measured as 0.3 W/m·K at 790 K. With the increase of the orientation factor, both electrical and thermal conductivities decrease, but the thermopowers are unchanged. As a consequence, the zT values remain unchanged in the polycrystalline samples despite the large variation in the degree of orientation.

The discovery of hydrogen-induced electronic phase transitions in strongly correlated materials such as rare-earth nickelates has opened up a new paradigm in regulating materials’ properties for both fundamental study and technological applications. However, the microscopic understanding of how protons and electrons behave in the phase transition is lacking, mainly due to the difficulty in the characterization of the hydrogen doping level. Here, we demonstrate the quantification and trajectory of hydrogen in strain-regulated SmNiO₃ by using nuclear reaction analysis. Introducing 2.4% of elastic strain in SmNiO₃ reduces the incorporated hydrogen concentration from ~10²¹ cm⁻³ to ~10²⁰ cm⁻³. Unexpectedly, despite a lower hydrogen concentration, a more significant modification in resistivity is observed for tensile-strained SmNiO₃, substantially different from the previous understanding. We argue that this transition is explained by an intermediate metastable state occurring in the transient diffusion process of hydrogen, despite the absence of hydrogen at the post-transition stage.

Effective synthesis of meta-stable materials challenging the thermodynamic limits will play a significant role in broadening the horizon in material designs and further explorations of their functionalities. Although d-band correlated rare-earth nickelate perovskites (ReNiO₃) have achieved promising applications, e.g., metal-to-insulator transition, artificial intelligence, and memory/logical devices, the thermodynamic instability and high vacuum-dependence in material synthesis have largely caused bottlenecks in these applications. Herein we demonstrate a vacuum-free and low cost chemical route to effectively synthesize single-crystalline ReNiO₃ thin films that further promote their device applications. It achieves high flexibility and convenience by adjusting the A-site compositions within the perovskites via single (i.e. Nd, Sm, Eu, and Gd), binary (i.e., Sm_1−xNd_x and Sm_1−xEu_x) and triple (i.e. Sm_1−x−yNd_xEu_y and Sm_1−x−yNd_xGd_y) rare-earth elements. The respective regulations in electronic structures, as probed via near edge X-ray absorption fine structure analysis, result in sharper metal-to-insulator transitions within a broad temperature range of 400 K, compared with their reported performances. Furthermore, we discover an overlooked thermistor transport behavior of ReNiO₃ within the binary A-site elements, which exhibits large temperature coefficients of resistance (>2%) across a broad range of temperatures (5–470 K). By overcoming the bottlenecks in material synthesis of ReNiO₃, the present work profoundly paves the way for device fabrication.