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Abstract The magnetoresistance (MR) in nonmagnetic materials continues to be a fertile research area in materials science. The search for giant, positive MR has been limited to a rather small window of materials such as high‐mobility semimetals in single‐crystalline form. Here, the observation of a very large positive MR in metallic Pt5P2 in polycrystalline form is reported. The observations reveal that improvement of the crystallinity results in a significant enhancement of the positive MR, exceeding 10 000% at 9 T, comparable to high‐mobility semimetals. Based on first‐principles calculations combined with magnetotransport and thermoelectric measurements, the Fermi surface of Pt5P2 is found to consist of a collection of multiple electron and hole pockets compensating one another, along with a characteristic pocket continuously connected to the adjacent Brillouin zone, together with possible topologically protected band crossings. This work extends the landscape of high MR candidate materials to polycrystalline metals, which demonstrates the importance of crystallinity and purity of the samples for the optimization of the MR.

Abstract A lead‐free piezoelectric material whose performance is comparable to that of market‐dominant lead‐based ones has been strongly desired for the development of environmentally friendly piezoelectric devices. In spite of intensive research activities, however, few effective alternatives have been found due to severe requirements such as the absence of conducting carriers screening the electric polarization. Here, a polar semiconductor, AgCrSe2, is demonstrated as a promising piezoelectric material even with a finite conductivity. The inverse piezoelectric effect for AgCrSe2 single crystals, of which carrier density can be tuned by subtle nonstoichiometry, is studied as a model material at room temperature. Although the inverse piezoelectric effect of the AgCrSe2 crystals tends to be suppressed as resistivity becomes lower, the maximum piezoelectric coefficients (d33) reach 100–1000 pC N−1, comparable to those reported for typical piezoelectric BaTiO3 and lead‐based perovskites. The large piezoelectric response in AgCrSe2 is attributed to the large polarizability inherent to the intercalated Ag+ ions acting as ionic carriers. The present results demonstrate that superionic polar semiconductors have a potential to be high‐performance lead‐free piezoelectric materials overcoming the screening effect by the enhanced polarizability.