Chen Li,Ke Liu,Dequan Jiang,Cheng Jin,Tianyao Pei,Ting Wen,Binbin Yue,Yonggang Wang
Negative thermal expansion (NTE) and zero thermal expansion (ZTE) properties are of great significance for the long-life stable operation of precision equipment. However, there are still existing challenges in finding new materials that exhibit NTE or ZTE over a wide temperature range. Here, we report negative, zero, and positive thermal expansion in NiAs-type, defective Cr1−δTe, containing three compounds: hexagonal CrTe, monoclinic Cr3Te4, and trigonal Cr5Te8. CrTe shows the NTE behavior from 280 to 340 K with the volume coefficient of thermal expansion αV = −27.6 × 10–6 K–1. Cr3Te4 shows the ZTE behavior over a wide temperature range of 180–320 K (αV = 0.16 × 10–6 K–1). And Cr5Te8 holds the PTE behavior over the whole temperature range (αV = 38.5 × 10–6 K–1). All of the samples show obvious anisotropic thermal expansion on heating. Combined with the magnetic measurements, it can be confirmed that the NTE and ZTE properties in ferromagnetic Cr1−δTe originate from the magnetovolume effect (MVE). Such NiAs-type, defective compounds with similar compositions but different structures provide a new perspective for tuning the NTE properties of materials and searching for new materials with ZTE over a wide temperature range.
Chen Li,Ke Liu,Cheng Jin,Dequan Jiang,Zimin Jiang,Ting Wen,Binbin Yue,Yonggang Wang
Cr1−δTe, as a unique series of defective compounds with a NiAs-type structure and periodic metal vacancies, has attracted intensive research interest because of its diversity in structure and property dependent on the tunable stoichiometric ratio. Another feature of these compounds is their ability to switch between NiAs- and MnP-type structures, in which there often exist composition-, temperature-, or pressure-induced phase transitions accompanied by intriguing physical property switching. Herein, we report the syntheses of a series of Cr1−δTe compounds with similar compositions but distinct crystal structures, their phase transitions, anomalous transport, and photoelectric conduction behaviors under high pressure (HP). For the three Cr1−δTe compounds with δ = 0, 0.25, 0.375, CrTe undergoes pressure-induced NiAs-to-MnP phase transition at around 15 GPa, while Cr3Te4 and Cr5Te8 undergo isostructural phase transitions at around 12 and 11 GPa, respectively. Electrical transport measurements indicate anomalous conduction behaviors: CrTe undergoes a semiconductor-to-metal transition at around 24 GPa, while Cr3Te4 and Cr5Te8 show unexpected metal–semiconductor–metal transitions sequentially upon compression. Besides, CrTe and Cr5Te8 exhibit pressure-induced n–p conduction-type switching at around 9 and 3 GPa, respectively, while Cr3Te4 shows p-type conductivity in the full pressure range. Local structure analyses based on in situ HP Raman spectra are performed to understand the structure and property evolutions of Cr1−δTe under HP. Defective transition-metal chalcogenides with pressure-induced NiAs-to-MnP phase transition and conduction-type conversion provide a potential platform for the rational design of photoelectric conversion devices.
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