Low-Temperature Structure and Dynamics in Cu₂Se

Abstract

We report 63 Cu and 65 Cu NMR on Cu 2 Se, a superionic conductor of interest for thermoelectric and other applications. Results demonstrate an initial appearance of ionic hopping in a narrow temperature range above 100 K, coinciding with the recently observed low-temperature phase transition. At room temperature and above this goes over to rapid Cu-ion hopping and a single motionally narrowed line both above and below the α − β structural transition. The low-temperature α ′ phase exhibits a sizable metallic shift, in contrast to the high temperature structures. The large variation in native carrier density, significantly enhanced at positions associated with Cu vacancies, indicates the strong influence of an impurity band on low-temperature electronic transport.

Promising bulk nanostructured Cu2Se thermoelectrics via high throughput and rapid chemical synthesis

“…Figure 6d represents the overall TE figure of merit ZT of both samples prepared. High power factor and low thermal conductivity result in an exceptionally high TE figure of merit ZT for Cu 2 Se_Nano_nl as compared to the 14 highest reported ZT values in Figure 8. Due to the methodology that preserved the nanostructure, ZT values as high as 2 at 900 K were achieved for the nanostructured Cu 2 Se, an almost 25%…”

“…Recently, copper selenide (Cu 2-x Se) has gained a renewed interest for the medium temperature range operation. Copper selenide structures have previously been used in photovoltaic cells and devices [2,3] and are being considered even in biological systems [4] . This renewed interest can be attributed to the low thermal conductivity of the material as well as to its phase changing nature, which is an interesting aspect of the material [5][6][7][8][9][10] .…”

“…Different methods have been used for the fabrication of these materials, including melt alloying [2,5,7,8,12,13] , 3 mechanical alloying (ball milling) [6,14,15] , self-propagating high-temperature synthesis (SHS) [10] , and chemical routes using aqueous solutions either directly [9,16,17] or solvothermally precipitated using autoclave [18] or organometallic precursors [4,19] . Some of these processes require high temperatures (above 1000 K), long processing times, which consumes a lot of energy, or inert atmosphere glove boxes for powder handling, which are expensive.…”

Solving the disordered structure of β-Cu2−xSe using the three-dimensional difference pair distribution function

“…Cu 2Àx Se is a promising thermoelectric material as it is cheap, non-toxic and has a high thermoelectric figure of merit (zT) (Eikeland et al, 2017;Liu, Yuan et al, 2013;Lu et al, 2015;Qiu et al, 2016;Nguyen et al, 2013;Brown et al, 2016;Brown, Day, Caillat & Snyder, 2013;Brown, Day, Borup et al, 2013;Dalgaard et al, 2018;Liu et al, 2012;Miyatani, 1973;Vučić et al, 1984Vučić et al, , 1981Vucic et al, 1982;Yu et al, 2012;Chi et al, 2014;Mahan, 2015;Sirusi et al, 2015;Sun et al, 2015;Kang et al, 2016). The compound has a phase transition at 350-410 K depending on the degree of Cu deficiency (Brown, Day, Caillat & Snyder, 2013;Chi et al, 2014;Eikeland et al, 2017;Liu, Yuan et al, 2013;Vučić et al, 1984).…”

Extremely low thermal conductivity and high thermoelectric performance in liquid-like Cu2Se1−xSx polymorphic materials

“…[25][26][27][28][29] For example, the crystal structure of the room temperature Cu2Se phase is still controversial up to now, although recently an average structure of the compound has been determined. 25,26,29,30 Upon heating, Cu2Se undergoes a reversible phase transition at around 400 K and transforms to the superionic phase with high symmetry cubic structure (space group Fm3 ̅ m). Similar to Cu2Se, the stoichiometric Cu2S compound also shows complicated temperature dependent structures, termed the monoclinic structure (for temperatures below 370 K), the superionic hexagonal structure (for temperatures between 370 K and 700 K), and the superionic cubic structure (for temperatures above 700 K).…”