Disorder-driven glasslike thermal conductivity in colusite Cu26V2Sn6S32

Abstract

The influence of structural disorder on the thermal transport in the colusite Cu 26 V 2 Sn 6 S 32 has been investigated by means of low-temperature thermal conductivity and specific heat measurements (2-300 K), 119 Sn Mössbauer spectroscopy and temperaturedependent powder inelastic neutron scattering (INS). Variations in the high-temperature synthesis conditions act as a key parameter for tuning the degree of disorder in colusite compounds. Intriguingly, we find that even samples previously thought to be fully ordered are in fact weakly disordered. Mössbauer data clearly evidence that Sn atoms do not solely occupy the 6c site of the crystal lattice but are present on possibly both the Cu and V sites, leading to a random distribution of these three cations within the unit cell. Increasing the disorder in these materials tends to lead to a smearing out of the main features in the phonon density of states measured by INS. Although the evolution of the inelastic signal upon warming is well described by a quasi-harmonic approximation, elastic properties calculations indicate large average Grüneisen parameters, consistent with those determined experimentally from thermodynamic data. Intriguingly, increasing the level of disorder results in a decreased average Grüneisen parameter suggesting that the lowered lattice thermal conductivity is not driven by enhanced anharmonicity. These results provide experimental evidence to support that the remarkable changeover in the lattice thermal conductivity from crystalline to glasslike is solely driven by enhanced disorder accompanied by local lattice distortions.

Structure, microstructure and thermoelectric properties of germanite-type Cu22Fe8Ge4S32 compounds

“…In agreement with the Seebeck coefficient, the electrical resistivity at room temperature varies such as: ST_SPS (760 mΩ cm) > ST_HP (640 mΩ cm) > MA_SPS (8.67 mΩ cm) > MA_HP (5.17 mΩ cm). The ST samples exhibit a higher electrical resistivity than the MA samples, consistent with a higher sulfur deficiency, similarly to what was previously reported on colusite[16,23].…”

“…[2] Beyond the promising results obtained for telluride-based materials which exhibit high performances, [3][4][5] the need to conciliate efficiency with environmental and cost constraints has triggered research toward copper-based sulfides, which, for most of them, benefit from containing eco-friendly and abundant elements. Tetrahedrite Cu 12 Sb 4 S 13 , [6][7][8][9][10][11][12][13][14][15] colusite Cu 26 V 2 Sn 6 S 32 , [16][17][18][19][20][21][22][23] stannoidite Cu 8 Fe 3 Sn 2 S 12 , [24][25][26] and bornite Cu 5 FeS 4 [27][28][29][30] are a few of the naturally occurring Cu-based sulfide minerals that have been reported to date for their promising thermoelectric properties.…”

“…One of the highest ZT value (i.e. 0.93 at 675 K) among sulfide materials [16,23] data refinements in this study were performed using the Le Bail method.…”

Toppling the Transport Properties with Cationic Overstoichiometry in Thermoelectric Colusite: [Cu26Cr2Ge6]1+δS32

“…Promising compositions include bornite Cu5FeS4, [7][8][9] germanite derivative Cu22Fe8Ge4S32, 10,11 stannoidite Cu8.5Fe2.5Sn2S12, 12 Cu2SnS3, 13 kesterite Cu2ZnSnS4, 14,15 Cu4Sn7S16, 16 CuFeS2 17 , tetrahedrites Cu12-xTxSb4S13 (T = Mn, Fe, Ni, Zn) [18][19][20][21] and colusites Cu26T2M6S32 (T = V, Nb, Ta, Cr, Mo, W; M = Sn, Ge). [22][23][24][25][26][27][28][29][30] In previous reports, the peculiar behavior of the electrical transport properties in colusite, in particular in Cu26Cr2-xTxGe6S32 (T = Mo, W) 29,30 and Cu26(V,Nb)2Sn6S32 [25][26][27] was investigated in details and revealed the importance of cationic ordering/disordering on the electrical and thermal transport properties. In Cu26Cr2Ge6S32, exceptional transport properties (PF = 1.94 mW m -1 K -2 at 700 K) were recently reported and explained by the presence of interstitial Cr cations forming mixed tetrahedral-octahedral [CrS 4 ]Cu 6 complexes, which influence the geometry of the conductive "Cu 26 S 32 " framework.…”

“…Such structural properties should make possible the realization of cationic over A c c e p t e d m a n u s c r i p t stoichiometry with disorder and consequently has a great potential for changing its transport properties. The effect of deviation from ideal stoichiometry was shown in Cu26(V,Nb)2Sn6S32 [25][26][27] by sulfur volatilization using high-temperature sintering processes: cationic over-stoichiometr y associated with atomic-scale defects/disordered states, including interstitial defects, anti-site defects, and site splitting was achieved. In particular, a high concentration of antisite defects decreases the thermal conductivity drastically, and consequently increased the ZT significantly.…”