Ultralowthermal conductivity draws great attention in avariety of fields of applications such as thermoelectrics and thermal barrier coatings.H erein, the crystal structure and transport properties of Cu 4 TiSe 4 are reported. Cu 4 TiSe 4 is au nique example of an on-toxic and low-cost material that exhibits al attice ultra-low thermal conductivity of 0.19 Wm À1 K À1 at room temperature.T he main contribution to the unusually low thermal conductivity is connected with the atomic lattice and its dynamics.T his ultralow value of lattice thermal conductivity (k L )c an be attributed to the presence of the localized modes of Cu, which partially hybridizew ith the Se atoms,w hich in turn leads to avoidance of crossing of acoustic phonon modes that reach the zone boundary with ar educed frequency.L ike ap honon glass electron crystal, Cu 4 TiSe 4 could also open ar oute to efficient thermoelectric materials,even, with chalcogenides of relatively high electrical resistivity and al arge band gap,p rovided that their structures offer as ublattice with lightly bound cations.
A new compound Cu 4 TiTe 4 in the Cu−Ti−Te ternary system is prepared using high-temperature solid-state synthesis and characterized by single-crystal X-ray diffraction and energy-dispersive X-ray spectroscopy. The average structure of Cu 4 TiTe 4 crystallizes in the cubic space group P4̅ 3m (cP9; a = 5.9484(1) Å) and adopts the Cu 4 TiSe 4 structure type. Like Cu 4 TiSe 4 , it shows positional disorder in one of the two Cu sites. The three-dimensional structure of Cu 4 TiTe 4 is viewed as a cubic close-packed (ccp) array of Te, where half of the tetrahedral holes are orderly occupied by three Cu and one Ti and the disordered Cu atoms effectively occupied 1/4 of the octahedral holes. The calculated density of states (DOS) discerns that the compound is a narrow-bandgap semiconductor, and the crystal orbital Hamilton population (COHP) analysis shows that though the individual Cu−Te short contact is relatively weak compared to the Ti−Te contact, Cu−Te bonds largely contribute toward the overall stability. Due to the unique atomic arrangements, some Te atoms in the unit cell have unsaturated coordination, which presents 5s 2 lone pairs on the Te atoms. This has been confirmed by the density of states (DOS) and electron localization function (ELF) calculations.
The newly synthesized ultralow thermal conducting Cu 4 TiSe 4 undergoes a temperature-induced reversible structural phase transition at À 92 °C. The room temperature structure exhibits cubic symmetry (space group P � 43 m with lattice parameter a = 5.6458(1) Å) and shows positional disorder in two Cu-sites (4e and 1a Wyckoff sites). In this report, a structural study was carried out on the same sample by means of single-crystal X-ray diffraction data (nitrogen-cryostat) collected at 131 K. At this temperature, Cu 4 TiSe 4 crystallizes in rhombohedral symmetry, space group R3 m, with a = 7.9538(5) Å and c = 19.4828(17) Å. The low temperature structure is fourfold twinned. In the low temperature phase, copper atoms at 9b and 3a Wyckoff sites (originally 4e and 1a Wyckoff sites in the room temperature structure) order partially in comparison to room temperature structure. This temperature induced phase transition induce slight increase of the thermal conductivity of the material.
Ultralowthermal conductivity draws great attention in avariety of fields of applications such as thermoelectrics and thermal barrier coatings.H erein, the crystal structure and transport properties of Cu 4 TiSe 4 are reported. Cu 4 TiSe 4 is au nique example of an on-toxic and low-cost material that exhibits al attice ultra-low thermal conductivity of 0.19 Wm À1 K À1 at room temperature.T he main contribution to the unusually low thermal conductivity is connected with the atomic lattice and its dynamics.T his ultralow value of lattice thermal conductivity (k L)c an be attributed to the presence of the localized modes of Cu, which partially hybridizew ith the Se atoms,w hich in turn leads to avoidance of crossing of acoustic phonon modes that reach the zone boundary with ar educed frequency.L ike ap honon glass electron crystal, Cu 4 TiSe 4 could also open ar oute to efficient thermoelectric materials,even, with chalcogenides of relatively high electrical resistivity and al arge band gap,p rovided that their structures offer as ublattice with lightly bound cations.
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