Universal Behaviors of the Phonon Thermal Conductivity Associated with Charge/Orbital Ordering in Transition-Metal Oxides

Thermal diodes, regulators, and switches: Physical mechanisms and potential applications

“…Huesler alloys 35 have shown r as large as 1.6 across a magnetostructural phase transition, and r ¼ 1.8 has been measured 36,37 across a structural phase transition near T ¼ 400 K in Cu 2-x Se. At high pressures (>10 GPa), r ¼3 has been observed in semiconductor-metal transitions 38 due to the enhanced electronic contribution to k. Charge-density wave (CDW) phase transitions can impact both the electron and the phonon contributions to k. For example, a 25% increase in k of octahedral TaS 2 was observed 39 at the CDW transition at 350 K, and similar r has been measured 40,41 in other CDW systems.…”

Strongly correlated oxides for energy harvesting

“…On the other hand, SrV 10 O 15 does not exhibit any ordering or a phase transition down to the lowest temperature [31]. The thermal conductivity for BaV 10 O 15 exhibits a large jump and further enhancement below the transition temperature (,130 K) associated with the orbital ordering [29], but such an anomaly is not observed and the thermal conductivity keeps decreasing down to the lowest temperature for SrV 10 This suppression of the phonon thermal conductivity can be utilized to enhance the figure of merit for the thermoelectric material, ZT ¼ S 2 σT=κ. For example, as shown in Figure 2(b), the phonon thermal conductivity of the hollandite Ba x Ti 8 O 16þδ with n ¼ 0:22 decreases with decreasing temperature, but starts increasing below ,200 K [10].…”

“…In this compound, a charge ordering of the d electrons at Ti sites occurs below ,200 K, where d electrons are located at every five Ti ions along the chain [10], and this charge ordering is responsible for the anomalies of various quantities at ,200 K shown in Figure 1. Accordingly, the phonon thermal conductivity above 200 K is suppressed by the fluctuation of the charge ordering, but it is enhanced with the charge ordering below 200 K. Compared with doped SrTiO 3 , Reproduced with permission from [11,29]. whose thermal conductivity at room temperature is n,0:02 W/Km and further increases with decreasing temperature, the phonon thermal conductivity of the hollandite titanates is one third smaller.…”

“…It was found that such orbital/charge fluctuation and their ordering affect the phonon thermal conductivity of materials. Namely, above the transition temperature of the orbital/charge ordering, the thermal conductivity is suppressed because the phonons that carry heat are scattered by the fluctuation of the orbital or charge, whereas once the orbital/charge ordering occurs, the source of the phonon scattering disappears and the thermal conductivity is enhanced accordingly [22][23][24][25][26][27][28][29]. Figure 2(a) shows one example of such behavior.…”

Temperature-dependent valence state within the metallic phase of BaV10O15 probed by hard x-ray photoelectron spectroscopy

“…11,18 With the increase in temperature above 120 K, the resistivity decreases along with a decrease in the positive Seebeck coefficient. 19 Very recently, the surface sensitive Scanning photoemission microscopy (SPEM) studies have shown that the metallic domains evolve with temperature above 120 K. 20 As the electronic properties at the surface are different from the bulk due to higher electron correlation and/or charge fluctuation, 21 22 Although the previous HAXPES study by Yoshino et al 13 is based only at the two temperatures i.e., below and above MIT, the NMR measurement revealed the electronic transition with the 3d electrons that couple differently in the three V sites of the triangular lattice across the intermediatetemperature phase. 17 There are localized magnetic V1 site and V2-V3 sites with the quasi-one-dimensional itinerant conductivity across the intermediate-temperature such that the later leads towards the formation of the trimers below 220 K. Further below MIT, the trimers are ordered while V 2+ -V 3+ charge order arrangement in the adjacent layers.…”