DWs results in anomalous behavior in the material compared to the mono domain state. In relaxorferroelectrics such as Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (PMN-PT) or Pb(In 1/2 Nb 1/2 )O 3 -Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (PIN-PMN-PT), manipulating DWs has been shown to achieve excep tional dielectric and piezoelectric proper ties, lending them as excellent candidates for several electromechanical applica tions including sensors, actuators, energy storage, drug delivery, diagnostic imaging, and nondestructive testing. [4][5][6][7][8][9] Among several DW engineering methods to achieve further enhancement, the use of alternating current poling (ACP) to manipulate domains has received con siderable attention due to its convenience and effectiveness. [10][11][12][13][14][15][16] Studies have shown that up to 40% enhancement in piezo electric coefficient (d 33 ) and 35% enhance ment in free dielectric constants using ACP compared to conventional direct cur rent poling (DCP) methods. [10][11][12][13][14][15][16] However, even though the DW engineeringbased piezoelectric property enhancement is consistent for ACP, a consensus on how the domain size (or DW density) influences the piezoelectric properties is still not reached. Conventional belief in ferro electrics has been that the property enhancement is due to high domain wall density. [17][18][19] Studies by Chang et al., [11] Xu et al., [16] Zhang et al., [13] and Sun et al. [10] on PMN-PT single crystals (SCs) show that the presence of monoclinic phases and finer domain sizes as a result of ACP are responsible for piezoelectric property enhancement, indicating that the con ventional wisdom still holds for relaxorferroelectrics. However, Acting like thermal resistances, ferroelectric domain walls can be manipulated to realize dynamic modulation of thermal conductivity (k), which is essential for developing novel phononic circuits. Despite the interest, little attention has been paid to achieving room-temperature thermal modulation in bulk materials due to challenges in obtaining a high thermal conductivity switching ratio (k high /k low ), particularly in commercially viable materials. Here, room-temperature thermal modulation in 2.5 mm-thick Pb(Mg 1/3 Nb 2/3 )O 3 -xPbTiO 3 (PMN-xPT) single crystals is demonstrated. With the use of advanced poling conditions, assisted by the systematic study on composition and orientation dependence of PMN-xPT, a range of thermal conductivity switching ratios with a maximum of ≈1.27 is observed. Simultaneous measurements of piezoelectric coefficient (d 33 ) to characterize the poling state, domain wall density using polarized light microscopy (PLM), and birefringence change using quantitative PLM reveal that compared to the unpoled state, the domain wall density at intermediate poling states (0< d 33

In this work, 0.95(K0.5Na0.5)NbO3–0.05(Bi0.5Na0.5)(Zr0.85Sn0.15)O3 (KNN–BNZS) lead‐free piezoelectric single crystals are grown by the solid‐state crystal growth (SSCG) method and their phase transitions and properties characterized. Addition of an appropriate amount of Li2CO3 and Bi2O3 sintering aids considerably promotes single‐crystal growth. Chemical composition of the single crystal is similar to the nominal composition with a slight deficiency in Na and K as evaluated by electron probe microanalysis (EPMA). X‐Ray diffraction (XRD), electrical property measurements, and temperature‐controlled Raman scattering reveal that the phase transition near room temperature is from a rhombohedral to an orthorhombic or tetragonal phase. Wide temperature range Raman scattering and dielectric properties consistently indicate transitions associated with the phase transformations or boundaries of a phase coexistence region and further details. Nontrivial frequency dependence of dielectric properties is successfully parameterized by capacitance relaxations described by Debye and Cole–Cole responses with special constant phase elements with fixed exponents of 0.5 (Warburg elements). High‐frequency dielectric constants and additional dielectric relaxations and dc relaxations are deconvoluted. The modeling parameters indicate the phase transition temperatures (or boundaries of the phase coexistence region) unambiguously. Polarization–electric field and bipolar strain–electric field hysteresis loops show that the single crystal is a classic lossy ferroelectric material.