Twin‐Peak Electrocaloric Effect in Ferroelectric Phase of Diffusive SrBi_1.98La_0.02Nb_2−xTa_xO₉ Materials

Abstract

Dual‐peak electrocaloric effect (ECE) exists over a broad range significantly below Curie temperature in co‐mixed SrBi1.98La0.02Nb2–x Ta x O9 (SBLNT) compositions exhibiting diffused phase transition. The indirect measurement method is used to study the influence of defect‐modified potential barrier distributions on the ECE of lead‐free SBLNT ceramics. Variation in isothermal entropy change and adiabatic temperature change with varying tantalum (Ta) content signify effects of morphology and impact of altered potential barriers in the parent lattice without involving structural phase transition. A unique appearance of twin‐peak positive–negative ECE in equally doped SrBi1.98La0.02Nb‐TaO9 matrix is attributed to doping‐induced formation of polar nanoregions and distinct dipole switching mechanisms controlled by evenly distributed Nb–Ta potential barriers in SBLNT lattice. Higher doping concentration reduces porosity and promotes electric polarization, thereby the relative cooling power gradually increases with increasing Ta at Nb sites.

Electrocaloric Effect Near Room Temperature in Lead-Free Aurivillius Phase Sr2bi4ti5o18 Upon La and Nb Codoping

“…Besides, in Pr 3+ doped SrBi 2 (Nb 0.2 Ta 0.8 ) 2 O 9, a strong disagreement between the direct and indirect ECE measurements was reported [18]. Much smaller ΔT values were reported from indirect ECE estimations in SrBi 1.98-La 0.02 Nb 2-x Ta x O 9 [20].…”

“…Hence, the challenge with Aurivillius phases is lowering T C towards room temperature while maintaining the excellent dielectric properties. Doping strategies have been reported to decrease T C , namely, A-site substitution with divalent cations (Ba 2+ , Sr 2+ ) and/or lanthanides [15][16][17][18][19][20], and B-site substitution with an appropriate choice of pentavalent cations (Ta 5+ , Nb 5+ ) [18,21,22]. At the same time, the introduction of structural disorder through chemical substitutions in ferroelectrics induces diffuse phase transitions which can be useful to expand the working temperature range of these materials as refrigerants [10,23,24].…”

“…Besides, above critical concentrations, chemical substitutions can lead to relaxor behavior, where instead of long-range ferroelectric domains, the strong compositional and structural disorder yields the formation of polar nanoregions with randomly distributed directions of dipole moments [25,26]. The EC properties of a few Aurivillius phases with different chemical substitutions bringing T C closer to room temperature and exhibiting relaxor behavior were surveyed [17][18][19][20]. Direct ECE measurements in either Pr 3+ or Ba 2+ doped SrBi 2 (Nb 0.2 Ta 0.8 ) 2 O 9 showed ΔT values ⁓ 0.5 K and ⁓ 0.8 K respectively, although no clear ECE peak was obtained in the temperature range around their T C [17][18][19].…”