Energy generation, distribution, and storage remain the key elements to worldwide sustainable development to decrease CO2 emissions. Therefore, it is crucial to develop effective technologies for the simultaneous conversion of energy from multiple sources with higher energy output. In particular, perovskites play a central role as an enabling technology, making them promising in numerous technological fields for their wide range of functional properties, such as ferroelectricity, thermoelectricity, photo-activity, superconductivity, electrocatalysis, ionic conductivity for electrolytes and cathodes, high energy density energy storage, and much more. Importantly, multifunctionalities can be simultaneously present, making perovskites an especially attractive material class for multimodal energy conversion. However, understanding structure-microstructure-property relationships is critical to better realize the full potential of the applicability of perovskite-based multifunctional materials. In particular, the determination of the origin of functional properties involves the characterization of materials across multiple length scales to include (i) average crystal structure, (ii) local scale structure, e.g., defects, dopants, (iii) nano-/microstructure, (iv) surface vs. bulk structure, (v) chemical and crystal structure homo-/heterogeneity. This presentation will discuss the influence of post-sintering annealing on the structure-property relationships of perovskites with a focus on energy conversions. In addition, a method to modulate the temperature stability of functional properties without altering the composition will be presented.

The perovskite-type (ABO₃) solid solution Na₀.₅Bi₀.₅TiO₃-xBaTiO₃ (NBT-xBT) is a promising lead-free ferroelectric material. Unpoled NBT-xBT compounds in the vicinity of the morphotropic phase boundary (MPB) (xMPB ~ 0.05-0.06), exhibit relaxor properties and a complex crystal structure with strong nanoscale inhomogeneities. The structural heterogeneity hinders the determination of the actual symmetry and hence, the average structure is best described as pseudo-cubic. Analyzing how the structure responds to external stimuli such as temperature, pressure or an electric field can reveal subtle structural features and ultimately lead to a better understanding of the structure at ambient conditions. We have performed high-pressure single-crystal X-ray diffraction (XRD) and Raman spectroscopic experiments on NBT-0.048BT up to 9 GPa, using the diamond-anvil-cell technique. We have detected a pressure-induced phase transition as well as preceding local scale structural transformations by complementary analyses of Bragg scattering, informative of long-range order, X-ray diffuse scattering (XDS), indicative of intermediate-range order and Raman scattering, sensitive to the intermediate-/short-range order. At ambient and low pressure, deviations from the average pseudo-cubic perovskite structure produce strong diffuse scattering, which gradually evolves into sharp Bragg peaks as pressure is increased. Between 4.4 and 5.5 GPa the appearance of additional Bragg peaks indicates a phase transition. The symmetry of the high-pressure phase is orthorhombic Pnma and it is characterized by anti-polar order of A-cation shifts and mixed octahedral tilting. The Raman data show that the change in symmetry is preceded by multistep local scale structural changes, comprising a reduction of the Ba-induced TiO₆ anisotropy at ~0.5-0.9 GPa, a development of anti-polar order of A-cation off-center displacements already at ~1.9 GPa and enhancement of octahedral tilting vibrations at ~2.7 GPa. Additional data sets collected for two NBT-xBT compositions below and above the MPB (x=0.013 and x=0.074) are currently being evaluated. The comparison of the high-pressure behavior of all three compounds will improve the understanding of the relation between the chemical composition and the nanoscale structural characteristics of NBT-xBT, which in turn is closely related to the material properties.

Rare-earth ion doped ferroelectric ceramic becomes a fascinating ground for pairing of electrical and luminescence properties. These materials come under the class of piezo-luminescent materials. Importantly, ferroelectric materials are electromechanically soft and as a result the presence of external stimuli like temperature, stress and electric field gives rise to structural modulation which eventually affects the luminescence. In the present work europium (Eu3+) and praseodymium (Pr+3) doped (Na0.41K0.09Bi0.5)TiO3 [NKBT] ceramics were fabricated and investigated for their luminescence response. The effect of external stimuli i.e. electric field on photo-luminescence was also investigated.
Solid-state reaction technique was used to synthesize (Na0.41K0.09Bi0.49Pr0.01)TiO3 and (Na0.41K0.09Bi0.49Eu0.01)TiO3 ceramics. The composition selected in the present comparison was found to exhibit optimum piezoelectric response with piezoelectric charge coefficient (d33) ~165 pC/N. The photo-luminescence study reveals the presence of distinctive emission bands between 550 to 660 nm related to 5D0 → 7Fj (j = 0 to 6) for Eu3+, and three characteristic bands near 540, 610, and 655 nm corresponding to 3Pj → 3H5 (j =0 to 2), 1D2 → 3H4, and 3P0 → 3F2 transitions in Pr+3 ions. On application of electric field, there is no noticeable shift in the position and shape of the photoluminescence line which is attributed to the shielding of 4f-4f transitions in Eu+3 and Pr+3 by 5s2 and 5p6 electrons in the outer shells. However, quenching of luminescence for hypersensitive 5D0 → 7F2 and 1D2 → 3H4 transition was observed on application of the electric field. The quenching effect is more evident for the Pr+3 doped sample. The structural studies reveal the emergence of irreversible long-range tetragonal ordering on application of electric field. The increase in the symmetry results in the observed quenching of luminescence. The present study further reveals that the ferroelectric and photo-luminescence lack one-to-one correlation. The photo-luminescence response is associated to the local site symmetry around the rare earth ions and it is independent of the development of long-range ordering required for ferroelectricity.