Vasant G. Sathe

Observation and interpretation of the temperature dependence of the prominent Raman modes of Mn 3 NiTa 2 O 9 (MNTO) from T = 8 to 300 K is reported and compared with those reported in the isostructural Mn 4 Ta 2 O 9 (MTO) with honeycomb lattice and trigonal symmetry. The systematic blue shift of all Raman modes of MNTO relative to those in MTO is interpreted to result from the reduced lattice parameters of MNTO due to Ni substitution. The smaller magnetic moment of Ni 2 + relative to that of Mn 2 + is the likely source of a few % reduction in the observed magnetic transitions ( T N ∼ 97 K and T S ∼ 46 K) of MNTO. The phonon frequencies ( ω ) and linewidths ( Γ ) were obtained by fitting each Raman mode with a single Lorentzian profile. For T > T N , the phonon frequencies decrease monotonically while the linewidths broaden with increasing temperatures, these results are well described by a three-phonon anharmonic decay model. With decreasing temperatures below T N , pronounced softening of nearly all Raman modes in MNTO is observed, becoming particularly significant below T S in contrast to MTO, where phonon hardening was reported. These anomalous phonon shifts are attributed to strong spin-phonon coupling ( λ ), which is positive in MNTO but negative in MTO. The reversal in the sign of λ in MNTO highlights a fundamental modification of the spin-lattice interaction induced by Ni substitution demonstrating that subtle structural changes can profoundly alter the coupling between lattice vibrations and magnetic order in the honeycomb magnetoelectric tantalates.

This study investigates the electronic and lattice properties of single-crystalline Bi2SeTe2(BST) to deepen the understanding of its topological behavior and defect-mediated transport mechanisms. Here, we combine results from low-temperature scanning tunneling microscopy and spectroscopy (STM/STS), temperature-dependent transport measurements, and Raman spectroscopy to establish a unified picture of defect-mediated electronic conduction. STM/STS reveal that intrinsic surface defects within the ordered threefold lattice induce nanoscale fluctuations in the Dirac node energy, significantly modulating the local electronic structure. Transport measurements identify variable-range hopping as the dominant conduction mechanism, with a crossover from Mott to Efros–Shklovskii regime at low temperatures. The temperature-dependent Raman anomalies correlate closely with this transport behavior. Together, these multi-probe measurements provide a unified understanding of defect-mediated charge transport in BST and clarify the role of intrinsic disorder in shaping its electronic properties, reinforcing its potential applications in quantum and spintronic devices.

The photo-dielectric effect with electric field-driven structural and phonon changes is demonstrated for BCTO. A remarkable correlation between the structural and dielectric properties is established.

Here, we report the dependence of the functional properties on textural and crystallographic phases of Aand B-site co-doped BaTiO 3 ceramic, (0.82)BaTi 0.88 Sn 0.12 O 3 -(0.18)Ba 0.7 Ca 0.3 TiO 3 [BST-18BCT], featuring polymorphic phase boundaries (PPB), by varying the sintering temperature (1350 • C to 1550 • C) . Increasing sintering temperatures non-monotonically increased grain size, as confirmed by scanning electron microscopy, while conversely, reduced sintering temperatures enhanced tetragonality, as revealed by Rietveld refinement and corroborated by HRTEM and SAED analyses. Notably, increasing sintering temperature led to a significant enhancement in remanent polarization (P r ), saturation polarization (P s ), and permittivity (ε ′ ), whereas reduction in coercive field (E c ). Specifically, the sample sintered at the lowest temperature (1350 • C) exhibited a nearly three-times increase in E c and the lowest dielectric loss (tanδ). The study highlights that sintering temperature plays a pivotal role in tailoring the ferroelectric properties of PPB compositions by influencing grain size, grain boundaries, and crystallographic parameters.

We report a real-time Stranski–Krastanov growth of reactively sputtered ScNx thin films on MgO(100). The epitaxial growth was limited to 5 nm at a substrate temperature (Ts) of ∼25 °C, while the self-sustaining epitaxial nature along the [100] azimuth was retained up to 25 nm in Ts = 250 and 500 °C samples due to enhanced adatom mobility. At Ts = 700 °C, the film showed a half order in situ RHEED pattern, with forbidden (hkl) planes indicating an N-deficient hcp ScN phase. The presence of defect densities, i.e., N vacancies and O interstitials, leads to a disorder in the ScNx system with a weak intergranular tunneling effect and the appearance of Raman relaxed first order transverse and longitudinal optical phonon modes and further leads to a metal like Seebeck coefficient. Higher grain boundaries at Ts = 25 °C and higher N out-diffusion at Ts = 700 °C pave the way for the incorporation of higher oxygen interstitial in these samples.