Lattice dynamics across the ferroelastic phase transition in Ba2ZnTeO6: a Raman and first-principles study.

Structural phase transitions drive several unconventional phenomena including some illustrious ferroic attributes which are relevant for technological advancements. On this note, we have investigated the ferroelastic structural transition of perovskite-type trigonal Ba2ZnTeO6 across Tc ∼ 150 K. With the help of Raman spectroscopy and density-functional theory (DFT)-based calculations, we report new intriguing observations associated with the phase transition in Ba2ZnTeO6. We observed the presence of a central peak (quasi-elastic Rayleigh profile), huge softening in the soft mode, hysteretic phonon behavior, and signatures of coexistent phases. The existence of a central peak in Ba2ZnTeO6 is manifested by a sharp rise in the intensity of the Rayleigh profile concomitant with the huge damping (or softening) of the soft mode (at ∼31 cm-1) near Tc, shedding light on the lattice dynamics during the phase transition. This is further corroborated by our phonon calculations that show that the soft mode (Eg) in the high-symmetry structure involving TeO6 octahedral rotation (with Ba and Zn translation) condenses into Ag and Bg modes in the C2/m low-symmetry phase. While most of the phonon bands split below Tc confirming the phase transition, we observed thermal hysteretic behavior of phonon modes, which signifies the first-order nature of the transition and the presence of coexisting phases as corroborated by our temperature-dependent X-ray diffraction and specific heat measurements.

Spin-phonon coupling in ferrimagnet spinel CoMn2O4.

Coupling of material properties provides new fundamental insights and possibilities toward multifunctional devices. The spinel structures display strong coupling between different order parameters, as a consequence, exhibiting many fascinating properties, such as multiferroicity and superconductivity. Here, we have investigated the structural, magnetic, and vibrational properties of mixed-spinel CoMn2O4 stabilized in distorted tetragonal structures as evidenced from x-ray diffraction measurements. Magnetization measurements reveal two ferrimagnetic phase transitions at 185 and 90 K. Raman scattering measurements reveal the renormalization of phonon parameters for a few phonon modes at low temperatures, arising from spin-phonon coupling. The obtained value for λS2 is ∼2 cm-1. The strength of spin-phonon coupling (λ) is estimated according to the spins involved in the corresponding lattice vibrations and discussed.

Sb3+ -Er3+ -Codoped Cs2 NaInCl6 for Emitting Blue and Short-Wave Infrared Radiation.

Cs2 NaInCl6 double perovskite is stable, environmentally benign and easy to prepare. But it has a wide band gap (5.1 eV), and therefore, does not show optical and optoelectronic properties in the visible and short-wave infrared (SWIR) region. Here we introduce such functionalities in Cs2 NaInCl6 by codoping Sb3+ (s-electron doping) and Er3+ (f-electron doping) ions. Sb3+ doping introduces optically allowed 5s2 → 5s1 5p1 electronic absorption at the sub-band gap level, which then emits blue photoluminescence with ≈93 % quantum yield. But f-f electronic absorption of Er3+ is parity forbidden. Codoping Sb3+ -Er3+ , leads to transfer of excitation energy from Sb3+ to Er3+ , yielding SWIR emission at 1540 nm. Temperature (6 to 300 K) dependent photoluminescence measurements elucidate the excitation and emission mechanism. A phosphor converted light emitting diode (pc-LED) fabricated by using the codoped sample emits stable blue and SWIR radiation over prolonged (84 hours) operation at 5.1 V.