Magnitude Of Lattice Distortion Research Articles

Lattice dynamics of the excitonic insulator Ta2Ni(Se1−xSx)5

Recently, we employed electronic polarization-resolved Raman spectroscopy to reveal the strongly correlated excitonic insulator (EI) nature of Ta2NiSe5, Volkov et al. [arXiv:2007.07344], and also showed that for Ta$_2$Ni(Se$_{1-x}$S$_x$)$_5$ alloys the critical excitonic fluctuations diminish with sulfur concentration x exposing a cooperating lattice instability that takes over for large x, Volkov et al. [arXiv:2104.07032]. Here we focus on the lattice dynamics of this EI family. We identify all Raman-active optical phonons of fully symmetric and ac-quadrupole-like symmetries and study their evolution with temperature and sulfur concentration. We demonstrate the change of selection rules at temperatures below the orthorhombic-to-monoclinic transition at Tc(x) that is related to the EI phase. We find that Tc(x) decrease monotonically from 328 K for Ta2NiSe5 to 120 K for Ta2NiS5 and that the magnitude of lattice distortion also decreases with the sulfur concentration x. For x < 0.7, the two lowest-frequency B2g phonon modes show strongly asymmetric lineshapes at high temperatures due to Fano interference with the broad excitonic continuum present in a semimetallic state. Within the framework of extended Fano model, we develop a quantitative description of the interacting exciton-phonon excitation lineshape, enabling us to derive the intrinsic phonon parameters and determine the exciton-phonon interaction strength, that affects the transition temperature Tc(x). We also observe signatures of the acoustic mode scattered assisted by the structural domain walls formed below Tc. Based on our results, we additionally present a consistent interpretation of the origin of oscillations observed in time-resolved pump-probe experiments.

The structural properties of LaRO3 (R=Cr, Mn, Fe): a first-principles calculation

We investigate the structural properties of three lanthanum transition-metal oxides (LaRO3, R=Cr, Mn, Fe) by the first-principles method based on density functional theory. The crystal parameters and electronic structures are calculated for three different oxides. The results show that the magnitude of lattice distortion is the strongest in LaMnO3, and the slightest in LaCrO3 among three compounds. The magnetic ground states of LaCrO3 and LaFeO3 are G-type antiferromagnet. However, for LaMnO3, GGA+U fails to predict the experimental ground state, and only the internal coordinates are relaxed and the experimental lattice constants are fixed, the LDA+U can reproduce the experimental A-type antiferromagnetic state. It is found that Fe-3d electrons in LaFeO3 occupy the localized orbital with the low energy, while Cr-3d and Mn-3d electrons in LaCrO3 and LaMnO3 occupy the relatively wide energy states due to the strong hybridization between d electrons and p electrons of oxygen atoms.

Determination of the range of lattice distortion in AIN sintered body by higher order laue zone pattern

To analyse the magnitude and range of lattice distortion which is responsible for the low thermal conductivity in aluminium nitride (AIN) crystal grains, the higher order laue zone (HOLZ) pattern of transmission electron microscopy was used. The HOLZ patterns obtained from various positions in the AIN crystal grain show that the AIN crystal lattice is distorted in the vicinity of the grain-boundary phase, and the magnitude of lattice distortion becomes large as it approaches the grain-boundary phase. Also, the range of distortion extends to approximately 300 nm from the grain-boundary phase.