Pressure-dependent Raman Spectroscopy Research Articles

Synthesis and Properties of the Ba2PrWO6 Double Perovskite.

We report details on the synthesis and properties of barium praseodymium tungstate, Ba2PrWO6, a double perovskite that has not been synthesized before. Room-temperature (RT) powder X-ray diffraction identified the most probable space group (SG) as monoclinic I2/m, but it was only slightly distorted from the cubic structure. X-ray photoelectron spectroscopy confirmed that the initial (postsynthesis) material contained praseodymium in both 3+ and 4+ charge states. The former (Pr3+) disappeared after exposure to UV light at RT. Photoluminescence studies of Pr3+ revealed that Ba2PrWO6 is an insulator with a band gap exceeding 4.93 eV. Pressure-dependent Raman spectroscopy excluded the possibility of a phase transition up to 20 GPa; however, measurements between 8 and 873 K signified that there might be a change toward the lower symmetry SG below 200 K. Electron paramagnetic resonance spectra revealed the presence of interstitial oxygen which acts as a deep electron trap.

Zero Thermal Expansion in Ta2Mo2O11 by Compensation Effects.

Although zero thermal expansion (ZTE) materials have broad application prospects for high precision engineering, they are rare. Here, a new ZTE material, Ta2Mo2O11 (αl = 0.37 × 10-6 K-1, 200-600 K), is reported. A joint study of high-resolution synchrotron X-ray diffraction, temperature- and pressure-dependent Raman spectroscopy, and first-principles calculations was performed to investigate the structure and dynamics of Ta2Mo2O11 with the aim of understanding its ZTE mechanism. Ta2Mo2O11 displays a layered structure, stacking along the [001] direction. Analysis of the phonon modes indicates that positive and negative contributions to thermal expansion are balanced, and a shrinkage occurs along the layers, while the interlayer distance expands with increasing temperature, thus giving rise to the ZTE behavior of Ta2Mo2O11. The present study provides a promising ZTE material and new insights into the mechanisms of thermal expansion.

Lattice dynamics of the topological Dirac semimetal LaAgSb2 with charge density wave ordering

LaAgSb$_{2}$ is a rare material, which offers the opportunity to investigate the complex interplay between charge density wave (CDW) ordering and topology protected electronic band structure. As both of these phenomena are governed by the structural symmetries, a comprehensive study of the lattice dynamics is highly desirable. In this report, we present the results of temperature and pressure dependent Raman spectroscopy and x-ray diffraction in single crystalline LaAgSb$_{2}$. Our results confirm that Raman spectroscopy is a highly sensitive tool to probe CDW ordering phenomenon, particularly the low-temperature second CDW transition in LaAgSb$_{2}$, which appears as a very weak anomaly in most experiments. The crystal orientation-dependent measurements provide the evolution of Raman modes with crystallographic symmetries and can be further studied through group symmetry analysis. The low-temperature x-ray diffraction data show the emergence of structural modulations corresponding to the CDW instability. The combined high-pressure Raman spectroscopy and synchrotron x-ray diffraction reveal multiple structural phase transitions through lowering of crystalline symmetries, which are also expected to lead to electronic topological transitions.

Indications for Lifshitz transitions in the nodal-line semimetal ZrSiTe induced by interlayer interaction

The layered material ZrSiTe is currently extensively investigated as a nodal-line semimetal with Dirac-like band crossings protected by nonsymmorphic symmetry close to the Fermi energy. A recent infrared spectroscopy study on ZrSiTe under external pressure found anomalies in the optical response, providing hints for pressure-induced phase transitions at $\approx$4.1 and $\approx$6.5 GPa. By pressure-dependent Raman spectroscopy and x-ray diffraction measurements combined with electronic band structure calculations we find indications for two pressure-induced Lifshitz transitions with major changes in the Fermi surface topology in the absence of lattice symmetry changes. These electronic phase transitions can be attributed to the enhanced interlayer interaction induced by external pressure. Our findings demonstrate the crucial role of the interlayer distance for the electronic properties of layered van der Waals topological materials.

Competition and coexistence of polar and non-polar states in Sr1−xCaxTiO3: an investigation using pressure dependent Raman spectroscopy

The competition and cooperation between ferroelectric and anti-ferro-distortion (AFD) instabilities are studied using pressure dependent Raman spectroscopy on polycrystalline powder samples of Sr1−xCaxTiO3(x = 0.0, 0.06, 0.25, 0.35). For x = 0.0 composition, a broad polar mode is detected in the Raman spectra above 6 GPa, while for x = 0.06 composition, the polar modes appear well above 9 GPa where the AFD modes showed strong suppression. In x = 0.25 and 0.35 composition, the application of small pressure resulted in the appearance of strong AFD modes suppressing the polar modes. At elevated pressures, re-entrant polar modes are observed along with the broad AFD modes and some new peaks are also observed, signifying the lowering of local symmetry. The reappearance of polar modes is found to be related to pressure induced symmetry disorder at local level, suggesting its electronic origin. The re-entrant polar modes observed at higher pressure values are found to be significantly broad and asymmetric in nature, signifying the development of ferroelectric micro regions/nano domains coexisting with AFD. The lower symmetry at local length scale provides a conducive atmosphere for coexisting AFD and FE instabilities.

A study on the Fermi resonance of phenol under the effects of pressure and temperature by Raman spectroscopy

The ν1–ν18a Fermi resonance (FR) of phenol were investigated by pressure-dependent Raman spectroscopy from atmospheric up to P=15.2GPa and temperature-dependent Raman spectroscopy from 40 down to T=−180°C, respectively. In the case of pressure, we found the Fermi coupling coefficient W, which were calculated based on the FR theory, exposed a value turnover between 1.912 and 2.244GPa in the process of increasing the pressure. This turnover phenomenon of the Fermi coupling coefficient W has been ascribed to the crystal structure of phenol evolving towards a more symmetric structure with pressure, from a structure like a pseudo-threefold helical chain at ambient pressure to like a ribbon arrangement at 1.912GPa, then to adopt a herringbone arrangement at much higher pressure. On the other hand, we also found the Fermi coupling coefficient W exhibited monotonic reduction without turnover points appearing by decreasing the temperature. The tendencies of the Fermi coupling coefficient W with temperature were in good agreement with the pressure dependence of the Fermi coupling coefficient W in the region of ambient to 1.912GPa, indicating that the effect of pressure and temperature on the FR of phenol in this region might be the same. A conformation evolving induced by pressure and temperature on the ν1 and ν18a FR of phenol have been analyzed.

Pressure-tuned vibrational resonance coupling of intramolecular fundamentals in ammonium azide (NH 4N 3)

The peculiar behavior of the symmetric stretch of the N 3-anion and the bending mode of the NH 4-cation in the high-pressure phase II (above 3 GPa) of the molecular ionic crystal NH 4N 3 has been studied by pressure-dependent Raman spectroscopy up to a pressure of 20 GPa. The spectra in the pressure range above 10 GPa reveal a remarkable intensity transfer between bands and a level repulsion (characteristic of vibrational resonance coupling) that is mediated by hydrogen bonding between the N 3- and NH 4-ions. These observations may be useful for theoretical studies of the potential energy surface and for studying the nature of hydrogen bonding in ammonium salts. With respect to the biological activity of the ammonium ion, the observed resonance coupling may be relevant to vibrational energy transfer in organic systems.

Pressure induced polymorphism in ammonium azide (NH 4N 3)

Pressure-dependent Raman spectroscopy studies reveal polymorph phase transition in simple molecular ionic crystal NH 4N 3 at pressure ≈3 GPa unobserved by recent ab initio evolutionary structure searches. Hydrogen bonding is spectroscopically evident in both low- and high-pressure phases. The strength of hydrogen bond appears to be modified at the phase transition: in the low-pressure phase NH 4N 3 behaves as system with very strong hydrogen bonding whereas changes of spectra with pressure in the high-pressure phase are indicative of weak or medium-strength hydrogen bonds. The high pressure phase is most likely thermodynamically stable at least up to pressure ≈55 GPa contradicting the ab initio studies predicting transformation of NH 4N 3 to nonmolecular hydronitrogen solid at 36 GPa.

Pressure-dependent Raman spectroscopy of metal-oxo multiple bonds in rhenium(V) and osmium(VI) complexes

The pressure dependence of stretching vibrations involving metal-oxo multiple bonds is reported and discussed. Metal–ligand vibrational frequencies involving double and triple bonds increase with pressure according to the nature of the metal center and ancillary ligands. Pressure-dependent luminescence spectra, at room temperature, of a trans-dioxo rhenium(V) complex exhibit resolved vibronic structure with a progression in a high frequency metal-oxo stretching mode that shows a change of the intensity distribution with pressure. Raman and luminescence spectra are used to rationalize the unusual decrease of the metal-oxo stretching frequency observed for two complexes at pressures over 30 kbar.