No abstract

We present a Raman scattering study of optical phonons in hexagonal BN for temperatures ranging from 80 to 600 K. The experiments were performed on high-quality, single-crystalline hexagonal BN platelets. The observed temperature dependence of the frequencies and linewidths of both Raman active E 2g optical phonons is analyzed in the framework of anharmonic decay theory, and possible decay channels are discussed in the light of density-functional theory calculations. With increasing temperature, the E high 2g mode displays strong anharmonic interactions, with a linewidth increase that indicates an important contribution of four-phonon processes and a marked frequency downshift that can be attributed to a substantial effect of the four-phonon scattering processes (quartic anharmonicity). In contrast, the E low 2g mode displays a very narrow linewidth and weak anharmonic interactions, with a frequency downshift that is primarily accounted for by the thermal expansion of the interlayer spacing.

A comparison between Raman spectra of polycrystalline Ca10(PO4)6(OH)2 and β-Ca3(PO4)2 is reported. Both compounds exhibit similar Raman spectra, which are dominated by the internal modes of the PO4 3- tetrahedra. However, several characteristic features of the Raman spectra allow us to establish a distinction between these two calcium phosphates. Besides the presence of peaks associated with vibrations of the OH- group in the Raman spectrum of hydroxyapatite, which are highly sensitive to sample crystallinity, other characteristic features such as the width of the PO4 3- internal bands can be used to distinguish between hydroxyapatite and β-Ca3(PO4)2.

Hexagonal boron nitride is a model lamellar compound where weak, non-local van der Waals interactions ensure the vertical stacking of two-dimensional honeycomb lattices made of strongly bound boron and nitrogen atoms. We study the isotope engineering of lamellar compounds by synthesizing hexagonal boron nitride crystals with nearly pure boron isotopes (B and B) compared to those with the natural distribution of boron (20 at%B and 80 at% B). On the one hand, as with standard semiconductors, both the phonon energy and electronic bandgap varied with the boron isotope mass, the latter due to the quantum effect of zero-point renormalization. On the other hand, temperature-dependent experiments focusing on the shear and breathing motions of adjacent layers revealed the specificity of isotope engineering in a layered material, with a modification of the van der Waals interactions upon isotope purification. The electron density distribution is more diffuse between adjacent layers inBN than in BN crystals. Our results open perspectives in understanding and controlling van der Waals bonding in layered materials.

No abstract

Hexagonal boron nitride (h-BN) is a layered crystal that is attracting a great deal of attention as a promising material for nanophotonic applications. The strong optical anisotropy of this crystal is key to exploit polaritonic modes for manipulating light-matter interactions in 2D materials. h-BN has also great potential for solid-state neutron detection and neutron imaging devices, given the exceptionally high thermal neutron capture cross section of the boron-10 isotope. A good knowledge of phonons in layered crystals is essential for harnessing long-lived phonon-polariton modes for nanophotonic applications and may prove valuable for developing solidstate 10 BN neutron detectors with improved device architectures and higher detection efficiencies. Although phonons in graphene and isoelectronic materials with a similar hexagonal layer structure have been studied, the effect of isotopic substitution on the phonons of such lamellar compounds has not been addressed yet. Here we present a Raman scattering study of the in-plane high-energy Raman active mode on isotopically enriched singlecrystal h-BN. Phonon frequency and lifetime are measured in the 80-600-K temperature range for 10 B-enriched, 11 B-enriched, and natural composition high quality crystals. Their temperature dependence is explained in the light of perturbation theory calculations of the phonon self-energy. The effects of crystal anisotropy, isotopic disorder, and anharmonic phonon-decay channels are investigated in detail. The isotopic-induced changes in the phonon density of states are shown to enhance three-phonon anharmonic decay channels in 10 B-enriched crystals, opening the possibility of isotope tuning of the anharmonic phonon decay processes.

The giant birefringence of layered h-BN was demonstrated by analyzing the interference patterns in reflectance and transmittance measurements in the mid-infrared to the deep ultraviolet energy range. The refractive index for polarization perpendicular to the c axis is much higher than the refractive index for polarization parallel to the c axis, and it displays a strong increase in the ultraviolet range that is attributed to the huge excitonic effects arising from the unique electronic structure of h-BN. Thus, h-BN is shown to exhibit a giant negative birefringence that ranges from −0.7 in the visible to −2 in the deep ultraviolet close to the band gap. The electronic dielectric constants for polarization perpendicular and parallel to the c axis were determined to be ε ⊥ ∞ = 4.95 and ε ∞ = 2.86, respectively. The anisotropy we find in high-quality h-BN is significantly larger than proposed in previous experimental studies but in excellent agreement with ab initio calculations.

We present a micro-Raman study on the hydration and carbonation of the main silicate phases of Portland cement, i.e. monoclinic dicalcium silicate (C 2 S) and monoclinic tricalcium silicate (C 3 S). We investigate the reaction products and the loss of crystallinity induced by hydration on these two compounds. In the CO 2 -contaminated pastes we find that calcite, aragonite, and vaterite are inhomogeneously formed. We study sample cross sections to evaluate the maximum depth at which CaCO 3 is formed. We find that carbonation is limited to the first 500-1000 µm from the surface in the C 3 S pastes, while in C 2 S pastes CaCO 3 is formed well beyond this depth. Our results show the great potential of Raman spectroscopy in the study of the chemistry of cements.