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.
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