Raman spectroscopy has advanced considerably in the last several years due to rapid developments in instrumentation and the availability of theoretical methods for accurate calculation of Raman spectra, thus enormously facilitating the interpretation of Raman data. This review is restricted to cover papers mainly published in the Journal of Raman Spectroscopy, which serve to give a fast overview of recent advances in this research field as well as to provide readers of this journal a quick introduction to the various subfields of Raman spectroscopy. It also reflects the current research interests of the Raman community. Similar reviews of highly active areas of Raman spectroscopy will appear in future issues of this journal. Copyright © 2007 John Wiley & Sons, Ltd.

Recent Advances in linear and nonlinear Raman spectroscopy I

In order to explore a new family of phosphate phosphors, the synthesis and crystal structure of a novel phosphate family, Ca19M2(PO4)14 (M = Mg, Zn, Mn), were investigated. Eu3+ doped Ca19Mg2(PO4)14 red phosphor was also successfully synthesized with the objective of application in ultraviolet-based light-emitting diodes (LEDs) and field emission displays (FEDs). The characteristic photoluminescence properties were studied in detail by photoluminescence excitation (PLE), emission (PL) spectra and decay times. The Ca19Mg2(PO4)14:0.06Eu3+ phosphor offers higher brightness and thermal stability than the commercial Y2O3:Eu3+. The unexpected temperature-dependent luminescence from higher 5D1 states was observed and was explained via the configuration coordinate diagram. The cathodoluminescence (CL) spectra as a function of accelerating voltage and probe current were also measured. Excellent degradation properties with good color stability were obtained by continuous low-voltage electron-beam excitation of the phosphor. The results indicate that the phosphor Ca19Mg2(PO4)14:0.06Eu3+ can be a suitable red-emitting phosphor candidate for LEDs and FEDs.

Synthesis, crystal structure and luminescence characteristics of a novel red phosphor Ca19Mg2(PO4)14:Eu3+ for light emitting diodes and field emission displays

Using a wet-chemical method and without any surfactants or templates, various 3D hierarchical superstructures of Cu2PO4OH were synthesized by simply adjusting the pH. The resulting hierarchical superstructures were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), high resolution-TEM (HRTEM), Fourier transform infrared spectroscopy (FTIR), Raman, and UV–Vis spectroscopy. With an increase in pH from 2.5 to 7.0, the morphology of Cu2PO4OH varied from microrods to walnut-shaped microspheres of self-assembled nanoparticles. A possible mechanism for the formation of the Cu2PO4OH hierarchical superstructures was also proposed. The optical properties of the Cu2PO4OH hierarchical superstructures were strongly related to their morphologies and the size of the assembled crystallites. We further demonstrated the useful photocatalytic activity of these complex structures in the degradation of methylene blue (MB) dye under visible light irradiation (λ > 410 nm). The best photocatalytic performance was achieved by Cu2PO4OH with a walnut-shaped morphology due to the excellent absorption of visible light as well as a high BET surface area.

Synthesis of Cu2PO4OH hierarchical superstructures with photocatalytic activity in visible light

Ferrous and ferric phosphate crystalline compounds and glasses were studied using Raman spectroscopy. A comparison of the spectra from crystalline and glassy ortho-, pyro-, and metaphosphates indicates that similar phosphate anions constitute the structures of the respective materials, and some information about the compositional dependence of the phosphate-site distributions in the glasses can be gleaned from relative peak intensities. A correlation exists between the average P–O bond distance and the Raman peak frequencies in the crystalline compounds, and this correlation is used to provide information about the structures of the iron phosphate glasses. For example, the average P–O bond distance is estimated to decrease from about 1.57 A for iron metaphosphate glasses (O/P∼3.0) to 1.54 A for iron orthophosphate glasses (O/P∼4.0). These bond distances are in good agreement with those reported from diffraction studies of similar glasses.

A Raman Study of Iron–Phosphate Crystalline Compounds and Glasses

A novel blue-emitting phosphor Na3Sc2(PO4)3:Eu2+ was successfully synthesized by solid-state reaction. The crystal structure, bandgap, and photoluminescence properties were investigated in detail. Under n-UV excitation, the NSP:Eu2+ phosphor exhibits a narrow blue emission peaking at 458 nm with a FWHM of about 50 nm. The concentration quenching mechanism of the NSP:Eu2+ is dominated by the dipole–dipole interaction. Moreover, the NSP:Eu2+ shows better PL intensity, colour purity and thermal quenching properties compared to the commercial BAM phosphor. Non-equivalence replacement of Eu2+ ions for Na+ ions causing defects, inducing the abnormal thermal quenching behaviour. All these results suggest that the NSP:Eu2+ phosphor can be a potential candidate for n-UV LED applications.

Structure, photoluminescence and abnormal thermal quenching behavior of Eu2+-doped Na3Sc2(PO4)3: a novel blue-emitting phosphor for n-UV LEDs

A linear empirical correlation was established between Raman stretching wavenumbers of phosphorus–oxygen bonds and their bond lengths in inorganic crystalline phosphates. Although established on samples of inorganic crystalline phosphates, the correlation can be applied to glassy and amorphous phosphate materials (GAMs). Their unpolarized vibrational spectra are often similar because they are determined largely by short-range order. The correlation was used to predict PO bond length in the α-form of Li3PO4, which is stable over only a small range of temperatures below the melting-point. It can also be used to estimate the length of PO single bonds, terminal PO and OPO chain bonds and terminal double bonds in many technologically important amorphous materials containing phosphate groups. This correlation is expected to offer invaluable insight into the structures of phosphate species for which diffraction or other spectroscopic techniques provide incomplete structural information. That would enhance the value of Raman spectroscopy as a complementary technique in structural studies of phosphates. Copyright © 2004 John Wiley & Sons, Ltd.

J. C. A. Boeyens

Papers

Correlation between Raman wavenumbers and P?O bond lengths in crystalline inorganic phosphates