XRD data manifest that the typical PCN structure persists in the R-CNx material also after the high temperature treatment (Fig. ). The reflections corresponding to the typical for PCN (100), (002) and even the less intense (300) and (004) planes are clearly distinguishable in the R-CNx diffractogram, although having somewhat lower intensity owing to partial amorphization of PCN under thermal treatment at 900 C (Fig. ). FTIR data also point to similar structure of Y-CNx and R-CNx as all the typical bands characteristic of PCN are also found in the spectrum of R-CNx (Fig. ), which is not the case, for instance, when PCN is treated in NaCl melt at high temperatures. Although no definite conclusion can be made from it, the appearance of another band at around 3400 cm -1 in the spectrum of R-CNx suggests the modification of H-bonding in the treated material (Fig. ). The FT-Raman spectra of these two materials confirm the observations obtained by the IR-and XRD-measurements. According to the Raman spectra, the structure of Y-CNx and R-CNx are practically the same with only minor changes in intensity ratios in the fingerprint region (Fig. ). The most noticeable difference between the two spectra is the stark decrease of intensity of the broad band between 2300 and 2500 cm -1 in R-CNx. This band can be assigned to a second-order vibration of graphite-like carbon nitride and can only be observed in ordered structures, which supports the XRD-observed amorphization of the PCN. The band at 980 cm -1 can be assigned to the ring-breathing mode of the triazines present in the CNx structure while the band at 1235 cm -1 is specific to the ν(C-N) stretching vibration of the bridgeng amine moieties in the heptazine structure. FT-Raman spectroscopy shows that the ratio of bridging amine to the sym-triazine breathing mode decreases in R-CNx when comparing to Y-CNx. This could indicate a replacement of the bridging amine moieties by some other bridging functionality, which is further supported by the intensity increase of the shoulder located at 1662 cm -1 relative to the main band at 1618 cm -1 (aromatic ring stretching). This increase in intensity could indicate an increase in exo unsatured functionalities. Due to the complicated spectra and the possible loss of unpolymerized triazine impurities during the heat-treatment, the Raman spectra do not allow for definitive conclusions regarding the precise chemical structure, but can at least confirm changes to the binding conditions and offer a hint that amine bridges are replaced by unsaturated functionalities. Neutron diffraction, possibly the most adequate tool for structural characterization of carbon nitrides composed of low electron density atoms, was further employed as a sensitive technique to detect the structural changes in PCN caused by the thermal treatment. The structure function F(Q) (Fig. ) as well as the calculated pair distribution function (PDF) data (Fig. ) obtained from the neutron diffraction, however, show no significant differences in the analysed samples. This suggests that the PCN structure is mostly preserved in the R-CNx material and no significant changes of the interlayer distances corresponding to the corrugation or planarization can be evidenced, which were previously suggested to be responsible for the colour change. The C 1s XP spectra for both materials are dominated by the peak at 283.8 eV attributed to adventitious carbon, while the maxima at 285.7, 287.3 and 290.1 eV are assigned to C-NHx, N=C-N, and COOH species respectively (Fig. ). The N 1s spectrum of Y-CNx can be fitted with four peaks having their maxima at binding energy values typical of PCN 397.8, 399.7, 401.5 and 404.1 eV, the latter one being ascribed to - excitation in the aromatic polyheptazine system (Fig. ). The peak at 401.5 eV corresponds to the nitrogen in C-NHx groups and the maximum 397.8 eV is attributed to the C-N=C aromatic species. The contribution centred at 399.7 eV, usually assigned to N-C3 groups, should be rather assigned to the sum of these species and the triazine impurities, since the ratio between the maxima at 397.8 and 399.7 eV is far below 6, the value one might expect to obtain for the heptazine structure. We assume that the Y-CNx sample contains certain amount of non-condensed impurities, which is in agreement with the XPS data obtained for melamine, which shows equal contributions at 398.4 and 399.7 eV in its XP N 1s spectrum. The respective spectrum of the R-CNx sample has, on the other hand, a more conventional profile, where the peaks corresponding to C-N=C, N-C3 and C-NHx are clearly distinguished at 398.1, 399.5 and 400.5 eV respectively (Fig. ). The direct 13 C NMR excitation spectra show a typical profile for heptazine based PCN materials with maxima in the range of 157-156 ppm and 165-164 ppm corresponding to the carbon atoms in N-C=N and -C-NHx groups, respectively (Fig. ). The reduction of the peak intensity and its slight shift from 156.7 to 157.4 ppm with respect to that of Y-CNx is evident in the spectrum of R-CNx (Fig. ). This shift might be attributed either to a higher degree of disorder due to heptazine fragmentation in R-CNx caused by the thermal treatment or to the formation of some new species in the material. According to the tabulated data of 13 C NMR chemical shifts, the appearance of imine-like or azomethine groups (C=N-) formed as the result of heptazine fragmentation is one of the conceivable possibilities. Another possibility is the formation of -C-N=N-C-azo functions that are known to promote visible-light absorption of tuned carbon nitrides. The ratio between the peaks assigned to N-C=N to that of C-NHx in the 13 C spectra is around 1.1 for Y-CNx and 0.8 for R-CNx. We assume that this might be attributed to a partial fragmentation of the heptazine structure in the thermally treated material, hence the loss of aromatic carbon-species. The 1 H-13 C CPMAS spectra show the enhancement of the peak intensity at 165-164 ppm confirming its assignment to the C-NHx moieties (Fig. ). The intensity of the peak at around 157.4 ppm in the spectrum of R-CNx is not increased significantly, suggesting that the new species formed after the treatment is not in the immediate proximity to any H atoms. This leads us to the conclusion that the formation of -C-N=N-C-azo linkages is more likely (Fig. ). The 1 H- N CPMAS spectra show the same set of peaks at -191, -224, -245 and -265 ppm for both samples, which can be assigned to the peripheral heptazine N atoms, central heptazine nitrogen, nitrogen of NH-groups and of NH2 species, respectively (Fig. ) . A reduction of the FWHM and the intensity of the peak ascribed to the NH2 functions can be observed for R-CNx compared to Y-CNx (Fig. ). The peak fitting shows that the contribution centred at about -274 ppm is significantly reduced in the case of the R-CNx sample (Fig. ).