One can observe that the sample 0F as prepared (Red curve, Fig.1.) presented majority phase of YF3, with relatively broad peaks possibly related to the presence of small size particles due the low temperature about 120℃ in the synthesis process, where the growth process of the single-crystals it was not efficient. The secondary phase present in this sample (Rhombus filled in green.) could not be identified, but can be that this corresponding to layers of NH4Y2F7 connected to YF3 through the NH4

+ cations forming

a weak N-H-F bond. Nevertheless, it has been successfully removed after heating treatment at 400 ºC [1] (Blue curve, Fig.1.), such that for this sample single crystalline phase of YF3 was obtained. No change in the structure was noticed due to the presence of dopants. The XRD and EDX measurements show that as the amount of fluorine increases, layers of NH4Y2F7 connect to YF3 through the NH4

+ cations forming a weak N-H-F bond. As

can be seen Figure 5, we tuned the crystal phase by varying the Y3+/NHF4 ratio, this tuning is very abrupt varying from 0 to 30% in excess of fluorine, , nevertheless, from 30 to 60 % the structure keeps itself stable. The increase in nitrogen with increasing amount of NH4F reported in table.1 confirms the formation of layers of NH4Y2F7 with increasing amount of NH4F in the synthesis of the crystals.

Furthemore, an analysis of the samples with heating treatment varying from 0 to 60% in excess of the ammonium fluoride precursor ( NH4F) made in this moment Fig.6, the samples with heating treatment show no appreciable differences in crystalline structure, all them crystalize to orthorhombic YF3 structure. This can be explained by the

decomposition of reaction:

NH4Y2F7 → 2YF3 + NH3(g) + HF(g)in the treatment temperature.

in the treatment temperature.

The heating treatment in 4000℃ is used to break N-H-F bond (i.e., to destroy the layer structure by liberating NH3 gas), thereby the structure of NH4Y2F7 would collapse due to decomposition of interweaving layers of NH4

+ cations in the treatment temperature.

The figures 1 and 2 show that the samples as prepared According to Rajeshwar and Secco [2]the NH4 Y2F7 structure may be visualized as layers of YF6 octahedra (built up by mutual sharing of cornerand edges) with interlaying layers of NH4

+ cations, therefore, we can explain the secondary phase present in the sample (Rhombus filled in green. Fig. 1.) and formed structure not identified (Fig. 2.) as the process of formation of layers NH4 Y2F7 not finished.

one change of structure is seen when increasing the quantity of the ammonium fluoride precursor although this change is very abrupt varying from 0 to 30% in excess of fluorine, nevertheless, from 30 to 60 % the structure keeps itself stable.

The measures FTIR ( red curve of the fig. 8 to 10, and 11) show the peak located at 1427 cm-1 which correspond to the NH4

+ cation of NH4 Y2F7, however, after heating treatment this peak disappear. Also, to compare the nitrogen quantities between the

The heating treatment in 400℃ is used to break N-H-F bond (i.e., to destroy the

The heating treatment in 400℃ is used to break N-H-F bond (i.e., to destroy the layer structure by liberating NH3 gas), thereby the structure of NH4Y2F7 would collapse due to decomposition of interweaving layers of NH4

+ cations

tables. 1. and 2., we can see one reduction with the heating treatment. This results confirm the decompostion of NH4 Y2F7 structure to orthorhombic YF3 structure.

Besides the broad band in the region of 3000-3600 cm-1 is due to water O-H stretching vibrations, and it suffers a strong reduction with the heating treatment. As the amount of excess of fluorine used increased, the luminescence emissions becomes smaller (Figure.7). In all cases, transition lines are attributed to levels of Nd3+.

The results suggest that with increasing the excess of fluorine favours the formation of structure of NH4 Y2F7 where the formation is finished at 30% , nevertheless, from 30 to 60 % the structure keeps itself stable. After heating treatment in all samples, the structure NH4 Y2F7 decomposes itself to orthorhombic YF3 structure.

[1] P. Ghosh, A. Patra, "Tuning of crystal phase and luminescence properties of Eu3+ doped sodium yttrium fluoride nanocrystals", Journal of Physical Chemistry C, 112 (2008) 3223-3231.[2]