1 Introduction

Dysprosium (Dy) is an element that belongs to the rare earth classification, it has the [Xe] 4f106s2 electronic configuration. This rare earth combines with the carbonate CO32- ion under suitable conditions, thus generating dysprosium dioxymonocarbonate Dy2CO33( s)^[1]. Considered the broad search applied at high-performance luminescent devices, photocatalysts, etc., associated at electronic and optical, and structural properties arising of the 4fs→4fs electronic intra-transition, it is possible to carry out detailed studies of this nanomaterial. Have been investigated due to their optical and structural properties, for which they are important with high potential associated with the wide range of applications ^[2]. Here, are some of the Dy2CO33( s) synthesis techniques: using ammonium bicarbonate as a precipitant ^[3], gravimetrically ^[4], sonochemical ^[5], via an amorphous precursor ^[6], among other interesting techniques, both chemical and physical ^[7]. Another inorganic nanomaterial that contains Dy3+ cation in its crystal structure is dysprosium oxide Dy2O3, which has been prepared by the following techniques; Vacuum Sintering ^[8], Rapid thermal annealing ^[9], Atomic Layer Deposition ^[10]. Because of its thermodynamic stability relative, Dy2O3( s) is considered a suitable nanocrystal for resisting corrosion in stainless steels at elevated temperature, is employed in microelectronic devices such as resistive switching devices ^[11]. It is important to point out that of the immense and overwhelming variety of synthesis techniques, it is sought to apply some that are simple and also nonpolluting, with the aim of obtaining nanomaterials at a relatively low cost. Despite the advantages of green synthesis, current methods face challenges with appropriate solvent selection, reaction temperature, reagents process chemical parameters that affect the synthesis process ^[12]. Here, we apply the green technique or Chemical Bath Deposit (CBD) in which the parameters of crystalline growth are easily controlled ^[13,14,15]. And so, we report a simple method for prepare Dy2CO33( s) powder, and subsequent thermal annealing temperature (TT) in air atmosphere at ∼600∘C to study Dy2CO33(a)→Dy2O3( s) transition. In this manuscript, we demonstrated that Dy2O3( s) could be successfully prepared by CBD technique from Dy2CO33( s) and it was worth noting that the size of Dy2O3( s) nanocrystals could be easily controlled by adjusting the synthetic parameters. In addition, the effects of some optical and structural properties on Dy2CO33(a)→Dy2O3( s) transition, were examined. We believe that this manuscript will bring a novel strategy to synthesis of Dy2CO33( s) and Dy2O3( s) nanocrystals. Dy2O3( s) it highly suitable for applications, such as in filters, modulators, switches, and anti-reflection coatings ^[16]. Due to the interesting optical properties of these nanomaterials, it is possible to investigate in detail the 4fs→4fs electronic intra-transitions located at UV-Vis-IR region, associated at Dy3+ cation. These are generally, reflectance (%R), Transmittance (%T) percentage and Absorbance, which have been examined. The analysis of the chemical kinetics of crystalline growth is carried out in the key stage and is based on the experimental synthesis applying the green chemistry or CBD technique. This paper presents preliminary results of crystalline growth in synthesizing inorganic materials to analyze the chemical equilibria in the growth of Dy2CO33( s) nanocrystals, which is carried out in aqueous solution with pH alkaline (∼8.3), the parameters remain constant during the evolution of the chemical equilibria except the reaction temperature. Two reaction temperatures are systematically chosen, ∼20∘C and ∼90 ∘C±2∘C. A systematic study of the optical and structural properties are considered. The presentation of chemical kinetic equilibria are generally key parameters to achieve the expected chemical product. This is done in principle in order to find the correlation of the proposed chemical kinetic mechanism with the structural properties and these in turn with the optical ones.

2 Chemical reactions and experimental procedure

The crystalline growth of Dy2CO33( s) nanocrystals is systematically prepared by CBD green technique and the theoretical experimental model is proposed as a first approximation, in a comprehensive manner according to the key stages previously presented ^{[13,14,15,17]}. A parameter of interest associated with the crystalline growth of nanoparticles in the solid phase is related to the van der Waals forces present in the aqueous solution in which the positive and negative ions are found. On other hand, the electrostatic van der Waals interactions associated with anions and cations ^[18]. A systematic study of the different electrostatic interactions in crystal growth is necessary for a better understanding of the chemical kinetic phenomenon ^[19]. It also shows charged and polar species through forming ion pairs, and other electrostatic interactions, which favor the creation of strong chemical bonds. The electrostatic interactions are complex because they depend on the molecular environment associated with the geometric distribution of the charge of the ion-cation and the symmetry of the electrostatic charge that surrounds the chemical species which is in equilibrium in some cases due to electrostatic interaction of the solvent and the ionic species. The energetic polarization of the ionic species (ions and cations, including the solvent) present in the aqueous solution correlates linearly with the magnitude of the electric field, which is difficult to predict exactly and is still under study ^[20]. For the kinetic mechanism considered here by us, the carbonate CO32- ion has been identified in aqueous solution through FT-IR studies, hydrolysis of thiourea SCNH22 generates CO32-, ion an NH3 molecules ^[13,15,21].

Here, we present a brief and simple exposition of the chemical properties of CO32- and OH- ions, with the aim of interpreting in a simple way the interactions in the formation of the chemical bonds and the synthesis of Dy2CO33( s) nanomaterial. The Lewis ionic molecular structure of CO32- ion, electronic delocalization structure has three π bonds (π bond, sp2 hybridization) ^[22]. The CO32- ion is found to show an ionic arrangement with trigonal molecular geometry in which the central carbon atom is surrounded by three oxygen (O 2- ions) with plane geometry. The configurational phenomenon typical in organic and inorganic molecules with conjugated double π-bonds and in turn, associated with π-resonance. In the molecular design of π-conjugated systems has been investigated. This physicochemical behavior and relative thermodynamic stability, which in principle quantitatively allows us to understand the different chemical equilibria associated with the products obtained ^[23]. We found in preliminary reports, rare earths cations prepared by CBD technique, present the following order OH- and CO32- of capture (under the same experimental conditions of chemical synthesis); Sm(OH)3<Ho(OH)3<NdOHCO3<ErOOH<CeO2^{[14,15,25,26]}. This capture order is linked to properties associated with electron affinity, ionic radii, electronegativity, to mention just a few of these intrinsic characteristics, and requires formal study. In aqueous solution, CO32- ion exist together in a dynamic equilibrium relative and carbon dioxide CO2(aq) soluble in water; which presents the thermodynamic equilibrium through the conversion to carbonic acid H2CO3(aq). The acid-base HCO3- equilibria, and the transport of electric charge from one species to another is complex and requires care since it formally depends on the medium and/or the type of solvent that it’s difficult to understand them without a quantitative model ^[27,28]. Tentatively, in crystal growth we propose that the alkaline medium, ionic radii, electronic affinity and synthesis conditions, of capture of OH- and CO32- ions are crucial in obtaining the corresponding hydroxide and/or carbonate ^{[14,15,25,26,29]}.

The equilibria in this acid-base system are shown below, without losing sight of the fact that they depend on the acid-base aqueous medium.

The ion exchange process slowly generates the chemical phenomenon with ionic and molecular rearrangements associated with the slow and gradual capture of OH- ion with simultaneous release of NH3 molecules. In the initial stage, the Dy(OH)3(aq) are present, it releases OH- ions and through molecular exchange with ligands (L) NH3, created indirectly the coordination complex DyNH363+ cation. It is at a key stage in chemical synthesis since the importance of the type of molecule L and its properties associated with size, location of donor electron pairs, electronegativity, etc., of L are considered, these roles include small inorganic molecules with ability to complexate with tunable polarity to preferentially bind crystallographic domains, to mention just some of the multiple parameters associated with crystal growth ^[30]. The following equilibria associated with the acidity constant (ΣlogK) for species considered here, is the following sequence of acidic species reported, ΣlogK=logk1k2k3k4=-17.62±0.07^[31].

The complex of coordination DyNH363+ cation, the following chemical equilibria is proposed. Here, we consider the key stage associated with the chemical kinetic mechanism in the chemical synthesis of the product. Parameters chemical such as pH, molecular hydration of the L, stirring, temperature and nucleation time and their implication in the study of such precipitation processes will be investigated ^[32].

According to the negative sign (-) of ΔG0, it is found that the probable and spontaneous thermodynamic equilibrium According to the chemical equilibria presented previously, the kinetics of the crystalline growth of Dy2CO33( s) and Dy(OH)3( s) nanocrystals

We mention the key chemical and physical parameters associated with crystalline growth (i) nucleation and particle growth, the clusters formed undergo rapid decomposition and growth within the context of rare earth nanocrystal synthesis ^[33,34], (ii) then the particles combine to grow up to nanopowder. (iii) The particles distribution becomes remarkably narrow during the first stages of coarsening and progressively turns to a small crystal time passes.

3 Results and discussion

Figure 1a) shows the X-Ray Diffraction (XRD) diffractograms of (a) Dy 20 and Dy 90 (b) Dy 20TT and Dy 90TT samples. Different relative intensity and broadening in crystalline reflections are generally assigned at nanocrystals ^{[13-15,17,24-6]}. Here, four different crystalline reflections are observed in the Dy 20 sample, located at 2θ∼19.89∘,29.71∘,46.16∘ and ∼50.97∘, while in the Dy 90 nanomaterial, five reflections located at angular positions 2θ∼19.46∘,29.59∘,45.77∘,55.08∘ and 59.67∘, respectively. The reflections correspond to those of the lanthanide-type rare-earth carbonates, and all the reflections can be indexed in the orthorhombic system ^[35], both nanomaterials are identified to Dy2CO23( s) nanomaterial, which is in agreement with Niasari et al. ^[5] and Nasrabadi1 et al. ^[2]. However, the plane crystalline located at 2θ∼51.0∘ (is indicated in the diffractogram by an asterisk in the upper part of the crystalline plane) can be readily indexed to a pure hexagonal phase of Dysprosium hydroxide Dy(OH)3(JCPDS19-0430)^[36]. A particular structural characteristic of nanomaterials is associated with the gradual widening and relative decrease in the crystalline planes, a situation here observed in our chemical compounds. A particular and important feature of the structural behavior of organic and inorganic materials is related to: crystalline stacking faults, grain boundaries, stoichiometry, and in general the intrinsic native crystalline defects, which are related in principle with the widening and decrease of the crystalline planes that inevitably originate during crystalline growth ^[36]. According to the experimental XRD diffraction patterns, it is possible to observe that a crystalline plane is oriented at (222) direction in both nanomaterials with TT and without TT, respectively, associated with the experimental conditions of synthesis and requires a deep and detailed study applying a suitable theoretical model. The preferred orientation and this is associated with the symmetrical structural part of the ionic and/or molecular packing. We discussed this structural phenomenon in a preliminary way in some previous reports for organic and inorganic nanocrystals ^[13,37,38]. The plane of lower relative crystalline energy is favored by optimal chemical and physical parameters, is generated by various parameters of crystalline growth related and plausibly raised considering ionic packing, which reaches a relative energy minimum ^[39]. However, the occurrence of these structural types appears to be random with no rules governing their structural formation. As a first approximation to these XRD experimental results, we deemed that, according to the reflections plane observed, structural symmetry associated as a result of the presence of the stereogenic carbon atom CO32-. In aromatic compounds, the molecular stability originates from the π-resonance effect ^[40]. The surface energy of the nanocrystals, is a relevant property of a crystal that is crucial to the understanding of various structural phenomena like surface segregation, roughening, catalytic activity, and the crystal’s equilibrium shape ^[41]. The main parameters affecting the structural properties, such as size, shape, concentration of nanoparticles, aggregation, are of interest to understand the optical behavior. We consider that some nanomaterials show the aforementioned preferential orientation, which is tentatively associated with the slow stirring of the solution containing the progenitor reagents and the long reaction time in crystal growth. The XRD of Dy 20 TT and Dy 90 TT samples are shown in Fig. 1b), according to (JCPDS Card No. 86-1327) in cubic bixbyite phase of Dy2O3(s) cubic phase is identified and no other reflections exists ^[42]. Both inorganic materials have the same cubic crystalline phase and preferential orientation of the (222) reflection. The effect of TT on Dy 20TT and Dy 90TT inorganic nanocrystals, can be understood according to the following plausible chemical reaction Dy2CO23⟹∼600∘CDy2O3( s)+3CO2( g)↑.

Figure 1 X-Ray Diffraction (XRD) diffractograms of a) Dy 20, Dy 90 and b) Dy 20 TT, Dy 90 inorganic nanocrystals. 

The grain size (GS) average is quantified numerically using the Scher GS=kλ/βcosθ eq., were λ is the wavelength of X-ray radiation (∼0.154Å), k the Scherer’s constant (k∼0.9),θ (in radians) the characteristic X-ray radiation and β is the Full Width at Half-Maximum (FWHM) of the crystalline plane (in radians). A set of physical parameters characterizes the nanocrystals: size, shape, structure, and optical properties to find some correlation between these. Table I and II presents a compilation of the numerical values of the mean grain size (GS) and the FWHM of Dy 20, Dy 90 and Dy 20 TT, Dy 90 TT samples. The numerical average value reported here for the GS of the nanocrystals is located at ∼2.32  3.25 nm for the Dy 20 sample and ∼2.79-21.78 nm in the case of Dy 90 nanocrystal, respectively.

Raman analysis was carried out for all samples. Figure 2 shows Raman spectra of (a) Dy 20 and Dy 90, (b) Dy 20 TT and Dy 90 TT nanocrystals. Four bands located at ∼150-1800 cm-  1 frequency range assigned to internal vibrations of CO32- ions: v1-symmetric stretching ∼1098 cm-1v3-asymmetric -C-O stretching at ∼1063 cm-1, were observed. The vibrational mode are shifted to slightly higher wavenumbers when compared to the vibrational mode in CO32- pure, due to the inner-sphere coordination of Dy3+ cation with CO32- ligands (L). The frequencies for the vibrational mode of CO32- ion situated at ∼1063 cm-1 and ∼1415 cm- 1 , are assigned when the first two modes nondegenerate. Lattice vibrational mode, V3- asymmetric -C-O stretching ∼1063 cm-1,v1-symetric O-C-O stretching ∼1098 cm-1, and the intensity of the v14 band, associated with lattice vibration parallel to the c-axis, correspond to the in-plane bending v4 of -C-O, out-of-plane bending v2 of O-C-O, doubly degenerate asymmetric stretching v3 of O-C=O, and the overtone of the out-ofplane bending mode 2v2 vibrations, respectively ^[43]. The splitting of the nondegenerate bands is generally an indication of nonequivalent CO32- ions ^[44]. Three weak Raman vibrational mode were found and assigned to Ag,E2g and E1g liberation modes. A vibrational spectrum of intra-and inter-metal and semimetal bonds study using Raman spectroscopy complexes of Ln-(OH)3 with symmetry analysis and vibrational mode assignments ^[45]. The primary vibrational mode in the solid phase for Dy(OH)3( s) occur at ∼397 cm-1 and that the vibrational mode are sharp and relatively narrow bands. The spectrum for Dy 20 and Dy 90 samples exhibit vibrational mode in the regions near those reported for Dy(OH)3. The sharp vibrational mode situated at ∼1040-1060 cm-1 result from the coordinated CO32-L^[4]. Raman spectrum of Dy 20 and Dy 90 nanocrystals shows for active vibrational modes that would indicate that O4 is a coordinated -OH ion. Metal hydroxides (M-OH) typically exhibit active Raman vibrational mode and deformation modes below ∼1200 cm-1, the observed average shift may be attributed to size effects or surface stress/strain ^[46]. For the Dy 20 TT and Dy 90 TT nanocrystals obtained, strong Raman vibrational modes were observed at ∼377 cm-1, which were assigned to the Fg mode ^[47]. The cubic phase was implied by the strong Raman intensity vibrational modes, with a large charge of polarizability appearing during the vibration. Raman active region and Dy2O3( s) cubic phase were in good agreement with previously published evidence ^[48]. Raman vibrational mode at ∼477 cm-1 and ∼611 cm-1 were assigned to the combination of Fg+Ag stretching vibration modes. For the C-type Dy2O3 with a space group of Ia-3, the irreducible representations were ascribed to 4 Ag+4Eg+14 Fg+5 A2u+5Eu+16 Fu, where Ag,Eg, and Fg are Raman active ^[49]. The Raman vibrational mode of the Dy 20TT and Dy 90 TT samples, shows a higher crystallinity due TT effect. The bridged oxygen modes band situated at range ∼300-600 cm-1 region includes the bands at ∼377 cm-1 and ∼477 cm-1 of Dy-O characteristic vibration ^[50], illustrating the Fg+Ag and A1 g modes, respectively of cubic phase Dy2O3( s)^[51].

Figure 2 Raman spectra of a) Dy 20 and Dy 90, b) Dy 20 TT and Dy 90 TT nanocrystals. 

Figure 3 displays the UV-Vis (a) Reflectance(%R), (b) Transmittance (%T) and (c) absorbance vs. wavelength (λ) spectrum of Dy 20, Dy 90, Dy 20 TT and Dy 90 TT. According to the optical behavior of (%R), (%T) and absorbance, let’s divide the graph into three different regions. The optical spectral of Dy 20 and Dy 90 , the %T located at two regions is directly observed. Examining the optical behavior in the %R spectra, we have the following: at λ∼200-750 nm  (∼6.20-1.65eV), the %R shows an increase with the λ(nm) until reaching a relative maximum of ∼68%, for Dy 20, Dy 90 and Dy 20 TT, Dy 90 TT of ∼75%. An increase in the %R is appreciated, which is associated with the effect of the TT with the structural properties of inorganic materials ^[52,53]. For located at range λ∼750 1750 nm  (∼1.65-0.70eV), shows oscillatory optical behavior of %R, with several relative maximums/minimum situated at range ∼100-20%, at ∼1750-2500 nm  (∼0.7-0.04eV)%R shows two isolated relative maximum, which decreases up to ∼20%. UV-Vis region (λ ∼200-750 nm), it shows an increase until reaching the relative maximum of ∼70%. Next, the %T shows oscillatory behavior with bands associated with 4fs→4fs electronic intratransitions, related at electron configuration [Xe] 4f96s0 of Dy3+ cation. The in-line %T curves of Dy 20 TT and Dy 90 TT samples, decreases, which is due to the incomplete grain growth observed and weak agglomeration of nanocrystals caused by the surface energy will lead to different grain boundary migration rate at crystal growth. The nanocrystals show the transparency, and their in-line %T reach ∼70.0% at ≥800 nm, For Dy 20 TT and Dy 90 TT samples, the optical behavior of R% andT%, is generally associated with 4f s→4f s electronic intra-transitions from one material to a different one, typical of TT materials ^[54]. Both nanomaterials, oscillatory behavior of R% and T% located at UV-Vis region situated at range ∼300-800 nm  (∼4.13-1.55eV) can be seen, which is identified with 4fs→4fs electronic intra-transitions corresponding to rare earths. The absorbance spectrum show absorption bands (AB) situated at λ(nm) and energy (eV) of ∼353 nm(∼3.51eV),∼389 nm(∼3.18eV),∼457 nm(∼2.71eV),∼801 nm(∼1.54eV),∼907nm(∼1.36eV),∼1095nm(∼1.13eV),∼1290 nm(∼0.96eV),∼1415 nm(∼0.87eV),∼1691nm(∼0.73eV) and ∼1940 nm(∼0.63eV). They are generally assigned to 4fs→4fs electronic intra-transitions of Dy3+ cation, at the ground state  6H15/2 into different excited states  6P7/2, 4 F7/2, 4I15/2, 4 F9/2, 6 F3/2, 6 F5/2, 6 F7/2, 6 F9/2+6H7/2 6 F11/2+6H9/2 and  6H11/2, respectively ^[55,56]. The AB were assigned to the electronic intra-transitions from ground state  6H15/2 to different excited states such as  4I13/2→4 F7/2,4I15/2,6 F3/2,6 F5/2,6 F7/2→6H5/2,6 F9/2→6H7/2,6 F11/2→6H9/2 and  6H11/2 of Dy3+ cation. The AB was observed at ∼1290 nm, classified as hypersensitive electron intra transition for Dy3+ cation in optical spectra  6 F11/2→6H15/2, at ∼1290 nm^[55]. Some of the AB have disappeared at UV-region and are very sparse at ∼353 nm 6H15/2→6P7/2,∼389 nm 6H15/2→4 F7/2 and ∼457 nm   6H15/2→4I15/2, and also they have very low intensity. The samples showed a hypersensitive intra-transition at ∼1290 nm∼0.96eV 6H15/2→6H11/2^[57]. The 4f s→4f s hypersensitive intra-transition at 1290 nm∼0.96eVH152→H112^[57]. The 4/5→4/5 and charge-transfer AB are observed for the most 4fn-series cations, the AB at Dy20 also, they have very low. Tentatively, according to this optical behavior, it is proposed that the dependence of free electron contribution to the dielectric function which can be the dependence of free electron contribution to the dielectric function which can be modified by changing at GS average. This statement is confirmed by our experimental results of XRD and the GS reported here. On the other hand, the optical phenomenon of the nanocrystal surface causes the Coulombic restoring force on the electron cloud becomes weaker has been systematically investigated ^[58].

Figure 3 UV-Vis spectra of a) Reflectance (%R), b) Transmittance (%T) and c) absorbance vs. λ(nm) of Dy 20, Dy 90, Dy 20 TT and Dy 90 TT nanocrystals, respectively. 

Figure 4 displayed Normalized Absorbance vs. E(hv) spectrum of (a) Dy 20 and Dy 90 (b) Dy 20 TT and Dy 90 TT, nanocrystals. The emission property of rare earth ions located at Visregion can be majorly attributed to their predominant non-hypersensitive and hypersensitive electronic intra-transitions. Optical and structural studies were investigated related to  4 F9/2→6H13/2 electronic intra-transitions in Dy3+ cation being an electric dipole transition is greatly sensitive ^[55]. On the other hand, Dy3+ cation populate the  4 F7/2 meta-stable state during rapid of non-radiative decay process due to the small energy gap energy between  4I15/2 and  4 F9/2 electronic states ^[59]. The electronic intra-transitions of Dy3+ cation present in yellow or blue regions of  4 F9/2→6H13/2 (electric dipole) and  4 F9/2→6H15/2 (magnetic dipole), respectively ^[60]. The electronic intra-transitions of  4 F7/2→6H13/2 is hypersensitive, while the intensity of electronic intra-transitions of  4 F9/2→6H15/2 is not quite sensitive. Dy3+ cation has been acknowledged for its white light production, which is appropriate at an acceptable yellow → blue (Y→B) intensity ratio. The absorption transition from the lower level  6H15/2 into the various energy levels  6H11/2,6 F11/2+6H9/2,6 F9/2+6H7/2,6 F7/2,6 F5/2, 4 F9/2,6 F3/2,4I15/2,4 F7/2, and  6P7/2. Table III and IV, shows 4fs→4fs electronic intratransitions of Dy 90, Dy 20 and Dy 90 TT, Dy20 TT nanocrystals. The 5s and 5p orbitals provide shielding effects on the 4f s electrons, generating as an optical response an intense AB in the different cations of the rare earths. Intense AB have been identified:  6H15/2 (blue),  6H13/2 (yellow) and  6H11/2→6H9/2 (red). Besides, the samples showed a hypersensitive transition at ∼1290 nm(∼0.96eV),6H15/2→6H11/2 with high intensity ^[61]. with high intensity ^[61]. Dy2O3( s) show AB located at ∼457 nm    (∼2.71eV), which were attributed to Dy3+ cation of  4 F9/2⟶6H15/2 electronic intra-transition ^[62]. The comparative analysis associated in principle with the chemical composition in the optical properties in these nanomaterials, in principle can be associated with the surrounding medium (atoms, molecules and ions), as well as intrinsic native crystal defects, stacking faults, grain boundaries, character of the stoichiometric balance, etc. These physicochemical parameters are observed and are generally assigned to changes in the optical properties of nanomaterials. The -O-H,CO32- ions, -O-O=O and Dy3+ cation, generate molecular polarization through the photo-response to the interaction of the external incident radiation, the result of this optical phenomenon generates shift towards higher energy ^[63]. Those AB, especially those at the Vis-light range, may increase the sensitivity of the composition and are apt to be used in several applications such as sensors and solar cells.

Figure 4 Normalized absorbance spectrum of (a) Dy 20 and Dy 90, (b) Dy 20 TT and Dy 90 TT nanocrystals. Table 3.4f→4f electronic intra-transitions of Dy 90 and Dy 20 nanocrystals. 

Table IV 4f→4f electronic intra-transitions of Dy 20 TT and Dy 90 TT nanocrystals. Figure 5 displayed (a) the real (n) and imaginary (k) parts of refractive index vs. Wavelength of (a) Dy 20, Dy 90 and (b) Dy 20 TT, Dy 90 TT nanocrystals. The refractive index (n) vs. λ(nm) of Dy 20, Dy 90 and 20 TT and Dy 90 TT samples. The electronic intra-transitions of greater relative intensity are seen in the Dy 20 and Dy 90 nanocrystals. The observed AB can, in principle, be justified by the optical and structural phenomena: 4fds→4fds electronic intra-transitions, native crystalline intrinsic defects as vacancies, grain boundaries, stacking faults, and stoichiometric and quantum confinement effect. It has been shown to a deviation of the local symmetry of the Dy3+ cation induced by the CO32- and OH- ion modifiers for O2- ion, such differences in the optical properties arising from the 20TT and Dy 90TT samples electronic intra-transition. These ions were previously identified using the FTIR technique (they are not shown here) ^{[13-15,17,24-26]}. The properties of the nanocrystals can be assigned to the effects of quantum and electronic confinement. The ABintensity was found to increase with the concentration of the CO32- and OH- ion. It can be related to the decrease in GS causes more atoms to be closer to the surface and thereby increasing the rate of trapping of photogenerated holes h+ at the surface, which in turn enhances the emission intensity. We consider that the different types of chemical bonding in the molecular configuration surrounding the central Dy3+ cation OH-,O-C-O,O-C=O and its optical response is associated with the interaction of external incident electromagnetic radiation, which creates a signal. much more intense optics in AB for the TT sample. In these nanomaterials, the signals have different relative intensity observed when directly compared to each other. However, intrinsic crystalline defects must be considered in a more formal way by applying an adequate theoretical model, which is underway by our research group.

Figure 5 Real (n) and imaginary (k) parts of refractive index (n) vs. wavelength of a) Dy 20, Dy 90 and b) Dy 20 TT, Dy 90 TT nanocrystals. 

Figure 6 displayed (αhv)2 vs. E(hv) spectra of (a) Dy 20 and Dy 90, (b) Dy 20 TT and Dy 90 TT nanocrystals. Tauc’s plot reveals an absorption edge located at range ∼4.66-5.17eV, it is believed that may be a wide optical band gap Egg of Dy2CO33 nanomaterial. The Dy2CO33( s) nanomaterial show Eg energy located at range ∼3.75-4.1eV, have recently been reported in the literature ^[2,64]. A systematic analysis of the optical behavior of these nanocrystals, it is possible to divide into three regions associated with intra-electronic transitions 4fs→4fs located in the UV-Vis region at range ∼1.5-3.5eV^[15], fundamental transition valence band (VB) and conduction band (CB)VB→CB, located at ∼4.66-5.17eV. The samples labeled with Dy 20 TT D and Dy 90 TT symbols show similar optical behavior at (αhv)2 vs. E(hv) spectra spectrum.It is confirmed that both materials are Dy2O3 (s), so the Eg∼4.6eV, which is very close to that reported for Dy2O3( s), were attained from rare-earth oxides, with a high thermal and chemical stability [65]. The concentration of native crystalline defects (vacancies, interstices, stacking faults etc.), related transition is due to the recombination of a photo-generated hole h+ and electron (e- that fits a singly ionized state of the defect, such as oxygen vacancy or oxygen interstitial.

Figure 6 (αhv)2 vs. E(hv) spectra of a) Dy 20 and Dy 90, b) Dy 20 TT and Dy 90 TT nanocrystals. 

4 Conclusions

Nanomaterials containing the Dy3+ cation were prepared at two different temperatures of ∼20∘C and ∼90∘C±2∘C, keeping the parameters constant during crystal growth and subsequent thermal treatment in an air atmosphere at ∼600±5∘C during 1.0 h. The influence of changing temperature of crystal growth on the structural and optical of the material were determined. The influence of changing temperature of crystal growth on structural and optical of the material were determined. The change in optical and structural properties are systematically discussed in this document, these are associated with the TT which produces DyCO33( s) nanocrystal. All these nanomaterials were characterized by DXR technique. It is found that Dy 20 and Dy 90 are identified in orthorhombic phase, while both samples TT present cubic phase. These nanomaterials present interesting optical properties: Transmittance (%T), Reflectivity (%R), Normalized Absorbance, real (n) and imaginary (k) parts refractive index and band gap energy Eg. The change in optical and structural properties are systematically discussed in this document, these are associated with the TT which directly produces the Dy2CO33( s)→Dy2O3( s) transition. These optical properties are associated with 4fs→4fs electronic intra-transitions and Eg located at UVVis-NIR region. Using the experimental results of absorbance, the Tauc equation is applied to quantify the Eg which is located at UV-region in all these nanomaterials. These optical properties are associated with 4fs→4fs intra-electronic transitions and Eg located at UV Vis-NIR region. The optical energy Eg of the nanocrystals for direct allowed transitions are decreases with TT located at 5.1-4.6 eV. The measured and calculated oscillator strengths for various electronic intra-transitions with Dy3+ ons, which is the highest for hypersensitive transition. This newly proposed nanomaterials may contribute to the development of photonic devices.

5 Statements and declaration

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.