Sensitized IR luminescence in Ca3Y2Ge3O12: Nd3+, Ho3+ under 808 nm laser excitation

Ceramics International 44 (2018) 6959–6967

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Sensitized IR luminescence in Ca3Y2Ge3O12: Nd3+, Ho3+ under 808 nm laser excitation

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Yana V. Baklanova , Andrey N. Enyashin, Lidiya G. Maksimova, Alexander P. Tyutyunnik, Alexander Yu. Chufarov, Evgeny V. Gorbatov, Inna V. Baklanova, Vladimir G. Zubkov Institute of Solid State Chemistry, UB RAS, 91 Pervomayskaya str., 620990 Ekaterinburg, Russia

A R T I C L E I N F O

A B S T R A C T

Keywords: A. Precursors: organic B. X-ray methods C. Optical properties DFT calculations

A promising series of infrared phosphors Ca3Y2-х-yNdxHoyGe3O12 (x = 0.0–0.3; y = 0.000015–0.15) with garnet structure has been prepared using a liquid-phase precursor method. The room temperature X-ray powder diffraction study and density functional theory (DFT) calculations show consistently that incorporated trivalent neodymium ions occupy calcium dodecahedral sites, whereas ousted part of calcium ions moves into yttrium/ holmium octahedral sites. DFT calculations and results of diffuse reflectance measurements reveal that an increase in the neodymium concentration and, consequently, the degree of cation substitution leads to a noticeable band gap reduction. The infrared luminescence in range from 1.0 to 3.4 µm for Ca3Y2-х-yNdxHoyGe3O12 is observed under 808 nm laser diode excitation. The trace amount of holmium ions and a low doping ratio of Ho3+ and Nd3+ ions allow revealing of intense emission at 2.0–3.4 µm region. The highest emission intensity is attained at x = 0.15 in Ca3Y2-хNdxGe3O12:Ho3+. The concentration quenching of Ho3+ emission in the Ca3Y1.85–yNd0.15HoyGe3O12 solid solutions is observed at the activator content of y > 1.5·10−4. A very weak red upconversion emissions at 650 and 740 nm were observed upon 808 nm laser diode excitation. The proposed mechanism for multistage process of energy transfer between the active centers involves the participation of Nd3+ ions as sensitizers of infrared luminescence of Ho3+ ions. The results indicate that the Ca3Y2-хyNdxHoyGe3O12 germanate can be considered as a promising material for near- and middle infrared phosphors.

1. Introduction Infrared (IR) lasers operating in the 2.0–3.0 μm radiation region find a wide application in laser radar systems, laser imaging, biomedical systems, remote sensing, monitoring and communication systems [1–3]. A high protection from external impacts and the impossibility of visualization for human eyes without special devices make the IR range very attractive for practical applications. However, the range of industrial emitters based on light-emitting diodes is limited. The interest in quantum-cascade lasers of first and second types is mainly focused on the mid-infrared region (3.5–13 μm) [4,5]. As an alternative, various possibilities for conversion of laser diode radiation (750–980 nm) into longer-wavelength emission using phosphors are considered to solve the problem in creating of long-wave IR radiation sources. One of the traditional methods to convert the near-infrared (NIR) emission into the short-wave infrared emission is based on the use of Stokes shift at the excitation and deexcitation of an activator, which is located in the optical host. During one-center excitation, the activator transforms from the ground state into excited state, further, nonradiative

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relaxation from excited state to metastable state occurs and, finally, a radiative transition from metastable state into the ground state proceeds with emission of IR photon. Effective activators for such process are Nd3+, Ho3+, Er3+, Tm3+ and Dy3+, and various materials with reduced phonon energy are used as optical hosts (glasses, crystalline inorganic compounds etc.) [6–20]. The transformation coefficient in such devices, i.e. the ratio between the number of excited and exciting photons, in principle, should not exceed unity. Nevertheless, schemes for radiation generation in the visible and near IR range with a transformation coefficient larger than unity do exist for the one-center emission model, too. They include, for example, the schemes employing a variety of cascade luminescence based on quantum cutting. The process of conversion of one UV photon into two photons of visible range was first reported for fluoride systems, such as YF3, LaF3, LiYF4 and α-NaYF4 [21–24]. Usually, the combinations of single-type activators, yet, being in two metastable states E1 and E2 are considered, for which the condition E1 = ~ 2E2 must be fulfilled and the final level of one emission transition is the initial level of another emission transition. The excitation of such activator with energy Eex > E1 is

Corresponding author. E-mail address: [email protected] (Y.V. Baklanova).

https://doi.org/10.1016/j.ceramint.2018.01.128 Received 23 November 2017; Received in revised form 12 January 2018; Accepted 16 January 2018 Available online 01 February 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

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crystallographic, electronic and luminescence properties during ET process within a new family of Ca3Y2-х-yNdxHoyGe3O12 (x = 0.0–0.3; y = 0.000015–0.15) solid solutions with garnet structure.

accompanied by emission of two photons with energy Eem < E2. A typical representative of such activators is Pr3+, which should be in a symmetrical weak crystal field of anions with the maximal coordination number of 10–12 [25]. In that case, the upper 1S0 (4f) excited state is lower than the excited 4f5d state. The multiphoton relaxation of the excited 4f5d state leads to the population of the metastable 1S0 (4f) level with subsequent emission of photons with wavelength of 400 nm for the 1S0 → (1I6, 3PJ) transition and 480–700 nm for 3PJ → (3FJ, 3HJ) [22,23]. As a rule, similar scheme is realized in the visible or NIR spectral regions [26–28]. Noteworthy, YNbO4:Tm3+ is found to exhibit not only the aforementioned two-photon relaxation, but also three- and four-photon relaxations [26]. The largest emission wavelength of 1820 nm was obtained for the transition 3F4 → 3H6. The schemes of crystalline cascade IR lasers were proposed already in 1970's [29]. The cascade mechanism in these lasers could be realized both on the basis of the described one-center activator with two metastable states and two-component media including a sensitizer and an activator. The quantum yield (QY) of luminescence in such systems is higher than unity [29]. The activators should have a well developed structure of closely lying levels in the IR range with large probabilities of spontaneous transitions, while the optical host should have a reduced phonon energy hω. The most suitable activators are Ho3+, Er3+, Tm3+, Pr3+, Nd3+, Sm3+, Eu3+ and Dy3+. However, apart the proposal, the investigations in this field have been not ever promoted. One of the promising directions for creation of IR emission is related to the application of the energy transfer (ET) mechanism during cooperative nonradiative cross-relaxation (CR) [30,31]. The cooperative mechanism is realized experimentally in La1-xCexF3 doped with Nd3+, Ho3+, Er3+ and Tm3+ [30,31]. Here, the simultaneous ET's from single donor (Ho3+, Tm3+, Nd3+ or Er3+) to two or more acceptors (Ce3+) is accompanied by multiplication and delocalization of excitation in the medium. Although in fluoride systems hω ~ 400 cm−1, the excited 2F7/ 3+ ) state could be expected to relax via emission of photons. Yet, 2(Ce due to a high concentration of acceptors (> 10%), the energy dissipation occurs via phonon emission. The cooperative nonradiative CR effect was used to suppress upconversion and strengthen 1.5–1.6 μm emission in Y2O2S:Er3+, Ce3+ phosphor [32]. In solid solutions Y1.7−6 < x < 5·10−2) during 0.90–0.98 μm excitation, xEr0.3CexO2S (1·10 the CR process Er3+ (4I11/2) + Ce3+ (2F5/2) → Er3+ (4I13/2) + Ce3+ (2F7/2) suppresses the visible anti-Stokes luminescence and enhances the Stokes IR luminescence in the 1.5–1.6 μm range. Notwithstanding a low content of acceptors in these solid solutions, the relaxation of the excited 2F7/2(Ce3+) state takes place with emission of phonons. This is due to the phonon energy value of the oxysulfide host hω ~ 750 cm−1 [33], which stimulates the transition 2F7/2 → 2F5/2 with ΔE ~ 2200–2400 cm−1 (~ 3hω) and emission of three phonons. The examples considered above are indicative of the generation of phonons during dissipation from excited state of acceptor. Though, it is not clear enough, in which cases a transition from the phonon mechanism to the photon one is possible. We have already mentioned earlier that, donors and acceptors should have a well developed structure of closely lying levels in the IR range with large probabilities of spontaneous transitions, the optical host should have a reduced phonon energy hω, and expected radiative transition is possible between states with ΔE > 3hω. However, the example of Y2O2S:Er3+, Ce3+ allows us to formulate an additional condition: in order to increase QY in the IR range, the upconversion processes in the visible range should be suppressed. These conditions have been met for the conversion of UV (340 nm) radiation into NIR (1 μm) at Y8V2O17 [34] and Ba4La6(SiO4)6O compounds [35] codoped with Eu3+ and Yb3+. It is revealed that the ER process occurs predominantly through the CR process Eu3+ (5D0) + Yb3+ (2F7/2) → Eu3+ (7F6) + Yb3+ (7F5/2). The analysis of IR (0.9–3.0 μm) luminescence properties of CaLa23+ [36] and Li7La3-xNdxHf2O12:Ho3+ [37] under xNdxGe3O10:Ho 808 nm laser excitation shows that ET process can take place at trace amount of holmium ions. In this paper we report the data on

2. Materials and methods 2.1. Preparation The polycrystalline samples of Ca3Y2-хNdxGe3O12 and Ca3Y2-х(x = 0.0–0.3; y = 0.000015–0.15) were prepared using liquid-phase precursor method. Y2O3 (99.98%, Giredmet, Russia), Nd2O3 (99.99%, Lanhit, Russia), Ho2O3 (99.99%, Giredmet, Russia) calcined previously at 700–900 °С for 5 h, CaCO3 (99.9%, Reachem, Russia) and GeO2 (99.5%, Germanium, Russia) were used as raw chemicals. The yttrium and neodymium oxides contained trace impurities of holmium, which were accurately defined by mass spectrometry on a PerkinElmer Elan 9000 ICP-MS: 7.8·10−5 mol% Nd3+ and 4.1·10−5 mol % Ho3+ in Y2O3, 5.7·10−5 mol% Y3+ and 4.1·10−4 mol% Ho3+ in Nd2O3, 1.1·10−3 mol% Y3+ and 6.6·10−4 mol% Nd3+ in Ho2O3. A stoichiometric amount of RE2O3 (RE = Y, Nd, Ho) and CaCO3 was dissolved in 3.5 М HNO3 solution, and GeO2 – in 3.1 М NH4OH solution. Then, these two solutions (acidic and alkaline) were combined under continuous stirring and heating at 150 °C. The stoichiometric amount of citric acid C6H8O7·2H2O sufficient for complex formation was added to the reaction mixture. The obtained solution was then heated at ~ 150 °C under continuous stirring till the formation of a dry residual solid. Calcination of the residual solid powder was performed in three steps at the temperatures 550 °C (12 h), 800 °C (10 h) and 1000 °C (10 h), with intermediate cooling and regrinding in an agate mortar at each stage. In contrast to Ca3Y2-хNdxGe3O12:Ho3+ (x = 0.0–0.3), holmium was introduced specially in selected amounts into solid solutions with a constant content of neodymium Ca3Y1.85yNd0.15HoyGe3O12 (y = 0.000015–0.15). yNdxHoyGe3O12

2.2. Characterization The microstructure and morphology of the samples were studied by scanning electron microscopy (SEM) using a JEOL JSM–6390 LA microscope. The representative SEM images of solid solution Ca3Y2-хyNdxHoyGe3O12 (x = 0.0–0.3; y = 0.000015–0.15) powders are gathered in Supplementary material (Fig. S1). The particle size distributions of the obtained samples were evaluated from the SEM images, collected in histogram form and fitted to log-normal distributions (Fig. S1). The rather large agglomerates (up to 20 μm) consist of small lamellar particles with an average particle size of 250 ± 60 nm, which is typical of combustion and solid state synthesis techniques [38–40]. This value for Ca3Y2-х-yNdxHoyGe3O12 corresponds well to results reported by Rao Bandi et al. [41]. All X-ray powder diffraction (XRD) patterns were collected on a STADI–P automated diffractometer (STOE) in transmission geometry equipped with a linear mini–PSD detector using Cu Kα1 radiation in the 2θ range from 5° to 120° with a step of 0.02°. Polycrystalline silicon (a = 5.43075(5) Å) was used as an external standard. The phase purity of the samples was checked by comparing their XRD patterns with those in the PDF2 database (ICDD), release 2016. The crystal structure of Ca3Y2Ge3O12 [42] was used as the starting model. The crystal structure refinements were carried out with the GSAS program suite [43,44]. Experimental, calculated and difference XRD patterns can be found in Supplementary material (Figs. S2–S8). The crystallographic data, details of Rietveld refinements for Ca3Y2-хNdxGe3O12 (x = 0.0–0.3) are listed in Table 1. Illustrations were produced in VESTA program [45]. The diffuse reflectance spectra were recorded in the range of 220–1100 nm on a Shimadzu UV-3600 UV–vis–NIR spectrophotometer using BaSO4 as a reference. The Ca3Y2Ge3O12 compound is transparent in the visible region and has a strong absorption of near-UV light (Fig. S9). The Ca3Y2-х-yNdxHoyGe3O12 (x = 0.0–0.3; y = 0.000015–0.15) 6960

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The experimental and equilibrium DFT GGA lattice parameters of oxides have been found in fair agreement with the average difference within ± 1%. The Ca3Y2Ge3O12 garnet lattice demonstrated the preservation of initial geometry after the optimization. The calculated cell parameter a = 12.98 Å is in fair agreement with experimental crystallographic data from recent work [42] as well as from our present study.

Table 1 Experimental values of optical band gap (Egap) for Ca3Y2-х-yNdxHoyGe3O12 (x = 0.0–0.3; y = 0.000015–0.015). x/y in Ca3Y2-х-yNdxHoyGe3O12

Egap, eV

0.0/0.0

0.15/0.0

0.30/00

0.15/0.00015

0.15/0.015

0.15/0.15

5.56

5.32

5.10

5.38

5.38

5.22

3. Results and discussion

data include the entire set of lines due to 4f–4f transitions from the Nd3+ ground states to a series of excited states (Fig. S10). The band at 808 nm associated with excitation of the 4F5/2+2H9/2 manifolds in Nd3+ matches a commercial laser diode. The transitions of Ho3+ ions correspond only to weak lines at ~ 450 nm, associated with the 5I8 → (5G6, 5F1) transitions, which can be observed on the spectrum for Ca3Y1.7Nd0.15Ho0.15Ge3O12 with a high concentration of holmium (Fig. S10). For Ca3Y2-х-yNdxHoyGe3O12 (x = 0.0–0.3; y = 0.000015–0.0015) compounds, no lines assigned to transitions in Ho3+ have been detected due to relatively negligible ion content. The optical band gaps of the samples, in assumption of direct transition, can be determined using the optical absorption coefficient evaluated from the Kubelka–Munk function [46,47], extrapolating the linear part of the plot of (F(R)·hν)2 versus hν (insets in Fig. S9). The optical band gap energies of Ca3Y2-хyNdxHoyGe3O12 are listed in Table 1. The room temperature (RT) luminescence spectra in the range from 1.0 to 3.4 µm under 808 nm laser diode excitation (P = 140 mV, KLMH808-120-5, FTI-Optronic JSC) in the 90° excitation geometry were recorded with a MDR-204 (LOMO-Photonica, Russia) monochromator equipped with an FPU-FS PbS (LOMO-Photonica, Russia) detector (diffraction grating 300 grooves/mm, optical shutter with a modulation frequency of 200 Hz, optical filter IRG7). The spectra were not corrected using а typical spectral response for PbS photoconductive detector. The spectral regions from 0.85 μm to 1.80 μm were recorded with the use of a photomultiplier tube (PMT module) H10330C-75 with InP/InGaAs photocathode (Hamamatsu). The laser power was controlled with a 13 PEM 001 facility (Melles Griot). The decay kinetics of excited neodymium states was recorded using the Picosecond optical pulse generator PLS-808 (InTech, Russia) and TCSPC/MCS Counter Module TCC2 (Edinburgh Instruments Ltd) with H10330C-75 (Hamamatsu). The cooled detector H10720-01 (Hamamatsu) was used to register the upconversion luminescence spectra. The spin-polarized calculations of all Ca3Y2−xMxGe3O12 (M = Sc, Y, La, Ac, Nd, Gd, Ho, Lu) garnets as well as their parent binary oxides were performed within the framework of the density-functional theory (DFT) as implemented in the SIESTA 4.0 software [48,49]. The exchange-correlation potential within the Generalized Gradient Approximation (GGA) with the Perdew-Burke-Ernzerhof parametrization was used. The core electrons were treated within the frozen core approximation, applying norm-conserving Troullier–Martins pseudopotentials. Valence states for main constituent elements and corresponding pseudopotential core radii (in brackets, in a.u.) were chosen as 4s2(3.29) 4p0(3.29)3d0(3.29) for Ca, 5s2(3.19)5p0(3.36)4d1(3.19) for Y, 4s2(2.48) 4p2(2.48)4d0(2.99) for Ge, and 2s2(1.45)2p4(1.45) for O. In all calculations the double-ζ polarized basis set was used. The k-point mesh was generated by the method of Monkhorst and Pack. The real-space grid used for the numeric integrations was set to correspond to the energy cutoff of 300 Ry. For k-point sampling, a cutoff of 15 Å was used. The high symmetry of a garnet lattice allowed to reduce the cubic cell to the bcc cell (i.e. to 4 stoichiometric units, 80 atoms), assisting the calculational performance. All calculations were performed using variablecell and atomic position relaxations, with convergence criteria corresponding to the maximum residual stress of 0.1 GPa for each component of the stress tensor, and the maximum residual force component of 0.05 eV/Å. Preliminary test calculations of binary oxides have revealed a good suitability of chosen approach for the description of geometry.

3.1. Crystal structure of solid solution Ca3Y2-хNdxGe3O12 and Ca3Y2-хyNdxHoyGe3O12 According to the X-ray diffraction data, the solid solutions Ca3Y2(x = 0.0–0.3) and Ca3Y2-х-yNdxHoyGe3O12 (y = 0.000015–0.15) are isostructural to Ca3Y2Ge3O12 [42], (sp. gr. Ia3d , Z = 8). Attempting to fabricate the Ca3RE2Ge3O12 (RE = La, Nd) germanates, no garnet-like structure was observed. The samples were represented mainly by the phase with Ca1.8La8.133(GeO4)6O2 apatite-like structure (sp. gr. P63/m, ICSD No. 59730 [50]), and sufficient amount of calcium germanate phases Ca5Ge3O11 (sp. gr. С1, ICSD No. 403085 [51]) and Ca2GeO4 (sp. gr. Pnma, ICSD No. 173460 [52]). The crystal structure of undoped and Nd3+/Ho3+ codoped yttrium garnet belongs to an insular motif of lattice structure (Fig. 1). Calcium, yttrium and germanium atoms are lodged at dodecahedral 24c (0, 1/4, 1/8), octahedral 16a (0, 0, 0) and tetrahedral 24d (0, 1/4, 3/8) sites, respectively. Oxygen atoms occupy the joint sites 96h (x, y, z). It should be noted that there is a significant contrast in the scattering power of calcium, yttrium and neodymium, which is proportional to the number of electrons and is 20, 39 and 60, respectively. At first steps of structure refinement Nd3+ atoms in the solid solution Ca3Y2-хNdxGe3O12 (x = 0.0–0.3) were placed in the Y3+ site. However, in such a model the thermal parameter of the calcium site turned out to be negative, while for the yttrium site it became large. The reason for this could be insufficient and excess electron density at these two sites, respectively. At the same time a very poor fit of XRD data was observed. Our structural studies established that all neodymium atoms occupy the calcium dodecahedral sites, while the ousted calcium atoms pass into the yttrium octahedral sites, i.е. the cation exchange is observed after substitution (Table 2). Such a model results in perfect fit of the observed XRD profiles, reasonable thermal parameters, and goodness of fit, χ2, dropped, for example for Ca3Y1.7Nd0.3Ge3O12, from 8.246 to 1.929. This phenomenon is accompanied by an increase in the cell parameter хNdxGe3O12

Fig. 1. The unit cell of Ca3Y2-хNdxGe3O12 garnet along [001]: GeO4 tetrahedra (purple), Y/CaO6 octahedrons (green), dodecahedron Ca/NdO8 (blue) and oxygen atoms (red balls). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Table 2 Crystallographic data and refinement parameters for Ca3Y2-хNdxGe3O12 (x = 0.0–0.3). x in Ca3Y2-хNdxGe3O12 x Cell parameters: a = b = c, Å, V, Å3 wRp, % Rp, % R(F2), % χ2 Y/Ca: 16a Ca/Nd: 24d Ge: 24d O: 96h

Frac. U*100 Frac. U*100 U*100 x/a y/b z/c U*100

0.0

0.05

0.10

0.15

0.20

0.25

0.30

12.8092(5) 2101.7(2) 3.6 2.52 3.53 1.98 1/0 2.73(2) 1/0 1.96(4) 2.53(3) −0.03558(17) 0.05744(16) 0.16128(18) 1.78(7)

12.8116(5) 2102.8(2) 2.35 1.67 3.33 2.607 0.975/0.025 2.80(2) 0.9833/0.0167 2.32(3) 2.56(2) −0.03578(12) 0.05738(11) 0.16068(13) 1.99(5)

12.8130(5) 2103.5(2) 2.48 1.66 2.97 5.828 0.95/0.05 2.69(2) 0.9667/0.0333 2.31(3) 2.54(2) −0.03581(10) 0.05723(10) 0.16091(11) 1.96(4)

12.8187(5) 2106.4(2) 2.85 2.06 3.69 2.568 0.925/0.075 2.62(2) 0.95/0.05 2.26(4) 2.45(3) −0.03582(14) 0.05703(13) 0.16116(16) 2.07(6)

12.8197(5) 2106.7(2) 1.88 1.36 2.79 2.213 0.9/0.1 2.55(2) 0.9333/0.0667 2.3(3) 2.41(2) −0.03588(10) 0.05663(9) 0.16102(11) 1.98(4)

12.8219(5) 2107.9 (2) 2.35 1.67 2.45 2.185 0.875/0.125 2.53(5) 0.9166/0.0833 2.37(6) 2.30(6) −0.03599(12) 0.05641(12) 0.16068(14) 2.02(7)

12.8243(5) 2109.1(2) 2.24 1.60 2.24 1.929 0.85/0.15 2.34(5) 0.9/0.1 2.34(6) 2.18(6) −0.03619(13) 0.05726(13) 0.16051(16) 1.93(7)

and volume. In the garnet lattice, a dodecahedron Ca/NdO8 shares (О–О) edges with two GeO4 tetrahedra, four Y/CaO6 octahedra and four other dodecahedra Ca/NdO8 (Fig. 1). The ionic radii of Y3+ (1.04 Å), Ca2+ (1.16 Å) for 6-fold coordination and Ca2+ (1.26 Å), Nd3+ (1.249 Å) for 8-fold coordination [53] point to an absence of significant changes in the interatomic distances in the octahedra and dodecahedra during cation substitution, forming the solid solutions Ca3Y2хNdxGe3O12 (Table S1). The localization of Ho atoms in the solid solutions Ca3Y2-х-yNdxHoyGe3O12 (x = 0.0–0.3; y = 0.000015–0.15) cannot be determined only by means of X-ray diffraction, since the concentration of holmium is too low. However, taking into account the value of the ionic radius of Ho3+ for 6-fold coordination (1.041 Å) and for 8-fold coordination (1.155 Å) we may assume Ho localization in the octahedral position. The optical band gap values (Egap) of the solid solutions Ca3Y2-хyNdxHoyGe3O12 decrease upon increasing neodymium and holmium concentration from 5.56 eV for undoped compound to 5.22 eV for x/y= 0.15/0.15. The change in the optical band gap values can be explained by the variations in the electronic structure and chemical bonds in these compounds, which is well confirmed by the results of quantum chemical calculations.

Fig. 2. Formation energies Eantisite for antisite defects in M-doped Ca3Y2Ge3O12 garnets (substitution of Y3+ on cation M3+ accompanied by exchange of M3+ on Ca2+). The cationic radius rion was suggested by Shannon for six-fold coordinated M3+ [47]. DFT GGA calculations.

3.2. DFT calculations

In general, the M3+ cations can be subdivided into two groups with a boundary value of rion ≈ 1.10 Å, depending on the ability to form antisite defect between metal sublattices of Ca3Y2Ge3O12. Particularly, Y3+ cations demonstrate the positive energy Eantisite = 0.93 eV. Hence, antisite exchange of Ca2+ and Y3+ is thermodynamically unprofitable. The amount of such defects estimated in accordance to Boltzmann distribution may not exceed ~ 0.02% even at the highest calcination temperature 1000 °C. The input of Ho3+ into Ca-sublattice is found also unprofitable with Eantisite = 0.17 eV. During thermodynamic equilibrium at 1000 °C and at room temperature the Y sublattice of Ca3Y2Ge3O12 should adopt ~ 80% and ~ 99.93% of injected Ho impurity, respectively. In turn, the energy of antisite defect with participation of Nd3+ is negative Eantisite = −0.19 eV. Consequently, the estimated part of Nd impurity in the Y sublattice can rich only ~ 18% and ~ 0.03% at 1000 °C and 273 °C, respectively, which confirms convincingly the data of XRD analysis at room temperature.

3.2.1. Thermodynamics of impurity distribution DFT calculations were employed to approve interpretation of structural data on the hosting of impurities either at octahedral Y or dodecahedral Ca sites of Ca3Y2Ge3O12 lattice. The bcc representation of a garnet cell allows to study the least content of impurity as x = 0.125 after single Y atom substitution, which fits well the order of the experimentally achieved x values. Apart of Nd and Ho, a large family of other related trivalent elements (M = Sc, Y, La, Ac, Gd, Lu) was considered to disambiguate the role of cationic radii in the distribution of impurity atoms between Y and Ca sites. To gauge the site preference of M, the relative energies of the cells with two possible substitutional defects were analyzed: first, simple substitution of Y3+ on M3+ and, second, substitution of Y3+ on M3+ accompanied by antisite exchange of M3+ on Ca2+. The resulting energies of antisite substitutional defects Eantisite have been plotted in Fig. 2 against the radius rion of cation M3+ in six-fold coordination [53]. The distinctly straight relationship can be recognized between Eantisite and rion. While Nd and Ho as elements with incomplete f-shell may display the peculiarities of chemical bonding, the Eantisite values for elements with closed or half-occupied f-shell obey the straight dependence on rion with even a higher correlation.

3.2.2. Electronic structure of Ca3Y2Ge3O12 and related solid solutions The electronic band structure and the density of states (DOS) calculated for pristine Ca3Y2Ge3O12 phase are visualized in Fig. 3. The compound possesses the calculated value of the band gap 3.32 eV with the direct transition at Г-point. The bottom of conduction band has a 6962

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Fig. 3. Band structure, total and partial densities of states (DOS) for the bulk Ca3Y2Ge3O12 garnet without doping (a), with Gd-doping into Y- (b) or Ca-sublattice (c), with La-doping into Ca-sublattice (d). DFT GGA calculations. Valent Ca4s, Y4d, O2p and Ge4p states are painted in red, cyan, yellow and blue, respectively. Gd5d and La5d states are factorized 20 times and painted in green. The bands with α- and β- spin states are painted in black and gray, respectively.

for the conduction band, while the valence states are updated by the narrow band of localized Gd4f-states with energy −5.8 eV below the Fermi level. The bottom of the conduction band is a mixture of Ca4s-, Y4d- and Gd5d-states. The majority of unoccupied Gd5d-states can be found at +4 eV, i.e. with lower energies, than Y4d-states. The site exchange of Gd3+ on Ca2+ implies the change of octahedral crystal field on dodecahedral field for Gd3+ ion. The latter causes a remarkable reorganization of unoccupied Gd5d-states (Fig. 3c). The calculated band gap is decreased to 2.99 eV due to the splitting off a single state from the bottom of conduction band. Moreover, this and the nearest states demonstrate a high DOS contributed mostly from Gd5dstates. The valence bands are not perturbed upon antisite defect: only the impurity-related band of Gd4f-states is shifted to −6.6 eV below the Fermi level. A qualitatively similar modulation of electronic properties of Ca3Y2Ge3O12 has been registered also for La and Lu dopants (Fig. 3d). Hence, the similar action of Nd- and Ho-dopants may be expected as well. Notwithstanding the type of the rare-earth cation and the hosting sublattice, the band gap of Ca3Y2Ge3O12 reduces on ~ 0.2–0.3 eV after M-doping. However, only antisite cationic exchange modifies drastically the bottom of the conduction band by M5d-states and leads to a more noticeable band gap reduction.

low DOS and consists of a mixture of Ca4s- and Y4d-states. The unoccupied band with high DOS is well resolved near +5 eV above the Fermi level and consists mostly of Y4d-states. The wide valence band at −2…−4 eV below the Fermi level is formed by dominant O2p-states. Such distribution of the near-Fermi level states reflects an essentially ionic character of interaction between GeO4 tetrahedra and Ca or Y atoms. The lower valence band at −4.5…−6 eV is a mixture of O2pand Ge4p-states. The separated and well localized band near −8 eV is formed by hybridized Ge4s4p and O2s2p states. These features evidence the predominantly covalent Ge-O overlaps within GeO4 tetrahedra. The discussion of electronic structure of Nd- or Ho-doped Ca3Y2Ge3O12 garnet after direct calculations at the level of DFT employed in recent work would be too speculative. To date the calculations using the more sophisticated DFT or GW schemes, accounting for the strong correlation of electrons at open f-shells, are too cumbersome for the large unit cells of garnets and are limited to binary rare-earth oxides [54]. However, several clues on the general modulation of the band structure upon a rare-earth doping can be obtained using the calculations of Ca3Y2Ge3O12 doped by La, Gd or Lu – the elements with closed f-shells of spin-orbitals. The electronic band structure and the density of states (DOS) calculated for Ca3Y2Ge3O12 doped by Gd in different sublattices are visualized in Fig. 3b and c. The substitution of Y on Gd does not perturb the general picture of the band structure of the matrix, although the magnetic moment is emerged due to 4f electrons of Gd3+. The calculated band gap of doped compound remains direct and it is equal to 3.23 eV. The most prominent removal of levels’ degeneracy is obtained

3.3. RT luminescence properties The RT luminescence spectra of Ca3Y2-х-yNdxHoyGe3O12 (x = 0.0–0.3; y = 0.000015–0.015) in the range from 1.0 to 3.4 µm under 6963

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F3/2 state of Nd3+ through the energy transfer to other ions. The most intensive lines centered at 2.0–2.3 µm, 2.7–2.9 µm are associated with the 5I7 → 5I8 and 5I6 → 5I7 transitions in holmium ion. A broad line with a lower intensity at 3.0–3.3 µm can be referred to (5F4, 5S2) → 5F5 transition in Ho3+, which implies the existence of UC process. Splitting of all emission lines is observed and ascribed to various sub-energy level transitions [55,56]. The use of PMT module with InP/InGaAs photocathode is capable to distinguish sub-energy level transitions on the luminescence spectra in the range 0.85–1.80 μm (Fig. 5). However, due to the spectral characteristics of the photomultiplier, some of the lowintensity sub-energy levels of 4F3/2 → 4I9/2 transitions in the range 0.86–0.95 µm were not registered. The high relative intensity and inhomogeneous broadening of lines at 2.0–3.0 µm could provide the broad wavelength tunable emission, which might contribute to improvement of the middle infrared emission properties [57]. Unfortunately, the branching ratio for transitions from 4F3/2 state cannot be accurately estimated even taking into account the spectral response of detectors, but it is possible to determine the effective line width as the integral emission intensity divided by the peak intensity. In case of Ca3Y1.85Nd0.15Ge3O12:Ho3+, the calculated effective width of the 2.1 and 2.7 μm emission lines is 144.4 and 132.1 nm, respectively. The effective line width for Ca3Y2-хNdxGe3O12:Ho3+ is significantly broader than that for germanate (67.5 nm) and chalcogenide (56 nm) glasses for 2.7 μm emission band [57], and also for tetragonal garnet Li7La3−xNdxHf2O12:Ho3+ (63.6 and 82.6 nm) for 2.1 μm and 2.7 μm emission bands [37]. It makes the germanate garnet Ca3Y23+ applicable for middle infrared broadband amplifiers. хNdxGe3O12:Ho Since holmium ions are contained mainly in initial neodymium oxide reagent, the doping of the Ca3Y2Ge3O12 host with Nd3+ ions leads correspondingly to a gradual increase of the holmium concentration in the samples. As it appears from the dependence of the emission intensity of the 1.3 and 2.1 μm lines on the Nd3+ concentration (Figs. 4 and 5, inset), the highest luminescence intensity is achieved at x = 0.15 in Ca3Y2-хNdxGe3O12:Ho3+. Concentration quenching of Ho3+ emission in solid solutions with a fixed content of neodymium Ca3Y1.85yNd0.15HoyGe3O12 (y = 0.000015–0.015) is observed at an activator content of y > 1.5·10−4 (Fig. 4, inset). The critical distance Rc of energy transfer between lanthanide ions can be calculated using the equation proposed by Blasse [58].

Fig. 4. RT luminescence spectra (λex = 808 nm) of Ca3Y2-хNdxGe3O12 (x = 0.0–0.3) (a) and Ca3Y1.85-yNd0.15HoyGe3O12 (y = 0.000015–0.015) (b) in the range from 0.9 to 3.4 µm obtained on PbS detector. The insets show the integral luminescence intensity of the 2.1 μm lines versus the Nd3+ and Ho3+ concentration in Ca3Y2-хNdxGe3O12 and Ca3Y1.85yNd0.15HoyGe3O12, respectively.

1

3V ⎤3 RC = 2 ⎡ , ⎢ 4 πx CN ⎥ ⎦ ⎣

(1)

where V is the volume of the unit cell, N represents the number of sites in the host unit cell that can be occupied by activator ions, and xc is the critical concentration. Taking the values of V = 2106.4 Å3, N = 8, and xc = 0.15, the critical distances Rc are found to be 14.97 Å for Ca3Y1.85Nd0.15Ge3O12:Ho3+. The luminescence decay curves measured for Ca3Y1.853+ concentration at the waveyNd0.15HoyGe3O12 with different Ho lengths of 930, 1055 and 1330 nm by 808 nm laser diode excitation exhibit nonexponential profiles (Figs. 6 and S11). Since it is not possible to describe the curves with single exponential function, the decay process of these samples is characterized by average lifetime (τ), which can be calculated by using the following equation: Fig. 5. Typical RT luminescence spectra (λex = 808 nm) of Ca3Y1.85-yNd0.15HoyGe3O12 (y = 0.000015–0.015) within the manifold 4F3/2 → 4I9/2, 4I11/2, 4I13/2 transitions obtained using PMT module with InP/InGaAs photocathode (a) and the integral luminescence intensity of the 1.25–1.45 μm lines versus the Nd3+ and Ho3+ concentration in Ca3Y2хNdxGe3O12 (b) and in Ca3Y1.85-yNd0.15HoyGe3O12 (c), respectively.

τ=

∫ I (t ) tdt I (t ) dt

,

(2)

where I(t) stands for the luminescence intensity at time t corrected for the background. The results of the fitting procedure of the luminescence decay curves are presented in Table 3. The average lifetime of the 4F3/2 state is typical for Nd3+ doped, Nd3+/Ho3+ and Nd3+/Er3+ codoped compounds [59–61]. The rapid decrease of the decay time upon raising the Ho3+ concentration shows that energy transfer from Nd3+ to Ho3+ is efficient.

808 nm laser diode excitation exhibit the infrared emission bands, which correspond to a series of 4f–4f transitions of the lanthanide ions (Fig. 4). The lines in the range 1.0–1.9 μm are attributed to transitions from the 4F3/2 state to 4I11/2, 4I13/2 and 4I15/2 in neodymium ion. The low intensity of these lines indicates a significant relaxation from the 6964

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Nd3+ and Ho3+ ions, e.g., Nd3+(4F3/2) + Ho3+(5I5) → Nd3+(4I13/2) + Ho3+(5F4, 5S2) [62]. Either of two processes can explain a weak intensity of 1.1 μm line, referred to the transition of Nd3+: 4F3/2 → 4I11/2. To estimate the order of the UC process, and therefore the number n of the pump photons required for excitation of the emitting state, the measurements of excitation power dependence were carried out. The upconversion emission intensity is proportional to the excitation power:

I (P ) ∞P n,

where n is the number of NIR photons absorbed to excite one upconversion photon [63]. The slope of the log-log plot of UC emission intensity versus pump power gives the n value. The double logarithmic plots of UC emission intensity for the composition Ca3Y1.85yNd0.15HoyGe3O12, as a function of excitation power are presented in inset, Fig. 7. Values of n obtained for Ca3Y1.85-yNd0.15HoyGe3O12 with different Ho3+ concentration are 1.10–1.28 and 0.90–0.96 for the corresponding 650 nm and 740 nm emission bands, which is indicative of the one-photon upconversion process to red emissions. The presence of emission lines from neodymium and holmium ions in the luminescence spectra of Ca3Y2-хNdxGe3O12:Ho3+ and Ca3Y1.85yNd0.15HoyGe3O12 indicates a multistage energy transfer between active centers. Taking into account the energy levels of these trivalent lanthanide ions [55,56,64], the following mechanism of energy transfer, which involves the participation of Nd3+ ions as sensitizers of infrared luminescence of Ho3+ ions, can be proposed (Fig. 8). Under 808 nm laser diode excitation, the electrons in the ground state 4I9/2 are excited to the Nd3+ states of 4F5/2 and 2H9/2. The subsequent nonradiative transition leads to the population of the lower metastable 4F3/2 state (E ~ 11,500 cm−1), from which the radiative transitions 4F3/2 → 4I9/2 (0.9 μm), 4F3/2 → 4I11/2 (1.05 μm), 4F3/2 → 4I13/2 (1.3 μm), 4F3/2 → 4 I15/2 (1.8 μm) occur (Fig. 8). The second process is the energy transfer from the 4F3/2 state of Nd3+ to the 5I5 state of Ho3+ with subsequent multiphonon relaxation to the 5I6 state, which provokes the emission at 2.7 µm. Finally, the most intensive emission peaks centered at 2.1 µm corresponds to the 5I7 → 5I8 transition in Ho3+. Moreover, the excited state absorption process "5I5 + one photon → 5F3" is expected to occur. The subsequent nonradiative transition populates the lower 5F4, 5S2 states, from which relaxes radiatively to the 5I7 state with red emission around 650 nm and to 5F5 with 3.2 μm emission. Afterwards, the 5F5 → 5 I8 transition brings red emission at 740 nm. It is worth noting that the

Fig. 6. Decay curves of the Nd3+: 4F3/2 → 4I13/2 (1330 nm) under excitation of 808 nm with different Ho3+ concentration (y) in Ca3Y1.85-yNd0.15HoyGe3O12 (y = 0.000015–0.15).

Table 3 Characterization of 0.000015–0.15).

F3/2 →

4

4

IJ transitions for Ca3Y1.85-yNd0.15HoyGe3O12 (y =

y in Ca3Y1.85-yNd0.15HoyGe3O12

4

F3/2 λpeak 4 F3/2 λpeak 4 F3/2 λpeak

→ I9/2 = 930 nm → 4I11/2 = 1055 nm → 4I13/2 = 1330 nm 4

0.000015

0.0015

0.015

0.15

160 μs

173 μs

135 μs

57 μs

153 μs

171 μs

130 μs

59 μs

154 μs

168 μs

133 μs

55 μs

(3)

Fig. 7. Typical upconversion emission spectra of the samples Ca3Y1.85-yNd0.15HoyGe3O12 (y = 0.000015–0.15). The lines correspond to transitions of Ho3+: 5F5 → 5I8 (650 nm) and (5F4, 5S2) → 5I7 (740 nm). The insets show the power dependence of the upconversion luminescence intensity for the Ca3Y1.85-yNd0.15HoyGe3O12 (y = 0.000015–0.15) sample in the low-power regime. The laser excitation wavelength is 808 nm.

Fig.

7

presents typical upconversion spectra of Ca3Y1.85under 808 nm excitation. The very weak red upconversion emission at 650 nm and 740 nm can be attributed to the transitions of Ho3+: 5F5 → 5I8 and (5F4, 5S2) → 5I7, respectively. The 5F5 or upper level can be populated by different processes, e.g., excited state absorption (ESA) from at least 5I5 level or CR process between yNd0.15HoyGe3O12

Fig. 8. Simplified energy level diagram, excitation and infrared emission schemes for Nd3+ and Ho3+ ions. Full and dotted arrows indicate radiative and nonradiative energy transfer processes, respectively. Zigzag and dash dotted lines denote nonradiative multiphonon relaxation and cooperative nonradiative cross-relaxation, respectively.

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diagrams do not include the inverse energy transfer process. Thus, neodymium ions in the solid solution Ca3Y1.85Nd0.15Ge3O12:Ho3+ with a trace amount of holmium ions act as sensitizers of holmium luminescence allowing conversion of 808 nm radiation into a set of emission lines in the middle IR regions. Moreover, the excessive intensity of Ho3+ emission in the wavelength range 2.0–2.3 μm indicates the existence of an additional cross-relaxation mechanism of energy transfer from Nd3+ to Ho3+ according to the scheme: Nd3+(4F3/2) + Ho3+(5I8) → Nd3+(4I15/2) + Ho3+(5I7). However, the decay curves of the Ho3+ levels in 3+ Ca3Y1.85Nd0.15Ge3O12:Ho under 808 nm excitation should be collected to confirm this energy transfer process.

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4. Conclusions The garnet-related Ca3Y2-х-yNdxHoyGe3O12 (x = 0.0–0.3; y = 0.000015–0.15) germanate prepared using liquid-phase precursor method crystallizes in the cubic group Ia3d , Z = 8. Both the room temperature X-ray powder diffraction study and the DFT calculations evidence that trivalent neodymium ions occupy calcium dodecahedral sites, whereas ousted calcium ions migrate into yttrium/holmium octahedral sites. Both the DFT calculations and the diffuse reflectance measurements reveal that an increase in the neodymium concentration and, consequently, the degree of cation substitution leads to noticeable band gap reduction. The luminescence spectra (λex = 808 nm) of the Ca3Y2-х-yNdxHoyGe3O12 germanate in the range of 1.0–3.4 μm consist of a series of lines corresponding to characteristic transitions in the neodymium and holmium ions. The residual concentration of holmium ions and the low doping ratio of Ho3+ and Nd3+ allow revealing of intensity emission in the range from 2.0 to 3.4 µm. The highest emission intensity is attained at x = 0.15 in Ca3Y2-хNdxGe3O12:Ho3+. The concentration quenching of Ho3+ emission in solid solutions with a fixed content of neodymium Ca3Y1.85-yNd0.15HoyGe3O12 is observed at an activator content of y > 1.5·10−4. The very weak red upconversion emission at 650 nm and 740 nm were observed upon 808 nm laser diode excitation. This result indicates that Nd3+ ions can be potentially used as sensitizers for Ho3+ ions to stimulate the intense near- and middle-infrared emission in this system. Possible energy transfer mechanisms between lanthanide ions have been briefly discussed. Acknowledgements The work was supported by the Russian Science Foundation (project No. 16–13–10111). The crystallographic study was carried out at the multiple–access center for X–ray structure analysis at the Institute of Solid State Chemistry, UB RAS (Ekaterinburg, Russia). Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ceramint.2018.01.128. References [1] I.K. Ile, R.W. Waynant, Mid-infrared biomedical applications, in: A. Krier (Ed.), Mid-Infrared Semiconductor Optoelectronics, Springer-Verlag London Ltd, London, 2006, pp. 615–634. [2] M. Pollnau, S.D. Jackson, Mid-infrared fiber lasers, in: I.T. Sorokina, K.L. Vodopyanov (Eds.), Solid-State Mid-Infrared Laser Sources, Springer-Verlag Berlin Heidelberg Ltd, Berlin, 2003, pp. 225–261. [3] P. Zhou, X. Wang, Y. Ma, H. Lü, Z.J. Liu, Review on recent progress on mid-infrared fiber lasers, Laser Phys. 22 (2012) 1744–1751. [4] B.H. Yang, D. Zhang, R.Q. Yang, C.-H. Lin, S.J. Murry, S.S. Pei, Mid-infrared interband cascade lasers with quantum efficiencies > 200%, Appl. Phys. Lett. 72 (18) (1998) 2220–2222. [5] R. Pecharromán-Gallego, An overview on quantum cascade lasers: origins and development, in: V.N. Stavrou (Ed.), Quantum Cascade Lasers, InTechOpen, Rijeka, 2017, pp. 3–24. [6] L. Nagli, O. Gayer, A. Katzir, Optical properties of Pr ions in silver halide crystals in

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[39]

[40]

[41]

[42] [43] [44] [45] [46]

[47] [48] [49]

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