Solid state synthesis of Perovskite-like calcium stannate structure activated with three rare-Earth metal ions Yb³⁺,Er³⁺,Но³⁺ has been studied. The formation of the CaSnO₃ : Yb³⁺,Er³⁺,Но³⁺ luminescent structure requires the following synthesis conditions: anneal temperature 1250 °C and duration at least 18 h. The luminescent properties of the specimens have been studied under 960 nm semiconductor diode laser excitation. The luminescence spectra contain bands in the visible and IR spectral regions. Yb³⁺ ions have been shown to act predominantly as sensibilizers capable of transferring part of absorbed energy to Er³⁺ and Но³⁺ ions thus intensifying their respective luminescence peaks. Er³⁺ ions also transfer part of absorbed energy to Но³⁺ ions leading to an increase in the intensity of the 1194 and 1950 nm IR luminescence bands. A schematic of possible energy transitions in the CaSnO₃ : Yb³⁺,Er³⁺,Но³⁺ system under 960 nm laser excitation has been suggested. The energy transfer mechanism between Yb³⁺,Er³⁺ and Но³⁺ions has been described in detail. The luminescence intensity of the luminophore has been studied at 994, 1194, 1550 and 1950 nm as a function of Но³⁺ion concentration. The peak intensity of the 1194 and 1950 nm bands is the highest at a Но³⁺ion concentration of 0.007 at.fr. It has been suggested to use the CaSnO₃ : Yb³⁺,Er³⁺,Но³⁺ luminescent structure for radiation sources capable of converting 960 nm IR radiation to ~2000 nm IR radiation.

luminescence, infrared luminophores, solid state synthesis, rare-Earth elements

CaSnO₃ base compounds pertaining to МSnO₃ type Perovskite-like stannates find increasing application due to their universal electromagnetic properties. Currently they are of interest for multiple fields of electronics engineering as materials of cathodes, thermally stable capacitors, photocatalysts, gas or humidity sensors etc. [1, 2]. The luminescent properties of rare-Earth metal stannates have been studied insufficiently yet, most works dealing with calcium stannate compounds emitting visible radiation [3–6]. There are only a few works on IR luminescence of calcium stannate compounds in the 900–1600 nm region [7, 8]. No information on the luminescent properties of Perovskite-like stannates in the 1700–2000 nm region was available to us at the time we made this work. Therefore study of the luminescent properties of CaSnO₃ base compounds in this spectral region is an important research task. Earlier experimental data for CaSnO₃ : Yb³⁺, CaSnO₃ : Er³⁺ and CaSnO₃ : Ho³⁺ single-activator luminescent structures and double-activator CaSnO₃ : Yb³⁺,Er³⁺ and CaSnO₃ : Yb³⁺,Ho³⁺ luminescent structures indicated energy transfer from Yb³⁺ ions to Er³⁺ и Ho³⁺ ones. This transfer increases the 1550 and 1950 nm luminescence band intensities, respectively. Below we consider the mechanism of energy transformation in the Yb³⁺–Er³⁺–Ho³⁺ combination and the possibility of increasing IR luminescence intensity near 1950 nm by energy transfer to Ho³⁺ ions from Yb³⁺ and Er³⁺ ones. To solve this task we synthesized series of specimens with the general formula (Ca_1-x-_y-_zYb_xEr_yHo_z)SnO₃ and studied their luminescent properties.

At the first stage we tested different solid state synthesis modes and selected optimum anneal temperature and time for pure CaSnO₃. The source components were calcium carbonate CaCO₃ and tin hydroxide Sn(OH)₂ which were dry mixed in the 1 : 1 stoichiometric ratio. After thorough grinding the charge was sieved (# 100 mesh), loaded into alundum crucibles and placed in a high-temperature furnace for heat treatment. The experiments showed that the formation of the pure CaSnO₃ phase requires specimen annealing in an oxidizing atmosphere at 1250 °C for 18 h [9].

At the second stage we obtained calcium stannate specimens activated with rare-Earth metal ions. The charge composition was calculated based on the formula (Ca_1-x-_y-_zYb_xEr_yHo_z)SnO₃. The x, y and z indices in the formula correspond to the content of each element in the luminophore in atomic fractions. Since the luminescent properties of the test structures are sensitive to impurities the CaSnO₃ : Yb³⁺,Er³⁺,Но³⁺ system was synthesized from special purity Yb₂O₃, Er₂O₃ and Ho₂O₃ rare-Earth element oxides that were added to the charge in the form of nitrate solutions. The Yb³⁺ ion concentration in the specimens was constant, 0.05 at.fr.. According to earlier data [10] this concentration provides for the highest 996 nm luminescence band intensity for the (Ca_0.95Yb_0.05)SnO₃ system under 960 nm excitation. The Er³⁺ ion concentration was 0.02 at.fr. and also constant. This concentration provides for the highest 1550 nm luminescence band intensity for the (Ca_0.93Yb_0.05Er_0.02)SnO₃ system under 960 nm optical laser excitation [11]. The Но³⁺ ion concentration varied from 0.00005 to 0.1 at.fr.. Calcium carbonate and rare-Earth element nitrate solutions were mixed in liquid state and then the specimens were placed into a drying chamber for 2 h. Then a tin hydroxide and flux mixture was added to the charge, and the mixture was thoroughly grinded and sieved. Then the charge was loaded into alundum crucibles and annealed at 1250 °C for 18 h. The SnCl₂ (3 wt.%), Li₂CO₃ (1 wt.%) and Na₂CO₃ (1 wt.%) fluxes were added to the luminophore charge for reducing the melting point and increasing the solution component diffusion rate. Furthermore Li⁺ and Na⁺ alkaline metal ions act as compensating impurities. They partially compensate the charge mismatch produced by Ca²⁺ ion substitution for Yb³⁺, Er³⁺ and Но³⁺ ions in CaSnO₃ lattice sites [5, 12, 13].

The phase composition of the as-synthesized specimens was studied on a DIFRAY-401 X-ray diffractometer (CuK_α radiation, Ni filter). The unit cell parameters were calculated using the Difract software. The grain size distribution of the powders was studied using a Microsizer-201A laser particle size analyzer. The luminescence and excitation spectra were taken in the 400–2100 nm range using an MDR-41 monochromator, FEU-62 and FEU-100 photoelectron amplifiers and a 960 nm pulse semiconductor diode laser.

Calcium stannate CaSnO₃ has a perovskite-like orthorhombic crystalline structure with the Pbnm space symmetry group. Calcium stannate doping with rare-Earth element ions when Yb³⁺, Er³⁺ and Но³⁺ ions substitute Ca²⁺ rare-Earth metal ions does not change the crystal lattice symmetry [14–16]. Figure 1 shows X-ray phase analysis data for the pure calcium stannate specimens synthesized at a constant temperature and different anneal times and for rare-Earth ion activated calcium stannate.

Figure 1.

Diffraction patterns of CaSnO₃ specimens synthesized with (1) 1, (2) 10 and (3) 18 h anneal time and (4) CaSnO₃ : Yb³⁺,Er³⁺,Но³⁺ specimen synthesized for 18 h. The synthesis temperature was 1250 °C for all specimens.

The position and intensity of the diffraction peaks correspond to the CaSnO₃ phase. The sharp diffraction peaks indicate the high crystallinity of the specimens. Grain size distribution analysis shows the average particle size of the (Ca_0.93-xYb_0.05Er_0.02Ho_0.007)SnO₃ structure to be 30.5 mm.

(Ca_0.93-xYb_0.05Er_0.02Ho_z)SnO₃ structure excitation with 960 nm visible laser radiation produces anti-Stokes luminescence in the 540–560 nm and 640–690 nm bands (Fig. 2).

Figure 2.

Visible luminescence spectra of (Ca_0.93-xYb_0.05Er_0.02Ho_z)SnO₃ specimens (z = 0.00005; 0.0005; 0.007) for 960 nm laser excitation: z = (1) 0.00005 at.fr.; (2) 0.0005 at.fr.; (3) 0.007 at.fr.

The 540–560 nm band does not show any clear holmium ion concentration dependence of luminescence intensity since this spectrum region is a superposition of luminescence peaks for multiple activators. The 545 nm peak for a 0.007 at.fr. Но³⁺ ion concentration is produced by the ⁵F₄→⁵I₈ transition in holmium ions. The two other peaks in this band for 0.0005 and 0.00005 at.fr. holmium ion concentrations are at 545 and 550 nm and produced by the ⁵F₄→⁵I₈ transition in holmium ions and the ⁴S_3/2→ ⁴I_15/2 transition in erbium ions, respectively. The luminescence intensity in the 640–690 nm band decreases with increasing Но³⁺ ion concentration in the luminophore. The luminescent peaks in this region are produced by the ⁴F_9/2→⁴I_15/2 transition in Er³⁺ ions. Similar anti-Stokes luminescence was observed in the visible spectral region earlier [8, 13].

Weak luminescence was observed in the ~740–770 nm near IR region. This luminescence is produced by the ⁵I₄→⁵I₈ transition in Но³⁺ ions (Fig. 3).

Figure 3.

740–775 nm luminescence spectra of (Ca_0.93x Yb_0.05Er_0.02Ho_z)SnO₃ specimens for 960 nm laser excitation: z = (1) 0.00005 at.fr.; (2) 0.0005 at.fr.; (3) 0.007 at.fr.

As the Но³⁺ ion concentration increases to 0.00005 at.fr. the luminescence intensity in this band increases and then decreases due to the concentration quenching of luminescence.

960 nm excitation of the material also produces IR luminescence bands peaking at ~996, 1194, 1550 and 1950 nm (Fig. 4).

Figure 4.

1000–2150 nm luminescence spectra of (Ca_0.93-x Yb_0.05Er_0.02Ho_z)SnO₃ specimens for 960 nm laser excitation: z = (1) 0.00005 at.fr.; (2) 0.0005 at.fr.; (3) 0.007 at.fr.

The radiation in these bands corresponds to the following transitions:

996 nm band: ²F_5/2→²F_7/2 transition in Yb³⁺ ions;

1194 nm band:⁵I₆→⁵I₈ transition in Ho³⁺ ions;

1550 nm band:⁴I_13/2→⁴I_15/2 transition in Er³⁺ ions;

1950 nm band:⁵I₇→⁵I₈ transition in Ho³⁺ ions.

Figure 3 shows that an increase in the Ho³⁺ ion concentration in the luminophore matrix increases the luminescence intensity in the bands corresponding to Ho³⁺ ions. However the bands corresponding to Yb³⁺ and Er³⁺ ions exhibit the contrary behavior. Earlier we reported that 811–960 nm laser excitation of specimens having a (Ca_1-xHo_х)SnO₃ structure produces no luminescence but generates luminescence in the (Yb³⁺, Но³⁺) and (Er³⁺, Но³⁺) bands for the (Ca_1-x-_yYb_xHo_y)SnO₃ and (Ca_1-x-_yEr_xHo_y)SnO₃ systems, respectively [10, 11]. Based on these data we concluded that these double-activator systems are activated through Yb³⁺/Er³⁺ ions acting as excitation centers. Part of energy is transferred from excited energy levels of Yb³⁺/Er³⁺ ions to Ho³⁺ ion levels followed by optical transitions in Ho³⁺ ions. The weak luminescence intensity of the (Ca_1-x-_yEr_xHo_y)SnO₃ specimens compared with the (Ca_1-x-_yYb_xHo_y)SnO₃ ones indicates that Yb³⁺ ions are more efficient sensibilizers than Er³⁺ ones under IR excitation.

The luminescence peak intensity redistribution (Fig. 4) also indicates energy transfer from Yb³⁺ and Er³⁺ ions to Но³⁺ ones in the (Ca_0.93-xYb_0.05Er_0.02Ho_z)SnO₃ luminescent system. The energy transformation mechanism in the (Ca_1-x-_y-_zYb_xEr_yHo_z)SnO₃ triple-activator system can be described as follows.

Luminescent system excitation with a 960 nm source supplies sufficient energy for the core level electrons in Yb³⁺ (²F_7/2) and Er³⁺ (⁴I_15/2) ions to transit to the ²F_5/2 and ⁴I_11/2 excited states, respectively.

The energy absorbed by Yb³⁺ ions is spent for phonon energy, the ²F_5/2 → ²F_7/2 radiative transition, energy transfer from the ²F_5/2 excited state of Yb³⁺ ions to top energy levels of Ho³⁺ and Er³⁺ ions (cooperative sensibilization) and resonance interaction between Yb³⁺ (²F_5/2), Ho³⁺ (⁵I₆) and Er³⁺ (⁴I_11/2) ions.

Yb³⁺–Ho³⁺ ion pairs exhibit the following energy transformation processes: electron transition from the ²F_5/2 excited level of Yb³⁺ ions to the ³K₈, ⁵F₂, ⁵F₃, ⁵S₂ and ⁵F₄ excited levels of Ho³⁺ ions followed by anti-Stokes luminescence, and resonance energy transfer from the ²F_5/2 excited level of Yb³⁺ ions to the ⁵I₆ excited level of Ho³⁺ ions followed by Stokes IR luminescence.

Yb³⁺–Er³⁺ ion pairs exhibit the following energy transformation processes: electron transition from the ²F_5/2 excited level of Yb³⁺ ions to the ⁴F_7/2 excited level of Er³⁺ ions followed by anti-Stokes luminescence, and resonance energy transfer from the ²F_5/2 excited level of Yb³⁺ ions to the ⁴I_11/2 excited level of Er³⁺ ions followed by Stokes IR luminescence in the 1550 nm band.

Er³⁺–Ho³⁺ ion pairs exhibit the following energy transformation processes: resonance energy transfer from the ⁴I_11/2 excited level of Er³⁺ ions to the ⁵I₆ excited level of Ho³⁺ ions followed by Stokes IR luminescence.

The diagram of the abovementioned energy transitions in the visible and IR regions is presented in Fig. 5. The diagram was plotted using earlier data on the electron state energies of Yb³⁺, Er³⁺ and Но³⁺ ions [8, 17].

Figure 6.

(Ca_0.93-xYb_0.05Er_0.02Ho_z)SnO₃ luminescence intensity as a function of Ho³⁺ concentration for (1) 994, (2) 1194, (3) 1550 and (4) 1950 bands.

To specify more exactly the luminescence intensity in the 994, 1194, 1550 and 1950 nm bands as a function of Но³⁺ ion concentration we synthesized an additional series of specimens with z = 0.001, 0.007, 0.01, 0.05 and 0.1 at.fr. (Fig. 6).

Figure 5.

Schematic diagram of energy transfer mechanism between Yb³⁺, Er³⁺ and Ho³⁺ ions in the (Ca_1-x-_y-_zYb_xEr_yHo_z)SnO₃ system.

The luminescence intensity in the bands corresponding to ytterbium (994 nm) and erbium (1550 nm) ions is the highest at low Но³⁺ concentrations and decreases gradually with an increase in the holmium concentration. On the contrary, the luminescence intensity in the 1194 and 1950 bands corresponding to radiative transitions in holmium ions increases and peaks at z = 0.007. Further increase in the holmium ion concentration in the luminophore matrix crystalline lattice intensifies concentration quenching of luminescence.

We used the best (Ca_0.943Yb_0.05Ho_0.007)SnO₃ double-activator composition as a reference specimen for assessing the radiation efficiency of the (Ca_0.93-x Yb_0.05Er_0.02Ho_z)SnO₃ system in the ~2000 nm band when measuring the luminescence spectra for the specimens of this series. The study showed that the (Ca_0.923Yb_0.05Er_0.02Ho_0.007)SnO₃ triple-activator composition has a higher luminescence intensity than the (Ca_0.943Yb_0.05Ho_0.007)SnO₃ reference specimen at a 0.007 at.fr. Но³⁺ ion concentration. If the (Ca_0.943Yb_0.05Ho_0.007)SnO₃ reference specimen intensity in the 1950 nm band is taken as 100%, the (Ca_0.923Yb_0.05Er_0.02Ho_0.007)SnO₃ triple-activator composition specimen intensity is 25% higher in this band. This accounts for the experimentally observed increase in the holmium ion luminescence intensity in the ~2000 nm band.

The interaction mechanism between Yb³⁺,Er³⁺ and Но³⁺ions in the calcium stannate crystal lattice was described. The possibility of increasing IR radiation luminescence intensity in the 1950 nm region by energy transfer from Yb³⁺ and Er³⁺ ions to Ho³⁺ ones was experimentally confirmed. The energy transfer mechanism and the radiative transition probability are largely controlled by the Ho³⁺ ion concentration in the luminescent composition. The highest luminescence intensity in the 1950 nm band corresponds to a 0.007 et.fr. Ho³⁺ ion concentration. The capability of the CaSnO₃ : Yb³⁺,Er³⁺,Но³⁺ luminescent system of converting near IR region radiation (960 nm) to greater wavelength radiation (~2000 nm) can be used for the fabrication of photoconverters, protective marks and markers and IR sources. 1500+ nm sources are widely used for information transmission via fiber optics communication lines and find increasing application in ophthalmology, detection/ranging and materials processing due to the eye-safety of this spectral region.