Existing calcium gallate CaGa₂O₄ based luminescent materials radiating in visible IR region have been reviewed. IR luminophores have been studied but slightly but their practical implementation is of interest. CaGa₂O₄ specimens activated with Yb³⁺ rare-earth ions have been synthesized using the solid-state method. The structure and luminescent properties of CaGa₂O₄ : Yb³⁺ have been studied. CaGa₂O₄ : Yb³⁺ excitation with 940 and 980 nm radiation generates luminescence in the 980–1100 nm region. Data on the electron level structure in Yb³⁺ ions suggest that the excitation and luminescence occur directly in the Yb³⁺ ions with only a passive role of the base lattice. The luminescence spectra contain three peaks at 993, 1025 and 1080 nm. These luminescence peaks are caused by electron optical transitions from excited to main state in Yb³⁺ ions. 993 nm band luminescence intensity has been studied as a function of Yb³⁺ activator ions concentration. Introduction of Na⁺ ions into the luminophore increases IR luminescence intensity. Optimum (Ca_1-x-_yYb_xNa_y)Ga₂O₄ luminophore composition has been suggested at which the 993 nm luminescence intensity is the highest.

luminophores, gallates, CaGa₂O₄, rare-earth elements, Yb³⁺.

The optical properties of gallates have been studied since long; particular attention is drawn by the effect of photoconductivity induced by UV excitation of the materials. Yttrium, lead, gadolinium, lithium, zinc, copper, lanthanum, barium and strontium gallates are known the photoconductivity [1, 2] and luminescent properties of which are of research interest [3–5]. However calcium gallate CaGa₂O₄ has been studied insufficiently in spite of its optical transparency in the visible region, high photoelectric stability and low price. On the other hand the wide band gap of CaGa₂O₄ (3.6 eV) and phonon oscillation frequencies in the 500–600 cm^-1 range [3] make this material quite promising for the synthesis of new luminophores. There have been few if any studies of the luminescent properties of calcium gallate. Most of them [3, 8, 9, 11, 12] have dealt with the luminescence and electroluminescence of these materials in the visible spectral region under UV excitation.

CaGa₂O₄ based compounds have high chemical stability and the structure and optical properties of CaGa₂O₄ allow their use as an efficient luminophore matrix. These compounds have also been claimed as promising materials for electroluminescent color displays [6, 7].

Intrinsic luminescence of CaGa₂O₄ at λ from 300 to 600 nm and impurity CaGa₂O₄ : Eu³⁺, Na⁺ luminescence bands at 588 and 612 nm generated by UV excitation at λ = 255 nm were described earlier [3]. It was found [3] that the luminophore base efficiently transfers its energy to Eu³⁺ ions and addition of Na⁺ ions greatly increases the intensity and duration of the impurity luminescence. Similar conclusions were also made elsewhere [8] where red luminescence (517 nm) was observed in CaGa₂O₄ activated with Eu³⁺ trivalent ions. Known are the yellow CaGa₂O₄ based luminophores activated with Bi³⁺ [9], Mn²⁺ and Dy³⁺ ions [10] as well as the green CaGa₂O₄ : Tb luminophore [11, 12].

Unlike visible region luminophores infrared (IR) CaGa₂O₄ based ones have been less studied.

It was reported [13] that CaGa_1.99Cr_0.01O₄ gallate has a well-developed system of traps. UV excitation causes charge accumulation and storage in deep traps. UV stimulation of traps generates wide-band Cr³⁺ radiation in the red and IR regions resembling anti-Stokes luminescence in lanthanides.

Co-activation with Cr³⁺ and Nd³⁺ ions [14] causes energy transfer from Cr³⁺ ions to Nd³⁺ ones resulting in a growth of the luminescence band at 1064 nm pertaining to Nd³⁺ ions [15]. It was suggested [14] to use these luminophores in advanced multifunctional applications for bio imaging.

Of interest is also a study [16] of Stokes CaGa₂O₄ based luminophores activated with Nd³⁺ ions and different sensibilizers (Bi³⁺, Eu³⁺, Cr³⁺) where the possibility of near-UV transformation of blue exciting radiation into near-IR region radiation was investigated. These processes are based on energy accumulation and transfer from sensibilizing ions to Nd³⁺ ions. Cr³⁺ ions have a broad excitation spectrum in the blue UV region and as reported earlier [16] most efficiently transfer energy to Nd³⁺ ions which in turn generate IR luminescence at ~1000 nm [16]. This material was suggested as a promising spectral converter for silicon solar cells.

The CaGa₂O₄ : Yb³⁺ excitation spectra [16] contained two bands: a wide one at λ = 200–255 nm corresponding to interband transitions in the CaGa₂O₄ base lattice and a peak series at 900–1000 nm caused by transitions in Nd³⁺ ions. Luminescence in Eu³⁺,Yb³⁺ : CaGa₂O₄, Bi³⁺,Yb³⁺ : CaGa₂O₄, Cr³⁺,Yb³⁺ : CaGa₂O₄ compounds was excited with 266 and 450 nm sources. Luminescence was excited through the luminophore base and sensibilizing impurity. CaGa₂O₄ : Yb³⁺ luminescence excitation with 900–1000 nm radiation was not studied [16]. However IR-excited IR luminophores are also of research interest and can be used for the fabrication of spectral photoconverters, latent images and markers.

Presented below are data on the luminescent properties of CaGa₂O₄ activated with Yb³⁺ions upon 940 and 980 nm laser excitation.

The raw materials for luminophore synthesis were special purity CaCO₃ calcium carbonate, Ga₂O₃ gallium oxide, Yb₂O₃ ytterbium oxide and Na₂CO₃ sodium carbonate. Due to the small required quantity of rare-earth Nd³⁺ ions they were introduced into the charge in the form of Yb(NO₃)₃ nitrate in concentrations of 0.3 to 15 mol.%. Along with the activator ions, charge compensating Na⁺ ions were introduced into the luminophore. Their concentration was the same as that of Nd³⁺ ions. The empirical formula of the synthesized luminophore was (Ca_1-x-_yYb_xNa_y)Ga₂O₄. The luminophore was synthesized using the solid-state method in a high-temperature furnace in an air atmosphere at 1250 °C for 18 h. These temperature and time parameters were selected based on experimental X-ray diffraction data on specimens synthesized under different conditions. Detailed description of α-CaGa₂O₄ and β-CaGa₂O₄ polymorphic phase formation in the 1050–1350 °C range was reported earlier [7]. The 1250–1350 °C range is the optimum one for synthesizing the CaGa₂O₄ phase at normal atmospheric pressure in the presence of oxygen gas.

In accordance with earlier data [12] three compounds may exist in the CaO–Ga₂O₃ system depending on the stoichiometric ratio: 3CaO ∙ Ga₂O₃, CaO ∙ Ga₂O₃ and CaO ∙ 2Ga₂O₃. To obtain the CaO ∙ Ga₂O₃ phase (hereinafter CaGa₂O₄) we added the CaO and Ga₂O₃ components in the 1 : 1 ratio.

Qualitative and quantitative phase analyses of the specimens were carried out on the basis of X-ray diffraction patterns taken on a DIFREI 401 X-ray diffraction instrument in CuK_α radiation with a Ni filter. The unit cell parameters were calculated using the Difract software.

The size distribution of the synthesized powders was studied with a Microsizer 201A laser analyzer [17].

The excitation spectra were analyzed using two MDR-24 monochromators and a photocell device for IR radiation detection. The luminescence spectra were taken with an MDR-204 monochromator and a PbS photocell device. The luminescence was excited with 940 and 980 nm semiconductor laser diodes.

The reference specimen for the spectral analysis of the synthesized specimens was L-54 industrial luminophore. The maximum luminescence band intensity ratio in the 980–1100 nm region for the test and reference specimens was used as the IR luminescence intensity measure.

Solid state synthesis of calcium gallate includes baking of two CaO and Ga₂O₃ oxides at 1250 °C for 18 h. The reaction formula is [18]

CaO + Ga₂O₃ → CaGa₂O₄. (1)

Calcium gallate (CaGa₂O₄) crystallizes predominantly in the orthorhombic modification, space group P21/c [19, 20]. The ionic radius of Ca²⁺ (0.099 nm) is comparable with that of the activator ions Yb³⁺ (0.086 nm) [21, 22], the Ga³⁺ ionic radius being far smaller, 0.062 nm [12]. Therefore upon introduction of Yb³⁺ ions into CaGa₂O₄ structure, these activator ions most probably occupy Ca²⁺ ion positions to form a substitutional solid solution. This however causes a charge mismatch that we compensated with Na⁺ ions by introducing them into the luminophore charge in the same quantity as Yb³⁺ ions. The Na⁺ ionic radius (0.097 nm) is almost the same as that of Ca²⁺, and this is also expected to reduce structure distortion in the base lattice.

Figure 1 shows X-ray diffraction patterns of pure and Yb³⁺ and Na⁺ doped calcium gallate. By way of comparison we presented the bar chart for CaGa₂O₄ borrowed from the ASTM international X-ray diffraction standard files, card No. PDF-140143.

The most prominent diffraction peaks in the X-ray diffraction patterns coincide with the typical peaks of CaGa₂O₄ (PDF-140143) as per the ASTM X-ray diffraction files. It can also be seen from Fig. 1 that the typical diffraction peaks of the pure and doped calcium gallate coincide. This is caused by the low Yb³⁺ and Na⁺ doping impurity concentrations which incorporate into the gallate lattice and occupy Ca²⁺ ion positions without causing considerable distortion to the material’s crystal lattice.

The size distribution of the powdered specimens synthesized under similar conditions showed that the mean particle size of pure CaGa₂O₄ is 17.9 µm. The mean particle size of CaGa₂O₄ activated with 5 mol.% Yb³⁺ ions is 18.5 µm. Introduction of Na⁺ ions into the luminophore in the same quantity increases the mean particle size but slightly to 21.7 µm. This is because Na₂CO₃ impurity acts as flux during solid state luminophore synthesis thus improving the interaction between solid solution components and accelerating mass transport and new phase formation. As a result the crystallites grow and the mean particle size in the specimen increases.

The excitation spectra of the (Ca_0,9Yb_0,05Na_0,05)Ga₂O₄ luminophore were taken for 1025 and 1080 nm luminescent bands which were selected based upon a preliminary study of specimen luminescence spectra excited with a 980 nm laser. The excitation wavelength used for excitation spectra measurement was varied in the 800 to 2000 nm range. The 1025 nm band in the excitation spectra corresponded to two peaks at 923 and 975 nm. For the 1080 nm band the maximum excitation intensity was at the same wavelengths (Fig. 2). The spectra also contained one more excitation band at ~910 nm but its peak was less clear.

Figure 1.

(a) Bar chart and experimental X-ray diffraction patterns of (b) pure CaGa₂O₄ and (c) doped CaGa₂O₄ : Yb,Na.

Figure 2.

(Ca_0,9Yb_0,05Na_0,05)Ga₂O₄ luminophore specimen excitation spectra: (1) 1025 and (2) 1080 nm.

As noted above the CaGa₂O₄ band gap is about 3.6 eV. To make the energy of valence electrons sufficient for electron transition to the conduction band one should excite the material with a 344 nm or smaller wavelength radiation. This may initiate interband transitions. These transitions in CaGa₂O₄ were described in an earlier study of intrinsic and extrinsic UV-excited visible luminescence [3, 16]. Since IR luminescence was excited in our experiment with photons having far greater wavelengths one can conclude that their energy would be insufficient for interband transitions. Thus electron transitions in CaGa₂O₄ : Yb³⁺ occurred directly in Yb³⁺ activator ions with only a passive role of the base lattice.

Excitation at the 910, 932 and 975 nm corresponds to energy transfers between Stark components of the ²F_7/2 and ²F_5/2 levels in Yb³⁺ ions. Since the excitation intensity for the 932 and 975 nm bands was the highest we used 940 and 980 nm semiconductor laser diodes for further luminescence studies of the test specimens.

We now consider excitation and luminescence in CaGa₂O₄ : Yb³⁺ in a greater detail using the electron level diagram of Yb³⁺ ions (Fig. 3). Data on the positions of the top Stark levels were borrowed from an earlier study of electron absorption spectra [18], and the structure of the bottom Stark sublevels of Yb³⁺ ions was studied using luminescence spectra [23, 24].

The main unexcited level ²F_7/2 consists of four Stark sublevels (1, 2, 3, 4) and the excited level ²F_5/2 is split into three sublevels (5, 6, 7). Arrows show potential electron transitions with energy absorption or release. Earlier absorption and luminescence spectra [18, 23, 24] were studied at LN or lower temperatures. This greatly reduced the thermal oscillation energy of the lattice atoms for more reliable matching of absorption peaks with the respective transitions between Stark components in Yb³⁺ ions. It was reported [23] that at 4.2 K transitions may occur from the main bottom Stark sublevel ²F_7/2 (1) to the excited Stark sublevels 5, 6 and 7 of the ²F_5/2 state (Fig. 3a).

Figure 3.

Structure of Stark sublevels in Yb³⁺ ions: (a) 4.2–77 K transitions [23, 24]; (b) 300 K transitions.

We studied the material at room temperature (300 K). The occupation of the Stark sublevels 2, 3 and 4 at this temperature is greater than at 4.2 K and therefore there is a higher probability of electron transitions from these levels to the ²F_5/2 multiplet sublevels (Fig. 3b). Electron transitions from the bottom sublevel 5 of the excited ²F_5/2 state to the sublevels 1, 2, 3 and 4 of the ²F_7/2 state is accompanied by energy release in the form of a photon, i.e., luminescence occurs. The same conclusions were made in a earlier study of the Stark structure of Yb³⁺ ion levels in (Yb_xY_1-x)₂Ti₂O₇ at 4.5–300 K [25].

The first stage of our investigation into the luminescent properties of CaGa₂O₄ : Yb³⁺ dealt with the luminescence spectra of the specimens and the effect of Na⁺ ions on the luminescence intensity. The luminescence spectra of the specimens contain three resolvable peaks at 993, 1025 and 1080 nm (Fig. 4). Radiation at these wavelengths is generated by electron transitions from the excited ²F_5/2 level to the main unexcited ²F_7/2 level in Yb³⁺ ions.

The 993 nm luminescence intensity in the specimen synthesized with flux was almost threefold higher than that in the specimen synthesized without flux. This confirms our assumptions concerning the effect of Na⁺ ions on the luminescent properties of CaGa₂O₄ : Yb³⁺. We will now consider in detail the process of charge compensation upon the formation of a substitutional solid solution.

Yb³⁺ ions in CaGa₂O₄ : Yb³⁺ occupy the positions of Ca²⁺ ions. Inovalent substitution produces electrically charged defects. Two Yb³⁺ ions substitute three Ca²⁺ ions thus generating one negatively charged defect V"_Ca and two positively charged defects Yb•_Ca the defect formation reaction being as follows:

$3{\mathrm{Ca}}^{2+}+2{\mathrm{Yb}}^{3+}\to V_{\mathrm{Ca}}^{’’}+2{\mathrm{Yb}}_{\mathrm{Ca}}^{\bullet }$. (1)

Ca²⁺ ion substitution for Na⁺ and Yb³⁺ ions in the lattice generates one positively charged defect Yb•_Ca and one negatively charged defect Na^°_Ca. The defect formation equation is as follows:

$2{\mathrm{Ca}}^{2+}+{\mathrm{Na}}^{+}+{\mathrm{Yb}}^{3+}\to {\mathrm{Na}}_{\mathrm{Ca}}°+\mathrm{Y}{\dot{\mathrm{b}}}_{\mathrm{Ca}}$. (2)

It is well-known [3] that Na⁺, Li⁺ and K⁺ ions not only act as charge compensators but also affect to a certain extent the crystal lattice, powder particle size and surface morphology of the material. The Na⁺ ionic radius (0.097 nm) is almost the same as that of Ca²⁺ ions (0.099 nm) so the structural distortions caused by their mutual replacement in the CaGa₂O₄ lattice are but minor. Thus introduction of Na⁺ ions into the luminophore minimizes the role of defects generated by violation of crystal electrical neutrality and size mismatch between Ca²⁺ and Yb³⁺ ions and hence increases the luminescence intensity in the 980–1090 nm region.

At the second stage we studied the luminescence intensity in the specimens as a function of Yb³⁺ ions concentration. Figure 5 shows luminescence spectra of (Ca_1-х-_yYb_хNa_y)Ga₂O₄ for different activator ion concentrations.

Figure 4.

(1) (Ca_0.9Yb_0.05Na_0.05)Ga₂O₄ and (2) (Ca_0.95Yb_0.05)Ga₂O₄ luminescence spectra excited with 940 nm radiation.

Figure 5.

(Ca_1-х-_yYb_хNa_y)Ga₂O₄ luminescence spectra excited with 940 nm radiation for different activator ion concentrations: (1) х = y = 0.5 mol.%; (2) 1 mol.%; (3) 5 mol.%; (4) 10 mol.%; (5) 15 mol.%.

The luminescence spectrum of (Ca_1-х-_yYb_хNa_y)Ga₂O₄ is a broad band with three clearly resolved peaks at 993, 1025 and 1080 nm and a few weak peaks. The luminescence intensity is the highest for the specimen with an Yb³⁺ ions concentration of 1 mol.%. The luminescence intensity decreases with an increase in the activator ion concentration. At Yb³⁺ ions concentrations of 10 and 15 mol.% the luminescence spectrum exhibits intensity redistribution between the 1025 nm peak which is strong at low Yb³⁺ and Na⁺ concentrations and the weak peaks in the vicinity of 1040 nm, in favor of the latter ones. This can be caused by structural distortions in the base lattice and new phase formation. To exclude the effect of occasional errors in the synthesis or study of the specimens we synthesized one more series of specimens with the same Yb³⁺ concentrations. The luminescence spectra of the specimens containing 10 and 15 mol.% Yb³⁺ ions exhibited a similar intensity redistribution.

Figure 6 shows 993 nm luminescence intensity as a function of Yb³⁺ ions concentration.

To obtain a more accurate concentration function we synthesized an additional series of specimens with the Yb³⁺ concentrations x = 0.003, 0.007, 0.01, 0.02 and 0.03. The highest 993 nm luminescence intensity in (Ca_1-х-_yYb_хNa_y)Ga₂O₄ was observed at the Yb³⁺ concentration x = 0.01. The optimum luminophore composition for this spectral band is (Ca_0.98Yb_0.01Na_0.01)Ga₂O₄.

Figure 6.

993 nm luminescence band intensity of (Ca_1-х-_yYb_хNa_y) Ga₂O₄ (x = y = 0.3; 0.5; 0.7; 1; 2; 3; 5; 10; 15 mol.%) as a function of Yb³⁺ ions concentration for 940 nm excitation.

Study of the CaGa₂O₄ : Yb³⁺ compound showed that excitation of the material with 940 and 980 nm radiation generates luminescence in the 980–1100 nm region with peaks at 993, 1025 and 1080 nm. The luminescence in this region is caused by optical transitions in Yb³⁺ ions. Experiments prove the potential for enhancing this IR luminescence by introducing the Na₂CO₃ compensating impurity into the luminophore. Study of the luminescence intensity as a function of activator ions concentration showed that the highest 993 nm luminescence intensity is at the Yb³⁺ concentration x = 0.01. The optimum luminophore composition was suggested to be (Ca_0.98Yb_0.01Na_0.01)Ga₂O₄.

The IR luminophores suggested in this work can find applications in biomedicine, laser engineering, marking expensive products and art pieces, military engineering and fiber optics.