The synthesized samples are formed by single crystal phase that we had earlier confirmed using the XRD data [27Chukova O, Gomenyuk O, Nedilko S, et al. Luminescence processes in A3IILaNb3O12 (AII = Ba, Sr) layered perovskites. Opt Mater 2014; 36: 1709-14.
[http://dx.doi.org/10.1016/j.optmat.2014.02.001] , 28Titov YuA, Slobodyanik NS, Polubinskii VV, Chumak VV. Mechanisms of the formation of layered A4B3O12 compounds from co-precipitated hydroxocarbonate and hydroxide systems. Theor Exp Chem 2011; 47: 394-8.
[http://dx.doi.org/10.1007/s11237-012-9233-2] ]. It was found that the A^II₃LaM₃O₁₂ (A^II = Sr, Ba; M = Nb, Ta) compounds are characterized by layered crystal structure, that can be described as formed by repetition of some packet of the lattice constituents in direction of axes Z. Each packet (it is named by some authors as “slab”) consists of three perovskite-like layers of the MO₆ octahedrons and A^II, La ions. The part of the packet, within the fragment shown in Fig. (1), is called as perovskite-like block [9Lichtenberg F, Herrnberger A, Wiedenmann K, Mannhart K. Synthesis of perovskite-related layered AnBnO3n+2 = ABOx type niobates and titanates and study of their structural, electric and magnetic properties. J Progress in Solid State Chem 2001; 29: 1-70.
[http://dx.doi.org/10.1016/S0079-6786(01)00002-4] , 11Zhang H, Wu Y, Meng S, Fang L. Crystal structure and microwave dielectric properties of a new A4B3O12-type cation-deficient perovskite Ba3LaTa3O12. J Alloys Compd 2008; 460: 460-3.
[http://dx.doi.org/10.1016/j.jallcom.2007.05.099] , 13Karsu EC, Popovici EJ, Ege A, et al. Luminescence study of some yttrium tantalate-based phosphors. J Lumin 2011; 131: 1052-7.
[http://dx.doi.org/10.1016/j.jlumin.2011.01.021] ]. Two of these blocks are completely shown in Fig. (1). (For clarity, we separated them by imaginary planes drawn with dashed lines.) So, we can say that every packet is formed by 3-layered blocks repeated infinitely in XY plane.

Fig. (1) The fragment of the AII3LaM3O12 (AII = Sr, Ba; M = Nb, Ta) compounds crystal structure.

The MO₆ groups belonging to the same packet are joined by vertices (Fig. 1), while there are no direct connections between the MO₆ octahedrons belonging to neighbour packets. They are linked through - O – (A^II, La) - O - interblock bond. This allows us to distinguish the octahedrons of inner layer (inner layer’s octahedrons) from the octahedrons of two neighboring layers, which we call as border layer’s octahedrons (Fig. 1).

Two of the A^II/La cations lie between blocks (one A^II/La cation lies above the block and other one lies below the same block). They can be called as non-block cations. The four A^II/La cations belong to the block, with one completely, and the other four cations belong also to the adjacent four blocks. Consequently, in the sum we have two cations that lie inside the block. They can be called as internal cations. The described location leads to more significant distortions of the border layer’s MO₆ octahedrons compared to the inner layer’s ones.

The distortion of the MO₆ groups may effect on luminescence characteristics. Therefore, distortion rates Δ for the (A^II, La)О₁₂ and of the MO₆ groups were calculated using crystallographic data from the (Table 1) and formulae (1) [31Shannon RD. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr A 1976; 32: 751-67.
[http://dx.doi.org/10.1107/S0567739476001551] ]:

Table 1
Some cryslallographic data for the A^II₃LaM₃O₁₂ (A^II = Sr, Ba; M = Nb, Ta) compounds.

(1)

where R_i are the cation - Oxygen distances, R is the average distance, n is a coordination number.

The results of calculations are available in the (Table 1). The data on the distribution of the total number of cations A^II and La between them (in %) are also given in the (Table 1). According to the data, the Sr₃LaNb₃O₁₂ crystal structure is characterized by partially ordered distribution of the Sr and La atoms, when the ions of larger radius (Sr²⁺ ions) are mainly located between the blocks, while the the ions of the smaller radius (La³⁺ ions) are inside the block. On the contrary, the statistical distribution of the alkaline-earth and La ions is characteristic for the Ва₃LaM₃O₁₂ (M = Nb, Ta) and Sr₃LaTa₃O₁₂ compounds [11Zhang H, Wu Y, Meng S, Fang L. Crystal structure and microwave dielectric properties of a new A4B3O12-type cation-deficient perovskite Ba3LaTa3O12. J Alloys Compd 2008; 460: 460-3.
[http://dx.doi.org/10.1016/j.jallcom.2007.05.099] , 27Chukova O, Gomenyuk O, Nedilko S, et al. Luminescence processes in A3IILaNb3O12 (AII = Ba, Sr) layered perovskites. Opt Mater 2014; 36: 1709-14.
[http://dx.doi.org/10.1016/j.optmat.2014.02.001] , 32Rother HJ, Kemmler-Sack S, Treiber U, Cyris W-R. Über hexagonale perowskite mit kationenfehlstellen. XXI. Die Struktur von Ba4Nb2WO12 und Ba3LaNb3O12. Z Anorg Allg Chem 1980; 466: 131-8.
[http://dx.doi.org/10.1002/zaac.19804660115] ].

Emission spectra of both niobate and tantalate samples are presented by wide bands in the 3.5 – 1.7 eV energy range (Fig. 2). Intensity of the tantalate samples emission at room temperature (RT) is quite low. The PL spectra of the Sr₃LaTa₃O₁₂ monitored at excitation E_ex = 6.2 eV contain three low intensity components with peak position near 2.5, 2.5 and 1.75 eV (Fig. 2, curve3). One more band at 2.85eV appears in the luminescence spectrum measured under excitation E_ex= 4 eV. The PL spectra of the Ba₃LaTa₃O₁₂ also contain the bands with position of maximum at 2.5, 2.05 and 1.75 eV, but intensity of the niobate samples luminescence is higher at about 5 -10 times if compare with the tantalates emission. The spectra consist of the wide bands at 2.8 and 2.85 eV for the Sr₃LaNb₃O₁₂ and Ba₃LaNb₃O₁₂ samples, respectively (Fig. 2).

The low temperature (T = 8 K) luminescence spectra of the tantalate and niobate samples are similar, but intensity of the niobates luminescence is higher at about 10 times. The main bands of the spectra of the niobates at excitations of E_ex = 6.2 eV reach a maximum near 2.75 - 2.7 eV (Fig. 3, curves 3, 4). The long wave length tails of these spectra contain also weak features around 2.1and 1.8 eV. Peak positions of the main PL bands for the tantalates at the noted excitation are at 2.6 – 2.55 and 2.7 – 2.65eV for the Sr₃LaTa₃O₁₂ and Ba₃LaTa₃O₁₂, respectively (Fig. 3). The PL spectra of the tantalates measured at excitation of E_ex = 5 eV show the maxima near 2.5 eV. The low intensity details and shoulder can be also seen at 2.25eV.

Fig. (2) The PL spectra of the AII3LaM3O12 (AII = Sr, Ba; M = Nb, Ta) compounds. T = 300 K; Eex = 4 (2), 5 (1), and 6.2eV (3, 4, 5).

Fig. (3) The PL spectra of the Ba3LaTa3O12 (1,5), Sr3LaTa3O12 (2,6), Ba3LaNb3O12 (3) and Sr3LaNb3O12 (4,7). T = 8 K; Eex = 6.2 (1 – 4), 4 eV (5 -7) for the left part of the Figure; Eex = 5 eV for the right part of the Figure.

The described above the PL peculiarities allowed us to suppose, that all spectra consist of two main bands with maxima positions near 2.9 and 2.5 eV, and variations of these components’ contributions determine the position of maximum of the total spectral profile. The relative contribution of the 2.5 eV component is obviously higher for the tantalates than for the niobates (see Fig. 3, left) and when excitation is performed at lower energy of excitation photons. You can pay attention to the curves 5, 7 on the Fig. (3) (left) (E_ex = 4 eV) and to the Fig. (3) (right) (E_ex = 5 eV). The 2.9 eV component is clearly becomes more pronounced, when the niobates are excited at E_ex = 6.2 eV (see Fig. 3, curves3, 4). Besides, we have to suppose that some others the PL bands of the lower intensity influence the spectral shape, particularly they effect on low energy side of the PL spectra (see, the range 1.5 - 2.25 eV of the curves 1, 2, 5, and 7 on the Fig. 3).

Three spectral ranges can be distinguished in the excitation spectra of luminescence monitored at room temperatures of the samples. (The energy of the radiation photons registered at the measuring of the excitation spectra is indicated below as E_reg.) There are 11.0 - 7.5, 7.5 - 4.5, and 4.5 - 3.5 eV spectral ranges, where the separated excitation bands are located. We are able to clearly state the difference behavior of the excitation bands with maxima near 5.9 and 4.0 eV. The first of them is of high intensity in the spectra of the niobates, while it vanishes in the spectra of the tantalates (please compare the curves1, 2 and 3, 4 on the Fig. 4). The band at 4.0 eV is strongly enhanced in the spectra of the Sr- containing compounds, if compare with the spectra of Ba- containing compounds. This last effect is observed for both the niobates and tantalates samples.

Fig. (4) Excitation spectra of the AII3LaM3O12compounds PL(AII = Sr, Ba; M = Nb, Ta). T = 300 K, Ereg = 2.4 (1), 2.9 (2-4) eV.

Excitation spectra registered at low temperature also consist of three main bands located in the spectral ranges near 11–7.0, 7.0–4.8, 4.8– 3.7eV. All the bands are complex and consist of at least two components at 9.0, 8.2; 5.9, 5.2; and 4.5, 4.0 eV, respectively (Fig. 5). The bands in the 11–7.0 eV range undergo minor changes depending on the type of the sample and on the E_reg value. For the tantalate compounds the bands at 4.5 and 4.0 eV are of lower intensity in the excitation spectra registered at 2.4 eV compared with the spectra registered at 2.9 eV. Similarly, for the niobate samples the maximum at 5.9 eV has lower intensity in the spectra registered at 2.4 eV compared to the spectra registered at 2.9 eV.

Fig. (5) Excitation spectra of the Ba3LaTa3O12 (1), Sr3LaTa3O12 (2), Ba3LaNb3O12 (3) and Sr3LaNb3O12 (4); T = 8 K, Ereg = 2.9 (left) and 2.4 eV (right).

The wide band photoluminescence spectra of the investigated tantalate compounds were not reported previously, but above described positions of their emission and excitation bands are similar to those observed previously for the some others layered perovskite-like tantalates [10Shimizu K, Tsujii Y, Hatamachi T, et al. Photocatalytic water splitting on hydrated layered perovskite tantalate A2SrTa2O7·nH2O (A = H, K, and Rb). Phys Chem Chem Phys 2004; 6: 1064-9.
[http://dx.doi.org/10.1039/B312620J] , 11Zhang H, Wu Y, Meng S, Fang L. Crystal structure and microwave dielectric properties of a new A4B3O12-type cation-deficient perovskite Ba3LaTa3O12. J Alloys Compd 2008; 460: 460-3.
[http://dx.doi.org/10.1016/j.jallcom.2007.05.099] , 17Srivastava AM, Ackerman JF, Beers WW. On the Luminescence of Ba5M4O15 (M = Ta5+, Nb5+). J Solid State Chem 1997; 134: 187-91.
[http://dx.doi.org/10.1006/jssc.1997.7574] ]. As it was shown formerly, luminescence properties of tantalates and niobates of the same structure are strongly similar and they are usually considered as caused by the same processes [13Karsu EC, Popovici EJ, Ege A, et al. Luminescence study of some yttrium tantalate-based phosphors. J Lumin 2011; 131: 1052-7.
[http://dx.doi.org/10.1016/j.jlumin.2011.01.021] , 15Wiegel M, Middel W, Blasse G. Influence of ns2 ions on the luminescence of niobates and tantalates. J Mater Chem 1995; 5: 981-3.
[http://dx.doi.org/10.1039/jm9950500981] -18Blasse G. The influence of crystal structure on the luminescence of tantalates and niobates. J Solid State Chem 1988; 72: 72-9.
[http://dx.doi.org/10.1016/0022-4596(88)90010-2] ]. The excitation and emission transitions in such compounds are known to be mainly related with octahedral niobate and tantalate molecular groups of their crystal lattices. The commonly accepted point of view is that reported two emission bands at 430 and 500 nm (it is equal 2.88 and 2.48 eV, respectively) are connected with electron transitions in MO₆^7- groups (M = Nb or Ta) located in two different sites of the perovskite – like blocks.

The PL spectra described by us in this work also reveal both noted above the PL bands near 2.9 and 2.5 eV. So, we have to regard these bands as caused by radiation transitions in two different MO₆^7- molecular groups of the A^II₃LaM₃O₁₂ compounds. In fact, we have already described above that the MO₆^7- molecular groups belonging to the inner layer of octahedrons and the molecular groups of the two border layers differ in the level of deformation, symmetry, and composition of the neighborhood environment. Thus, we consider that indicated above differences in the positions of the PL bands and their temperature behavior are caused by different distortion rates of the corresponded MO₆^7- groups (Table 1). Really, the MO₆^7- groups of the inner layer are close to the O_h octahedral symmetry; whereas stronger distortion of the MO₆^7- groups located on the border layers reduces their symmetry to C_3v [34Liu GK, Vikhnin VS, Kapphan SE. Anharmonic metastable charge transfer vibronic exciton in potassium tantalate. J Phys Conf Ser 2005; 21: 173-6.
[http://dx.doi.org/10.1088/1742-6596/21/1/028] ]. The reduction of the MO₆^7- molecular octahedrons symmetry changes probabilities of the radiation transitions and allows some transitions of lower energy those were forbidden for the octahedrons of the O_h symmetry. In this scheme, we can assign the 2.9 and 2.5 eV emission bands to the T₂→ A₁ and E → A₁ transitions in the MO₆^7- groups of the O_h and C_3v symmetries, respectively (Fig. 6). Taking into account that relative contribution of the 2.9 eV component is lower for the tantalate compounds, than for the niobate ones, we assume that undistorted tantalate octahedrons, TaO₆^7-, are characterized by lower probability of the T₂→ A₁ transitions than undistorted niobate octahedrons, NbO₆^7-. We also have observed that the 2.5 eV band is characterized by higher intensity than intensity of the 2.9 eV band at T = 8K (Fig. 3). This agrees with assumption that strongly distorted octahedral groups are usually characterized by more intensive luminescence [14Blasse G, Reveau B. The luminescence of potassium silico-niobates. J Solid State Chem 1980; 31: 127-30.
[http://dx.doi.org/10.1016/0022-4596(80)90014-6] , 15Wiegel M, Middel W, Blasse G. Influence of ns2 ions on the luminescence of niobates and tantalates. J Mater Chem 1995; 5: 981-3.
[http://dx.doi.org/10.1039/jm9950500981] ]. However, the 2.5 eV PL band was not practically observed at room temperature. This can be caused by higher probability of the thermo-activated radiation-less transitions for the more distorted niobate groups [26Blasse G, Brixner LH. Luminescence of perovskite-like niobates and tantalates. Mater Res Bull 1989; 24: 363-6.
[http://dx.doi.org/10.1016/0025-5408(89)90222-5] ].

Fig. (6) Scheme of the lowest excited and ground energy levels for the MO67- groups and related self-trapped excitons, as well as of the possible electronic absorption and radiation transitions.

The low intensity emission features more clearly observed at room temperature around 2.1 eV and near 1.8 eV are identified as luminescence of the RE³⁺ ions traces. The presence of uncontrolled Eu³⁺ and Pr³⁺ impurities was confirmed by direct laser excitation of the tantalates [35Chukova O, Nedilko S, Polubinskii V, Scherbatsky V, Titov Y. Synthesis and luminescent investigation of the Sr3LaTa3O12 layered perovskite, IEEE Xplore Proceedings of Oxide Materials for Electronic Engineering International Conference 2012; 209-10.
[http://dx.doi.org/10.1109/OMEE.2012.6464897] ]. We estimate the impurity concentration as lower then 10^-6%, according to described previously measurements of the luminescence of the solids containing uncontrolled impurities at such low concentrations of the latter [36Boyko R, Chukova OV, Gomenyuk OV, Nagornyi PG, Nedilko SG. Origin of red luminescence of some phosphate crystals contained chromium and titanium ions. Phys Status Solidi 2005; 2(c): 712-5.
[http://dx.doi.org/10.1002/pssc.200460272] , 37Mogilevsky R, Nedilko S, Sharafutdinova L, et al. Luminescence study of grown sapphire: from starting material to single crystal. Phys Status Solidi 2009; 6(c): s179-83.
[http://dx.doi.org/10.1002/pssc.200881323] ].

The behavior of the excitation spectra depends heavily on the sample composition, if compared to the PL properties (Figs. 4, 5). It is clearly seen from the excitation spectra measured at 300 K: the excitation band in 6.5 – 4.8 eV energy range is observed only for niobate compounds. The low temperature excitation spectra for the tantalate samples contain the 5.2 eV band, but the 5.9 eV band is observed only for the niobates. So, the properties of the excitation bands in the 6.5–4.8 eV energy range noticeably depend on the M cation type, it is Nb or Ta. We suppose that noted excitation is caused by absorption electron transitions within the MO₆^7- groups followed with charge transfer from Oxygen to the central Nb or Ta atoms. There are transitions with electron charge transfer O^2-→M⁵⁺ from 2p Oxygen orbitals to 5d orbitals of the Ta⁵⁺ or 4d orbitals of Nb⁵⁺ions. The corresponded transitions could be A₁ → E and A₁ → T₂ type, for the 5.9 and 5.2 eV bands, respectively. The above described mechanism of excitation and emission, in fact, determines excitonic nature of observed luminescence. First, the excitation leads to some separation of the charge, then self – localization is realized and, finally, excitation energy releases with luminescence quanta emission. This description is illustrated by the simple scheme of energy levels for the MO₆^7- groups and formed self-trapped exciton (Fig. 6).

In fact, we suppose, that luminescence mechanisms in these systems are more complicated. We have pointed out that earlier when discussed the PL spectra as shown in Fig. (3), and the main PL bands of higher energy and the PL components of lower energy, however, with a weak intensity (1.5 - 2.25 eV) have been discussed. Such situation is typical for intrinsic luminescence of many oxide crystals of chemical composition AXO_n, and the PL was usually explained as superposition of luminescence of regular XO_n (n = 4, 6) molecular groups (higher energy bands) and of emission of some defect related luminescence centers (lower energy bands) [19Chukova O, Nedilko S. Study of RE-impurity effects on exciton luminescence of PWO4 single crystals grown by Czochralski method. Opt Mater 2013; 35: 1735-40.
[http://dx.doi.org/10.1016/j.optmat.2013.05.019] , 38Hizhnyi YuA, Nedilko SG. Investigation of the luminescent properties of pure and defect lead tungstate crystals by electronic structure calculations. J Lumin 2003; 102-103: 688-93.
[http://dx.doi.org/10.1016/S0022-2313(02)00625-7] , 39Hizhnyi Yu, Oliynik A, Gomenyuk O, et al. The electronic structure and optical properties of ABP2O7 (A = Na, Li) double phosphates. Opt Mater 2008; 30: 687-9.
[http://dx.doi.org/10.1016/j.optmat.2007.02.009] ].

The oxygen vacancies are the predominant type of the defects in complex oxides. That is why, we assume that the low energy PL bands can be related to luminescence of the defected MO₆^7- group that lost Oxygen -MO₅^5-group, or with the MO₆^7-octahedron located near the mentioned defective group.

The regular MO₆^7-groups can be affected not only by the Oxygen vacancy in neighbor Ta(Nb) – Oxygen octahedron. The vacancies in the cation sublattice, namely A^II and La vacancies, can influence electronic states of these groups.

Defect related luminescence, as a rule, is effectively excited in the spectral range below the absorption edge and its excitation is not so effective in the range of band to band electronic transitions. So, according to the data shown on the (Figs. 3 and 4), we can believe that the excitation bands located in the range 3.7 – 4.8 eV are responsible for excitation of luminescence centers related with Oxygen vacancies. Besides, other factors should also be discussed.

We have seen that intensity of the excitation spectra in the 4.7–3.7 eV energy range depends on the A^II cation type: that is Sr or Ba. So, the 4.0 eV excitation band is more intensive in the spectra of the Sr-containing samples, while the 4.5 eV band increases in the spectra of the Ba-containing samples. Therefore, the absorption transitions responsible for the long wavelength part of the excitation spectra can be occurred on the MO₆^7- groups affected by the A^II cation vacancies - V_AII. Such possibility can be realized effectively, if luminescence is of exciton nature. Important role of the cations of lattice in luminescence processes in complex center “Cation – Molecular group” was described before for some other oxide crystals, e.g. for molybdates and tungstates [19Chukova O, Nedilko S. Study of RE-impurity effects on exciton luminescence of PWO4 single crystals grown by Czochralski method. Opt Mater 2013; 35: 1735-40.
[http://dx.doi.org/10.1016/j.optmat.2013.05.019] , 40Fujita M, Itoh M, Mitani H. Sangeeta, Tyagi M., Exciton transition and electronic structure of PbMoO4 crystals studied by polarized light. Phys Status Solidi 2010; 247: 405-10. [b].
[http://dx.doi.org/10.1002/pssb.200945447] ], and also for the other type tantalates [34Liu GK, Vikhnin VS, Kapphan SE. Anharmonic metastable charge transfer vibronic exciton in potassium tantalate. J Phys Conf Ser 2005; 21: 173-6.
[http://dx.doi.org/10.1088/1742-6596/21/1/028] ]. As we pointed above, cation distribution in the Sr₃LaNb₃O₁₂ crystal structure differs from the one for the other compounds (Table 1). This feature correlates with behavior of the 4.0 eV excitation band which is intensive only in the spectra of Sr₃LaNb₃O₁₂. In such a case, according to data about the cation distribution (Table 1), we can assign the 4.0 eV excitation band to absorption transitions in the centers involving the border MO₆^7-octahedron and vacancy of cation, MO₆^7--V_AII. Then, the 4.5 eV excitation band may be caused by transitions in the center involving the inner MO₆^7-octahedron and related cation vacancies.