Luminescence of Sb3+ in closed shell transition metal oxides

Journal of Luminescence 208 (2019) 394–401

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Luminescence of Sb3+ in closed shell transition metal oxides

T

Philippe Boutinaud Clermont Université Auvergne, SIGMA Clermont, Institut de Chimie de Clermont-Ferrand, BP 10448, 63000 Clermont-Ferrand, France

A R T I C LE I N FO

A B S T R A C T

Keywords: Charge transfer Photoluminescence Sb3+

The spectroscopic activity of Sb3+ is investigated at 77 K in CaTiO3, CaZrO3, SrZrO3, YNbO4, SrNb2O6 and M'YTaO4. The data are interpreted in the frame of a Sb3+-to-transition metal (Mn+) charge transfer process that is formalized on the basis of a semi-empirical equation involving the energy of the corresponding electronic transition, the electronegativity of Sb3+ (the electron donor) and of Mn+ (the electron acceptor) and crystal structure-related parameters.

1. Introduction The luminescence of trivalent antimony has been the subject of extensive investigation in the past [1–8] before being almost forgotten by the scientific community. The most remarkable phosphor at that time was the doubly doped Sb3+,Mn2+ halophosphate Ca10(PO4)6(F,Cl) that shows such bright luminescence that it has been used as a commercial phosphor in lamps for decades. Sb3+ was shown to be responsible for the blue emission and for the sensitization of the yellow emission of Mn2+ in this material, thus giving white light upon excitation at ≈ 250 nm in the Sb3+ levels [5]. The Stokes shift associated with the Sb3+ emission in this phosphate is as large as 19,000 cm−1, which should normally be concomitant with a low quenching temperature of the corresponding luminescence [9]. To date, it is still unexplained why Sb3+ shows such intense luminescence at room temperature in this material [10]. Trivalent antimony has a 5 s2 electron configuration and thereby belongs to the mercury-like family, like for instance the isovalent Bi3+ ion. The spectroscopy of these cations is commonly described in terms of ns2 ↔ ns1 p1 interconfigurational transitions. Regular absorption takes place from the ground state 1S0 to the excited states 3P1 (A transition) 3P2 (B transition) and 1P1 (C transition). The spin-allowed C transition is usually intense but its wavelength is in general located far in the UV. The B transition is spin forbidden and has low intensity. The A transition has, in contrast, appreciable oscillator strength through the spin-orbit mixing that takes place between 3P1 and 1P1. Regular emission occurs from 3P1,0 levels. Optical transitions with a charge transfer character have also been reported in ns2 cations, but essentially in the case of Bi3+ [11,12]. The corresponding absorbing state results from the transfer of an electron from the ns2 shell of Bi3+, which is oxidized, to the empty d° shell of a nearby cation Mn+, which is reduced. Since a Coulomb interaction remains between the bismuth cation and the electron, the metal-to-

metal (MMCT) state should be regarded as an impurity trapped-exciton state. As reviewed a few years ago, the host lattices in which the above redox process is commonly observable are oxidic lattices that contain closed-shell transition metal cations (Mn+) that form complex anionic units (i.e. titanates, vanadates, niobates, tantalates, molybdates etc.) [12]. It has been recently shown that the A and MMCT transitions of Bi3+ have comparable energy in such closed shell transition metal oxides, but owing to its larger Stokes shift, the MMCT emission is usually observed at longer wavelength relative to the regular A emission [13]. The similarity of behavior between Sb3+ and Bi3+ has been demonstrated in several reports in the past; especially we know that the luminescence of both ions shows strong dependence with temperature, excitation wavelength and nature of the nearby environment. For both, the optical properties can be affected by the stereochemical activity of the s2 lone pair, i.e. with the degree of asymmetry of the coordination polyhedron and related Jahn-Teller effects. Our motivation in the present paper is to check if MMCT transitions are present in closed-shell transition metal oxides doped with Sb3+. To our knowledge, the luminescence of Sb3+ in closed shell transition metal oxides is unknown and, correlatively, MMCT transitions have not been identified yet for this cation. Generally speaking, the attribution of the optical signals in solids doped with ns2 cations is not straightforward. This task is even more complex in the present case considering that if Sb3+ and Bi3+ present similarities, they also have noticeable differences: (1) the former has a much smaller ionic radius and is not known with coordination numbers larger than 6 [14]. This suggests an off-centered position of Sb3+ in large crystal sites. This, in fact, accounts for the larger Stokes shifts of the Sb3+ emission with respect to the Bi3+ emission and its sensitivity to the Jahn-Teller effect. (2) The small Sb3+ ion may also substitute for the Mn+ cations of the host lattices to create antimonate units. In some cases, this substitution could even contribute to the electro-neutrality of the material, if we consider for instance

E-mail address: [email protected]. https://doi.org/10.1016/j.jlumin.2019.01.003 Received 6 November 2018; Received in revised form 31 December 2018; Accepted 2 January 2019 Available online 04 January 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.

Journal of Luminescence 208 (2019) 394–401

P. Boutinaud

Table 1 Raw materials and experimental conditions used for the synthesis of several closed-shell transition metal oxides doped with Sb3+. For all, the antimony source was Sb2O3 (Alfa Aesar, > 99.6%). Host lattice

Raw chemicals (supplier, analytical grade)

Temperature (°C)/duration (h)

JCPDS

CaTiO3 CaZrO3 SrZrO3 YNbO4 SrNb2O6 M'-YTaO4

CaCO3 (Alfa Aesar, > 99%), TiO2 (Aldrich, > 99.5%) CaCO3 (Alfa Aesar, > 99%), ZrO2 (Aldrich, > 99%) SrCO3 (Aldrich, 99.9%), ZrO2 (Aldrich, > 99%) Y2O3 (Aldrich, 99.9%), Nb2O5 (Aldrich, 99.9%) SrCO3 (Aldrich, 99.9%), Nb2O5 (Aldrich, 99.9%) Y2O3 (Aldrich, 99.9%), Ta2O5 (Aldrich, 99.9%)

1100/12 1100/12 1200/10 1500/12 1200/12 1200/10

22-0153 72-7550 70-0283 72-2077 45-0227 9-0341

m

cationic substitutions like (A2+,Sb3+)(M4+,Sb3+) or (A+,Sb3+) (M5+,Sb3+). Two semi-empirical models will be used in this work to help in ascribing the origin of the luminescence signals: the environmental factor model, that has been used by Wang et al. and Amer et al. to estimate the energy of the A and C transitions of Bi3+ in solids [13,15] and the MMCT model that has been used to estimate the energy of MMCT transitions of Bi3+ in closed-shell transition metal oxides [12] and that will be adapted here to Sb3+. In the frame of the present work, we will limit ourselves to systems in which the transition metal adopts an octahedral coordination (i.e. vanadates, molydates or tungstates with zircon or scheelite crystal structure will not be treated here).

oxygen) in each given unit XmLn. It is calculated as QL = n Q X , where QX is the effective charge of the considered cation X. In the present work, this charge was taken as the bond valence sum of atom X in its polyhedron and was obtained from the crystal structure of the host lattice using VESTA software [18]. The necessary values of bond valence parameters were obtained from the work of O'Keeffe and Brese [19]. The calculation of the fractional covalency and volume polarization of a given A(i)-O(i) bond, where A(i) is a cation site and O(i) is an oxygen site, is described elsewhere [17] and will not be reproduced here for sake of brevity. 3.2. The MMCT model

2. Experimental The MMCT model, as previously introduced [12], provides information on the energy of the Bi3+ → Mn+ metal-to-metal charge transfer, which formally corresponds to the transition Bi3+ (s2) Mn+ (d°) → Bi4+ (s1) M(n-1)+ (d1), from the knowledge of some structural properties of the oxidic host lattice. This energy (in cm-1) is given by the following empirical equation [12]:

3+

Several closed-shell transition metal oxides doped with Sb were prepared in this work by standard solid-state reaction at high temperature. For all, the transition metal belongs to the second or third family and their coordination number CN' = 6. The raw chemicals and experimental conditions (temperature/duration) used for the synthesis are given in Table 1. The nominal doping rate was 1 mol%. This content was chosen not too high to limit the formation of Sb-related dimers and not too low to insure incorporation of Sb3+ within the host lattices in spite of the evaporation of Sb2O3 at high temperature. In this connection, care was taken to confine the thermal treatment at the bottom of a 50 cm-long sealed silica tube. All compounds were checked by X-ray diffraction to be single phased with good crystallinity. The corresponding JCPDS standards are listed in Table 1. The photoluminescence emission and excitation spectra were collected at 77 K using a CW spectrometer consisting of a combination of two Jobin-Yvon/Horiba TRIAX 180 and TRIAX 550 monochromators (Symphony system). The light source was a 450 W xenon lamp and the detection was a N2 cooled CCD camera for emission spectra and a Hamamatsu R928 PMT for the excitation spectra. The spectra were corrected for instruments responses using sodium salicylate.

n+ CN χCN (M ) ⎤ ′ MMCT (Bi3 + − M n +) = kCN ⎡χCN (Bi3 +) − αCN ⎢ ′ ( ) ⎥ d X corr ⎦ ⎣

3. Theoretical backgrounds 3.1. The environmental factor (EF) model The basic idea here is to correlate the absorption properties of a dopant in a solid with the chemical environment that is experienced by this dopant in its crystal site. The action of the environment at the considered crystal site (X) is formalized by the calculation of an environmental factor he(X), defined as: Nx

he (X ) =

∑ fc (X −L) α(X −L) QL2 1

(2)

where χCN(Bi ) and χCN'(M ) are the electronegativities of Bi and Mn+ in CN and CN'-fold coordination, respectively, as compiled by Li and Xue [20]. dcorr(X) is the shortest Bi3+–Mn+ interatomic distance corrected for the doping effect, defined as dcorr(X) = dhost(X) + 1/2 [r (Bi3+)−r(X)], where dhost(X) is the shortest distance separating the Mn+ site(s) and the cation site(s) X available for Bi3+ in the host lattice. r(Bi3+) and r(X) are the crystal radii of Bi3+ and of the host cation X CN that is substituted to Bi3+. kCN ’ and αCN ′ are crystal-structure related quantities defined and tabulated in [12]. Eq. (2) is a modified form of the equation introduced by C.K. Jorgensen in the early 70's to calculate the charge transfer (CT) energy between a ligand donor and a metal acceptor, from the knowledge of the optical electronegativities of the involved atoms [21]. This calculation relates to an inner-sphere charge transfer in which a direct bond is established between the donor and the acceptor. This differs from the MMCT process in which the orbital overlap between the involved atoms is expected small or inexistent. In this so-called outer-sphere CT scheme, the distance between the atoms at their closest possible approach (i.e. the parameter dcorr(X) in Eq. (2)) critically affects the energy of the charge transfer, the latter becoming larger upon increasing the donor-acceptor distance [22]. Further, in contrast with the original equation of Jorgensen which is valid only for CN hexa-coordinated metallic hexahalides, the quantities kCN' and αCN ′ in Eq. (2) account for any coordination number CN and CN' of the cations contained in the host crystal structure. 3+

(1)

n+

3+

4. Results and discussion

In this equation, fc(X-L) and α(X-L) represent respectively the fractional covalency and the volume polarization of each chemical bond separating cation X to its nearby ligands L in binary units to which the host lattice is initially decomposed. The procedure for lattice decomposition in binary units is described in details elsewhere [13,16,17] and not repeated here. QL is the effective charge carried by ligand L (here

This section is organized as follows: in Sections 4.1. and 4.2., the luminescence properties of Sb3+ in the transition metal oxides listed in Table 1 are presented. Owing to the relatively low efficiency of Sb3+ luminescence at room temperature, all spectra were collected at 77 K. At this temperature, self-luminescence of the corresponding host 395

Journal of Luminescence 208 (2019) 394–401

P. Boutinaud

that shows the difference between the normalized excitation spectra of CaTiO3–Sb3+ taken at 500 nm (*) and 570 nm (¤), respectively. Owing to this plot, we locate the lowest excitation signal at ≈ 355 nm (28,170 cm−1) in CaTiO3–Sb3+. In the case of the zirconates, the excitation features are located much below the fundamental excitation. They are ascribed to Sb3+. We note here that the position of the excitation band in CaZrO3–Sb3+ is consistent with that of CaZrO3–Bi3+ [32]. The resulting emission is, however, significantly red shifted, revealing a larger Stokes shift in the case of Sb3+. The situation is likely the same for SrZrO3–Sb3+ relative to SrZrO3–Bi3+. For the zirconates, the excitation spectrum does not vary with the monitored wavelength (* and ¤).

lattices is possible. This issue will be discussed based on the archival literature. In Section 4.3., the EF and MMCT models will be applied to the Sb3+ doped compounds, i.e. those prepared in this work and others selected from the literature, for which the MMCT has not yet been identified as a possible excitation/emission process. Dealing first with the EF model, the values of the environmental factors he(X) are calculated using Eq. (1) for each crystal site X that is considered as a possible doping site for the Sb3+ ions in the compounds. Our aim here is to assess whether an empirical relationship can be established between he(X) and the energy of the A transition of Sb3+, as done a few years ago by Wang et al. for Bi3+ [15]. For this, we have focused on lattices in which the luminescence properties have already been reported in the literature and interpreted in terms of ns2 ↔ ns1 p1 interconfigurational transitions (i.e. A-like transitions in practice). These lattices either do not contain transition metals or contain trivalent d° cations for which the electronegativity χCN'(M3+) with M = Sc, Y, Lu and La is rather small, i.e. the corresponding MMCT is likely at relatively high energy. This, however, does not mean that the luminescence is obviously of A character in these compounds [13]. To fix this issue, the MMCT model, after being adapted to Sb3+, will be applied to the compounds listed in Table 1 and to the literature compounds containing Sc3+, Y3+, Lu 3+ and La3+. Again, our motivation is to formalize a semi-empirical equation similar to Eq. (2) from which the excitation energy of Sb3+ could be predicted from the knowledge of χCN'(Mn+) and dcorr(X).

4.2. Luminescence of Sb3+ in niobates and tantalates Niobates and M'-type tantalate are examples of lattices in which the transition metal is pentavalent. In this case, the χ6(Nb5+) and χ6(Ta5+) values exceed that of χ6(Zr4+) and χ6(Ti4+) [20]. Like for titanates and zirconates, the luminescence of undoped YNbO4, SrNb2O6 and M'YTaO4 is of CT character. For YNbO4, the excitation band peaks at 265 nm for an emission at 420 nm [33,34]. We did not find information on the luminescence of undoped SrNb2O6, but found that CaNb2O6 shows an emission at 500 nm upon excitation at 275 nm [35]. We expect a similar behavior in SrNb2O6. For M'-YTaO4, the excitation peaks at 230 nm for an emission at 340 nm [34]. Doping with Bi3+ leads to a band-like emission in the blue (≈ 475 nm) in YNbO4 for a low-lying excitation reported at 310 [36], 314 [37] or 324 nm [13]. M'YTaO4–Bi3+ glows in the green region (515 nm) for an excitation peaking at 343 nm [13]. To our knowledge, the luminescence of SrNb2O6–Bi3+ has not been reported yet, but CaNb2O6–Bi3+ has been shown to emit at 490 nm for an excitation peaking at 325 nm [13]. Doping with Sb3+ yields broad emissions that appear to be red shifted with respect to Bi3+ (Fig. 2). In the niobates, these bands consist of at least two overlapping contributions, as evidenced by the corresponding excitation spectra (* and ¤). The features located in the red spectral region (¤) are excited at ≈ 335 nm. They are ascribed to Sb3+. The other feature in the green region (*) are excited at ≈ 280 nm and below are likely host-related. In YTaO4–Sb3+, the red band observed in Fig. 2 and its corresponding excitation are ascribed to Sb3+.

4.1. Luminescence of Sb3+ in titanates and zirconates with perovskite structure Titanate and zirconate perovskites like CaTiO3, CaZrO3 and SrZrO3 are examples of compounds in which the transition metal is tetravalent. At low temperature, well-crystallized undoped CaTiO3 is characterized by a blue (TiO6)8− emission at 470 nm in correspondence with a broad extra bandgap excitation expanding below 345 nm. The corresponding lowest excitation maximum is located at 330 nm [23,24]. Defects-related violet and green emissions at ≈ 405 and 520 nm are also observable upon sub-bandgap excitation (≥ 350 nm) [24]. Doping with Bi3+ generates a broad yellow emission at ≈ 570 nm in correspondence with a host excitation at 335 nm and a shoulder at ≈ 370 nm ascribed to Bi3+ → Ti4+ MMCT [25,26]. The corresponding Stokes shift is estimated at ≈ 9000 cm−1. Undoped CaZrO3 is known to show optical absorption below 245 nm with a regular emission in the blue at 430 nm [27]. The situation is similar in undoped SrZrO3 [28]. Other works report on a photoluminescence emission at 395 nm (consisting of 3 overlapping contributions) excited at 246 nm in highly crystallized SrZrO3 [29]. Doping with Pb2+ induces an emission in the UV (365 nm, excited at 285 nm in CaZrO3 or 360 nm, excited at 275 nm in SrZrO3 [30,31]). These features were ascribed to A transitions. Doping with Bi3+ leads to a UV emission in CaZrO3 (≈ 390 nm) for a corresponding excitation at ≈ 310 nm. These features also possess an A character [32]. As shown in Fig. 1, the emission spectra of Sb3+-doped CaTiO3, CaZrO3 and SrZrO3 collected upon excitation at 330 nm consist of a broad greenish signal covering the 450–650 nm region. The corresponding excitation spectra cover a wide wavelength range from ≈ 350–250 nm (instrument limit) with a maximum of the lowest-lying excitation band positioned at 330 nm for CaTiO3–Sb3+, 310 nm for CaZrO3–Sb3+ and 305 nm for SrZrO3–Sb3+. In the titanate, the excitation matches the host fundamental excitation and is ascribed accordingly. The corresponding green emission at ≈ 510 nm (solid line) is likely due to intrinsic defects [24]. The luminescence contains a weaker contribution in the yellow spectral range (≈ 570 nm). The intensity of this yellow band is reinforced upon excitation at 360 nm (dotted line). This new emission resembles the luminescence of Bi3+ in CaTiO3 [26] and is ascribed to Sb3+. It is produced essentially by sensitization through the host but also upon sub-bandgap excitation (arrow in Fig. 1). This is more clearly evidenced in the inset of Fig. 1

4.3. Application of the EF and MMCT models to oxidic compounds doped with Sb3+ We have compiled in Table 2 the energy Eexp(Sb3+) of the lowestlying photoluminescence excitation signals of Sb3+ in a selection of oxidic compounds whose crystal structure is available from ICSD database [38]. The environmental factors he(X) were correspondingly calculated. Note that we have discarded a few lattices owing to uncertainties regarding their crystal structure. This firstly concerns YBO3. The different refinements that were carried out on the vaterite structure of this compound led to space groups P63/m [39] and C2/c [40] with 8fold coordinated Y3+ sites. This differs from space group P63/mmc reported in [6] in which the Y3+ sites are 6 and 6 + 6 fold-coordinated. This unclear issue motivated us to eliminate YBO3 from our list of lattices. For the same reason, only the high temperature calcite polymorph of LuBO3 was maintained in our list. Secondly, the phosphates M (PO3)3–Sb3+, with M=Gd, Y, Lu, Sc are known to possess many polymorphs, which creates uncertainties regarding their exact crystal structure. For instance, it is stated that La(PO3)3 has the crystal structure of Nd(PO3)3 (space group C2221) while the others (M=Gd, Y, Lu, Sc) have the crystal structure of Yb(PO3)3 (space group P21/c) [2]. These attributions were based on the work of Hong dated from the midseventies [41,42]. More recently, however, a new space group (R-3 h) has been proposed for Yb(PO3)3 [43], which raises perplexities. Further, we found in ICSD database that Sc(PO3)3 is described in none of 396

Journal of Luminescence 208 (2019) 394–401

P. Boutinaud

Fig. 1. Excitation (left) and emission spectra (right) of the Sb3+-doped titanate and zirconate perovskites at 77 K. Symbols * and ¤ indicate the monitored wavelengths for the excitation spectra. Emission spectra in solid lines were collected upon excitation at 330 nm. The emission spectrum in dotted line was collected upon excitation at 360 nm (arrow). The inset shows the difference between the normalized excitation spectra of CaTiO3–Sb3+ collected at 570 nm (¤) and 500 nm (*), respectively.

the above space groups, Gd(PO3)3 is described in the space group of Nd (PO3)3 instead of that of Yb(PO3)3, Lu(PO3)3 is described in space group R-3 h and Y(PO3)3 exists with both space groups R-3 h and P21/c. In this puzzling context, we only retained the member La(PO3)3 within our list of literature lattices. The optical data related to the selected lattices were picked up from the literature or from Sections 4.1 and 4.2. In most cases, the archival literature ascribed the lowest lying excitation to A transitions. Noticeable exceptions are AlPO4–Sb3+ for which it has been concluded that Sb3+ is more probably located in interstitial sites than in the regular tetrahedrally-coordinated Al3+ sites [3], and LaOCl–Sb3+ that shows two emissions, a violet one ascribed to single Sb3+ ions (i.e. A transitions) and a green one ascribed to Sb3+ dimers [7]. The data corresponding to Sb3+ dimers were not included in Table 2. In the specific case of Y2O3–Sb3+, the two lowest excitation signals were ascribed to A transitions for sites C2 and S6 [44]. This situation resembles that of Y2O3–Bi3+ [36,45]. A third excitation band is also present at 38,460 cm−1 in Y2O3–Sb3+ in correspondence with a Stokes shift of 17000 cm−1. This signal was ascribed to Sb5+ [44] but this assignment will be reconsidered in the following. In the orthoborates ScBO3 and LuBO3 and in the phosphates LnPO4

(Ln = Sc, Y, Lu), the lowest-lying excitation features show a doublet (or triplet) structure which is ascribed to the crystal field splitting of the 3P1 level or to the action of a Jahn-Teller effect [4,6]. In the calcite-structured borates, the barycenter of 3P1 is located at ≈ 33,000 cm−1. Higher lying states at ≈ 40,000 cm−1 are also present [6]. In the zircon phosphates, the situation looks more puzzling. The work of Grafmeyer et al. [1], locates three excitation bands at 40,800, 41,700 and 43,300 cm−1 in YPO4–Sb3+ after spectral decomposition at low temperature. These three bands were ascribed to the 1P1 state based on the theoretical work carried by Soules et al. in fluorophosphates [46]. This attribution is however inconsistent with a crystal field splitting associated with a D2d site for the Sb3+ ions from which only two bands are expected. This prompted Grafmeyer et al. to postulate a descent of point symmetry at the Sb3+ site to account for their experimental data. Whatever, these assignments differ drastically from the attributions made a few years later [4]. In this paper, the 40,800 cm−1 excitation of YPO4–Sb3+ was ascribed to the 3P1 state. Correspondingly, two emission bands (UV at 295 nm and visible at 395 nm) were observed with associated Stokes shifts of 6900 cm−1 and 15,500 cm−1. Their intensity ratio was found independent of the Sb3+ content but varied strongly

Fig. 2. Excitation (left) and emission spectra (right) of the Sb3+-doped niobates and tantalate at 77 K. Symbols * and ¤ indicate the monitored wavelengths for the excitation spectra. Emission was collected upon excitation at 330 nm. 397

Journal of Luminescence 208 (2019) 394–401

P. Boutinaud

Table 2 Experimental excitation energy of Sb3+ in oxidic lattices, nature of the doping site X (with corresponding coordination number CN or CN') and corresponding values of the environmental factor he(X). The electronegativities of d° metals (M) are picked up from [20]. The values of dcorr(X) (see text) are given only for lattices in which an MMCT transition is possible. When necessary, the involved donor-acceptor sites is indicated as (X(Sb)-M). Only the shortest inter-atomic distances are compiled. Host lattice LaOCl Y2O3

Eexp (Sb3+) (cm-1)

LuPO4 LaPO4 AlPO4 LiLaP4O12

40,000 27,400 30,300 38,460 41,000 40,800 41,700 43,300 40,815 42,765 35,760 43,480

LaP3O9 Sr3(PO4)2

41,500 [2] 40,000 [48]

Ca10(PO4)6F2

38,365 [3]

ScBO3

CaZrO3

32,700 [6] 40,000 [6] 33,400 [6] 41,000 [6] 42,000 sh [6] 30,300a h 28,170a sh 32,260a

SrZrO3

32,800a

SrNb2O6

30,120a

YNbO4

30,300a

M’-YTaO4

30,300a

ScPO4 YPO4

LuBO3 LaBO3 CaTiO3

[7] [44] [44] [44] [4] [1,4] [1] [1] [4] [3] [3] [3]

he (X)

χCN' (Mn+)

dcorr (X) (Å)

(9) La Y3+ (6) - C2 3+ Y (6) - S6

3.01 1.61 1.61

1.26 1.34 1.34

3.55 3.46 3.45

Sc3+ (8) Y3+ (8)

0.97 1.14

1.35 1.29

3.53 3.63

Lu3+ (8) La3+ (9) Al3+ (4) Li+ (6) La3+ (8) La3+ (8) Sr(1)2+ (12) Sr(2)2+ (10) Ca(1)2+ (9) Ca(2)2+ (7) Sc3+ (6)

1.08 1.13 2.50 2.16 1.32 1.23 1.01 0.29 0.72 1.18 1.49

1.37 1.26 n.a. n.a. 1.28 1.28 n.a. n.a. n.a. n.a. 1.41

3.60 3.91 n.a. 3.57 (Li(Sb)-La) 5.40 (La(Sb)-La) 4.11 n.a. n.a. n.a. n.a. 3.74

Lu3+ (6)

1.66

1.43

3.87

La3+ (9) Ca2+ (8+4) Ti4+ (6) Ca2+ (8+4) Zr4+ (6) Sr2+ (8+4) Zr4+ (6) Sr2+ (8) Nb(1)5+ (6) Nb(2)5+ (6) Y3+ (8) Nb5+ (6)

0.53 1.31 1.75 1.19 1.78 1.46 1.53 1.25 2.18 3.09 1.04 1.29

Y3+ (8) Ta5+ (6)

1.06 1.28

1.26 n.a. 1.73 n.a. 1.61 n.a. 1.61 n.a. 1.86 1.86 1.29 1.29 1.86 1.86 1.29 1.29 1.92 1.92

3.78 3.00 3.91 3.08 4.02 3.19 4.12 3.27 3.26 3.73 3.63 3.41 3.60 3.54 3.55 3.36 3.55 3.45

X (CN or CN' if X = Mn+) 3+

(Ca(Sb)-Ti) (Ti(Sb)-Ti) (Ca(Sb)-Zr) (Zr(Sb)-Zr) (Sr(Sb)-Zr) (Zr(Sb)-Zr) (Sr(Sb)-Nb) (Nb1(Sb)-Nb1) (Nb2(Sb)-Nb1) (Y(Sb)-Y) (Nb(Sb)-Y) (Y(Sb)-Nb) (Nb(Sb)-Nb) (Y(Sb)-Y) (Ta(Sb)-Y) (Y(Sb)-Ta) (Ta(Sb)-Ta)

n.a.: not applicable h: host-related signal sh: shoulder. a This work.

crystal radius of Sb3+ is known for coordination numbers lower or equal to 6 only. For this reason, the value r(Sb3+) = 0.90 Å [14] was used each time we have CN ≥ 6 or CN' = 6 (i.e. in case of an insertion of Sb3+ in the Mn+ sites like Ti4+, Zr4+, Nb5+ or Ta5+). Plotting the lowest values of Eexp(Sb3+) against he(X) reveals a decreasing trend but with an important dispersion of the data preventing the determination of an empirical equation. Several factors may account for this large dispersion:

with temperature. Below 25 K, the emission is mainly UV; between 25 and 40 K, the visible emission increases at the expanse of the UV one; at T > 50 K, only the visible band is observed; then at T > 180 K, the UV band reappears while the visible emission starts to decrease. Similar observations were made in ScPO4–Sb3+ and LuPO4–Sb3+ [4]. This behavior was ascribed to a dynamical Jahn-Teller effect. We will suggest alternative interpretations of these data in the following. The case of LiLaP4O12–Sb3+, at last, raises other perplexities. In [47], two excitation bands are reported at 31,745 and 41,670 cm−1 for an emission at 28,570 cm−1, thus giving a very small Stokes shift of 3175 cm−1. A few years later, the 31,745 cm−1 band was no longer observed and the excitation maximum for the 28,570 cm−1 emission was located at ≈ 43,480 cm−1, thus raising a Stokes shift of ≈ 14,900 cm−1. Despite such a large Stokes shift, the luminescence was ascribed to A transition, invoking a Jahn-Teller effect at the Sb3+ site [3]. The calculation was performed for every cation site X that it susceptible to be occupied by Sb3+, including Mn+ sites, but excluding the very small P5+ and B3+ sites. We have also added in Table 2 some useful data for the calculation of Sb3+ → Mn+ MMCT energies, namely χCN' (Mn+) [20] and dcorr(X). Here, the values of dcorr(X) were obtained using dcorr (X)= dhost(X)+ 1/2 [r(Sb3+)−r(Mn+)], where X is the cation site occupied by Sb3+ and r(Sb3+) is the crystal radius of Sb3+ in the considered site. As mentioned in the Introduction Section, the

(1) there are uncertainties on the energy position of the excitation bands due to the fact that the experimental data were collected at various temperatures in the literature and were (or not) spectrally corrected for instruments responses. A good illustration of that is given by the three values of excitation positions in YNbO4–Sb3+ from 310 to 324 nm (Section 4.2.), corresponding to an uncertainty of roughly ± 700 cm-1. (2) for a few lattices, the nature of the doping site is not fixed. In Table 2, we have calculated he(X) for all cation site X that Sb3+ may occupy in the lattice, without preconceived idea on the “most probable or acceptable situation”. This concerns especially LiLaP4O12, Sr3(PO4)2, Ca10(PO4)6F2 and all compounds investigated in the present work for which more than one cation site is potentially (although not necessarily) available for Sb3+. 398

Journal of Luminescence 208 (2019) 394–401

P. Boutinaud

emission, an assignment of this signal to a Sb3+ → Y3+ CT is meaningful. The Stokes shift in LiLaP4O12–Sb3+ is also important (14,900 cm−1) but the large mismatch between experimental and calculated MMCT energies (3800 cm−1) suggests that the luminescence in this compound is in fact not of MMCT character. Here, the MMCT activity is hampered by the large Sb3+–La3+ distance in the lattice. In the other La-containing lattices in which the Stokes shifts are also large (19,500 cm−1 in LaP3O9–Sb3+, 14,000 cm−1 in LaPO4–Sb3+, 19,500 cm−1 in LaBO3–Sb3+, 15,900 cm−1 in LaOCl–Sb3+), the Sb3+ → La3+ CT process is a relevant option. We found that the excitation signals in the zircon-structured LnPO4–Sb3+ (Ln = Sc, Y, Lu) are also consistent with values calculated by mean of Eq. (3). Considering the multiplet character of the excitation in these compounds, it is likely that A and MMCT states coexist at similar energies. Dealing with YPO4–Sb3+, which is the best documented in the series, the presence of an MMCT state lying closely to the A states would in fact explain why three excitation bands are observed by Grafmeyer et al. although only two are expected in D2d point symmetry. In this compound, there is clear evidence that the UV emission is produced upon excitation at 40,800 cm−1 [1,4]. The corresponding Stokes shift is 6900 cm−1, which is consistent with an A-type transition. Between the two remaining excitations in Table 2, only the one at 41,700 cm−1 is reproduced by means of Eq. (3). This signal could have an MMCT origin. The corresponding emission is the visible one with a Stokes shift of 15,500 cm−1. As shown in [1,4], this emission is produced at the expanse of the UV emission as temperature is raised. Instead of invoking a Jahn-Teller effect of the A state, this phenomenon can be understood by a thermal assisted population of the MMCT state from the A state, the energy mismatch between them being less than the quantum corresponding to one lattice phonon. The situation is depicted schematically in Fig. 4 (path (1)). As temperature is raised further, the MMCT state starts to be thermally quenched by cross-over to the ground state 1S0 (path (2) in Fig. 4). Since the A state has a smaller Stokes shift, the quenching of this state starts at higher temperature. In consequence, the contribution of the UV (A-type) emission relative to the visible (MMCT type) emission grows up. The situation is likely similar in LuPO4–Sb3+ and ScPO4–Sb3+. At last, for all compounds investigated in the present work (Table 1), the experimental excitation energies are well reproduced by Eq. (3) for Sb3+ incorporated in the divalent or trivalent cation sites. The incorporation of Sb3+ in sites Ti4+, Zr4+ of the perovskites is not reproduced by Eq. (3) but the substitution of Sb3+ to Nb5+ or Ta5+ in SrNb2O6 (except site Nb2), YNbO4 and YTaO4 is. This, of course, does not allow concluding that such substitutions do occur effectively in the lattices. This specific point needs to be further checked. The Stokes shifts in all these compounds exceed 12,500 cm−1, except in CaTiO3–Sb3+ (10,600 cm-1). This is consistent with previous results showing that the specific composition CaTiO3–Bi3+ also has a moderate

(3) some of the excitation signals are erroneously ascribed to A transitions and therefore their energies do not depend (in first order approximation) on the value of he(X). This may concern all lattices in Table 2 that incorporate a d° metal cation Mn+ (= Ti4+, Zr4+, Nb5+, Ta5+, Sc3+, Y3+, Lu3+, La3+) which may act as an electron acceptor for a MMCT transition involving Sb3+ as an electron donor. To our knowledge, such a process has never been considered for Sb3+ in the literature. In some of these lattices (i.e. Y2O3, ScPO4, YPO4, LaP3O9, ScBO3, LaBO3, etc…), the only possible doping site is a transition metal site, so the MMCT process (if any) is of the type M (Sb)-M. Some other lattices incorporate more than one type of transition metal (e.g. Y3+ and Nb5+ in YNbO4) so that more than one MMCT process may occur. Taking YNbO4–Sb3+, we may have Sb3+ → Y3+ and Sb3+→ Nb5+ MMCTs, the energy of which depending on the electronegativity of the cations and the shortest distance between them. This, of course, depends on the considered doping site (either Y3+ or Nb5+) for Sb3+. Therefore, this raises the four following situations for YNbO4–Sb3+: Y(Sb)-Y, Nb(Sb)-Y, Y (Sb)-Nb and Nb(Sb)-Nb. The same holds for M'-YTaO4–Sb3+. The different cases are detailed in Table 2. If we assume that the MMCT model introduced for Bi3+ is general enough to be transposable to Sb3+, then we should observe a linear variation of the Sb3+ → Mn+ χ

(M n +)

′ , where dcorr(X) pertains now CT energy with the quantity CN dcorr (X ) 3+ to Sb . The corresponding semi-empirical equation should write:

n+ CN χCN (M ) ⎤ ′ MMCT (Sb3 + − M n +) = kCN ⎡χCN (Sb3 +) − αCN ′⎢ ′ ( ) ⎥ d X corr ⎦ ⎣

(3)

CN αCN ′

where kCN' and have the same meaning than in Eq. (2). Since Sb3+ is not known for coordination numbers larger than 6, the value of the electronegativity χCN (Sb3 +) will be taken for CN = 6, i.e. χ6 (Sb3 +) χ

(M n +)

= 1.476 [20]. We show in Fig. 3 the Eexp(Sb3+) - CN ′ plot that we dcorr (X ) obtain for the oxidic lattices listed in Table 2 containing d° cations. A linear trend is evidenced and obeys Eq. (3) with an accuracy CN of ± 1500 cm−1 for kCN ′= 40,665 and αCN ′= 1.32. A few data in Table 2 deviate however notably (by 7000 cm−1 or more) from this interval of uncertainty. This concerns especially the lowest-lying excitation signals of Y2O3, ScBO3 and LuBO3 that are more probably of A character, in agreement with the literature assignments. This is consistent with the rather low values of their corresponding Stokes shifts (7900 cm−1 for ScBO3–Sb3+, 10,700 cm−1 for LuBO3–Sb3+ and 4000 cm−1 (S6 site) or 11,000 cm−1 (C2 site) for Y2O3–Sb3+). In contrast, the higher-lying excitation signals in these compounds are well predicted by Eq. (3). This is especially true for the 38,460 cm−1 excitation of Y2O3–Sb3+ that was previously associated to Sb5+ [44]. With a Stokes shift as large as 17,000 cm−1 for the corresponding

Fig. 3. Eexp(Sb3+) -

χCN ′ (Mn +) dcorr (X )

plot for Sb3+-doped oxidic lattices containing d0 Fig. 4. Schematic configurational coordinate diagram for YPO4–Sb3+. See text for details.

transition metals. The solid line pertains to Eq. (4) with kCN' = 40,665 and CN αCN ′ = 1.32. The dotted lines mark the limits of the model. 399

Journal of Luminescence 208 (2019) 394–401

P. Boutinaud

M'-YTaO4 is governed by a Sb3+-to-transition metal (Mn+) charge transfer. This is the very first demonstration of the occurrence of a MMCT process in oxidic compounds doped with Sb3+. Acknowledgments This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] J. Grafmeyer, J.C. Bourcet, J. Janin, J.P. Denis, L. Loriers, Luminescence properties of Sb3+ in yttrium phosphates, J. Lumin. 11 (1976) 369–380. [2] E.W.J.L. Oomen, R.C.M. Peeters, W.M.A. Smit, G. Blasse, The Luminescence of the Sb3+ ion in Ln(PO3)3 (Ln=Sc,Lu,Y,Gd,La), J. Solid State Chem. 73 (1988) 151–159. [3] E.W.J.L. Omen, W.M.A. Smit, G. Blasse, Luminescence of the Sb3+ ion in calcium fluoroapatite and other phosphates, Mater. Chem. Phys. 19 (1988) 357–368. [4] E.W.J.L. Omen, W.M.A. Smit, G. Blasse, Jahn-Teller effect in the Sb3+ emission in zircon-structured phosphates, Chem. Phys. Lett. 112 (1984) 547–550. [5] G. Blasse, Luminescence of calcium halophosphate-Sb3+,Mn2+ at low temperatures, Chem. Phys. Lett. 104 (1983) 160–162. [6] E.W.J.L. Oomen, L.C.G. Van Gorkom, W.M.A. Smit, G. Blasse, Luminescence of Sb3+ in rare earth orthoborates LnBO3 (Ln=Sc,Y,La,Gd,Lu), J. Solid State Chem. 65 (1986) 156–167. [7] L.I. Van Steensel, G. Blasse, The luminescence of Sb3+ in LaOCl, J. Alloy. Compds 232 (1996) 60–62. [8] E.W.J.L. Oomen, G.J. Dirksen, W.M.A. Smit, G. Blasse, On the luminescence of the Sb3+ ion in Cs2NaMBr6 (M=Sc,Y,La), J. Phys. C: Sol. State Phys. 20 (1987) 1161–1171. [9] D.L. Dexter, C.C. Klick, G.A. Russell, Criterion for the occurrence of luminescence, Phys. Rev. 100 (1955) 603–605. [10] G. Blasse, Classical phosphors: a Pandora's box, J. Lumin. 72–74 (1997) 129–134. [11] G. Blasse, Optical electron transfer between metal ions and its consequences, Struct. Bond. (Berl.) 76 (1991) 153–187. [12] P. Boutinaud, Revisiting the spectroscopy of the Bi3+ ion in oxide compounds, Inorg. Chem. 52 (2013) 6028–6038. [13] M. Amer, P. Boutinaud, On the character of the optical transitions in closed-shell transition metal oxides doped with Bi3+, Phys. Chem. Chem. Phys. 19 (2017) 2591–2596. [14] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Cryst. A32 (1976) 751–767. [15] L. Wang, Q. Sun, Q. Liu, J. Shi, Investigation and application of quantitative relationship between sp energy levels of Bi3+ ion and host lattice, J. Sol. State Chem. 191 (2012) 142–146. [16] J.S. Shi, Z.J. Wu, S.H. Zhou, S.Y. Zhang, Dependence of crystal field splitting of 5d levels on hosts in the halide crystals, Chem. Phys. Lett. 380 (2003) 245–250. [17] J.S. Shi, S.Y. Zhang, Barycenter of energy of Lanthanide 4fN-15d configuration in inorganic crystals, J. Phys. Chem. B 108 (2004) 18845–18849. [18] K. Momma, F. Izumi, VESTA, A. Three-dimensional Visualization, System for electronic and structural analysis, J. Appl. Crystallogr. 41 (2008) 653–658. [19] M. O'Keeffe, N.E. Brese, Bond valence parameters for solids, Acta Crystallogr. B 47 (1990) 192–197. [20] K. Li, D. Xue, Estimation of electronegativity values of elements in different valence states, J. Phys. Chem. A 110 (2006) 11332–11337. [21] C.K. Jørgensen, Modern Aspects of Ligand Field Theory, North-Holland Publishing Co., 1971. [22] A. Vogler, H. Kunkely, Photochemistry of transition metal complexes induced by outer-sphere charge transfer excitation, in: J. Mattay (Ed.), Photoinduced Electron Transfer II. Topics in Current Chemistry, 158 Springer, Berlin, Heidelberg, 1990. [23] P. Boutinaud, E. Pinel, M. Dubois, A.P. Vink, R. Mahiou, UV-to-red relaxation pathways in CaTiO3:Pr3+, J. Lumin. 111 (2005) 69–80. [24] A. Alzahrani, A. Samokhvlalov, Conventional and cryo-synchronous luminescence spectra of orthorhombic calcium titanate, J. Lumin. 178 (2016) 430–436. [25] P. Boutinaud, E. Cavalli, Predicting metal-to-metal charge transfer in closed-shell transition metal oxides doped with Bi3+ or Pb2+, Chem. Phys. Lett. 503 (2011) 239–243. [26] P. Boutinaud, E. Cavalli, R. Mahiou, Photon conversion in Bi3+/Pr3+ co-doped CaTiO3, J. Phys.: Condens. Matter 24 (2012) 295502–295509. [27] S.K. Gupta, P.S. Ghosh, N. Pathak, R. Tewan, Nature of defects in blue light emitting CaZrO3: spectroscopic and theoretical study, RSC Adv. 5 (2015) 56526–56533. [28] V.M. Longo, L.S. Cavalcante, R. Erlo, V.R. Mastelaro, A.T. de Figueiredo, J.R. Sambrano, S. de Lazaro, A.Z. Freitas, L. Gomes, N.D. Vieira Jr., J.A. Varela, E. Longo, Strong violet–blue light photoluminescence emission at room temperature in SrZrO3: joint experimental and theoretical study, Acta Mater. 56 (2008) 2191–2202. [29] Z. Wang, J. Zhang, G. Zheng, X. Peng, H. Dai, violet-blue afterglow luminescence properties of non-doped SrZrO3 material, J. Lumin. 144 (2013) 30–33. [30] A.W.M. Braam, G. Blasse, Luminescence of Pb2+-ions in calcium zirconate (CaZrO3), Solid State Comm. 20 (1976) 717–719. [31] G. Blasse, A.W.M. Braam, M. Heerschop, Influence of crystal structure of ions with s2 configuration, J. Solid State Chem. 20 (1977) 63–65. [32] A.M. Srivastava, Luminescence of Bi3+ in the orthorhombic perovskites CaB4+O3

Fig. 5. Eexp(Sb3+)-he(X) plot for cation sites X in Sr3(PO4)2 (except Sr(2)), Ca10(PO4)6F2 (except Ca(1)), Y2O3, ScBO3, LuBO3, ScPO4, YPO4, LuPO4 (■), La3+ sites in LaPO4 and La(PO4)3 (○) and sites Ti4+ and Zr4+ in the perovskites (marked ☐). The solid line pertains to Eq. (4), see text. The dotted lines mark the limits of the model.

Stokes shift (≈ 9000 cm-1) compared to other Bi3+-doped transition metal oxides [13]. Noteworthy, this new description of the luminescence behavior of Sb3+ in closed-shell transition metal compounds based on the MMCT process opens the possibility to locate the 1S0 ground state of trivalent antimony relative to the host fundamental states, as done recently for Bi3+ [49]. We show in Fig. 5 the Eexp(Sb3+)-he(X) plot corresponding to the Sb3+-doped lattices that should normally be of A character if we follow the conclusions of the above analysis. This concerns Sr3(PO4)2, Ca10(PO4)6F2, LiLaP4O12, Y2O3, ScBO3 and LuBO3. The zircon phosphates ScPO4, YPO4 and LuPO4 are also added since we suspect the A and MMCT states to be located in the same spectral range. The corresponding data are marked (■) in the figure. A linear trend is observed but with a rather important dispersion ( ± 4000 cm−1). The solid line reproduces the following empirical equation:

EA (Sb3 +, cm−1) = 66700–22050 he (X )

(4)

In this equation, EA is the energy corresponding to the regular A (1S0–3P1) transition and 66,700 cm−1 is the energy of the 3P1 state in the free ion [50]. The quantity 22,050he(X) (in cm−1) is perceived as a host-induced energy depression acting on 3P1. Despite the rather large uncertainties that we have on the calculated EA values, we point out that many data in Table 2 are not reproduced by Eq. (4). This concerns especially the La3+ site of LaBO3 and LaOCl and the divalent, trivalent and pentavalent cation sites of the compounds listed in Table 1. This gives credit to the fact that the luminescence of Sb3+ in these compounds is likely of MMCT character. Sites Al3+ in AlPO4, Sr(2) in Sr3(PO4)2, Ca(1) in Ca10(PO4)6F2 and La3+ in LiLaP4O12 are also out of the interval of uncertainty. In the case of AlPO4 and Ca10(PO4)6F2, this confirms the conclusions of [3,5] from which Sb3+ is more probably located in interstitial sites in the former and in the Ca(2) sites in the latter. At last, sites La3+ in LaPO4 and La(PO4)3 (marked ○) and sites Ti4+ and Zr4+ in the perovskites (marked ☐) are within the ± 4000 cm−1 limit of the model. In the lanthanum phosphates, this suggests that A and MMCT states are lying close to each other, just like in the zircon-structured phosphates. In the perovskites, this suggests that the (hypothetical) occupation of the transition metal sites by Sb3+ would preferentially lead to A-type transitions instead of MMCT transitions. At last, the system LiLaP4O12–Sb3+ is described neither by Eq. (3) nor Eq. (4) and remains puzzling. The presence of Sb3+ pairs is an alternative that deserves future study in this compound. 5. Conclusion We have shown that the luminescence of Sb3+ in the closed-shell transition metal oxides CaTiO3, CaZrO3, SrZrO3, YNbO4, SrNb2O6 and 400

Journal of Luminescence 208 (2019) 394–401

P. Boutinaud

[33]

[34]

[35]

[36] [37] [38]

[39] [40]

[41]

(B4+ = Zr, Sn): crossover from localized to D-state emission, Opt. Mater. 58 (2016) 89–92. S.K. Lee, H. Chang, C.-H. Han, H.-J. Kim, H.G. Jang, H.D. Park, Electronic structures and luminescence properties of YNbO4 and YNbO4:Bi, J. Solid State Chem. 156 (2001) 267–273. W.J. Schipper, M.F. Oogendrop, G. Blasse, The luminescence and X-ray storage properties of Pr3+ and Ce3+ in YNbO4 and M′-YTaO4, J. Alloy. Compds 202 (1993) 283–287. P. Boutinaud, E. Cavalli, M. Bettinelli, Emission quenching induced by intervalence charge transfer in Pr3+- or Tb3+-doped YNbO4 and CaNb2O6, J. Phys. Condens. Matter 19 (2007) 386230–386240. G. Blasse, A. Bril, Investigations on Bi3+‐activated phosphors, J. Chem. Phys. 48 (1968) 217–222. S.H. Shin, D.Y. Jeon, K.S. Suh, Charge-transfer nature in luminescence of YNbO4:Bi blue phosphor, J. Appl. Phys. 90 (2001) 5986–5990. G. Bergerhoff, I.D. Brown, Crystallographic Databases Information Content, Software Systems, Scientific Applications, in: F.H. Allen, G. Bergerhoff, R. Sievers (Eds.), International Union of Crystallography, Bonn, Germany, 1987. G. Chadeyron, M. El Ghozzi, R. Mahiou, A. Arbus, J.C. Cousseins, Revised structure of the orthoborate YBO3, J. Solid State Chem. 128 (1997) 261–266. J. Lin, D. Sheptyakov, Y. Wang, P. Allenspach, Structures and phase transition of vaterite-type rare earth orthoborates: a neutron diffraction study, Chem. Mater. 16 (2004) 2418–2424. H.Y.P. Hong, Crystal structures of neodymium metaphosphate (NdP3O9) and

ultraphosphate (NdP5O14), Acta Crystallogr. B 30 (1974) 468–474. [42] H.Y.P. Hong, The crystal structure of ytterbium metaphosphate YbP3O9, Acta Crystallogr. B 30 (1974) 1857–1861. [43] N. Yu Anisimova, V.K. Trunov, N.B. Karmanovskaya, N.N. Chudinova, Synthesis and crystal structure of new modification of Yb(PO3)3, Izvestiya Akademii Nauk SSSR, Neorg. Mater. 28 (1992) 441–444. [44] A.M. van de Craats, G. Blasse, the influence of d1° ions on the luminescence of bismuth(III) in solids, Mater. Res. Bull. 31 (1996) 381–387. [45] G. Boulon, Processus de luminescence dans les oxydes et les orthovanadates de terres-rares polycristallins activés par l’ion Bi3+, J. Phys. (Paris) 32 (1971) 333–347. [46] T.F. Soules, T.S. Davies, E.R. Kreidler, Molecular orbital model for antimony luminescent centers in fluorophosphates, J. Chem. Phys. 55 (1971) 1056–1064. [47] G. Blasse, G.J. Dirksen, The luminescence of broad band emitters in LiLaP4O12, Phys. Stat. Sol. (b) 110 (1982) 487–494. [48] F. Wen, J. Chen, J. Moon, J.H. Kim, J. Niu, W. Li, Hydrothermal synthesis and luminescent properties of Sb3+-doped Sr3(PO4)2, J. Sol. State Chem. 177 (2004) 3114–3118. [49] R.H.P. Awater, P. Dorenbos, The Bi3+ 6s and 6p electron binding energies in relation to the chemical environment of inorganic compounds, J. Lumin. 184 (2017) 221–231. [50] C.E. Moore, Atomic Energy Levels, Nat. Bur. Ref. Dat. Ser., Nat. Bur. Stand. (US) 35/ v. (1971) III.

401