1. Introduction

Due to the large number of possible valence states, strong interaction with the surrounding lattice and, in some cases, cluster formation, as dopant in inorganic matrices, Bismuth exhibits a broad variety of optoelectronic properties and potential applications [1–9]. Contrary especially to Bi³⁺ and Bi⁰, however, research on Bi²⁺-doped materials is still rare [10–14]. Because of the relatively high oxidation tendency, only a handful of matrix materials are know in which efficient stabilization of the Bi²⁺ ion is possible. On the other side, its electronic band structure [10] clearly suggests application in alternative red- or orange emitting phosphors that can be excited by blue or UV light emitting diodes (LEDs). Consequently, Bi²⁺-doped strontium borates have been considered recently for this application [13].

In the late 1990s, MBPO₅ (M = Ca²⁺, Sr²⁺ and Ba²⁺) was proposed by Srivastrava as an alternative host for Bi²⁺ [12]. Spectroscopic examination at low temperature revealed the known excitation bands of ²P_1/2→²S_1/2 (UV), ²P_1/2→²P_3/2 (2) (blue) and ²P_1/2→²P_3/2 (1) (orange) and emission in the red for BaBPO₅:Bi²⁺, whereas ²P_1/2→²S_1/2 could not be seen in CaBPO₅: Bi²⁺ and SrBPO₅: Bi²⁺ [12].

According to Srivastava’s report, heat treatment at reducing atmosphere was necessary to obtain red emission from Bi²⁺ centers in MBPO₅. On the other hand, more recent observations from Liang and Su suggest differently [15,16]: they have shown what was later described as an abnormal reduction process of Sm³⁺ and Eu³⁺ to Sm²⁺ and Eu²⁺, respectively, to occur under oxidizing atmosphere when either of the two was incorporated in an MBPO₅ matrix. Following this idea and considering the fact that at elevated temperature, Bi₂O₃ readily decomposes into BiO [2,9], the aim of the present study is to evaluate whether MBPO₅:Bi²⁺ can be synthesized in the same way.

Lattice constants of MBPO₅ were originally reported by Bauer ([17,18], Table 1). Kniep et al. [19] as well as Rulmont and Tarte [20], however, suggested that the structure of MBPO₅ is in fact close to trigonal stillwellite, (Ce,La,Ca)BSiO₅, with only half the c value as compared to Bauer’s first results. CaBPO₅ and SrBPO₅ [21] were consequently assigned to space group P3₁21 with a = 6.6799Å and c = 6.6121Å, and a = 6.8488Å and c = 6.8159Å (Table 1 [19], ) respectively. On the other hand, single-phase synthesis was not achieved in these early works. Space-group P3₁21 was confirmed by Shi et al. who reported lattice constants a = 7.1094Å and c = 6.9895Å for BaBPO₅ that was prepared via a standard solid state reaction (Table 1 [22], ). Contrary, in a more recent study, Pushcharovsky et al. assigned the structure of BaBPO₅ to P3₂2 [23]. Since knowledge on the crystal structure is of high importance for understanding and assigning spectroscopic properties and, particularly, electron-phonon interactions, these reported discrepancies deserve further consideration. In the present study, in situ high temperature X-ray diffraction (HT-XRD) was therefore employed to first determine optimum synthesis conditions for the MO-B₂O₃-P₂O₅ system, and to then synthesize single phase MBPO₅:Bi²⁺ in air. Rietveld refinement was used to confirm that all three compounds belong to the trigonal space group P3₁21. Excitation and emission properties were quantified at room temperature by photoluminescence spectroscopy, and the appearance of multi phonon side bands of ²P_1/2→²P_3/2 could be related to the crystal structure.

Table 1. Lattice parameters of M_0.995BPO₅: 0.005Bi (M = Ca²⁺, Sr²⁺ and Ba²⁺)

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2. Experimental

Doped and un-doped samples of MBPO₅ (M = Ca²⁺, Sr²⁺ and Ba²⁺) were prepared via solid state reaction. For that, analytical grade reagents CaCO₃, SrCO₃, BaCO₃, H₃BO₃, NH₄H₂PO₄ and Bi₂O₃ were used as raw materials. Individual batches of 20 g were weighed according to nominal compositions M_1-xBPO₅: xBi (x = 0, 0.005) and mixed thoroughly. Boric acid and ammonium-dihydrogen-phosphate were added in excess of 3% to compensate for volatilization losses. Batches were pre-calcined at 500°C for 5 h and ground for a second time. Subsequently, a small sample was taken from each batch for HT-XRD examination. For that, about 200 mg of the powder were suspended in isopropanol, and about two drops of this suspension were brought on a platinum substrate which acted as both sample holder and heating element. Finally, the major part of the batches of Ca_1-xBPO₅: xBi (x = 0, 0.005) and Ba_1-xBPO₅: xBi was heat-treated at 900°C for 10 h, Sr_1-xBPO₅: xBi at 1000°C for 10 h. In all heating steps, heating rates of 1.4 K/min were applied.

Crystal structures of the obtained samples were examined by room-temperature XRD. Dynamic and static optical emission and excitation spectra were recorded with a high-resolution photoluminescence spectrometer (Horiba Jobin Yvon Fluorolog-3), using a static Xe lamp (450 W) and a Xe flashlamp (75 W) as excitation sources. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet Impact 420 spectrometer on samples embedded in KBr pellets at room temperature.

3. Results and discussion

3.1 Crystal structure

HT-XRD patterns for pre-calcined Ba_0.995BPO₅: 0.005Bi are shown in Fig. 1 . At temperatures below 800 °C, the spectra are dominated by the two precursor phases β-Ba(PO₃)₂ (JCPDS Card No. 43-518) and Ba₄B₂O₇ (JCPDS Card No. 24-84). Starting at around 850 °C, diffraction peaks due to planes (200) (2θ = 28.88), (003) (2θ = 38.48), and (121) (2θ = 40.80) of BaBPO₅ begin to appear. These grow continuously with further increasing temperature up to > 900 °C, where they are joined by additional reflexes at 2θ = 29.30 (012) and 2θ = 38.82 (202). Approaching 1000°C, most BaBPO₅ reflexes start to disappear as a result of structural breakdown. At 1050°C, planes (012) and (003) can no longer be indexed. This suggests a preferred reaction temperature in the range of 900-950 °C. On the other hand, it explains why synthesis according to Srivastava’s procedure (at 1000°C [12], ) does not result in a single-phase material. Similar results are found for the synthesis CaBPO₅ (onset of growth at ~800 °C, optimum at 900-1100 °C, breakdown at 1150 °C). SrBPO₅ exhibits a significantly more narrow synthesis window, i.e. 950-1000°C and breakdown occurs already at 1050°C due to the competing phase Sr₂P₂O₇ (JCPDS Card No. 75-1490).

 

Fig. 1 HT-XRD patterns of Ba_0.995BPO₅: 0.005Bi mixture between 800 and 1200°C. Red circles denote the refelection peaks due to BaBPO₅ (JCPDS Card No. 19-96); blue circles, β-Ba(PO₃)₂ (JCPDS Card No. 43-518); and green circles, Ba₄B₂O₇ (JCPDS Card No. 24-84). Note that this data was taken in situ during heating. For kinetic reasons, BaBPO₅ is not observed in single phase state.

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Based on above results, Ca_0.995BPO₅: 0.005Bi, Sr_0.995BPO₅: 0.005Bi and Ba_0.995BPO₅: 0.005Bi were synthesized as single-phase materials as already described. Their room-temperature XRD patterns are shown in Fig. 2 . Rietveld refinement (FullProf [24,25] software package) was performed on the basis of space group P3₁21. Calculated diffraction patterns are compared to experimental data in Fig. 2, and corresponding lattice parameters are given in Tab.1. High consistency between calculated and experimental data clearly confirms that all three compounds are isostructural, confirming conclusions from Refs [19,22]. As a result of increasing d spacing and, hence, increasing lattice parameters, in the series M = Ca²⁺→Sr²⁺→Ba²⁺, all diffraction peaks gradually shift towards lower θ. This is a direct consequence of increasing cation radius, i.e. from 1.00 Å for Ca²⁺ to 1.18 Å for Sr²⁺ and 1.35 Å for Ba²⁺ (for sixfold coordination), and agrees well with calculated data (column 2 and 3 in Table 1).

 

Fig. 2 Rietveld refinement of XRD patterns of M_0.995BPO₅: 0.005Bi (M = Ca²⁺, Sr²⁺ and Ba²⁺). Data (-o-) and fit (––), the difference profile and expected reflection positions are displayed.

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FTIR spectroscopy reveals a corresponding picture. The network of MBPO₅ consists essentially of corner-sharing BO₄ tetrahedra in infinite loop-branched chains, whereby along the c-axis, free corners of BO₄ are connected by PO₄ tetrahedra to form a 3-dimensional network ([19], Fig. 3(b)). FTIR spectra of all three compounds are therefore dominated by analogous absorption peaks that can be assigned to those BO₄ and PO₄ groups (Fig. 3(a) ). Peaks at ~1200, ~1100, ~1050, and ~970 cm⁻¹ originate from asymmetric stretching ν ₃ modes of BO₄ and PO₄; ~900 cm⁻¹ is due to ν ₃ (BO₄) + ν ₁(PO₄); ~850/~800 and ~750cm⁻¹ are asymmetric and symmetric B-O-P stretching vibrations, respectively; ~650/~600 cm⁻¹ are due to symmetric vibration/bending modes of BO₄; ~560/520cm⁻¹ is due to the bending mode ν ₄ of PO₄; ~470 cm⁻¹ is the deformation mode ν ₂ of PO₄ and BO₄ [20,22,26,27].

 

Fig. 3 FTIR Spectra of M_0.995BPO₅: 0.005Bi (M = Ca²⁺, Sr²⁺ and Ba²⁺) (a) and sketch of the corresponding structure (b).

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3.2 Optoelectronic properties

Room-temperature excitation and emission spectra of MBPO₅:Bi²⁺ (prepared in air) are shown in Fig. 4 . Positions of excitation and emission peaks are given in Tab. 2 and, where applicable, compared to Srivastrava’s original data (prepared in N₂/H₂, spectra taken at 15 K). Consistent with the expectations from Srivastrava’s data, red emission was observed in all three compounds. This clearly evidences that Bi²⁺ can be stabilized in MBPO₅ even when synthesized in air. Noteworthy, UV excitation peaks (²P_1/2→²S_1/2) could be detected also in CaBPO₅:Bi²⁺ and SrBPO₅: Bi²⁺, at 231 and 234 nm, respectively, for the first time (Fig. 4 and Table 2 ).

 

Fig. 4 Excitation and emission spectra of M_0.995BPO₅: 0.005Bi (M = Ca²⁺ (1, λ_em = 660nm; 2, λ_ex = 416nm), Sr²⁺ (3, λ_em = 660nm; 4, λ_ex = 413nm)and Ba²⁺ (5, λ_em = 660nm; 6, λ_ex = 430nm;)) at room temperature. The spectral region of 320-650nm was amplified 10 times for clarity.

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Table 2. Spectroscopic data and fluorescence lifetime of Bi²⁺ in M_0.995BPO₅: 0.005Bi (M = Ca²⁺, Sr²⁺ and Ba²⁺) to/from ground state ²P_1/2.

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Formation and stabilization of Bi²⁺, even under oxidizing atmosphere, may be explained on basis of a charge compensation model [16,17,28]. When Bi³⁺ is doped into the lattice of MBPO₅, due to the good match of ionic radii, it will be incorporated on M²⁺-sites. Such a [Bi³⁺ _M2+]^· defect requires negative charge compensation, what can be achieved only via the creation of a M²⁺ vacancy [V_M2+]”, or an interstitial oxygen defect [O_i]”. At sufficiently high temperature, these negative defects will be thermally activated and, eventually, captured by [Bi³⁺ _M2+]^·, thus reducing it to [Bi²⁺ _M2+]^x In this way, the charge imbalance due to the aliovalent substitution of Bi³⁺ for M²⁺ will be removed. As discussed in Refs [16,17], the shielding environment of PO₄-BO₄ loops prevents future oxidation and, hence, stabilizes the [Bi²⁺ _M2+] ^x defect.

Detailed examination of the emission spectra reveals shoulder peaks towards lower energy, at 676nm, 688nm and 686nm for CaBPO₅:Bi²⁺, SrBPO₅:Bi²⁺ and BaBPO₅:Bi²⁺, 1080, 1066 and 1024cm⁻¹ apart from the dominating peak, respectively. This frequency shift corresponds to the asymmetric stretching modes of BO₄ and PO₄ anion groups (Fig. 3). The shoulder peaks should therefore be assigned to the asymmetric stretching vibration of the respective anion groups. Corresponding phonon side bands occur for the excitation transitions from ²P_1/2 in all three compounds (insets of Fig. 4). Noteworthy, single-phonon side bands were recently observed in the excitation and emission spectra of Bi²⁺-doped SrB₄O₇ and SrB₆O₁₀ [13].

While in the excitation spectra of CaBPO₅: Bi²⁺ and SrBPO₅: Bi²⁺, the phonon side bands are strongly overlapped by the main peaks of electronic excitation from ²P_1/2, a more complex situation can be identified for BaBPO₅:Bi²⁺. In the uncorrected excitation spectrum taken at 15K, Srivastava found a vibronic side band centered at 587nm, 959cm⁻¹ higher than the lowest energy band at 622nm, but he did not notice the side band of the transition at 432 nm band. However, the present data clearly reveals the side bands of ²P_1/2→²P_3/2 (1) and ²P_1/2→²P_3/2 (2) at 562 nm, 1639 cm⁻¹ above the main peak, and at 372 nm, 3626cm⁻¹ above the corresponding main peak, respectively.

Crystal refinement reveals that in BaBPO₅, only one Ba²⁺-site is present. As shown in the inset of Fig. 5 , it is surrounded by three types of oxygen, four O₁, four O₂ and two O₃. The average bond length is 2.8687 Å. Assuming that Bi²⁺ is indeed incorporated into the MBPO₅ lattice only on Ba²⁺-sites, there should be only a single type of Bi²⁺ emission centers present in doped material. This is confirmed by time-resolved fluorescence spectroscopy (Fig. 5 and Fig. 6 ): for BaBPO₅: Bi²⁺, the fluorescence decay curve follows a first order exponential decay equation (Fig. 5). Excitation at wavelengths 619, 562, 432 and 372 nm always leads to the emission at 641 nm. Emission lifetimes for at all these different excitation wavelengths fall within an error range of ± 1μs around an absolute value of 22 µs, what corresponds to the experimental error of the employed set-up. Similar behaviour is found for CaBPO₅: Bi²⁺ and SrBPO₅: Bi²⁺ (column 5 of Tab. 2).

 

Fig. 5 Fluorescence decay curve of Ba_0.995BPO₅: 0.005Bi prepared in air. Excitation and emission wavelengths are 267 nm and 641nm, respectively. The red curve it a fit of the experimental data to a first order exponential decay equation (lifetime of 22 μs, correlation coefficient 99.84%). Inset: coordination environment of the Ba²⁺ site (green: oxygen ions; red: Ba²⁺).

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Fig. 6 Time resolved spectra of Ba_0.995BPO₅: 0.005Bi. Excitation wavelength is 430nm. Delay times are listed in legend.

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Figure 6 shows emission spectra of Ba_0.995BPO₅: 0.005Bi²⁺ after excitation at 430 nm as taken after different delay times (room temperature). With increasing delay time, emission intensity monotonically decreases while the line shape remains unchanged. Adding to the data shown in Fig. 5, this observation represents further evidence that only a single emission center is present in Ba_0.995BPO₅: 0.005Bi²⁺. Excitation bands at 619 nm, 430 nm, 562 nm and 372 nm all belong to the same active center, and represent the transition of ²P_1/2→²P_3/2 (Table 2) and corresponding vibronic sidebands.

The vibronic transition probability is determined by two contributions: so-called M and Δ processes [29,30]. The M process describes a vibrationally induced forced eletric dipole transition: coupling with an infrared active vibrational mode leads to opposite parity wavefunctions. The Δ process represents the Frank-Condon principle. It arises from a change in the equilibrium position of the excited state against the ground state. Vibronic transitions that involve multiple phonons are expected only for the Δ process, especially at low temperature. For instance, in Na₅La(MoO₄)₄:Pr³⁺, for the transition of ³H₄→³P₂ (Pr³⁺), two vibronic sidebands exist 800 cm⁻¹ and 1620 cm⁻¹ above the zero phonon line (at 4.2 K). These can be ascribed to one- and two-phonon replicas due phonon-electron coupling of the molybdate stretching vibration at ~800cm⁻¹) [31].

The highest-energy phonon of BaBPO₅ is the v ₃-mode of the BO₄ group at ~1194 cm⁻¹ (Fig. 3). This is much lower than the energy distance of the vibronic sidebands from the respective main peak (1639cm⁻¹ and 3626cm⁻¹). The sidebands are therefore assigned to multi-phonon interactions with the ²P_1/2→²P_3/2 transition.

Vibronic sidebands are much less pronounced in CaBPO₅:Bi²⁺ and SrBPO₅:Bi²⁺, what is a clear sign for different electron-phonon coupling strength [13]. Qualitatively, this can be confirmed further by considering the Huang-Rhys factor S [31]. S represents a measure of electron-phonon coupling strength and can be estimated from (2S-1)·ħω = Δυ [32],where ħω is the maximum phonon energy and Δυ the Stokes shift. Values of ħω can be taken from FTIR data, 1194 cm⁻¹ for BaBPO₅, 1207 cm⁻¹ for SrBPO₅ and 1211 cm⁻¹ for CaBPO₅ (Fig. 3), Δυ -data are listed in column 4 of Tab. 2. Using these data, values of 0.73, 0.76, and 1.04 are derived for S of BaBPO₅, SrBPO₅ and CaBPO₅, respectively. This means that the strength of electron-phonon coupling indeed increases from BaBPO₅ to SrBPO₅ to CaBPO₅.

4. Conclusions

In summary, Bi²⁺-doped MBPO₅ (M = Ba²⁺, Sr²⁺, Ca²⁺) were synthesized in air via a solid-state reaction at 900-1000°C. The reduction process of Bi³⁺ to Bi²⁺ occurs for reasons of charge compensation, and Bi²⁺-ions are stabilized on M²⁺-sites as a result of the surrounding crystal lattice. All three compounds are isostructural. Their crystal structure belongs to space group P3₁21. Fluorescence decay behaviour indicates that in accordance with crystallographic data, only one type of Bi²⁺-emission center is present in MBPO₅. In all compounds, the transition of ²P_1/2→²S_1/2 was directly observed. For ²P_1/2→²P_3/2, vibronic sidebands were observed in both excitation and emission spectra. In the case of BaBPO₅:Bi²⁺, one-, two- and three- phonon processes could clearly be identified. Strong absorption in the blue spectral range suggests high interest for application in WLED devices, for example in combination with a blue LED chip and/or a yellow-emitting phosphor.