Luminescence and Scintillation in the Niobium Doped Oxyfluoride Rb₄Ge₅O₉F₆:Nb

Abstract

A new niobium-doped inorganic scintillating oxyfluoride, Rb₄Ge₅O₉F₆:Nb, was synthesized in single crystal form by high-temperature flux growth. The host structure, Rb₄Ge₅O₉F₆, crystallizes in the orthorhombic space group Pbcn with lattice parameters a = 6.98430(10) Å, b = 11.7265(2) Å, and c = 19.2732(3) Å, consisting of germanium oxyfluoride layers made up of Ge₃O₉ units connected by GeO₃F₃ octahedra. In its pure form, Rb₄Ge₅O₉F₆ shows neither luminescence nor scintillation but when doped with niobium, Rb₄Ge₅O₉F₆:Nb exhibits bright blue luminescence and scintillation. The isostructural doped structure, Rb₄Ge₅O₉F₆:Nb, crystallizes in the orthorhombic space group Pbcn with lattice parameters a = 6.9960(3) Å, b = 11.7464(6) Å, and c = 19.3341(9) Å. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements suggest that the niobium is located in an octahedral coordination environment. Optical measurements inform us that the niobium dopant acts as the activator. The synthesis, structure, and optical properties are reported, including radioluminescence (RL) measurements under X-ray irradiation.

1. Introduction

A commonly observed process in inorganic materials is luminescence, where UV light-excited electrons return to the ground state via the emission of light. Less common is the additional ability of some of these materials to scintillate, where X-ray, γ-ray, or energetic particles are the source of excitation. These materials find use as X-ray phosphors and scintillators for positron emission tomography (PET) [1], computer tomography (CT) scanners [2], and more recently, by homeland security for improved nuclear detection systems.

Traditional approaches for the discovery of new luminescent (and potentially scintillating) inorganic materials include trial-and-error incorporation of chemical elements such as Ce³⁺ into the crystal structure. These elements can either fully occupy a crystallographic site, thus creating an intrinsic scintillator, or can change a small percentage of the host composition thus creating an extrinsic scintillator. In the latter case, often the location of the dopant in the crystal structure cannot be determined with certainty.

It is known that several d⁰ cations, such as Nb⁵⁺, Ta⁵⁺, or W⁶⁺, can exhibit intense luminescence and scintillation as self-activated luminescent oxides, with MgWO₄ being a well-known example [3]. Our research group has explored the synthesis of new oxide and oxyfluoride materials via several different synthetic routes, resulting in the discovery of new luminescent and scintillating materials. This has recently led to the discovery of a new self-activated luminescent silicate Rb₄Ta₂Si₈O₂₃ that exhibits bright blue scintillation when exposed to X-rays [4]. These efforts were continued, targeting oxyfluoride type materials that are less abundant in the literature likely due to the relative ease with which oxides containing inherently luminescent ions can be prepared using traditional solid-state syntheses or hydrothermal methods. In contrast, creating oxyfluoride environments for luminescent ions is much more challenging, as it requires a delicate balance between oxygen and fluorine content. For this reason, we pursued flux crystal growth using metal fluoride reagents in air to encourage the formation of oxyfluorides. Using such approaches, our group has explored the synthesis of new oxide and oxyfluoride materials via several different synthetic routes aimed at the discovery of new luminescent and scintillating materials [5,6,7,8,9,10,11]. This resulted in luminescing and scintillating flux-grown crystals of lanthanide-containing silicates Cs₃RESi₄O₁₀F (RE = Eu and Tb) [12], whose optical properties support the proposed idea that the incorporation of fluorine into the crystal structure can lead to increased fluorescence intensity. In addition, it led to a new hydrothermally synthesized tungsten-based oxyfluoride, BaWO₂F₄, exhibiting intense, intrinsic, X-ray scintillation, where the isolated WO₂F₄²⁻ species functions as the luminescence and scintillation center [13].

These discoveries prompted us to perform additional syntheses of new oxyhalide, especially oxyfluoride, materials, and to target the incorporation of inherently optically inactive cations, Nb⁵⁺, Ta⁵⁺, or W⁶⁺, which are, however, known to luminesce when located in a mixed oxide fluoride coordination environment [14,15,16]. Herein, we report the synthesis, crystal structure, and optical properties of a new host material, Rb₄Ge₅O₉F₆, which, when doped with the inherently optically inactive Nb⁵⁺ through high-temperature flux growth to form Rb₄Ge₅O₉F₆:Nb, luminesces in a bright blue color under UV-light excitation, and also scintillates under X-ray exposure. The photoluminescence quantum yield is reported, and the scintillation response was studied by radioluminescence (RL) measurements. The local coordination of the Nb dopant was investigated by extended X-ray absorption fine structure (EXAFS).

2. Experimental Section

2.1. Reagents

RbCl (BeanTown Chemical, Hudson, NH, USA, 99%), GeO₂ (Alfa Aesar, Tewksbury, MA, USA, 99.999%), and Nb₂O₅ (Alfa Aesar, 99.9%) were used as received. RbF (Strem, Newburyport, MA, USA, 99.8%) was also used as received, although found to be HRbF₂ following powder X-ray diffraction analysis.

2.2. Crystal Growth

Single crystals of Rb₄Ge₅O₉F₆ were grown by layering a mixture of 2 mmol GeO₂ beneath a mixture of 14 mmol of RbCl and 12.5 mmol of RbF. Reagents were added to a cylindrical silver crucible (1.2 cm D × 5.7 cm H) that was heated to 900 °C in air, held at that temperature for 12 h, slowly cooled to 450 °C at a rate of 6 °C/h, and finally cooled to room temperature by turning off the furnace. The solidified flux matrix was dissolved in water aided by sonication, and the products were isolated via vacuum filtration. Thin, colorless plate crystals were produced along with large amounts of AgCl powder, which was removed using a concentrated solution of sodium thiosulfate. Crystals of Rb₄Ge₅O₉F₆ grown in these reactions do not exhibit any luminescence properties under UV exposure.

Performing the same crystal growth procedure, but adding Nb₂O₅ as a dopant by layering a mixture of 2 mmol GeO₂ and 0.5 mmol Nb₂O₅ beneath a mixture of 14 mmol of RbCl and 12.5 mmol of RbF, resulted in single crystals of Rb₄Ge₅O₉F₆:Nb. Again, thin, colorless plate crystals (Figure 1) were produced along with AgCl, which was removed from the crystals using sodium thiosulfate. This time, however, the crystals exhibited significant luminescence under UV exposure.

2.3. Single-Crystal X-ray Diffraction

Single-crystal X-ray diffraction data were collected on crystals produced from reactions with and without Nb₂O₅, at 300(2) K on a Bruker (Madison, WI, USA) D8 QUEST diffractometer equipped with an Incoatec (Geesthacht, Germany) IμS 3.0 microfocus radiation source (MoKα, λ = 0.71073 Å) and a PHOTON II area detector. The crystals were mounted on a microloop using immersion oil. The raw data reduction and absorption corrections were performed using SAINT+ and SADABS programs [17,18]. Initial structure solutions were obtained with SHELXS-2017 using direct methods and Olex2 GUI [19]. Full-matrix least-square refinements against F² were performed with SHELXL software (Version 2018/3, Georg-August Universität Göttingen, Göttingen, Germany) [20]. The structures were checked for missing symmetry with the Addsym program implemented within PLATON software (Version 200322, Utrecht University, Utrecht, The Netherlands), and no higher symmetry was found [21].

Initial refinement of the structure of the undoped crystal with all anion sites modeled as oxygens resulted in an R1 value of 2.20%, with the highest residual electron density peaks (max peak = 1.23 e⁻/ų) being adjacent to three of the anion sites, and a formula of Rb₄Ge₅O₁₅, which has an excess charge of −6. The excess negative charge and the residual electron density peaks adjacent to the anion sites were both clear indicators that the material was an oxyfluoride. Freely refining the occupancy of all the oxygen sites resulted in five oxygen sites with occupancies close to unity (103–104%) and three oxygen sites, those with the high adjacent residual electron density peaks, with much higher occupancies (124–126%). Freely refining these latter three sites as fluorine sites resulted in occupancies near unity (102–103%). Fixing all anion site occupancies to unity then resulted in the charge balanced formula of Rb₄Ge₅O₉F₆, the highest residual electron density peaks (max peak = 0.728 e⁻/ų) being located among the Rb atoms between the slabs, and a final R1 value of 1.52%. The crystallographic data and results of the diffraction experiments for Rb₄Ge₅O₉F₆ and Rb₄Ge₅O₉F₆:Nb are summarized in

Table 1. Crystallographic data for Rb₄Ge₅O₉F₆ and Rb₄Ge₅O₉F₆:Nb.

Empirical Formula	Rb4Ge5O9F6	Rb4Ge5O9F6:Nb
Temperature (K)	302(2)	303(2)
Wavelength (Å)	0.71073	0.71073
Space group, Z	Pbcn, 4	Pbcn, 4
Unit cell dimensions (Å)	a = 6.98430(10) b = 11.7265(2) c = 19.2732(3)	a = 6.9960(3) b = 11.7464(6) c = 19.3341(9)
Volume (Å3)	1578.50(4)	1588.83(13)
Density (calculated) (g/cm3)	4.051	4.025
Absorption coefficient (mm−1)	21.768	21.626
F(000)	1736	1736
Crystal size (mm × mm × mm)	0.06 × 0.03 × 0.02	0.04 × 0.03 × 0.005
Theta range for data collection (°)	3.40 to 36.31	3.39 to 31.61
Index ranges	−9 ≤ h ≤ 9, −16 ≤ k ≤16, −27 ≤ l ≤ 24	−9 ≤ h ≤ 9, −16 ≤ k ≤ 16, −27 ≤ l ≤ 27
Reflections collected	36,504	106,148
Independent reflections	2297 [R(int) = 0.0410]	2317 [R(int) = 0.0890]
Data/restraints/parameters	2297/0/120	2317/0/119
Goodness-of-fit on F2	1.144	1.089
Final R indices [I > 2sigma(I)]	R1 = 0.0152, wR2 = 0.0335	R1 = 0.0245, wR2 = 0.0513
R indices (all data)	R1 = 0.0164, wR2 = 0.0338	R1 = 0.0335, wR2 = 0.0563
Largest diff. peak and hole	0.728 and −0.464 e−/Å3	1.464 and −0.873 e−/Å3

. No differences in the structural models for Rb₄Ge₅O₉F₆ and Rb₄Ge₅O₉F₆:Nb were observed, and no additional electron density was located that could indicate the location of the Nb in the Rb₄Ge₅O₉F₆:Nb structure. Select bond lengths for Rb₄Ge₅O₉F₆ and Rb₄Ge₅O₉F₆:Nb are listed in

Table 2. Selected interatomic distances (Å) for Rb₄Ge₅O₉F₆ and Rb₄Ge₅O₉F₆:Nb.

Interaction	Rb4Ge5O9F6	Rb4Ge5O9F6:Nb
Ge1-F1	1.8290(12)	1.828(2)
Ge1-F2	1.8252(12)	1.831(2)
Ge1-F3	1.8374(12)	1.841(2)
Ge1-O1	1.8237(14)	1.833(3)
Ge1-O2	1.8569(14)	1.863(3)
Ge1-O5	1.8527(14)	1.861(3)
Ge2-O1	1.7168(14)	1.714(3)
Ge2-O2	1.7607(10)	1.727(3)
Ge2-O3	1.7232(14)	1.7614(18)
Ge2-O4	1.7777(13)	1.779(3)
Ge3-O4	1.7727(13)	1.772(3)
Ge3-O4	1.7727(13)	1.772(3)
Ge3-O5	1.7269(14)	1.728(3)
Ge3-O5	1.7270(14)	1.728(3)

.

2.4. Powder X-ray Diffraction (PXRD)

Powder X-ray diffraction data were collected on a Bruker D2 Phaser powder X-ray diffractometer using Cu Kα radiation. The step scan covered the angular range 5–65° 2θ in steps of 0.04°. Experimental and calculated PXRD data for Rb₄Ge₅O₉F₆:Nb are provided in Figure S1.

2.5. Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)

ICP-OES was performed using a Perkin Elmer (Waltham, MA, USA) Avio 200 spectrometer on digested samples of Rb₄Ge₅O₉F₆ and Rb₄Ge₅O₉F₆:Nb. The digestion process was carried out with a PerkinElmer’s Titan MPS microwave sample preparation system. Crystalline samples of around 10.0 mg were loaded in Teflon^TM vials with 8 mL of aqua regia (ratio mixture of 8 mL HNO₃, 4 mL of HCl, and 3 mL of DI water). The Teflon^TM vials were placed in a microwave-programmable oven. The temperature increased to 175 °C by 5 °C/min and 30 bars. Then, the samples were held at this temperature for 10 min followed by cooling to 50 °C at 1 °C/min. The samples were then cooled down to room temperature without rate control for two hours before removing the vials. The dissolved samples in aqua regia solutions were diluted to achieve the appropriate concentrations using 5 wt% HNO₃ in water. The calibration curves were measured for Rb, Ge, and Nb in various concentrations and using appropriate optical mode (by radial or axial detectors). The molar ratio of the mixed salts was determined by ICP–OES for digested samples calibrated by the High–Purity Standards (HPS). The overall precision was ±1 mole%.

2.6. Optical Properties

The photoluminescence response was determined using an Edinburgh Instruments (Livingston, United Kingdom) FLS1000 photoluminescence spectrometer on ground single crystals of Rb₄Ge₅O₉F₆:Nb. The emission scan was collected at an excitation wavelength of 254 nm. The photoluminescence quantum yield was measured using an Edinburgh Instruments FS5 Spectrofluorometer with an SC-30 integrating sphere.

2.7. Scintillation

Radioluminescence (RL) measurements were carried out using a customer-designed configuration of the Freiberg Instruments (Frieberg, Germany) Lexsyg spectrofluorometer equipped with a Varian Medical Systems (Crawley, United Kingdom) VF-50J X-ray tube with a tungsten target. The X-ray source was coupled with a Crystal Photonics CXD-S10 photodiode for continuous radiation intensity monitoring. The light emitted by the sample was collected by an Andor Technology (Belfast, United Kingdom) SR-OPT-8024 optical fiber connected to an Andor Technology Shamrock 163 spectograph coupled to a cooled (−80 °C) Andor Technology DU920P-BU Newton CCD camera (spectral resolution of ~0.5 nm/pixel). Powdered sample was filled into ~8 mm diameter, 0.5 mm deep cups, thus allowing for relative RL intensity between different samples. Bismuth germanium oxide (BGO) powder (Alfa Aesar Puratronic, 99.9995% (metals basis)) was used as a reference. RL was measured under continuous X-ray irradiation (W lines together with bremsstrahlung radiation) 40 kV, 1 mA) with an integration time of 5 s. RL measurements as a function of temperature were executed under continuous heating with a 0.5 °C/s heating rate up to 450 °C and a 10 s integration time. Thus, temperature increased by 5 °C during the acquisition of each spectrum. Spectra were labeled by the starting acquisition temperature. Spectra were automatically corrected using the spectral response of the system determined by the manufacturer.

Scintillation images of Rb₄Ge₅O₉F₆:Nb single crystals were taken using a digital camera inside a Rigaku (Tokyo, Japan) Ultima IV diffractometer equipped with a Cu Kα source (λ = 1.54018 Å).

2.8. Extended X-ray Absorption Fine Structure (EXAFS)

Specimens for EXAFS analysis were prepared for a known crystalline sample, Cs₅Nb₃O₃F₁₄, and for Nb-doped Rb₄Ge₅O₉F₆, by dilution of the sample with BN to achieve approximately 1 absorption length attenuation in transmission geometry at the Nb K edge. Samples were measured at the MRCAT beamline (10-ID) at the Advanced Photon Source. Both transmission and fluorescence signals were collected in parallel for both specimens and the fluorescence data were corrected for self-absorption. Data analysis was performed using the Demeter suite [22].

3. Results and Discussion

3.1. Crystal Structure

The host structure, Rb₄Ge₅O₉F₆, crystallizes in the orthorhombic space group Pbcn with lattice parameters of a = 6.98430(10) Å, b = 11.7265(2) Å, and c = 19.2732(3) Å. The crystal structure is composed of germanium oxyfluoride layers consisting of isolated GeO₃F₃ octahedra that are connected by cyclic Ge₃O₉ units (Figure 2c); the cyclic Ge₃O₉ units were previously observed in the optically inactive (NH₄)₄[(GeO₂)₃(GeO_1.5F₃)₂]·0.67H₂O [23]. The GeO₃F₃ octahedra in Rb₄Ge₅O₉F₆, shown in Figure 2a, have three Ge-O bonds with distances of 1.821(2)–1.859(2) Å, and three, on average, slightly shorter Ge-F bonds with distances of 1.825(2)–1.8371(19) Å. Cyclic Ge₃O₉ units, shown in Figure 2b, consisting of corner sharing GeO₄ tetrahedra with Ge-O bond lengths of 1.717(2)–1.7791(19) Å, connect the GeO₃F₃ octahedra through each oxygen atom to form the Ge₅O₉F₆⁴⁻ germanium oxyfluoride layers. The fluorine atoms terminate the layers and are oriented towards the Rb atoms, which separate the layers. The layers repeat along the c-axis to form the overall structure, shown in Figure 3.

3.2. Optical Properties

Crystals of Rb₄Ge₅O₉F₆:Nb were found to exhibit room-temperature luminescence when exposed to UV light. The emission spectrum for Rb₄Ge₅O₉F₆:Nb, when excited at 254 nm, is shown in Figure 4. It can be seen that the material exhibits fluorescence with one broad emission peak in the 375–650 nm range. The broad peak is centered at ~478 nm, which lies within the blue region of the visible spectrum and is consistent with the blue emission observed in crystals exposed to 254 nm UV light, shown in the Figure 4 inset. The photoluminescence quantum yield was measured on a powder sample and was found to be 17.9%. Due to the lack of a luminescent center and the absence of luminescence in the pure Rb₄Ge₅O₉F₆ host crystals, but the presence of luminescence in Rb₄Ge₅O₉F₆:Nb, we conclude that Nb doping is responsible for the observed luminescence. To confirm and quantify the presence of Nb in Rb₄Ge₅O₉F₆, the elemental composition of the crystals was measured using ICP-OES. Several single crystals grown from reactions containing Nb₂O₅ were digested in aqua regia and analyzed for Rb, Ge, and Nb concentrations. For comparison and as a control, single crystals grown in the absence of the Nb₂O₅ precursor, which exhibit no room temperature luminescence, were also digested in aqua regia and the same analysis was performed. It was determined that the crystals grown in the presence of Nb₂O₅ contained an average concentration of 0.82(6)% atomic percent Nb (cationic), while the sample grown in the absence of Nb yielded a measured concentration of <0.01% atomic percent, which is within the experimental uncertainty of the technique. The experimental results clearly indicate the presence of Nb within the crystals and corroborate the importance of Nb for the luminescence activation of Rb₄Ge₅O₉F₆.

Crystals of Rb₄Ge₅O₉F₆:Nb were also found to scintillate upon exposure to X-rays (Cu Kα radiation) and the blue scintillation is shown in Figure 5. The scintillation response of powdered Rb₄Ge₅O₉F₆:Nb was investigated under X-ray irradiation through RL measurements. Figure 6 shows the RL spectrum for Rb₄Ge₅O₉F₆:Nb, with the most intense spectrum acquired just above room temperature. The similarities of this spectrum to the emission spectrum under 254 nm light are clear, with the maximum of the RL spectrum occurring at 480 nm. Integral emission was determined to be ~2% of BGO powder at room temperature.

The scintillation thermal quenching was investigated through the acquisition of RL spectra at high temperatures (Figure 6) [24,25,26,27]. As expected, the peak intensity was quenched with increasing temperature, and no emissions were observed above about 400 °C. Figure 7 highlights the behavior of peak intensity as a function of temperature. A short plateau up to about 350 K followed by a continuous decay was fitted using the Mott-Seitz model, where the RL intensity I is given by: I = I_o/(1 + Ce^−W/kT), with I_o being the intensity at room temperature, W the activation energy for non-radiative recombination, k Boltzman’s constant, T the temperature in K, and C a constant [28]. It is important to note that transport losses are included in the scintillation process and that such effects end up being incorporated in the proposed thermal quenching analysis. These results are shown in Figure 7 as a continuous red line and yielded a thermal quenching activation energy of 0.38 eV.

3.3. EXAFS

The low Nb concentration within crystals of Rb₄Ge₅O₉F₆, <1% as determined by ICP-OES, contributed to a small amount of extra electron density relative to the undoped structure. Consequently, the location of Nb was not determinable using single-crystal X-ray diffraction data. To investigate the Nb local coordination environment, EXAFS measurements were performed. The results of the Nb K-edge experiments suggest Nb to be located in a six-fold coordination environment with bond lengths of approximately 2.0 Å. The approach included fitting the EXAFS signal for the known crystalline material, Cs₅Nb₃O₃F₁₄, which had a large edge jump and clear EXAFS signal from the absorption edge near 19 keV out to 19.8 keV, providing EXAFS data out to approximately 15 Å⁻¹. The Nb ions in Cs₅Nb₃O₃F₁₄ formed pairs of corner-shared NbOF₅ octahedra and, although different from the coordination environment of Ge in Rb₄Ge₅O₉F₆, (GeO₃F₃), which is expected to be occupied by Nb, Cs₅Nb₃O₃F₁₄ provides a reliable standard for evaluating the data fitting approach. The results of measurements in fluorescence mode were fitted to the known structural model by refining only the amplitude reduction factor and the Debye-Waller factor for the first coordination sphere; see Figure 8. Figure 9 demonstrates that the single crystal model fits the EXAFS data well. The first shell bond lengths include one short Nb-O bond near 1.7 Å and 5 Nb-F bonds ranging from 1.9 to 2.2 Å.

The EXAFS spectrum for Rb₄Ge₅O₉F₆:Nb was of lower intensity, Figure 9, with a small absorption edge step, rendering the amplitudes unreliable for direct determination of the coordination number. However, the bond lengths can be easily extracted from these results using a single-path fit. The parent structure has three distinct Ge sites, two of which are tetrahedral with 1.74 and 1.75 Å average Ge-O bond lengths and one of which is octahedral with an average Ge-O bond length of 1.84 Å. Nb⁵⁺ replacing a Ge ion leads to three possible Nb configurations: all octahedral, all tetrahedral, or a mix of both. The model yields an average Nb first coordination shell at 2.03(4) Å, suggesting that the coordination is 6-fold. A related oxide structure with similar polyhedral linkages, Cs₂Nb₄O₁₁, has average Nb-O distances of 1.85 Å in tetrahedral coordination and 1.97 Å in octahedral coordination [29]. The EXAFS data suggest octahedral coordination for the niobium given that the bond lengths derived from the EXAFS data are close to those found for octahedral Ge-F and Ge-O bond lengths and, in fact, slightly longer, which is consistent with the fact that they represent Nb-F and Nb-O bonds.

4. Conclusions

In summary, a new inorganic scintillating material, Rb₄Ge₅O₉F₆:Nb, was discovered and prepared in single crystal form through niobium doping using a high-temperature RbCl/RbF flux. The crystal structure consists of layers containing GeO₃F₃ octahedra connected by cyclic Ge₃O₉ units, with the layers separated by Rb atoms. The crystals emit bright blue light under UV light exposure and, additionally, emit blue light under X-ray radiation. The observed fluorescence is caused by a small percentage of Nb incorporated into the structure that likely occupies an octahedral coordination environment, as suggested by EXAFS experiments.