Phase Relations and Solid Solutions in the YbBO₃–ScBO₃ System

Abstract

The YbBO₃–ScBO₃ system was studied across the selected compositional range by means of solid-state synthesis, powder X-ray diffraction (XRD), differential scanning calorimetry (DSC), and photoluminescence spectroscopy. XRD of annealed samples at 1300–1400 °C revealed that the system reaches equilibrium and consists of two phases, YbBO₃ and ScBO₃, separated by a two-phase region. The lattice parameters show a limited solubility between Yb³⁺ and Sc³⁺ ions. DSC measurements display a broad endothermic feature at approximately 1480 °C, corresponding to the eutectic point. Near-infrared emission excited at 975–980 nm originates from Yb³⁺ ions and shows the highest intensity for pure YbBO₃ and for the Sc-rich composition, while intermediate samples exhibit weaker luminescence.

1. Introduction

The growing demands of photonics and energy applications require inorganic host materials that combine strong optical performance with chemical and thermal robustness [1,2,3]. Rare-earth-based materials meet these requirements by providing stable luminescence and reliable electronic behavior, enabling their use in lasers, phosphors, and sensors. Among NIR emitters, Yb³⁺ is particularly attractive: its simple two-multiplet 4f configuration produces efficient ~0.92–1.10 µm emission with low sensitivity to non-radiative processes and concentration quenching [4,5,6,7]. In suitable host compounds, Yb³⁺ exhibits broad emission bands and millisecond-scale lifetimes—features highly desirable for ~1 µm lasers and NIR phosphors. Emission near 1 µm is highly relevant since it falls within the first biological transparency window (700–1200 nm), enabling deep-tissue imaging and biomedical diagnostics. At the same time, this range remains compatible with silicon photodetectors (up to ~1100 nm) and is widely utilized in high-power Yb³⁺-based lasers for industrial and medical applications [8,9,10].

Orthoborates with the general formula RBO₃, where R denotes a trivalent rare-earth cation (Ln³⁺), are considered promising host lattices due to their high thermal and chemical stability [11,12]. Their rigid BO₃ framework and moderate phonon energies help suppress multiphonon relaxation and support efficient Ln³⁺ luminescence. These favorable characteristics are well documented for a variety of RBO₃ hosts, such as YBO₃: Eu³⁺ and (Y,Gd)BO₃: Eu³⁺, as well as Tb³⁺/Eu³⁺-activated LuBO₃ compositions, all of which demonstrate efficient rare-earth incorporation and strong luminescent response [13,14,15,16,17].

In addition to Eu³⁺ and Tb³⁺, many borate hosts have been co-doped with Yb³⁺ to exploit energy transfer mechanisms. For example, YBO₃ co-doped with Yb³⁺ and Er³⁺ exhibits red, green, and blue upconversion bands under 980 nm excitation, where Yb³⁺ acts as a sensitizer transferring energy to Er³⁺, enabling cooperative Yb³⁺–Yb³⁺ luminescence [18]. In Li₆Y(BO₃)₃: Yb³⁺, UV excitation into the Yb³⁺ charge-transfer band yields NIR emission (~972 nm), while IR excitation at high Yb³⁺ content generates blue–green emission via cooperative processes, potentially involving trace Er³⁺ [19]. Similar behavior is observed in YAl₃(BO₃)₄: Yb³⁺, Tm³⁺, where 980 nm excitation produces intense upconversion due to sequential photon absorption by Tm³⁺ sensitized by Yb³⁺ [20]. LaBO₃: Ce³⁺, Yb³⁺ enables UV-to-NIR downconversion via inter-ion charge transfer, achieving over 60% efficiency [21]. GdBO₃: Tb³⁺, Yb³⁺ is also known for efficient quantum cutting with up to ~182% quantum yield [22].

Within the RBO₃ family, ScBO₃ crystallizes in a trigonal calcite-type structure (R-3c) and serves as a robust end-member. It can be grown as single crystals and can form limited solid solutions with other rare-earth borates, depending on ionic size mismatch and structure compatibility [23,24,25]. In pseudobinary RBO₃–ScBO₃ series, three distinct behaviors can be identified depending on the ionic radius of R and the structural polymorphism of the RBO₃ end member. For larger lanthanides (Sm, Eu), the system stabilizes mixed borate compounds such as RSc(BO₃)₂ and RSc₃(BO₃)₄ [26,27]. For intermediate-sized ions such as Tb³⁺, broad mutual solubility and composition-dependent optical properties have been reported [28]. For the smallest rare-earth ions, the phase behavior depends strongly on polymorphism: Lu-Sc forms continuous calcite-type solid solutions [29,30], whereas Yb-Sc shows only limited Yb³⁺ substitution into ScBO₃ [23]. These contrasting results indicate that not only ionic radius but also structural polymorphism of the RBO₃ end-members plays a decisive role in phase behavior across the RBO₃–ScBO₃ series. A comparison with other RBO₃-ScBO₃ systems further highlights the unique character of YbBO₃–ScBO₃.

The YbBO₃–ScBO₃ system remains poorly studied. Yb³⁺ (VI, 0.868 Å) differs from the ScBO₃ end member based on both ionic radius (Sc³⁺ VI, 0.745 Å) and the polymorphism of YbBO₃, raising the question of whether mixing YbBO₃ with ScBO₃ yields continuous solid solutions, only limited mutual solubility, or new intermediate phases—as was shown for Eu/Sm systems [31]. This question is directly relevant for photonics, since the distribution of Yb³⁺ between phases, possible self-absorption, and defect states strongly affect NIR emission efficiency.

2. Results and Discussion

2.1. Crystal Structure

Powder X-ray diffraction (XRD) patterns were collected for the xYbBO₃–(1 − x)ScBO₃ series (x = 0.1–0.9) after annealing at 1100 °C, 1300 °C and 1400 °C (Figure 1). Reference data for the end members were taken from standard structures: P6₃/mmc for YbBO₃ (a = 3.735 Å, c = 8.747 Å) [32] and R-3c for ScBO₃ (a = 4.748 Å, c = 15.262 Å) [33]. After annealing at 1100 °C, the reaction of the starting oxides is incomplete and the reflections of the target borates are only weakly developed. In contrast, annealing at 1300–1400 °C yields sharp, well-resolved patterns containing only reflections of YbBO₃ and ScBO₃. These datasets were used to evaluate lattice parameters and composition-dependent peak shifts.

The evolution of a and c for the xYbBO₃–(1 − x)ScBO₃ series at 1300 °C and 1400 °C is shown in Figure 2. For ScBO₃, both parameters increase smoothly with YbBO₃ content up to x ≈ 0.8. At higher Yb fractions, the further increase of the ScBO₃-related parameters becomes very slight, while values corresponding to the YbBO₃ lattice appear, indicating the onset of a YbBO₃ + ScBO₃ two-phase region rather than a continuous solid solution. Incorporation of Yb³⁺ into ScBO₃ produces a small lattice expansion, whereas partial substitution of Yb³⁺ by the smaller Sc³⁺ (0.745 Å, CN = 6) in YbBO₃ leads to a minor contraction. Across a narrow interval of intermediate compositions, reflections of both structure types coexist, matching the two-phase field in the phase diagram. Peak positions shift slightly with composition, evidencing only limited mutual solubility between the end members; complete miscibility is not achieved even at 1400 °C.

The limited solubility between Yb³⁺ and Sc³⁺ can be attributed to their significant ionic radius mismatch: 0.868 Å for Yb³⁺ and 0.745 Å for Sc³⁺ [31]. This difference is close to the ~15% threshold outlined by the Goldschmidt rule [34], above which the formation of continuous solid solutions becomes increasingly unfavorable. Experimental observations support this interpretation: homogeneous single-phase samples with higher Yb³⁺ content were only obtained at elevated synthesis temperatures, indicating that increased thermal energy is required to overcome lattice strain arising from ionic size disparity. No phase separation occurred upon cooling, which enabled preservation of high-temperature single-phase states.

2.2. Thermal Properties

Differential scanning calorimetry (DSC) curves for the xYbBO₃–(1 − x)ScBO₃ series are shown in Figure 3. In general, all samples display a smooth endothermic background up to ~1400 °C with no sharp anomalies, indicating the absence of intermediate compound formation and confirming the phase stability observed by XRD. However, for pure YbBO₃ and the composition x = 0.8, an additional endothermic peak appears near 1000 °C, corresponding to a polymorphic transition between the low-temperature and high-temperature modifications of YbBO₃. According to [35], this transition occurs at approximately 1040 °C and represents a phase transformation. No such features are detected for Sc-rich samples, confirming that scandium borate remains structurally stable in this range. At higher temperatures, a broad endothermic effect extending to ~1480 °C is observed for all compositions. This temperature corresponds to the eutectic point on the YbBO₃–ScBO₃ phase diagram, marking the onset of partial melting. Thus, the DSC data confirm that, within the studied temperature range, the system exhibits a simple binary behavior with a eutectic reaction at ~1480 °C and only one polymorphic transformation characteristic of YbBO₃.

The phase diagram of the YbBO₃–ScBO₃ system, shown in Figure 4, was derived from XRD and DSC data. To construct the full high-temperature section of the phase diagram shown in Figure 4, in addition to XRD and DSC results, literature data were used on the melting behavior and thermal stability of the end members—YbBO₃ and ScBO₃. Specifically, the melting points of the mentioned compounds, as well as their thermal decomposition behavior, were taken from published sources [35,36]. XRD results revealed that at 1300–1400 °C the system reached equilibrium and contained only two stable phases, YbBO₃ and ScBO₃, separated by a narrow two-phase region (YbBO₃ + ScBO₃).

2.3. Luminescence

The luminescence spectra of YbBO₃–ScBO₃ solid solutions were studied at room temperature in the wavelength range of 900–1100 nm (Figure 5a). The main emission peak, corresponding to the ²F_5/2 → ²F_7/2 transition of Yb³⁺, lies around 975–980 nm and is marked in Figure 5a. Classical spectra of Yb³⁺ typically exhibit a series of narrow bands in the near-infrared region, arising from transitions between the spin-orbit components of the ²F_7/2 and ²F_5/2 multiplet [37]. The studied solid solutions exhibit characteristic features associated with the electronic transitions of Yb³⁺ ions. A detailed analysis of the spectra revealed several discrete peaks, indicating splitting of the Yb³⁺ energy levels under the influence of the crystal field of the surrounding lattice [38]. At low YbBO₃ concentrations (x = 0.1–0.6), when Yb³⁺ acts as a dopant in the ScBO₃ matrix, narrow peaks are observed, corresponding to transitions within the Yb³⁺ ion in the specific environment of ScBO₃. These peaks are clearly defined and indicate well-defined energy levels.

As the YbBO₃ content increases (x = 0.7–1), the luminescence spectra undergo significant changes, the peaks broaden, and the spectral structure becomes less distinct. This is possibly due to the formation of solid solutions, in which the environment of the Yb³⁺ ions becomes more inhomogeneous [39]. Furthermore, at high Yb³⁺ concentrations, concentration quenching of luminescence is observed, caused by energy exchange between Yb³⁺ ions, leading to non-radiative transitions and, consequently, a decrease in luminescence intensity [40].

Figure 5b shows the dependence of the integrated luminescence intensity on the solid solution composition. The maximum integrated intensity is observed at low YbBO₃ concentrations (x = 0.2), when Yb³⁺ ions effectively luminesce in the ScBO₃ matrix. Interestingly, at low YbBO₃ concentrations (x = 0.1–0.6), the position of the main peak remains virtually constant, while the intensity of the side peaks decreases. Although the ScBO₃ (calcite-type) structure contains one site for Sc³⁺ ions, the incorporation of Yb³⁺ ions can lead to local distortions of the crystal lattice and the formation of defects that affect the crystal field around Yb³⁺ [41]. A decrease in the intensity of the side peaks may indicate that, as the Yb³⁺ concentration increases, defects or distortions of a certain type accumulate. This, in turn, can lead to the formation of a spectrum increasingly resembling the spectrum of pure YbBO₃, where a certain type of Yb³⁺ environment dominates. In the gray zone (x = 0.7–0.9), the coexistence of ss–ScBO₃ and ss–YbBO₃ solid solutions is observed, which may also influence the luminescence pattern.

The position of the maximum luminescence intensity peak also exhibits a compositional dependence (Figure 5c). For ScBO₃-rich compositions, the peak position remains virtually unchanged, indicating that the ground state of the Yb³⁺ ions remains similar. However, for YbBO₃-rich compositions, a shift to shorter wavelengths is observed. This shift may be due to a change in the crystal field around the Yb³⁺ ions upon transition from the ScBO₃ to the YbBO₃ structure, which affects the energy levels of the Yb³⁺ ions and, consequently, the position of the luminescence peak. In the gray zone (x = 0.7–0.9), corresponding to the phase coexistence region, a transitional behavior of the peak position is observed.

3. Experimental Procedures

3.1. Synthesis

Polycrystalline samples of the (xYbBO₃–(1 − x)ScBO₃, x = 0–1) series were synthesized by a two-stage solid-state method in Pt crucibles. In the first stage, YbBO₃ and ScBO₃ were prepared from stoichiometric mixtures of Yb₂O₃ (99.9%), Sc₂O₃ (99.0%), and H₃BO₃ (99.8%), with a 2 mol% excess of H₃BO₃ added to compensate for its partial volatility during heating. The reagents were manually ground in an agate mortar using a hand pestle and then heated at 650 °C for 5 h to decompose H₃BO₃. In the second stage, the obtained YbBO₃ and ScBO₃ powders were reground in the same manner and annealed at 1100 °C, 1300 °C and 1400 °C for 12 h. After annealing, the samples were rapidly cooled to room temperature by removing the crucibles from the furnace and exposing them to ambient air.

3.2. Powder X-Ray Diffraction

Powder samples of the xYbBO₃–(1 − x)ScBO₃ series were examined in the Bragg-Brentano geometry using Cu Kα radiation at room temperature. Diffraction patterns were collected on a TD3700 diffractometer (Dandong Tongda Science & Technology Co., Ltd., Dandong, China) over the 2θ range of 10–60°. The unit cell parameters of the samples were refined by the Le Bail method using the The GSAS-II software package (version 5793) [42], with single-crystal data employed as starting models for the refinements. X-ray diffraction patterns were obtained for samples annealed at 1100 °C, 1300 °C and 1400 °C. At 1100 °C, the reflections of the target borate phases were only weakly developed, indicating incomplete reaction of the starting oxides. The data for samples annealed at 1300 °C and 1400 °C were used for further analysis of phase composition and lattice parameters. To assess the phase composition, a comparison with the reference profiles of individual phases was used.

3.3. Thermal Properties Analysis

The thermal properties of powdered samples of the xYbBO₃–(1 − x)ScBO₃ system with compositions x = 0, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 1.0 Yb were investigated by differential scanning calorimetry (DSC) using a STA 449 F5 Jupiter thermal analyzer (Netzsch Pumpen & Systeme GmbH, Waldkraiburg, Germany). Approximately 50 mg of powdered samples were placed in platinum crucibles and heated under flowing argon from room temperature to 1500 °C at a rate of 20 K·min⁻¹. An empty platinum crucible was used as a reference.

3.4. Luminescence Properties

The emission spectra of the compounds were recorded using an The InVia Raman microscope (Renishaw plc, Wotton-under-Edge, UK) under the excitation of a 785 nm CW diode laser in the range of 900–1100 nm. While the main absorption of Yb³⁺ (²F_7/2 → ²F_5/2 transition) typically peaks near 975–980 nm, this transition forms a broad absorption band spanning ~800–1080 nm in crystalline and glassy hosts, allowing for appreciable absorption even near 785 nm [43,44,45]. Additionally, in some co-doped systems (e.g., Ho/Tm/Yb-doped zirconates), absorption features at 785 nm have been directly observed [46].

4. Conclusions

The YbBO₃–ScBO₃ system, in the subsolidus temperature range of 1300–1400 °C, shows two single-phase regions of YbBO₃ and ScBO₃ separated by a narrow two-phase field (YbBO₃ + ScBO₃). X-ray diffraction confirmed that equilibrium is achieved at 1300–1400 °C without the formation of intermediate compounds. Thermal analysis shows a single eutectic reaction at ~1480 °C, marking the onset of partial melting. Photoluminescence measurements show near-infrared emission typical of Yb³⁺, with the strongest intensity observed for pure YbBO₃ and for the Sc-rich composition containing about 20 mol.% YbBO₃ (x ≈ 0.2), while intermediate samples exhibit noticeably weaker luminescence. These findings outline a consistent subsolidus section of the YbBO₃–ScBO₃ phase relations rather than a full equilibrium diagram and highlight its relevance as a model system for structure–property correlations in rare-earth borates. ScBO₃ emerges as a robust, transition-free host that accommodates appreciable Yb³⁺ and is promising for multi-element rare-earth doping and NIR-emitting applications.