${\mathrm{YAlO}}_3$ (YAP) is recognized as a laser host crystal due to its excellent mechanical strength and anisotropy that inhibits birefringence effects. Additionally, YAP crystals as scintillators are increasingly used in the fields of high-energy physics, nuclear physics, and positron emission tomography (PET) medical imaging^[1–3]. Rare-earth element-doped YAP (RE:YAP) has emerged as a widely used crystal material. The research work on RE:YAP first appeared in 1973. Weber et al. reported the excitation and fluorescence spectra of ${\mathrm{Ce}}^{3+}$$5\mathrm{d}\to 4\mathrm{f}$ in Ce:YAP crystals and the decay time ($\tau =16\mathrm{ns}$) of 5d levels at room temperature^[4]. After that, Shim et al. grew the Yb:YAP fiber single crystals using a modified micro-pulling-down (μ-PD) method, and obtained that the decay time corresponding to charge-transfer (CT) luminescence of ${\mathrm{Yb}}^{3+}$ was 0.8 ns^[5]. In order to study the trapping and recombination process of free carriers in YAP crystals doped with various rare-earth ions, Vedda et al. carried out a series of studies on ${\mathrm{Eu}}^{3+}$-, ${\mathrm{Tm}}^{3+}$-, ${\mathrm{Yb}}^{3+}$-, ${\mathrm{Ce}}^{3+}$-, ${\mathrm{Pr}}^{3+}$-, and ${\mathrm{Tb}}^{3+}$-doped YAP crystals, assuming that the traps and recombination centers were part of the defect complexes^[6]. Under $\gamma$-ray excitation, Pr:YAP exhibits a light yield of 5600 ph/MeV and a decay time of 9 ns^[7]. Although the light yield of Pr:YAP is lower than that of Ce:YAP (17000 ph/MeV)^[8], its rapid decay performance has attracted much attention. Takayuki et al. found that the doping concentration of ${\mathrm{Pr}}^{3+}$ ions affected the light yield, and using ${}^{137}\mathrm{Cs}$ to excite a YAP sample doped with 0.05% (mole fraction) Pr could achieve a light yield of 20400 ph/MeV^[3]. Nikl et al. calculated that the intrinsic scintillation efficiency of Pr:YAP was 1.5 times that of the standard Ce:YAP sample, yet its light yield was significantly lower than that of Ce:YAP. This was because the lifetime of the 192 K peak-related trap in Pr:YAP was about 1 ms longer than that of the 115 K peak-related trap in Ce:YAP. This longer lifetime resulted in an enhanced delayed radiative recombination process during the transport phase of the scintillation mechanism, which in turn led to a decrease in the light yield of Pr:YAP^[9]. Enhancing the light yield could potentially be achieved by suppressing the relevant traps.

The further optimization of Pr:YAP remains an issue that needs to be addressed. In view of this, a thorough investigation into the point defects of Pr:YAP crystals holds significant importance for optimizing the performance and enhancing application efficiency. For the characterization of defects in crystals, photoluminescence (PL) decay and thermoluminescence (TL) can be employed. TL testing can demonstrate the luminescence dynamics of intrinsic defects and rare-earth ions in YAP crystals. Yaroslav et al. combined density functional theory (DFT) and TL experiment to investigate the trap depth of intrinsic point defects of ${\mathrm{RAlO}}_3$ (R=Y, La, Gd, Yb, Lu). It was also clarified that the TL peaks in Mn:YAP crystals were attributed to intrinsic defects (${\mathrm{V}}_{\mathrm{O}}$ in composite defect), rather than to the doped ions^[10]. Furthermore, the structural parameters of YAP are as follows: $a=5.329\mathrm{\AA }$, $b=7.370\mathrm{\AA }$, and $c=5.179\mathrm{\AA }$. YAP crystals possess a perovskite structure with the PNMA space group. Due to the anisotropy of crystal structure, there is a phenomenon that the laser threshold and gain coefficient vary with the crystal orientation in the field of laser crystals. For instance, $b$-axis orientation can obtain stronger laser output efficiency in Tm-doped YAP crystal^[11].

Research on the anisotropy of RE:YAP primarily focuses on identifying significant variations in laser properties across different crystallographic axes^[11–13]. Notably, in the field of scintillators, there is currently limited research on the defect anisotropy of Pr:YAP crystals. For bulk scintillators, the anisotropy of crystals may not be a primary concern. Nevertheless, for thin wafers, the influence of defect anisotropy becomes significant and must be taken into account. Based on relevant literature, we focus on the detailed calculations of TL data, considering defects corresponding to various trapping depths. Furthermore, from the perspective of lattice structure, we calculated the atomic offsets relative to different crystallographic planes in order to elucidate the fundamental reasons underlying the anisotropy arising from defects.

In this paper, we comparatively analyzed the point defects in the (100), (010), and (001) planes of Pr:YAP crystals. Raman spectroscopy was employed to determine the variations in fine structure. Inductively coupled plasma (ICP)-atomic emission spectroscopy (AES) testing was used to detect the content of Pr atoms in crystals. Using PL and X-ray excited luminescence (XEL) spectra, we analyzed the emission intensities of the ${\mathrm{F}}^{+}$ center and ${\mathrm{Pr}}^{3+}$ ions in different crystal planes. PL decay measurements were performed to distinguish the differences in the luminescence mechanisms of ${\mathrm{Pr}}^{3+}$ ions in the three planes. Furthermore, the use of TL measurements confirmed the difference in defect distributions of the (100), (010), and (001) planes.

A ${\mathrm{YAlO}}_3$ crystal doped with 1% (atom fraction) Pr was grown by the Czochralski (Cz) method. The stoichiometric mixture of ${\mathrm{Y}}_2{\mathrm{O}}_3$ (99.999%), ${\mathrm{Al}}_2{\mathrm{O}}_3$ (99.999%), and ${\mathrm{Pr}}_6{\mathrm{O}}_{11}$ (99.999%) powders were used as starting materials and weighed accurately under the formula ${\mathrm{Pr}}_x{\mathrm{Y}}_{1-x}{\mathrm{AlO}}_3$ ($x=0.01$). The molten raw materials were contained in an iridium crucible that was heated inductively at a frequency of 2.5 kHz. The Cz-grown $⟨111⟩$-oriented YAP was used as a seed. The melt temperature in the crucible exceeded the melting point of YAP (1875°C). The crystal was grown at a pulling rate of 0.6 mm/h under an Ar atmosphere, and the as-grown Pr:YAP crystal is shown in Fig. 1. Transparent samples with dimensions of $10\mathrm{mm}\times 8\mathrm{mm}\times 1\mathrm{mm}$ were cut from air-annealed (12 h at 1200°C) crystal. Crystals grown in argon developed oxygen vacancies. Annealing at 1200°C for 12 h in air removed stresses and color centers caused by oxygen deficiency. The crystal surfaces perpendicular to the $⟨100⟩$, $⟨010⟩$, and $⟨001⟩$ axes were polished for use in experiments.

To identify the generated phases, powder X-ray diffraction (XRD) analysis in the angular range from 10° to 90° was recorded by the X-ray diffractometer (Ultima IV Diffraction, Rigaku, Japan) with an X-ray source (40 kV, 40 mA). The concentration of ${\mathrm{Pr}}^{3+}$ ions was determined by ICP-AES. Raman spectra were measured by the RENISHAW inVia Raman microscope with a 488 nm laser source. The absorption spectra were recorded by a PerkinElmer Lambda 1050 UV/VIS/NIR Spectrometer (Massachusetts, USA) at room temperature. The PL spectra, XEL spectra, and PL decays were measured by an Edinburgh Instrument FLS1000 PL spectrometer. XEL spectra were recorded by an X-ray generator operated at 50 kV and 15 µA. TL tests were carried out at the Nankai University with a heating rate of 12 K/min. During the TL measurements, a current of 1000 µA and a voltage of 40 kV were applied, yielding a corresponding absorbed dose rate of X-rays at 283 mGy/s. With a test duration of 18 min, the absorbed dose of X-rays amounted to 305,640 mGy.

The XRD pattern of the annealed Pr:YAP crystal is shown in Fig. 2. Compared with the JCPDS No. 70-1677 card of the YAP crystal, Fig. 2 shows that the annealed crystal is an orthorhombic structure with space group $D_{2h}^{16}$, and no impurity phase is detected. This indicates that doping with a small amount of ${\mathrm{Pr}}^{3+}$ has no effect on the crystal structure of YAP. The unit cell parameters calculated from the XRD spectrum are $a=5.33205\mathrm{\AA }$, $b=7.36951\mathrm{\AA }$, and $c=5.18250\mathrm{\AA }$. The slight increase in unit cell volume compared to the standard card values is attributed to the substitution of ${\mathrm{Y}}^{3+}$ by ${\mathrm{Pr}}^{3+}$, where the ionic radius of ${\mathrm{Pr}}^{3+}$ (1.013 Å)^[14] is larger than that of ${\mathrm{Y}}^{3+}$ (0.95 Å)^[15].

In order to compare the concentration of Pr atoms in the (100), (010), and (001) crystal planes, the ICP-AES method is employed to determine the Pr atoms in the crystal. The results are shown in the Supplement 1. It is found that the Pr concentrations in (100), (010), and (001) planes are 0.45% (atom fraction), 0.54% (atom fraction), and 0.54% (atom fraction), respectively.

Around the first Brillouin center of YAP with perovskite structure, the symmetry group theoretical analysis allows 60 optical modes^[16], $$\mathrm{\Gamma }=7A_{1g}+7B_{1g}+5B_{2g}+5B_{3g}+8A_{1u}+8B_{1u}+10B_{2u}+10B_{3u}.$$

Twenty-four Raman-active modes are $7A_{1g}$ modes, $7B_{1g}$ modes, $5B_{2g}$ modes, and $5B_{3g}$ modes, although some modes have not been measured^[16]. As shown in Fig. 3, the Raman spectra in the range of $100–600{\mathrm{cm}}^{-1}$ are obtained from the laser perpendicular to the incident (100), (010), and (001) planes. Figure 3 shows seven $A_{1g}$ modes, three $B_{1g}$ modes, four $B_{2g}$ modes, and two $B_{3g}$ modes of the three planes, respectively. The Raman peaks and vibrational modes are listed in the Supplement 1.

In undoped YAP, the $x$-stretching mode of ${\mathrm{O}}_1$ atom is assigned to the Raman peak of $A_{1g}$ mode ($350{\mathrm{cm}}^{-1}$), and the in-phase stretching mode of the oxygen cage is related to $A_{1g}$ mode ($550{\mathrm{cm}}^{-1}$), while $A_{1g}$ mode ($150{\mathrm{cm}}^{-1}$) contributes to the $x$-axis rotation of all atoms^[17]. There are three Raman peaks at around $150$, $350$, and $550{\mathrm{cm}}^{-1}$ in (100), (010), and (001) planes. We note that the peaks at $\sim 150$ and $\sim 550{\mathrm{cm}}^{-1}$ are the weakest in the (001) plane, which shows that both the rotation along the $x$ direction and the in-phase stretching are affected. The strongest peak near $350{\mathrm{cm}}^{-1}$ is located in the (001) plane, indicating that the $x$-stretching of ${\mathrm{O}}_1$ atom is enhanced.

In the Pr:YAP crystal lattice, the ${\mathrm{Pr}}^{3+}$ ions substitute the position of ${\mathrm{Y}}^{3+}$ ions and then form the ${[{\mathrm{PrO}}_8]}^{13-}$ ligand with O atoms^[18]. Comparing our data with that reported by Suda^[17], the lattice distortion caused by doping with rare-earth elements leads to a slight shift of Raman peaks. The differences in the intensities of Raman spectral peaks across the three planes reveal significant anisotropy in the vibrational, rotational, and symmetric properties of the molecules.

The absorption spectra of (100), (010), and (001) planes are shown in Fig. 4. There exists an absorption edge at 230 nm attributed to ${\mathrm{Pr}}^{3+}$$4\mathrm{f}\to 5{\mathrm{d}}_1$, and ${{}^3\mathrm{H}}_4\to {{}^3\mathrm{P}}_{0,1,2}$ transitions are observed within the range of 450–500 nm. Additionally, the weak absorption peak at 600 nm is related to ${{}^3\mathrm{H}}_4\to {{}^1\mathrm{D}}_2$^[1]. Butaeva et al. proved that the 250 and 270 nm absorption peaks in Pr:YAP were caused by the electron (${\mathrm{Fe}}^{3+}$) traps^[19], with the impurity ${\mathrm{Fe}}^{3+}$ ions potentially originating from the raw materials or furnace^[20]. Chen et al. attributed the intense absorption of Fe:YAP crystals in the 200–300 nm range to the absorption by Fe ions^[21]. In pure YAP, absorption peaks associated with oxygen vacancies have been observed at 240 and 290 nm^[22,23]. Consequently, the absorption band within the 250–300 nm range is related to both ${\mathrm{Fe}}^{3+}$ and oxygen vacancies. The absorption band within 300–400 nm is related to ${\mathrm{Pr}}^{4+}$ or ${\mathrm{O}}^{-}$ center. For Pr, Si:YAP crystal before and after annealing, Zhuravleva et al. supposed that the absorption band at 400 nm was due to the hole (${\mathrm{Pr}}^{4+}$ or ${\mathrm{O}}^{-}$) traps rather than ${\mathrm{F}}^{+}$ centers^[24]. As can be seen from Fig. 4, the absorption spectrum basically does not exhibit anisotropy, but there are slight differences in the absorption bands related to ${\mathrm{Fe}}^{3+}$ ions on different crystal planes. In the literature, the bandgap value of YAP obtained by measuring absorption spectra is about 7 eV^[10]. To obtain the band gap of Pr:YAP, calculations are performed based on the absorption spectra presented in Fig. 4. The calculation method referenced in Ref. [10] is used to establish a correlation between ${(\alpha h\nu )}^{1/n}$ and hν. Subsequently, the linear segment of this correlation is extrapolated until it intersects the abscissa, and the intersection point is the bandgap value. As indicated in Ref. [18], Pr:YAP exhibits direct transitions (with $n=1/2$), leading to a calculated bandgap energy ($E_g$) of $5.11\mathrm{eV}\pm 0.05\mathrm{eV}$.

Figure 5 shows the PL spectra obtained from the (100), (010), and (001) planes of Pr:YAP crystal. During the process of PL testing, no saturation phenomenon was observed in the detector. The emission peak around 500 nm shown in the inset is related to the f-f transition of ${\mathrm{Pr}}^{3+}$ ions. The transition mechanism of the ${\mathrm{F}}^{+}$ center is a complex process. Due to the overlap of excitation bands from different types of oxygen vacancies, it is difficult to determine the true excitation peak of the ${\mathrm{F}}^{+}$ center in YAP. Zorenko et al. reported that the excitation peak and emission peak of the ${\mathrm{F}}^{+}$ center in YAP were located at 4.3 and 3.49 eV, respectively^[25]. The emission peak obtained using 288 nm excitation in Fig. 5 is located at 355 nm, which is basically consistent with the reported data of luminescence of ${\mathrm{F}}^{+}$ centers (one electron in an oxygen vacancy). In the PL spectra, the strongest emission peak is observed on the (001) plane, while the weakest emission peak is observed on the (100) plane. This indicates that the concentration of the corresponding ${\mathrm{F}}^{+}$ centers, among the three crystal planes, decreases from high to low in the order of (001), (010), and (100).

Figure 6 shows the XEL spectra of the (100), (010), and (001) planes at room temperature. The two main emission peaks at 246 and 280 nm are caused by the transition from the $5{\mathrm{d}}_1$ level to ${{}^3\mathrm{H}}_{4,5}$, ${{}^3\mathrm{H}}_6$, and ${{}^3\mathrm{F}}_x$ of ${\mathrm{Pr}}^{3+}$^[1]. The 4f-4f transition is observed at around 490 nm ($3{\mathrm{P}}_x$, $x=0,1,2\to {{}^3\mathrm{H}}_4$)^[3]. The emission band at 350 nm is associated with the ${\mathrm{F}}^{+}$ center. By integrating the intensity of the emission band within the 320–400 nm range, it is found that the (001) crystal plane exhibits the strongest emission intensity. This result is consistent with the intensity order of the ${\mathrm{F}}^{+}$ center emission observed in the PL spectra. However, in the XEL spectra, the slight difference in peak intensity of the ${\mathrm{F}}^{+}$ center is not sufficient to explain the anisotropy of ${5\mathrm{d}}_1\to 4\mathrm{f}$ luminescence. Therefore, it is inferred that other point defects in the crystal may affect ${\mathrm{Pr}}^{3+}$ luminescence. We also obtain the emission spectra through excitation using a 213 nm laser, as shown in Fig. S1 in the Supplement 1.

PL decay tests are conducted on the (100), (010), and (001) plane samples, revealing decay time associated with the ${\mathrm{Pr}}^{3+}$${5\mathrm{d}}_1\to 4\mathrm{f}$ transition to be approximately 7 ns in all three planes (see details in the Supplement 1). The almost equal decay time indicates that there is no anisotropy in the luminescence mechanism, which is consistent with Ding’s research result^[26].

Based on the definition of areal ion densities of LYSO crystals^[26], it is found that the anisotropy of the crystal structure has a significant impact on the areal ion densities. The values of YAP structural parameters are $a=5.329Å$, $b=7.370Å$, and $c=5.179Å$, and the number of atoms in the YAP unit cell is 20. The ratio of relative areal ion density of the (100), (010), and (001) planes is about 52.4%:72.5%:50.9%. The (001) plane has the lowest ion density, resulting in the lowest chemical bond density. The largest interionic separation of this plane leads to weak bonding forces between ions, making the chemical bonds prone to breaking and introducing more defects. Conversely, the (010) plane exhibits the highest ion density, thereby contributing to the stablest chemical bonds that are difficult to break. Lu et al. calculated that the surface energies of YAP (100), (010), and (001) surfaces were 1.91, 3.32, and $1.96\mathrm{J}/{\mathrm{m}}^2$, respectively^[27], with the formation energies of the (100) and (001) planes being basically the same and the formation energy of the (010) plane being the highest. Therefore, the (010) plane has the highest areal ion density, which coincides with the phenomenon that this plane has the highest surface energy and requires the greatest energy for defect formation.

The anisotropy of areal ion density can lead to variations in the introduction of defects across different crystal planes. The anisotropy effect is restricted by both bulk effect and surface effect, and the defect-related TL glow peak is largely influenced by surface effect. Ding et al. conducted TL tests on Ce:LYSO, and calculated the integrated intensity of the TL glow peaks in the (200), (020), and (002) crystal planes to be 56:100:95, indicating that there were obvious differences in the concentration of defects contained in the three planes^[26]. The anisotropy in the distribution of defects leads to differences in the TL glow peaks. Therefore, TL tests are performed on the (100), (010), and (001) planes of Pr:YAP at temperatures ranging from 78 to 500 K, as illustrated in Fig. 7. Since the XEL test was conducted at room temperature, only the TL peaks near and above room temperature are taken into consideration. The ratio of integrated intensity of the TL curves for the (100), (010), and (001) planes is 45.6:34.5:46.0. This suggests that the defect concentration of the (010) plane is the weakest, while the defect concentrations of the (001) and (100) planes are similar, with the (001) plane having the highest number of defects. This phenomenon aligns with the ranking of areal ion density strength among the three crystal planes. The lowest areal ion density in the (001) plane is prone to introducing more defects, leading to the maximum TL integral intensity. Conversely, the highest areal ion density in the (010) plane makes it difficult for chemical bonds to break, resulting in the minimum TL integral intensity.

The trap depth corresponding to the TL peak is calculated by the initial rise method, and the curves of $\ln I$ and $1/T$ are drawn according to the formula $I(T)=C\exp (-E_T/\mathrm{KT})$. The initial rise segment is selected as the range where the TL intensity is 5% to 10 % of the maximum intensity of the TL peak. This initial rise segment approximately forms a straight line. By fitting this linear segment and obtaining the slope ($E/K$), the trap depth can be obtained^[28]. The selection of the initial rise method^[29] for calculation stems from its clever exploitation of the functional relationship during the low-temperature ascending segment of the TL peak. This feature allows for the calculation of trap depths independently of the frequency factor and the kinetic order, thus attracting an increasing number of researchers to adopt this method for determining trap depths in materials^[30–32]. E_T denotes the shallowest depth in the trap distribution corresponding to the peak. Therefore, by applying the initial rise method to the TL curve in this study, we are able to identify the shallowest trap depth among the several components of the TL peak.

The temperature corresponding to the TL glow peak, the calculated integrated intensity of the TL glow peaks, and the trap depths are recorded in Table 1. The error range for E is $\pm 0.03\mathrm{eV}$. Meanwhile, the calculated trap depths and corresponding electron traps in the literature are listed^[10]. Given the low concentration of doped ${\mathrm{Pr}}^{3+}$ ions and the minor difference in ionic radii between ${\mathrm{Pr}}^{3+}$ and ${\mathrm{Y}}^{3+}$, it is feasible to utilize the trap types calculated for pure YAP for comparative purposes within an acceptable error range, thereby validating the trap types associated with the observed trap depths. Notably, despite being complex defects, those listed in column 5 are primarily composed of antisite defects or oxygen vacancies. Therefore, from the perspectives of shallow and deep energy levels, these composite defects correspond to the trap depth data presented in column 3.

Table 1 indicates that the complex defects primarily consist of shallow antisite defects where Y atoms replace the Al sites. In order to explain the anisotropy of antisite defect distribution within different crystal planes, we calculate the offset parameters of Al atoms^[26]. The offset parameters of Al atoms against (010) and (001) planes are shown to be the largest and smallest, respectively (see details in the Supplement 1). This suggests that Al atoms tend to scatter in the (001) plane, where defects caused by the substitution of Al atom by a Y atom are most likely to occur. The anisotropy of the structure leads to differences in the offset parameter of Al atoms and variations in areal ion density. Consequently, the antisite defect ${\mathrm{Y}}_{\mathrm{Al}}$ is most likely to appear in the (001) plane, which is consistent with the antisite defect concentrations calculated in Table 1.

Furthermore, as evident from Table 1, the complex defects of the (100) plane are closely related to oxygen vacancies. This conclusion is also supported by the TL curve, which shows peaks of the same shape shifting towards the high-temperature region in the (100) plane. This shifting indicates a significant presence of oxygen vacancy defects, which combine with shallow defects, thereby deepening the trap depths of the shallow defects. The electron traps with depths of 0.105 and 1.033 eV can be formed by a ${\mathrm{V}}_{\mathrm{O}}+{\mathrm{V}}_{\mathrm{Y}}+{2\mathrm{Y}}_{\mathrm{Al}}$ complex. In this defect, two ${\mathrm{Y}}_{\mathrm{Al}}$ ions are located closest to the ${\mathrm{V}}_{\mathrm{O}}$ and ${\mathrm{V}}_{\mathrm{Y}}$, allowing them to trap two electrons, which is different from the ${\mathrm{F}}^{+}$ center, which traps one electron. In the (100), (010), and (001) planes, the intensity ordering of ${\mathrm{V}}_{\mathrm{O}}$-related defects that trap one electron is consistent with the intensity ordering of the ${\mathrm{F}}^{+}$ center luminescence peak in the PL spectra.

In YAP crystals, the main intrinsic defect ${\mathrm{Y}}_{\mathrm{Al}}$ accounts for a high proportion^[33], and the presence of such defects can impact the scintillation luminescence process of the crystals. The following inferences have been made about the behavior of the antisite defects competing with ${\mathrm{Pr}}^{3+}$ for electron trapping. Vedda et al. proposed that the existence of antisite defects reduced the light yield of scintillators^[34]. The valence states and atomic radii of Ce and Pr are similar, and the relationship between ${\mathrm{Pr}}^{3+}$ and antisite defects can be inferred by referring to the literature on ${\mathrm{Ce}}^{3+}$ ion-doped scintillators. Kitaura et al. conducted a TL test on Ce:GAGG and calculated that the defect with a trap depth of 0.25 eV may be an antisite defect, which was related to the reduction of ${\mathrm{Ce}}^{3+}$ peak intensity^[35]. In YAG crystals, Drew et al. demonstrated that high-concentration doping of Ce could regulate the competition between Ce and ${\mathrm{Y}}_{\mathrm{Al}}$, reduce the ultraviolet luminescence related to antisite defects, and simultaneously increase the visible light emission related to Ce^[36]. Accordingly, a competitive relationship is observed between the Ce luminescent center and the shallow antisite defect. From this observation, it can be logically deduced that ${\mathrm{Pr}}^{3+}$ ions similarly engage in competition with the ${\mathrm{Y}}_{\mathrm{Al}}$-related defects within YAP crystals. The trap depth of ${\mathrm{Y}}_{\mathrm{Al}}$-related defects in this paper is less than 0.5 eV, which is significantly far from the trapping depth of ${{}^1\mathrm{S}}_0$ (2 eV) mentioned in Ref. [37]. Therefore, there will be no resonance energy transfer between them. In the previously mentioned XEL spectra, the ${\mathrm{Pr}}^{3+}$${5\mathrm{d}}_1\to 4\mathrm{f}$ emission peak and the ${\mathrm{F}}^{+}$ center emission peak can be observed, but no ${\mathrm{Y}}_{\mathrm{Al}}$ emission peak is detected. This is attributed to the ${\mathrm{Y}}_{\mathrm{Al}}$ emission peak near 218 nm gradually weakening with increasing temperature and eventually quenching at room temperature^[38], which explains the absence of ${\mathrm{Y}}_{\mathrm{Al}}$ emission in the XEL spectra measured at room temperature.

When the crystal interacts with X-rays and becomes ionized, electrons transition from the valence band to the conduction band, creating corresponding holes in the valence band. The ${\mathrm{Pr}}^{3+}$ emission centers capture these electrons and holes, and give rise to scintillation light through radiative transitions. Simultaneously, the ${\mathrm{V}}_{\mathrm{O}}-$, ${\mathrm{V}}_{\mathrm{Y}}-$, and ${\mathrm{Y}}_{\mathrm{Al}}$-related complex defects also trap electrons. In the XEL spectra, the integrated intensity ratio of the ${\mathrm{Pr}}^{3+}$${5\mathrm{d}}_1\to 4\mathrm{f}$ emission peak among the (100), (010), and (001) planes is 40.0:32.9:27.1. TL peaks at high temperatures correspond to deep-level defects, where capturing electrons excited by X-rays becomes excessively difficult. Therefore, only shallow defects with trap depths less than 0.5 eV are considered. Among the defects calculated from the TL curves, it can be observed that the shallow defects of the (100) plane are ${\mathrm{Y}}_{\mathrm{Al}}+{\mathrm{V}}_{\mathrm{O}}$ and ${\mathrm{V}}_{\mathrm{O}}+{\mathrm{V}}_{\mathrm{Y}}+{2Y}_{\mathrm{Al}}$, which correspond to TL peaks at 354 and 429 K, respectively. For the (010) plane, the shallow defects are represented by ${\mathrm{V}}_{\mathrm{Y}}+{\mathrm{Y}}_{\mathrm{Al}}$, corresponding to the 298 K peak. Similarly, the shallow defects of the (001) plane are ${\mathrm{V}}_{\mathrm{Y}}+{\mathrm{Y}}_{\mathrm{Al}}$, corresponding to the 294 K peak. The ratio of the integrated intensity of the shallow defects in (100), (010), and (001) planes is 23.7:27.1:36.0. We calculated and compared the relative proportions of ${\mathrm{Pr}}^{3+}$${5\mathrm{d}}_1\to 4\mathrm{f}$ emission intensities and shallow defect concentrations; the results are presented in Fig. 8. The figure clearly demonstrates that among these three crystal planes, the integrated emission intensity of ${\mathrm{Pr}}^{3+}$${5\mathrm{d}}_1\to 4\mathrm{f}$ exhibits a ratio of 1.48:1.21:1, and the integrated intensity ratio of shallow defects is 1:1.14:1.52. Notably, a certain degree of numerical proximity is observed between 1.14 and 1.21, as well as between 1.52 and 1.48. This phenomenon suggests that the most likely primary cause of the variation in ${\mathrm{Pr}}^{3+}$${5\mathrm{d}}_1\to 4\mathrm{f}$ emission intensity is the anisotropic distribution of shallow defects in different planes.

As previously demonstrated, the (001) plane exhibits the highest concentration of shallow-level complex defects, which engage in competitive electron capture, ultimately leading to a reduction in the concentration of electrons trapped by the ${\mathrm{Pr}}^{3+}$ 5d level. Consequently, in this (001) plane, the integrated intensities of the 246 and 280 nm emission peaks, which correspond to the ${\mathrm{Pr}}^{3+}$${5\mathrm{d}}_1\to 4\mathrm{f}$ transition in the XEL spectra, are the weakest. Conversely, although the Pr atom concentration in the (100) plane is lower than that in the other two planes, it possesses the lowest concentration of shallow complex defects, resulting in the strongest ${5\mathrm{d}}_1\to 4\mathrm{f}$ emission. This further proves the influence of defect anisotropy on luminescence properties.

A series of tests are conducted on the (100), (010), and (001) planes of the Pr:YAP crystal. The Raman spectra exhibit anisotropy in the vibrational modes of the molecules, whereas no significant anisotropy is observed in the absorption spectra. The ICP test results indicate that the Pr atom concentration in the (100) plane is 83% of that in the (010) and (001) planes. In the XEL spectra, the (100) plane displays the highest emission intensity of the ${\mathrm{Pr}}^{3+}$${5\mathrm{d}}_1\to 4\mathrm{f}$ transition. Conversely, in the PL spectra, the strongest emission of the ${\mathrm{F}}^{+}$ center is observed in the (001) plane. The PL decay test indicates that the decay time of the ${\mathrm{Pr}}^{3+}$${5\mathrm{d}}_1\to 4\mathrm{f}$ transition remains approximately 7 ns in all three planes, signifying the absence of anisotropy in its luminescence mechanism. Further TL analysis has revealed the defect distributions across three planes. The intensity order of ${\mathrm{V}}_{\mathrm{O}}$-related defects capturing an electron in the (100), (010), and (001) planes is consistent with that of the ${\mathrm{F}}^{+}$ center emission peaks in the PL spectra. Based on the analysis of TL and XEL tests, ${\mathrm{Pr}}^{3+}$ centers are found to competitively trap electrons with complex defects (${\mathrm{V}}_{\mathrm{O}}-$, ${\mathrm{Y}}_{\mathrm{Al}}-$, and ${\mathrm{V}}_{\mathrm{Y}}$-related). The (100) plane exhibits a higher concentration of deep-level defects, primarily oxygen vacancies, which combine with antisite defects. This complex defect deepens the trap depth of the shallow-level antisite defects, consequently reducing the number of shallow-level defects that compete with ${\mathrm{Pr}}^{3+}$ for electron capture. In contrast, calculations of the Al atom offset parameter indicate that ${\mathrm{Y}}_{\mathrm{Al}}$ defects preferentially occur on the (001) plane. This plane simultaneously exhibits the lowest areal ion density and the highest concentration of shallow-level complex defects, resulting in the weakest emission from ${\mathrm{Pr}}^{3+}$. This discovery underscores the intimate connection between defects and luminescence properties, demonstrating that defect anisotropy impacts luminescence intensity. For scintillator crystal thin wafer, particular attention should be paid to the impact of defect anisotropy on their performance.