Photoluminescence associated with the site occupations of Ho3+ ions in BaTiO3

A nominal (Ba1−xHox)Ti1−x/4O3 (x = 0.01) (BHTH) ceramic with a single-phase tetragonal structure was prepared at 1400 °C using the solid-state reaction method. The analysis on the defect chemistry revealed that the real formula of BHTH is (Ba1−xHo3x/4)(Ti1−x/4Hox/4)O3 with Ba vacancies via electron paramagnetic resonance (EPR). Photoluminescence (PL) was investigated on the basis of excitation with different wavelength lasers. The results indicated that under 488-nm excitation, PL and Raman scattering can occur simultaneously as two distinct optical processes for BHTH ceramic powders, and the strongest PL band at 564 nm was discovered and verified to originate from the 5G6/5F1 → 5I7 transition of Ho3+ ions on the Ti sites in the BaTiO3 lattice. Upon 532- and 638-nm excitations, three PL bands of 5F4/5S2 → 5I8, 5F5 → 5I8, and 5F4/5S2 → 5I7 transitions are attributed to the contributions from Ho3+ ions on the Ba sites. The common Raman spectrum of BaTiO3 can be observed without PL disturbance using 785-nm excitation wavelength. The PL effect may provide a probe for the site occupations of Ho3+ ions in widely-used BaTiO3 dielectric ceramics co-doped with Ho3+ and other dopants.

ions in BaTiO 3 is still an unresolved scientific problem. Moreover, a distinct exhibition between PL and Raman scattering needs to be also clarified.
In this work, a nominal (Ba 1−x Ho x )Ti 1−x/4 O 3 (x = 0.01) ceramic was prepared at 1400 °C using the solid-state reaction method. The defect chemistry is discussed. On the basis of excitation with different wavelength lasers (488-785 nm), photoluminescence and Raman scattering can occur simultaneously as two distinct optical processes for BHTH ceramic powders. The reasonable laser wavelength is indicated for observation for the common Raman phonon modes of Ho-doped BaTiO 3 . A rare non-ground-state transition corresponding to 5 G 6 / 5 F 1 → 5 I 7 of Ho 3+ was discovered and associated with the site occupations of Ho 3+ ions in BaTiO 3 , which may provide a probe for the site occupations of Ho 3+ ions in BaTiO 3 -based dielectric ceramics.

Experimental
The initial materials were analytical-reagent chemicals of BaCO 3 , TiO 2 , and Ho 2 O 3 (99.9%). Ho-doped BaTiO 3 ceramics were prepared using the solid-state reaction method according to the nominal formula (Ba 1−x Ho x ) Ti 1−x/4 O 3 (x = 0.01) named as BHTH. The weighed powders were dry-mixed and ground. Then, the powders were calcined at 1100 °C for 5 h to decarbonate. The calcined powders were mixed with a small amount of PVA aqueous solution and pressed at a pressure of 200 MPa into pellets (Φ12 mm). These pellets were sintered at T s = 1400 °C for 12 h, and cooled at a cooling rate of −200 °C/h from 1400 °C to 700 °C and then furnace-cooled to room temperature. The pellets were densified into crack-free ceramics. In addition, a nominal (Ba 1−x Ho x )Ti 1−x/4 O 3 (x = 0.01) (BH1T) ceramic was prepared at T s = 1300 °C for 12 h for analysis on site-occupation-related photoluminescence of Ho 3+ ions in the BaTiO 3 lattice.
Powder X-ray diffraction (XRD) data were recorded from 20° to 85° and in steps of 0.02° using a DX-2700 X-ray diffractometer (Dandong Haoyuan) at room temperature. Lattice parameters were calculated with MS Modeling (Accelry Inc.) using Cu Kα 1 radiation (λ = 1.540562 Å). Microstructure was observed using an EVOMA 10 scanning electron microscope (Zeiss) operated at 15 keV. Photoluminescence (PL) and Raman spectra of the ceramic powders were recorded using a LabRAM XploRA Raman spectrometer (Horiba Jobin Yvon) with 532-(green) and 638-nm (red) lasers and an inVia Raman spectrometer (Renishaw) equipped with 488-(blue-purple) and 785-nm (red) lasers. The laser power level was adjusted to 0.1-10% (Filter) of the normal output of 25 mW because of a huge difference in spectral intensity. The accumulation time and resolution are 2 s and 2.7 cm −1 , respectively. Electron paramagnetic resonance (EPR) measurements were performed at room temperature using an EMX Plus X-band spectrometer (Bruker) operating at 9.84 GHz. The gyromagnetic factor (g) was calculated by the relationship hν 0 = gβH, where h is the Planck constant (h = 6.626 × 10 −34 J·s), ν 0 is the microwave frequency, β is the Bohr magnetron (β = 9.262 × 10 −24 J/T), H is the magnetic field strength.

Results and Discussion
Crystal structure, site occupation and microstructure. Figure 1a shows the XRD pattern of (Ba 1−x Ho x ) Ti 1−x/4 O 3 (x = 0.01) ceramic (BHTH) prepared at T s = 1400 °C. BHTH has a single-phase tetragonal perovskite structure, indicating that Ho 3+ ions are incorporated sufficiently into the perovskite lattice. This result is in accord with the solubility limit of x = 0.014 for (Ba 1−x Ho x )Ti 1−x/4 O 3 at T s = 1400 °C reported by Makovec et al. 10 .
The lattice parameters (a, c) and unit cell volume (V 0 ) is shown in Fig. 1a. It is inferred that the expansion in V 0 caused by the occupations of the Ti 4+ sites by Ho 3+ ions should be greater than the contraction in V 0 caused by the occupations of the Ba 2+ sites by Ho 3+ ions on the basis of the BO 6 octahedrons skeleton characteristic of the perovskite lattice and ionic size comparisons between 12-CN Ba 2+ and Ho 3+ and between 6-CN Ti 4+ and Ho 3+ ( Table 1). The V 0 of BHTH (64.44 Å 3 ) is slightly greater than that of the tetragonal BaTiO 3 (V 0-BT = 64.41 Å 3 ) (JCPDS Cards No. 6-526), implying that Ho 3+ ions enter the Ti sites in part except for some Ho 3+ ions on the Ba sites. Figure 1b shows the EPR spectrum of BHTH. Ho 3+ (4f 10 ) is a non-Kramers ion, which is EPR silent in theory. A very strong EPR signal at g = 1.974 appears in BHTH and was observed in Dy-and Er-doped BaTiO 3 27, 28 . This signal is assigned to ionized Ba-vacancy defects (V Ba ) 27,28,37,38 . A g = 2.004 signal associated with Ti vacancies is absent for BHTH, revealing that Ti-vacancy defects cannot be formed in the nominal (Ba 1−x Ho x )Ti 1−x/4 O 3 (BHTH) with deliberately designed Ti vacancies; Ho 3+ ions transfer from the Ba sites to the Ti ones and results in appearance of Ba vacancies in BHTH. This result is in good agreement with the previous XRD result. Figure 1c and d shows the SEM images on the surface and the interior of BHTH. The surface of the ceramic sintered exhibits an inhomogeneous microstructure ( Fig. 1c) with grains (2-20 μm), but the interior in the ceramic shows a fine-grained feature (2-6 μm) (Fig. 1d). This illustrates that the grain growth on the surface is faster than the interior.
Defect chemistry. For the nominal (Ba 1−x Ho x )Ti 1−x/4 O 3 (x = 0.01) ceramic (BHTH) sintered at T s = 1400 °C, the above XRD result gives evidence of a single-phase perovskite ceramic and the partial occupations of the Ti sites by Ho 3+ ions (Fig. 1a); the EPR result reveals that Ti vacancies in BHTH are completely filled by Ho 3+ and the transfer of Ho 3+ ions from the Ba sites to the Ti ones gives rise to the presence of Ba vacancies (Fig. 1b). It is obvious that the formula cannot be described as (Ba 1−x Ho x )Ti 1−x/4 O 3 with Ti vacancies suggested by Makovec et al. 10 . Ho 3+ is known to show an amphoteric behavior, substituting for both Ba and Ti sites in BaTiO 3 10-12 . Thus, the

Ho Ho
Ba Ti to meet the requirement of the lattice electroneutrality.
Thus, a very strong EPR signal at g = 1.974 is present in BHTH (Fig. 1b). The real formula of BHTH is determined to be (Ba 1−x Ho 3x/4 )(Ti 1−x/4 Ho x/4 )O 3 with double-site substitutions and Ba-vacancy compensation, i.e., that at the doping level of x = 0.01, 0.75 at.% Ho 3+ ions are substituted on the Ba sites and 0.25 at.% Ho 3+ ions on the Ti sites. The quantitative information on the site occupations of Ho 3+ ions in BHTH can be drawn from the EPR and XRD results. The defect chemistry of BHTH is the same as that of BaTiO 3 doped with Dy 27 and Er 28 . An interesting phenomenon is that Ho (AN = 67) is between Dy (66) and Er (68) in atomic number (AN).  Fig. 2d. In the detectable range, these PL bands originate from the 4f-4f inner-shell emission of Ho 3+ in BaTiO 3 . Upon 638-nm excitation, Ho 3+ ions are excited through one-photon absorption from 5 I 8 to 5 F 5 ; the strongest emission band at 653 nm (Fig. 2a) is caused by the 5 F 5 → 5 I 8 transition. The near infrared (NIR) emission band (755 nm) associated with the 5 F 4 / 5 S 2 → 5 I 7 transition might originate from the following process: Ho 3+ ions are excited through one-photon absorption from the ground state 5 I 8 to the excited state 5 F 5 that relaxes nonradiatively to the lying excited state 5 I 7 via continuous multi-phonon relaxation processes ( 5 F 5 -5 I 4 -5 I 5 -5 I 6 -5 I 7 ). Subsequently, Ho 3+ ions were excited again through one-photon absorption (excited state absorption (ESA) process or energy transfer (ET) process) from 5 I 7 to the excited state 5 F 3 that relaxes nonradiatively to the emitting level 5 F 4 / 5 S 2 . Finally, the 5 F 4 / 5 S 2 → 5 I 7 transition occurs (Fig. 2d). Each emission band shows a multiplet feature because of the Stark components of the ground state 5 I 8 and the excited states 5 F 4 / 5 S 2 and 5 F 5 5, 31, 32, 35 . Upon 532-nm excitation, three PL bands with different intensities can be observed (Fig. 2b). The emission mechanism of Ho 3+ in BHTH does not go through two-step ESA or ET process, but one-step one-photon absorption from 5 I 8 to 5 F 4 / 5 S 2 that relax nonradiatively to the emitting level 5 F 5 , which is the reason that the intensity of the NIR (755 nm) emission band under 532-nm excitation is higher than that under 638-nm excitation. BHTH exhibits strong green (545 nm) with weak red (653 nm) and NIR (755 nm) emission bands associated with 5 F 4 / 5 S 2 → 5 I 8 , 5 F 5 → 5 I 8 and 5 F 4 / 5 S 2 → 5 I 7 transitions, respectively, of Ho 3+ ions 31,32 . The multiplet feature of bands is also caused by Stark splitting of the energy levels. It can be seen from the insets in Fig. 2a and b that the corresponding emission bands upon excitation with different wavelengths exhibit the same spectral structure.

Photoluminescence and Raman spectra.
Pure Raman spectrum was observed in BHTH using 785-nm excitation wavelength, as shown in Fig. 2f. This spectrum of BHTH, which does not show any emission transition line and is similar to that reported for the tetragonal BaTiO 3 (Fig. 2e), respectively. No signal was present in the range of 850-1000 nm.
Upon 488-nm excitation, two bands were observed and the intensity of the main PL band at around 564 nm is 40 times that of the weak band at around 500 nm (Fig. 2c). The band at 564 nm, which has not been reported for Ho-doped BaTiO 3 31, 32 , was observed for BHTH (Fig. 2c). This band is attributed to the 5 G 6 / 5 F 1 → 5 I 7 transition (Fig. 2d). The following analyses give its emission mechanism, and indicate that the band at 564 nm is associated with Ho 3+ ions on the Ti sites and the weak band at 500 nm originates from Raman scattering of BHTH.
It is well known that when the excitation is changed to a different wavelength, the Raman bands shift the same amount correspondingly, while the PL lines stay on the same absolute wavenumber 28,43 . Upon 638-and 532-nm excitations, it can be seen that both 5 F 5 → 5 I 8 and 5 F 4 / 5 S 2 → 5 I 7 transition bands at around 653 and 755 nm ( Fig. 2a and b) stay on the same position, respectively, illustrating the PL nature of these two bands. Upon 488-nm (20491.8 cm −1 ) and 785-nm (12738.9 cm −1 ) excitations, Raman band is easy to be identified by comparison of band positions. Figure 2f shows their spectra, in which the right figure shows the weak band at 500 nm in Fig. 2c inset. When the excitation is changed from 488 nm to 785 nm, the shift of 7752.9 cm −1 between two Raman signals was observed for BHTH. Moreover, both spectra exhibit the nearly same spectral structure as the tetragonal BaTiO 3 (Fig. 2f). Thus, the band at around 500 nm is confirmed to originate from Raman scattering in the case of excitation at 488 nm.
The 5 G 6 / 5 F 1 → 5 I 7 transition most likely originates from ground state absorption plus phonon-assisted transition (Fig. 2d). The 5 F 3 energy level can be populated from 5 I 8 via 488-nm laser energy absorption (one-photon). As the energy differences between 5 F 3 and 3 K 8 / 5 F 2 and between 3 K 8 / 5 F 2 and 5 G 6 / 5 F 1 is about 250 and 1050 cm −1 , respectively, the multi-phonon assisted energy transfer (MPA-ET) corresponding to one A 1 (TO 2 ) phonon absorption ( 5 F 3 → 3 K 8 / 5 F 2 ) and then two A 1 (TO 3 ) phonons absorption ( 3 K 8 / 5 F 2 → 5 G 6 / 5 F 1 ) can occur. The absorbed phonons can be detected in light of the Raman spectrum in Fig. 2c inset or Fig. 2f. Normally, the three-phonon absorption is enough for the energy gap between 5 F 3 and 5 G 6 / 5 F 1 . Finally, the emission band at 564 nm is due to the transition from 5 G 6 / 5 F 1 to the second ground state level 5 I 7 .
On the basis of the above analyses, an important finding is that PL and Raman scattering under 488-nm excitation can occur simultaneously as two distinct optical processes for BHTH ceramic powders. One can also see that the PL signals from the 5 F 4 / 5 S 2 → 5 I 8 transition under 532-nm excitation are so intense that they overwhelm the traditional Raman spectra of BaTiO 3 (Fig. 2b and Fig. 2c inset). The option for laser wavelength is an important factor in observation of Raman signals. Photoluminescent origin associated with site occupations of Ho 3+ in BaTiO 3 . To investigate the PL origin associated with the site occupations of Ho 3+ ions in the host BaTiO 3 lattice, a BaTiO 3 ceramic doped with Ho 3+ on the Ba site is required to be prepared. In the Ti-rich case of Ba/Ti < 1, Ho 3+ was considered to enter the Ba site for (Ba 1−x Ho x )TiO 3 sintered at T s = 1320-1360°C in air [12][13][14] . For this reason, we prepared a nominal (Ba 1−x Ho x )Ti 1−x/4 O 3 (x = 0.01) ceramic (BH1T) sintered at T s = 1300 °C. BHTH and BH1T have the same stoichiometric proportions in components.
The XRD pattern and EPR spectrum of BH1T as well as a comparison in PL spectra between BHTH and BH1T under 488-nm excitation are shown in Fig. 3. BH1T also exhibits a single-phase tetragonal perovskite structure (Fig. 3a) like BHTH, implying a complete incorporation of Ho 3+ ions into the host BaTiO 3 lattice. The V 0 of BH1T (64.25 Å 3 ) is less than those of BHTH (64.44 Å 3 ) and the tetragonal BaTiO 3 (64.41 Å 3 ), suggesting that Ho 3+ ions are dominantly substituted for the Ba sites.
The EPR monitoring for BH1T shows coexistence of both Ba vacancies and Ti ones, marked by two signals at g = 1.974 and 2.004, respectively (Fig. 3b), implying that Ho 3+ may transfer mutually between the Ba site and the Ti one during ceramic sintering and cooling. This behavior is similar to that of Lu 3+ -doped BaTiO 3 44 . In this case more Ho 3+ ions in BH1T can be present on the Ba 2+ sites relative to BHTH, resulting in a stronger donor effect. The Mn 2+ sextet signal is therefore caused by the reduction of Mn 4+ /Mn 3+ to Mn 2+ impurities in BH1T 45,46 .
In a detectable wavelength range, one can see that both PL bands at around 564 nm are nearly the same in spectral structure and different in intensity for BHTH and BH1T, while there is no difference between both Raman bands in a lower-wavelength range at around 500 nm (Fig. 3c). It is obvious that the PL band at around 564 nm does not relate to Ho 3+ ions on the Ba sites. The above XRD and EPR results reveal that Ho 3+ ions in BH1T are dominantly substituted for the Ba sites, which is in good agreement with the site occupations of Ho 3+ at a lower sintering temperature of T s = 1320-1360°C in air [12][13][14] . Similar to the doping behavior of Dy 3+ or Er 3+ in BaTiO 3 27,28 , a small number of Ho 3+ ions in BH1T inevitably enter the Ti sites. For our BHTH, 0.25 at.% Ho 3+ ions are substituted on the Ti sites. The PL intensity is proportional to the concentration of Ho 3+ ions. The PL intensity of the 5 G 6 / 5 F 1 → 5 I 7 transition of BH1T is one-twentieth of BHTH (Fig. 3c), which matches the concentration of Ho 3+ ions on the Ti sites in BH1T and BHTH. Thus, Ho 3+ ions on the Ti sites are responsible for the 5 G 6 / 5 F 1 → 5 I 7 transition at around 564 nm.
It is inferred from the PL intensity that 0.0125 at.% Ho 3+ ions are substituted on the Ti sites in BH1T, though no quantitative information on the site occupations of Ho 3+ ions on the Ti sites or Ba ones can be drawn directly from the EPR and XRD results. The real formula of BH1T may be approximately described as (Ba 1−x Ho x−δ ) (Ti 1-x/4 Ho δ )O 3 (δ is a small quantity, δ = 0.0125%) with Ba vacancies and Ti ones. Hence, the change in local environment and crystal field at which Ho 3+ lies plays a decisive role in PL.
PL band at 564 nm as a probe for dielectrics in MLCC. Temperature-stable dielectric materials in MLCC often adopt the core-shell structured BaTiO 3 ceramics, in which a fine grain consists of a tetragonal BaTiO 3 core and a cubic surface layer consisting of Ho 2 O 3 and other oxide, such as Ho/Mg 7, 47 , Ho/Mn 48 , Ho/ Zr, 49,50 Ho/Dy 8 etc. The incorporation of additives ions into BaTiO 3 particles will reduce dielectric permittivity and dielectric-temperature stability. The sintering process plays an important role in controlling incorporation of Ho 3+ into BaTiO 3 particles. Although at a high sintering temperature (e.g. T s = 1550 °C 10,15 ) Ho 3+ ions are considered to substitute for the Ti sites, even at a lower T s Ho 3+ ions inevitably enter the Ti sites in part, as observed in Fig. 3c. It is more expensive and complicated to detect the amount of Ho 3+ ions dissolved in the BaTiO 3 lattice using an ion implantation technique with an accelerating voltage of 500 keV and secondary-ion mass spectrometry (SIMS) 11 . The PL of the 5 G 6 / 5 F 1 → 5 I 7 transition is sensitive to Ho 3+ on the Ti sites in BaTiO 3 . Thus, the application of PL under 488-nm excitation may provide a probe for the occupations of Ho 3+ ions on the Ti sites in BaTiO 3 ceramics co-doped with Ho 3+ and other dopants. Accordingly, the application of PL under 532-or 638-nm excitations may provide a probe for the occupations of Ho 3+ on the Ba site.

Conclusions
The nominal (Ba 1−x Ho x )Ti 1−x/4 O 3 (x = 0.01) (BHTH) ceramic was prepared at 1400 °C using the solid-state reaction method. BHTH exhibits a single-phase tetragonal perovskite structure. The study on the defect chemistry indicates that the defects in BHTH are Ba-site Ho 3+ ( • Ho Ba ), Ba vacancies ( ″ V Ba ), and Ti-site Ho 3+ ( ′ Ho Ti ). The real formula of BHTH is expressed by (Ba 1−x Ho 3x/4 )(Ti 1−x/4 Ho x/4 )O 3 . The change in local environment and crystal field at which Ho 3+ lies plays a decisive role in photoluminescence (PL) of Ho 3+ ions. Upon 532-and 638-nm excitations, three PL bands corresponding to 5 F 4 / 5 S 2 → 5 I 8 , 5 F 5 → 5 I 8 , and 5 F 4 / 5 S 2 → 5 I 7 transitions are attributed to the contributions from Ho 3+ ions on the Ba sites in the BaTiO 3 lattice. On the contrary, Ho 3+ ions on the Ti sites are responsible for the 5 G 6 / 5 F 1 → 5 I 7 transition under 488-nm excitation, and moreover, PL and Raman scattering can occur simultaneously as two distinct optical processes. The common Raman spectrum of BaTiO 3 can be observed without PL disturbance using 785-nm excitation wavelength. The PL signals under 532-nm excitation are intense enough to conceal the traditional Raman phonon modes of BaTiO 3 . The application of PL may provide a probe for the site occupations of Ho 3+ in BaTiO 3 dielectric ceramics co-doped with Ho 3+ and other dopants.