Nowadays, white light-emitting diodes (W-LEDs), due to their compact size, eco-friendliness, high brightness, low energy consumption, and long lifetime, are regarded as a viable substitute for traditional incandescent lamps and fluorescent lamps^[1]. At present, the most popular W-LED device is phosphor-converted white LEDs^[2,3]. However, the phosphor of W-LEDs still has many drawbacks, such as large light decline, poor uniformity, deficiency of red emission^[4], a short lifetime, and poor physical and chemical characteristics, which is unfavorable for the performance of the W-LED^[1]. Recently, some researchers devoted their attention to the fields of non rare earth (non-RE) such as Bi ion-doped phosphors^[5–7] to solve the problems above, and have made great progress in the light decline and deficiency of red emission. However, there are still more problems than a single crystal can solve^[8]. Therefore, it is urgently required to seek more stable single crystals to act as substitutes for the red fluorescent materials.

Europium ions have attracted a great deal of attention for their wide use in the field of display and illumination since 1960s. As one of the most frequently used red-emitting activators, the Eu3+ ion mainly shows characteristic emissions with high intensity from the transitions of D05–F7J (J=0, 1, 2, 3, 4, 5, 6)^[9,10], and has a long fluorescent lifetime^[11].

CGA single crystals have a perovskite structure that belongs to the family of ABCO4 compounds, where A=Ca, Sr, or Ba, B=a rare-earth element, and C=Ga, Al, or a transition element^[12,13]. The crystal’s structure is a three-dimensional net composed of connected AlO6 octahedra. A and B ions occupy the same site with a site occupancy factor equal to 0.5. These ions are coordinated by nine oxygen ions^[14,15]. Therefore, the dopant ions are distributed in a disorderly manner in the CGA single crystal and occupy the sites without or deviated from inversion symmetry^[2]. Due to the high thermal conductivity (6.9Wm−1K−1 along the a-axis and 6.3Wm−1K−1 along the c-axis), favorable thermal expansion coefficient (10.1×10−6 and 16.2×10−6K−1 along the a- and c-axes, respectively), and high mechanical strength (Mohs hardness 6), lanthanide-doped CGA crystals are good candidates for fluorescent and laser materials^[16,17]. To date, all of the reported results concerning Ln-doped CGAs are focused on the spectroscopic properties of IR-relevant Ln3+ ions such as Nd3+, Er3+, Tm3+, and Yb3+^[18–21]. Therefore, in this Letter, we present a report concerning the spectroscopic properties of Eu3+-doped CGA single crystals.

CGA single crystals can be grown by the Czochralski (CZ) method. Due to the strong thermal expansion anisotropy, the crystal growth process is nontrivial, and the obtained boules often posses cleavages parallel to the (001) crystallographic plane^[22]. However, the floating zone (FZ) method possesses some advantages over the CZ method in the growth of these sorts of materials. FZ is much faster and cheaper due to the lack of expensive crucibles. Moreover, the several-millimeter-diameter crystals grown by the FZ method possess a rod-like shape and are thought to have potential as a material for miniature optical device applications because light can pass through the cylindrical crystal without complicated processing^[23].

In this Letter, we present the results of different investigations carried on Eu3+-doped CGA single crystals grown by the FZ method. The x ray diffraction (XRD), rocking curve, segregation coefficient, and spectra properties of the Eu:CGA crystal were studied.

A CaGdAlO4 single crystal doped with Eu3+ (2 at. %) was grown by the FZ technique. The starting materials were CaCO3, Gd2O3, Al2O3, and Eu2O3 with 5 N purity. They were mixed with stoichiometric proportions. Afterwards, anhydrous ethanol was added, and the obtained slurry was well ground in a ball mill. The mixed powder was dried and pressed into rods (~130 mm long, 10 mm in diameter) by a cold isostatic press (applied pressure: 200 MPa, pressing time: 2 min). The obtained rods were sintered at 1250°C for 20 h for decarbonation in air. Single-crystal growth was carried out in an optical floating-zone furnace (IRF01-001-00, Quantum Design Corporation, America). A CGA single crystal bar (4mm×4mm×50mm) oriented in the 〈100〉 direction was used as a seed. The sintered rods were rotated at 10 rpm, and the single crystal was grown at a speed of 3–4.5 mm/h in the air atmosphere. Moreover, in order to assure the uniform composition of the molten zone, an opposite rotation at the same rotation speed is necessary^[24].

Long and geometrically homogeneous textured cylindrical rods have been obtained [see Fig. 1(a)]. The rod was cut along the (001) plane into ~1 mm thick slices, as shown in Fig. 1(b). As shown in Fig. 1(a), the Eu:CGA single crystal is transparent and slightly brown in color. Color is relevant to the colored centers, which are related to the oxygen vacancies in the crystal structure. Therefore, the piece was annealed in H2 for 20 h at 1250°C to eliminate the colored center. As shown in Fig. 1(b), the right piece, which was transparent, was annealed in H2, compared to the left, which is an as-grown crystal.

The crystal structure of Eu:CGA was analyzed by XRD using Cu Kα radiation (D8-Advance, Bruker, Germany). Data was recorded over a range of 10°–70° with a 0.02° step. The rocking curve for the crystal was measured by a high resolution x ray diffractometer (D8-Discover, Bruker, Germany). The segregation coefficient of the Eu3+ ion in the CGA crystal was measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES). The absorption spectra were measured by a Cary 5 E Varian spectrophotometer. The emission spectra were measured by exploiting a DongWoo Optron DM750 monochromator coupled to an R-928 Hamamatsu photomultiplier. The luminescence decay kinetics were measured with the use of a Coherent Inc. Libra femtosecond laser coupled with a light conversion OPerA optical parametric amplifier (OPA) as an excitation source and a Princeton Instruments ACTON 2500i monochromator coupled with a Hamamatsu C5680 streak camera. All spectroscopic measurements were carried out on the annealed samples. All measurements were performed at room temperature. For polarized experiments, a Harrick PGT-S1 V Glan-Tylor polarizer was used.

The XRD patterns of the grown crystal and appropriate Joint Committee On Powder Diffraction Standards (JCPDS) database entry (no. 24-0192) are shown in Fig. 2. The diffraction patterns of the obtained material are in very good agreement with the database record. The cell parameters of the Eu:CGA and pure CGA crystals were calculated from the diffraction data and are listed in Table 1. The calculated parameters for both the doped and pure CGA are quite similar, which is evidence of the integrity of the Eu:CGA structure. The incorporation of dopant ions leads to the extension of the unit cell. This fact can be easily explained: the larger Eu3+ ions act as substitutes for the smaller Gd3+ ions in the crystal structure (rEu=1.120Å, rGd=0.938Å). The site occupancy is expected to be determined largely by considerations of ionic size. In consequence, Eu3+ ions are expected to preferentially occupy Ca2+/Gd3+ sites because the radius of Eu3+ (1.120 Å) is closer to those of Ca2+ (1.18 Å) and Gd3+ (0.938 Å), and is much larger than that of Al3+ (0.535 Å). In addition, the similar valency is the other favorable factor to determine the site occupancy, which makes it preferred that the Gd3+ ions in the CaGdAlO4 host be replaced by Eu3+ ions^[25].

The result of the rocking curve for the Eu:CGA crystal is shown in Fig. 3. The only peak indicates that the as-grown crystal is a single crystal. The full width at half-maximum of the rocking curve for the Eu:CGA crystal is about 17 arcsec, which shows the high crystallinity of the crystal grown by the FZ method.

The segregation coefficient of Eu3+ in the Eu:CGA crystal can be calculated from the following formula: K0=Cs/Cl,(1)where Cs and Cl are the Eu3+ concentrations in the crystal and melt, respectively, when there is a solid–liquid equilibrium^[20,26]. Approximately, we calculate Cs as the concentration at the growth starting position in the crystal, and Cl as the doping concentration. The element analysis results are listed in Table 2. The segregation coefficient of Eu3+ was calculated to be 0.96. The reason is that the radius of Eu3+ is larger than the radius of Gd3+, leading to the slight lattice distortion and difficulties in Eu3+ replacing the Gd3+. The obtained value of the segregation coefficient suggests that Eu3+ should be uniformly distributed along the crystal boule^[27].

The polarized absorption spectra of Eu:CGA in the range of 300–475 nm are shown in Fig. 4, Both spectra are dominated by a strong line occurring at 399 nm that is related to the F07→L65 transition. The absorption bands lack a refined structure due to the random crystal environment, which affects the influence of the crystal field on the Eu ions. The Eu ions are located in a site that is randomly occupied by Gd3+; therefore, the Eu ions distributed in a disorderly manner in the Eu:CGA single crystal. All tetragonal crystals are optically uniaxial. For those crystals, the optical axis is parallel to the crystallographic c-axis. The differences between the σ and π polarization absorption spectra reflect the anisotropy of Eu:CGA crystal. The effective absorption coefficients of the F07→L65 transition at 399 nm for σ polarization and π polarization were calculated to be 3.85 and 7.57cm−1, the full widths at half-maximum were 1.58 and 1.62 nm, and the absorption cross sections σabs were calculated to be 1.61×10−20 and 3.17×10−20cm2, respectively.

Figure 5 shows the emission spectra of the Eu:CGA in the region from 500–750 nm under the 399 nm excitation. Some characteristic emission lines correspond to the transitions from the D05 excited state to the ground states F7J (J=0, 1, 2, 3, 4): D05→F07 (581 nm), D05→F17 (591 nm, 599 nm), D05→F27 (613 nm, 621 nm), D05→F37 (658 nm), and D05→F47 (691 nm, 701 nm). Among these emission bands, 621 nm (D05→F27) is observed to be the most intense. From the emission spectra, we can only find the emission transitions from the D05 state to F7J. While the transitions from the D5J to the F7J multiplet are expected in theory, the absence of emissions could be due to the existence of higher energy phonons in the CGA, and also could be due to fast nonradiative multiphonon relaxations from these levels to the D05 level.

The transition D05→F17 is a magnetic dipole transition and is allowed with selection rule ΔJ=±1 according to the parity selection rules, while the electric dipole emission transition D05→F27 is forbidden based on selection rule ΔJ=±2^[13]. If an Eu3+ ion occupies a site with inversion symmetry, the orange emission due to the allowed magnetic dipole transition D05→F17 will be dominant. If it occupies the non-inversion symmetry environment, the D05→F27 transition is not strictly forbidden, but it is a hypersensitive and forced electric dipole transition. Then, the electric dipole transition D05→F27 at 621 nm with a bright red emission is dominant. The dominance of the D05→F27 emission transition at 621 nm suggests that Eu3+ is located at a non-inversion symmetry position in CGA, basically agreeing with the random crystal environment of Eu3+ ions^[28].

Crystal field splitting for rare-earth ions is usually small as compared to the gaps between J multiplets belonging to a definite LJ term, defined as the LSJ terms (Here the L, S, J represent the orbital quantum number, spin quantum number and total angular momentum quantum number respectively)^[29]. In this case, the splitting lines observed at the peaks at 591, 621, 658, and 701 nm (shown in Fig. 5) can be attributed to the crystal field spectra of Eu3+. The space group for the CaGdAlO4 tetragonal crystals is I4/mmm. The Ca2+ and Gd3+ sites have C4v point symmetry. Crystal field splitting for the following lines is expected from the theoretical analysis: F17 splits into 2 levels (electric dipole transition and magnetic dipole transition), F27 splits into 2 levels (electric dipole transition), F37 splits into 2 levels, and F47 splits into 4 levels, but we can only find 3 levels at 701 nm, for the other is too weak to see. Only one emission peak at 581 nm corresponding to the D05→F07 transition was detected, reflecting that the Eu3+ ions occupy only one kind of crystallographic site^[30], which is also consistent with the results of the cell parameters we calculated in Table 1.

The luminescence decay characteristic of Eu:CGA excited by femtosecond laser pulses (λex=399nm) at room temperature is shown in Fig. 6. The decay curve could be well fitted with a single exponential function, I=I0exp(t/τ) (I0 is the initial intensity at t=0, and τ represents the 1/e lifetime)^[25]. The decay time was calculated to be 1.02 ms, which is a moderate value for oxide materials doped with Eu3+ ions such as CaGd4O7 nanocrystalline phosphors (0.76 ms)^[31], tellurite ceramics (0.93 ms)^[32], zinc-tellurite glasses (0.98 ms)^[33], and CsZnPO4 monophosphate (0.98 ms)^[34]. All these results indicate that Eu3+ doped in a CGA crystal can be an effective luminescence material.

In conclusion, we successfully grow an Eu:CGA single crystal by the FZ method. The XRD, x ray rocking curve, segregation coefficient, polarized absorption spectra, emission spectra, and luminescence decay kinetics are used to characterize the sample. The result of XRD indicates the structural integrity of the Eu:CGA, and the crystal shows a full width at half-maximum as small as 17 arcsec by the x ray rocking curve measurement. Both results reveal that the single crystal we grow is of high quality. The segregation coefficient is estimated to be 0.96, close to 1, which is helpful in improving the efficiency of the fluorescence. From the spectrum, the f–f transitions of Eu3+ in the host lattice are assigned and discussed in detail. The absorption cross sections are calculated to be 1.61×10−20 and 3.17×10−20cm2 at 399 nm for σ polarization and π polarization. Two lines at 599 and 621 nm are observed in the emission spectra of the D05–F71 and D05–F72 transitions, and the latter line is much stronger than the former line, which is due to the random crystal environment of Eu3+ ions in a CGA single crystal. The decay time of Eu:D05 is measured to be 1.02 ms, slightly longer than some other Eu3+-doped media. All the results show that Eu:CGA crystal has good optical characterization and promises to be an excellent red fluorescence material.