Fabrication and optical properties of Er 2-x Yb x Zr 2 O 7 transparent ceramics

: In this study, transparent Er 2-x Yb x Zr 2 O 7 (x = 0-2.0) ceramics were successfully prepared by vacuum sintering at 1850 °C for 6 h. The phase composition, microstructure evolution and optical transmittance of the transparent ceramics were investigated. The powders and ceramics demonstrate a single-phase defective fluorite structure. All the ceramics exhibit excellent optical performance and the highest transmittance can reach to 76% at 1100 nm when x=1.0. The upconversion and infrared emission under 980 nm exciting were measured and discussed as well. The Er 2-x Yb x Zr 2 O 7 (x=0-2.0) transparent ceramics exhibit efficient upconversion photoluminescence emission in the visible wavelength region at 524 nm, 541 nm, 650 nm and downconversion emission at 1525 nm, 1565 nm, 1646 nm.


Introduction
In recent years, pyrochlore transparent ceramic has received a high degree of attention and widespread concern from researchers in many different fields because of its unique and excellent properties. Pyrochlore ceramics exhibit the advantages of high melting point, good chemical stability, good ionic conductivity, low thermal conductivity and high catalytic activity. Therefore, these compounds exhibit various application prospects, such as thermal barrier coatings, solid oxide fuel cells, catalysts, scintillator matrix materials, nuclear waste solidification, solid laser materials and other fields [1][2][3][4][5][6]. The general formula of pyrochlore is A2B2O7, in which A is generally trivalent cation (lanthanide element and Y element) while B is tetravalent cation (usually Zr, Hf, Ti) [7][8][9][10][11]. The structure depends on the radius ratio and temperature of the rare earth cation and anion. Some studies have pointed out that it indicates the disordered defective fluorite structure when rA/rB <1.46 [12]. When 1.46 ≤ rA/rB ≤ 1.78, it shows the ordered pyrochlore structure. It is the monoclinal structure when rA/rB > 1.78. Because the structures of pyrochlore (space group: Fd-3M) and defective fluorite (space group: Fm-3m) are cubic systems, ceramics with these structures can be make transparent by solid-state sintering in vacuum.
The rare earth ions show extremely complex linear spectra. Different matrix materials can make rare earth ions in different spatial distribution, different coordination environment and different transfer efficiency, so that they have different luminescence properties [13]. The Er 3+ ion is an ideal up-conversion activator ion. They have a stepped distribution of energy levels, longer lifetime and appropriate energy level spacing. But Er 3+ ion has poor absorption capacity and small absorption cross section. Therefore, the conversion rate of Er 3+ ions is usually improved by adding sensitizers [14]. Yb 3+ is an ion that has only one excited energy level. In the case of high concentration doping, it has a certain tolerance and will not decrease the luminescence intensity due to the cross relaxation process [15]. At the same time, Yb 3+ ion has no excited state absorption and is beneficial to the application of high density excitation. The combination of Er 3+ and Yb 3+ enhances the emission intensity of Er 3+ due to the high absorption cross section of Yb 3+ and Eb 3+ , and the energy is effectively transferred from Yb 3+ to Er 3+ . Therefore, Yb 3+ -Er 3+ co-doped materials have attracted much attention. In recent years, researchers have made many explorations such as SrLaGa3O7:Er/Yb, NaBiF4:Yb/Er, β-NaYF4: Er/Yb, Yb/Er:YAG, Y2Sn2O7:Yb 3+ /Er 3+ [16][17][18][19][20].
Optical elements with high doping density and morphology can be obtained by ceramic technology. Ji et al. have successfully prepared transparent La2Hf2O7 ceramic with maximum in-line transmittance higher than 70% by vacuum sintering [21]. Wang et al. have successfully prepared La2−xLuxZr2O7 (x = 0-2.0) transparent ceramics by solid-state reactive sintering in vacuum [22]. It can be found that phase transition from pyrochlore to defective fluorite occurred with the increase of Lu content (x). Zhao et al. have successfully synthesized La2-xGdxZr2O7 (x=0-2.0) transparent ceramics [23]. The x = 1.0, 1.2 and 1.6 samples exhibit high optical transmittance in the 450 nm-600 nm range and the La0.4Gd1.6Zr2O7 sample shows the highest transmittance of 83.84%. The transparent Gd2-xNdxZr2O7 (x=0.4-2.0) ceramics were successfully prepared by Li et.al, in which the highest transmittance can reach to 78% at 1000 nm when x = 0.4 [24]. The aim of this work is to fabricate Er2-xYbxZr2O7 (x=0.4-2.0) transparent ceramics with high optical quality. We attempt to explore the effects of Er-Yb with different contents on the luminescence performance of ceramics and the possibility of its use as laser material and upconversion material. The preparation technique, morphology, crystal structure and transmittance of the powders and final transparent ceramics were investigated.

Fabrication of Er2-xYbxZr2O7 (x=0.4-2.0) transparent ceramics
Erbium nitrate (Er(NO3)3·6H2O, 99.9%, Aladdin industrial Co. ltd.), Ytterbium nitrate (Yb(NO3)3·6H2O, 99.9%, Aladdin industrial Co. ltd.), Zirconium nitrate (Zr(NO3)4·5H2O, 99.9%, Aladdin industrial Co. ltd.) and glycine were employed as the raw reactants. Firstly, the stoichiometric amounts of the four ingredients were dissolved in deionized water and stirred to clarified state. Appropriate amount of ammonia was slowly added to the mixture until the pH was adjusted to 4. Then, the solution was heated in a water bath until the solution changed to a sol-gel state. After that, it was transferred to a muffle furnace and then calcined at 300°C in air to obtain fluffy powder. Subsequently, the primary fluffy powders were calcined at 1200°C for 2 h and then ball milled in deionized water for 24 h with water as the medium. The ball milled slurry was sieved, dried and preformed into thin discs. Finally, the samples were vacuum sintered at 1850°C for 6 h and annealed at 1500°C for 5 h in air. The obtained ceramic samples were double-side polished to a thickness of 1 mm for subsequent tests.

Characterizations
The phase composition and crystal structure of the calcined powders and final ceramic samples were measured by X-ray diffractometer (XRD, X' Pert PRO, PANalytical B.V., Netherlands). The thermal-etched surface and the fracture surface of transparent ceramics were observed by scanning electron microscopy (SEM, Quanta 450F, FEI, USA), where the back-scattered electron (BSE) mode was conducted to analyze the grain sizes. The elemental distribution was measured using energy dispersive X-ray spectrometer (EDX) attached with the SEM equipment. The transmittance spectra of final ceramics were measured by solid ultraviolet-visible spectrophotometer (UV-VIS, Solidspec-3700, Shimadzu, Japan). The luminescence spectra were characterized at room temperature by the OMN15015i spectrometer (Zolix, China), iHR 320 spectrometer (Jobin-Yvon, France) and 980 nm laser diode (LD, LEO photoelectric, China). -xYbxZr2O7 (x=0-2.0)  Subramanian's theory, all the ceramics exhibit the defective fluorite structure [12].   Fig. 5(a) are further characterized using EDX elemental spotting analysis with the results listed in Fig. 5(b). The Er/Yb ratio of two points are 1.07:1 and 1:1.09, which all are close to 1:1. Studies on LaLuZr2O7 ceramics prepared by Z.J. Wang and La1.28Er1.28Zr2O7 ceramics prepared by W.W. Zhao found that differences in atomic radius ratio and element content resulted in the formation of both defective fluorite and calcite phases [27,28]. Two different phases are obviously observed in different grayscale. However, ErYbZr2O7 ceramics have a single missing fluorite phase, so two different grayscales have nothing to do with the phase structure. It may be because Er and Yb have different atomic radii, which make it impossible for the elements to be evenly distributed in ErYbZr2O7 ceramics. Moreover, the enrichment of Er or Yb elements is small due to the close atomic radii of Er and Yb.

Optical performance analysis
The illustration in Fig. 6 shows the macroscopic photos of Er2-xYbxZr2O7 (x=0-2.0) transparent ceramics. All the samples exhibit excellent transmittance in the visible region and the text under the samples can be clearly seen. In-line optical transmittances of Er2-xYbxZr2O7 ceramics were tested with the results shown in Fig 6. It should be mentioned that all the samples exhibit near infrared transmittance higher than 70%, where the highest transmittance can reach to 76% at 1100 nm when x=0. The sample shows the lowest transmittance reach to 70% when x = 0.4, which may be due to the presence of pores. Many absorption peaks can also be observed in the transmittance curve, which are located at the wavelength range from 200 nm to 2000 nm. The x=2.0 sample exhibit absorption peaks at 913 nm and 980 nm, which are related to the level excitation of Yb 3+ from 2 F7/2 to 2 F5/2 in 4f-4f. Absorption bands lying at 350-1000 nm wavelength range correspond to transition from 4 I15/2 ground level of Er 3+ ions to the 2 H9/2, 4 F7/2, 2 H11/2 and 4 S3/2 upper levels, while those at 1400-1650 nm correspond to 4 I15/2 → 4 I13/2 transitions [29][30][31][32][33].
Efficiency of Yb 3+ →Er 3+ resonant energy transfer during sensitized luminescence strongly depends on the concentration ratio of involved ions. Comparison of luminescence intensity of some spectral lines in ceramic samples containing different Yb/Er ratio allows us to study the influence of Er 3+ concentration on luminescent properties of ceramics. Fig. 7 shows the upconversion luminescence (UCL) spectra of Er2-xYbxZr2O7 (x=0-2.0) transparent ceramics in the 500-750 nm region under 980 nm excitation. All spectra show two characteristic bands of Er 3+ in 500-750 nm region. The bright green and red luminescence of the samples can be observed. The peaks in the green region of 510-540 nm and 544-580 nm are attributed to the 2 H11/2 → 4 I15/2 and 4 S3/2 → 4 I15/2 transitions of Er 3+ ions while the peaks in the red region of 625-720 nm can be assigned to the 4 F9/2 → 4 I15/2 transition [34,35]. And the intensity of red luminescence is significantly higher than that of green luminescence. It is noted that the emission spectra of x=0 and x = 2.0 samples present no peaks in upconversion luminescence spectrum. The x = 1.0 and x = 1.6 samples demonstrate basically the same peak shape and peak strength in green area. The x = 1.6 sample exhibits the highest luminescence intensity in the red region. intensities at 1525 nm in the sample of x = 1.0 is lower than in that the sample of x=1.6. And the fluorescence intensity gradually increases as x increases from 1.0 to 1.6. It shows the highest luminescence intensities at x=1.6, which is due to the increase of Yb 3+ concentration shortens the average distance between Yb 3+ and Er 3+ ions. Broad emission bandwidths are advantageous for tunable laser output and make Er0.4Yb1.6Zr2O7 transparent ceramic a promising 1.5 μm solid-state tunable laser media. Fig. 9 shows the energy level diagram of Er 3+ /Yb 3+ ions and the possible energy transfer fluorescence mechanism of the ceramics. The electron of Yb 3+ ions jumping from ground level ( 2 F7/2) to excited level ( 2 F5/2) when the Er2-xYbxZr2O7 (x =0-2.0) transparent ceramics were pumping by 980 nm laser. Then, energy transfer occurs between Yb 3+ ions and nearby Er 3+ ions due to the electron populated in 2 F5/2 is not stable. The excited states of Er 3+ for upconversion emission include energy transfer (ET) and ground/excited state absorption (GSA/ESA). The specific process is as follows: the Yb 3+ ions at 2 F7/2 state undergo a transition to its 2 F5/2 state by absorbing one 980 nm photon, release energy in the form of photon, adjacent Er 3+ capture this photon and jump from 4 I15/2 to 4 I11/2 simultaneously. On the one hand, multi-phonon relaxes from 4 I11/2 to 4 I13/2 state. After receiving the energy equal to a 980 nm photon from the Yb 3+ ions, the Er 3+ ions jump from 4 I13/2 to 4 F9/2. On the other hand, the Er 3+ ions populated in 4 I11/2 capture another photon from Yb 3+ ion jump to the 4 F7/2 level state with more higher energy. However, electrons located in excited 4 F7/2 state is not stable. And part of energy would be lost by non-radiative relaxation as the electron jumped to 4 S3/2, 2 H11/2, 4 F9/2. Radiative transition of 2 H11/2/ 4 S3/2 → 4 I15/2 of Er 3+ ions generate green UCL emissions of the ceramics. The electrons populated in 4 F9/2 would jump to ground state of 4 I15/2 and release energy by emitting red light.
Radiative transition of 4 I13/2 → 4 I15/2 generate near infrared emission at around 1526 nm, which is useful in many fields, especially for near infrared laser applications and optical communication.

Conclusions
Er2-xYbxZr2O7 (x=0-2.0) transparent ceramics were successfully fabricated by vacuum sintering at 1850°C for 6h. All the ceramics exhibit the defective fluorite structure and the grain size is about 100 μm. All the samples are transparent and the highest in-line transmittance is 76% when x=1.0. The ceramics exhibite green and red upconversion emission, as well as broad-bandwidth near-infrared emission. The intensity of red luminescence is significantly higher than that of green luminescence. This study confirms that the Er2-xYbxZr2O7 (x =0-2.0) transparent ceramics are promising optical material with efficient visible upconversion and downconversion photoluminescence properties.