Structural and Spectroscopic Features of the Bixbyite-Type Yttrium Scandate Doped by Rare-Earth Ions

: Yttrium scandate crystal ﬁber has been obtained through laser-heated pedestal growth. The crystal belongs to a bixbyite crystal structure and crystallizes in Ia 3 space group. X-ray diffraction method shows a lattice parameter of a = 10.228(1) Å. Factor-group analysis of YScO 3 Raman spectra points to high degree of disorder in crystal structure of the new compound. Spectral-kinetic investigation of the crystal ﬁbers doped by rare-earth ions points to the presence of two independent active optical centers of rare-earth ions. Moreover, the character of rare-earth impurities’ distribution is independent on a rare-earth ionic radius size.


Introduction
In recent years, there has been a global tendency to replace radio communication lines within satellite constellations, as well as between satellites and ground objects [1].There is a fundamental problem of searching for an active laser medium for use in atmospheric and space optical communication systems (FSO).To organize space or long-range atmospheric optical communication, laser systems with high energy characteristics and, therefore, laser media that can withstand extreme optical, radiational and thermal loads are needed.
High melting rare-earth-doped sesquioxides can be promising materials for FSO applications.Due to the disordered crystal structure, rare-earth-doped mixed sesquioxides (such as yttrium scandate YScO 3 ) have inhomogeneously broadened spectral lines [2,3] which provides an advantage for its highly efficient pumping with modern diodes with low requirements for their thermal stabilization.Instead of the Czochralski method [4,5], we propose the use of the laser-heated pedestal method, which is more suitable for the growth of the high-melting crystals of good optical quality and small size required for use in equipment.In this approach, crystal synthesis is accompanied by high-temperature heating with laser radiation and subsequent rapid cooling.
Yttrium scandate has a polymorphic nature.Its high-temperature modification [24,25], similar to yttria [26] and scandia [27], belongs to a bixbyite structural type with the space group Ia3.The crystal structure includes one site of oxygen ion with local symmetry Crystals 2022, 12, 1745 2 of 8 C 1 (Wykoff position 48e) and two symmetrically independent sites of cations with local symmetry C 3i (Wykoff position 8b) and C 2 (Wykoff position 24d).The ionic radii of yttrium (0.9 Å), scandium (0.83 Å) and rare-earths are close, and the substitution of basic cations by rare-earth impurities does not require charge compensation and it is crystallochemically unforbidden.
Here we have synthesized and characterized the YScO 3 crystal fibers, pure and doped by 0.1 at.% Nd 3+ and Tm 3+ ions.Nd 3+ and Tm 3+ ion situate in different ends of lanthanoid group and have a maximal different ionic radius.Nd 3+ belongs to large rare-earth ions while Tm 3+ is a small ion.Moreover, Nd 3+ is a Kramers ion with an odd number of electrons, and Tm 3+ belongs to non-Kramers ions, with an even number of electrons.Therefore, Nd 3+  and Tm 3+ are maximally different ions and would presumably have different spectral behavior, which is related to the local structure of optically active centers.We have used selective laser spectroscopy methods, in which the luminescence spectra are recorded at selective excitation and, conversely, the excitation spectra are measured at selective detection.This approach allows us to trace local changes at included ion-activators.In this work, we have focused on studying the influence of rare-earth ion radius size on spectroscopic properties and the character of rare-earth impurities' distribution.

Results and Discussion
To grow the crystal fiber, high pure powders of scandium and yttrium oxides have been mixed in an agate mortar up to a homogeneous state.The mixture of Y 2 O 3 , Sc 2 O 3 and RE 2 O 3 (Tm 2 O 3 or Nd 2 O 3 ) was used as precursor to grow RE-doped crystal fibers.Then, the mixture has been transferred to a platinum crucible and annealed at a temperature of 1100 • C for 48 h.The powder rod of Y 2 O 3 -Sc 2 O 3 composition has been obtained at the pressure of 20 MPa.The powder rod had length of 50 mm and diameter of 1 mm.
The rod was fixed vertically, and its tip was heated and melted at CO 2 laser irradiation.An yttria-stabilized zirconia (YSZ) crystal was used as a seed.It was dipped into the melt and raised slowly, pulling a crystal fiber with an approximately round cross-section from the melt.At a growth rate of about 80 mm/h, a YScO 3 crystal fiber with a diameter of 0.85 mm and length of 50 mm has been obtained.
The image in back-scattered electrons, the brightness of which depends on the average atomic number and the density of the substance, demonstrates the homogeneity of the crystal composition (Figure 1a).According to X-ray spectral analysis (SEM-EDS) the amounts of Sc and Y are almost equal and unchanging across the fiber (Figure 1b).Therefore, the crystal composition may be presented as Y 0.95 Sc 1.05 O 3 , while taking into account three oxygen atoms per formula unit.
Structural characteristics of the crystal fiber, as well as powders of Y 2 O 3 and Sc 2 O 3 precursors, have been obtained with the X-ray phase analysis.The X-ray pattern of YScO 3 includes good narrow peaks that point to a high degree of crystallinity (Figure 1c).According to X-ray data, the YScO 3 crystal fiber is homogeneous and belongs to the bixbyite structural type (space group Ia3), similar to yttrium and scandium sesquioxides (Figure 1c, insert).The lattice parameters of YScO 3 , Y 2 O 3 and Sc 2 O 3 were obtained as a (YScO 3 ) = 10.228(1)Å, a (Y 2 O 3 ) = 10.604(1)Å and a (Sc 2 O 3 ) = 9.841(4) Å, respectively.Due to the low concentration of Nd 3+ and Tm 3+ ions in the YScO 3 host (about 0.1 at.%), RE impurity does not influence bixbyite-type crystal structure, phonon structure and crystal quality.XRD (Supplementary Materials Figure S1, Table S1) and Raman spectra (Figure S2) of the pure and rare-earth doped YScO 3 crystal fibers have been added as Supporting Information.Table 1 shows indexation of the most intensive peaks.Structural characteristics of the crystal fiber, as well as powders of Y2O3 and Sc2O3 precursors, have been obtained with the X-ray phase analysis.The X-ray pattern of YScO3 includes good narrow peaks that point to a high degree of crystallinity (Figure 1c).According to X-ray data, the YScO3 crystal fiber is homogeneous and belongs to the bixbyite structural type (space group Ia3), similar to yttrium and scandium sesquioxides (Figure 1c, insert).The lattice parameters of YScO3, Y2O3 and Sc2O3 were obtained as a (YScO3) = 10.228(1)Å, a (Y2O3) = 10.604(1)Å and a (Sc2O3) = 9.841(4) Å, respectively.Due to the low concentration of Nd 3+ and Tm 3+ ions in the YScO3 host (about 0.1 at.%), RE impurity does not influence bixbyite-type crystal structure, phonon structure and crystal quality.XRD (Supplementary Materials Figure S1, Table S1) and Raman spectra (Figure S2) of the pure and rare-earth doped YScO3 crystal fibers have been added as Supporting Information.Table 1 shows indexation of the most intensive peaks.The micro-Raman spectra have been obtained for Y 2 O 3 and Sc 2 O 3 powders and the YScO 3 crystal fiber (Figure 1d).
The unit cell of the bixbyite-type yttrium scandate includes 40 atoms (8 formula units).Therefore, factor-group analysis leads to the following distribution of the 120 phonon modes in the Brillouin zone point Γ = 0: After subtracting 1F u of the acoustic modes there remain 16F u of the infra-red active, (4A g + 4E g + 14F g ) of the Raman-active and (5A u + 5E u ) of the optically inactive phonon modes.In the Sc 2 O 3 → YScO 3 → Y 2 O 3 series, we observed "mass effect" when vibrational modes are shifted to the range of lower frequencies with increasing cation mass.The spectral lines of yttrium scandate are broadened due to the presentence of both Y 3+ and Sc 3+ cations in statistically equal structural positions.Due to partial overlap, a number of experimental lines are lower than predicted with factor-group analysis.Table 2 shows identification of the Raman-active modes [28].Time-resolved luminescence excitation spectra of Nd 3+ :YScO 3 have been obtained for 4 I 9/2 → 4 F 5/2 + 2 H 9/2 electro-dipole transitions in the Nd 3+ ion at selective detection of 896 nm ( 4 F 3/2 → 4 F 9/2 transition) at a temperature of 77 K (Figure 2a).Two intensive peaks are observed at 821. 4  Time-resolved luminescence excitation spectra of Nd 3+ :YScO3 have been obtained for 4 I9/2 → 4 F5/2 + 2 H9/2 electro-dipole transitions in the Nd 3+ ion at selective detection of 896 nm ( 4 F3/2 → 4 F9/2 transition) at a temperature of 77 K (Figure 2a).Two intensive peaks are observed at 821.4 and 823.8 nm.The ratio of line intensities depends on delay in registration.Figure 2b shows the luminescence emission spectra of Nd 3+ ions being measured in the 4 F3/2 → 4 I9/2 electron transitions at 77 K. Nd 3+ ions have been excited at 823.6 and 821.4 Figure 2b shows the luminescence emission spectra of Nd 3+ ions being measured in the 4 F 3/2 → 4 I 9/2 electron transitions at 77 K. Nd 3+ ions have been excited at 823.6 and 821.4 nm, which corresponds to a maximum of peaks being observed in luminescence excitation spectra.
Analysis of the spectra allows us to distinguish two independent optical centers of Nd 3+ ions.The luminescence kinetics have been recorded on the 3 F 3/2 → 4 I 9/2 electron transition at two different excitation wavelengths (823.6 and 821.4 nm).In both cases, decay curves are described by exponential law with lifetimes of τ 1 (77 K) = 290 µs (at 823.6 nm excitation) and τ 2 (77 K) = 250 µs (at 821.4 nm excitation).The close lifetimes and relatively equal intensities of the spectral lines obtained from two different centers indicate the same C 2 symmetry in both cases.As is known [29], electro-dipole transitions are forbidden for rare-earth ion occupied centro-symmetric positions, in particular, the C 3i position in the bixbyite structure.The luminescence for these centers must be non-active.We assume that both of the centers have local symmetry of C 2 .Probably, one of them forms as a result of the local substitution of the Y 3+ cation by an Nd 3+ ion, and another one is a result of the local substitution of the Sc 3+ cation by an Nd 3+ ion in the YScO 3 host.
The Tm 3+ ion has the same behavior in Tm 3+ :YScO 3 .Luminescence excitation spectra of Tm 3+ :YScO 3 have been obtained for of 3 H 6 → 3 H 5 magneto-dipole transitions in the Tm 3+ ion at selective detection of 1945 nm ( 3 F 4 → 3 H 6 transition) at a temperature of 77 K (Figure 2c).Two intensive peaks are observed at 1202.4 and 1209.4 nm.Ratio of line intensities depends on delay in registration in time-resolved spectra.This indicates the presence of two optical centers of Tm 3+ ion with two different lifetimes.
Figure 2d shows luminescence spectra of Tm 3+ ions being measured in the 3 F 4 → 3 H 6 electron transition at selective excitation of 1202.4 and 1209.4 nm.The luminescence kinetics have been recorded on the 3 F 4 → 4 H 6 electron transition at two different excitations of 1202.4 and 1209.4 nm.In both cases, decay curves are described by exponential law with lifetimes of τ 1 (77 K) = 4.25 ms (at 1202.4 nm excitation) and τ 2 (77 K) = 3.7 ms (at 1209.4 nm excitation).The close lifetimes and relatively equal intensities of the spectral lines obtained from two different centers indicate the same C 2 symmetry in both cases.One of Tm 3+ optical center forms as a result of the local substitution of the Y 3+ cation by a Tm 3+ ion, and another one is a result of the local substitution of the Sc 3+ cation by a Tm 3+ ion in the YScO 3 host.

Materials and Methods
To grow the YScO 3 crystal fiber, the laser heated pedestal growth (LHPG) method has been used [25,30,31].This approach allows us to synthesize crystal in the form of a fiber and to obtain high-melting materials and compounding with disordered crystal structure.To prepare RE 2 O 3 −Y 2 O 3 −Sc 2 O 3 powder, preform scandium, yttrium and rare-earth (Nd 3+ or Tm 3+ ) oxides (purity of more than 99.999%, Sigma Aldrich, Burlington, MA, USA) were well mixed in a stoichiometric proportion (of mole ratio RE 2 O 3 /Y 2 O 3 /Sc 2 O 3 = 0.1:99.9:100) in an agate mortar to form a homogeneous mixture.The mole ratio of 1:1 was used to prepare undoped Y 2 O 3 −Sc 2 O 3 powder preform.Then, the mixture was transferred to a platinum crucible and annealed at 1100 • C for 48 h in an air atmosphere to exclude the presence of H 2 O.The final powder mixture was pressed at 20 MPa to form a rod with about 1 mm diameter and 50 mm length.
Visualization and chemical composition analysis of the YScO 3 crystal fiber were carried out on the EVO 10 scanning electron microscope (Zeiss Microscopy GmbH, Jena, Germany).The samples were examined at an accelerating voltage of 20 kV with a working distance of 9.0 mm in low vacuum operation (EP = 50 Pa) using a LaB6 cathode.
To obtain SEM imaging of the sample, part of the crystal fiber was placed onto a carbon tape.The observations of the sample were carried out at a beam current of 85 pA.The images were captured in backscattered electron mode (BSE) to assess sample homogeneity (Figure 1a).
The semi-quantitative estimation of the main cation (Y 3+ and Sc 3+ ) ratio was based on the data of the Smart METEK EDX spectrometer.Scanning was carried out in line-scan mode at a step of 2 µm and a beam current of 149 pA.
A part of the crystal fiber has been ground in an agate mortar and characterized with X-ray phase analysis.The X-ray pattern has been recorded on the Bruker D8 Discover A25 DaVinci Design diffractometer in the range of 2θ = 15-80 • with a step of 0.02 • and an exposition time of 1.0 s.The Siemens KFL X-ray tube with Cu Kα (U = 40 kV, I = 40 mA, λ = 0.154 nm) irradiation has been used as the X-ray source.The spectra have been resolved with the EVA 2.1 software and identified with the PDF-2 2011 database.The TOPAS 4.2 software has been used to index spectral peaks and to determine lattice cell parameters.
Micro-Raman spectra of the YScO 3 crystal fiber and precursor (Y 2 O 3 and Sc 2 O 3 ) powders have been recorded on the EnSpectr R532 express analyzer equipped with the Olympus microscope (40× objective) at a laser excitation of 532 nm.The laser beam has been focused on a core of the fiber.Each spectrum has been obtained as the difference between the spectrum taken with the laser running and the dark spectrum.The subtraction of the first spectrum from the second one was carried out automatically using the EnSpectr Pro software.The final Raman spectra were obtained by averaging 20 runs acquired during the total time of 90 s (4.5 s for a run).
Spectral-kinetic characteristics of the Nd 3+ -and Tm 3+ -doped YScO 3 crystal fibers have been obtained with the selective spectroscopy method.Luminescence spectra have been recorded on the MDR-23 monochromator.The Hamamatsu R5108 и FEU-83 photomultipliers have been used as detectors.The LQ529B-LP 604 (Solar LS, Belarus) laser complex has been used as the excitation source.

Conclusions
A YScO 3 crystal fiber has been obtained through laser-heated pedestal growth (LHPG).According to X-ray spectral analysis (SEM-EDS), the amounts of Sc and Y are almost equal and unchanging across the fiber.The YScO 3 crystal fiber is homogeneous and belongs to the bixbyite structural type (space group Ia3).The factor group analysis predicted 22 Ramanactive phonon modes.However, due to partial overlap, a number of experimental lines are lower than predicted with factor-group analysis.Luminescence excitation and luminescence spectra of Tm 3+ :YScO 3 and Nd 3+ :YScO 3 show the presence of two active centers of the rare-earth ions in both cases.We suppose that one of rare-earth optical centers forms as a result of the local substitution of the Y 3+ cation by an RE 3+ ion, and another one is a result of the local substitution of the Sc 3+ cation by an RE 3+ ion in the YScO 3 host.Therefore, the nature of the inclusion of a rare-earth impurity into the YScO 3 bixbyite-type crystal structure does not depend on the ionic radius size of rare-earths.

9 Figure 1 .
Figure 1.(a) An image of the YScO3 crystal fiber captured in backscattered electron mode (BSE).A scalebar is 0.5 mm; (b) Stoichiometric coefficients of Y 3+ (blue one) and Sc 3+ (red one) ions calculated based on three oxygen atoms per formula unit; (c) The X-ray pattern of the YScO3 crystal fiber (green one) and Y2O3 and Sc2O3 precursors (blue and red ones, respectively).Insert: bixbyite-type crystal structure of YScO3.Symmetrically diverse MO6 polyhedra both centered by statistically distributed Y 3+ and Sc 3+ ions are shown as orange (C3i site) and blue (C2 site); red balls are O atoms; (d) Raman spectra of Y2O3 and Sc2O3 precursors and YScO3 crystal fiber (blue, red and green lines, respectively).

Figure 1 .
Figure 1.(a) An image of the YScO 3 crystal fiber captured in backscattered electron mode (BSE).A scalebar is 0.5 mm; (b) Stoichiometric coefficients of Y 3+ (blue one) and Sc 3+ (red one) ions calculated based on three oxygen atoms per formula unit; (c) The X-ray pattern of the YScO 3 crystal fiber (green one) and Y 2 O 3 and Sc 2 O 3 precursors (blue and red ones, respectively).Insert: bixbyite-type crystal structure of YScO 3 .Symmetrically diverse MO 6 polyhedra both centered by statistically distributed Y 3+ and Sc 3+ ions are shown as orange (C 3i site) and blue (C 2 site); red balls are O atoms; (d) Raman spectra of Y 2 O 3 and Sc 2 O 3 precursors and YScO 3 crystal fiber (blue, red and green lines, respectively).

Table 1 .
Miller indices (hkl), interplanar distances d, Bragg angles 2θ and Bragg peak intensities I of the Y 2 O 3 , Sc 2 O 3 and YScO 3 with a bixbyite-type structure.
and 823.8 nm.The ratio of line intensities depends on delay in registration.