Incorporation of the Ce3+ activator ions in LaAlO3 crystals: EPR and NMR study

This work reports the results of EPR and NMR study of the Ce3+ incorporation in LaAlO3 single crystals grown by the micro-pulling-down method in the range of the Ce concentrations from x=0.1 at.% up to 100 at.%. From EPR measurements, Ce3+ g tensor parameters were obtained as function of Ce concentration. The g tensor has orthorhombic symmetry even in the trigonal phase (x<10 at.%) suggesting that the incorporation of Ce at La site lowers lattice symmetry near this ion. The local properties of the La1-xCexAlO3 crystals were further studied by 27Al and 139La high-resolution NMR measurements. It was found that 139La chemical shift has the Fermi contact interaction origin. It linearly increases with Ce concentration up to 165 ppm at x = 0.5. Due to this strong Fermi contact interaction, separated peaks corresponding to different Ce-O-La spin transfer passways are resolved in the 139La NMR spectra. On the other hand, no Fermi contact interaction is visible in 27Al NMR spectra. However, these spectra contain satellite peak which intensity linearly increases with increase of Ce concentration leaving position of this peak unchanged. This was interpreted as manifestation of crystal structure modification in vicinity of Ce ions in agreement with EPR data. Thus, optical properties of Ce3+ in LaAlO3 will be namely determined by the local crystal structure near this ion.


Introduction
The search of new materials with heavy ions suitable for applications as scintillators is an important topic of today physics and chemistry. Presently scintillators have been employed in high-energy physics, environmental monitoring, industrial defectoscopy, geological survey and oil well logging, astronomy and others [1,2]. One of the promising class of such scintillation materials belongs to aluminium perovskites [3]. The most known example of the perovskite scintillators is the Ce-doped YAlO3. However, it has low density and effective atomic number of 5.37 g/cm 3 and 33.5, respectively, and the resulting low stopping power for high-energy photons is the major disadvantage of this material.
In this respect, LuAlO3 (LuAP) perovskite with much higher density 8.4 g/cm 3 and effective atomic number Zeff=64.9 could be highly promising material. However, due to a severe instability of the perovskite phase in the process of crystal growth from the melt, resulting in a high price for the crystals, mixed (Lu0.7Y0.3)AP:Ce scintillators were chosen for industrial-scale production [4].
Another perovskite material with also high density is the compositional analogue LaAlO3 (LaAP), whose crystal growth is much easier due to almost ideal value of Goldschmitt tolerance factor [5]. Its growth technology [6] is well established as this crystal extensively uses as substrate for thin films epitaxial growth [7]. There are a number of papers which devoted optical absorption and luminescence of Ce-doped LaAlO3, e.g. [6,8,9,10]. While some crystals with low Ce concentration (up to 1 at.%) show characteristic Ce 3+ 4f-5d transition emission [6,8], in general, the radioluminescence efficiency of the Ce-doped LaAP crystals was found to be very low. This was explained as due to strong ionization of the Ce 3+ 5d excited states which are all located in the conduction band [11].
The most studied rare-earth impurity in LaAlO3 is Gd 3+ which Electron paramagnetic resonance (EPR) spectra were measured and analyzed in details in both single crystals [12] and powders [13].
This ion substitutes for La at simple trigonal symmetry. LaAlO3 and GdAlO3 create Gd1-xLaxAlO3 solid solution where the band gap gradually decreases with increase of La content [14]. LaAlO3 also creates similar solid solution with CeAlO3 [15]. However, to the best of our knowledge, Ce 3+ EPR spectrum was never measured in LaAlO3 crystals or even in the La1-xCexAlO3 solid solution in spite that in other aluminum perovskites (e.g. YAlO3, LuAlO3) the Ce 3+ EPR is well visible [16,17]. Because the optical properties of Ce-doped LaAlO3 are still not elucidated we decided to perform detailed investigations of Ce 3+ EPR in the La1-xCexAlO3 solid solutions in the range of Ce concentration from x=0.001 up to x=1.0.
Incorporation of Ce ions into LaAlO3 lattice was also studied via measurements of 139 La and 27 Al solidstate Nuclear magnetic resonance (NMR). NMR method is of particular useful in quantification of the site occupancy of atoms in a material and in study of chemical bonds. In particular, NMR showed that the Ce 3+ spin transfer (and thus charge transfer) to La 3+ is much stronger than to Al 3+ . Both EPR and NMR also suggest LaAlO3 crystal structure transformation in vicinity of Ce ions even at low their concentration in the trigonal phase.
However, as suggested in [18], a small amount (below the X-ray diffraction sensitivity) of other aluminate phases is not excluded. Their presence was confirmed by scanning electron microscopy for some of samples studied in [18].
EPR measurements were performed in the X-band (9.4 GHz) with a commercial Bruker EMX spectrometer at the temperatures 6-296 K. 27 Al static and magic-angle spinning (MAS) NMR spectra and 139 La MAS NMR spectra of powdered samples were recorded at room temperature in the magnetic field of 11.75 T using a wide-bore Bruker Avance III HD spectrometer ( 27 Al Larmor frequency ≈ 130.4 MHz, 139 La Larmor frequency ≈70.7 MHz), with a nonselective single-pulse excitation. The samples were packed into ZrO2 rotors of 2.5 mm in diameter and were rotating at 20 kHz MAS speed.
LaAP at T > 813 K has simple cubic perovskite structure belonging to the Pm-3m space group [19]. Below TC = 813 K, it undergoes phase transition to the rhombohedral R-3c structure related to rotation of the oxygen octahedrons around one of the [111] trigonal axes of the cubic phase through the same angle φ [19], with half of the polyhedral rotating clockwise around the trigonal axis, and half rotating anticlockwise (Fig. 1). The rotation angle is quite large below 50 K, it reaches the value of 5.7 0 . There is only one crystallographic site (6a sites) for La cations. Ce 3+ substituting for La 3+ has respectively also one crystallographic position in the lattice as illustrated in Fig. 1. This ion has outer electronic shell 4f 1 and electron spin S = 1/2.
In contrast to LaAlO3, the sequence of the reversible phase transitions I4/mcm ↔ Imma ↔ R-3c ↔ Pm-3m has been detected in CeAlO3 and solid solutions formed in the pseudo-binary system CeAlO3−LaAlO3 [15]. Below approximately 10 K, where the Ce 3+ EPR spectra can be measured, the lattice symmetry in the (1-x)LaAlO3−xCeAlO3 solid solutions changes from the trigonal R-3c to orthorhombic Imma and finally to the tetragonal I4/mcm as the CeAlO3 contents increases to approximately x=0.5 [15].

Ce 3+ EPR spectra in LaAP:Ce
As an example, Fig. 2 presents EPR spectra measured in selected LaAP:Ce crystals at different temperatures. At low Ce concentration (0.001Ce and 0.005Ce; 0.1% and 0.5%, respectively) the EPR spectra mainly contain narrow Gd 3+ resonances, which spectral parameters are well studied in LaAP [12,13]. The expected for these Ce concentrations very strong and narrow Ce 3+ spectral lines are not visible. Only in the crystal with the Ce concentration 0.5% two characteristic but very broad lines appear below 9 K (Fig. 2), which could belong to Ce 3+ , in spite that Gd 3+ lines are narrow, indicating good crystalline quality of the crystal. The Ce 3+ spectral lines are thus expected to be very narrow like, for instance, in YAlO3 [16], where the Ce 3+ linewidth is only 13-20 G for the same Ce concentration. This fact speaks in favor that Ce 3+ resonance lines are essentially broadened by the magnetic dipole interaction between spins located close to one other but are not broadened by crystal imperfections and, thus, the Ce ions do not homogeneously distributed in crystal but prefer to create Ce-rich regions.
Further increase of the Ce concentration leads to substantial increase of the Ce 3+ EPR intensity without marked change in linewidth (LaAP:0.05Ce sample, Fig. 2).
A drastic change in the Ce 3+ spectrum is seen at the Ce concentration of 20% (LaAP:0.2Ce sample, Fig. 2). The spectrum is split into many components which are sited on a very broad line at 3-7 kG magnetic field region. This corresponds to change of the crystal structure of the La1-xCexAlO3 solid solution from the rhombohedral R-3c to the tetragonal Imma phase at x>0.1 [15]. Numerous EPR transitions from Ce 3+ -Ce 3+ dimers appear as well that makes the spectrum analysis practically impossibly. Because our LaAP:Ce crystals were not singledomain, it was problematically correct determination of the Ce 3+ g factors from EPR spectra measured in crystals only. Therefore, EPR spectra were also measured in powders of grinded crystals. The EPR spectra measured in the grinded crystals for Ce concentration from x=0.005 up to x=0.5 are shown in Fig. 3a. In powders, EPR spectrum is averaged over all random orientations of crystallites. Only characteristic features (peaks) are presented in spectra at the fields corresponding to g-factors principal values [20] as it is well seen in the spectrum for LaAP:0.05Ce sample and its simulation (Fig. 3a). The simulation was performed by using the computer program "Powder" [21] and the spin Hamiltonian for a paramagnetic ion with the spin S = 1/2: where B is magnetic field, g is g tensor with three principal values g1, g2, and g3, S is the spin operator with three components along g tensor principal axes. The program "powder" uses bispiral approach to obtain directions homogeneously distributes in sphere. For the search of resonance magnetic fields and transition probabilities for reference directions, exact diagonalization of spin Hamiltonian matrices is used [21].
It is reasonable to assume that one of the principal axis (corresponding to g3 value) is oriented along the trigonal axis for the samples with low Ce concentration (R-3c symmetry) in analogy with Ce 3+ EPR in YAlO3 crystal [22], which crystal structure is very close to that of LaAlO3. It must rotate to the [001] tetragonal axis for samples with high Ce concentration according to the I4/mcm symmetry at x ≈ 0.5. But the explicit orientations of the principal axes could not be determined from our data due to multidomain structure of our crystals. Determined g factors values for all studied compositions are listed in Table I. One can see that the Ce 3+ spectrum even in the trigonal phase (x = 0.01 −0.1) is described by three non-equal g factor components that corresponds to the orthorhombic local symmetry at Ce ion, i.e. 6 incorporation of Ce at La site lowers lattice symmetry in vicinity of the Ce ion. This is in contrast with Gd 3+ ions (4f 7 , S=7/2) which also substitute for La but their EPR spectra are described assuming the pure trigonal (axial) symmetry [12]. The same takes place for Nd 3+ ions (4f 3 , S=1/2) which EPR spectrum is also axial (g = 2.12, g⊥ =2.68) [23]. At higher Ce concentration (0.5 > x > 0.1), symmetry of the lattice becomes orthorhombic Imma and g tensor corresponds to this symmetry as well as in the tetragonal I4/mcm phase at x > 0.5, i.e., Ce 3+ EPR spectrum is described by three non-equal components.
Note that a narrow peak at 3.2 kG seen in LaAP:0.005Ce and LaAP:0.01Ce samples originates from unidentified defect. It is visible only at T<9 K (Fig. 3b, upper panel) and is not present at Ce concentration larger about 5% (Fig. 3b, down panel), when the Ce 3+ spectrum becomes dominated in intensity. Its g factor is equal to 2.115 that is very close to g factor value measured for Nd 3+ in LaAP [23]. Therefore, most probably, this background impurity is Nd 3+ . The simulated spectrum is shown for the LaAP:0.05Ce sample: spectra (1) and (2)  -Ce 3+ exchange coupled pairs. To prove this assumption, we calculated powder spectrum expected from the Ce 3+ -Ce 3+ dimers. The following spin Hamiltonian is used: where Ji are the spin-spin interaction constants including the contribution from both the dipole-dipole and anisotropic exchange interactions. Because only one crystallographic site exists for Ce ions, we can take g1 = g2 = g. The spin Hamiltonian (2) assumes that both g and spin-spin interaction tensors have the same principal axes, which could not be completely valid as the explicit orientation of the g tensor axes is unknown. Therefore, the spin-spin interaction constants can be considered as effective quantities renormalized to the common with g tensors axes. However, because g tensors are fixed in the powder spectrum simulation, the overall error in the Ji values is expected to be in the range of 20-25%.
The simulation of the Ce 3+ -Ce 3+ pairs in the LaAP:0.05Ce sample is shown in Fig. 3a, spectrum (2). The calculated spectrum well describes the main peak at 700 G and few other weak peaks at 1900 and 4050 G. The following spin-spin interactions constants were determined: Jz = -1770×10 -4 cm -1 , Jx = 1490×10 -4 cm -1 , Jy = 280×10 -4 cm -1 . They are in the range measured for Ce 3+ pair centers in YAlO3 [16] and in LaCl3 [24], for instance, (Jz = 0.1 − 0.6 cm -1 ) taking into account that in both these crystals the largest g factor is almost two times bigger than that in LaAlO3 (dipole interaction is proportional to As the Ce concentration increases to x=0.3−0.5, contribution of the Ce 3+ pairs as well as Ce 3+ triads and complexes with more number of Ce ions coupled by the dipole and exchange interaction becomes dominated over the contribution of single Ce ions into spectrum. As a result, the Ce 3+ spectral lines become very broad. Moreover, the anisotropy in the spectrum decreases due to exchange averaging [25] that is reflected in the decrease of the g3 -g1 difference (Table I).

139 La NMR study
LaAlO3 − CeAlO3 solid solutions contain 27 Al (nuclear spin I = 5/2, magnetic dipolar moment µ informative. NMR method is often used to quantify the site occupancy of atoms in a material and in study of chemical bonds [26]. NMR frequency is also very sensitive to hyperfine fields produced by paramagnetic ions [27,28]. Therefore, NMR is very desirable in the investigation of Ce ions incorporation into LaAlO3 lattice. Separately, 139 La MAS NMR spectrum of x=0.005Ce sample is shown in Fig. 4b together with the fitted spectrum performed in the Bruker "SOLA" program [29]. From the fitting, the quadrupole constant Quadrupole parameters are in agreement with those measured in single crystal using quadrupole satellite transitions [30].The fit giving these EFG parameters seem fairly plausible, thought it is evident that there is also another very weak contribution on the left side of the central line which can be assigned to the presence of Ce. This feature increases with the Ce content increase. Indeed, the positive frequency shift has paramagnetic origin (Fermi contact interaction) and is proportional to concentration of Ce paramagnetic ions. It can be usually written as [27]: where f0 is the Larmor frequency, A is electron-nuclear interaction constants of the nuclear spin with electron spin (it is proportional to the electron spin density at the nucleus), 〈Sz〉 is the time-averaged electron spin projection along the quantization axis. Because the Fermi contact interaction is generally additive, the overall paramagnetic shift can be obtained by summing the shifts induced by each magnetic ion, so it should be proportional to concentration of paramagnetic ions. Indeed, the shift of the centerof-gravity of 139 La spectrum linearly increases with Ce concentration (Fig. 5). The contribution of different 139 La nuclei into spectrum intensity was calculated with using the following relation: 6 12 12! 6! ( , , ) (1 ) (1 ) , where the distribution of Ce ions over 6 nearest and 12 next-nearest to La positions, the first and second coordination spheres, respectively, was assumed. Here x is the Ce concentration, n is number of Ce ions in the first coordination sphere, and k is number of Ce ions in the second coordination sphere.  including also possible changes of lattice parameters. In addition, there is also anisotropic magnetic dipole interaction between nuclear and electron spins (also referred to as "pseudocontact" shift [31]), which could be only partly averaged by the magic-angle spinning. The dipole interaction also increases with increase of Ce concentration leading to broadening of NMR line. 27 Al static NMR spectra for the lowest Ce contents x = 0.005 and 0.01 are shown in Fig. 8a. The of Al when the content of Ce is higher. The fitting of the spectrum for x = 0.005Ce (Fig. 8b) gives CQ = 0.16 MHz and η = 0 in accordance with [30], where these parameters were directly obtained from single-domain crystal measurements. This, however, differs from CQ = 0.17 MHz and η = 0.6-0.8 given in [32]. The large asymmetry of EFG tensor η = 0.6-0.8 reported in [32] seems to be not reliable for

27 Al NMR study
LaAP as the EFG tensor must be axial at Al site (η= 0) due to the rhombohedral R-3c symmetry. For higher concentrations of Ce (x = 0.05 -0.2), Fig. 9, position of the main maximum does not change visibly but the lines broaden due to increased dipole interaction with paramagnetic Ce 3+ ions. In addition to the line broadenig, an asymmetric feature (additional maximum) appears on the right side (upfield) from the central maximum. Its intensity grows with rising Ce content, but its position does not visibly change and it seems that no other additional maxima rise. This additional maximum (which makes the spectrum asymmetric) is shifted from the central maximum by about 90 ppm, that is 11.8 kHz.  The spectrum for 50 % is broader than for 100%, which is consistent with the assumption that the atomic order in the 50% is poorer due to many different configurations of Al neighborhood. Also, the spectrum for x = 1.0 of Ce is partly exchange narrowed due to exchange interactions between Ce 3+ spins. This is well manifested in disappearance of broad wings at spectral line.   The behavior of 27 Al spectrum in the La1-xCexAlO3 solid solution thus suggests that the chemical shift for 27 Al nuclei has "structural" origin rather than magnetic origin, i.e. is related to change of crystal structure in the vicinity of Ce ions that was also reflected in the orthorhombic symmetry of the Ce 3+ g tensor. This structure (at least, locally) is obviously close to the crystal structure of CeAlO3 as the 27 Al satellite line in the La1-xCexAlO3 solid solutions is centered at the same frequency as the 27 Al spectrum in CeAlO3. These our data confirm that even at low concentration of the Ce 3+ dopant, it can not be considered as an isolated impurity in LaAlO3. This means that, the 4f ground and 5d exited energy levels of Ce 3+ substantially depend on the local crystal structure near this ion and, obviously, are located within the valence and conduction bands.

Conclusions
Detailed Ce 3+ EPR measurements were done in the La1-xCexAlO3 solid solutions in form of single crystals and powdered samples at Ce concentration from x = 0.001 up to x = 1. The distinct Ce 3+ EPR signal was observed only at Ce concentration bigger of x = 0.005. This could be related to two facts: (i) main part of the Ce ions was in the non-paramagnetic Ce 4+ state and (ii) too broad spectral lines due to partial clustering of Ce ions. Ce 3+ spin Hamiltonian parameters were determined as a function of Ce concentration, including the spin-spin interaction constants between nearest Ce 3+ ions. It was found that even in the trigonal phase (x = 0.01 −0.1) the Ce 3+ EPR spectrum is described by g tensor which corresponds to the orthorhombic local symmetry at Ce ion, i.e. incorporation of Ce at La site lowers lattice symmetry in vicinity of the Ce ion. This is in contrast with Gd 3+ and Nd 3+ ions which also substitute for La, but their EPR spectrum is described assuming the pure trigonal (axial) symmetry [12,23]. In contrast to 139 La NMR, 27 Al MAS NMR spectrum does not show visible chemical shift related to Fermi contact interaction. However, an additional satellite peak appears in the 27 Al MAS spectra, which intensity linearly increases with increase of Ce concentration in the La1-xCexAlO3 solid solution.
The position of this peak does not depends on Ce concentration up to x = 1 suggesting that it originates from regions locally distorted by Ce ions. Crystal structure of these regions is close to the crystal structure of CeAlO3 as the 27 Al satellite line in La1-xCexAlO3 solid solutions is observed at the same frequency as 27 Al NMR line in CeAlO3. This suggests that even at low Ce concentration, Ce ions prefer to create local regions with modified crystal structure that is also reflected in the orthorhombic symmetry of the Ce 3+ g tensor, which must be axial according to the R-3c space group of LaAlO3 lattice.
Most probably, Ce influences positions of oxygen ions due to completely different outer electronic shells of La (5d 1 6s 2 ) and Ce (4f 2 5d 0 6s 2 ). Note that this effect can not be seen in 139 La spectra due to strong Fermi contact interaction and broad spectral lines.