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aPhys. Status Solidi C 10, No. 7–8, 1119–1122 (2013) / DOI 10.1002/pssc.201200830
ESR study of Ce3+ ions in a CaGa2S4 single crystal Ittetsu Kitajima*, 1, Takeo Takizawa**, 1, Chiharu Hidaka1, and Shigetaka Nomura2 1 Department of Physics, College of Humanities and Sciences, Nihon University, Sakurajosui 3-25-40, Setagaya-ku, Tokyo 156-8550,
Japan 2 Department of Electrical Engineering, Tokyo University of Science, Chiyoda-ku, Tokyo 102-0073, Japan
Received 10 October 2012, revised 8 December 2012, accepted 25 February 2013 Published online 10 May 2013
Keywords EPR, rare earth element, CaGa2S4, single crystal ** Corresponding author: e-mail [email protected], Phone: +81 3 5317 9772, Fax: +81 3 5317 9772 ** e-mail [email protected], Phone: +81 3 5317 9772, Fax: +81 3 5317 9772
ESR measurements have been carried out to determine the local environment and the ground state of Ce3+ ions in the orthorhombic CaGa2S4 at 4.2 K. More than 30 ESR lines are observed with almost the same angular depend-ence. The local symmetry and the anisotropic g-tensor for the four strongest ESR signals indicate that Ce3+ ions substitute the three independent Ca sites. A part of Ce3+
ions at the Ca2+ sites and a neighbouring sulphur vacancy are expected to form complex centers in view of the other weak ESR lines observed. The ground state of Ce3+ in the host lattice is shown to consist of mostly φ±1/2 by fitting the ESR spectra using the spin Hamiltonian. The upper 2F7/2 state is thought to be slightly mixed into the ground state 2F5/2 judging from a small difference in the fitting.
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction The compound CaGa2S4 doped with a rare earth element (REE) is a promising phosphor for illu-mination devices because of its efficient and various lumi-nescence [1-3]. Trivalent REE ions doped into the crystal lattice of CaGa2S4 mostly replace the divalent cation sites. Thus, there always arises discrepancy in valence between dopants and replaced ions, which induces vacancies in the neighbourhood of dopants to keep the total charge neutral-ity. The replacing ions and resulting vacancies may nor-mally form complex centers as well as lattice distortions in the crystal. In the case of CdIn2S4 doped with Ce3+ ions, the complex center was observed to be formed as a pair of a Ce3+ ion and a neighbouring sulphur vacancy [4]. Since this kind of vacancy generally affects luminescence prop-erties, it is very important to investigate the fundamental properties of dopants or complex centres such as local symmetries in order to eventually improve the emission ef-ficiency.
ESR measurement is one of the suitable tools for inves-tigation of the dopants and the complex centers. We have chosen a Ce3+ ion as an ESR probe to obtain the local symmetry around REE3+. It is well known that Ce3+ ions doped into CaGa2S4 work as both activators and sensitizers
for luminescence [1-3, 5-8]. They show efficient blue-green double band emission due to the transition between the 5d and 4f states which split into the ground state 2F5/2 and the upper one 2F7/2. In view of ESR measurements, a Ce3+ ion has only one 4f - electron with S = 1/2 and no nu-clear spin, so that the spin Hamiltonian (SH) includes ESR parameters of an anisotropic g-tensor with no hyperfine structure. It is also known that the lowest levels of the Ce3+ in Kramer’s doublet differ from sample to sample due to the change in the crystal field [9]. Thus, the information of the ground state of Ce3+ is also useful to explain the lumi-nescence phenomena. Here, we report the results of ESR investigation of the local symmetry around Ce3+ ions to-gether with the identification of the ground state of Ce3+ in CaGa2S4.
2 Experimental A single crystal of CaGa2S4 doped
with Ce (0.2 mol%) was grown by the Horizontal Bridg-man method in a vacuumed quartz tube (13 mmφ × 150 mm). The starting materials used here were powders of CaS (purity: 99.99 %), Ga2S3 (synthesized using Ga and S of six nines purity), and Ce2S3 (purity: 99.9 %). The de-tailed preparation method has been reported in Ref. [5].
1120 I. Kitajima et al.: ESR study of Ce3+ ions in a CaGa2S4 single crystal
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The crystallographic axes were determined using the re-flection Laue method. A sample was typically cut into a piece of 2.5 × 2.9 × 1.0 mm3 and mounted on a quartz rod in a sample holder. ESR measurements were carried out using a JEOL JES-FA300 X-band ESR spectrometer with 100 kHz modulation at 4.2 K. To determine the anisotropic g-tensor, the angular dependencies of the ESR signals were measured by rotating the crystal around each of the three principal axes with a step of 2° from 0 to 90° and a step of 10 or 30° from 90 to 180°.
3 Results and discussion 3.1 ESR spectra Figure 1 shows a typical ESR spec-
trum of Ce3+ ions in a single crystal of CaGa2S4 measured by rotating the sample around the a-axis, where the static magnetic field is initially along the b-axis. To remove the superposition of the peaks with each other, the sample is rotated by 10°, that is, the direction of the magnetic field is deviated from the b-axis by the same amount. The four strongest ESR lines are designated as A signals. More than 30 signals including weak ones are observed in the meas-urements. Figure 2 shows the angular variations of the A signals for three kinds of rotations around the a-, b- and c-axes, where (a) represents A-1, (b) A-2 and A-3, and (c) A-4, respectively. The initial directions of the static magnetic field are in the [010], [001] and [010] directions for the re-spective rotations. As we discuss in the following section, signals A-2 and A-3 in Fig. 2(b) are thought as a pair. Weak signals due to Ce3+ ions other than the A signals ex-hibit almost the same angular behaviour as shown in Fig. 3. These signals are originating from the magnetically in-equivalent sites. Thus, various kinds of complex centers are thought to be formed in the crystal, resulting in more than 30 ESR signals. Hereafter, we focus on the A signals and determine the anisotropic g-tensor.
3.2 Substitution sites of Ce3+ ions To obtain the
g-values, the following SH
B ,b= ◊ ◊H g SH (1)
was used, where βB is the Bohr magneton, H the external magnetic field, g the anisotropic g-tensor, and S the effec-tive spin operator (where S = 1/2). The principal axis of the tensor is described by three Euler angles (α, β, γ). We used EasySpin software for the calculation of the g-values and the Euler angles [10]. The g-values of the A signals were calculated to be g// ≈ 0.8 and g⊥ ≈ 2.3 ~ 2.8, as summarized in Table 1 together with the Euler angles and the other pa-rameters explained later. Signals A-2 and A-3 have almost the same g-values and their principal axes coincide with each other under the two-fold rotation. Since the ortho-rhombic crystal of CaGa2S4 has the two-fold symmetry for all crystallographic axes, these two signals are considered to arise from a single cation site. Thus, the number of the substitution sites of Ce3+ ions is expected to be three for the A signals. There are three independent Ca2+ sites in
CaGa2S4, being labelled as Ca1, Ca2 and Ca3 [11]. In a previous work of the ESR studies for Mn2+ and Eu2+ ions doped in CaGa2S4, those ions were concluded to replace the three Ca2+ sites [12, 13]. Following the discussion simi-lar to that in the previous work, Ce3+ ions are considered to substitute the Ca2+ sites. The Ce3+ ions are also thought to replace the Ca2+ ions rather than the Ga3+ ones in view of the ionic radii being 1.14, 1.12, and 0.47 Å, respectively. The numbers of atomic positions of Ca1 and Ca2 in a unit cell are the same and that of Ca3 are just twice of them [11]. Since signals A-2 and A-3 show spectra of the same ESR intensities with two-fold symmetry, we can infer that A-1 and A-4 in Table 1 correspond to Ca1 or Ca2 and both A-2 and A-3 to Ca3.
A-1 A-2 A-3 A-4
gx 2.253 2.335 2.330 2.524
gy 2.822 2.763 2.756 2.583
gz 0.8245 0.7824 0.7841 0.7944
Σgi 5.900 5.880 5.870 5.901
α 63.6 87.1 -81.6 60.4
β 0.65 3.51 2.40 0.65
γ -62.5 -85.9 82.3 -61.8
C1 -0.114 -0.114 -0.113 -0.015
C2 0.955 0.950 0.950 0.965
C3 0.035 0.048 0.046 0.037
ΣCi2 0.927 0.917 0.917 0.933
C3 (calc) 0.099 0.108 0.112 0.098
ESR
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28002700260025002400230022002100Magnetic field [G]
A-1 signal
A-2 signal A-3 signal A-4 signal
Figure 1 Typical ESR spectrum of Ce3+ ions in a CaGa2S4 single crystal at 4.2 K. The microwave magnetic field is a-long the a-axis and the static magnetic field is deviated from the b-axis by 10°.
Table 1 Calculated g-values and Euler angles from the SH fitting and the orthorhombic wave function coefficients using linear combination of the cubic ones [13], where C3 (calc) is estimated using Eq. (3) in the text for the four strongest ESR signals named as A signals.
Phys. Status Solidi C 10, No. 7–8 (2013) 1121
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As above-mentioned, weak signals other than the A signals show almost the same angular dependence as the A signals (see Fig. 3). This result indicates that Ce3+ ions are also at Ca2+ sites, but their local environments slightly dif-fer from those of the A signals. Therefore, about 20 ESR
signals except for the A signals may be attributed to the sites consisting of a Ce3+ ion and a neighbouring sulphur vacancy in the crystal. Since every Ce3+ site locates at the center of a decahedron made of eight sulphurs, the number of conceivable configurations for the complex center is eight. Taking into account of three independent Ca2+ sites in the unit cell, the number of different centers amounts to 24, which may be the origin of the weak ESR signals showing similar angular dependences.
3.3 Ground state of Ce3+ The ground state of Ce3+
ions in the orthorhombic field can be calculated from the basic wave functions in the cubic field as reported by Lew-is [14]. Since the ground state in the cubic field 2F5/2 splits into a quartet Γ8 and a doublet Γ7, the orthorhombic bases φ± for 2F5/2 are defined by the linear combination of the cubic ones as follows
52
8
12
372
5 1 31 2 32 2 2
5 5 1 36 2 26Γ125 5 1 3Γ ,6 2 26
C C C
j
j
j
j j j j
±
±
±
±
± ± ±
Ï= ± +ÔÔ
ÌÔ = ±ÔÓÏ
= - ±ÌÓ= + +
∓
∓ (2)
where the admixture coefficients of the cubic wave func-tions have been determined by the degeneracy condition of energy for the orthorhombic field. In order to evaluate the relation between the anisotropic g-values obtained and the coefficients of the wave functions, we calculate the eigen values of the SH in Eq. (1) to obtain the diagonal elements of the g-tensor using the effective spin operator for the 2F5/2 ground state. The summation of the three g-values de-rived is given by the following relation using a coefficient C3 in Eq. (2),
23
726 ,7i
ig C= -Â
which can be used for the check of the coefficients [14]. The values of the coefficients C1, C2 and C3 in Eq. (2) for the A signals are shown in Table 1. The coefficients of the wave functions clearly show that the ground state of Ce3+ ions in this lattice consists of mostly φ±1/2 state. However, the coefficients do not satisfy the normalization condition (C1
2 + C22 + C3
2 = 1). The C3 values obtained above also slightly deviate from those obtained using Eq. (3) as shown in the bottom line in Table 1. Thus, the upper state 2F7/2 must be mixed into the ground state 2F5/2 in spite of the large energy difference of about 2200 cm-1, as proposed by some authors [14-16]. In the present work, the mixture is also expected by the deviation in the g-values calculated as gz = 6/7 and gx = gy = 18/7 for the φ±1/2 state [9].
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Figure 2 Angular variations of signals (a) A-1, (b) A-2 and A-3, and (c) A-4, respectively. The filled- and open-symbols show the experimental results and the solid lines are the cal-culated ones using the parameters in Table 1. The red circles and lines denote that the rotation around the a-axis with the magnetic field initiated at the [010] crystallographic axis. The blue squares and green triangles express the rotation around the b- and c-axis; the magnetic field of the former is initially in the [001] and the latter in the [010], respectively.
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1122 I. Kitajima et al.: ESR study of Ce3+ ions in a CaGa2S4 single crystal
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4 Summary To investigate the local symmetries
around the Ce3+ ions and the ground state of Ce3+ in CaGa2S4, ESR measurements of Ce3+ doped CaGa2S4 have been carried out at 4.2 K. More than 30 signals are ob-served with almost the same angular behaviours. From the SH fitting of the A signals, it is concluded that Ce3+ ions substitute the divalent Ca sites. About 20 ESR signals ex-cept for the A signals may be attributed to several kinds of complex centers consisting of a Ce3+ ion and a neighbour-ing sulphur vacancy. The calculation of the ground state of Ce3+ ions shows that the lowest state in this lattice consists of mostly φ±1/2 state and the admixture of the upper 2F7/2 state to the ground 2F5/2 state is suggested.
Acknowledgements This work is partly supported by the grant for the private universities for 2009-2013 from the ministry of education, culture, sports, science and technology, Japan.
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Angle of rotation [º ]Figure 3 Angular dependence of weak signals except for the A signals rotated around the a-axis from the [010] to [001] crystallographic axis.
