The Ce-doped rare-earth oxyorthosilicate solid solutions are new, insufficiently explored field of scintillation materials researches. The first mixed oxyorthosilicate scintillate crystals, Lu_1-yMe_yA_1-xCe_xSiO_5-z_Z, were proposed in 1996, where A is Lu and at least one element selected from the group of the rare-earth elements, Me is element selected from the group consisting of K, Ca, Ti, Zr, Sn, Hf, As, V, Nb, Sb, Ta, Mo, and W [1]. The first solid-solutions of oxyorthosilicate scintillate crystals, (Lu_1-xGd_x)₂SiO₅, were investigated in [2, 3]. Later, the lutetium-based oxyorthosilicate crystals of non-stoichiometry composition, LFS, containing at least one element selected from the group consisting of Ca, Gd, Sc, Y, La, Eu and Tb, were patented, [4]. Lutetium fine silicate, LFS, is a brand name of this family of Ce-doped solid-solution scintillation crystals comprising lutetium and crystallizing in the monoclinic system, space group C2/c, Z = 8. First mentioning of LFS crystal has occurred in 2004, [5].

In the Lu₂SiO₅ structure, there are two crystallographically non-equivalent sites with coordination numbers 6 and 7 for Lu³⁺ in oxyorthosilicate lattice host, whose unit cell contains 64 ions: eight lutetium ions (Lu1-type) are in the six-oxygen coordination, eight Lu3+ ions (Lu2-type) are in the seven-oxygen coordination, eight Si⁴⁺ ions and the forty O^2- ions of five types, O1, O2, O3, O4, O5 (eight O^2- ions in each type). Lutetium ions stand two right systems of points: Lu(1)O₆ is disordered octahedron; Lu(2)O₇ is disordered octahedron with one forked vertex, at that an average interatomic cation-anion distance is about 1.05 times larger in the Lu(2)O₇ polyhedron than that in the Lu(1)O₆ polyhedron. Si⁴⁺ ions are in disordered tetrahedron. The structural formula of a lutetium oxyorthosilicate crystal is (Lu1)(Lu2)SiO₅.

A content of the host elements in the crystals was measured by the ICP-MS method and the LECO combustion analysis method; concentration of the impurities was measured by the GDMS method. The crystals were grown by Czochralski technique from the melts containing a silicon oxide, a lutetium oxide, an yttrium oxide, a cerium oxide, and a scandium oxide of the different weights ratios. In particular, the crystals grown from the melts of Lu_2.06Si_0.97O_5.03, Lu_1.99Si_1.005O_4.995, Lu_1.98Ce_0.02SiO₅, and Ce:Ca:Lu_2+2y-zY_zSi_1-yO_5+y compositions were studied. The X-ray study of the samples cut from top and bottom of the crystals was performed. Precise measurement of the lattice parameters of the samples cut from the crystals grinded to powder allowed specifying structural chemical compositions of the crystals. The compositions of all crystallographic sites of oxyorthosilicate structure, namely Lu1, Lu2, Si, and oxygen, were taken into consideration.

Variation of cerium composition along a crystal length and related to it variation of structure parameters was found and explained. Distribution of Ce-ions in the Lu1 and Lu2 sites in the defect structure of lutetium oxyorthosilicate is established. Measured distribution coefficient of Ce³⁺ ions in LFS-3 crystal equals 0.365. The distribution coefficients in Ln₂SiO₅ calculated with Brandle’s empirical formula [6] for the Ce³⁺-ions in Ce2 and Ce1 sites are 0.39 and 0.17, respectively. Thus, the relative population of each site in crystal grown from a melt of the Ce:Ca:Lu_2+2y-zY_zSi_1-yO_5+y composition is found to be about 62% for Ce2 and 38% for Ce1 in contrast to the relative population in LSO crystal: 55% and 45% for Ce2 and Ce1 sites, respectively [7].

The kind of point defects and non-stoichiometry in oxyorthosilicates are discussed. Existing experimental evidence suggests that in Lu-oxyorthosilicate with different Lu/Si ratios, vacancies of the Lu1 and Lu2 sites and possibility of interstitial ions presence are the point defects, while O vacancies and conduction-band electrons are the primary oxygen deficient defects.

References

[1] A.I. Zagumennyi, Yu.D. Zavartsev, and P.A. Studenekin, US Patent 6,278,832 (1996)

[2] I.A.Kamenskikh, V.V. Mikhailin, I.H. Munro, D.Y. Petrovukh, D.A. Shaw, P.A. Studenikin, A.N. Vasil'ev, A.I. Zagumennyi, Yu.D. Zavartsev, Radiation Effects and Defects in Solids 135 (1995) 391-396

[3] G.B. Loutts, A.I. Zagumennyi, S.B. Lavrishev, Yu.D. Zavartsev, P.A. Studenikin “Czochralski growth and characterization of (Lu_1-xGd_x)₂SiO₅ single crystals for scintillators” J. Crystal Growth 174 (1997) 331-336

[4] A.I. Zagumennyi, Yu.D. Zavartsev, and S.A. Kutovoi, US Patent 7,132,060 (2004)

[5] Th.K. Lewellen, M.L. Janes, R.S.Miyaoka, A.F.Zerrouk, “Initial evaluation of the scintillator LFS for positron emission tomograph applications” DOI:10.1109/NSSMIC.2004.1466296, Nuclear Science Symposium Conference Record, 2004 IEEE Vol.5 (2004) 2915-2918

[6] C.D. Brandle, A.J. Valentino and G.W. Berkstresser “Czochralski growth of rare-earth orthosilicates (Ln₂SiO₅)” J. Cryst. Growth 79 (1986) 308-315

[7] H. Suzuki, T.A. Tombrello, C.L. Melcher, and J.S. Schweitzer “Light emission mechanism of Lu₂(SiO₄)O:Ce” IEEE Transactions on nuclear science 40 #4 (1993) 380-383

[8] L.Pidol, O.Guillot-Noel, A.Kahn-Harari, B.Vianna, D.Pelenc, D.Gourier, Journal of Physics and Chemistry of Solids 67 (2006) 643-650

* Calculated with Brandle’s empirical formula for rare-earth orthosilicates

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