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S.J. Zhang and Z.X. Cheng Institute of Crystal Materials and National Laboratory of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China (Received 20 March 2002; accepted 2 July 2002)

SmCa4O(BO3)3 (SCOB) crystallizes in the noncentrosymmetric monoclinic space group Cm with cell parameters a ⳱ 0.8129(5), b ⳱ 1.6076(1), c ⳱ 0.3584(1) nm, ␤ ⳱ 101.36(2)°, and Z ⳱ 2. Raman results showed that the characteristic spectra of SCOB are mainly contributed by the B–O triangles and partly by the Ca(1)–O octahedra. The structural rigidity of SCOB is mainly ascribed to the B–O bond stretching and bending modes and partly by the Ca(1)–O bonds. The rigid structure of the B–O triangles and the quasi-rigid structure of the Ca(1)–O octahedra are necessary to the SCOB crystal as a laser host. The large distortions of the Sm–O and Ca(2)–O octahedra intensify the polar forces and anisotropic lattice forces, which generally imply the best nonlinear properties possible for SCOB as a nonlinear optical material and piezocrystal. Thermal measurements show a larger value of the specific heat and three small expansion coefficients, which show that SCOB can allow a large temperature gradient, especially in the Y direction. Displacement parameters indicated that the thermal ellipsoids of the Sm and Ca(2) atoms had significantly large components along the b direction. Synthetical study indicated that the SCOB crystal should be outstanding as an efficient self-frequency doubled material.

I. INTRODUCTION

Some rare-earth (RE) borates and their solid solutions, such as NdxGd1−x Al3(BO3)4 and YbxY1−x Al3(BO3)4,1,2 and YCa4O(BO3)3 (YCOB), GdCa4O(BO3)3 (GCOB), SmCa4O(BO3)3 (SCOB), and Nd- and Yb-doped YCOB and GCOB,3– 6 have attracted much attention. RE calcium oxyborate is attractive due to its advantages of a broad transparency range, a short ultraviolet absorption edge, a low laser threshold, and a high damage threshold. It exhibits a small beam divergence and a large angular acceptance. They are nonhygroscopic and chemically stable with good mechanical properties, which allow for easy optical polishing, and can be grown to large sizes. These oxoborates are not only good laser host materials but also allow the development of efficient self-frequency doubled lasers. Self-frequency doubling in Nd- and Yb-doped YCOB was demonstrated in continuous wave and pulsed lasers.7–9 The nonlinear optical properties—e.g., the effective nonlinear coefficient, limit of type-I second and/or third harmonic generation wavelength, birefringence, noncritical phase-matching, etc.—in YCOB and GCOB, are interesting and good J. Mater. Res., Vol. 17, No. 9, Sep 2002

practice.4,5,10,11 In this paper, attention is focused on the structure–property relationship of SCOB studied by Raman scattering and thermal measurement. II. EXPERIMENTAL

SCOB melts congruently and was grown using the Czochralski technique. The starting reagents were prepared by mixing 4 N pure Sm2O3, CaCO3, and B2O3 powders at stoichiometric ratios for about 3 h. The excess of a sma