CRYSTAL GROWTH, CRYSTAL STRUCTURE AND LINEAR OPTICAL PROPERTIES OF K2Re (NO3)5 . 2 H2O (RE = La, Ce, Pr, Nd)
S. Rühle, L. Bohaty, P. Held, H. Hellwig
Institute of Crystallography, University of Cologne,
Zülpicher Str. 49 b, D-50674 Köln, Germany,
e-mail: bohaty@kri.uni-koeln.de
Keywords: K2 Re (NO3)5 . 2 H2O, crystal growth, linear optics, crystal structure
The isomorphic nitrates K2 La (NO3)5 . 2 H2O and K2 Ce (NO3)5 . 2 H2O were already described by Fock [1] more than 100 years ago (1894). The morphological crystallography is also presented in the famous Chemische Krystallographie by Groth (1908) [2]: the fine orthorhombic polar morphology consists of {111}, {011_} and {011}. Eriksson et al. (1980) [3] solved the crystal structure in space group Fdd2. They showed that the main structural unit is the distorted icosahedron La(NO3)5 (H2O)2]2-. Dryuchko and Shevchuk (1984) [4] could synthesize the isomorphic Nd-compound; the isomorphic Pr-salt [5] was first found in our laboratory (1993). We also measured the pyroelectric effect and its temperature dependence for all four compounds. In 1993, Ebbers et al. [6] discovered the exceptional nonlinear optical properties of the La- and Ce-compounds.
In this contribution we report on the crystal growth of all four compounds (RE=La, Ce, Pr, Nd) and of mixed crystals K2 La1/2Ce1/2(NO3)5 . 2 H2O and K2 (La1-xNdx )(NO3)5 . 2 H2O (x=0.05). We have grown crystals with dimensions up to 5 cm in a period of eight to ten weeks at 38o C using an evaporation technique of HNO3-containing aqueous solutions. Furthermore, we present (re-)determination of the crystal structure of all four compounds from X-ray single crystal diffraction data.
As a basis for our nonlinear optical and electro-optical investigations we have determined the refractive indices of high precision and optical absorption spectra in a wavelength range from 0.3 to 2.5 mm. Our results confirm, refine and extend the data of Ebbers et al. (1993). We fit the data to a Sellmeier equation of type:
n2 = D1 + D2/l2 - D3) - D4 l2
ni | D1 | D2 | D3 | D4 | |
K2 La (NO3)5 . 2 H2O | n1 | 2.205(1) | 0.0133(4) | 0.0363(3) | 0.0083(3) |
n2 | 2.327(1) | 0.0173(5) | 0.0381(5) | 0.0098(3) | |
n3 | 2.392(1) | 0.0183(6) | 0.0391(4) | 0.0118(4) | |
K2 Ce (NO3)5 . 2 H2O | n1 | 2.215(1) | 0.0131(5) | 0.041(4) | 0.0085(3) |
n2 | 2.344(2) | 0.0172(8) | 0.045(5) | 0.0103(4) | |
n3 | 2.407(2) | 0.0182(9) | 0.045(5) | 0.0120(4) | |
K2 Pr (NO3)5 . 2 H2O | n1 | 2.218(1) | 0.0135(5) | 0.036(3) | 0.0084(3) |
n2 | 2.349(1) | 0.0180(6) | 0.039(3) | 0.0101(4) | |
n3 | 2.411(2) | 0.0191(6) | 0.039(3) | 0.0118(4) | |
K2 Nd (NO3)5 . 2 H2O | n1 | 2.220(1) | 0.0135(4) | 0.036(3) | 0.0084(3) |
n2 | 2.352(1) | 0.0182(5) | 0.038(2) | 0.0099(3) | |
n3 | 2.414(1) | 0.0191(6) | 0.038(3) | 0.0117(4) |
The Sellmeier equation for the Nd- and Pr-compounds fits the refractive indices and their dispersion in non-absorbing wavelength regions well.