Einkristalle des Neodym(III)-meta-Borats Nd(BO₂)₃ und -ortho-Borats Nd[BO₃]/Single Crystals of the Neodymium(III) meta-Borate Nd(BO₂)₃ and ortho-Borate Nd[BO₃]

Abstract

Pale violet, transparent, air- and water-resistant single crystals of the neodymium oxoborates Nd[BO 3 ] and Nd(BO 2 ) 3 emerged from reactions of Nd 2 O 3 , NdF 3 or NdCl 3 , and B 2 O 3 in variable molar ratios during attempts to synthesize neodymium halide borates (e. g. Nd 2 F 3 [BO 3 ] or NdCl(BO 2 ) 2 ). The preparations were carried out at 700 °C in sealed tantalum capsules using CsCl as a flux, or at 850 °C in evacuated silica ampoules with an excess of NdCl 3 as fluxing agent. Neodymium ortho-borate λ -Nd[BO 3 ] (orthorhombic, Pnma; a = 573.51(5), b = 505.64(4), c = 809.16(7) pm; Z = 4) is isotypic with aragonite-type CaCO 3 containing quasi-planar, discrete [BO 3 ] 3 − triangles (d(B-O) = 138 pm, 3×) and Nd 3+ in ninefold oxygen coordination (d(Nd-O) = 239 - 269 pm, CN = 9). In the crystal structure of neodymium meta-borate Nd(BO 2 ) 3 (NdB 3 O 6 ; monoclinic, C2/c; a = 983.24(9), b = 809.32(7), c = 637.71(6) pm, β = 126.639(8)°; Z = 4) both [BO 3 ] 3 − triangles (d(B-O) = 133 - 142 pm, CN = 3) and [BO 4 ] 5 − tetrahedra (d(B-O) = 145 - 149 pm, CN = 4) are present. They share common corners forming chains of the composition 1 ∞ {([(B1)(O1) e 2/1 (O2) e 2/1 ][(B2)(O1) e 1/1 (O2) e 1/1 (O3) t 1/1 ]2) 3− } (≡ 1 ∞ {(BO 2 ) − }) which run parallel to the [101] direction and are interconnected by Nd 3+ cations in tenfold coordination of oxygen atoms (d(Nd-O) = 238 - 280 pm, CN = 10).