1. N. A. Tyukavkina, V. I. Lutskii, A. K. Dzizenko, and V. A. Pentegova, KhPS [Chemistry of Natural Compounds],4, 249, 1968.
2. L. H. Briggs and L. D. Colebrook, Spectrochim. Acta,18, 939, 1962.
3. H. Erdtman, Svensk Kem. Tid.,56, 26, 1944.
4. H. Erdtman, Svensk. Kem. Tid.,56, 95, 1944.
5. G. Linstedt, Acta Chem. Scand.,4, 1042, 1950.