Serna Galán, JulioTroev, T.Petkov, M.Alemany, C.2023-06-202023-06-201994-02-151. N. A. Lange (ed.), “Handbook of Chemistry” (McGraw-Hill, New York, 1967) p. 250. 2. “The Condensed Chemical Dictionary”, 8th Edn (Van Nostrand Reinhold, New York, 1971) p. 160. 3. L. A. Hardie, Amer. Mineralogist 52 (1967) 171. 4. P. Gay, Mineral. Mag. 35 (1965) 354. 5. Idem. ibid. 35 (1965) 347. 6. G. A. Lager, Th. Armbruster, F. J. Rotella, J. D. Jorgensen and D. G. Hinks, Amer. Mineralogist 69 (1984) 910. 7. J. D. C. McConell, D. M. Astil and P. L. Hall, Mineral. Mag. 51 (1987) 453. 8. L. Bragg (ed.), “Crystal Structures of Minerals”, Vol. IV, “The Crystalline State” (Bell, 1965) p. 139. 9. R. W. G. Wyckoff, in “Crystal Structures”, 2nd Edn, Vol. 3 (John Wiley, New York, 1965) pp. 18 and 642. 10. B. F. Pedersen, Acta Crystallogr. B38 (1982) 1074. 11. W. F. Cole and C. J. Lancucki, ibid. B30 (1974) 921. 12. M. E. Lines and A. M. Glass, in “Principles and Applications of Ferroelectrics and Related Materials” edited by W. Marshall and D. H. Wilkinson (Clarendon, Oxford, 1977) p. 608. 13. P. Kirkegaard, N. J. Pedersen and M. Eldrup, “PATFIT-88”, Risø-M-2740 (1989). 14. J. Serna, Ferroelectrics 129 (1992) 157. 15. J. Serna, unpublished work (1988–1990). 16. R. E. Green and R. E. Bell, Can. J. Phys. 35 (1957) 398. 17. K. P. Singh and R. M. Singru, Phys. Lett. 33A (1970) 463. 18. S. J. Tao, in Proceedings of 5th International Conference on Positron Annihilation, edited by R. R. Hasiguti and K. Fujiwara (Japan Institute of Metals, Aoba Aramaki, Sendai, 1979) p. 429.0022-246110.1007/BF00351403https://hdl.handle.net/20.500.14352/58603Bassanite (CaSO_4•½ H_2O) and anhydrite (CaSO_4) are the low-temperature products of gypsum (CaSO_4·2H_2O) dehydration, which are obtained at about 373 and 433 K, respectively. These sulphates have non-centrosymmetric crystallographic point groups, but dielectric measurements do not reveal any piezo- or ferroelectric characteristic, and they practically behave like linear dielectrics. Positron lifetime spectra exhibit the existence of two different positron states, besides a free positron state. There is positronium formation in the three sulphates, and there is also evidence for the presence of a highly populated positron bound state which may be a complex state associated with positrons bound to SO_4^2- ions. Parameter S estimated from the Doppler curve and the average positron lifetime show unquestionably the sensitivity of positrons to the phase transitions gypsum-bassanite and bassanite-anhydrite.journal articlehttp://dx.doi.org/10.1007/BF00351403http://link.springer.com/metadata only access535Materials ScienceMultidisciplinaryÓptica (Física)2209.19 Óptica Física