The effects of heating and dehydration on the crystal structure of hemimorphite up to 600° C

Abstract

Abstract The crystal structure of hemimorphite, Zn4Si2O7(OH)2 · H2O, contracts upon dehydration caused by heating. X-ray intensity data were collected at 300°C, at 600°C and then at 22°C. Anisotropic least-squares refinements of 532, 255, and 432 reflections in the space group Imm2, and Z = 2, resulted in unweighted R-factors of 0.046, 0.074, and 0.061, respectively. The following cell parameters were obtained during crystal alignment on the diffractometer: 22°C1 300°C 600°C 22°C a 8.367(5) 8.337(5) 8.268(5) 8.206(4) b 10.730(6) 10.724(6) 10.784(8) 10.815(6) c 5.115(3) 5.116(4) 5.113(3) 5.089(2) V 459.2(4) 457.4(5) 455.9(5) 451.6(4) The structure is a framework consisting of interconnected corrugated sheets composed of 3-membered rings of one SiO4 and two Zn(OH)O3 tetrahedra. Contraction of the structure above 300°C is due to dehydration, and positive thermal expansion is observed in the dehydrated form. The structural mechanism of contraction is dependent upon the collapse of the cavities interconnected parallel to c towards the position of the expelled water molecule. The decrease in unit cell volume (ΔVuc ) is directly correlated with volume loss in the cavities (ΔVc ) and the increase in volume of the Zn tetrahedra (ΔVz ): ΔVuc ∼ 8ΔVz + 2ΔVc . Zeolites with structural characteristics like hemimorphite show similar changes during molecular exchange reactions.