The low-temperature structure and phase transition of hemimorphite, Zn4Si2O7(OH)(2)center dot H2O

The low-temperature structure of hemimorphite, Zn4Si2O7(OH)(2). H2O, was in vestigated by neutron diffraction at 20 K using the "Time-Of-Flight" (TOF) method with a four-circle single-crystal diffractometer and a position-sens itive detector at the Intense Pulsed Neutron Source (IPNS) at the Argonne N ational Laboratory, IL. The reversible second-order phase transition, which had previously been determined at 98(2)K by optical measurements, was conf irmed by two TOF neutron histograms acquired at 20 K and 120 K. Whereas the 20 K histogram showed superlattice reflections hkl with k, l = n/2 and int ensities up to 30 sigma(I), these reflections were absent in the 120 K data set. In contrast to the room-temperature structure of hemimorphite (Imm2, a similar to 8.37 Angstrom, b similar to 10.73 Angstrom, c similar to 5.12 Angstrom), the additional reflections of the low-temperature structure lead to a supercell with a = 8.354(1) Angstrom, b = 21.519(4) Angstrom c = 10.2 40(1) Angstrom. According to the general reflection conditions, the low-temperature superst ructure of hemimorphite was refined in space group Abm2 (= Acm2 = Aem2) wit h Rw(F-2) = 0.058. Whereas the room-temperature structure shows dynamically disordered hydroxyl and H2O groups, the low-temperature structure is chara cterized by an ordered arrangement of non-equivalent hydroxyl groups and ro tated H2O molecules linked by an enhanced H bond system along the channels of the structure, thus leading to a doubled c lattice parameter. A partiall y ordered arrangement of the channels across the structure (using the [1/2, 1/4, 1/4] or [1/2, 1/4, -1/4] translation vectors) results in doubling of the b lattice parameter. According to space group symmetry and additional n on-space group reflection conditions Okl with k + l = 4n and h00 with h = 2 n, an order/disorder (OD) structure model with microdomains and twinning is proposed.