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

Abstract

The low-temperature structure of hemimorphite, Zn₄Si₂O₇(OH)₂ · H₂O, was investigated by neutron diffraction at 20 K using the “Time-Of-Flight” (TOF) method with a four-circle single-crystal diffractometer and a position-sensitive detector at the Intense Pulsed Neutron Source (IPNS) at the Argonne National Laboratory, IL. The reversible second-order phase transition, which had previously been determined at 98(2) K by optical measurements, was confirmed 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 intensities up to 30 σ(I), these reflections were absent in the 120 K data set. In contrast to the room-temperature structure of hemimorphite (Imm2, a ∼ 8.37 Å, b ∼ 10.73 Å, c ∼ 5.12 Å), the additional reflections of the low-temperature structure lead to a supercell with a = 8.354(1) Å, b = 21.519(4) Å, c = 10.240(1) Å.

According to the general reflection conditions, the low-temperature superstructure of hemimorphite was refined in space group Abm2 (= Acm2 = Aem2) with Rw(F²) = 0.058. Whereas the room-temperature structure shows dynamically disordered hydroxyl and H₂O groups, the low-temperature structure is characterized by an ordered arrangement of non-equivalent hydroxyl groups and rotated H₂O molecules linked by an enhanced H bond system along the channels of the structure, thus leading to a doubled c lattice parameter. A partially 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 non-space group reflection conditions 0kl with k + l = 4n and h00 with h = 2n, an order/dis-order (OD) structure model with microdomains and twinning is proposed.