A neutron-diffraction study of hemimorphite

The cold neutron chopper spectrometer at the Spallation Neutron Source-A review of the first 8 years of operation

The first eight years of operation of the Cold Neutron Chopper Spectrometer (CNCS) at the Spallation Neutron Source in Oak Ridge is being reviewed. The instrument has been part of the facility user program since 2009, and more than 250 individual user experiments have been performed to date. CNCS is an extremely powerful and versatile instrument and offers leading edge performance in terms of beam intensity, energy resolution, and flexibility to trade one for another. Experiments are being routinely performed with the sample at extreme conditions: T ≲ 0.05 K, p ≳ 2 GPa, and B = 8 T can be achieved individually or in combination. In particular, CNCS is in a position to advance the state of the art with inelastic neutron scattering under pressure, and some of the recent accomplishments in this area will be presented in more detail.

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

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.

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

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(F 2) = 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.