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

Author(s)

Eugen Libowitzky, Arthur J. Schultz, D. M. Young

Abstract

The low-temp. structure of hemimorphite, Zn4Si2O7(OH)2.H2O, was investigated by neutron diffraction at 20 K using the time-of-flight (TOF) method with a 4-cycle single-crystal diffractometer and a position-sensitive detector at the intense-pulsed-neutron source (IPNS) at the Argonne National Lab., IL. The reversible 2nd-order phase transition, which was previously detd. at 98(2) K by optical measurements, was confirmed by 2 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 s(I), these reflections were absent in the 120 K data set. In contrast to the room temp. structure of hemimorphite (Imm2, a » 8.37, b » 10.73, c » 5.12 .ANG.), the addnl. reflections of the low-temp. structure lead to a supercell with a = 8.354(1), b = 21.519(4), c = 10.240(1) .ANG.. According to the general reflection conditions, the low-temp. superstructure of hemimorphite was refined in space group Abm2 (= Acm2 = Aem2) with Rw(F2) = 0.058. Whereas the room temp. structure shows dynamically disordered OH and H2O groups, the low-temp. structure is characterized by an ordered arrangement of nonequivalent OH groups and rotated H2O mols. 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 addnl. non-space group reflection conditions 0kl with k+l = 4n, and h00 with h = 2n, an order/disorder (OD) structure model with microdomains and twinning is proposed.

Organisation(s)

Department of Mineralogy and Crystallography

External organisation(s)

Argonne National Laboratory

Journal

Zeitschrift für Kristallographie - Crystalline Materials

Volume

213

Pages

659-668

No. of pages

10

ISSN

2194-4946

Publication date

1998

Peer reviewed

Yes

Austrian Fields of Science 2012

105113 Crystallography, 105116 Mineralogy

Portal url

https://ucrisportal.univie.ac.at/en/publications/f9488b6e-2344-4aa1-b766-10287c9c5a5b