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Temperature-dependent neutron powder diffraction study of the Ba(OD)(2) polymorphs: a new low-temperature phase
1Laboratorium für Kristallographie, ETH Zentrum, Sonneggstrasse 5, CH-8092 Zürich, Switzerland. alexandra@kristall.erdw.ethz.ch
Acta Crystallographica. Section B, Structural Science
|November 22, 2001
Summary
Neutron powder diffraction revealed that barium dihydroxide-d exhibits unusual phase transitions. A metastable alpha phase transforms into another metastable phase via a reversible order-disorder mechanism driven by deuterium atom displacement.
Area of Science:
- Solid-state chemistry
- Materials science
- Crystallography
Background:
- Barium dihydroxide-d (Ba(OD)2) exists in low-temperature beta and high-temperature alpha phases.
- Understanding phase transitions is crucial for materials properties and applications.
Purpose of the Study:
- To investigate the structural behavior of alpha- and beta-Ba(OD)2 across a wide temperature range (10–552 K).
- To elucidate the mechanism behind the observed phase transitions in the metastable alpha phase.
Main Methods:
- Neutron powder diffraction was employed to analyze the crystal structures.
- Temperature-dependent studies were conducted from 10 K to 552 K.
Main Results:
- The beta phase (P2(1)/n) is stable at low temperatures.
- The alpha phase (Pnma) undergoes a reversible orthorhombic-to-monoclinic (P2(1)/n) transition between 100–150 K.
- This transition is attributed to an order-disorder mechanism involving deuterium atom displacement.
Conclusions:
- The alpha phase transforms into a new metastable alpha(m) phase (P2(1)/n) below 150 K.
- This transformation is driven by deuterium atom disorder, leading to reduced symmetry.
- The observed phase transition behavior in barium dihydroxide-d is unusual for a metastable system.