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A variable-temperature study of a phase transition in barbituric acid dihydrate
1School of Natural Sciences-Chemistry, Bedson Building, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, England.
Acta Crystallographica. Section B, Structural Science
|July 26, 2005
Summary
Barbituric acid dihydrate exhibits a temperature-dependent crystal structure. Below 200 K, it adopts a monoclinic phase, transitioning to an orthorhombic structure above 220 K.
Area of Science:
- Crystallography
- Materials Science
- Chemical Physics
Background:
- Barbituric acid dihydrate (C4H4N2O3*2H2O) crystal structure was previously reported as orthorhombic (space group Pnma).
- Previous studies by Al-Karaghouli et al. (1977) and Jeffrey et al. (1961) established the orthorhombic structure.
Purpose of the Study:
- To investigate the temperature-dependent crystal structure of barbituric acid dihydrate.
- To identify phase transitions and structural changes with varying temperatures.
Main Methods:
- Single-crystal X-ray diffraction at variable temperatures.
- Analysis of crystal structure and phase transitions.
Main Results:
- At temperatures below 200 K, barbituric acid dihydrate forms a non-merohedrally twinned monoclinic crystal structure (space group P2(1)/n).
- A phase transition from monoclinic to orthorhombic occurs between 200 K and 220 K, specifically around 216-217 K.
- The monoclinic beta angle is temperature-dependent, decreasing towards 90 degrees with increasing temperature.
- Differences in crystal packing, including water molecule and carbon atom displacement, exist between the monoclinic and orthorhombic phases despite similar hydrogen-bonding motifs.
Conclusions:
- Barbituric acid dihydrate displays a reversible phase transition influenced by temperature.
- The crystal structure exhibits distinct monoclinic and orthorhombic phases with unique packing arrangements.