{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,2]],"date-time":"2026-10-02T05:15:51Z","timestamp":1790918151170,"version":"4.1.0"},"reference-count":0,"publisher":"International Union of Crystallography (IUCr)","issue":"4","license":[{"start":{"date-parts":[[2005,7,19]],"date-time":"2005-07-19T00:00:00Z","timestamp":1121731200000},"content-version":"vor","delay-in-days":0,"URL":"http:\/\/creativecommons.org\/licenses\/by\/2.0\/uk\/legalcode"},{"start":{"date-parts":[[2005,7,19]],"date-time":"2005-07-19T00:00:00Z","timestamp":1121731200000},"content-version":"tdm","delay-in-days":0,"URL":"http:\/\/creativecommons.org\/licenses\/by\/2.0\/uk\/legalcode"}],"content-domain":{"domain":["iucr.org","wiley.com","iucrj.org"],"crossmark-restriction":true},"short-container-title":["Acta Crystallogr B Struct Sci","Acta Cryst B","Acta Cryst Sect B","Acta Crystallogr B","Acta Crystallogr Sect B","Acta Crystallogr Sect B Struct Sci"],"accepted":{"date-parts":[[2005,5,31]]},"published-print":{"date-parts":[[2005,8,1]]},"abstract":"<jats:p>The crystal structure of barbituric acid dihydrate (C<jats:sub>4<\/jats:sub>H<jats:sub>4<\/jats:sub>N<jats:sub>2<\/jats:sub>O<jats:sub>3<\/jats:sub>\u00b72H<jats:sub>2<\/jats:sub>O) has twice been reported as orthorhombic, space group <jats:italic>Pnma<\/jats:italic>, with all atoms (except for CH<jats:sub>2<\/jats:sub> H atoms) lying on the mirror plane [Al-Karaghouli <jats:italic>et al.<\/jats:italic> (1977). <jats:italic>Acta Cryst<\/jats:italic>. B<jats:bold>33<\/jats:bold>, 1655\u20131660; Jeffrey <jats:italic>et al.<\/jats:italic> (1961). <jats:italic>Acta Cryst<\/jats:italic>. <jats:bold>14<\/jats:bold>, 881\u2013887]. The present study has found that at low temperatures, below 200\u2005K, the crystal structure is no longer orthorhombic but is non-merohedrally twinned monoclinic, space group <jats:italic>P<\/jats:italic>2<jats:sub>1<\/jats:sub>\/<jats:italic>n<\/jats:italic>. This phase is stable down to 100\u2005K. Above 220\u2005K the crystal structure is orthorhombic, and between 200 and 220\u2005K the structure undergoes a phase change, with the monoclinic-to-orthorhombic phase transition itself taking place at around 216\u2013217\u2005K. The size of the \u03b2 angle in the monoclinic structure is temperature dependent; at 100\u2005K \u03b2 is around 94\u00b0 and it decreases in magnitude towards 90\u00b0 as the temperature increases. Although the hydrogen-bonding motifs are the same for both crystal systems, there are significant differences in the crystal packing, in particular the out-of-plane displacement of the two water molecules and the <jats:italic>sp<\/jats:italic>\n                  <jats:sup>3<\/jats:sup>-hybridized C atom of barbituric acid.<\/jats:p>","DOI":"10.1107\/s0108768105017258","type":"journal-article","created":{"date-parts":[[2005,7,25]],"date-time":"2005-07-25T14:13:08Z","timestamp":1122300788000},"page":"464-472","update-policy":"https:\/\/doi.org\/10.1107\/cm_01","source":"Crossref","is-referenced-by-count":43,"title":["A variable-temperature study of a phase transition in barbituric acid dihydrate"],"prefix":"10.1107","volume":"61","author":[{"given":"Gary S.","family":"Nichol","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"William","family":"Clegg","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"329","published-online":{"date-parts":[[2005,7,19]]},"container-title":["Acta Crystallographica Section B Structural 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