X-ray studies of structural changes in melanophlogite with varying temperature
| Accession number;06A0010672 |
| Title;X-ray studies of structural changes in melanophlogite with varying temperature |
| Author;NAKAGAWA TAKESHI(Kanazawa Univ., Kanazawa, Jpn) KIHARA KUNIAKI(Kanazawa Univ., Kanazawa, Jpn) FUJINAMI SYUHEI(Kanazawa Univ., Kanazawa, Jpn) |
Journal Title;J Mineral Petrol Sci
|
Journal Code:G0150B
|
ISSN:1345-6296
|
|
VOL.100;NO.6;PAGE.247-259(2005)
|
| Figure&Table&Reference;FIG.6, TBL.7, REF.28 |
| Pub. Country;Japan |
| Language;English |
| Abstract;The a and c unit cell dimensions of melanophlogite (MEP) have been determined in the temperature range -50 to 700.DEG.C., showing a different expansion behavior for the low temperature .ALPHA.-phase. The c-axis length, 2c, which is smaller than a-axis length in the .ALPHA.-phase, shows a steep rise reaching the value of a at the tetragonal (.ALPHA.)-cubic (.BETA.) transition temperature at about 65.DEG.C., and then remains nearly constant until about 500.DEG.C., after which contraction occurs. The crystal structures of .ALPHA.-MEP (space group=P42/nbc) and .BETA.-MEP (space group=Pm3n) were refined using a least-squares refinement of a harmonic structure factor expression, using single crystal X-ray diffraction data measured at seven temperatures from -50 to 200.DEG.C. (four points for the .ALPHA.-phase and three points for the .BETA.-phase). The average Si-O distance decreased from 1.593.ANGS. at -50.DEG.C. down to 1.573.ANGS. at 63.DEG.C. (the estimated transition point). It then remained nearly constant in the .BETA.-phase. The Si-O bond distance corrected using a simple rigid body motion model remained nearly constant at 1.611.ANGS. in the temperature range -50 to 200.DEG.C., indicating that the negative temperature dependence is due to strong distortions of the probability density functions of the O atoms. The atomic mean-square displacement, (u'2'), of the O atoms increased steeply with increasing temperature up to the .ALPHA.-.BETA. transition point. The low-high (.ALPHA.-.BETA.) transformation in MEP is driven by a mechanism involving atom disorder beginning in the low-temperature phase. (author abst.) |
|
|
