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Melnikova, S. V.
Influence of atmospheric humidity on the symmetry and phase transitions of layered potassium oxyfluorides K2NbOF5 center dot H2O / S. V. Melnikova, N. M. Laptash // Phys. Solid State. - 2013. - Vol. 55, Is. 6. - P. 1231-1235, DOI 10.1134/S1063783413060243. - Cited References: 8. - This study was supported by the Council on Grants from the President of the Russian Federation (grant no. NSh-4828.2012.2). . - ISSN 1063-7834

РУБ Physics, Condensed Matter
Рубрики:
OXOPEROXOFLUORO COMPLEXES
   MO V
   CRYSTAL STRUCTURE
   OXOFLUORO
   MONOHYDRATE
Аннотация: Crystals of K2NbOF5 center dot H2O have been grown, polarization optical investigations have been performed, and the birefringence and rotation angle of the optical indicatrix have been measured in the temperature range 100-400 K. It has been found that, depending on the degree of atmospheric humidity, the layered K2NbOF5 center dot H2O crystal at room temperature can be in three states, namely, A, B, and C, which differ in symmetry and properties of the crystal. The K2NbOF5 center dot H2O crystal in the A state exists at a relative humidity RH = 90-100% and undergoes a first-order improper ferroelastic phase transition P a dagger" C2/m, which is accompanied by strong anomalies of the optical characteristics, twinning, and shear strain x (6) at temperatures T (2) (a dagger") = 308 K and T (2) (a dagger) = 313 K. The most stable state of the K2NbOF5 sdH(2)O crystal is the B state (RH = 20-90%), which retains the monoclinic symmetry C2/m in the temperature range 100-370 K. In a dry atmosphere (RH = 0-20%) or at T (1) a parts per thousand 370 K, the crystal becomes anhydrous (K2NbOF5) with the symmetry P4/nmm (the C state). The difference between the crystals in the states A and B is explained by the presence or absence of water molecules in interlayer spaces.

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Держатели документа:
Russian Acad Sci, Siberian Branch, LV Kirensky Phys Inst, Krasnoyarsk 660036, Russia
Russian Acad Sci, Inst Chem, Far Eastern Branch, Vladivostok 690022, Russia

Доп.точки доступа:
Laptash, N. M.; Мельникова, Светлана Владимировна
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Melnikova, S. V.
The role of structural and nonstructural water in oxyfluoride K2WO2F4 center dot H2O / S. V. Melnikova, A. D. Vasilev, A. G. Kocharova // Phys. Solid State. - 2011. - Vol. 53, Is. 12. - P. 2435-2440, DOI 10.1134/S1063783411120146. - Cited References: 16. - This work was supported by the Presidential Grant of the Russian Federation NSh-4645.2010.2, by the Russian Foundation for Basic Research, project no. 09-02-00062, and by the Integration Project of the Siberian Branch of the Russian Academy of Sciences no. 101. . - ISSN 1063-7834

РУБ Physics, Condensed Matter
Рубрики:
PHASE-TRANSITIONS
   OXOPEROXOFLUORO COMPLEXES
   MO V
   CRYSTAL STRUCTURE
   OXOFLUORO
   (NH4)(2)NBOF5
   (NH4)(2)WO2F4
   MONOHYDRATE
   DISORDER
Аннотация: X-ray structural and polarization optical investigations have been performed, and birefringence and rotation angles of the optical indicatrix phi (b) and phi (c) of the K2WO2F4 center dot H2O crystal have been measured in the temperature range of 100-600 K. The structure and symmetry of compounds at room temperature have been refined. It has been established that the layered crystal K2WO2F4 center dot H2O can exist in two states (A and B) depending on the atmospheric humidity and undergoes the sequence of reversible and irreversible phase transformations G (3)↔ G (2)↣ G (1)↣ G (0). The sequences of changes in the phase symmetry P (1) over bar↔C2/m↣ {4/nmm for samples A and m↔ C2/m↣P4/nmm for samples B havve been found. The second older proper ferroelastic phase transition (P(1) over bar↔ C2/m) at T-03 = 2700-290 K (G (3)↔ G (2)) is accompanied by twinning and appearance of the shift deformation x (6). The crystal system of the substance for the B crystals remains invariable after the second-order phase transition G (3) a dagger" G (2). The irreversible first-order phase transition G (2)↣ G (1) occurs in a temperature range T (02) a parts per thousand 350-380 K; it is accompanied by the loss of the crystallization water, which then is reduced easily from the atmosphere for a day. The substance decomposes at T (01) approximate to 510 K (G (1)↣ G (0)). The distinction between the A and B crystals has been explained by the presence or absence of free water in interlayer spacings.

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Держатели документа:
[Melnikova, S. V.
Vasilev, A. D.
Kocharova, A. G.] Russian Acad Sci, LV Kirensky Phys Inst, Siberian Branch, Krasnoyarsk 660036, Russia
ИФ СО РАН
Kirensky Institute of Physics, Siberian Branch of the Russian Academy of Sciences, Akademgorodok 50, Krasnoyarsk 660036, Russian Federation

Доп.точки доступа:
Vasilev, A. D.; Kocharova, A. G.; Кочарова, Алла Георгиевна; Мельникова, Светлана Владимировна
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