IR Studies of Thermally Stimulated Structural Phase Transformations in Cryovacuum Condensates of Freon 134a

Abstract

The method of cryovacuum condensation of thin gas films and, in particular, condensation involving the formation of glassy states, provides ample opportunities for precise control and maintenance of the formation conditions of studied samples. This has prompted researchers to formulate and address the question of the relationship between the formation conditions/structure of molecules and the degree of kinetic stability of cryocondensates, including organic glasses. This study focused on thin films of Freon 134a cryovacuum condensates condensed on a cooled metal substrate comprised of gas at deposition temperatures ranging from 16–100 K and gas phase pressures from 10–4 to 10–6 Torr. A comparison between the vibrational spectra of Freon 134a in the gas phase and in the cryocondensed state is provided. The results of IR spectrometric studies of cryovacuum condensates of Freon 134a (2.5 μm thick) in the frequency range 400–4200 cm–1 are presented. Based on the obtained spectra and data on their thermally stimulated transformations, an assumption is made that, in the temperature range 16–60 K, Freon 134a cryocondensates are in an amorphous state with different degrees of amorphization. At the vitrification temperature of 70 K, transition from the amorphous glassy state to the state of the supercooled liquid phase takes place, followed by its crystallization into the state of a plastic crystal. In the temperature range of 78–80 K, the transition of a plastic crystal to a crystalline state with a monoclinic lattice begins.

Refractive Index at Low Temperature of Tetrachloromethane and Tetrafluoroethane Cryovacuum Condensates

“…When the deposition temperature is below T = 55 K, one obtains an amorphous phase, as previously shown. 8,9 Therefore, a continuous increase in the refractive index is observed in the temperature range of the amorphous state from 16 to 50 K. Then, at temperatures below the glass transition temperature but above 50 K, a sharp change in the refractive index is observed, similar to what was observed for ethanol, methane, and nitrous oxide 23,24 followed by another step at the monoclinic crystallization above 80 K.…”

“…Depending on both the temperature of its deposition and the subsequent thermal history, , Freon 134a cryofilms can exist in various states. Specifically, thin films of Freon 134a obtained by direct cryocondensation at T = 16 K from the gas phase have been observed to undergo multiple structural transformations of different natures in the temperature range from 70 to 90 K. In particular, a typical glass transition from a structural glass state (amorphous solid) to a supercooled liquid (SCL) state is observed.…”

“…Although the physical properties of tetrafluoroethane in the gas phase have been studied with some detail, 4−7 its thermophysical and optical characteristics in the condensed state at low temperatures have seldom been studied. 8,9 On the other hand, tetrachloromethane or carbon tetrachloride (CCl 4 ) is a tetrahedral molecule, which has an interesting phase diagram as many other small-molecule organic materials. The phase diagram of CCl 4 has been investigated thoroughly in the low-temperature range.…”

“…1,1,1,2-tetrafluoroethane is nowadays widely used for domestic and industrial refrigeration systems, as well as air conditioning systems, liquid refrigeration systems, and heat pumps. Although the physical properties of tetrafluoroethane in the gas phase have been studied with some detail, − its thermophysical and optical characteristics in the condensed state at low temperatures have seldom been studied. , …”

Multiple glass transitions in vapor-deposited orientational glasses of the most fragile plastic crystal Freon 113

“…As far as we know, only ethanol, tetrachlormethane and Freon 134a, all of them exhibiting PC phases, have been studied by vapor deposition. [23][24][25][26] In all these works, only the SG was formed at deposition temperatures below Tg. On the contrary, we show that vapor deposition at T<Tg of the highly fragile Freon 113 yields structural and orientational glasses in the as-deposited state depending on the temperature range of deposition.…”