Calorimetric and Other Studies of Phase Transitions in Solids

Chihara, H.

doi:10.1002/bbpc.19830870303

Ber Bunsenges Phys Chem

1983

DOI: 10.1002/bbpc.19830870303

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Calorimetric and Other Studies of Phase Transitions in Solids

H. Chihara

Abstract: Kernquadrupolresonanz / Phasenum wandlungen / ThermodynamikPrecision calorimetry can provide detailed information about the phase transitions in solids which is utilized to give insight into the nature of transitions and to check the model theories for their validity. Examples of studies are reviewed in this vein on the soft librational mode transition in p-chloranil, the orientational glass transition in carbon monoxide, and the order-disorder transition in hydrogen halides.

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The nature of ammonium ion disorder in ammonium tetrafluoroaluminate, NH4AlF4

White

Wagner

1985

The Journal of Chemical Physics

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In order to clarify previous conflicting reports concerning the existence of an order-disorder phase transition in NH4AlF 4 at T -150 K, the heat capacity of this compound has been measured from T = 15 to 305 K, by adiabatic calorimetry. The calorimetric results show a small anomaly in Cp centered at T = 157 K (~ = 0.02 R), and this is attributed to a soft mode phase transition associated with slight changes in the (AlF 4): -layers. Analysis of the contribution of ammonium ion disorder to the heat capacity indicates that the ammonium ion disorder is gradually thermally excited, and can be adequately described by a three-dimensional Einstein oscillator for which (J E -400 K. There is no evidence of an order-disorder phase transition association with the ammonium ions.

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The nature of ammonium ion disorder in ammonium tetrafluoroaluminate, NH4AlF4

White

Wagner

1985

The Journal of Chemical Physics

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The phase transitions of ferroelastic K2SeBr6: A calorimetric and x-ray diffraction study

Abriel

White

1990

The Journal of Chemical Physics

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Guinier x-ray powder diffraction data in the temperature range from T = 10 to 290 K, and heat capacity measurements from T = 40 to 280 K indicate three phases for K z SeBr 6 . The high-temperature cubic structure [K 2 PtCI 6 -type, space group Fm3m; a = 10.4211 (7) A at T= 290 K] transforms at 237.8 ± 0.3 K with t::.Str = (1.36 ± 0.02) J K -I mol-I to a tetragonal phase with a Rb 2 TeI 6 -type structure [space group P4lmnc; a = 7.313(1) A, c = 10.474(2) A at T= 220 K). A second phase transition at 215 ± 1 K [t::.Str = (0.54 ± 0.01) J K -1 mol-l] results in a monoclinic phase with a K2 TeBr6 -type structure [space group P2 1 /n; a = 7.265(3) A, b = 7.265(3) A. c = 10.438(4) A. P = 89.66(5)" at T= 80 KJ. Both transitions can be interpreted in terms of softening of rotary phonons at the X edge of the Brillouin zone. From the thermal parameters of a single crystal xray structure determination at T = 293 K, this low-temperature instability can be predicted. The results agree with comparable data offerroelastic K2 TeBr6 and refute some earlier speculations concerning the phase transitions in K2 SeBr 6' J FIG. 1. K, ptel. structure. Small spheres are K, octahedra are Ptel. (SeBr. ). J. Chern. Phys. 93 (11).

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Calorimetric and Other Studies of Phase Transitions in Solids

Cited by 2 publications

References 27 publications

The nature of ammonium ion disorder in ammonium tetrafluoroaluminate, NH4AlF4

The nature of ammonium ion disorder in ammonium tetrafluoroaluminate, NH4AlF4

The phase transitions of ferroelastic K2SeBr6: A calorimetric and x-ray diffraction study

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