C. W. Garland

Order—Disorder Phenomena. III. Effect of Temperature and Pressure on the Elastic Constants of Ammonium Chloride

The adiabatic elastic constants of single‐crystal ammonium chloride have been measured at 20 Mc/sec as functions of temperature and pressure in the region of the lambda transition. At atmospheric pressure, data were obtained over the range of temperature from 150° to 320°K. At five temperatures, evenly spaced between 250° and 310°K, measurements were made as the pressure was varied from 0 to 12 kbar. The values at 300°K and atmospheric pressure are: c11 = 3.815, c44 = 0.8878, C′ = 1.4698, in units of 1011 dyn cm−2. A hysteresis of the critical temperature, equal to ∼0.9°K at atmospheric pressure, was observed; this hysteresis disappears rapidly as the hydrostatic pressure is increased. The results are in good agreement with those predicted for a compressible Ising model. In particular, it is concluded that the order—disorder transition in ammonium chloride is not of a simple lambda type. It is proposed that NH4Cl is unstable in the immediate vicinity of its critical point and undergoes a first‐order trans...

Nonadiabatic scanning calorimeter

A high-resolution computerized calorimeter capable of fully automatic operation in either ac or relaxation modes is described. Emphasis is given to a new version of the relaxation technique in which the heater power is ramped linearly in time. This improvement results in superior performance and convenience in studying both first- and second-order phase transitions and allows quantitative evaluation of latent heats as well as pretransitional heat capacity variations. Examples are given for the use of this calorimeter in the study of liquid crystal phase transitions.

Infrared Investigation of Structural and Ordering Changes in Ammonium Chloride and Bromide

Infrared spectra of NH4Cl, ND4Cl, NH4Br, and ND4Br have been recorded at numerous temperatures between 21 and 300°K using thin sublimed films, single‐crystal sections, and pressed disks. At 21°K each spectrum contains a large number of sharp intense bands which can be assigned to combinations involving the internal vibrational modes of the ammonium ion, the librational (torsional oscillation) mode, and various lattice modes. Observation of the first and second librational overtone makes it possible to discuss the potential barrier hindering the rotation of the ammonium ion. Barrier heights of 1860 cm−1 (5.32 kcal mole−1) for NH4Cl and ND4Cl and 1520 cm−1 (4.35 kcal mole−1) for NH4Br and ND4Br were calculated from the Gutowsky–Pake–Bersohn model, but the experimental frequencies show that the libration is more anharmonic than this model would predict. The temperature dependence of an anomalous component of the ν4 bending fundamental was studied in detail. The intensity of this component can be directly cor...

Order‐Disorder Phenomena. VII. Critical Variations in the Length of NH4Cl Single Crystals at High Pressures

A high‐precision capacitance method has been used to investigate variations in the length L of an NH4Cl single crystal in the vicinity of its order—disorder transition line. At low pressures, there is a small first‐order discontinuity ΔL superimposed on the lambdalike variation in L. At 255.95°K and 1491.8 bar, L varies continuously but κT and α appear to diverge. At higher pressures, the variation in L at the transition becomes progressively more gradual as the pressure increases. The linear isothermal compressibility in the immediate vicinity of the transition is well represented along an isotherm by κ L = (κ lim ∼L + B φ −y) / (1 + 2 pB φ −y), where φ ≡ |  p−p c  | /  p c and pc is the transition pressure for the given isotherm. The quantity κL has been assigned the constant value 1.57 × 10−6bar−1 for all isotherms; B and y (which are assumed to be independent of pressure) vary with the temperature. The linear isobaric coefficient of thermal expansion is fairly well represented along an isobar by α L =...

High-resolution ac calorimetry and critical behavior at phase transitions

Abstract An AC calorimeter technique operating at very low frequencies has been developed for the study of second-order phase transitions. Only a small amount of sample is required (50 to 100 mg), and samples with low thermal conductivity, such as insulator crystals, fluids and liquid crystals, can be investigated at I atm and at pressures up to ∼3000 bar. Two versions of this AC calorimeter will be described: a manually operated high-precision (±0.05%) calorimeter with a computerized data-acquisition system, and a fully computerized calorimeter with good precision (±0.2%) which can be operated in a scanning mode with linear drift rates in the range 0.01 K/hour to 1 K/hour. Experimental results and a discussion of their analysis will be presented for order-disorder transitions in ionic crystals (ammonium halides), consolute-point phase separation in binary liquids (3-methylpentane + nitroethane), orientational ordering in aqueous micelle solutions (cesium perfluoro-octanoate), and a variety of liquid crystal systems (with emphasis on transitions involving nematic and various smectic phases).

Ultrasonic Velocity and Attenuation in K H 2 P O 4

The velocity and attenuation of ultrasonic shear waves have been investigated at temperatures above

Generalized Pippard Equations

{T}_{C}={121.8}_{2}\ifmmode^\circ\else\textdegree\fi{}

Temperature and Pressure Dependence of the Elastic Constants of Ammonium Bromide

K in single-crystal KDP. An elastic Curie-Weiss law,

Ultrasonic investigation of the nematic‐isotropic phase transition in MBBA

({{s}_{66}}^{E}\ensuremath{-}{{s}_{66}}^{P})=\frac{D}{(T\ensuremath{-}{T}_{C})}

Order—Disorder Phenomena. I. Instability and Hysteresis in an Ising Model Near Its Critical Point

, is obtained with an elastic Curie constant