Renard, R. H.

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V. ULTRASONIC PROPERTIES OF SOLIDSProf. C. W. GarlandR. H. RenardC. F. YarnellA. ORDER-DISORDER LAMBDA TRANSITION IN AMMONIUM CHLORIDEAT HIGH PRESSURESThe lambda transition in NH4C1, which involves an ordering of the orientations ofthe NH 4 ion, has been extensively studied by the ultrasonic pulse technique. Both lon-gitudinal and tranverse (shear) acoustic velocities have been measured over a wide rangeof frequencies (5-60 Mc) and temperatures (150-3200K) at 1 atm, with special emphasison the region close to the lambda point (243 0 K). At a fixed frequency of 20 Mc and atfive temperatures spaced between 250 0K and 310 0 K, the pressure dependence of thevelocities has now been measured between 1 atm and 12, 000 atm, with emphasis on theregion where the isotherms cross the lambda line. (At 310 0 K the lambda transitionoccurs at -10, 000 atm; thus our pressure work gives data on both the ordered anddisordered phases at a series of temperatures.)The results of these velocity measurements as a function of pressure are shown inFigs. V-1, V-2, and V-3. The McSkimin pulse-superposition method was used, and thevalues of the three independent elastic constants are known within an error of less than0. 05%. As seen in Figs. V-1 and V-2, the shear constants c 4 4 and C' undergo a rapidincrease when the pressure is increased near the lambda line. This variation is relatedto the volume changes that occur in the crystal upon ordering. Indeed, it can be shownthat c 4 4 and C' show very little variation with temperature or the degree of orderingif the volume is maintained constant. The anomalous variations of the shear constantswith temperature, which were observed previously at constant pressure,1 can now beshown to be due to the anomalous volume changes and not to the ordering per se. Sincethese shear constants provide a sensitive probe for the behavior of the volume uponordering, it is interesting to note the rapidly changing character of the anomaly near thelambda point at high pressures.Longitudinal waves are strongly coupled to the ordering process, and c 1 1 values at1 atm show an abrupt dip to a finite minimum at T (and thus the adiabatic compress-ibility goes through a sharp but finite maximum). Our new data in Fig. V-3 show thistype of finite dip in c I 1 upon crossing the lambda line with increasing pressure. Noteagain the change in the behavior of the anomaly at higher pressures. For c 1 1,' there isstill an appreciable influence of the volume, but the entire anomaly is by no means alldue to volume changes. Figure V-4 shows the variation of c1 1 with temperature at a3 -1constant pressure of 1 atm and at a constant volume of 34. 15 cm mole . The mostimportant difference between these two curves is the way in which the minimum valueis approached from temperatures above and below. Thus, these new data, together withQPR No. 77I.0LUo.]o(I, 0zo 0.-800 3000 6000 9000 12,000PRESSURE (bar)Fig. V-I. Shear elastic constant c 4 4 ver-sus pressure at (1) 250. 7,(2) 265.0 ° , (3) 280.00, (4) 295.00,and (5) 308. 0K.5.05-OS4.5 540(23.0 0 3000 6000 9000 12.000PRESSURE (bar)Fig. V-3. Compressional elastic constantC 1 versus pressure at (1) 250.70,(2) 265.00, (3) 280.00, (4) 295.0,and (5) 308*K. Because of veryhigh attenuation near the lambdaline, data could not be obtainedthrough the lambda point exceptat 308°K; for other temperaturesthe anticipated behavior is shownby the dashed lines.12.000PRESSURE (bar)Fig. V-2. Shear elastic constant C' ver-sus pressure at (1) 250. 70,(2) 265.00, (3) 280.00, (4) 295.00,and (5) 308. 00 K.4.501-B: ''""~ A"Si4§Sia'3.501--100 150 200 250 300Fig. V-4. Variation of c1 1 with temper-ature at (a) constant pressureof 1 atm, and (b) constant vol-ume of 34.15 cm mole - 1 . Soliddots represent new 1-atm meas-urements at 20 Mc/sec, andwere obtained with the McSkiminmethod; open circles representcorrected data of Garland andJones 1 at 5 and 15 Mc/sec.QPR No. 771 --I I L ~_~I C I _-(V. ULTRASONIC PROPERTIES OF SOLIDS)our previous data, permit us to separate the effect of ordering, volume changes, andtemperature changes. A detailed analysis of the effect of ordering alone (at constantvolume and temperature) is in progress and will be reported elsewhere.R. H. RenardReferences1. C. W. Garland and J. S. Jones, J. Chem. Phys. 39, 2874 (1963).QPR No. 77

Ultrasonic Properties of Solids

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Ultrasonic Properties of Solids

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