Contains research objectivesJoint Services Electronics Programs (U. S. Army, U.S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E

Garland, C. W.

Contains research objectives.Joint Services Electronics Programs (U. S. Army, U.S. Navy, and U.S. Air Force) under Contract DA 28-043-AMC-02536(E

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XI. ULTRASONIC PROPERTIES OF SOLIDSAcademic and Research StaffProf. C. W. GarlandDr. D. B. NovotnyGraduate StudentsD. Eden R. T. Ruettinger B. B. WeinerP. E. Mueller N. E. Schumaker R. A. YoungD. D. SnyderRESEARCH OBJECTIVESThe general goal of our ultrasonic investigations is to obtain information about bulksystems near phase transitions or critical points. In particular, ultrasonic velocitiesgive direct information about the equilibrium thermodynamic properties, and ultrasonicattenuation provides data about the dynamical behavior. The majority of the work isconcerned with order-disorder lambda transitions in solids, but fluids near their criti-cal point are also being studied. During the past year, considerable progress has beenmade. Indeed, the work on NH4Br and potassium dihydrogen phosphate (KDP) is nowessentially complete.During our high-pressure work on NH4Br, a new ordered phase was discovered inthe region T ~ 1800 to 230'K and p > 2000 bar. The acoustic properties of this phasewere completely determined, and the regions of the phase-transition lines were care-fully studied. Data are now available on the previously known disordered-cubic toordered-tetragonal lambda transition, as well as on the new order-disorder lambdatransition and the order-order transition between the new high-pressure phase and thelow-pressure tetragonal phase. X-ray and infrared data have also been obtained at1 bar for temperatures down to 20"K. In contrast to the much simpler system NH4 C1,the NH4Br results clearly show that many complexities can occur in cooperative phe-nomena involving orientational ordering.Ultrasonic measurements are now complete on KDP in the paraelectric phase aboveits Curie point (Tc=121. 8K). Data on the sheer velocity as a function of temperatureshow that c 6 6 goes to zero at T according to an elastic Curie-Weiss law. Further-more, attenuation measurements were made at several frequencies and the ultrasonicloss was interpreted in terms of a very short, temperature-independent relaxation time-12(T10-12 sec). Data are not yet available below T c , because of the very large acousticlosses between 90 and 122 0 K. Such losses may be caused by domain scattering, andfurther work on the ferroelectric phase, with larger poling fields used, is in progress.Velocity and attenuation measurements are currently being made on single-crystalquartz near its a-P transition at 847"K. Longitudinal velocities in both the x and zdirections show a very sharp dip at the lambda temperature, but no clear indication ofa discontinuity. Preliminary attenuation results indicate that the relaxation time forthis system may be quite long, perhaps of the order of 10- 7 sec.Interest in the liquid-vapor critical point is still very great, and work is in progresson xenon near its critical point at 16. 60 C and 58 bar. Recent measurements on heliumThis work is supported by the Joint Services Electronics Programs (U. S. Army,U. S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E).QPR No. 88(XI. ULTRASONIC PROPERTIES OF SOLIDS)suggest that the adiabatic sound velocity may go to zero at a critical point, in contradic-tion to the classical theoretical view that it will approach a finite minimum. We plan to--6measure the velocity for AT/Tc values as small as 3 X 10 6 in order to test this behavior.Accurate acoustic attenuation measurements are also planned over a wide range of fre-quencies in order to obtain information about the dynamical response of a fluid near itscritical point. All equipment for this work has been assembled and tested. Control andmeasurement of the temperatures and pressure are excellent, and the ultrasonic gearis also working well. Unfortunately, difficulties with the variable-path cell have neces-sitated the redesign of several features of the cell. These modifications are almostfinished, and measurements should begin soon.In order to make direct measurements of the volume of an NH 4 C1 single crystal nearits lambda point at high pressures, we have designed and built a special capacitance cellcontaining two 1-pF capacitors. One capacitor is rigidly mounted and serves to deter-mine the dielectric constant of the gas at each pressure. The other capacitor has avariable gap, since one of its parallel plates is attached to the NH4Cl crystal. When thesample length decreases, the gap will increase by the same amount. Since one can detectcapacitance changes of 1 part in 105 , this method provides a very sensitive means ofstudying volume changes.C. W. GarlandPublic ationsa C. W. Garland and R. Renard, "Ultrasonic Investigation of the Order-Disorder Tran-sition in Ammonium Chloride, " Critical Phenomena, Proceedings of a conferenceheld in Washington, D.C., April 1965, NBS Misc. Publ. No. 273, pp. 202-210 (1966)." C. W. Garland and N. E. Schumaker, "Effect of Ordering on the Infrared Spectrum ofAmmonium Chloride," J. Phys. Chem. Solids 28, 799 (1967).C. W. Garland and R. A. Young, "Order-Disorder Phenomena VI: Anomalous Changesin the Volume of Ammonium Chloride," J. Chem. Phys. (in press).QPR No. 88

Ultrasonic Properties of Solids

XI. ULTRASONIC PROPERTIES OF SOLIDSAcademic and Research StaffProf. C. W. GarlandDr. E. LitovGraduate StudentsA. Bonilla P. E. MuellerD. Eden B. B. WeinerRESEARCH OBJECTIVESThe general goal of our ultrasonic investigations is to obtain information about bulksystems near phase transitions or critical points. In particular, ultrasonic velocitiesgive direct information about the equilibrium thermodynamic properties, and ultrasonicattenuation provides data about the dynamical behavior. The majority of the work isconcerned with order-disorder lambda transitions in solids, but fluids near their liquid-vapor critical point are also being studied. During the past year, progress was madein a variety of areas.1. High-pressure work on single-crystal ammonium bromide is now complete. Theadiabatic elastic constants were measured at 20 MHz as functions of temperature andpressure in the region from 180 0 K to 240°K and from 0 to 6 kbar. A new high-pressureordered phase, denoted OII, was discovered in this temperature range. The acousticproperties of this new OII phase were investigated with emphasis on the regions of thefirst-order phase transition from the OI phase to the ordered tetragonal phase, and thelambda transition from the Oii phase to the disordered cubic phase.2. The ultrasonic data obtained previously on potassium dihydrogen phosphate in itsparaelectric phase have been subjected to a careful review and analysis. The velocityof ultrasonic shear waves at temperatures above T = 121. 82K shows that there is anelastic Curie-Weiss law, (s 6 6 E-s 6 6 P) = D/(T-T ), with an elastic Curie constant D-11 -l 2equal to 6. 31 X 1011 dyn cm deg. The attenuation data are consistent with a cooper-ative relaxation time at constant stress - which varies as T = 24 X 10 1 2 /(T-T ) sec.cA new cell for the investigation of KDP below Tc (including improved provisions forpoling the crystal with an electric field) has been built, and Dr. E. Litov will soon beginmeasurements on the ferroelectric phase of KDP.3. The velocities of 30-MHz and 45-MHz longitudinal waves propagating in quartzparallel to the X- and Z-axes were measured as a function of temperature from 500'Cto 600 C, and their variations were studied with special care in the immediate regionof TX = 574°C. No unambiguous discontinuity (indicating a first-order instability) couldbe established from our data, although the recent Brillouin data of Shapiro and Cumminsdo indicate such an effect. It would appear that very precise temperature stability andresolution is required. Attenuation of 20-MHz and 60-MHz longitudinal waves propa-gating parallel to the X-axis was also measured. These attenuation data are difficultto interpret, but seem to suggest intense acoustic scattering arising from domainsThis work is supported by the Joint Services Electronics Programs (U. S. Army,U. S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E).QPR No. 92 143(XI. ULTRASONIC PROPERTIES OF SOLIDS)(microtwinning) over a range of approximately 20 near T. Outside this region thereis a much lower attenuation that is roughly proportional to the frequency, and does notconform to the expected critical variation in the relaxation time.4. The attenuation of a longitudinal wave propagating in the [100] direction of NII4Clwas measured as a function of temperature and pressure in the vicinity of the lambdaline. At seven temperatures between 241. 0OK and 270. 2'K measurements at 10,20, and 30 MHz were made as the pressure was varied from 1 bar to 3500 bar.One-atmosphere attenuation results were reinterpreted, by using recently obtainedhigh-frequency velocity values from Brillouin scattering (~17 GHz) to calculate therelaxation strength. In the disordered phase, TSV behaves approximately as A/(T-T )0 . 6along the 1-atm isobar. In the ordered phase, the TSV behavior at one atmosphereis more complicated and does not conform to simple theoretical models. Lines ofconstant attenuation in the ordered phase lie parallel to the lambda line; however,lines of constant attenuation in the disordered phase converge rather strongly towardthe lambda line at high pressures. Thus, it appears that the dynamical propertiesfollow a law of corresponding states in the ordered phase, but not in the disorderedphase.5. Much progress has been made in the past year on the liquid-vapor critical pointin Xenon, although recurrent experimental problems have prevented the collection ofultrasonic data until recently. Preliminary data on Xenon indicate that the "sing-around" method of velocity measurement will not work well without drastic modification.Therefore, the velocity measurements are being made by observing relative changes inthe time delay of an arbitrarily selected cycle in the amplified RF signal from thereceiving crystal as the transmitting crystal is moved by known amounts. The peak-to-peak amplitudes measured at the same time and on the same cycle are used to calculatethe attenuation. Uncertainties in the delay measurements are ~±0. 01 Fsec, and thosein the relative spacing of the transducers are ~+0. 002 mm. Measurements at 16. 94 0 C(~0.35°C above the critical temperature) over a narrow range of pressure (58.6-57.7 atm)have yielded values of the velocity ranging from 99 msec-1 to 122 msec -1, and valuesof the attenuation ranging from 255 dB cm to 5 dB cm On the basis of these mea-surements, it is expected that data can be obtained in regions where the attenuation-1is as large as 500 dB cm-16. By making direct measurements of the change in volume with pressure on asingle crystal of NfI 4 C1 at several constant temperatures, it is anticipated that wecan confirm the existence of a first-order transition which has been strongly sug-gested by elastic-constant measurement near the lambda line. Such a first-orderinstability associated with the NH 4C lambda transition has been predicted on thebasis of a compressible Ising model, but a direct observation of the phenomenonand an investigation of the way in which 6V changes along the lambda line is impor-tant. A great deal of operating experience has been obtained with the gas com-pressor capable of generating pressures up to 3.4 kbar. The sample cell, whichconsists of a precision 3-lead, parallel-plate capacitor whose lower plate is sup-ported by the crystal, has been constructed, assembled, and tested. The capacitanceis measured by an impedance bridge that is driven at 1 kHz and a phase-sensitive"lock-in" amplifier. Thus, all components seem to be operative and data shouldbe forthcoming soon.C. W. GarlandQPR No. 92 144(XI. ULTRASONIC PROPERTIES OF SOLIDS)PublicationsC. W. Garland and R. A. Young, "Order-Disorder Phenomena VI: Anomalous Changes ain the Volume of Ammonium Chloride," J. Chem. Phys. 48, 146 (1968).C. W. Garland, Book Review of "Ultrasonic Absorption," by A. B. Bhatia, in Phys.Today 21, 101 (1968).C. W. Garland and R. A. Young, "Elastic Constants of Ammonium Bromide II. High-pressure Ultrasonic Investigation of the Phase Transition," J. Chem. Phys.(scheduled for December 15, 1968 issue).C. W. Garland and D. B. Novotny, "Ultrasonic Velocity and Attenuation in KH2P0O4 ,"Phys. Rev. (scheduled for January 10, 1969 issue).QPR No. 92 145

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Ultrasonic Properties of Solids

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