Garland, C. W.

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V. ELASTIC AND THERMAL PROPERTIES OF SOLIDSProf. C. W. GarlandR. H. RenardC. F. YarnellRESEARCH OBJECTIVESThere is a variety of problems in the physics and chemistry of solids in which pho-nons (lattice vibrations) play an important role. The theory of lattice dynamics can beused to calculate many bulk properties of a crystalline material if appropriate inter-atomic force constants are known. The adiabatic elastic constants of a single crystal,which can be measured with high precision by ultrasonic pulse techniques, provide directinformation about the dispersion curves and frequency spectrum at low frequencies, andthus determine the low-temperature thermal properties. They also provide a way oftesting or evaluating the force constant parameters in various force models of a solid.Recently, attention has been focussed on the elastic properties of crystals nearlambda-point transitions. There are "anomalous" changes in the elastic constants of asolid near the critical temperature for an order-disorder transition; such changes arerelated to the detailed structural changes that occur and to the nature of the interatomicforces involved.Experiments performed in the past year showed that there are important variationsin the elastic constants of NH 4 C1 around the lambda-point transition. These variationsare caused partly by the change in the volume of the lattice which occurs at the lambda-point transition. To resolve these contributions, the pressure dependence of the elasticconstants of NH 4 C1 will be studied at several temperatures. A high-pressure systemcapable of achieving pressures up to 10, 000 atmospheres has been built for this study.A large constant-temperature bath was built in order to obtain good control of the tem-perature down to -30 C. To improve the accuracy of the measurements, the ultrasonicpulse method of H. J. McSkimin was adopted. Preliminary measurements of the elasticconstants of NH 4 C1 as a function of pressure at room temperature have been made.NH 4 Br and K2 SnC16 have lambda-point transitions at a convenient temperature.Crystals of NH4Br and K2SnC16 are being grown. The elastic constants of these crystalswill be measured as a function of temperature and pressure and the results comparedwith those for NH4 C1.C. W. GarlandPublicationsC. W. Garland and J. R. Wilt, Infrared spectra and dipole moments of Rh 2 (CO) 4 C12and RhZ(CO) 4 Br 2 , J. Chem. Phys. 36, 1094 (1962).C. W. Garland and K. C. Park, Low temperature elastic constants of GalliumArsenide, J. Appl. Phys. 33, 759 (1962).L. L. Isaacs and C. W. Garland, Heat capacity of crystalline polyethylene from1. 80 K to 5. 30 K, J. Phys. Chem. Solids 23, 311 (1962).C. W. Garland, Infrared spectra of adsorbed molecules, PB Report 149, 515 (1962),U. S. Department of Commerce, Office of Technical Services, Washington, D. C.D. P. Shoemaker and C. W. Garland, Experiments in Physical Chemistry (McGraw-Hill Book Company, New York, 1962).C. W. Garland and J. S. Jones, Elastic constants of Ammonium Chloride near thelambda transition, J. Chem. Phys. (in press).QPR No. 72

Elastic and Thermal Properties of Solids

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Elastic and Thermal Properties of Solids

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