The attenuation of low‐frequency (≈︁ 103 Hz) sound vibrations and the elastic modulus dispersion in mono‐ and polycrystalline niobium hydrides with hydrogen contents H/Nb ≈︁ 0.8 are investigated over the temperature range from 160 to 450 K. Sound attenuation and elastic modulus dispersion are anomalously high (Q−1 ≈︁ 0.14, ΔG/G0 ≈︁ 0.5) in the region of orthorhombic β‐phase. Such a strong anelastic relaxation is observed only during the motion of Bloch walls in ferromagnets. Such characteristics of the spectrum as a) sharp attenuation decrease outside the β‐phase region; b) a wide range of relaxation times in monocrystalline samples and its narrowing to a single spectral line in polycrystals;; c) a strong dependence of relaxation rate upon hydrogen concentration (sceeding the fourth power) show that the anelastic relaxation in the β‐phase of niobium hydride is caused by domain boundary motion under the action of elastic stresses.
Spectra of the attenuation of low-frequency oscillations of solid 4He were studied in the temperature range 0.3-2.2 K. The volume in which helium was solidified was a capillary made of stainless steel with a diameter of 0.5 mm, wall thickness of 0.1 mm,and length of 30 mm. The capillary was a quarter-wave vibrator; its resonance frequency in the transverse mode of oscillations was about 500 Hz. Excitation and detection of oscillations were made by means of a method described elsewhere. ! Attenuation of vibratorfree oscillations and its resonance frequency at a deformation amplitude of 10 -6 were measured.Solid helium was obtained by the blocked capillary method, and after solidification it was annealed at temperatures near the melting curve for 3 h to remove stresses created at crystallization. Crystals were obtained at pressures from 59 to 69 atm. The temperature in the above-mentioned range was maintained by pumping out 3He.The attenuation spectra consist of two relaxation maxima at temperatures 2 and 1.4 K at P = 58.9 atm and a background attenuation increasing with decreasing temperature below 0.8 K. An increase of pressure to 65.6 atm leads to a shift of the maxima to 1.9 and 1.65 K.At a pressure of 69.12atm the high-t~mperature maximum was at 1.75 K, while the low-temperature maximum was shifted to the temperature range below 0.3 K.As a rule, attenuation of oscillations of solids in the sound frequencies is caused only by the diffusion of defects in the crystal lattice. Since at helium temperatures heat-activated diffusion is practically absent, the appearance of relaxation maxima permits us to assume that a subbarrier mechanism of defect motion is involved.If, however, we suppose that diffusion is of an activation character (with a very low energy of activation), then an increase of pressure applied to the crystal can only decrease the frequency of diffusion jumps, which 5 9 1976 Plenum Publishing Corporation, 227 West 17th Street, New York, N.Y. 1001 I. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission of the publisher.
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