Materials Science and Engineering A 442 (2006) 151–155 Acoustic resonances of relaxation nature in CsI single crystals in the temperature range 2–20 K S.N. Smirnov, P.P. Pal-Val ∗ , V.D. Natsik B. Verkin Institute for Low Temperature Physics & Engineering NAS of Ukraine, 47 Lenin Avenue, Kharkov 61103, Ukraine Received 5 September 2005; received in revised form 27 January 2006; accepted 9 February 2006 Abstract Acoustic relaxations in CsI single crystals are investigated in the temperature range 2–20 K at frequencies (1–7) × 10 5 Hz. The absorption peak found earlier at temperatures 3–5 K, which is caused by preliminary plastic deformation at room temperature, is studied in detail. A new relaxation resonance that corresponds to a thermally activated process with the activation energy U ≈ (6.1–8.9) × 10 -3 eV and the attempt frequency ν 0 ≈ 2 × 10 9 to 8 × 10 10 s -1 is revealed in the temperature interval 8–10 K. © 2006 Elsevier B.V. All rights reserved. Keywords: Acoustic relaxation resonances; Cesium iodide; Low temperatures 1. Introduction Acoustic spectroscopy methods at liquid helium temperatures allow studying some local structural rearrangements in crys- tals which are controlled by overcoming very small potential barriers of about 10 -2 to 10 -3 eV. Low-temperature relaxation resonances (internal friction peaks and modulus defects in the acoustic susceptibility) may be regarded as experimental evi- dence of existence of such rearrangements. In some cases, when interpreting concrete experimental data, it is possible to attribute a certain microscopic mechanism for low-energy structural rear- rangements using, for instance, the concept of geometrical kink motion along dislocation lines and overcoming of the second order Peierls relief [1–4]. Up to now, acoustic anomalies asso- ciated with dislocations in the low-temperature range T ∼ 10 K have been registered in metals [3–8] and in an alkali halide crys- tal CsI [9,10]. When studying low-temperature acoustic properties in CsI single crystals, it has been established that the plastic deforma- tion ε pl ≈ 3% at room temperature initiates an acoustic relax- ation peak that is localized in the temperature interval 3–5 K at elastic waves frequencies f ≈ (1–7) × 10 5 Hz [10]. An analysis has shown that the peak is caused by a thermally activated relax- ation process with the activation energy U ≈ 1.9 × 10 -3 eV and ∗ Corresponding author. Tel.: +38 57 3308513; fax: +38 57 3403370. E-mail address: palval@ilt.kharkov.ua (P.P. Pal-Val). the attempt frequency ν 0 ≈ 6.7 × 10 8 s -1 . In the present work, the investigation of low-temperature acoustic properties of CsI single crystals has been continued on two other series of samples. A systematic study of the peak caused by plastic deformation has been carried out and a new peak of internal friction local- ized in the temperature interval 8–10 K has been found. The new peak is characterized by different relaxation parameters and is, apparently, of other physical nature. 2. Experimental Acoustic measurements were made by the two-component composite piezoelectric vibrator technique in the frequency range f ≈ (1–7) × 10 5 Hz in a temperature interval 2–20 K. Lon- gitudinal standing waves were excited in the samples at fre- quencies close to first, third, fifth, and to seventh harmonics of piezoelectric quartz. Details of the experimental technique are described in [10,11]. The samples studied were right-angle par- allelepipeds with dimensions 2.2 mm × 2.2 mm × 13 mm; their longitudinal axis was parallel to the crystallographic orientation [1 1 0]. Annealing of samples was carried out in air at T = 773 K during 8 h. The strain amplitude of ultrasonic vibrations ε 0 varied in the range 2 × 10 -7 to 5 × 10 -5 . Amplitude dependence was exam- ined at a constant temperature with increasing ε 0 . Results of measurements were processed by a technique pro- posed by the authors in [11]: after a preliminary testing of 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.219