ELASTIC PROPERTIES OF AMORPHOUS AND CRYSTALLINE B1-XCX AND BORON AT LOW-TEMPERATURES

We present measurements of the internal friction (Q(-1)) and speed of sound variation (delta upsilon/upsilon(0)) of amorphous boron (a-B) an d amorphous B9C (a-B9C). The elastic properties of these materials, wh ich can only be produced as thin films, are consistent with those of o ther amorphous solids measured to date and exhibit good agreement with the tunneling model (TM) of amorphous solids. The TM parameter (P) ov er bar gamma(t)(2)/rho upsilon(t)(2) extracted from the elastic data h as the same order of magnitude as that observed for all amorphous soli ds studied to date; a review will be presented. Using the results from the elastic measurements, we calculate the T-2 thermal conductivity i i expected in the TM regime (T less than or equal to K) for a-B. The p redicted thermal conductivity falls within the expected range for amor phous solids and agrees with the thermal conductivity of the crystalli ne icosahedral boride MB68-delta (M=Y, Gd), which has been previously shown to exhibit glass-like excitations. We have also measured the int ernal friction and speed of sound variation of bulk polycrystalline c- B1-xCx at low temperatures (0.07 K<T<10 K). The elastic properties evo lve towards the behavior characteristics of amorphous solids for incre asingly carbon-deficient (x<0.20) specimens. The magnitude of the inte rnal friction for the most carbon-deficient crystalline c-B1-xCx sampl e (x=0.1, c-B9C) is comparable to that for a-B and n-B9C, thereby conf irming the inherent glass-like vibrational properties of carbon-defici ent c-B1-xCx. Such behavior supports the glass-like character of carbo n-deficient c-B1-xCx high temperature (T>50 K) thermal transport repor ted previously and provides the first experimental evidence for the pr esence of two-level systems (TLS) in these crystalline solids. However , discrepancies with the tunneling model are present; the data for c-B 1-xCx bear some similarity to those for amorphous metals in which elec tronic relaxation channels are active, although details are still uncl ear. Previous studies have shown that the TM quantity C = P gamma(t)(2 )/rho upsilon(t)(2) (''tunneling strength'') is essentially independen t of the material's shear modulus G = rho upsilon(t)(2) over a factor of similar to 17. The elastic data presented in this work now extend t he observed independence of the tunneling strength, C, over a factor o f similar to 70 in shear modulus. (C) 1998 Elsevier Science S.A.