Non-Debye excess heat capacity and boson peak of binary lithium borate glasses Yu Matsuda a, ⁎, Hitoshi Kawaji b , Tooru Atake b , Yasuhisa Yamamura a , Shuma Yasuzuka a , Kazuya Saito a , Seiji Kojima a a Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan b Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503, Japan abstract article info Keywords: Alkali borate glasses; Low-temperature heat capacities; Specific heat; Boson peak; Low-energy excitation; Vibrational density of states; Relaxation calorimetry; Non-Debye The non-Debye excess heat capacities of binary lithium borate glasses with different Li 2 O compositions of x = 8, 14 and 22 (mol%) are investigated to understand origin of the boson peak. The low-temperature heat capacities are measured between 2 and 50 K by a relaxation calorimeter. The experimental non-Debye heat capacities with x = 14 is successfully reproduced using the excess vibrational density of states measured by inelastic neutron scattering. This finding indicates that the non-Debye heat capacities of lithium borate glasses originate from the excess vibrational density of states measureable by inelastic neutron scattering. Moreover, it is demonstrated that all of the excess heat capacity spectra lie on a single master curve by the scaling using boson peak temperature and intensity. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The structures of glasses are random without any translational symmetry, and the lack of long-range order in the glassy network leads to physical properties markedly different from those of crystalline solids [1–3]. One of the significant differences is in the low-temperature heat capacity (C p ) [4]. Although C p for ideal crystals at low-temperatures is well described by the Debye T 3 law as an elastic continuum approximation, that of glass presented in a C p /T 3 –T plot shows a broad maximum centered at approximately 5–10 K above the Debye expectation, known as the “non-Debye excess C p ”. The origin of such a high C p for glasses has been discussed in connection with the low-energy excess vibrational density of states (VDoS), known as the “boson peak”, which is usually observed by inelastic neutron scattering (INS) or by Raman scattering. Despite considerable effort [1–4], the origins of the non-Debye excess C p and boson peak still remain serious open questions in condensed-matter physics and materials science. From the experimental viewpoint, the non-Debye C p of the alkali- metal borate glass family except for lithium borate glasses (LiB) has been studied. Results for pure B 2 O 3 glass [5–7], sodium borate glasses (NaB) [8] and cesium borate glasses (CsB) [9] have been reported. Moreover, the alkali-metal dependence for a specific composition (x = 14) [10] has recently been published. In particular, in Ref. [9] the non-Debye C p of CsB was discussed by comparison with that of NaB, and different behaviors were revealed. The hump in the excess C p of NaB shifted to a higher temperature, and the hump itself became much less intense with increasing Na 2 O. It was proposed that the excess C p of NaB is related to the propagating nature of the correlated excess VDoS, which are acoustic-like or strongly coupled with the acoustic phonons of borate network structures. In contrast, it has been revealed that the position of the hump for CsB does not change with the composition, indicating a localized nature for the vibrations resulting in the excess C p of CsB. The intensity of the hump of CsB exhibits an anomalous maximum in the Cs 2 O composition dependence. To obtain deeper insight into the origins of the non-Debye excess C p and the boson peak of the alkali-metal borate glass family, it is highly desirable to first study the composition dependence of the non- Debye C p of binary LiB as a fundamental system of the borate family. However, there are almost no reports of detailed studies on the low- temperature C p of LiB with reliable absolute values. In addition to the experimental viewpoint mentioned above, the LiB system itself is of considerable interest. It has been revealed that the physical properties of LiB such as density [11], sound velocity [11,12], stretched exponentiality [13], and Raman [14] and INS spectra [15] drastically vary with the composition, in contrast with CsB, and they show maxima or minima in their composition dependences owing to changes in the intermediate structure units. The Angell's fragility of LiB also markedly varies from strong to fragile [16], indicating that the LiB system can cover a wide variety of glass-forming materials from the viewpoint of strong-fragile classification. In this study, we investigate the non-Debye C p at low-temperatures and the boson peak of the LiB system as a function of Li 2 O com- position. The main purpose of this paper is to present the com- position dependences of the low-temperature C p of LiB as absolute Journal of Non-Crystalline Solids 357 (2011) 534–537 ⁎ Corresponding author. Present address: Glass Research Center, Central Glass Co. Ltd., Matsusaka, Mie 515-0001, Japan. E-mail addresses: hiiro_s721@yahoo.co.jp (Y. Matsuda), kojima@bk.tsukuba.ac.jp (S. Kojima). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.06.066 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol