Pressure effect and superconducting properties of lithium ternary borides Hiroyuki Takeya a, * , Mohammed El Massalami b , Kazuto Hirata a , Takashi Mochiku a , Shigeru Kasahara a , Kazumasa Togano a a National Institute for Materials Science, Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan b Instituto de Fisica, UFRJ, CxP68-528, 21945-970 Rio de Janeiro, Brazil Available online 28 March 2007 Abstract Li 2 Pd 3 B and Li 2 Pt 3 B show superconducting transition at the temperatures, 7.4 K and 2.6 K, respectively. These materials have the same cubic structure (P4 3 32) composed of distorted octahedrons, whose corners are occupied by Pd/Pt. This structure does not have mirror or inversion symmetry along any directions. It is a very interesting feature of those materials in relation to the symmetry of super- conductivity. The samples were prepared by a two-stage arc-melting method. Their superconductivity is induced by Li addition. Con- trolling the Li stoichiometry is a key to synthesize high quality samples in the preparation process of arc-melting because Li is volatile at high temperatures. The specific heat measurements were carried out by heat-pulse relaxation and semi-adiabatic methods. Pressure effect was measured with a SQUID magnetometer using a low temperature hydrostatic micropressure cell at pressures up to around 1 GPa. The superconducting transition temperatures of both Li 2 Pd 3 B and Li 2 Pt 3 B decreased linearly with pressure. The pressure dependence, dT c /dP, were 0.091 and 0.090 K/GPa, respectively. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Li 2 Pd 3 B; Li 2 Pt 3 B; Heat capacity; Pressure effect 1. Introduction Research of boride compounds with light elements has been encouraged since the discovery of MgB 2 . We have been searching new superconducting materials composed of light elements and transition metals. Those compounds are expected to reveal interesting physical properties. Superconductivity was recently confirmed in Li 2 Pd 3 B and Li 2 Pt 3 B around at 7.4 K and 2.6 K, respectively [1]. Two of these compounds consist of distorted octahedral coordi- nations of Pd/Pt–B in the cubic lattice structure with the space group of P4 3 32, which lacks both of mirror and inversion symmetry. In this report, their superconducting properties of heat capacity and pressure effect are mentioned. 2. Experimental Samples of Li 2 Pd 3 B and Li 2 Pt 3 B were prepared by a two-step arc-melting method described in Ref. [1,2]. Heat capacity was measured using relaxation-type and semi-adi- abatic calorimeters. Pressure dependence of superconductivity was measured using a micropressure cell in the SQUID magnetometer. The pressure cell was made of hardened beryllium–copper (Be–Cu) alloy. A sample was placed in the cell filled by Daphne oil as a pressure-transmitting fluid. At room tem- perature, force was transmitted to high pressure region up to 1 GPa by a Be–Cu pushing piston, which was clamped with a Be–Cu bolt. Sn as a manometer was also placed in the cell at a distance of 40 mm from the sample. The pres- sure at a low temperature was determined from the super- conducting transition temperature T c of Sn whose pressure dependence is well-known. 0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.03.162 * Corresponding author. Tel.: +81 29 859 2318; fax: +81 29 859 2301. E-mail address: takeya.hiroyuki@nims.go.jp (H. Takeya). www.elsevier.com/locate/physc Physica C 460–462 (2007) 89–90