Properties of CaB6 single crystals synthesized under high pressure and temperature

Abstract

Pure CaB 6 single crystals are synthesized under high pressure (1 GPa) and temperature (1050°C). The temperature-dependence of electric resistivity and Hall coefficient from 2 to 300 K shows that the CaB 6 single crystals are conductors with semi-metallic behavior and electron carriers. Band structure calculations indicate that the conduction and valence bands meet at the X point at the Fermi level, which is consistent with the experimentally determined conducting behavior of CaB 6 single crystals. Calculations of state density suggest that the states at the Fermi level originate from the 2p orbital of the B atoms and the 3d orbital of the Ca atom. Magnetization measurements show the paramagnetic nature of the CaB 6 . The micro-hardness of CaB 6 is 24.39 GPa, and the Raman spectra of CaB 6 yield three sharp peaks at around 780.9, 1138.9, and 1282.1 cm −1 for T 2g , E g , and A 1g , respectively. The specific heat of the crystal is measured and found to be well described by the Debye and Einstein combined model. The fitting results show Debye and Einstein temperatures are 1119 and 199 K, respectively. high pressure, CaB 6 , semi-metallic, single crystals PACS: 75.40.-s, 72.15.-v, 78.30.-j

Seebeck effect near quantum MIT in Eu1 − xCaxB6

“…A similar contribution to Seebeck effect from quasilocal phonon mode has been observed earlier in the related compound ZrB 12 14. The estimation of Θ E for CaB 6 happens to be much lower than the Einstein temperature Θ E = 199 K found from heat capacity 15. However, our value is in a reasonable agreement with the frequency ħω E = 9.3–10.8 meV, which determines the contribution of phonon resonant scattering to thermal conductivity 16.…”

Electrical transport properties ofCaB6

“…The temperature variation of the Hall coefficient reported in Ref. [18] follows the trend of our results although no Al flux had been used in crystal growth. The authors of this report attribute such behavior to a semi-metallic nature of CaB 6 .…”

“…However, it has been shown that their concentration in CaB 6 would be negligible for growth under thermal equilibrium conditions [17]. Recent results for a CaB 6 single crystal grown by a liquid Ca-solid B reaction under high temperature and high pressure conditions are in line with this conclusion [18]. The temperature variation of the Hall coefficient reported in Ref.…”

Clathrate‐Like Alkali and Alkaline‐Earth Metal Borides: A New Family of Superconductors with Superior Hardness

“…Elemental calcium (Ca) is one potential candidate because of its suitable atomic radius (1.80 Å) and high reactive activity with boron (i.e., CaB, CaB 2 , CaB 4, and CaB 6 ). [19,20,31] Considering the requirement for large boron content in clathrate-like framework, [9,13,29,30] we thus focus our investigation in this work on the clathrate structures in the CaB x (x = 1-12) systems throughout the pressure range of 0-200 Gpa by the USPEX and CALYPSO codes. [32][33][34][35][36][37] Here we report a pressure-stabilized Cmcm CaB 7 phase at 141 Gpa, constituted by a B 23 cage with one centered calcium atom, behaving as the first clathrate structure in AMB systems.…”

“…Alkali or alkaline-earth metals (AM) possessing high chemical activity are expected to form stable AM borides (AMB) with uncommon stoichiometries and exotic properties at the relatively lower P-T condition, which can be due to the effective adjusting of d-electron bands at pressures. [15][16][17][18] For instance, metallic calcium borides of P4/mbm CaB 4 [19] and Pm-3m CaB 6 [20] have been fabricated at 1-2 GPa and about 1300 K, which is much lower than that required P-T conditions for the synthesis of TMB systems. Very recently, we also successfully produced a novel honeycomb-like NaB 4 structure at 1.5 GPa and 1000 K, [21] predicted by our theoretical calculations as a pressure-stabilized phase at 80 GPa that can be quenchable under ambient conditions, [22] enabling fabrication of large-size productions (several centimeters in diameter).…”