Study of α ↔ β transformation in the dimorphic clathrate Ba8Ga16Sn30

Abstract

The intermetallic clathrate Ba8Ga16Sn30 (BGS) is dimorphic, possessing α-phase (type-VIII) and β-phase (type-I). To determine the stability ranges of the two phases, we performed a study of the phase diagram by combining differential thermal analysis (DTA) and power X-ray diffraction (XRD) on both samples of bulk single crystals (≅1 mm) and powdered crystals (<50 µm). It was found that bulk samples of both α- and β-phases melt congruently at 500°C, but a powder specimen of β-phase has a broad endothermic DTA peak at 466°C, just below the melting point. Intensive studies on this event revealed a partially reversible and strongly kinetically controlled β → α structural phase transition. This transition has been confirmed by in situ synchrotron XRD at SPring-8. After heating the powder specimens to 530°C and cooling to room temperature, the main phase remained intact in both modifications. When the soaking temperature was increased from 530 to 910°C, a weak exothermic peak for nucleation of Ba(Ga/Sn)4 appeared at about 570°C in the cooling process. The main phase of the resulting materials was found to be β-phase, irrespective of the starting phases and grain sizes. Based on the results of DTA and XRD, we constructed a tentative Sn–Ba(Ga/Sn)4 pseudo-binary phase diagram. It suggests that large single crystals of β-BGS can be grown from the Sn flux without being disturbed by the α ↔ β structural phase transition.