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Abstract:
We present the evolution of the low-temperature thermodynamic,
galvanomagnetic and thermoelectric properties of the type-I clathrate
Ba8NixGe46-x-y square(y) with the Ni concentration studied on
polycrystalline samples with 0.0 <= x <= 6.0 by means of specific heat,
Hall effect, electrical resistivity, thermopower and thermal
conductivity measurements in the 2-350 K temperature range and supported
by first-principles calculations. The experimental results evidence a 2a
x 2a x 2a supercell described in the space group Ia (3) over bard for x
<= 1.0 and a primitive unit cell a x a x a (space group Pm (3) over
barn) above this Ni content. This concentration also marks the limit
between a regime where both electrons and holes contribute to the
electrical conduction (x <= 1.0) and a conventional, single-carrier
regime (x > 1.0). This evolution is traced by the variations in the
thermopower and Hall effect with x. In agreement with band structure
calculations, increasing the Ni content drives the system from a
nearly-compensated semimetallic state (x = 0.0) towards a
narrow-band-gap semiconducting state (x = 4.0). A crossover from an
n-type to a p-type conduction occurs when crossing the x = 4.0
concentration i.e. for x = 4.1. The solid solution Ba8NixGe46-x-y
square(y) therefore provides an excellent experimental platform to probe
the evolution of the peculiar properties of the parent type-I clathrate
Ba8Ge43 square(3) upon Ge/Ni substitution and filling up of the
vacancies, which might be universal among the ternary systems at low
substitution levels.
