The hexagonal cell dimensions a and c of 2H‐MoS2, 2H‐MoSe2 and 2H‐WSe2 have been measured over the temperature range 20 to 800 °C. In all three compounds a and c increased non‐linearly with temperature, the linear thermal expansion coefficient of c being greater than that of a. Above 500°C the compounds begin to decompose in vacuum.

The electrical conductivity and Hall coefficient perpendicular to the c-axis of hexagonal MoS,, MoSe,, and WSe, are measured over the temperature ranges 120 to 117OK and 140 to 820K, respectively. The derived temperature-dependent carrier densities are described by nexp (-E/IcT) for temperatures less than 800 K indicating (thermal) energy gaps of 0.38 eV in n-type MoS,, 0.27 eV in n-type MoSe,, and 0.19 eV in p-type WSe,. Above room temperature the carrier mobilities are given by p -T-a, where a z 1.5 in MoS,, MoSe, and 2.7 in WSe,. The high-temperature (>800 K) thermal activation energies of conduction correspond to energy gaps of 1.27, 0.95, and 1.33 eV in MoS,, MoSe,, and WSe,, respectively. Similar measurements on MoTe, crystals grown by bromine vapour transport demonstrate the effects of included bromine on the low-temperature conduction processes.Es werden die elektrische Leitfahigkeit und der Hall-Koeffizient senkrecht zur c-Ache fiir hexagonales MoS,, Nose, und WSe, im Temperaturbereich 120 bis 1170 K bzw. 140 bis 820 K gemessen. Die abgeleiteten temperaturabhingigen Ladungstragerdichten werden durch n -exp (--E/kT) fiir Temperaturen kleiner als 800 K beschrieben, was (thermische) Energielucken von 0,38 eV in n-leitendem MoS,, 0,27 eV in n-leitendem MoSe,, und 0,19 eV in p-leitendem WSe, ergibt. Oberhalb Raumtemperatur werden die Triigerbeweglichkeiten durch p -T -a gegeben, wobei a = 1,5 in MoS,, MoSe, und 2,7 in WSe, betragt. Die thermischen Aktivierungsenergien der Leitfiihigkeit bei hohen Temperaturen (>SO0 K) entsprechen Energielucken von 1,27, 0,95 und 1,33 eV in MoS,, MoSe, bzw. WSe,. Ahnliche Messungen an MoTe,-Kristallen, die mit Bromdampftransport geziichtet wurden, zeigen den Einflu5 von Bromeinschlussen auf den Leitungsproze5 bei tiefen Temperaturen.

The hexagonal cell dimensions a and c of 2H-MoS2 and 2H-WSe2 have been measured over the temperature range 10 to 320 K. In both compounds a increased linearly and c non-linearly with temperature, the linear coefficient of thermal expansion in the c direction being greater than that in the a direction for 2H-MoS2. In 2H-WSe2 the linear coefficients of thermal expansion in the a and c directions were approximately equal.

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The optical constantsof synthetic 2H-MoS, have been determined over the range 0.05 to 3.75 eV. The absorption edge consists of two series of exciton bands due to direct allowed transitions. The Zeeman splitting and diamagnetic shift of these exciton lines are measured together with the effects of crystal confinement and temperature; the derived exciton parameters are different from those of molybdenite. Oscillatory magneto-absorption occurs a t the high energy side of each exciton series which gives a measure of the carrier masses.