The d.c. conductivity of NaI has been measured in the intrinsic and extrinsic temperature regions. The formation energy Es for the Schottky defects and the energy Em+ for the cation vacancy migration have been determined. The Em+ value has been found to be slightly higher than that required for the motion of the 'bound' vacancy in the system NaI + Ca++. The formation energy Es obtained is smaller than that of NaF, NaCl, and NaBr as was expected.

The transversal magnetoresistance was studied in single crystals of natural magnetite for different temperatures between 300 and 77 °K and in fields up to 10 kG. For temperatures above the transition point, i.e. in the cubic phase, the magnetoresistance is positive and shows a maximum at 5 kG. This effect becomes stronger the nearer temperature approaches the transition point. Below the tranisition point, i.e. in the orthorhombic phase, the magnetoresistance is negative increasing with magnetic field and with temperature up to the transition point. The effect of temperature change increases with increasing fields.

Bauxite samples collected with permission from currently active mining areas in the Parnassos-Ghiona Mts district were studied using both conventional (optical microscopy and powder XRD) and, for the first time in the literature, micro-scale advanced non-destructive spectroscopic techniques, in particular Laser μ-Raman. The results showed that the different bauxite types (diasporic or boehmitic) can easily be identified regardless of sample type (porous white-grey or massive redbrown) by recording spectra in the low-wavenumber region (250 cm' - 600 cm' ) where distinct bands of the natural AlOOHpolymorphs are easily discernible (448 cm' for diaspore and 362 cm' for boehmite). The method described herein could equally be applied in the laboratory as well as in-situ at the mines for quick and accurate phase determination, in order to bypass the laborious and time-consuming indirect bulk techniques (such as XRD) routinely used to this date.

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Magnetite, a ferrimagnetic semiconductor of inverse spinel struct,ure has attracted much attention due to a phase transition occurring at low temperatures. The transition temperature reported differs for the various properties under investigation. I n the present paper measurements are reported of the Hall effect and resistivity in the range from 77 to 380 OK.The 105 "K Hall resistivity isotherm and the ones for higher temperatures follow the usual trend for ferromagnetic materials. Contrary, the lower temperature isotherms, up to 100 OK, exhibit irregular behaviour, involving sign revcrsal. If we accept the average of the two temperatures, i.e. the value of 103 OK, as our transition temperature we observe that it lies, by some degrees, below the values found in theliterature. Oniy part of the difference can be attributed to a systematic error of our thermocouple. The previously mentioned sign reversal shifts to stronger magnetic fields as the temperature increases from 77 t o 100 OK.Magnetit, ein ferrimagnetischer Halbleiter mit inverser Spinelstruktur hat durch seinen Phaseniibergang bei niedrigen Temperaturen starke Beachtung gefunden. Die veroffentlichten Ubergangstemperaturen unterscheiden sich bei der Untersuchung verschiedener Eigenschaften. I n der vorliegenden Arbeit wird iiber Messungen des Hall-Effekts und der Leitfiihigkeit im Temperaturbereich zwischen 77 bis 380 "K berichtet. Die Isothermen des Hallwiderstandes bei 105 "K und hoheren Temperaturen folgen dem iiblichen Verlauf fur ferromagnetische Materialien. In) Gegensatz dazu zeigen die Isothermen fur tiefe Temperaturen bis 100 "K irregulares Verhalten, einschlieDlich Vorzeichenumkehr. Wenn man den Mittelwert der beiden Temperaturen, d. h. den Wert von 103 "K als in diesem Fall giiltige nbergangstemperatur akzeptiert, stellt man fest, daD er etwas unterhalb der Werte liegt, die in der Literatur angegeben werden. Nur ein Teil dieser Differenz kann einem systematischen Fehler des Thermoelements zugeschrieben werden. Die oben erwahnte Vorzeichenumkehr verschiebt sich zu starkeren Magnetfeldern, wenn die Temperatur von 77 bis 100 "K gesteigert wird.