  Diamond properties down to the quantum-size region are still poorly
understood. High-pressure high-temperature (HPHT) synthesis from
chloroadamantane molecules allows precise control of nanodiamond size. Thermal
stability and optical properties of nanodiamonds with sizes spanning range from
$<$1 to 8 nm are investigated. It is shown that hypothesis about enhanced
thermal stability of nanodiamonds smaller than 2 nm is incorrect. The most
striking feature in IR absorption of these samples is the appearance of an
enhanced transmission band near the diamond Raman mode (1332 cm$^{-1}$).
Following previously proposed explanation, we attribute this phenomenon to the
Fano-effect caused by resonance of diamond Raman mode with continuum of
conductive surface states. We assume that these surface states may be formed by
reconstruction of broken bonds on the nanodiamond surfaces. This effect is also
responsible for the observed asymmetry of Raman scattering peak. he mechanism
of nanodiamond formation in HPHT synthesis is proposed, explaining pecularities
of their structure and properties.
