Along with above-considered s,p elements (Be, B, C, N, O, Al, etc.) that can enter as impurities into transition metal carbides and nitrides or form solid solutions with them, one of the most widespread impurities found in refractory compounds is H, which is present in the atmosphere during compound synthesis and enters into the composition of final products. Moreover, transition metal hydrides are sometimes used as starting agents (Pavlov, Zainulin and Alyamovsky, 1976) in the production of carbides or nitrides and their solid solutions.

Thus one faces a problem in studying the influence of hydrogen upon the characteristics (including the electronic ones) of transition metal carbides and nitrides. A rather large group of carbide- and nitride-based hydride phases formed in M−(C,N)−H systems has been obtained. However, until recently, attention has been focussed mainly on the methods of synthesis of H-containing phases, on optimal hydrogenation conditions for refractory compounds, as well as on studies of their physico-chemical properties (thermal expansion coefficient, lattice constants, phase composition, etc.) as a function of the dissolved hydrogen concentration (Pavlov et al, 1976; Samsonov, Antonova and Morozov, 1970; Samonov, Upadkhaya and Neshpor, 1974; Shveikin et al, 1984).

An important problem arising in the description of hydrogenated MX phases is that of the localisation sites of H atoms in the matrix. H-containing carbides and nitrides have been investigated rather extensively by the NMR, X-ray and neutron diffraction methods (see Rundqvist Tellgren and Andersson (1984)). The data obtained show that H atoms occupy crystal lattice positions, which are maximally distant from the p elements and are situated at least 2 Å from one another.