Characteristics of the thermodynamic and kinetic properties of NbH0.83 in the phase transition region

Measurements were made of the specific heat (in the temperature range 80-40 0 K), electrical resistivity (4.2-300 K), thermo-emf (4.2-300 K), thermal c onductivity (7-300 K), magnetic susceptibility (4.2-400 K), and lattice par ameters (30-300 K) of the alloy NbH0.83. The lambda' --> lambda --> beta ph ase transitions were studied. It was established that the structure of the niobium matrix of the hydride remains unchanged as a result of these transi tions. It is shown that the beta --> lambda phase transition (one-dimension al hydrogen ordering) is a three-stage one and is accompanied by a substant ial change in the shear modulus of the hydride. The lambda' phase existing at T < 110 K was determined. The electron thermal conductivity kappa(el) ca lculated for the lambda' phase in the range T < 23 K is 25% higher than the measured thermal conductivity. In order to explain this fact and also the discontinuity in the concentration dependence of the coefficient of thermal expansion of NbHx for x similar or equal to 0.83-0.84 and the approximatel y 1.5% compression of the NbH0.84 volume it is assumed that in the region o f x(c) in the lambda' phase the topology of the Fermi surface of the NbHx i nterstitial alloy changes substantially. (C) 2000 MAIK "Nauka/Interperiodic a".