INFLUENCE OF DEVIATIONS FROM THE CRYSTAL LATTICE PERIODICITY ON THE SEMICONDUCTOR-METAL PHASE TRANSITION IN VANADIUM DIOXIDE.

An experimental investigation was made of the influence of departures from the lattice periodicity on the semiconductor-metal phase transition in VO//2. There was no definite critical size r of a region with a periodic distribution of atoms (crystallite) below which the phase transition was absent although there were grounds to expect the influence of r on the phase transition in VO//2 to appear only for very low values of r (amounting to hundreds and, possibly, tens of angstroms). The occurrence of the phase transition was demonstrated for amorphous samples. Deviations from periodicity in the distribution of atoms (reduction in r) broadened the temperature range DELTA t//t within which there were changes in the optical and electrical properties associated with the phase transition; moreover, departures from periodicity increased the width of the thermal hysteresis region DELTA t//H and reduced the effective temperature of the phase transition t**e**f**f//t. The increases in DELTA t//t and DELTA t//H were attributed to, respectively, the spatial inhomogeneity of the factors influencing t//t and to the thermodynamic nonequilibrium of the state of matter with a disordered distribution of atoms. The lowering of t**e**f** f//t was explained by destabilization of the insulator state because of the destruction of the characteristic low-temperature order of the VV pairs and consequent delocalization of the 3d electrons located near these VV molecules because of the presence of structure defects. The spectrum of the changes in the optical properties at the phase transition was practically the same in polycrystalline and amorphous samples and this, together with the occurrence of the phase transition in the amorphous samples, was evidence of a decisive contribution made to the phase transition (and the associated changes in VO//2) by the components of the electron structure determined by the short order.