Two types of point defects (Shottky's vacancies and Frenkel's pairs) are well known. This paper is dedicated to the description of the third type of defects, coexisting with two named before. There are a number of physical effects which are determined just by these short-living lattice excitations, which obtain name "unstable vacancy – interstitial pairs" or "scintillating pairs". Frenkel's pair includes vacancy (v) and interstitial (i), being independent one from other, despite of the obvious fact, that Frenkel's pair components origin is their birth at one and the same site of crystal lattice. The famous paper of G.Vineyard et al (1959), using computer simulation of radiation damaging of crystals, showed, however, that in a vicinity of v the restricted space presents in which i can not exist. This region (r is its radius) had been named "instability zone" (IZ) in which any i recombines with v without activation at any low temperature. The great number of experiments with different crystals, using mostly the effect of saturating radiation defects concentration, confirmed the IZ existence. Its presence is determined by Coulomb or elastic i-v interaction, cutting of the periodic potential of i migration energy at the definite distance from v. Some years ago we showed that not only attractive potentials but repulsive as well form to absolute squeezing of i from IZ . Both reasons of IZ formation lead to the same result: the decreasing of defects' saturation concentration. But under ionizing irradiation the effect of focusing atomic collisions is taking place, leading to appearance of long distances between the point of birth of i and v and real point of knocked out i stop. The crystallographic criterion of radiation stability proposed is as following: L<hkl> < r<hkl>,where L is the free path length of crowdion, and down indices represent Miller indices for given crystallographic direction. A big number of compounds with loose crystal structure, where defocusing atomic lenses present shows extremely high radiation stability. Number of radiation stable semiconductor devices had been performed using materials of this materials series. Just loose structured semiconductors were the subject of special interest of outstanding Moldavian scientist S.I.Radautsan, now late. The IZ existence determines also a lot of non-radiation phenomena (diffusion, quenching, high temperature heat capacities and thermal conductivity as well as properties of super-ionic conductors). All thest are consequences of existence of special type of equilibrium point defects – short-living unstable pairs i-v, being the third type of defects in solids in addition to Shottky's and Frenkel's defects. Materials science perspectives will be discussed.